2011 Yamaha 70 4 Stroke

[K'jun]

I currently have a 18 southfork with a tall transom running a 70 yamaha 4 stroke with a 15 pitch prop, I'm hitting the rev limiter just by bumping the trim once while on step. Would it be worth repitching the prop to a 17? I run about 39 full throttle now, but would expect to get more out of the motor if i trim it up but thats when it hits the rev limiter, looking for opinions

 

[Sea Rider]

Welcome to Iboats,

With deck weight evenly distributed, trim to achieve engine to work at 90?, (perpendicular to water level) once on plane and leave it there. At which throttle setting is engine hitting the rev limiter and at which rpm ? If engine doesn't sit at the sweet transom height engine will rev more while at 90? too. A 17 pitch will lower revs, but will not know by how much if no tach readings are available to compare them to the wot safe range for that 70 HP. Assume you have a installed tach, right ?

Happy Boating

 

[steelespike]

It does appear you at least need a 17" prop.We need your recorded rpm.Guessing 12% slip with a 15" prop,2.33 ratio 39 mph calculates to about 7000 rpm. I would guess 39 isn't accurate
we need a gps speed.

 

[K'jun]

Will try to find out this afternoon, i did install a tach but haven't ran it since. I will download a gps speedometer on my phone and get the info for you all this evening. Thanks for the reply

 

[steelespike]

I think the motor is mounted too high.Tell us about the south fork. What kind,dry weigh,t all up weight?
Deep V ,flat bottom?
It may be venting when you trim. It may be closer to venting without trim.
We need the light load wot rpm and gps speed.You may just need more prop.