need help choosing prop for my starcraft runabout

[xjdriver]

Hello, i have a 64 starcraft sea mist 16 footer with a 75' 70hp evinrude running a 13 3/4 x15 prop and it is wayy over propped, any advice on the right size prop is appreciated , im trying to get ready for summer.

 

[steelespike]

Re: need help choosing prop for my starcraft runabout

I think you have more problem than being over propped.
A 15" prop is actually a little on the low side.
Your boat doesn't strike me a particularly heavy.Are you over loaded?
If you can give us an idea of your performance, rpm and speed,hole shot,your typical load.
A 15 is pretty close to what I would expect.

 

[xjdriver]

Re: need help choosing prop for my starcraft runabout

With 2 people she ran 33mph on the second trim hole, the tach read 3200rpm, i know i dont belive it either, but the tach is set up correct. Hole shot is good, but at cruising speed on plane the motor seems like its lugging. I just picked up a photo tach from harbor fraght to varify rpm.

 

[steelespike]

Re: need help choosing prop for my starcraft runabout

Using 2.42 gear ratio a 15" prop at 3200 10% slip would do about 16 mph.
If you think about it at 3200 a boat is maybe barely on plane.Think you will find your tach isn't accurate.
Sometimes if you note the position of the switch on the back and operate it a few times to clean the contacts it will help.

 

[xjdriver]

Re: need help choosing prop for my starcraft runabout

Okay, thanks alot becouse that rpm and speed didnt seem right to me, i will test it with the gun as soon as i get the chance to, how does 33mph gps speed at wot sound to you? I think i should be seeing around 40. It does about 27-28 at 3/4 throttle with 4 people.

 

[Mi duckdown]

Re: need help choosing prop for my starcraft runabout

Need to nail down the rpm and speed. GPS for speed.

 

[xjdriver]

Re: need help choosing prop for my starcraft runabout

Speed is by gps, i will try to figure out whats up with the tach tommorow, it jumps around at idle but is steady at higher rpm

 

[steelespike]

Re: need help choosing prop for my starcraft runabout

33 is probably a little slow and 40 is a possibility.Keep in mind your running a 38 year old motor.
Some things to check.Is the throttle opening all the way?Is the motor in excellent tune?
Is the prop in excellent condition?If it is the original prop it is likely well worn and a new prop
could improve performance.Is the bottom clean and smooth?
Is the antivent plate(just above the prop) about even with the bottom of the transom.

 

[xjdriver]

Re: need help choosing prop for my starcraft runabout

Throttle is opening all the way, new plugs and power pack, 120psi on all 3, the carb throttle shafts have a little play, with the middle bieng worse.i will be doing wot timing before i take it out next time. The prop is stainless and looks good with some knicks on the leading edges. Bottoms clean and the anti vent plate is about even with the bottom of the boat.

 

[steelespike]

Re: need help choosing prop for my starcraft runabout

Some aluminum boats can be hard to push considering their relatively light weight.
I suppose its all those rivets and convolutions of the bottom.
If your not getting any venting issues you may be able raise the motor more.
If we can get your rpm it would help.
a fresh prop calculation indicates you could need more prop for more speed.
At 33 mph guessing 12% slip your rpm may be about 6000.
Is the hole shot really strong?

 

[xjdriver]

Re: need help choosing prop for my starcraft runabout

Next time i take her out i will get correct rpm's. Hole shot is strong

 

[steelespike]

Re: need help choosing prop for my starcraft runabout

The strong hole shot is a good sign.