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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 |
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