Sisu 22 Prop

No Wahalla

Jun 28, 2012
Long Island, NY
Boat Make
Sisu 22
This may be a better Forum for this Thread;

Hello All,

I'm looking for some advice regarding a straight inboard 1979 Sisu 22. I purchased the boat and removed the original old and tired 145 hp Volvo gasoline inboard engine. I then installed a marinized 77 hp OM-617 Mercedes 5 cyl diesel engine - engine found in older 1980-85 Mercedes 300D cars - with a 1.91:1 Velvet Drive gear attached to the original 1" stainless steel shaft. I had the 13" X 11 - i'm almost positive its an 11 pitch - prop ballanced and re-conditioned by a good prop place in Queens, NY, put it all together and launched her hoping for an economical cruise speed of around 7-9 kts with a top speed of at elast 10 kts WOT. To be honest the engine is performing very nicely with good fuel economy but at the mid range or what I consider the economic RPM's I am only getting around 6-6.5 kts and at WOT around 8.5-9.5 if i'm lucky and she is light. The engine it rated at around 4000 rpm's when it is in a car and im getting around 3,600-3,700 when I put the throttle to the dash which I can live with considering its application. I know that this particular engine is much heavier than what was designed for the boat and what is available today by Yanmar, Volvo and Cummins.

I think I can up the prop size to 14" and still have sufficient clearance. Does anyone out there have a similar boat with a 14" prop and if so at what pitch and what is turning it? The goal here as always it to maximize my engine performance in both speed and economy. Nevertheless it is a great little boat but if I can get her to cruise around 8-10 in the mid range RPM's it would be lovely.

Thanks for all of the continual advice and support,

No Wahalla


1st Mate
Sep 5, 2011
if your not getting 4k rpm's as rated you need a smaller prop. You might find that you go slower as well.

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