Help with intentionally over-propping a sailboat for fuel efficency?

[fstbttms]

Originally Posted by Jim Cate

...I think that you are making a big mistake in your plans.

As reinventing the wheel usually is.

[YankeePSC34]

The flaw in your calculations is that you ignore the torque. The torque delivered by the engine with the tranny, and the torque required by the prop. Those have to be matched.
Another flaw is that the engine running time is ignored. If you go 36 nm with 4 knots and I drive with 6 knots, that makes 50% difference in running time. When your diesel is still downing diesel, I am downing lagers.

[Pete7]

Originally Posted by calderp

Would love to have these, but no idea where to get them for a 25-year old engine that no one gives a damn about anymore. Regardless I'm just aiming for a ballpark which will then be fudged a bit more depending on what I can find used!

In that case, use something like the little Beta or volvo diesel engine graphs, they are not going to be that different. How about these?

http://www.betamarine.co.uk/seagoing...0-SDS-0311.pdf

Engine database - Find your engine : Volvo Penta

Originally Posted by charliehows

intentionally overpropping - why dont you try to get the elusive best match of prop, engine and hull? On a 1gm fuel efficiency is a ridiculous concept -

Good point, generating say 20hp at 2500 revs we use 1.5 litres an hour. A 1GM will use what.... a pint perhaps? you probably drink beer faster than that engine uses diesel.

[Mystic38]

1. over-propping will not give better fuel efficiency.

2. determining the sweet spot for your engine/boat/prop combo, will give better fuel efficiency. I have a Sabre 38 with a W38 and with my 16" maxprop will do 0.45gph @2200rpm @5.2kts in calm seas.. i can go faster and i can go slower, but for nmpg thats my number..

3. You do NEED to be able to run at rated engine speed..consider headwinds and chop.. an over propped boat will present a severe overload to the engine and also not make headway as ably as a correctly pitched prop.

4. if you run for long hours significantly below rated engine speed (and i do) simply wait for dead calm seas and run at full speed for an hour.. its the simplest way to clean an engine out.

5. a three bladed prop is more fuel and thrust efficient than two blade.. concerned about drag?..get a folding or feathering prop.

6. want better control in a marina?.. a feathering prop is the only answer, as it provide a higher percentage of thrust in reverse relative to a fixed, or folding prop

[calderp]

Ok, ok, point taken, it's a bad idea. I suppose the fundamental flaw is that I'm already way underpowered, so this idea of ramping up the prop size to get more thrust at the speed I want to run the engine will just overload things. Thanks for clarifying the issue! I guess now I need to decide whether the three-blade is worth the trouble. I would certainly stick with the 2-blade if it was going to make a 1/2kt difference, probably even 1/4kt but again I just find it hard to believe that a blade the size of my hand is going to do anything to my speed, especially if it's freewheeling. Can any 2-blade proponents point me to some evidence? Someone must have done a test or study on this, right? Feathering/folding are out of the question for me.

[CPseudonym]

Originally Posted by calderp

I just find it hard to believe that a blade the size of my hand is going to do anything to my speed,

Why? You thought one would increase your speed quite a bit earlier.

Props are part science, part black art, and above all else a compromise. Not at all unlike the boats they propel through the water. Remove yours and take it and your data to a good local prop shop. 10 minutes with a prop man will get you more factual information than a 300 post count forum thread.

He may recommend the 3 blade you have in mind or perhaps tweaking your existing prop for far less money and better performance according to your parameters.

[djmarchand]

Wow, I haven't tried to read this whole thread, but I see that you came to the wise conclusion that you are severely underpowered and all the prop tweaking in the world isn't going to change that fact.

Your current prop is probably close and maybe even overpropped. I plugged in you numbers into boatdiesel's prop calculator and used a 13" diameter and it gave me a 7" pitch as opposed to your current 13x9 prop.

And why would you want to change from a 2 blade to a 3 blade prop? Gerr tells us that 2 blades are more efficient that 3. The only reason people use 3, 4 or 5 blade props is to fit into the prop aperture. If you can fit a 13" prop then stick with a 2 blade, particularly as it is severely undersquare right now.

And finally back to your original premise- improving efficiency by overpropping. You will never be able to measure the fuel consumption impact on 10% overpropping. If you had enough power to start with, you might overprop 10% and move from 0.06 gal/hp hr to 0.055 gal/hp hr at the same speed through the water and running 200 rpm less (look at the specific fuel consumption cuirve for the Yanmar). You might save a gallon in 20 hours of motoring. It will take a long, long time to pay for an new prop at that savings.

Overpropping only makes some sense when you have much more horsepower than you need to reach hull speed and you don't ever want to run at hull speed. You currently have about half of the hp needed to make hull speed.

David