Repowering my 365

[fernando]

When I bought my boat two years ago the original engine had been replaced with a 30 hp yanmar. It turns out the setup for the yanmar was for a direct drive with a 2:1 reduction on the transmission. This was then connected to the original V drive which appears to also have a 2:1 reduction. The end result is that my boat can't do more than 3 knots which has put me in a dangerous situation more than once. I've been seriously considering repowering with a new westerbeke 44 but a couple of the cruisers and liveaboards here in Salinas have told me to switch my  V drive to a 1:1. The prop shop in San Juan crunched numbers and suggested I also switch to a larger four bladed prop instead of what I believe is the original three bladed prop. Considering my total lack of mechanical knowledge I could really use advise from anybody who may have any experience with something like this.
thanks in advance,

Fernando Pla
S/V Vejigante
Salinas, Puerto Rico

[Trinity]

Hi Fernando,
Some of the guys on this site pour motor oil on their cornflakes, and should be able to help out with all the ratios.  It would seem to me that a 30hp Yanmar should move you at faster than 3kts if everything else is ok.  I'm sure the bottom's clean and doesn't have a lot of muck on it, right?

Here's some engine chatter from sailnet...might shed some light:
http://www.sailnet.com/forums/gear-main ... 107-a.html
" Westerbeke 4-107
Trek, I think Nereus32 has summed it up nicely. And in the hopes this might add an additional data point for you, I recently spoke with a Scot who repowered his Pearson 40, which was previously powered with a 4-107. He also chose the new W-44B and he was very pleased. As you probably know, the P40 was way overbuilt for its intended purpose and is roughly a 11-ton boat. He commented that the engine was much quieter, far smoother and a great deal more powerful; as I recall, he had a fixed 3-blade prop. Also, he found the geometry on the 44B to make it a suitable 4-107 replacement. Hope that helps...

Jack"

[Jim S]

Repowering is complicated issue that is easily overlooked.  The variables are propeller size, engine horsepower and reduction to the propeller.  We repowered Phantom in 2005 with a Westerbeke 35Dthree (31hp), Hurth transmission 10M (1.48:1), and a  RV 10 V-drive (1.67:1).  This is combined with a 18X13 RHP.  The 30 hp engine will be enough if you get the propeller size and reduction correct.   It is easy to place a large engine into a boat and use very little of the hp (ie low rpm) for propulsion.  Correct sizing will allow a smaller engine to run at the rpm it was designed for long life.  Email me direct if you wish more about this subject.

[Jim S]

Fernando, is it possible that the Yanmar is operating with a different direction revolution than the original engine?  If so, the new engine is operating in reverse to create forward propulsion.  Reverse gears are frequently higher reduction than forward and would compound your speed dilemma.  I personally feel that the 30 hp engine will be fine with proper reduction and propeller sizing.  Walters does produce a 1:1 reduction v-drive that may be appropriate for fyour situation.  if you are going to use your main engine for other energy uses, ie. hydraulics, high amp alternators, watermakers, refrigeration; then larger horsepower would be indicated.  

Remember, a large slow moving, high-pitch propeller is more efficient than a same sized high-speed, low-pitch propeller.  Your mechanic may well be dead-on with a large four blade, high pitched propeller doing very well.  You will be dragging a big "wheel" when sailing and the engine is off.  Depends on where you put your priorities.  I would check the direction of rotation, though.