Olivier,
Let me add a little info regarding the engine/gear/prop question.
Assuming a 19% higher gear ratio, if you can run the engine at 19%
higher RPM you will have identical performance of the prop/hull, as the
prop RPM will be the same. That means the power demanded by the prop is
identical, thus the load on the engine will be the same. The principle
at work is that power = torque x RPM, so if you go up in engine RPM,
the engine torque will go down proportionately to give the same power
output. (There may be a small change in engine efficiency at the
different speed/torque combination, but no more than a few percent,
meaning fuel consumption may change a few percent, possibly for the better.)
The only question is can the engine run 19% faster without hitting the
limit. The 3HM35F appears to have a limit of 3,200 RPM, so if with your
old gear you were under 2,624 RPM (=3,200 x 2.14/2.61) at your normal
cruising speed, you would be able to achieve that with the new gear at
3,200 or less. If you run above 2,600 RPM now, you won't be able to
match the speed you have with the old gear. And running the engine at
higher RPM may cause it to wear a little faster, but if you're within
the 3,200 limit, and the torque will be lower, shouldn't be too big an
issue.
If you approach this with the new gear ratio and want to run the engine
at the old RPM, you'd have to increase pitch to give the same boat
speed, as several people have pointed out. Now your torque will
increase, possibly exceeding the torque limit at your old RPM. You'd
want to check with a prop shop, who can give you that kind of info.
Neil Gallagher
Weatherly, 35-1
Glen Cove, NY
On 1/2/2020 9:32 AM, Olivier Chatot via CnC-List wrote:
Thank you for the comments!
I see that my understanding was basically correct. The shaft RPM will
be lower than before, and using a larger prop or a prop with greater
pitch would counter this difference.
I'll update you later if I end up using the transmission with a
different gear ratio. I won't get a new propeller right away to get a
chance to test the new transmission with the old prop.
I think there's a chance the gear ratios are close enough that I'll
get the same speeds at still reasonable engine RPMs. What I'm thinking
is that lower prop RPM also means that the prop slip will be lower, so
even though the prop RPM would be 19% lower, the boat speed is likely
to decrease by less than 19%.
Thank you,
Olivier Chatot
C&C 38-3
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