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Sterndrive ratio choice, Please Help!!

O

Opentraffic

New member
Hi there
Im new to this forum but have found your other posts very informative and helpful.

So my dilemma....

I have a 1994 Wellcraft eclipse 196 sports cuddy boat that has recently required a new sterndrive.

The boat was originally a 305 but at some point has been repowered with a 350 using the stock 1:1.65 ratio sterndrive.

My question is.. Do I replace the sterndrive with the similar 1:1.62 or go for the correct match to the 350 and use the 1:1.47 ratio??

On paper the boat weights 2750 lbs

Reached 42 knots on a 19 pitch stainless @ appox 4800rpm, maybe a little higher.

Thank you in advance for any help
 
1.47 - you'll have to reprop, but that's the way to go
1.62 delivers way too much torque to the lower, and the w/p impeller will be turning too slow for a 5.7
 
If my understanding is correct your prop shaft rpm will increase by about 300 rpm. That means you engine rpm will drop by about 400-450 rpm which means you'd need to increase pitch by a bit more than 2". However, adding a 4th blade will lower the engine rpm another 100 rmp or so. Prop material will also affect wot rpm.

I just dont see how, as the previous poster said, that impeller speed change will affect engine temp that much. You haven't been experiencing overheating now have you? The 305 and 350 are the same block afterall...
 
I suppose we should ask if OP is boating at sea level or up in the mountains.

Part of the reason Merc uses lower overall ratios is to get the gpm of cooling water up on higher HP units. More HP demands more impeller speed (they all use the same impeller)
PPS or some other parts supplier used to have a chart online showing how much 70F seawater it took to cool various marine engines at full HP.
As I recall, if you compared that chart to merc's flow specs model by model, there wasn't a whole lot of room for error.

Personally, if ever on the fence, I would go with a lower overall ratio (1.47 in this case). I want all the cooling flow there is. About 10% more water flow in this scenario.
 
Hi guys

I do all my boating at sea level, in the sea and in the rivers.

I agree that the high hp of the 350 will produce a higher operating temperature and thus the cooling is important.
The engine has not been overheating as far as I'm aware but I wound not dismiss extra cooling!

I was running a stainless 3 blade 19 pitch lazer 2.

With our river trips I was considering a 4 blade Ali to prevent any damage as you see a fair amount of submerged debris!!!
 
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