prop sizing

[richvan]

I am trying to figure out what prop to try next. I have a 2001 searay 280 sundancer with 2 v6 4.3 liter engines, alpha 1 outdrives. The props on the boat now are 21 pitch but I dont know the dia. The engines will only run 3000 rpms and wot is suppose to be 4400 to 4800. Any advice on how to fix this ? I looked at some of the prop selection guides online but it appears you need to know the gear ratio which I have no idea

Thanks in advance

 

[jestor68]

Re: prop sizing

Your gear ratio(s) are likely 1.81; possibly 1.62. Both ratios come up with a similar result.

What kind of props are you using now; aluminum or steel?

Some experts will be along shortly to help you.

 

[richvan]

Re: prop sizing

Steel

 

[steelespike]

Re: prop sizing

What is the gps speed at that 3000? Some cells have a gps app.What is the hp of the motors?
With the hp,and wot rpm and gps speed we can get an idea if the tachs are accurate.
To gain 1800 rpm you would need about 13" pitch props. At 4800 rpm, 1.81 ratio, that would produce about 28 mph
perhaps reasonable for a 28'? ft boat.