Change prop or gear ratio
 

Hi all
Would like some help in deciding whether i should try various props or just spend
more hard earned cash on changing the gear ratios. I was lucky enough to buy a 1996 Shaba 180
In brilliant condition quite cheap ,reason being the 4.3lx mercruiser engine had had it.
So have now had fitted a brand new 4.3 mpi lump,which now fully ran/broken in is great,But
I'm getting 47 mph( GPS) at 5200 rpm using a 14 x21 prop,Too much rpm I know.getting on the plane
Isn't a problem gets there nice and quick,so is the rpm high cos I haven't changed the gear ratio I'm
On the original 1.81. Will changing up to a 23 pich prop solve my problem,or go to the expense as
I have read on forums here change the gear ratio to 6:62 just cos its now a mpi version.like to know
just how much difference top end mph rpm,etc this would make.
Thanks to all,appreciate all the great info found no the this site
Paul,
From The South Coast UK

Re: Change prop or gear ratio
 

Throw a 23p at least on it , but I'd put a 25p on it and see what it does. Best senario your looking at 52-54 mph. Just remember the more pitch ....the less holeshot:wink_thumbup:
The prop is way cheaper than a regear job:shakehead:
I went thru this on a 4.3 on another boat(17' VIP) that had 5.0 gearing. I wound up running a 20p on it , and it ran about 48-50mph. That was at 6000ft+:wink_thumbup:
My 4.3 Diablo I have now ran about the same( motor blew-bad build from shop) with a 19p 4 blade stainless steel on it.

JMO :)

Re: Change prop or gear ratio
 

Therein lies the problem cuz the MPI version of the 4.3 used the 1.62 gear ratio. You could try (borrowing might be a better way) a 23" prop as that should bring your rpms down to about 4800 which is at the max of the recommended range and you might get 1-2 mph but my math is a little questionable on this one. I'm not certain if you will actually gain any top end because of the higher gear ratio. You will however lose some grunt out of the hole....

Re: Change prop or gear ratio
 

depending on where your located, but many prop shops will allow you to "test" some before you buy them. Of course their used props.

here Sheffield's will send you with 3 props with an imprint of your credit card. try em. pick the one that you think is best. return the other two and theyll send you out with two more, pick the best and buy it.

Re: Change prop or gear ratio
 

Thanks for the advice think ill try the prop changing option first.
Thanks again
Paul