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post #21 of Old 04-16-2013
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Re: Overpropped; What to do?

I bought a boat with an undersized engine and an over-propped prop. To complicate matters further, I have an engine that is governed for 4000rpm but only rated for continuous usage at 3000rpm under load. I've given this question a lot of though and asked a lot of experts. It's amazing to me the number of conflicting opinions on the matter!

From what I've distilled, here are my salient take away points:

With a clean bottom and a warmed up engine, with good fresh fuel, and an accurate tachometer, you should be able to reach your max rated rpm with WOT (wide open throttle or full throttle).

For me, the debate was 3K or 4K. I've settled on 3k being the only number that made sense. So, for me, at full throttle after a haulout, I should be able to reach 3,000rpm. Check and make sure your Yanmar is rated to 3,600 rpm continuous working rpm. It might just be governed at 3,600 and able to run for "intermittent usage" at that rpm. If so, that's not your target.

Now, if you're ABLE to reach max-rated RPM you can congratulate yourself because you're not driving up hills in 5th gear and ruining your engine. What about "cruising RPM"? Most of us want to go hull speed. Ideally, with the prop you have now pitched perfectly, the RPM that pushes you at hull speed will coincide with max efficiency of your fuel/torque/HP curves. But does it?

In order to hit the above RPM perfectly, it requires the correct SIZED engine. If it's too big, your cruising RPM will be low on the efficiency curves. If this is the case, do not despair, as you can push the boat faster than hull speed and you have the option to (inefficiently) turn extra diesel into a little extra speed when you need it.

Of course, with an oversized engine comes a price. It will be running too cool most of the time, causing carbon to build up in places you don't want it. The easy solution to this problem is to run the engine for 4 min or so at 80-100%max rpm at the end of each time you use it. That will keep the carbon from building up.

Now if you're like me, and you have the correct pitch figured out to keep your engine healthy, but at that pitch, your boat goes less than hull speed at cruising RPM, you have the choices of: 1. Accept that you will motor slower. 2. Run at higher than recommended rpm possibly wearing out the engine sooner. 3. Over-prop the engine slightly, and risk wearing it out further.

So, check your Yanmar manual closely. What's your MAX CONTINUOUS RATED RPM? That's the one you want to be able to hit at WOT. 3,600 seems a little fast for continuous usage to me, but then again Yanmar has been doing a lot of things in the last 20 years that I don't really approve of...


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Last edited by MedSailor; 04-16-2013 at 11:35 PM.
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