[Engines] Determining proper HP for a given boat design. - SailNet Community

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Old 10-31-2007
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[Engines] Determining proper HP for a given boat design.

A good friend of mine has a steel 36,000 lb. Goderich 40, a Bill Wallstrom design from the late '70s. Four of them were built. His original and underpowered for the job Volvo 35 HP is coming out and he is proposing putting in a Yanmar 75 HP turbo.

What he has asked me to find out is if anyone knows of some sort of Web resource that can properly match an "ideal" engine to a specific type of boat, given the usual designer's parameters, keel type, displacement, etc. He is thinking of going with the 75 to turn alternators, PTO for compressor, etc. and simply to have "extra" to push his three blade Autoprop that his old Volvo didn't have (although it could push him well enough at hull speed in flat water).

Does anyone know of websites with data along these lines? He could probably live with a 60 HP, but the price difference between a Yanmar 50 and a 75 is only a couple of grand.
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Val, if he calls Yanmar with the boat's detail, they'll calculate it for free for him.

If they don't let me know I ask Pinto, my yanmar/crew/friend dealer and he will calculate it for you
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Itís not the kind of thing that is solved with a simple individual formula so a webpage approach is unlikely and I donít know of any website that tries it. Is he mathematically inclined? If he is I can recommend some books that deal with this or point him to a collection of formulas.
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Old 10-31-2007
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Valiente,


Your friend might poke around here a bit:

http://boatdiesel.com/index.cfm?cfapp=31

I'm not certain, but one of the formulas he may be looking for might be called the "propeller exponent."

Nigel Calder did a nice article in this month's (OCT) Ocean Navigator -- entitled "Improving Low Speed Efficiency " -- which dealt in part with engine size selection. From the stand point of fuel consumption and efficiency, I was surprised at the magnitude of the penalty for going with TOO LARGE of an engine. So that "couple grand" price spread between the 60 and 75 would continue to grow over the life of the engine. Plus, without additional tankage, the range of the boat under power would go down.

So many variables...
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Originally Posted by JohnRPollard View Post
Valiente,


Your friend might poke around here a bit:

http://boatdiesel.com/index.cfm?cfapp=31

I'm not certain, but one of the formulas he may be looking for might be called the "propeller exponent."

Nigel Calder did a nice article in this month's (OCT) Ocean Navigator -- entitled "Improving Low Speed Efficiency " -- which dealt in part with engine size selection. From the stand point of fuel consumption and efficiency, I was surprised at the magnitude of the penalty for going with TOO LARGE of an engine. So that "couple grand" price spread between the 60 and 75 would continue to grow over the life of the engine. Plus, without additional tankage, the range of the boat under power would go down.

So many variables...
I'll check that out, thanks...I am expecting my copy of ON very soon.

That's exactly the sort of thing we were discussing, because his Autoprop was (even with a DriveSaver) very hard on the old Volvo, wearing out cones in the transmission and such, but the opposite is true, of course, that he might "lug" with too large an engine that he never gets into the proper power band.

This is why I decided to stick with my 52 HP, despite my 30,000 weight: with a four-blade VariProp, I will have a great deal of torque at low speeds, but I should be able to push the engine a little harder if I need speed. Right now with a fixed 18 x 13 three-blade, I can get to 6.4 knots at 2,100 rpm. With a four-blade 19 x 15 (the new prop), I suspect I'll run 200 rpm faster, which will be more efficient.
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I am wary of turbos. Do we need them. Is the weight penalty of a normally aspirated engine really that bad?
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Originally Posted by Rockter View Post
I am wary of turbos. Do we need them. Is the weight penalty of a normally aspirated engine really that bad?
I totally agree.
This seems to be an application where torque is more a question than speed and horsepower (Torque x rpm) and wheight not the biggest problem. Then it might get down to engine room size, what is room to put in, but I would have chosen and 'old' normally aspirated (non turbo) engine.
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I am wary of turbos. Do we need them. Is the weight penalty of a normally aspirated engine really that bad?
Yeah, I've been making the same comments to him. Added mechanical complexity to what end? He already has excellent engine room ventilation, and it's a steel full-keeler...I've been hinting that maybe the Cummins 66 HP is a better choice, but he's saying that Yanmar is so widespread he'll never have to search for parts.

I think turbo is great for race boats where every gram counts, but both he and I are far less affected by 100 kilos in the engine bay than most, 100 kilos being 0.3% of our total displacement.
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Turbos aren't really an ideal solution for a marine application IMHO. To really benefit from a turbo, you need to open the throttle up more... and that is pretty hard on a marine use diesel. A better approach is generally to get a properly sized normally aspirated engine. It will be a bit heavier and larger, but it will also last longer IMHO.
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Old 11-01-2007
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INHO..The turbo is an excelant choice for a sailboat.. been working woth turbos for a number of years, and installed one on every vehicle I owned .. its a sorce for un-used eneregy.
A turbo is, as we all know, are advetised as a performance add-on for most cars, but turbos have been used for truck applications all the way back to the 30s & 40s.. Its a sorce for tapping into power at LOW RPM as well as High..
On a boat, the ability to run a couple extra degrees of pitch while still operating at the same RPM or even lower and still keeping the same speed would be a positive swap..
On a desplacement boat...Catch the word "displacement" your pushing water, and that takes power.. and if you can create more power with less motor, thats a savings...
And what I've heard, the new Yanmar with a turbo, it only weeps fuel.
and the cost of fuel is getting higher every day,
I would change mine out in a minute, if this old war-horse would ever give out... I've got a perkins 4108 and the new yanmar is a perfect swap but with this Perkins, It will probably be here long after I'm gone..
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