Yes, most small, naturally aspirated diesels like ours have their max HP near their maximum rated RPM, assuming air, fuel and other factors are as designed. They may be capable of higher output beyond the rated max RPM, but only for a short time, as most marine and industrial engines are tuned for lower continuous RPMs than on road engines (where brief higher output is more typical/useful). I know from unfortunate experience that some small diesels can exceed 6000rpm without harm for short periods, but diesel combustion doesn't really lend itself to these engine speeds that well... :)
-- Shawn Wright shawngwri...@gmail.com S/V Callisto, 1974 C&C 35 https://www.facebook.com/SVCallisto On Thu, Jan 2, 2020 at 4:55 PM Josh Muckley via CnC-List < cnc-list@cnc-list.com> wrote: > When I look at my engine's manual, it gives a graph of horsepower to RPM. > The way I see it, the 2 are more or less directly related. If you are > unable to reach max RPM then you are by default also unable to reach max > horsepower. > > Alternator output current is also a function of RPM. Lower RPM = lower > maximum alternator power, seen as lower max charge current. > > Josh Muckley > S/V Sea Hawk > 1989 C&C 37+ > Solomons, MD > > On Thu, Jan 2, 2020, 12:05 PM Neil Gallagher via CnC-List < > cnc-list@cnc-list.com> wrote: > >> 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 >> > >> >> >> _______________________________________________ >> >> Thanks everyone for supporting this list with your contributions. Each >> and every one is greatly appreciated. If you want to support the list - >> use PayPal to send contribution -- https://www.paypal.me/stumurray >> >> _______________________________________________ > > Thanks everyone for supporting this list with your contributions. Each > and every one is greatly appreciated. If you want to support the list - > use PayPal to send contribution -- https://www.paypal.me/stumurray > >
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