Two other points to consider, even if the new ratio allows you maintain your cruising speed within a safe engine RPM:
1. You will be able to go slower while in gear at idle, which might be an advantage. My speed at idle is a bit over 2 knots, which is a bit fast for negotiating tight spaces, so I have to pop out of gear to scrub off speed sometimes. 2. You will have less power on demand at the top end when you might need a burst of power, such as yesterday when I backed out of my slip in 25+ knots of wind, and then had to spin the bow through the wind, which required two attempts and full throttle. I've considered whether I could have backed all the way out, but by the time I steer clear of my neighbour to leeward, there is very little room left to swing the other way. I have not installed a tach yet, but my engine has a wider power band than most diesels (900-4800RPM), so I'm thankful at times like this! Shawn Wright shawngwri...@gmail.com S/V Callisto, 1974 C&C 35 https://www.facebook.com/SVCallisto On Thu, Jan 2, 2020 at 9:05 AM 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
