moving to a 13 x19 ??

[bentech]

Hi, i have a 17' Starcraft boat powered by a Yamaha 85hp 2 stroke engine. The prop on the engine is a 13x17 (3 blades).
With this set up, i have reach 44,73 mph (speed mesured by GPS) and the engine rev 6000 rpm at WOT.


My question is: Can i expect reach 48-50 MPH with a 13x19 prop ??

 

[SkiDad]

I would say more like 46 - 47

 

[steelespike]

Your slip is only 7% possible but is really low. About 12% is more typical.Usually when slip is odd the Tach becomes suspicious.
I believe your speed but it does seem high.Tell us about the Starcraft.
The 85 is a bit of an odd ball I don't find any in the Yamaha performance reports. Using the 90 I find your speed reasonable but
your rpm seems really high.Same gear ratios .I do wonder if the speed might be aided by wind or current.
It does appear you need at least a 19" prop, also seems high I'm surprised that your theoretical speed could be in the upper 40s.
Will be interesting to see your results.

 

[Sea Rider]

Was the max GPS speed & wot rpm achieved with you alone and lightly loaded boat ? If that's the factory delivered prop with that engine you're at the max rated rpm which is fantastic. Difficult to tell how much wot rpm will be reduced by going to a 19 pitch. The near rule of thumb that states 100-200 rpm per 1 less pitch reduction doesn't always works with light to medium light applications and could end in the near min side of wot rpm range.

With 19 pitch should achieve slight better top end speed with reduced wot rpm than before. Don't expect a wow speed increase though. The only way to find that out will be buying that 19 pitch prop and testing it under trial. Is there a 18 pitch available ?

BTW, Welcome to Iboats,

Happy Boating

 

[bentech]

Thanks!!


When i did thoses test, i was alone in the boat, calm wind and almost no waves.
I was able to get 41 mph today with 2 others peoples in the boat with me.

The the boat get on plane very fast and one of my freind was beside me and the others was
on the back seat.

BTW, each time i launch the boat, i put the power tilt in half way to be sure to clear the ground but i
notice that the motor have go down by itself by the time i get back from parking the truck, get in the boat and start the engine. Is it normal (like a kind of safety) or
my tilt cylinder have a intrenal valve or seal problem ??

 

[Sea Rider]

Engine should stay at the tilted position it was left. Seems a power tilt cylinder failure, probably internal o'rings are shot already or have low amount of oil inside power tilt. If oil is low and old change complete amount for fresh one.

If engines achieves max wot rpm with you alone, it's easy to plane with 3 up, personally would stay with that prop. If going for more pitch to achieve slight more top end speed will sacrifice max rpm and hole shot, the more weight or passengers you add to the combo the worse will get.

Happy Boating