Here's a fun problem?

[Steve Hornberger]

I am at a point where I must rebuild my Yanmar 3HM35F engine or replace it. ;-( This particular engine model is out of production, though. :-( The boat currently has a 17 X 15 two bladed prop and one inch shaft that provides adequate performance for this 35 Hp engine. Of course, for several years, there were also many questions about this particular boat and proper prop sizes.Am considering replacing the old engine with the new Yanmar 3JH2(B)E, which is a 39 hp engine. If I do that, I absolutely want to marry the engine to the proper prop to do the work. So, without changing the shaft diameter, what prop would be recommended for the engine and the boat to attain maximum normal cruising speed (6.8 to 7.2 knots in 1 - 2 foot chop and 10-15 knots headwind) for the boat at a reasonable operating RPM and at the same time produce the power necessary in nastier Chesapeake chop to maintain 6 plus knots, which tends to be short and steep in stiffer winds?This new model engine produces 39 hp at 3600 RPM. So, I suspect normal operating RPM at 80 to 90 percent RPM would equate to 2900 - 3250 PRM operating range.The new engine can be viewed at www.mackboring.com under marine engines.

 

[Kevin]

:>) :>l :>( Good the bad the...

Dear Steve,Thanks for the note. Are you using the same reduction gear? In your case the reduction gear, I would think, should be kept the same if possible. If so the propeller should spec out to a 17” x 16” two blade. Of coarse this is also dependent upon whether the alternator you have chosen is not larger then the one before and also that you are not adding additional devices to the engine which could use Hp. I would look at the torque curve of the new engine to see what the available Hp is at the maximum torque level and compare it to the old engine. You may find that at 2,700 Rpm’s, the normal cruise for each of the engines, that each engine may achieve torque and Hp levels which may end up being very close to each other. If this were the case you may see yourself using the same current propeller. The advantage of the new engine will be its ability of sustaining speed when confronted with heavy winds or seas. As you begin to throttle up you will increase Hp helping you out in extreme conditions. Another factor is how bad is the current motor. It may not have the capability of producing the Hp or torque efficiently enough in order to use it as a comparison to the new motor.I have been specing out propellers for almost 20 years now and your particular scenario has always been the most difficult to do. As with most equipment past experience is always very helpful in making that educated guess. With no past experience to fall back on, we are resorting back to the ol-bag-of-bones prop specing. Its not that it is impossible it is just we have little to no comparison information in order to “guarantee” any results. In my opinion I see no problem in maintaining or even increasing the vessels performance underway using either scenario of re-building or re-powering. All in all I would not rule out at least one pitch change to make this new system work at peak performance. If you go to my web site www.pyiinc.com and fill out one of our propeller recommendation sheets we should be able to get you very close to the right unit. Hope this helps some. Sincerely, Kevin L. Woody

 

[Ed Schenck]

Folding 3-blade?

Then the manufacturer's warranty would require the prop to be right for the boat/motor combination. So long as you are spending that much money you could get the best motoring AND the best sailing with a folding 3-blade.