significance of gear reduction ratios when swapping transmissions

[Mike Two Moons]

I am faced with the distinct possibility of having to retire my Kanzaki-Hurth KWB10E transmission (attached to Yanmar 3HM35F) and replace it with a Yanmar KM35PS, as advise by the local mechanic.

Apart from the aparent mounting hurdles I forsee, I would appreciate some wisdom as to the different gear reduction ratios between the units

HBW10E 2.14:1
KM35PS 2.36:1 or 2.61:1

What is the ramification of changing the ratios or is 2.14 and 2.36 close enough? What performance issues can I anticipate?

The possibility of rebuilding the old unit here in Trinidad is slim, but it may just end up in my luggage heading home.

Thanks

[rslifkin]

That's a small enough change that it wouldn't require a drastic change in prop selection or have a huge effect on boat performance. However, with the lower gear ratio you'll find that you'll need a little more RPM for a given boat speed and you may find that you're now underpropped (engine can reach full RPM at less than full throttle). So you may need to add an inch of prop pitch to compensate.

[Jammer]

I would expect that the 2.61:1 would require a prop change for satisfactory performance and the 2.36:1 might require a prop change. You could defer the prop change for a while if you're willing to give up a knot or so while motoring.


If you have a feathering prop, most of those are variable pitch and can be adjusted. If you have a fixed prop, they're cheap enough that just replacing it is the best move. If you have a folding prop you may be able to have the pitch modified by the manufacturer, if not, replacement would be costly.

[DougR]

If you take 3000 engine RPM for example, and divide by the 2.14 ratio and by the 2.36 ratio you will find a difference of 131 shaft rpm between the two ratios at 3000 engine rpm.

I’ll take a wild guess and say you have a 12” pitch on your prop....

131 shaft rpm x 12” pitch = 1.29 kts of speed without any slip. Deduct half of that for 50% slip and the difference in motoring speed at 3000 rpm is .6 knots.

If you have a prop shop add 1.25” of pitch to your prop you are right back where you were with your original gearbox.