Less pitch or not ?

[denlou]

I posted a similiar question about a week ago on the outboard forum and two posters said I should drop 2" pitch.

I have a 16'4" Wellcraft Airslot with a hull weight of 1060 w/o mtr. powered by a 1986 Johnson 150 J150TLCDC with a 14 1/4 x 19 stainless cupped prop.

The cav. plate is even with the bottom of the hull and at full throttle I get 5000 rpm.

Typically we have 2 people, 12 gal. gas, 1 bat. and probably another 100 lbs. in gear.

Every online prop calc. tells me I'm under propped and should be running a 21 pitch.

The motor and carbs have been gone through (rebuilt) and the hull is not holding any water.

Any advice is appreciated, Thankyou.

 

[steelespike]

Re: Less pitch or not ?

Prop calculators are only as accurate as the information they are designed to use.
For instance there are 16 ft boats that weigh 700 or 800 lbs and others that might weigh 1600 could be deep V with a high number deadrise or relatively flat with a pad super fast bottom.Taller top sides more windage.etc etc.Does the "airslot" have some sort of unusual bottom?Model for model an aluminum boat will be generally run slower than a similar fiberglass boat and may have higher slip numbers.
What is your speed at 5,000? Rpm is kind of useless without mph figures.
I would raise the motor till just before venting becomes a problem.just be sure it pumps water.
Be sure the carb is opeing all the way.motor is well tuned.
I would think that should make 50+ and turn probabnly 5,500+ as well.
Does it respond well to trimming I would think the bow would come up and it would run on its tail end.

 

[denlou]

Re: Less pitch or not ?

Prop calculators are only as accurate as the information they are designed to use.
For instance there are 16 ft boats that weigh 700 or 800 lbs and others that might weigh 1600 could be deep V with a high number deadrise or relatively flat with a pad super fast bottom.Taller top sides more windage.etc etc.Does the "airslot" have some sort of unusual bottom?Model for model an aluminum boat will be generally run slower than a similar fiberglass boat and may have higher slip numbers.
What is your speed at 5,000? Rpm is kind of useless without mph figures.
I would raise the motor till just before venting becomes a problem.just be sure it pumps water.
Be sure the carb is opeing all the way.motor is well tuned.
I would think that should make 50+ and turn probabnly 5,500+ as well.
Does it respond well to trimming I would think the bow would come up and it would run on its tail end.

The Airslot hull is for all intent a deep V that has tunnels on each side for lift.
I have not had a chance to check the speed by gps but an educated wag it's running over 50.
The acceleration from a 2800 cruise to wot shoves you back in your seat very firmly.
The hull is pretty much out of the water at wot and if I trim up too much the bow starts getting a little unstable, moving back and forth left to right maybe 6" each way.
I understand this is caused by what teleflex calls engine flutter, so a dual cable steering is in my future.
I'll try moving the motor up and see if that helps my rpms.
Thankyou

 

[Dhadley]

Re: Less pitch or not ?

Man, there's a blast from the past! The Airslot was a hull design that was way ahead of it's time for sure.

You said the AV plate was even with the bottom at wide open. That's not how the X dimension is measured. Where is the AV plate in relation to the keel when the AV plate is parallel to the keel on the trailer, out of the water?

 

[denlou]

Re: Less pitch or not ?

I really wasn't clear in what I wrote.
The av plate is level with the keel while the boat is sitting on the trailer.
I checked that with a straight edge.