3 GM Prop spec?

Tranona

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You need to know the gearbox ratio, the weight and waterline length of the boat, the diameter of prop you have space for and the number of blades.

Then put it into the Propcalc programme from Cstle Marine. This will give you a good start - then go to a reputable prop supplier and take his recommendation - so if its not right he is responsible!
 

MASH

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Its probably the KM type gearbox (as I'm pretty sure it has a cone clutch not a plate type). I've just looked in the manual and there is a choice of three ratios. I have no idea which, and as as it is 2000 miles away will struggle to find out..Snookered.
 
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Tranona

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Not easy from 2000 miles away - but is easy if you can see the top of the gearbox as it is on a plate. Essential to know as it makes a massive difference to the diameter of prop you can use. Measuring the existing prop will give you a guide, then you can do the calcs based on each ratio. I expect it will be either a 2 or 2.6 as don't think there is room to swing a big enough prop to use a 3:1
 
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