RPM Difference
 

If you had a 30" prop on a standard Bravo 1.50 drive turning 5000 RPM's. What would be the RPM's with the same prop but with a 1.36 gear? I'm just looking to compare the RPM difference between the 2 Ratio's.

about 1000rpm. If it turned 5000 with a 1.5, then it will turn about 4000 with the 1.36. You would need to drop down about 4" of pitch to turn the same 5000 rpm with the 1.36.
Eddie

also I would stay with the 1.50

actually i think the math says about 514 revs so more like 4500 all things being equal. i.e to say that the motor has the power at 4500 to pull the longer gear to the same top speed.

About 600rpms.

here is a link for you to play with
http://www.rbbi.com/folders/prop/propcalc.htm
I have used it alot, it works pretty good
larry

Not going to change the ratio, trying to figure out whats going on. Suppose to have 1.50 drives boat, but only turning less than 4000 rpms. This boat should be turning 5000-5200 rpms with a 30" prop. Dealer changed lower units on Bravo drives so the serial number will not match up with ratio.

Why don't you just take the drive of,put in gear and turn the input shaft,,that way u know for shure what u have.

Input shaft 15 rev. / Prop shaft 10 rev. = 1:50 !!!!:cool:

Good ieda, Boat right now in shop for some other issues so I'm going to have them check for ratio also. Just trying to get some info so I might be able to talk with a little knowledge to the shop.

there are a million things that will cause that....

are you saying it USED to do that and now won't or that this is a new motor / boat combination that seems to be under performaing ? seems to me that you should be able to post all your specs... length, beam, type of hull, esitimated hp, drive type and current prop and someone here is going to say " oh yeah, ive got one of those and this is what it does...

i remember getting my boat together for the first time and couldn't see the speed or the revs that it should and without changing any hard parts, but just trimming and dialing things in went from 4200 rpm to 5400 rpm. what i had was a couple of knowledgeable people telling me that their identical combination went 60 mph... so why was mine going 45 ? was just mistakes i was making in set up and operation...

Maybe the foot has differamt gears than your old one?

do what daredevil says

Stevesxm, Your right the boat is a 2006 42 Ex with 525's, My last boat was a 2005 42 Ex with 525's. The reason I'm asking this question is because the 2005 would run 30' props to 5000-5200 rpms. The 2006 will not go past 4000 rpms. I know the dealer switched the lower ends on the Bravo's with another boat and I'm hoping they did not put 1.36 on by mistake.

The 30" props that are on the boat are the ones they issued with the boat at time of purchase. I'm just thinking that the drive ratio's are incorrect,because my 2005 Ex would turn 30" props 5000 rpms all day long.

sounds to me like your logic is impeccable

If you knew me, you would be giving me TOOOO MUCH CREDIT!!!!

1000-1200 rpms ?????? They might even be 1.25's.

Did you buy ANOTHER boat? :ernaehrung004:

YES!

I hope I did not make a mistake.

Mistakes can always be fixed. Just takes $$$$:evilb::evilb: Would you like to borrow a pair of 26 pitch Bravo's to see how they run? I won't be in the water for another week or two.

forgive my ignorance as i am not a prop expert or even knowledgeable but wouldn't just putting on shorter props to bring up the revs be the same as just putting on shorter tires on a car ? i.e ... yes the revs would come back but the top speed wouldn't be there ? for example... for the sake of discussion if the motor spins 4000 w/ 1.36 gears he is getting 2950 prop revs w/ a 30.

now a 28 is roughly 6 % less prop which would require only 6 % more revs for a "break even" or roughly 4250 ... lets say it actually does that... but what he really needs is 25 % more revs so that would imply an impossibly small prop... and no top speed at all....

now.. i understand that this whole thing is not precisely linear like it is w/ tires but it is substanatially so.... and you have the added unassailable fact that this combination worked flawlessly in what are essentially identical circumstances on the first boat...

i think something else is fundementally wrong. this seems like way too much " problem" for an incremental prop change...

i don't have any meaningful suggestion or solution but it just doesn't add up for me somehow.

Your right, not trying to fix the problem with the props. The drive train is the first place to start. The boat is in the shop as we speak and one way or another they will find out is going on.

Maybe it's something in the engine managment system that's not letting the engines come up to full RPM'S.

It would be like a car with taller gears and smaller dia. tires. In the end they would travel the same distance and speed. However, common wisdom seems to be that 1.5 ratio is more efficient in our speed range. Just suggesting. To me it seems like more is wrong than just gears. If the former rpm was 5000-5200, with the new ratio, the new rpm would be 4500-4700. 1.50-1.35 is not a very big change.

Might of found the problem, No compression in #7 Starboard Motor, Bad Fuel pump on Port Motor. Sometimes we're looking in all the Wrong places. Will know more when they pull the motor out.

I'm off to a great season with a new to me BOAT.

Are these Bravo 1s ? I was thinking the ratio is determined in the upper?