gear ratio

[sander25nl]

I have a maxum 1900SR year 1999 speeedboat.
Recently i change the engine , a volvo penta MPI 270HP , V8 , but the leg is a volvo SX cobra from a 6 cilinder ratio 1.78(or1.79) .
First we used an original 23 pitch volvo prop , and it felt like a lot of slip . , did about 45 mph 5000RPM
Changed to 4 blade mercury revolution prop stainless steel 21 pitch, top speed now is 50 miles/h , with 3 people in the boat , also 5000RPM (RPM limiter comes in around 5100-5200)
In my opinion the boat should run faster with 270HP.
The gear ratio for a V8 should be 1.50 , what will exactly happen when i change the gear ratio to 1.5 ? with the 21 pitch revolution prop .

thanks in advance !!

Sander
Netherlands

 

[steelespike]

Re: gear ratio

Can't be too specific but the 1.50 will turn the prop faster if my math is right 25%
assuming the motor can turn it up to its rated rpm, at 270 hp it sure should.
your rpm and speed will seem more logical and prop selection should be easier.
I think with a "speedboat" you would eventually end up with a 23 or 25" pitch.
You may want to find a dealer that will let you exchange props.
A stab in the dark speed approaching 60.This is all seat of the pants there are pros here that can likely be more specific.