Originally Posted by Northsearibber
Engine is a Yamaha f20 2013. Prop size is 9.25 x12.Speed - would have to let you know exactly but about 12/13 mph.
OK - Yamaha says your engine max rpm is 5000-6000 rpm. With a 2.08 gear ratio. So at 5000 rpm your prop should be spinning 2400 times per minute. Assuming no slip that would mean your prop moves forward 12x2400" a minute = 23.7 knots. Bearing in mind that 10% slip is about the best you will get you should be getting 20ish knots (23 mph). On a small boat it feels like more. You think you are only getting 1/2 that - so I think there is a problem.
Is it sitting bow up?
Is it generating a huge wake behind it?
Its going to take actual tacho and speed data to work out more - and ideally some video footage so people can see how she is sitting.
Originally Posted by PeterM
I'm running a 25 hp 4 stroke on a 3.6m rib.
I just think your asking/expecting too much of 20hp.
I'm running a 20 HP 2-str on a 3.9m and think he's not!