Hey D1ck quick question.... What are the pros and cons of different ration drives? Is a 1.50 better than a 1.36 and if so why? I am trying to get an understanding of the ratios and how they work.
Thanks,
Chuck

In the past, the experience I have been involved with and what I have seen other do, is dependant on a lot of things. In Mark's example, well taking a second look. I think you mean a 23" prop and 1.36, verses a 25" prop and 1.5 ratio. But even still. The old school of thought was that the common ratio is 1.5. When you run out of props then you need to go to the 1.36. That was in the days when the Bravo 1 or Mirage was limited to certain pitch size. Now a days they make them up into the mid 30's or so. Not sure how big, not there myself. But in a line of props, there seemed to be a favorite, that worked well. Say the 25" Mirage was a good one, then to stick with it.. when you hit the other side of 6k on the tach, you could go to a 27" or change the gear ratio to 1.36. Well if the 27" prop worked great, that was the cheap way to go. If not people would try the 1.36 and compare.
Now there are so many props and most people get them labbed by someone or another, that ratio falls into play more when you are getting to high on the rpm's.
I have an XR drive that was running 950p and a 32" prop. When the motor was bumped to 1170hp, we went to 1.36 gears. When it was bumped to 1400hp, the rpm range was extended past the 6k mark.
In the end it takes a comparsion test of your boat, prop and engine combination to tell if one works better than another. I can only go by what my friends and myself have tried and has sucess with. Most of us stick with the 1.5 ratio, because we are not hitting the big numbers..

I hope this helps.. and I am sure there is more feedback on this subject from more experienced people..

D*ck

So what you are saying is that the 1.36 gear ratio is for higher horsepower and a lower rpm range????

Yes, that is right. Most of us, myself included dont need to run 1.36 gears. Judging from your avatar, your soon to be 650 hp engine wont need 1.36 gears. Save those gears for the big horsepower guys.

Chuck,
The 1.36 would be for much higer hp to keep rpms down. Instead of running a 36" prop and 1.5 ratio, you could run a 32" prop and 1.36 ratio and turn about the same rpm.. ( I didnt do the math on this example, so it may be way off)


Mark,
A 1.5:1 ratio = motor turns 1500 rpm the prop turns 1000 rpm.
So a 1:1.75 = motor turns 1000rpm, the prop turns 1750rpm.. This would be overdrive!! Normally it is the other way around.

Heres my take on it since no has mentioned tq multiplication-if you had a motor that put out 750ftlbs of tq,the 1.50 drive would have 1125ft lbs of tq turning the prop at tq peak (aside from frictional loses),the 1.36 drive would have 1020ft lbs of tq, with the same motor used for this example you would have 10.3% more tq with the 1.50 gears to accelerate you but 10.3% less prop shaft speed once motor reaches its redline. Most single engine boats like the 1.50 ratio and accelerate slower and top end slower when switched to 1.36 gears. The 1.36 gears however seem to be much more durable then 1.50 gears in bravo applications. A boat can be propped down to make up for the loss of tq from the taller 1.36 gears but the next problem is once you spin a typical bravo prop much past 4000 rpms you run into new problems with effieciency/cavitation (from what I have read). Smitty

Basic rule of thumb is to run a 1.50 ratio until you need run more than 32 pitch prop.

Smitty,

I think you meant 6000 rpm, right.

What I have heard is if you try to spin a typical,run of the mill merc prop much past 4000 rpm's (which would be 6000 motor rpm's with a 1.50 drive ratio) it starts getting ugly. So if you had a choice between spinning a 27 pitch 4400 rpms with your motor at 6000 w/1.36 gears or spinning a 30 pitch 4000 rpms with your motor at 6000 rpm's with a 1.50 drive you would be better off with the 1.50 setup,Smitty

OK, that makes sense now. It's just that I am used to referring to engine rpm not prop. Same thing, different way to view it.