I am currently having my 1.50's changed to 1.35's because of the very problem you mentioned. The 1.50's kill top end speed.
The drive ratio on my bravo one is 1.36 I'm currently looking at some different boats and most have 1.5 ratio. My Baja cruiuses at about 40mph@3200rpm. Seems as though the other Ratio revs the engine higher to go the same mph, which is better? any feed back on this subject.
Thanks, Posey
I am currently having my 1.50's changed to 1.35's because of the very problem you mentioned. The 1.50's kill top end speed.
All depends on the hull design,weight & h.p.Most boats with 1.50 are made to run well with that ratio & change to higher with substantial engine upgrades.Hope 3200 rpm isn't WOT on your present boat.Sounds like a V-8 was traded for 4-popper.
[This message has been edited by not guilty (edited July 31, 2001).]
Whats the difference? eg: A boat spins a 28 with 1.5 gears at 5000 RPMs , changes to 1.36s and drops down in prop size to a 24. Engine RPMs remain the same, prop RPMs go up but its should still moving the same amount of water, same boat speed. Is there a speed advantage to spin a small prop faster? My thinking is that on a car, if if you change differential gears to a lower ratio to increase acceleration, you lose top speed. Put a set of taller tires (changing the prop), your back where you were and have gained nothing. I'm running 1.5s now and as I increase HP I plan on just changing prop pitch. Am I missing something?
It's my understanding, that you would switch to the 1.36s if you are getting close to running a 30" prop, as there are less choices in the 30"-34" range.
I think it also has something to do with the prop slippage, efficiency, and high-torque engines utilizing the extra revs.
http://www.goldenplasma.f2s.com/foru...es/happy25.gif
OK, I get it. Just because the monster under my hatch could spin a 42 dosen't mean I'll be able to find one. Thanks for the info.
Once you start running props bigger than 30" they become less efficient. I am running a 32 with 1.50's, and have friends running 34 and 36's. The slip increases with these bigger props. You are better off spinning a smaller prop faster. with my new gears I can drop to a 30" and maybe even a 28". Iwill have to do some testing and see. the problem is alot of people are running the new xr drive which only comes in the 1.50 ratio. I am going to teauge platinum foot gears in the 1.35 ratio, and I hope they hold up.
Thanks for the info,my Baja is 27ft 502mag (carbed)probably around 400hp. WFO is 5000rpm @ 68-70mph 1.36ratio swingin a 21p Mirage 3 blade. I tried a 25p and it couldn't pull it. I guess more pitch you need more HP. With a 1.5 ratio you can swing more pitch with less HP moving the boat faster at the same RPM? Right?
Posey
In theory spinning a larger prop faster should produce more speed. Like I stated above the bigger props seem to be less efficient from experience. Jumping from a 21" to a 25" prop is substantial. I would try to dial in a smaller prop with your current ratio. You can have your prop reworked by a reputable prop shop and maybe able to get your max rpm out of your current prop. I don't know what max rpm on your motor is, i would guess it would be no higher than 5200 on a stock motor. Call a prop shop and ask them about different prop options. I don't think changing to 1.50's would help you. my.02.
Hey Thanks MrHavasuCat I would like to try reworking my Prop. Merc. says max. rpm is 4800-5000 on my '92 502mag sounds like I'm there, might push to 5200 but I'd rather not. RPM's a killer. Do you know a prop shop thats good and has a good turn around time?
Posey