Need prop suggestion due to gear ratio change

Stjohns

Recruit
Joined
Oct 24, 2017
Messages
2
I took the drive unit from a 97 volvo penta 4.3gl, unit sx-c1 1.66 ratio and moved it over to a 2005 volvopenta 3.0 gs gear ratio 1.97.

Based on tbe smaller engive using the incorrect ratio how can i accomidate for this with a prop. Id really like a better top end. Prop suggestions please??
 

porscheguy

Chief Petty Officer
Joined
Jan 17, 2013
Messages
441
If you made this swap solely for the pursuit of speed, I?m afraid you?re going to be disappointed. Most boats with that engine and drive are running 17-19 pitch props at the highest. With your current ratio, you?d have to go down probably 4 inches in pitch. You want to pick up speed. If you?re serious about that, you need a stainless prop. The selection of stainless performance props in a 13-15 pitch is fairly limited. The other problem you face with props less than 15P is that they tend to be for applications like 50-70HP outboards on lightweight boats. They have small diameters of 10-11 inches. Prop diameter isn?t discussed much because most of the time it isn?t a huge factor, but I?ll tell you this, you?ll go slower with a 13Px10? prop. I looked and Volvo has nothing stainless in a 13P. Mercury has the enertia and possibly a mirage plus. But those are massive in diameter. You?ll likely lose performance due to the increased drag of the blades large surface area. Quicksilver has the Silverado but diameter my be an issue and the Silverado is no performance prop. I don?t know about the secondary prop companies like solas and Michigan Wheel. They might have something you can use. If your old drive is still in working order, my suggestion would be to sell the new drive and use the money to put a better prop on the the old one.

Out of curiosity, with the old drive what size prop were you using?
Top speed?
Engine rpm at top speed?
What type of boat?
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
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Jul 23, 2011
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47,306
welcome aboard

I seriously doubt you will find a prop that will work with a 1.66:1 drive and a 3.0

what made you put a big-block / diesel gear set on 3.0 4-banger?
 

Stjohns

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Oct 24, 2017
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2
The outdrive went to **** on the 3.0, and i had a Wellcraft with the 6 cyl just sitting there with rotten floor and other various problems, so it was purly an economic decision to do the swap. The 3.0 is in a glastron 195, no fing idea why youd put a 3.0 in that boat. Bit its what i got
 

Bondo

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Apr 17, 2002
Messages
70,468
The outdrive went to **** on the 3.0, and i had a Wellcraft with the 6 cyl just sitting there with rotten floor and other various problems, so it was purly an economic decision to do the swap. The 3.0 is in a glastron 195, no fing idea why youd put a 3.0 in that boat. Bit its what i got

Ayuh,.... Welcome Aboard,.... In that case ya need a motor to go with that drive, or another 3.0l drive for that motor,....

There's no props out there that can make up the differences yer lookin' at,....
 

jimmbo

Supreme Mariner
Joined
May 24, 2004
Messages
12,944
To get a better top end, the easiest, most reliable way, and often really the only way, is a bigger engine. A stainless prop is not likely to do much, if anything.
To get the most from your current set up, a prop that allows your engine to hit the rpm where it makes its peak hp will yield the best speed. Propping above or below that rpm, even though it may be within the WOT range will result in a lower top speed.
You are using a gearcase with a lower numerical ratio(taller) so to compensate, a prop with lower pitch will be needed to get the rpms back to where they should be. But with a lot of props as pitch drops so does efficiency.
Since you haven't provided any info like your engine WOT rpm range, what rpms you were getting with old gearcase, and what pitch was the prop on it, it really is difficult to make any recommendation.
The ratio change results in a approximate increase of propshaft rpm by 18% relative to engine rpm, so a decrease in pitch by about 17% from what was on the old drive would be a reasonable starting point.
Getting rid of unnecessary ballast will help. Wife and kids, the Mother-in-law, especially the Mother-in-law are to left ashore or tossed overboard. Mistresses and Girlfriends are allowed to remain board.
 
Last edited:

QBhoy

Fleet Admiral
Joined
Mar 10, 2016
Messages
8,286
Hi
Unless I?ve missed it, I don?t know what boat you have. If it?s a fairly light speedboat like a bow rider of the usual 17/18ft....surely you would get away with a 17? prop and just be careful when loading the boat up ?
It?s never going to be great at pulling a mono skier from deep water, but might just be ok.
 
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