13.75x15 on 15 foot fiberglass w/70 hp OB sound right?

[cougar15]

Hi, This is my first season with a boat and I am thinking about buying a new prop but I was wondering if I am running the best prop now or if I should make a change...

I have an old 15.5 foot fiberglass tri-hull runabout with a 70hp evinrude OB. The current prop is a 13.75x15 aluminum. The hole shot is pretty good and the top end is 31.5 mph. Does this sound right for my configuration? I don't have a tach so I really have no idea if I am running in the correct rpm range...

 

[critter34]

Re: 13.75x15 on 15 foot fiberglass w/70 hp OB sound right?

I removed a 70hp off of a 17ft tri haul to put on a aluminum boat the prop was a 13.25 x17 pitch and the boat moves real fast .My book says for a general purpose application between 14 -16ft
use a 13 x 19pitch for max speed of around 38mph

 

[jimmbo]

Re: 13.75x15 on 15 foot fiberglass w/70 hp OB sound right?

A tach is necessary to determine if the engine is loaded correctly

 

[Captain Tuna]

Re: 13.75x15 on 15 foot fiberglass w/70 hp OB sound right?

I have a 15' tri hull with 3 cyl 60 hp Rude (same basic motor that you have). I started out with a non cupped 17P aluminum 5100-5200 rpm. Switched to a slightly cupped 15P aluminum 5200 rpm. Ended up with a 13P ss Turbo 5200-5300 rpm. MPH stayed about the same 30-32. The difference was a MAJOR improvement in both hole shot and handling! Jerks me up on one ski!
Two other thinks made BIG differences in my boats performance. Removing the hydrofoil and raising the motor. Gained almost 3mph and cured some nasty steering issues.
I'd guess the 15P is probably right.......get a tach!