Do these numbers seem reasonable?

[Larry3215]

I have a 17' aluminum Sea Nymph. Total weight is aprox 2300-2400 lbs fully loaded.
Power is a 1986 70 hp Evinrude
Prop is a 13.25x17 aluminum thats a little rough and needs replacing.

I tached it today (new Tiny Tach) and at full throttle my best RPM (playing with trim) was 5650 at 35 mph on GPS.

Running those numbers through one of the prop calc programs comes up with just a hair under 6.8% for slip.

Do all those numbers seem reasonable? That slip % sounds low low to me. I've always heard slip ranges from about 10% and up, so thats why Im a little concerned that I may have something off.

Any comments appreciated.

Thanks,

Larry

 

[steelespike]

Re: Do these numbers seem reasonable?

Sure looks to me like your doing really well.We know your speed is accurate,prop size and gear ratio.
If I have to guess slip I usually use 10% not that far off from your almost 7%, thats what I got too.
Tach may be a little low but surely within reason.If you use 10% slip at 35 you get about 5800 rpm.
Your hp is rated at 5500 if your rpm is really 5800 it doesn't seem unreasonable.

 

[Larry3215]

Re: Do these numbers seem reasonable?

Thanks! Now I can shop for a replacement prop with confidence