prop height with a setback plate?

[biggy boy]

Hi there!
I will be adding a six inch setback plate on my 16' Vanguard Phantom ski boat.
How many inches do I raise the motor up for a six inch plate? I think I read somewhere 1 inch up for every 3 inches back. Does this sound right?

The motor on the boat is a 1988 Suz DT 115 with a 13-1/4 17
prop. The WOT is 5600 RPM. Currently the motor is set with the cav plate 1/2 inch above the bottom of the boat. I can trim it right out with no loss in power or over revving. I will be adding a pressure gauge to monitor my water pressure.

Thanks
Glen

 

[walleyehed]

Re: prop height with a setback plate?

Somewhere between 1-1/2 to 2" will be a good starting point.

 

[biggy boy]

Re: prop height with a setback plate?

Somewhere between 1-1/2 to 2" will be a good starting point.

Hi
Thanks for the reply Kenny!
I presume the 1-1/2 to 2" is the hight of the cav plate above the boat bottom.
I'm also going to block off the upper water intake plate, the one on the underside of the cav plate.

Thanks
Glen

 

[walleyehed]

Re: prop height with a setback plate?

Make a run without the water intake blocked first to get a water pressure reading for a base-line, then note the pressure on the first runs at the starting heigth.

 

[biggy boy]

Re: prop height with a setback plate?

OK good plan.
I'm hoping to have the boat back together by the end of the weekend.
Right now the top is off, just put in a new transom last week. Finishing off the gelcoat touch-up.
I'm eager to get one more run out of the boat before it gets to cold here.

Thanks
for your help.
Glen

 

[biggy boy]

Re: prop height with a setback plate?

Hi there!
I took the boat out for it's first run since installing the 6 inch setback plate.
I was able to run the trim all the way up without any drop in water pressure and no over revving or cavitation. The water pressure is 15 PSI while at speed about 5 at idle.
(New gauge) The gauge is tied into the tattle tail line.
I am currently set at 6 inches back with the cav plate 2 inches above the bottom of the hull.
I just broke 50 mph on the GPS 8) That is with two people and a half tank of gas 30 litres.
The prop in a raker 13.5 x 20 and the RPM is 5500 (new gauge). A bit on the low side. There is no porpoising at all. I'm wondering if I should move the gas tank to the back?
So what do I do next? raise the puppy up one more hole?

Thanks
Glen

 

[walleyehed]

Re: prop height with a setback plate?

You guessed it!
another hole and test...sounds like your hull is responding well to the changes.

 

[biggy boy]

Re: prop height with a setback plate?

OK raised the motor up again, it turned out the height i had it set at the last time was 1.5 inches above the bottom of the hull to the cav plate, not 2"!
It is now at 2 1/4 inches above. I also put extenders on where the trim piston rods push against. This extra gives me another inch of trim height.
I can now run a consistent 50 MPH and the rpm is at 5800, up 200 RPM from the last runs.
At 2/3rds throttle and full up trim I get a rooster tail about the height of the motor, but at wOT the tail goes away. I have video of the boat at WOT taken from shore and in the boat, but they are to big of files to post.
Anyway I'm not getting most of the boat out of the water (can't see light underneath it). It is up on plane and all, just not riding on the back of the hull. This is with the 13.5 x 20 Raker and six inches of setback.

I have put the boat way for the winter but will be back at it in the spring, hopefully with an hydraulic jack plate. :devil:

Do I need more height or more trim?

Thanks
Glen

 

[walleyehed]

Re: prop height with a setback plate?

"Usually" if you can use full (stock) trim with no venting, you can still go up for more efficiency.
There will come a point where it may want to blow-out(vent, actually) on hole-shot, then hook-up again, then may vent at part throttle (cruise) but hook-up at WOT.
As long as you have the same water pressure, you have a couple of options...you can stay right there or go for some higher technology prop(s) and maybe gain a bit more, but "at" that point, you're getting it pretty well dialed in.

 

[biggy boy]

Re: prop height with a setback plate?

No venting and no drop in water pressure yet.
Should I try going up a bit more in vertical height?
Or do I need to switch to a newer tech prop?
The boat jumps right out, great hole shot. no cavitation at start or at any speeds.

Thanks
for all of your help Kenny!!
Glen

 

[walleyehed]

Re: prop height with a setback plate?

Maximize what you have now, first....
When you reach the point of excessive venting or handling issues/water pressure issues, you can decide at that point whether you want to move up to another prop.
I wouldn't change anything but height until you get a reaction that is not in the positive direction.
Keep track of everything you can data wise so you know the total gain in the end when you reach the sweet spot.