Propping a boat with no existing prop?

[Mike0537]

I recently bought a 1985 Larson Senza V175 with a closed bow. It is a project boat that doesn't run. It has the Mercruiser 488 with an Alpha One drive. There is no prop on the drive so i need a starting point. I would like to be able to pull a tube or a skier and get up to 40-45 mph. I'm assuming I need somewhere around a 19 pitch, but I don't know what size. Please help me find a good starting point!

 

[jimmbo]

Yeah try a 19, and you better have a accurate tach. You also need to know the WOT rpm range for your engine. Once you have an initial starting point using the 19, you will know whether you need more pitch or less pitch

 

[ahicks]

When it comes to Merc I/O runabouts my experience has been the 19 may be close, but with skiing in the picture, my first guess would be a 17".

 

[Mike0537]

WOT is 4200-4600 rpm. I'll go with the 19 pitch, but what about diameter? I'm actually a Master BMW tech so obtaining accurate RPMs won't be a problem.

 

[ahicks]

The 19" props are only going to be available in a fairly small range of diameters. There's not a lot of difference from one extreme to the other, but if you want to be focused on the prop's characteristics, with all else being equal, the larger the diameter, the better the hole shot, with the downside will be increased drag. Opposite true when considering the smaller diameter. It won't have the hole shot, but will likely have higher top end. Likely splitting hairs over how much though....

 

[jimmbo]

Let the prop engineers worry about the dia.