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What gains if any?
The basics. I've got a 1985 seaway V hull that weighs in around 2k. It is powered by an OMC 1.6 L seadrive. Full throttle range is listed as 4500 to 5500 RPM and has a 13 1/4 X 17 three bladed prop on it. The prop's condition is such that it has to be replaced come spring. I will, at some point, have the existing one repaired and use it for a spare. I fish. I don't foresee towing or pulling anything, skiers, float toys, etc, behind the boat. While high speed is nice my main interest is efficiency. Especially in fuel consumption and range. As it sits the boat runs about 30MPH at 5500 RPM and falls off the plane right around 20 MPH at 4600RPM. These figures may, considering the condition of the present prop, improve slightly with a new prop of the same size. I don't know that for sure though. Looking at the figures it appears to me that, though I would like to be able to maintain the plane down to about 17 MPH, this is about as good as it gets for the boat/engine combination. Is it? Would I be able to realize something more efficient with a different prop, and, more importantly for making my decision, What effects be if I were to change to a four blade and or change the pitch, maybe even change the size if that is possible? Stainless steel is not an option. I fish the rocky coast around Boston harbor and would rather replace a prop rather then the lower end should I get stupid. PLEASE, just comments on the matter at hand and try not to end up in an infantile name calling contest of no value. Thank you -- MikeG Heirloom Woods www.heirloom-woods.net |
On Sat, 18 Dec 2004 11:01:15 -0500, MikeG
wrote: In article 1103384689.d0185e76b5ed39533aed94649c4268d6@teran ews, says... If the prop you have now is the one that was spec'd as OEM, you're not likely to see dramatic differences by changing pitch or diameter or number of blades. You indicate you are getting full RPMs -5500- with your present prop. Changing prop pitch will change that. There are devices you can bolt onto your lower unit that may give you the ability to plane the boat at a lower speed...a 20 mph minimum planing speed seems awfully high to me, but I'm not familiar with your boat. I have a 25' boat that weighs 3+ tons and it planes at around 17 mph. A four-blader will give you some additional stern lift. The planning speed thing was what was bothering me also. It's the main reason I got to thinking about re propping. The present prop is in pretty sad shape. I'm hoping that even without changing anything a new prop of the same spec's will show an improvement in that area. We went through this a couple of weeks ago, only we were discussing wakes. The question I have is why do you need to have a planing speed of 17 mph. Efficiency is really a function of more than just planing the boat. It's more a function of hull style, engine rpm, prop size, etc. Unless you have direct injection, you aren't going to gain much in GPH if that is what you are after. A four blade will give you better take off, certainly, but will it help maintain a 17 mph planing speed? I don't think so. Later, Tom - whose first guests are arriving and will be back later - much later. :) |
Mike,
I have a 1985 21 ft Sea Ray 210C (cuddy cabin) with the OMC drive. Although the boat already had hydraulic trim tabs I found that I got a significant improvement in lowering the planning speed by adding one of those trim fins to the lower unit. It is much more effective at getting the bow down and the boat on plane than anything else that I've tried. I would suggest that you invest in one of these fins first. After you try it I'm sure that you will just replace the prop without changing the pitch. -- Charley "MikeG" wrote in message ews.com... The basics. I've got a 1985 seaway V hull that weighs in around 2k. It is powered by an OMC 1.6 L seadrive. Full throttle range is listed as 4500 to 5500 RPM and has a 13 1/4 X 17 three bladed prop on it. The prop's condition is such that it has to be replaced come spring. I will, at some point, have the existing one repaired and use it for a spare. I fish. I don't foresee towing or pulling anything, skiers, float toys, etc, behind the boat. While high speed is nice my main interest is efficiency. Especially in fuel consumption and range. As it sits the boat runs about 30MPH at 5500 RPM and falls off the plane right around 20 MPH at 4600RPM. These figures may, considering the condition of the present prop, improve slightly with a new prop of the same size. I don't know that for sure though. Looking at the figures it appears to me that, though I would like to be able to maintain the plane down to about 17 MPH, this is about as good as it gets for the boat/engine combination. Is it? Would I be able to realize something more efficient with a different prop, and, more importantly for making my decision, What effects be if I were to change to a four blade and or change the pitch, maybe even change the size if that is possible? Stainless steel is not an option. I fish the rocky coast around Boston harbor and would rather replace a prop rather then the lower end should I get stupid. PLEASE, just comments on the matter at hand and try not to end up in an infantile name calling contest of no value. Thank you -- MikeG Heirloom Woods www.heirloom-woods.net |
Based on what you use the boat I would suggest the following:
1. Install whale tail. This will help maintain plane at lower speeds. 2. Increase pitch to a 19. This will get your rpms down, increase top speed, and increase effeciency by traveling more distance per rpm. You obviously dont want to go below 4500 on top end and the 19 will put you at about 5200. 3. Going to a 4 blade is a matter of choice. It will cost more to purchase and repair. It will help on the acceleration. And will probably help on the planing speed but not as much as the whale tail. In this case - I would probably stay w/ the 3 blade. As for SS. There is a rubber hub that will break free long before you tear up the gears in the outdrive if all that hits is the prop. Small strikes - you will never know you did w/ the SS and the aluminum will have the ear tore off. SS will also give you a little better performance as the blade design is more efficient. Bottom line - Whale Tail and a SS 3 blade 19 pitch. You won't believe the difference. -- Tony my boats and cars at http://t.thomas.home.mchsi.com "MikeG" wrote in message ews.com... The basics. I've got a 1985 seaway V hull that weighs in around 2k. It is powered by an OMC 1.6 L seadrive. Full throttle range is listed as 4500 to 5500 RPM and has a 13 1/4 X 17 three bladed prop on it. The prop's condition is such that it has to be replaced come spring. I will, at some point, have the existing one repaired and use it for a spare. I fish. I don't foresee towing or pulling anything, skiers, float toys, etc, behind the boat. While high speed is nice my main interest is efficiency. Especially in fuel consumption and range. As it sits the boat runs about 30MPH at 5500 RPM and falls off the plane right around 20 MPH at 4600RPM. These figures may, considering the condition of the present prop, improve slightly with a new prop of the same size. I don't know that for sure though. Looking at the figures it appears to me that, though I would like to be able to maintain the plane down to about 17 MPH, this is about as good as it gets for the boat/engine combination. Is it? Would I be able to realize something more efficient with a different prop, and, more importantly for making my decision, What effects be if I were to change to a four blade and or change the pitch, maybe even change the size if that is possible? Stainless steel is not an option. I fish the rocky coast around Boston harbor and would rather replace a prop rather then the lower end should I get stupid. PLEASE, just comments on the matter at hand and try not to end up in an infantile name calling contest of no value. Thank you -- MikeG Heirloom Woods www.heirloom-woods.net |
First, I'd like to thank all the responders. You've touched on all the
points I've been pondering. I think the general trend of the collective wisdom is that, with the figures I supplied, I can possibly see some gains in efficiency but I am pretty close to the boat/motor combination being at a point where gains won't be significant. Rather then try to reply to each post individually let me see if I can cover them with this post. Trim tabs - I'd dearly love them if only to get rid of the starboard list when I am in the boat alone, which is most of the time. However if afraid they won't make this years budget. Four bladed prop - An idea I am playing with but, with the figures given and keeping the same diameter and pitch, what will that do to top end speed and RPM? Lower planning speed - This is the tricky one for me so let me know where I am going wrong. My boat has a planing hull as opposed to a displacement hull. It is designed to work most efficiently up on a plane rather then pushing the water out of the way al la displacement hull. The boat falls off the plane and starts pushing water below 20 MPH/4700 RPM. A lower planing speed may mean it takes longer to get to a fishing spot but if that means one or two more trips a month in the gas budget I can live with it. So, and this is where I can see I may be looking at things wrong, a lower planing speed means lower RPM s with the boat still operating efficiently, planning rather then displacing, and lower RPM's mean better mileage. Whale tail or similar - A budgetable item I have been playing with the idea of. The things I've read on these additions have all been advertisements giving glowing reports on them. However, life being what it is, you can't pay Peter with out stealing from Paul. What is the down side to be considered? Thanks again all Mike -- MikeG Heirloom Woods www.heirloom-woods.net |
On Mon, 20 Dec 2004 10:42:42 -0500, MikeG
wrote: ~~ snippage ~~ Lower planning speed - This is the tricky one for me so let me know where I am going wrong. My boat has a planing hull as opposed to a displacement hull. It is designed to work most efficiently up on a plane rather then pushing the water out of the way al la displacement hull. The boat falls off the plane and starts pushing water below 20 MPH/4700 RPM. A lower planing speed may mean it takes longer to get to a fishing spot but if that means one or two more trips a month in the gas budget I can live with it. So, and this is where I can see I may be looking at things wrong, a lower planing speed means lower RPM s with the boat still operating efficiently, planning rather then displacing, and lower RPM's mean better mileage. I think you are way over estimating the difference 3 mph will make on fuel consumption. Even after several long runs, I doubt if you are going to save a significant amount of fuel savings such that you will be able to make additional trips to the fishing grounds. What's the difference between 17 and 20 in terms of RPM - a 5/600 at the most? Not enough to justify tinkering with planing speeds. My Ranger is 20 foot, has a FICHT (which I admit is probably more efficient than EFI or a carbureted engine), but still, the difference between 1800 and 2300 is pretty insignificant. I don't have the figures right at hand, but as I remember, the difference was in the very low tenths in terms of GPH. Now having said that, I normally run at 30 mph which is right under 4k - there I can see savings over the max speed of 42/43 mph at 5500. But again, the difference is in tenths - closer to a full GPH, but still tenths. I'll look for the data when I return from fishing this afternoon and give you some examples. Whale tail or similar - A budgetable item I have been playing with the idea of. The things I've read on these additions have all been advertisements giving glowing reports on them. However, life being what it is, you can't pay Peter with out stealing from Paul. What is the down side to be considered? For my money, these things suck. The main reason is that they work even when you don't want them to work. And you lose the ability to close trim the motor at speed. On my Princecraft with the Johnson 25, yeah - they make a difference in a lightly loaded boat by keeping the bow down. For your application, it wouldn't be something that I would consider. Later, Tom |
I haven't been following this thread, but you can add SmartTabs for below
$200.00 ..maybe even below $150.00 depending on size ( buy the "scratch and dent" specials directly from the manufacturer, Nauticus ). I have them on two smaller boats. They DO keep you on plane longer and at lower speeds and they DO get you up on plane faster. I surely didn't buy them for cost (fuel) savings but rather to get up and out from the shallows. The over-all effects of Trim Tabs ( any brand of trim tabs ) is so very positive that I think every boatowner/manufacturer ought to consider adding them. -- RichG manager, Carolina Skiff Owners Group on MSN http://groups.msn.com/CarolinaSkiffOwners .. |
On Mon, 20 Dec 2004 20:42:10 GMT, "RichG"
wrote: I haven't been following this thread, but you can add SmartTabs for below $200.00 ..maybe even below $150.00 depending on size ( buy the "scratch and dent" specials directly from the manufacturer, Nauticus ). I have them on two smaller boats. They DO keep you on plane longer and at lower speeds and they DO get you up on plane faster. I surely didn't buy them for cost (fuel) savings but rather to get up and out from the shallows. The over-all effects of Trim Tabs ( any brand of trim tabs ) is so very positive that I think every boatowner/manufacturer ought to consider adding them. -- RichG manager, Carolina Skiff Owners Group on MSN http://groups.msn.com/CarolinaSkiffOwners I'm curious as to what kind of boat you have to make that claim. On my Ranger, trim tabs would be useless because of the transom design. On my Contender, I guarantee you that the trim tabs have nothing to do with getting on or off plane faster or longer. Lateral stability at speed yes - planing, no. Trim tabs aren't the end all for planing problems which is why I asked what kind of boat you have. They work for some, but not all. I have a suspicion that Mike's might not be one of those. Later, Tom |
"Gene Kearns" wrote in message ... On Sat, 18 Dec 2004 11:01:15 -0500, MikeG wrote: The planning speed thing was what was bothering me also. It's the main reason I got to thinking about re propping. The present prop is in pretty sad shape. I'm hoping that even without changing anything a new prop of the same spec's will show an improvement in that area. The direction of this thread seems to have made a left turn and gotten me a bit lost.... The *prop,* per se, has *nothing* to do with the speed at which a boat planes. While, you may well pick up some efficiency and/or top end with a new prop, it won't change the planing speed at all. the prop has nothing to do with the speed at which the boat planes, but it does affect the ability of the boat to maintain that speed before falling off plane. For instance, his boat may actually plane at 17 mph...but his current prop can't maintain the boat on plane at 17 mph, and it quickly falls off plane. It sounds to me like 20 mph is simply the minimum speed at which he's able to *maintain* plane...and he's calling that his minimum planing speed. Changing props won't change the speed at which the boat planes, but it very well may change the boat's ability to maintain a lower planing speed. I'd say try the 4-blade. If he drops the pitch, the engine will overrev...unless he also increases diameter to compensate. Of course, he could also have a prop shop add cupping to the blades, but then it becomes even less of a science and more of trial and error. |
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