I believe we just went through this....about five times. What "designers" are you referring to? You seem to be thinking that the engine was designed for a specific boat. It's not. The maximum rated power curve developed by the engine OEM is there to cover installation in a range of boats. A small overprop in your boat might be an underprop in another make/model....and the cooling system has been tested to accommodate that range of installations. Again, the "designers" (of the engine) planned for different operating scenarios/installations. They provide the boat manufacturer with installation information to insure the installation doesn't screw up what the engine guys already addressed. If the installation is wrong...the engine folks won't warranty the engine. Excess capacity across the rpm range isn't a "safety cushion" as you call it. It's power capacity that's been tested as safe to use. The term "cushion" is often used in the context of "cruise prop with derated engine" as a way to convey that virtually all engine installations have approved capacity far in excess of prop demands. I urge you to get some engine and prop charts and think about what they depict.
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I believe we just went through this....about five times. What "designers" are you referring to? You seem to be thinking that the engine was designed for a specific boat. It's not. The maximum rated power curve developed by the engine OEM is there to cover installation in a range of boats. A small overprop in your boat might be an underprop in another make/model....and the cooling system has been tested to accommodate that range of installations. Again, the "designers" (of the engine) planned for different operating scenarios/installations. They provide the boat manufacturer with installation information to insure the installation doesn't screw up what the engine guys already addressed. If the installation is wrong...the engine folks won't warranty the engine. Excess capacity across the rpm range isn't a "safety cushion" as you call it. It's power capacity that's been tested as safe to use. The term "cushion" is often used in the context of "cruise prop with derated engine" as a way to convey that virtually all engine installations have approved capacity far in excess of prop demands. I urge you to get some engine and prop charts and think about what they depict.
Nomad Willy
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Skid you ask for suggestions? Have I got any?
I have only one and that's to prop to rated power and forget about all this nonsense of over propping. Yes nonsense because it makes no sense. It makes no sense economically as the fuel saved by over propping is so small. There really isn't enough to be gained from the practice to justify it. So it should be recognized as a bad practice and not as a tricky thing really smart people do to to beat the system And get ahead of the game.
In the post above you said .. "A small overprop in your boat might be an underprop in another make/model.." No way that can be. My engine is rated for 37hp at 3000rpm so if it's installed in another mans boat how could it be rated different?
I have only one and that's to prop to rated power and forget about all this nonsense of over propping. Yes nonsense because it makes no sense. It makes no sense economically as the fuel saved by over propping is so small. There really isn't enough to be gained from the practice to justify it. So it should be recognized as a bad practice and not as a tricky thing really smart people do to to beat the system And get ahead of the game.
In the post above you said .. "A small overprop in your boat might be an underprop in another make/model.." No way that can be. My engine is rated for 37hp at 3000rpm so if it's installed in another mans boat how could it be rated different?
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Nomad Willy
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Skidgear,
I've been thinking and I see that technically you're right.
Consider my Mitsubishi S4L2. It is rated at 37hp @ 3000rpm and marineized by Klassen. Vetus markets the same engine and calls it 42hp @ 3000rpm. Westerbeke also markets the same engine and calls it 44hp @ 3000rpm. Vetus lists in it's catalog or brochure and calls out a rating like MSO.... or something like that. All three engines are rated at 3000rpm but all have different hp ratings. If I had purchased either the Westerbeke or the Vetus I would have propped it the same as I did my Klassen .... WOT at 3000rpm. And with all three I would have the same prop. But I don't know the continuous ratting of any one of the three including my own Mitsu. I suppose the engines could have different injectors or compression ratios but I was told by someone that should know that they are the exact same engine. They are the same bore and stroke ect. I still don't know how the three engines can have different power ratings. Perhaps the "formula" they use to establish horsepower is a bit different. I think they all do the same amount of work and have the same ability to turn a prop ... I was assured of that by my "expert" source.
When I was shopping for my engine (when I repowered 10 yrs ago) I determined that my minimum power needed for my Willard 30 boat was 32hp. Seemed awful low and I was afraid it wouldn't be enough but was fairly well convinced my number was right. Decided too much power was better than too little (by about 50-1) and one of the reasons I chose the 37hp Klassen/Mitsu was that it came closer to my calculation of 32hp. Now I'm convinced they all have the same power. But I'd chose the Mitsu if I had it to choose again .. mostly for other reasons.
The only real problem w this situation is that most chose an engine for their boat based on horsepower. A "power calculator" is often mentioned here on TF that guys are using on boatdiesel and if one horsepower is not one hp and can vary one could go a bit astray choosing an engine. A 7hp differece on a 40hp engine would be 17 or 18hp on a 100hp engine. A 120hp FL could only be 100hp ... or 135. And engines are chosen by hp ratings.
At any rate things aren't as black and white as I thought but choosing an prop for one's boat shouldn't be any different. I wonder what the OP is thinking at this point?
eyschulman
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I don't think one can win this debate. If an individual feels that they can save $ or improve the performance of their boat and that it makes a significant difference by deviating from the engine manufacturers instructions that is there business. Whether the deviation is in propping or oil change frequency again their business until I or some enlightened buyer or insurance inspector comes along. I would always sooner buy a used boat from the person who followed the manual rather then the expert who knows better.
.... what, no more "enlightened" pseudo-technical hypotheses that won't withstand rigorous engineering scrutiny?
sunchaser
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Fortunately this scrutiny is being performed by successful boat builders and engine manufacturers who generally ignore over propping discussions as it has no positive benefit for new builds. They are too busy to read or even know of these TF threads.
This isn't about new builds. That said, it was those brilliant boat builders and engine manufacturers who conspired to installed twin turbo-intercooled 375s in a bunch of Grand Banks and others with similar hulls....simply to add one or two knots to the top end speed. Design by marketing. The same folks who brought you over-propped boats straight from the factory as I seem to recall from past discussions on this subject. Of course they ignore their past blunders....there's no profit in fixing them.
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sunchaser
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Assuming the GBs were propped correctly, why is that a blunder? I've been on several GBs where the big engines performed flawlessly, granted burning lots of fuel but that was the owner's choice. The market for these large vessels with big motors seems pretty strong with ever more being built and sold. And with today's modern diesels throttling back to 1200 RPM or so and speeding along at 9 knots does not break the bank.
Until Obama outlaws enjoying one's success in frivolous ways the Flemings of the world will thrive, both at 9 knots and 18. Wish I could do that, like lucky Ed in his Devlin.
Until Obama outlaws enjoying one's success in frivolous ways the Flemings of the world will thrive, both at 9 knots and 18. Wish I could do that, like lucky Ed in his Devlin.
eyschulman
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As I have said before I am not an expert in this field and feel that the manufacturers parameters of propping and maintenance are what I should rely on. If I were to take it on myself to change those parameters such as to purposefully over prop I would not be certain that the fuel delivery sx-the cooling and lube systems would have the ability to correct for my change to where the toque and heat curves would go. I have faith in the down rating of a engine done by the manufacturer where changes are made to compensate for this shift, such as fuel pump delivery and stops on rated WOT rpm. If you have certainty that is fine, but even if I believed in your certainty my question would still be why bother to tamper? In every technological debate their are two or more sides often with lots of opposing numbers graphs etc. I leave it up to the engineers who make the engines to decide what's right in this situation. I suspect if a over proper butted heads with them they might unload their technical data and the trump card no warranty.
The boat that was incorrectly propped from the factory was a GB36. So who knows if the boats you were on were propped correctly...
I'm actually not thinking of "large vessels with big motors" ....or "todays modern diesels". A 42 GB (with twin 375s) is not a large boat and the engines are anything but modern diesels. As you surely know Grand Banks stopped building these semi-displacement, monster engine kludges years ago. That combination is a perfect candidate for cruise propping if the owner wanted to limit cruise speed to 13-15 kts, and save some fuel by upping prop size and knocking off a few knots at the top end.
I'd think Grand Banks would want to help owners move the design point to a speed range more in tune with the times.
Yeah, whatever. I spent 25 years analyzing, evaluating, ground testing and flight testing re-engined, re-propped, re-rotored, re-geartrained, re-rated, de-rated aircraft of all types. I understand your trepidation, but it's really not all that complicated. I'm primarily coming at this from the technical side because it pisses me off that folks think it's not technically viable. It most certainly is. Whether in makes sense from a fiscal point of view depends on many many variables. That said, the boat I mentioned in my last post, does look like a good candidate. I wouldn't jack with a warrantied boat either. Nor would I mess with an engine equipped with a modern electronically controlled fuel system. But for the old, out of warranty, slobbering pig category...
By the way, if you think some insurance inspector/surveyor has the slightest clue about any of this you are mistaken.
Over and out.
kulas44
Guru
Skidgear, I agree with you but we will not win this debate. If we all just had EGT gauges we would see where we get max efficiency, assuming we knew what to look for. We could prop our "semi planing" I love that term, boats to whatever speed we wanted to make and be as economical as possible at that speed. Set the guv at that speed and stop worrying about it. If we wanted our 375 Cats in our GB to hit WOT rated rpm and burn that much fuel, ok, but we are just spinning them more than necessary at 8 mph. If we propped our boat at 8mph to make a decent EGT and set the guvna at that speed we would NEVER be overpropped, just more efficient. You will never get most folks to understand that, and, the word "clueless" is unacceptable 
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BryanF
Senior Member
Skidgear, I agree with you but we will not win this debate. If we all just had EGT gauges we would see where we get max efficiency, assuming we knew what to look for. We could prop our "semi planing" I love that term, boats to whatever speed we wanted to make and be as economical as possible at that speed. Set the guv at that speed and stop worrying about it. If we wanted our 375 Cats in our GB to hit WOT rated rpm and burn that much fuel, ok, but we are just spinning them more than necessary at 8 mph. If we propped our boat at 8mph to make a decent EGT and set the guvna at that speed we would NEVER be overpropped, just more efficient. You will never get most folks to understand that, and, the word "clueless" is unacceptable
Yup you say it better than I do. It is an envelope of operation not a simple straight line. There are numerous points inside the envelope.
sunchaser
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OK OK. At least we now have a target audience, GB owners who have 20+ year old 3208s rated at 375 HP. As an aside, is over propping common on a fixed pitch prop driven airplane?
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sunchaser
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Skidgear, IYou will never get most folks to understand that, and, the word "clueless" is unacceptable
Fortunately some very smart boat and engine professionals fit into the presumed clueless envelope when it comes to rational prop selection. Your point on EGT gauges is interesting, especially for the electronic engines of today using it as a digital input control point. Ya gotta love progress.
dimer2
Moderator Emeritus
No it's not common as you need full RPM/power during takeoff....but (there is always a but) with variable speed props, you will then adjust pitch to cruise RPM and maximize speed/fuel consumption. As trawlers don't require full RPM to get going, theoretically they can skip the 'takeoff' phase and go directly to the higher pitch 'cruise' setting, which is only over propped at an RPM above that cruise setting. IMHO of course....
kulas44
Guru
I was joking with the "clueless" remark. I do understand the reasoning behind a company that is going to carry the warranty on a new engine requiring it to reach rated rpm in the boat it is installed in. I would not do it differently. I also understand that it takes x amount of fuel to go x mph in a boat of x length and weight. It doesnt make a huge difference in fuel usage if our 3208 is spinning 1200 or 900 rpm. However, if I dont exceed my self imposed 8 mph speed limit and watch my EGT then I can probably get 8 mph at 700 rpm (assuming I am propped for it) and reduce fuel usage a good bit, and keep cylinder temps in the right range (not engine temp, totally different). That being said, an engine that can reach rated full rpm will not be overloaded and is the way most should do it. Fool proof is good. I dont really know if the savings in fuel and engine wear is worth the effort when over propping even when done correctly. I like the quieter aspect.
skipperdude
Guru
One thing I haven't seen mentioned is exhaust back pressure. High back pressure will severely reduce RPM at WOT. If not installed a gauge would need to be placed on the exhaust. Cat should be able to give you the correct exhaust back pressure.
Sd
Sd
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FF
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>If an individual feels that they can save $ or improve the performance of their boat and that it makes a significant difference by deviating from the engine manufacturers instructions that is there business.<
What most folks actually do is to attempt to operate the vessel at the best place for the power required.
Sadly it is usually a guess as most engine converters or mfg. do not supply a FUEL MAP, which would allow the most efficient setup with no guessing .
A HP / prop theory graph is not a FUEL MAP , and is almost useless if a good setup is desired.
At times the closest one can come is the Mfg HP /Torque curve , and the rule of thumb that most engines are efficient at or near the torque peak.
IF they are loaded to use the HP/torque produced .
AS direct comparisons of efficiency and engine suitability for any use can be done with a FUEL MAP , it is easier to get the latest plans for a T88 nuke bomb.
Setting an engine up to operate in the best fuel burn area of a mfgs fuel map has nothing to do with setting an engine to pull published max rpm .
It is expected that the engine will be operated as it was set up to do , period.
What most folks actually do is to attempt to operate the vessel at the best place for the power required.
Sadly it is usually a guess as most engine converters or mfg. do not supply a FUEL MAP, which would allow the most efficient setup with no guessing .
A HP / prop theory graph is not a FUEL MAP , and is almost useless if a good setup is desired.
At times the closest one can come is the Mfg HP /Torque curve , and the rule of thumb that most engines are efficient at or near the torque peak.
IF they are loaded to use the HP/torque produced .
AS direct comparisons of efficiency and engine suitability for any use can be done with a FUEL MAP , it is easier to get the latest plans for a T88 nuke bomb.
Setting an engine up to operate in the best fuel burn area of a mfgs fuel map has nothing to do with setting an engine to pull published max rpm .
It is expected that the engine will be operated as it was set up to do , period.
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