Exchanging Drives & Gear Ratios

[Paul180289]

Drive ratio Hi can anyone help please, I want to change my drive, I have a 24foot extreme powerboat weighs 2.1tons engine is a merc 525 Racing with a xr 1 1.50 ratio spinning a 4 blade 30 pitch Erving to 5,100 at wot. I want to change to a bravo 3 xr drive but not sure what ratio I need and What prop size. Would be great if some one could help me out thanks

 

[alldodge]

Howdy
A 2.1 ton (4200 pound) boat is not that heavy with a 525, I have a 4800 pounder with a 300 HP 454 and it does 60 MPH using a B1 and 1.5 drive

I put your info into a prop calculator and if you have 10% slip then your speed should be around 87 MPH. Going to a B3 would be as simple as swapping your lower B1 for a B3. The issue right now is I'm wondering if you actually have a 1.5 drive.

The B3 does not have a XR version, other then its and XR upper and a standard B3 lower. I currently have this setup in my Formula and I'm 10K lb a 2.0 ratio and 530HP

What is your WOT GPS speed?

 

[Paul180289]

Hi thanks for getting back to me, my actual gps it 85.6mph at wot

 

[alldodge]

That puts your slip at 11, and isn't bad, but should be able to get a bit better. I'm still back to the 30 pitch is really high and wonder if the reason for this is your trimming up to high. Not trying to steer you away from a B3 but would like to get some more baseline if you OK with it.

What boat is it?
Also is the prop a 3, 4 or 5 blade?
Find out where on the trim gauge that the ventilation plate is flat and level with the hull.
Then take the boat and keep the vent plate in this flat location, and go WOT and not speed and RPM. Don't try to trim up or down to get more or less RPM/speed

 

[Paul180289]

It’s a extreme 24 made by extreme marine
prop is a bravo 4 blade

Iv been told I should have a 28 as I should be revenge to 5,300 to 5,400 at wot

the trim rim is about 2/3 on the gauge anything under revs drop and lose speed anthing over its loses grip and speed

 

[alldodge]

Your at 11 percent with the 30 pitch turning 5100 rpm. Going to 28 pitch, at 5400 with same slip reaches 84 mph. Figuring you will reduce slip at least 2 percent (9 %) with the 28 your speed should be 86.8 mph.

I think your your trying to break the 90 mph line. A B3 should reduce the slip but where in the area of fine tuning. Do see a B3 lower with 15/19 gears and similar props, but this will get expensive faster and will send you a PM with a name and number of a builder in Illinois which I also use.

 

[Paul180289]

I’m happy with the speed it does now and 90% of the time the water is never flat enough to go to them speeds. Main reason I want to change is for stability and handling.

Two of my friends have same boat one with a yanmar on a b3 and the other with a Volvo dpx duo and both of them in rough water track so much straighter than mine. I get quite a bit of toque steer when in and out of the water

 

[alldodge]

OK, sorry for side tracking your question.

That being the case I would get the XR upper should be 19/16 gears for standard XR upper.
To stay with the 1.50 ratio you will need a B3 lower with gears 15/19

You will need to use 28 pitch because this is the highest pitch available for B3. Hill Marine makes a 4x4, and BBlades and Mercruiser makes a 4X3

The 4x4 and 4x3 will increase lift a bit more

Would be nice if we could get a 1.35 lower but 15/17 gears are not made for the B3, and the XR only comes with 19/16 uppers

 

[alldodge]

Should also add, that if you get props from Hill Marine, they exchange props for free, your only cost is sending the props back to them, no restock fee.

Others sell their props, but most do charge a restock fee

Hill will also add or reduce some cup at no charge to help tune in your props. At least that is the way it worked when I bought my set.

 

[Paul180289]

Ok thank you for your help much appreciated