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Mercruiser 165 reduction / prop sizing

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Dreadnaught

New member
Ok folks, so now I've bought a Mercruiser 165, recently rebuilt and sounding good. How to go about choosing a reduction gear and prop?

My boat is a 35' X 8'6" displacement cruiser, a 10 knot boat. Straight shaft inboard. It should have a large wheel, like a sailboat. With the Mercruiser I am replacing a Fort Flathead, 95 hp, which ran through a 1.9 to 1 gear to spin a 20 dia X 16 pitch three-blade prop.

What reduction do I need to spin the same propeller? Or what combination should I use? I already have a 1.5 to 1 reduction gear in hand...

Thanks!
 
Dreadnaught said:
Ok folks, so now I've bought a Mercruiser 165, recently rebuilt and sounding good. How to go about choosing a reduction gear and prop?

My boat is a 35' X 8'6" displacement cruiser, a 10 knot boat. Straight shaft inboard. It should have a large wheel, like a sailboat. With the Mercruiser I am replacing a Fort Flathead, 95 hp, which ran through a 1.9 to 1 gear to spin a 20 dia X 16 pitch three-blade prop.

What reduction do I need to spin the same propeller? Or what combination should I use? I already have a 1.5 to 1 reduction gear in hand...

Thanks!
Click to expand...

Ayuh,.... The motor originally ran through a 1.81:1 or 1.65:1 gear set, turnin' a prop somewhere 'tween a 15x15 to a 14x23,...
Most likely a pitch of 17" or 19"....
 
The Merc 165 was referred to in both Merc's own 4 cylinder (aka 470) and the GM 250 cu in inline 6 cylinder.
You do not qualify this for us.

Since this is an I/B straight shaft installation, you may want to contact Mercury Marine for a suggestion.


.
 
It is a 6 cylinder.

Thanks... It looks like I need a pretty high reduction. Looking around at guides on how to calculate these things. Does anyone know where to find torque curves, power curves, or similar data on the Merc 165? Or know where the peak efficiencies are? What is its most efficient RPM?
 
You won't find any published power curves due to the variables involved...

The vessel you describe will be plenty of "load" on the engine....as far as the gear and the prop selection/recommendation, find a local "modern" prop shop in your area...they will have 'proven' calculators and will also be able to help you dial in the prop when you get to that stage...

Personally, I think the 1.5:1 gear is NOT enough reduction for a boat that large...BTW, if you can get a good weight estimate, the prop shop can give you a better answer.
 
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