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Old 02-12-2013, 07:05 PM   #121
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Engine manufacturers don't recommend it.
I rather doubt that Ford and Lehman were calling American Marine in Singapore in 1966 and warning them not to overprop their engine.

Most boat engines in the kinds of boats we have were derived from truck, industrial, or agricultural engines. As such, I daresay they worked a hell of a lot harder at times than they do in our our toy boats.

The Ford Dorset diesel, which was designed to be a truck engine, proved a miserable failure at this because the high-rpm, high-heat, constantly varying loads that are experienced in over-the-road trucks were too much for it.

But it found a new lease on life as an industrial (cranes, pumps, generators) and agricultural (primarily combines) engine. As such it was not subjected to the high rpm and constantly changing high loads of truck service, but I suspect that while the loads were more constant they were probalby often more than what our slighly over-propped boats demand.
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Old 02-12-2013, 07:12 PM   #122
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are you guys sure about this? I think its common for the yard to prop boats more for the shipping weight than the loaded outfitted weight. Would make sense since they have no control over how the customer will outfit or load the vessel unless he stipulates during the build. If this were true then when loaded the boat would appear to be over propped wouldnt it.

Could be, although our OA was "correctly" propped by the factory for the current operating weight...and it's a tad on the porky side. But that's not what I'm talking about. Considering how the majority of the slobbering overpowered fuel pig dinasaur semi-plaing inboard diesel hulls from the late 80s and 90s are mostly operated these days, they'd be far better off if over propped and "throttle" stopped. Better for the engines, and maybe even a little improvement in fuel burn.
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Old 02-12-2013, 07:32 PM   #123
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I resemble that person psneeld.

GB may do what ever they want to do and because they do X,Y or Z dosn't mean it's good practice. I'm very surprised to hear they do over prop though.

Engine manufacturers don't recommend it.

By the way I'd rather you called me by name than "zealous". It's rude.
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Old 02-12-2013, 08:02 PM   #124
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Could be, although our OA was "correctly" propped by the factory for the current operating weight...and it's a tad on the porky side. But that's not what I'm talking about. Considering how the majority of the slobbering overpowered fuel pig dinasaur semi-plaing hulls from the late 80s and 90s are mostly operated these days, they'd be far better off if over propped and "throttle" stopped. Better for the engines, and maybe even a little improvement in fuel burn.
You are right. My experiance with io powered boats was that over propped the top speed was about the same but at a lower rpm but with better mid range fuel economy. Gas v8's like to run around 3000 with a top rpm 4-5000 depending upon manufacturer and tune.
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Old 02-12-2013, 08:49 PM   #125
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They still do. My 2009 47' EU was over-propped by about 2".
IPS or straight shaft?
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Old 02-12-2013, 08:55 PM   #126
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I resemble that person psneeld.
By the way I'd rather you called me by name than "zealous". It's rude.

Don't sweat it Eric, after all you were a prime contender for the HOF. Just keep wading thorugh the flack. You were the OP on this rather decent thread that has more substance than single vs twin. I wish more people were zealous and dedicated like you are.
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Old 02-12-2013, 10:08 PM   #127
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IPS or straight shaft?

The GB41/GB43 is the pod-drive boat. The GB47 has conventional drive albeit a different hull from previous GBs.
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Old 02-12-2013, 11:49 PM   #128
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Thank you very much Tom.

I had been planing for about a week to post this thread and was hoping it would get interesting. It has and even though the OP I put up hasn't been addressed (relative to what I was looking for) a lot of meaningful discussion has come to pass. It should have spawned a thread on prop overloading but the way I expressed my OP it seemed to fit.

The concept I tried to express in the OP is complicated and when I write about it frequently need to resist the temptation to scrap it all and start over. I wonder if I've made myself clear. I'll try it in a sentence. The big low pitch prop dumps it's load too quickly (at reduced rpm) to be as efficient as it is at full load.

I think it may be as simple as too low of a prop loading produces a prop that is less efficient. At full throttle it may be most to very efficient but at reduced rpm the loading goes so far down that the efficiency of a smaller prop can/may be more than the big prop. Too much "parasite" drag of water flowing over blades w/o enough loading to make good thrust.
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Old 02-13-2013, 12:37 AM   #129
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A fixed-pitch prop is always going to be a compromise except at one power setting. The only practical answer with regards to prop efficiency is a variable pitch propeller. I don't know if a constant-speed propeller is feasible in the water but it certainly solves all sorts of issues in the air. But a variable pitch prop is just as effective since after all, it's doing the same thing. Just that someone has to do it rather than it doing it on its own.

Give the kind of boats most of us have and how we use them I rather doubt that the cost and knowledge that would be necessary to incorporate one or two variable pitch props and use them effectively would be offset by what would probably be a fairly small gain in efficiency. They might make more sense with more return on a fast boat like Don's or an Eastbay or the like.
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Old 02-13-2013, 11:45 AM   #130
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are you guys sure about this? I think its common for the yard to prop boats more for the shipping weight than the loaded outfitted weight. Would make sense since they have no control over how the customer will outfit or load the vessel unless he stipulates during the build. If this were true then when loaded the boat would appear to be over propped wouldnt it.
Yes, I think that's what was going on with mine. With a light ship it was fine, but fully loaded, not so much. And propped for a light ship, the boat goes faster and looks better in magazine reviews.
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Old 02-13-2013, 11:50 AM   #131
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Aren't there other factors involved here like cylinder temperature that determine combustion eficiency? Dosen't the compression, temp, ratio of reactants also determine the type and amount of pollutants and of course if to rich raw fuel emitted into the air and or water.
Yes, of course these things play a role. But the first order issue is to be sure there is enough air in the cylinder to burn the fuel that gets injected. It's fundamentally different from a gas engine where you need to always "ingest" a quantity of the correct ratio of fuel and air or it won't go POP.
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Old 02-13-2013, 12:22 PM   #132
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a lot of meaningful discussion has come to pass
Yes - great thread for us Newbies to read over - twice.
Thanks guys.
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Old 02-13-2013, 01:33 PM   #133
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Yes, I think that's what was going on with mine. With a light ship it was fine, but fully loaded, not so much. And propped for a light ship, the boat goes faster and looks better in magazine reviews.
Exactly. You hit the nail sqaure. I gained lots of experiance with props on tinker toys< i/o powered boats under 30 feet> back in the eighties and ninties and you can make all the calculations you want but you won't know if your calc's are right untill you slap a prop on and try it. I found the best way was to run her up with a normal load for the owner with a new or trued prop on and take a note of the speed and engine rpm. If you try this with a mugged up prop even though it may look good to you you won't get good numbers. With trawlers it isnt as easy to change props as with io's but you might try taking your prop to a prop shop to see what condition it is in? If its in good shape you may as well have the guy take some of the pitch out of it since it sounds like youi are way over proped and then do a run to see how she runs.
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Old 02-14-2013, 07:09 AM   #134
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Besides the coat and maint on a CPP on a small boat the larger size of the hub may hurt performance.

For a "round the world" boat 45+ ft a CPP would pay for itself , on the ICW 200hr cruiser it would be many decades to break even.
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Old 02-14-2013, 06:25 PM   #135
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Yes, of course these things play a role. But the first order issue is to be sure there is enough air in the cylinder to burn the fuel that gets injected. It's fundamentally different from a gas engine where you need to always "ingest" a quantity of the correct ratio of fuel and air or it won't go POP.
I think diesel engines need the same thing, you can't go pop without fuel and an oxidizer. No matter how much compresion you have the fuel does nothing unless you are above the lower explosive limit and that can never happen without an oxidizer at the right concentration. I think idealy its 14.5:1 for diesel engines. If this were not true you wouldnt be able to put out a match with gasoline but it works every time.
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Old 02-14-2013, 06:32 PM   #136
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.....
For a "round the world" boat 45+ ft a CPP would pay for itself,
.....
.
I dont believe that is true. For most such boats it is close to a zero sum game.

Think of it like this: if you increase the pitch of your CPP, you will lower engine rpm for the same speed. But the engine has higher load and will then use more fuel than it otherwise would at that rpm. It all comes down to the BSFC map. The CPP will only benefit you if you can get to a sweet spot for efficiency, or away from an inefficient zone.

CPP's make sense where hull resistance that the engine/prop has to overcome is variable. Eg an empty fishing boat versus one with a full catch. For recreational users, I see the key application for CPP as a motorsailer. Wind strength and direction, and hence sail thrust, is variable. You can tweak engine/CPP to top-up the thrust required to get the speed you want. You can feather the prop for minimal drag if using sail alone.
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Old 02-14-2013, 07:25 PM   #137
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I dont believe that is true. For most such boats it is close to a zero sum game.

Think of it like this: if you increase the pitch of your CPP, you will lower engine rpm for the same speed. But the engine has higher load and will then use more fuel than it otherwise would at that rpm. It all comes down to the BSFC map. The CPP will only benefit you if you can get to a sweet spot for efficiency, or away from an inefficient zone.

CPP's make sense where hull resistance that the engine/prop has to overcome is variable. Eg an empty fishing boat versus one with a full catch. For recreational users, I see the key application for CPP as a motorsailer. Wind strength and direction, and hence sail thrust, is variable. You can tweak engine/CPP to top-up the thrust required to get the speed you want. You can feather the prop for minimal drag if using sail alone.

Increasing the pitch is like increasing the load your right. And the increase in load will use more fuel but you are also going slightly faster than you were with the old pitched prop. The savings isnt great but it was great enough on io powered boats of customers that they noticed the fuel saveings and brought it to my attention. I know how this all works with io's and would love to experiment with trawlers this way. The thing is a trawler already has a very eficient engine and hull so I'm not sure the gain would be at all noticable. Maybe on a long trip it would be
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Old 02-15-2013, 06:49 AM   #138
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"But the engine has higher load and will then use more fuel than it otherwise would at that rpm."

Most engines have a better fuel burn at lower rpm higher load, within limits.

It takes X amount of fuel to spin the prop at a desired RPM and the longer burn time of lower RPM helps extract energy.

Yes, the correct solution is a BMEP or Fuel Map, but most engine converters do not publish anything to help optimize a setup.

Big engines , 1000HP and up no problem finding information,
but truck and lawn implement converters are very tight with info.

All most publish is a useless theoretical prop hp and theoretical fuel burn .
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Old 02-15-2013, 06:01 PM   #139
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"But the engine has higher load and will then use more fuel than it otherwise would at that rpm."

Most engines have a better fuel burn at lower rpm higher load, within limits.

It takes X amount of fuel to spin the prop at a desired RPM and the longer burn time of lower RPM helps extract energy.

Yes, the correct solution is a BMEP or Fuel Map, but most engine converters do not publish anything to help optimize a setup.

Big engines , 1000HP and up no problem finding information,
but truck and lawn implement converters are very tight with info.

All most publish is a useless theoretical prop hp and theoretical fuel burn .
that makes sense. And your right if you have a super duper high hp fire breather there is all kinds of data out there but not for a little old 5.9L ford lehman puttin along with only 120hp.
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Old 02-15-2013, 07:05 PM   #140
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For a "round the world" boat 45+ ft a CPP would pay for itself ...
Exactly the opposite is true. CP wheels on that class of boat are so rare as to be non-existent.
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