For any kind of help with a prop suggestion , we would need to know :
What motor ?
What has been done to the motor , if anything ( mofications )
Are you running a tuned pipe or cannister muffler or a gutted one ?
What prop are you using now ? ( what number is on it )
How much does the boat weigh ( aprox )
Did you change anything , or is it the way it came RTR ?
Has the prop been balanced and sharpened yet ?
Have you put any back-cut on the prop ? ( allows the engine to load up faster without losing top end )
Did you change the pitch by cupping the prop ?
Was there any changes made to the current prop at all ?
( also - if you have no idea as to what I am referring to about modifing a prop , it always best to leave it to a prop bender . These DO get EXTREMELY sharp and can make hamburger out of your fingers , plus the berrillium is BAD for you - most props are copper which are easier to work with verses the S.S. ones which are a b*tch to do . Plus the copper ones are cheaper
and have a wider variety to choose from )
Here are a few basics to go by :
If a stock Homie engine - try a Prather 275 or 280 with the diameter cut and a back-cut on it, also a Octura 470/3
If a Zenoah - try a Prather 270 , or 275 or a Octura 472 , or a 7016/3
The list is honestly kind of BIG .... but the ones above are kind of basic choices . This list is by no means complete , since each boat is always slightly different from the next and the best way to find out , is by trial and error method .
If you live near a lake where they have a local club , it would be a good idea to go to your local hobby shop , see if there is any clubs close by and grab a hold of your boat , jump in your car and give them a visit when they are running ( usually on the weekends ) . Most of the guys are super and will help you out and will throw on a prop or two of their own to get you knowing which prop would work best . They will also let you know who voo-doo'ed the prop so you can get the same cutting on one ( voo-doo'ed - what prop benders do to the props to get the best results out of them . )