Changing propeller pitch - Mercruiser Bravo II

[jointventureII]

Dear forum,

I have a Mercruiser Bravo II, 1:2.2 ratio, 21p x 17.75 diameter prop. This produces 26.5 knots flat out on a 25ft, 3000kg planing hull.

The engine, a VM diesel 4.2, 320hp, should rev to 3900 but goes to 4000 where it's on the limiter and there's some throttle still to go. If it were allowed to go as far as it wanted I suspect it'd get to about 4200 but this is pure guesswork.

It'll get to the limiter easily even when dirty underneath and loaded. Therefore I want to change the prop.

For the Bravo II, the next step up would be 23p x 17.5 diameter (2 inches p more, 0.25 inches diameter less) however I'm thinking this might not even be enough and going for the 25p x 17.25

This would be a full 4 inches of pitch more (by my calculations that's 600 rpm lost....) but then 0.5 inch diameter less (which I think is around 300 rpm gained?)

Net result using my very ropey method would be 300 rpm less which I think would put me on the 3900 rpm max that the engine is supposed to reach.

Thoughts?

 

[Hot Property]

I re propped my 150 VTEC from 17 to 19" pitch.

It was exactly the same prop but just 2" coarser.

I expected a drop of 200 rpm per inch so 400.

Turns out it dropped 700 revs!

My Honda tacho may need calibration so im connecting it via nmea 2000 to my chart plotter to check accuracy.

As always it's a black art!

 

[jointventureII]

So I went ahead and did this.
The 25p prop produces exactly the same results - 26.5 knots flat out, 4000 rpm and on the limiter.

Back to the drawing board.

 

[Hot Property]

So I went ahead and did this.
The 25p prop produces exactly the same results - 26.5 knots flat out, 4000 rpm and on the limiter.

Back to the drawing board.

Is your boat actually 3,000kg?

My saver 690 had a list weight of 1700 plus outboard at around 300 so 2,000kg. I suspect it was actually nearer 2.5 tons.

Did you change just the pitch or the propeller brand? they are all different.....

black art as per post #2

 

[jointventureII]

It's 3200kg, measured by the crane that launched it last week.

I think the problem lies with the gear ratio. 2.2:1 is too much for this setup, a (relatively) low revving diesel. Usually the diesel ratios are 1.36, 1.5 or 1.8.

2.2 ratio @ 4000 rpm turns the propshaft 1818 times / min. It needs to be higher than this (which would be then coupled with a 19 or 17 pitch prop).

 

[Rhonda Snow]

If you’re changing pitch, remember that each inch of pitch is worth about 200 rpm. Lowering the pitch will increase rpm and vice versa. If you need more speed, a higher pitch is the way to go. [Unwelcome content removed]

 

[jointventureII]

The Mercruiser Bravo manual gives 150rpm for every 2 inches pitch change (mainly because the Bravo 2 propeller series drops 0.5 inch diameter for every 2 inches of pitch).

I've spoken to a prop specialist and the problem seems to be the aluminium. He thinks the blades will be flattening out slightly and a stainless prop would solve this.