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Old 07-29-2014, 05:02 AM   #61
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I've noted occasional reference to horsepower required to move a given boat at a given speed. In fact it is pounds of propeller thrust that moves the boat...or airplane. Keeping that in mind helps keep the units of measure straight and is important when looking at the various efficiencies (prop, drive line and engine) as you work backward to the engine fuel map.
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Old 07-29-2014, 08:00 AM   #62
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Terminology can get out of hand in the one or two paragraph world of the internet forum for sure....but then so can facts and opinions in general.

Here's something to feed the thoughts of those that really either enjoy all of this or think they actually know things for sure...

I was reminded when it was shared with me that even toy boat designers buy tank time. Many of the principles written about apply, maybe on a different scale, but are probably considered as well by any designer working on recreational vessels where similar concerns are thought about.

http://www.mandieselturbo.com/files/...4_02%20low.pdf
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Old 07-29-2014, 08:29 AM   #63
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it is indeed thrust that moves the boat however that thrust is produced by engine torque. People are altering propellers with unknown effect on prop efficiency in order to change engine operating parameters in another unknown manner since they don't know actual fuel consumption changes. A big part of the problem with these discussions is that they start with what we have information about, the engines but we don't have needed fuel use data. RPM curves are misinterpreted to infer fuel use since actual fuel information is unavailable.
The blind men are indeed describing the elephant.
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Old 07-29-2014, 09:16 AM   #64
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I'm not so sure...some opinions are being built around charted data....

More like astronomers versus visionaries describing the universe...both known and unknown...
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Old 07-29-2014, 10:03 AM   #65
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psneeld, Excellent link, thank you.
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Old 07-29-2014, 10:04 AM   #66
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it is indeed thrust that moves the boat however that thrust is produced by engine torque.

That thrust is produced by the propeller. The propeller is turned by the engine/gearset combination.
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Old 07-29-2014, 10:08 AM   #67
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Nothing need be an unknown about over propping in boats or gear choices for industrial uses. With a bit of research and money spent, the risk/reward for one's particular case can be determined. Several on this forum are experts in this regard but understand the pitfalls of raising their hand too high on the internet.

As an aside, prop and engine selection for production boats like SeaRay, Carver, Bayliner etc is normally done for a half load condition. Then comes full tanks, monstrous dinghies, towing an even bigger dinghy, pots, pans, washers, dryers, fishing chairs and before you know it the vessels weight has sky rocketed. Then the black smoke and overheating begins. Curing the ills associated with over propping a large planing style vessel can cost a lot, especially if the engines have been cratered.

Quite simply, this discussion is about the pros and cons of wrong propping. On a lightly used trawler operating at less than 50% load, the engine will not unduly wear out if an inch or two out in prop size. But if your engine is right sized like Mark's Coot, best you right size the props.
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Old 10-14-2014, 01:00 AM   #68
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We just went back in the water after a haulout at Townsend Bay Marine in Port Townsend. I took the advice of many on this forum and reduced pitch by 2" on our 44" wheel. I had been fine with a bit of over propping, since we run at around 1/3 of available horsepower most of the time. My thought was that if you run at low power settings, the likelihood of experiencing any adverse effects from over propping are going to be pretty minimal, and I still believe that is correct.

However, I can report my run back from P.T. today was very encouraging and validated the idea that it is not a bad idea to avoid over propping. My results were the expected increase in max rpm from reducing pitch from 1925 of 2200 to 2075. Still a bit light, but close enough for now. EGT at max rpm, however, declined by 125 degrees, and this can only be good. The sweet spot for cruising (from feel) went from 1350 to 1475 rpm, yet fuel economy at the increased rpm is about the same per knot as it was with the more aggressive pitch at a lower rpm, so I don't think economy changed. I'll know more once I collect more comparative data, but for now, I am a converted over pitcher to a reformed right pitcher. The engine just seems "happier".
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Old 10-14-2014, 01:42 AM   #69
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I'll know more once I collect more comparative data, but for now, I am a converted over pitcher to a reformed right pitcher. The engine just seems "happier".
That is precisely what I experienced. The whole vessel seems "happier" including the skipper.

Thanks for sharing your findings.

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Old 10-14-2014, 09:19 AM   #70
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Delfin:
Thanks for your actual conditions report. I'll be interested in hearing your results with heavy head seas that tend to stall the boat.
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Old 10-14-2014, 10:19 AM   #71
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Originally Posted by Delfin View Post
We just went back in the water after a haulout at Townsend Bay Marine in Port Townsend. I took the advice of many on this forum and reduced pitch by 2" on our 44" wheel. I had been fine with a bit of over propping, since we run at around 1/3 of available horsepower most of the time. My thought was that if you run at low power settings, the likelihood of experiencing any adverse effects from over propping are going to be pretty minimal, and I still believe that is correct.

However, I can report my run back from P.T. today was very encouraging and validated the idea that it is not a bad idea to avoid over propping. My results were the expected increase in max rpm from reducing pitch from 1925 of 2200 to 2075. Still a bit light, but close enough for now. EGT at max rpm, however, declined by 125 degrees, and this can only be good. The sweet spot for cruising (from feel) went from 1350 to 1475 rpm, yet fuel economy at the increased rpm is about the same per knot as it was with the more aggressive pitch at a lower rpm, so I don't think economy changed. I'll know more once I collect more comparative data, but for now, I am a converted over pitcher to a reformed right pitcher. The engine just seems "happier".

I found the same thing, I went from 22X20 to 22X17 (4 blade) and wot went from 1800ish to 2100ish cruse went from 1400 to 1550 using less fuel 1.6 to 1.4 and runs cooler and seems happier (huh?) while making a slight increase in noise. Speed at cruse is the same. Still a little tight (OP) but I feel the re-pitch was worth it, I would likely be spot on with a 3 blade.

I think nothing wrong with a very slight OP but others are much smarter then me so YMMV.
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Old 10-14-2014, 11:15 AM   #72
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I think I was the one that said my engine seems "happier" w a lightly pitched prop. At the time I was at 3050rpm on a 3000rpm engine (50rpm underpropped). I felt stupid saying my engine "felt happier" being such a subjective statement and thought no one would have any positive feedback from such a statement. But I couldn't find any objective way of expressing what I was experiencing. And there was no positive feedback .. until now.

Now I think I can. The engine is less loaded and the sound and feel of the engine is experienced by all on board. And if you jump back and say the boat's going the same speed and hence under the same load you'd be right BUT ... Each power stroke will be of less force as there are more of them and vibration will be less as there are more less forceful power strokes. Imagine how much vibration you'd have w a 100 cylinder engine .. or a one cylinder engine.

When I worked in the powerhouse (1400hp Enterprise engine) in Alaska at times the electrical load on the engine doubled instantaneously when they turned on the high pressure pumps on the dredge. The engine managed to handle that sudden load increase w only 2 or 3 rpm loss but the overwhelming experience being in the powerhouse at that time was the instantaneous increase in noise. Seemed to double or more. Incredible increase in noise and basically no increase in rpm. This engine was a straight eight w a flywheel weighing several tons.

But in our trawlers, cars and trucks higher rpm seems to generate more noise. The noise seems to be more rpm related .. less load related. SCOTTIEDAVIS wrote "seems happier (huh?) while making a slight increase in noise. Speed at cruse is the same."

But excluding such special events like harmonics and resonance (that are part of a n external condition) the amplitude of noise w more cylinders or a higher frequency (more rpm) is less. Higher frequency in our boats is a product of higher rpm so it would seem there should be less noise but the reverse seems to be true. Can anybod offer an explanation? Higher rpm is usually associated w increased power. But if it is not .. if we compare w the same load or amount of power being produced .. higher rpm and a constant load .. the noise still seems to increase w higher rpm. Like a truck going up a hill under high load. Downshift and maintain the same speed obviously w the same engine load but w less power per engine stroke the noise seems to increase ...... why?
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Old 10-14-2014, 11:53 AM   #73
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Thanks for sharing Delfin. As you have noted in the past, your Cat engine's basic design was intended as a genset. The rotating and up/down moving parts were designed to be in sync at 1500 or 1800 RPM so very long service hours and optimum fuel economy could be derived. It makes sense that the closer the engine gets to this design RPM the happier it sounds.

Would the on engine alternator have greater capacity? Is your Cooling system pump RPM related?
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Old 10-14-2014, 12:15 PM   #74
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I'm going to join the "my boat felt happier" by reducing my prop pitch group. The PO of Sherpa installed a 16x12 with the current engine, which was way overpitched and only allowed 2900 WOT. I've since installed a 16x8 and can achieve 3600 RPM WOT (maximum WOT RPM per Volvo). I certainly can "feel" the difference and experience less vibrations and reduced prop walk. I haven't experienced an appreciable difference in speed or economy. My cruise RPM is adjusted for about 53% load as shown in my engine's load/RPM graph.

I also have peace of mind that I can deliver needed HP in an emergency and I am not causing pre-mature wear while cruising. I'll stick with manufacturer recommendations and prop accordingly--to each his own though.
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Old 10-14-2014, 12:23 PM   #75
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Another vote for not over propping. I am so convinced that the motor is happier with lighter propping that I use under propping. I advise this only with common rail fully electronic motors that are used at less than peak out put such as trawlers and boats used mainly at hull speed. For boats that travel at high speed most of the time the desire is to pull as much HP as possible and that does not work with under propping.On older motors running at lower loads this may cause fuel build up, on the modern motor the fuel burn is automatically adjusted to the load. The net result of underpropping a common rail motor is similar to down ratting and using less fuel and less HP somewhat like taking an M4 and making it an M1 or M2
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Old 10-14-2014, 12:30 PM   #76
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Thanks for sharing Delfin. As you have noted in the past, your Cat engine's basic design was intended as a genset. The rotating and up/down moving parts were designed to be in sync at 1500 or 1800 RPM so very long service hours and optimum fuel economy could be derived. It makes sense that the closer the engine gets to this design RPM the happier it sounds.

Would the on engine alternator have greater capacity? Is your Cooling system pump RPM related?
Tom, I hadn't thought about the alternator output increasing, but of course that would have to be so. Temps have never seemed much of a problem at whatever rpm. I suppose the thermostat dominates cooling efficiency, but when open the coolers are sufficient to keep us in range. In southern waters, the difference might well be noticeable. Hope to find out....
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Old 10-14-2014, 05:32 PM   #77
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Your alternator output may not have increased much at all. Their output curves are typically like the one below. Ideally you have the alternator pulley sized so that alternator max rpm is just reached at engine WOT rpm. That way you get as high as possible output at your engine cruise rpm.


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Old 10-14-2014, 08:25 PM   #78
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I can't believe, when faced with solid written evidence that propping to put your engine in its lower specific fuel consumption area at cruise speed can be trumped by "feeling" the engine is "happier". Sounds like something my wife would say...
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Old 10-14-2014, 09:38 PM   #79
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Brookse,
An engine "feeling happier" has been "felt" by numerous members. And slightly better fuel burn or a wash has also been reported.

Every boat is different though. If your boat had a 6-354 Perkins engine and you over propped it you almost certainly would experience a lower fuel burn assuming good or lucky choices were made re gear ratio, prop configuration and other variables that effect fuel consumption. Just because you're over propped dosn't mean you're guaranteed better fuel burn. There's only a range that will prove beneficial and on some configurations better fuel economy may not be achieved. Obviously a mechanically controlled engine over propped will burn less fuel at WOT simply because of lower rpm. Over propping will make a mechanical engine more efficient at some engine speeds and less efficient at others. Lots of variables.

Over propping is like over drive in a stick shift car. The reason for over drive is mostly for fuel economy and the same laws of physics apply to both boats and cars. The effect electronic controls have on this question is not something I know about. But I don't doubt that the previous posters on their fuel burn statements. I believe some of the skippers here on TF about burning more or less fuel but I don't believe many when they talk about specific gph consumption .. too many variables.
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Old 10-15-2014, 01:09 AM   #80
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I can't believe, when faced with solid written evidence that propping to put your engine in its lower specific fuel consumption area at cruise speed can be trumped by "feeling" the engine is "happier". Sounds like something my wife would say...
When I say my engines feel better I am leaving out the observations of my pyro gauges boost gages and the load readings along with the fuel burn data constantly on display. What happens with my motors with less prop pitch is more rpm to get to the same speed and same load. Any fuel burn difference at a given speed and load which might be very slightly affected is so small it does not register and is more theoretical than practically measured. If my props were over pitched the data would show it and engines would not be felling happy. The pyro would develop a fever and the load would be overbearing even the coolant might start complaining.
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