A: "What prop is best for my boat" again :) 2300 SR 5.7 efi

[don_her]

Well i wouldn't go as far as to get 4 props but 2 i would not mind 1 for skiing and 1 for top speed is do-able.

[don_her]

So, it's been a while

This year we've had a huge service/overhaul done to our boat. It now runs 76km/h (47mph) with the 13.25*19 high five prop, it then runs at nearly 5000rpm!

So i've tried a 3 bladed, 14*21 prop. The RPM's dropped to 4100 and top speed was about 65km/h (40mph)

From what i've understood:
Going up in pitch means a drop of 150-200 rpm per pitch up. So in my case 300-400 rpm drop.
Going down 1 blade means an increase of 50-100 in rpm. So in my case 100-200 rpm increase.

Then, at best this should have resulted in a 100rpm drop, and at worst in a 300rpm drop.

How could i've lost about 800rpm's and 11km/h (7mph)? Or is it due to the increase in diameter from 13.25 to 14? Any help would be appreciated

[mmwjr]

Yes 3 years has been awhile.

Are both props stainless steel? 5 blade vs 3 blade? Yes increasing the diameter adds load which drops rpm, however the loss of 800 rpm does not make sense.

Is the 3 blade cupped?

So both of these runs were recent? Any other changes between the runs?

[don_her]

No, the 5 blade was stainless and the 3 blade is aluminium.

No other changes, runs made within minutes of eachother.

From what i've been searching and finding it probably is the change in diameter, apparantly 1" increase in diameter is the same effect as 2.5" in pitch.
So i guess i gotta find a 13.25 diameter 21" pitch 3 or 4 bladed prop and see what that does.

[mmwjr]

Aluminum is lighter than SS therefore the 3 blade performance just seems out of line but I won't argue with your data. Yes the diameter is an impact but less blades, lighter vs increase of diameter and pitch should balance.

Have you plugged you data into mercruisers prop calculator to see what it recommends?

Good luck

[Calguy]

Have a 1995 2400 SCR with a Mercruiser Horizon 350 Bravo 2 drive. Boat is on a lift so no problems with fouled hull. Currently running a Quicksilver 18.25 19 pitch. WOT at 5200 RPM top speed of 46 mph. Cruise speed of 30 mph at 3400 RPM.

[mmwjr]

The OP has a 23 foot bow rider and based on his prop size I believe it's an Alpha drive so your boat set up is in a different ballpark.

[don_her]

Yes, it's got the puny alpha

I've gone through the prop selector numerous times, thing is... It won't let you put in if you've got a 3-4-5 bladed prop and what diameter etc.