Trying to figure prop pitch needed
 

I am borrowing a Ski-Pro boat for winter while I work on my new engine,
its a 19ft ski style boat with a mercruiser 4.3 V6 and alpha drive.
The hull is intresting in the fact it has a couple of small sponsons in front,
, a small central fin , a fairly large flat pad running from middle of boat to the back and lots of rocker ..
This is what bottom looks like:
https://sphotos-b.xx.fbcdn.net/hphot...01393813_n.jpg
The factory specs have it at 53mph with the190hp V6,65mph with the 300/350 V8, and 80 mph
with the 425/496HO

I have not run it yet but owner says best he has seen is 48mph with the V6,
prop is in bad shape and is a 19p , seeing as the V6 Alpha has a 1.8 ratio
and the prop being a 19P plus the V6 having an rpm range of 4400-4800rpm
something doesnt add up or am I doing my prop math wrong? Sounds like to me he was overevving the engine to be able to get top speed ? I was thinking more along a 21p or 23p
to run low 50s with a 1.8 ratio?

A 21" pitch prop, with the 1.8 drive and motor turning 4800 rpm with 10% slip would equate to about 48 mph. I'd say the speed he advertised with a 19" prop is inflated or in error. I'm not sure a V-6 even with a 1.8 drive could swing a 21, let alone a 23" prop. It might, but I'm just not sure.

A 19" prop is where I'd start and go from there.

I agree a 21 P prop would be required to get into the upper 40's.
I also agree that power in that boat will be hard pressed to spin 23 inches of pitch and 21 might be tough too.

Get a baseline with what come with the propeller and we can discuss the options from there.

Brett

Skiboat Magazine test of same boat with same power shows it at 53mph on GPS with two people on board.
Unfortunately it doesnt say what prop they ran.
The 19p prop on it is very beat up,would rather not use it so as not to damage the drive from vibration ..
Guess I will need to find someone nearby with a 19 or 21 i can borrow to test..
Thanks..
Frank

We have a test propeller program. Check it out. We make it very simple.

http://bblades.com/propeller-test-program/

Brett

Thanks Brett..
Dont forget V6s have a 1.81 ratio I believe.
Here is what I have so far, on a 19p Sterndriver it turned 5600rpm,
way too much rpm considiering Mercury reccomends 4400-4800
it also didnt respond well to trim, very little trim made it blow out.
I then tried a 24p Quicksolver it turned 4400 rpm but you could tell engine wasnt thrilled at WOT
The Quicksilver had more bow lift as well getting the boat further out of the water which is important since driver sits almost at front of boat.
Im thinking a Mirage Plus 21p would be a good all around prop for this thing?

Do not take for granted a 1.98...2.0 gearing..1.6 is also avaible. The results you have posted so far make sense aside from a 24p ss turning 4400 rpm unless its a 2.0 gearing

Remeber one thing the 4.3 is a fairly high hp engine for its displacement but has very little touqre at high rpm. So how do you want it to perform...midrange punch or total top end..very different approach for each character.. By the way if its a 97 or newer it would vortec heads 4bl carb making for about 240 at the crank and 210 at the prop..efi maybe a tad more. and one weak cam either way.

That hull is simply a shallow bottom v with fins ...extra drag.

i would like it to be within mercurys reccomended rpm range and be a good all around prop.. could care less about top end..
the engine is 4bbl with centerbolt valvecovers but dont know if they are vortec.

And the gear ratio....that is big thing with low tourqe engine's....they cannot overpower a prop..How many people in a load..back to tourqe again..

Here 1.66 gearing @ 4800rpm 20p prop nets 55...no slip

1.98 gearing @ 4800rpm 20p prop nets 46 mph no slip

Big differnce there: here's a prop calc use 9% slip the right gearing and find the pitch..Your numbers above reflect pitch and rpm but no gearing and speed...Make's for wild guessing...

http://www.go-fast.com/Prop_Slip_Calculator.htm

gear ratio is 1.81 as it says in my first post..
its stamped on the upper on the side..
Only had a gps onboard one time with the 19p sterndriver
4000rpm it said 38 mph had a fuel problem that day so I could not run it wot.

Well 1.81 @4000 38mph is 5% slip quite good. But not enough to really now how that engine pushes a prop on that hull. So here's a WAG a 22 would put you at 52 with the same slip..very close to the posted test speed...And you had a 24 at 4400.

23 would get 4600
22 would get 4800

If the slip remain's constant which is should Solas Hr titan makes a very good 22p 3 blade SS for that combo...http://boatpropellers.iboats.com/Mer...t_id=138527619 Top end prop



Or a Stilletto or Turbo would well in a 21 work well also...same money virtually same prop with one difference the Precsion props are big lifting prop..shallow v could be a little bumpy or porpise a bit

Midrange Stilletto 20p 4blade baypro 2 http://www.boatownersworld.com/Props...pro_series.htm Great midrange prop it will act like a 21 3blade in terms of slip...maybe a 50 mph prop on that boat. I use a 20 on a 20' 4.3 mpi engine with 1.66 gearing actually makes you feel ike your in a small hotrod.