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Trim tabs size question
Hello. I have a 29' Chaparral (great boat) with twin v-6's and Volvo
duoprops. It comes on plane extremely fast if I work the trim tabs full and use the outdrive tilt. You almost don't even know the bow is raising! My question is...in lieu of the damn gas prices, I'm wondering if I could get better mileage if I added larger trim tabs so that the boat would plane at a slower speed. Right now, I have to get it to 3,000 rpm's to bring it down on plane. Any thoughts? Thanks! |
Larger trim tabs will help the boat to stay on plane at lower speeds but
they will create more drag which will defeat the purpose. For best fuel economy - you need to use minimal tabs (just enough to keep the boat level) and use enough throttle to keep the boat on plane. Throttle gives you more speed. Tabs create less speed due to drag. Speed vs throttle setting is what controls fuel economy. The more speed you have for a given throttle setting the better fuel economy you will have. W/ tabs buried to stay on plane you are creating a lot of drag and load on the engine. This really reduces economy. At least that is my (and several friends) experience w/ larger boats. -- Tony my boats and cars at http://t.thomas.home.mchsi.com "Big Daddy" wrote in message m... Hello. I have a 29' Chaparral (great boat) with twin v-6's and Volvo duoprops. It comes on plane extremely fast if I work the trim tabs full and use the outdrive tilt. You almost don't even know the bow is raising! My question is...in lieu of the damn gas prices, I'm wondering if I could get better mileage if I added larger trim tabs so that the boat would plane at a slower speed. Right now, I have to get it to 3,000 rpm's to bring it down on plane. Any thoughts? Thanks! |
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BD,
Since your question about getting the boat on plane is really about optimal fuel use, I'm wondering if you have fuel flow meters on the engines. If not, check out this link where several different manufacturer's products are shown: http://www.boatersland.com/fuelflow.html "Big Daddy" wrote in message m... Hello. I have a 29' Chaparral (great boat) with twin v-6's and Volvo duoprops. It comes on plane extremely fast if I work the trim tabs full and use the outdrive tilt. You almost don't even know the bow is raising! My question is...in lieu of the damn gas prices, I'm wondering if I could get better mileage if I added larger trim tabs so that the boat would plane at a slower speed. Right now, I have to get it to 3,000 rpm's to bring it down on plane. Any thoughts? Thanks! |
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Short Wave Sportfishing wrote in message ... On 21 Nov 2004 17:35:54 -0800, (Big Daddy) wrote: Hello. I have a 29' Chaparral (great boat) with twin v-6's and Volvo duoprops. It comes on plane extremely fast if I work the trim tabs full and use the outdrive tilt. You almost don't even know the bow is raising! My question is...in lieu of the damn gas prices, I'm wondering if I could get better mileage if I added larger trim tabs so that the boat would plane at a slower speed. Right now, I have to get it to 3,000 rpm's to bring it down on plane. Any thoughts? Based on my experience, I've never used trim tabs to bring a boat on or off plane. On my Contender, it's more a question of lateral balance in unfavorable sea conditions and for keeping proper bow entry in seas. Then again, I have outboards, so the conditions may be different. I don't think larger trim tabs are going to help much because of drag issues. I'm not all that familiar with I/Os, but with outboards, plane is more a function of proper engine trim and throttle setting. On my Ranger, a 20 footer which doesn't have trim tabs, it's all about engine angle and throttle setting. On the Contender, I hardly twitch the tabs unless I absolutely have to. I would think that larger trim tabs would create more drag, thus less performance, thus more gas used. Later, Tom IMO and personal experience, the results of using trim tabs and probably their optimum size varies from boat to boat, so there is no generic answer. Like Tom pointed out, trim tabs are primarily for lateral positioning due to heeling into the wind or unbalanced lateral loads. Many boats that are under or marginally powered benefit from lowering the tabs to help get on plane, but it is a balance of lifting the stern and generating more drag. If the OP's boat is underpowered, my guess would be that larger tabs might help lift the stern to get on plane which will the significantly reduce overall hull drag. Once on plane, he should slowly raise the tabs back up, while watching his tach until he finds the "sweet spot" of max RPM for a fixed throttle setting. Somewhere I think I remember reading that tabs should have an inch of width for every foot of boat length. Just my opinion. Eisboch |
On Mon, 22 Nov 2004 13:13:13 GMT, "Eisboch"
wrote: Many boats that are under or marginally powered benefit from lowering the tabs to help get on plane, but it is a balance of lifting the stern and generating more drag. I went through this exercise with my previous boat (Bertram 33 sportfish). With a full load of fuel and a lot of gear on board it was difficult to get the bow down to the correct attitude. The boat was planing but cruising speed was a knot or two short of what it should have been and the boat didn't feel quite right. The folks at Bennet tabs were helpful in recommending the correct size: http://www.bennetttrimtabs.com/ If the OP's boat is underpowered, my guess would be that larger tabs might help lift the stern to get on plane which will the significantly reduce overall hull drag. Once on plane, he should slowly raise the tabs back up, while watching his tach until he finds the "sweet spot" of max RPM for a fixed throttle setting. That's about right. I would start by setting my throttles at correct cruising RPMs and then adjust the tabs for best speed. That usually causes RPMs to increase so I would then throttle back slightly and readjust. Whether or not a particular boat will be helped by larger tabs is somewhat problematic. It depends on the boat, how it is powered, how heavily it is loaded, and weight distribution. |
Harry Krause wrote in message news:1101130176.uzrdBXuEidsC/CCUbMhMLQ@teranews... When, exactly, is a boat on plane and how can you tell, precisely, when this happens? It certainly is easy enough to tell in small boat, but it's not always so easy to tell on a larger one. My Parker, for example, breaks onto plane at around 17-19 mph, but seems to remain on plane down to about 13 mph...in that the bow is still raised a bit, the wake still is fairly flat, and if there is any chop, the spray is tossed off in the usual manner. But that exact moment of being on or off plane seems difficult to determine... I don't know what the exact definition of being on plane is. In my mind, it's when the boat speed is sufficient to "climb" up and over the bow wake and the hull section that meets the water shifts significantly aft. I'd be interested in other, more accurate definitions. On my boats, it's more of a "feel" thing. Eisboch |
On Mon, 22 Nov 2004 22:27:31 GMT, "Eisboch"
wrote: Harry Krause wrote in message news:1101130176.uzrdBXuEidsC/CCUbMhMLQ@teranews... When, exactly, is a boat on plane and how can you tell, precisely, when this happens? It certainly is easy enough to tell in small boat, but it's not always so easy to tell on a larger one. My Parker, for example, breaks onto plane at around 17-19 mph, but seems to remain on plane down to about 13 mph...in that the bow is still raised a bit, the wake still is fairly flat, and if there is any chop, the spray is tossed off in the usual manner. But that exact moment of being on or off plane seems difficult to determine... I don't know what the exact definition of being on plane is. In my mind, it's when the boat speed is sufficient to "climb" up and over the bow wake and the hull section that meets the water shifts significantly aft. I'd be interested in other, more accurate definitions. On my boats, it's more of a "feel" thing. I don't think there is an exact definition in nautical terms, but if I had to guess, it probably means "to glide", in this case, "to glide across the surface". Which would mean that the hull has to be sufficiently lifted to properly "glide". On my Ranger, I have zero bow lift - once the boat is powered up and hits about 2800 or so, it just....um....goes. RPMs come up, speed increases quickly and I'm "gliding". Actually at full speed, I have to trim the engine in to get it to turn - it won't turn otherwise. :) The Contender is a little different. I go get bow lift, but I actively trim the engines on take off which reduces bow lift and when I do it right (and I'm pretty good at it now), the boat just kind of zooms up and we're off to the Race. Get it - off to the Race? Never mind. Anyway, that's my opinion on the definition - to glide across a surface with minimal effort. Later, Tom |
Short Wave Sportfishing wrote in message ... The Contender is a little different. I go get bow lift, but I actively trim the engines on take off which reduces bow lift and when I do it right (and I'm pretty good at it now), the boat just kind of zooms up and we're off to the Race. Get it - off to the Race? Never mind. I get the Point. Eisboch |
On Mon, 22 Nov 2004 08:29:29 -0500, Harry Krause
wrote: When, exactly, is a boat on plane and how can you tell, precisely, when this happens? ================================== Good question. We all know it when we see it, but the formal definition is tougher. I think it relates to how much of the hull is riding above the water as opposed to plowing through it. That implies that planing could be defined as a percentage of hull lift versus displacement, for example 50% or more of the hull normally below the water, rises above water level when "on plane". This jibes reasonably well with the popular phrase "out of the hole", meaning "on plane". Another definition might involve flow separation at the transom. Most times a boat "on plane" will have little or no transom surface directly touching the water even though the bottom of the transom is still below the water line. It is also tempting to define planing as a ratio of actual speed to so called "hull speed" but there are plenty of heavy displacement boats that exceed theoretical hull speed by wide margins without actually giving the appearance of being on plane. Motor yachts and fishing trawlers come readily to mind as examples. They do it through sheer power, pushing aside a lot of water as opposed to actually rising above it. By the time a boat is going over two or three times its theoretical hull speed however, it usually gives the appearance of being on plane regardless of weight. There might be another mathematical approach worth considering. As a boat begins to exceed theoretical hull speed it takes huge increases in power to increase speed by relatively small percentages. Typically power required varies as the cube of speed. As a boat begins to plane however, the power required still increases exponentially with speed, but at a slower rate, more like the square of speed opposed to the cube. Using that approach, planing would be defined as the mathematical inflection point where the speed/power exponent begins to decrease. Comments? |
Wayne.B wrote in message ... On Mon, 22 Nov 2004 08:29:29 -0500, Harry Krause wrote: When, exactly, is a boat on plane and how can you tell, precisely, when this happens? ================================== Good question. We all know it when we see it, but the formal definition is tougher. I think it relates to how much of the hull is riding above the water as opposed to plowing through it. That implies that planing could be defined as a percentage of hull lift versus displacement, for example 50% or more of the hull normally below the water, rises above water level when "on plane". This jibes reasonably well with the popular phrase "out of the hole", meaning "on plane". Another definition might involve flow separation at the transom. Most times a boat "on plane" will have little or no transom surface directly touching the water even though the bottom of the transom is still below the water line. It is also tempting to define planing as a ratio of actual speed to so called "hull speed" but there are plenty of heavy displacement boats that exceed theoretical hull speed by wide margins without actually giving the appearance of being on plane. Motor yachts and fishing trawlers come readily to mind as examples. They do it through sheer power, pushing aside a lot of water as opposed to actually rising above it. By the time a boat is going over two or three times its theoretical hull speed however, it usually gives the appearance of being on plane regardless of weight. There might be another mathematical approach worth considering. As a boat begins to exceed theoretical hull speed it takes huge increases in power to increase speed by relatively small percentages. Typically power required varies as the cube of speed. As a boat begins to plane however, the power required still increases exponentially with speed, but at a slower rate, more like the square of speed opposed to the cube. Using that approach, planing would be defined as the mathematical inflection point where the speed/power exponent begins to decrease. Comments? I am not sure I'd buy paragraph #2 which relates to flow separation at the transom. Certainly true of a speedy, planning hull, it is also true of some of the boats you described in paragraph #3. I've seen and been on large, heavy, but highly powered boats that certainly are not planning, yet are going fast enough to have none of the transom in the water, other than at the water line. In fact, if powerful enough, they will create a "roostertail", but still be plowing through the water. I like your first explanation. Your math approach is probably correct, but I get a headache doing it in my head. Eisboch |
Around 11/22/2004 8:07 PM, Eisboch wrote:
Wayne.B wrote in message ... On Mon, 22 Nov 2004 08:29:29 -0500, Harry Krause wrote: When, exactly, is a boat on plane and how can you tell, precisely, when this happens? ================================== Another definition might involve flow separation at the transom. Most times a boat "on plane" will have little or no transom surface directly touching the water even though the bottom of the transom is still below the water line. I am not sure I'd buy paragraph #2 which relates to flow separation at the transom. Certainly true of a speedy, planning hull, it is also true of some of the boats you described in paragraph #3. I've seen and been on large, heavy, but highly powered boats that certainly are not planning, yet are going fast enough to have none of the transom in the water, other than at the water line. Yeah, my dad's Chris does that, even though we don't usually take her above 8 or 9 knots, and she's definitely not on a plane then. Aside: I can remember when we were breaking in the new Crusader about 12 years ago and we were supposed to run it up to full throttle for a while. Commando was, if not fully up, darn near up on a plane and making well over 15 knots, almost keeping up with the Mukilteo-Clinton ferry. It wasn't a pretty sight since she was really unstable and tended to wallow from side to side; my dad said it was like watching a little old lady hitch up her skirts and try to run. :) -- ~/Garth - 1966 Glastron V-142 Skiflite: "Blue-Boat" "There is nothing - absolutely nothing - half so much worth doing as simply messing about in boats." -Kenneth Grahame, The Wind in the Willows |
Harry Krause wrote:
Eisboch wrote: Short Wave Sportfishing wrote in message . .. On 21 Nov 2004 17:35:54 -0800, (Big Daddy) wrote: Hello. I have a 29' Chaparral (great boat) with twin v-6's and Volvo duoprops. It comes on plane extremely fast if I work the trim tabs full and use the outdrive tilt. You almost don't even know the bow is raising! My question is...in lieu of the damn gas prices, I'm wondering if I could get better mileage if I added larger trim tabs so that the boat would plane at a slower speed. Right now, I have to get it to 3,000 rpm's to bring it down on plane. Any thoughts? Based on my experience, I've never used trim tabs to bring a boat on or off plane. On my Contender, it's more a question of lateral balance in unfavorable sea conditions and for keeping proper bow entry in seas. Then again, I have outboards, so the conditions may be different. I don't think larger trim tabs are going to help much because of drag issues. I'm not all that familiar with I/Os, but with outboards, plane is more a function of proper engine trim and throttle setting. On my Ranger, a 20 footer which doesn't have trim tabs, it's all about engine angle and throttle setting. On the Contender, I hardly twitch the tabs unless I absolutely have to. I would think that larger trim tabs would create more drag, thus less performance, thus more gas used. Later, Tom IMO and personal experience, the results of using trim tabs and probably their optimum size varies from boat to boat, so there is no generic answer. Like Tom pointed out, trim tabs are primarily for lateral positioning due to heeling into the wind or unbalanced lateral loads. Many boats that are under or marginally powered benefit from lowering the tabs to help get on plane, but it is a balance of lifting the stern and generating more drag. If the OP's boat is underpowered, my guess would be that larger tabs might help lift the stern to get on plane which will the significantly reduce overall hull drag. Once on plane, he should slowly raise the tabs back up, while watching his tach until he finds the "sweet spot" of max RPM for a fixed throttle setting. Somewhere I think I remember reading that tabs should have an inch of width for every foot of boat length. Just my opinion. Eisboch When, exactly, is a boat on plane and how can you tell, precisely, when this happens? It certainly is easy enough to tell in small boat, but it's not always so easy to tell on a larger one. My Parker, for example, breaks onto plane at around 17-19 mph, but seems to remain on plane down to about 13 mph...in that the bow is still raised a bit, the wake still is fairly flat, and if there is any chop, the spray is tossed off in the usual manner. But that exact moment of being on or off plane seems difficult to determine... Crumbs this lying idiot doesn't even have the brains to properly understand the magazine test he tried to steal this from, as self serving evidence he owns a boat:-) Too too sad:-) K |
On Mon, 22 Nov 2004 22:38:45 -0500, Wayne.B
wrote: On Mon, 22 Nov 2004 08:29:29 -0500, Harry Krause wrote: When, exactly, is a boat on plane and how can you tell, precisely, when this happens? ================================== Good question. We all know it when we see it, but the formal definition is tougher. I think it relates to how much of the hull is riding above the water as opposed to plowing through it. That implies that planing could be defined as a percentage of hull lift versus displacement, for example 50% or more of the hull normally below the water, rises above water level when "on plane". This jibes reasonably well with the popular phrase "out of the hole", meaning "on plane". That makes sense, but I'm more inclined to describe "plane" as the least amount of hull in the water at cruise. I'm also not sure you can apply the word to boats like yours and Mr/Mrs E's as they are not true planing hulls. To my mind, which is a strange and curious place I'll admit, the act of planing on water is similar to how an airplane works. You use a lot of power to get the boat up on plane, but once there, you throttle back to maintain speed and attitude. To do that on the water, more hull has to be out than in. Yes/No? Another definition might involve flow separation at the transom. Most times a boat "on plane" will have little or no transom surface directly touching the water even though the bottom of the transom is still below the water line. I can't get into this one as I've been on big sport fisher's that clearly weren't planing, but plowing, although easily and without much bow lift. When you apply a lot of power to a small area, you will get voids and that is essentially what is happening in this circumstance. It is also tempting to define planing as a ratio of actual speed to so called "hull speed" but there are plenty of heavy displacement boats that exceed theoretical hull speed by wide margins without actually giving the appearance of being on plane. Motor yachts and fishing trawlers come readily to mind as examples. They do it through sheer power, pushing aside a lot of water as opposed to actually rising above it. By the time a boat is going over two or three times its theoretical hull speed however, it usually gives the appearance of being on plane regardless of weight. I need to give that one some thought. My initial reaction is that power can't replace the concept of the least amount to do the most work which would imply that brute power is not a factor in planing. There might be another mathematical approach worth considering. As a boat begins to exceed theoretical hull speed it takes huge increases in power to increase speed by relatively small percentages. Typically power required varies as the cube of speed. As a boat begins to plane however, the power required still increases exponentially with speed, but at a slower rate, more like the square of speed opposed to the cube. Using that approach, planing would be defined as the mathematical inflection point where the speed/power exponent begins to decrease. Comments? Oh - I like it. The problem is that you'd have to build a set of universal variables to account for weight, length, beam, deadrise, lift strakes, power, blade pitch or perhaps solve for the individual variables, then use those as plug in's for an equation for different speeds and power settings.... ~~ mutter - naval architecture - grumble ~~ Later, Tom |
On Tue, 23 Nov 2004 04:07:40 GMT, "Eisboch"
wrote: Wayne.B wrote in message .. . On Mon, 22 Nov 2004 08:29:29 -0500, Harry Krause wrote: When, exactly, is a boat on plane and how can you tell, precisely, when this happens? ================================== Good question. We all know it when we see it, but the formal definition is tougher. I think it relates to how much of the hull is riding above the water as opposed to plowing through it. That implies that planing could be defined as a percentage of hull lift versus displacement, for example 50% or more of the hull normally below the water, rises above water level when "on plane". This jibes reasonably well with the popular phrase "out of the hole", meaning "on plane". Another definition might involve flow separation at the transom. Most times a boat "on plane" will have little or no transom surface directly touching the water even though the bottom of the transom is still below the water line. It is also tempting to define planing as a ratio of actual speed to so called "hull speed" but there are plenty of heavy displacement boats that exceed theoretical hull speed by wide margins without actually giving the appearance of being on plane. Motor yachts and fishing trawlers come readily to mind as examples. They do it through sheer power, pushing aside a lot of water as opposed to actually rising above it. By the time a boat is going over two or three times its theoretical hull speed however, it usually gives the appearance of being on plane regardless of weight. There might be another mathematical approach worth considering. As a boat begins to exceed theoretical hull speed it takes huge increases in power to increase speed by relatively small percentages. Typically power required varies as the cube of speed. As a boat begins to plane however, the power required still increases exponentially with speed, but at a slower rate, more like the square of speed opposed to the cube. Using that approach, planing would be defined as the mathematical inflection point where the speed/power exponent begins to decrease. Comments? I am not sure I'd buy paragraph #2 which relates to flow separation at the transom. Certainly true of a speedy, planning hull, it is also true of some of the boats you described in paragraph #3. I've seen and been on large, heavy, but highly powered boats that certainly are not planning, yet are going fast enough to have none of the transom in the water, other than at the water line. In fact, if powerful enough, they will create a "roostertail", but still be plowing through the water. I like your first explanation. Your math approach is probably correct, but I get a headache doing it in my head. I've got the ultimate definition. Planing - the act of making a lot of wood shavings out of a large piece of wood. Later, Tom "Beware the one legged man in a butt kicking contest - he is there for a reason." Wun Hung Lo - date unknown |
On Tue, 23 Nov 2004 07:51:27 -0500, Harry Krause
wrote: With a 225 four stroke, my Parker with a half load of fuel, two guys aboard, and some "stuff" will hit between 39 and 40 mph at WOT, according to my GPS. And that number is what's on spreadsheet Parker sent me a year or so ago. On the Yamaha "performance" site, the same boat with a 300 hp two stroke DFI will hit 44.1 mph under similar load conditions...a 33% increase in horsies only produces about 5 mph in top end. There are, however, more worthwhile differences in the rpm/cruise speed curge. ================================================== =========== OK, let's do the math. An increase from 225 to 300 hp is a ratio of 1.33 (300 / 225). Taking the square root of 1.33, we get 1.15, and 39 mph times 1.15 is 45 mph. That agrees fairly well with the notion that speed increases in proportion to the SQRT of hp at planing speeds. Theoretical hull speed for a Parker 25 is about 6 mph which you could probably achieve with a 15 hp outboard in calm conditions. To go from 6 mph to 12 mph, double the speed in non-planing conditions, takes a lot more power, perhaps 8 times as much if power required is varying at the cube root of speed. That would imply 120 hp which sounds about right, perhaps half throttle on your 225. At some speed between 12 mph and 20 mph, probably around 18 or 19, the boat should begin planing and the power increase required for more speed should drop back more towards the first set of numbers. That's the basis for my conjecture that a mathematical definition might have some merit. The problem of course is measuring the transition without a whole boatload of instrumentation. :-) For another data point, I have a 12 foot inflatable which will plane off at 13 or 14 mph when lightly loaded, using about half throttle on a 15 hp Merc. |
Wow. Thanks for all the great comments. Let me clarify a few
things... I don't even need to use the trim tabs to get on plane, even with full tank and many people. I guess the duoprop twin outdrives do the trick. I will check into the flowmeters, but I think the reason I don't buy them is that I'd like to get good at finding what you have all called "the sweet spot" by watching the gps speed vs rpm while making adjustments. All I can think of is all the gas I can buy for the 2 or 3 hundred dollars for flow meters for my twins! (I hope that isn't a dumb answer). What I really should have asked is; Would I get better fuel mileage by running on plane at a slower speed with larger trim tabs versus running at a faster minimum planing speed with the tabs I have now. I think you have all answered my question. Leave it alone. The tabs are Bennet and installed new on the boat. The boat runs great the way it is. I really like the advice to watch rpm and speed while changing things. Thanks a lot. Hey, one more quick one. Has anyone else noticed in some of the magazine boat reviews where they show the rpm, boat angle, and mpg that in some tests, the best mpg that they measure is when the boat angle is at it's steepest, like 5 degrees? That seems odd to me. That, I would think is the very worst angle for good gas mileage and yet they show it as the best. Thanks Wayne Short Wave Sportfishing wrote in message . .. On 21 Nov 2004 17:35:54 -0800, (Big Daddy) wrote: Hello. I have a 29' Chaparral (great boat) with twin v-6's and Volvo duoprops. It comes on plane extremely fast if I work the trim tabs full and use the outdrive tilt. You almost don't even know the bow is raising! My question is...in lieu of the damn gas prices, I'm wondering if I could get better mileage if I added larger trim tabs so that the boat would plane at a slower speed. Right now, I have to get it to 3,000 rpm's to bring it down on plane. Any thoughts? Based on my experience, I've never used trim tabs to bring a boat on or off plane. On my Contender, it's more a question of lateral balance in unfavorable sea conditions and for keeping proper bow entry in seas. Then again, I have outboards, so the conditions may be different. I don't think larger trim tabs are going to help much because of drag issues. I'm not all that familiar with I/Os, but with outboards, plane is more a function of proper engine trim and throttle setting. On my Ranger, a 20 footer which doesn't have trim tabs, it's all about engine angle and throttle setting. On the Contender, I hardly twitch the tabs unless I absolutely have to. I would think that larger trim tabs would create more drag, thus less performance, thus more gas used. Later, Tom |
On Tue, 23 Nov 2004 10:24:06 -0500, Harry Krause
wrote: With a smaller, open boat, you can see when you are on plane. On the Parker, that's pretty much impossible, though there are certain "feels" at various speeds. ================================================== == It's been my experience that the transition point gets much fuzzier as boat weight and length increase. With something like a Hatteras 53 weighing 50,000 pounds or so, they never really look or feel like they are on plane even when going 20 knots. My Bertram 33 weighed about 23,000 pounds and was clearly on plane at 13 to 14 knots with a little help from the trim tabs. It really didn't FEEL like it was on plane however until it got over 20 kts. At that point the hull was well out of the water and it had the feel of gliding over the water instead of plowing it aside. |
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On Tue, 23 Nov 2004 18:47:36 -0500, Wayne.B
wrote: On Tue, 23 Nov 2004 10:24:06 -0500, Harry Krause wrote: With a smaller, open boat, you can see when you are on plane. On the Parker, that's pretty much impossible, though there are certain "feels" at various speeds. ================================================= === It's been my experience that the transition point gets much fuzzier as boat weight and length increase. With something like a Hatteras 53 weighing 50,000 pounds or so, they never really look or feel like they are on plane even when going 20 knots. My Bertram 33 weighed about 23,000 pounds and was clearly on plane at 13 to 14 knots with a little help from the trim tabs. It really didn't FEEL like it was on plane however until it got over 20 kts. At that point the hull was well out of the water and it had the feel of gliding over the water instead of plowing it aside. AH HA!!! See - "gliding".... WHOO HOO!!!! I love it when I'm right. :) Later, Tom |
On Mon, 22 Nov 2004 10:58:57 -0000, "BrianR"
wrote: "JohnH" wrote in message .. . On 21 Nov 2004 17:35:54 -0800, (Big Daddy) wrote: Hello. I have a 29' Chaparral (great boat) with twin v-6's and Volvo duoprops. It comes on plane extremely fast if I work the trim tabs full and use the outdrive tilt. You almost don't even know the bow is raising! My question is...in lieu of the damn gas prices, I'm wondering if I could get better mileage if I added larger trim tabs so that the boat would plane at a slower speed. Right now, I have to get it to 3,000 rpm's to bring it down on plane. Any thoughts? Thanks! I use no tabs unless I want the bow down for rough water, i.e., to give a little smoother ride while on plane. John H How does this answer his question? Brian Better than your post! John H On the 'PocoLoco' out of Deale, MD, on the beautiful Chesapeake Bay! |
On Sun, 21 Nov 2004 22:11:43 -0500, Harry Krause
wrote: JohnH wrote: On 21 Nov 2004 17:35:54 -0800, (Big Daddy) wrote: Hello. I have a 29' Chaparral (great boat) with twin v-6's and Volvo duoprops. It comes on plane extremely fast if I work the trim tabs full and use the outdrive tilt. You almost don't even know the bow is raising! My question is...in lieu of the damn gas prices, I'm wondering if I could get better mileage if I added larger trim tabs so that the boat would plane at a slower speed. Right now, I have to get it to 3,000 rpm's to bring it down on plane. Any thoughts? Thanks! I use no tabs unless I want the bow down for rough water, i.e., to give a little smoother ride while on plane. John H Nice that you have tabs and only use them on rough water, Herring, but...that isn't what the poster asked. Oh...and his boat is much heavier than yours, and is powered differently. D'oh. I can see that this response is much more your style, Harry. Do you think it helped answer his question? John H On the 'PocoLoco' out of Deale, MD, on the beautiful Chesapeake Bay! |
On Wed, 24 Nov 2004 00:00:19 GMT, Short Wave Sportfishing
wrote: AH HA!!! See - "gliding".... ========================= Yes, that light feeling of skipping across the wave tops. I know it well, and like it as much as anyone else, BUT many heavy boats are truly "on plane" well before that point is reached. And many heavy boats never reach that point even though they start planing somewhere between 13 and 20 knots. Which brings us back to the original question in a circular sort of way: When is a boat "on plane" ? I'd be inclined to vote for 2.5X the theoretical hull speed. Easy enough to calculate, and sort of intuitive. It may not be dead right for every boat but it should be in the ball park. |
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Wayne.B wrote in message ... On Tue, 23 Nov 2004 10:24:06 -0500, Harry Krause wrote: With a smaller, open boat, you can see when you are on plane. On the Parker, that's pretty much impossible, though there are certain "feels" at various speeds. ================================================== == It's been my experience that the transition point gets much fuzzier as boat weight and length increase. With something like a Hatteras 53 weighing 50,000 pounds or so, they never really look or feel like they are on plane even when going 20 knots. My Bertram 33 weighed about 23,000 pounds and was clearly on plane at 13 to 14 knots with a little help from the trim tabs. It really didn't FEEL like it was on plane however until it got over 20 kts. At that point the hull was well out of the water and it had the feel of gliding over the water instead of plowing it aside. I get a similar feel with the Navigator. At 15 knots or so I can sense that it is making a transition, and at 19 to 20 knots it sort of glides and will bank into turns. I tweak with the tabs until the boat "feels good" and it usually will add a knot or so to my speed. The Navigator has very little bow rise however, compared to a Hat that typically takes a bow to the sky attitude. The Navigator hull seems to lift uniformly and maintains a similar attitude at 19 knots as it does sitting in the slip. One of the claims of fame of the designer, Jules Marshall, is an efficient hull design that allows a decent cruise speed with smaller engines. At 19 knots it burns approximately 26 gals/hr with the twin 370 hp Volvo's and displacing somewhere around 42000 lbs loaded up. Eisboch |
On Wed, 24 Nov 2004 05:07:52 GMT, "Eisboch"
wrote: At 19 knots it burns approximately 26 gals/hr with the twin 370 hp Volvo's and displacing somewhere around 42000 lbs loaded up. ==================================== That is excellent fuel economy for a boat that size going 19 kts. Most Hatt 53s would be burning over 40 gph. |
Wayne.B wrote in message ... On Wed, 24 Nov 2004 05:07:52 GMT, "Eisboch" wrote: At 19 knots it burns approximately 26 gals/hr with the twin 370 hp Volvo's and displacing somewhere around 42000 lbs loaded up. ==================================== That is excellent fuel economy for a boat that size going 19 kts. Most Hatt 53s would be burning over 40 gph. It really is and it verified the fuel burn graph provided by Navigator when I bought the boat. I was a little dubious of it at first but made several checks on our trip to Florida when we would spend 8 hours or so at an average of 19 knots. The graph was right on the money. Eisboch |
Wayne.B wrote in message ... On Wed, 24 Nov 2004 05:07:52 GMT, "Eisboch" wrote: At 19 knots it burns approximately 26 gals/hr with the twin 370 hp Volvo's and displacing somewhere around 42000 lbs loaded up. ==================================== That is excellent fuel economy for a boat that size going 19 kts. Most Hatt 53s would be burning over 40 gph. A Hatt 53 is also significantly heavier I think. Eisboch |
On Tue, 23 Nov 2004 22:01:12 -0500, Wayne.B
wrote: On Wed, 24 Nov 2004 00:00:19 GMT, Short Wave Sportfishing wrote: AH HA!!! See - "gliding".... ========================= Yes, that light feeling of skipping across the wave tops. I know it well, and like it as much as anyone else, BUT many heavy boats are truly "on plane" well before that point is reached. And many heavy boats never reach that point even though they start planing somewhere between 13 and 20 knots. Which brings us back to the original question in a circular sort of way: When is a boat "on plane" ? I'd be inclined to vote for 2.5X the theoretical hull speed. Easy enough to calculate, and sort of intuitive. It may not be dead right for every boat but it should be in the ball park. I'll vote for gliding. :) Seriously, sounds good to me. Later, Tom |
On Tue, 23 Nov 2004 22:01:12 -0500, Wayne.B
wrote: On Wed, 24 Nov 2004 00:00:19 GMT, Short Wave Sportfishing wrote: AH HA!!! See - "gliding".... ========================= Yes, that light feeling of skipping across the wave tops. I know it well, and like it as much as anyone else, BUT many heavy boats are truly "on plane" well before that point is reached. And many heavy boats never reach that point even though they start planing somewhere between 13 and 20 knots. Which brings us back to the original question in a circular sort of way: When is a boat "on plane" ? I'd be inclined to vote for 2.5X the theoretical hull speed. Easy enough to calculate, and sort of intuitive. It may not be dead right for every boat but it should be in the ball park. I just thought of something - would this be true of displacement hulls? Later, Tom |
On Tue, 23 Nov 2004 22:10:12 -0500, Wayne.B
wrote: On 23 Nov 2004 15:31:50 -0800, (Big Daddy) wrote: Hey, one more quick one. Has anyone else noticed in some of the magazine boat reviews where they show the rpm, boat angle, and mpg that in some tests, the best mpg that they measure is when the boat angle is at it's steepest, like 5 degrees? That seems odd to me. That, I would think is the very worst angle for good gas mileage and yet they show it as the best. ============================ Depends on the boat and how it is loaded. 5 degrees is not all that much but it is probably enough to get the forward third of the boat out of the water and thus reducing hull friction. If you increase the angle too much the stern squats and digs in, creating a different kind of drag. I would also expect that a certain amount of bow rise helps the boat climb onto the surface of the water just as a small angle of attack helps an airplane climb through the air. In each case the downward flow deflection creates an upward lifting force. My Ranger has zero bow lift. There is a slight tilt when power is applied, but for the bow to actually "lift" is damn near impossible - I know, I've tried raising and lowering the motor, different tilt angles. As power is applied, the boat just lifts itself out of the water sans bow lift. I've never been able to figure that out. Most bay boats in my experience have similar bow lift to bass boats - this one doesn't. Later, Tom |
On Wed, 24 Nov 2004 11:26:29 GMT, Short Wave Sportfishing
wrote: Which brings us back to the original question in a circular sort of way: When is a boat "on plane" ? I'd be inclined to vote for 2.5X the theoretical hull speed. Easy enough to calculate, and sort of intuitive. It may not be dead right for every boat but it should be in the ball park. I just thought of something - would this be true of displacement hulls? ========================================== A lot of so called displacement hulls could probably plane to some extent if you could get enough power to the water. There are many heavy sailboats that will briefly plane if you can get them surfing down a wave with a lot of wind in the spinnaker. I suspect there are some hulls, tugboats come to mind, that would be hard pressed to plane regardless of power. The hull shapes just don't have enough flatness to them. Lack of planar areas? It's difficult to skip a round rock. |
On Wed, 24 Nov 2004 11:29:02 GMT, Short Wave Sportfishing
wrote: My Ranger has zero bow lift. There is a slight tilt when power is applied, but for the bow to actually "lift" is damn near impossible - I know, I've tried raising and lowering the motor, different tilt angles. As power is applied, the boat just lifts itself out of the water sans bow lift. I've never been able to figure that out. Most bay boats in my experience have similar bow lift to bass boats - this one doesn't. ============================ I think it's all in the hull shape. |
On Wed, 24 Nov 2004 10:01:32 -0500, Wayne.B
wrote: On Wed, 24 Nov 2004 11:29:02 GMT, Short Wave Sportfishing wrote: My Ranger has zero bow lift. There is a slight tilt when power is applied, but for the bow to actually "lift" is damn near impossible - I know, I've tried raising and lowering the motor, different tilt angles. As power is applied, the boat just lifts itself out of the water sans bow lift. I've never been able to figure that out. Most bay boats in my experience have similar bow lift to bass boats - this one doesn't. ============================ I think it's all in the hull shape. This hull is based and identical to the 620 series Ranger bass boats. Those have considerable bow lift. :) Live long and prosper, Tom |
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