Starcraft hull vs. cavitation plate height

[natemeins]

Hey fellas.
My buddy has a Bayliner Capri with a Johnson 110. Good pulling boat, but really has a hard time with plowing out of the hole, and when it's on plane it has little to no trim before it starts porpoising like crazy. Also, his fuel economy is pretty terrible. After doing a bit of research we had a look at his cav plate at cruise and sure enough it buried. Luckily for him, his motor mount allows it to be moved up or down with a series of holes where the mount bolts go through. As of yesterday his plan was to try to move the motor up a hole or two. On my old 14' flat bottom with a Johnson 30, the cav plate skimmed across the top of the water, just perfect as it should at cruise.
So I had my '71 Sprint with a '71 'Rude Lark 50 out yesterday, and for S & G's I had a look at my cav plate at cruise. Interestingly, it was buried. Every now and then I could see it, but for all intents and purposes, it was under water. This sort of makes sense as to how I can pin the motor tilt to the next to highest tilt angle and still jump out of the water and not cavitate. Another hint was that a few weeks ago I tried out a "standard" size prop for my motor, a 13-1/4" x 17. It was a dog! Terrible performance, motor couldn't make good revs. It currently runs a 13-3/4" x 15, engine sounds like it's screaming along at WOT (no idea of revs, no tach...yet). GPS shows 27-28 mph with just me and some gear in the boat.
I took a few measurements, and with the motor tilt pin set in the middle hole (of 5), the cav plate is nearly dead nuts level with the bottom of the keel, and the lower unit leading edge is approx. 7" from the back of the transom. My research into motor heights tells me that for every 12" of offset of the motor, the cav plate should be moved up 1".
Those of you with tinny 'crafts, where do you expect and/or have your motor heights at? With this type boat should I still expect to have the cav plate skimming the top of the water? After seeing how easy it was to move my buddy's motor up, I went to look and do the same for mine. Unfortunately, my bolt holes are not adjustable. What sort of methods could I use to move the cav plate height up, say 1/2"? I thought of placing a block of wood between the transom and saddle of the mount, but of course new bolt holes would have to be drilled, and that's assuming I wouldn't wind up "snowmanning" the holes. I plan to rebuild the transom this winter, so if I can/should move it up, I can always make the transom height a little bit taller. Is there a way to experiment with height prior to drilling into the transom?
Would love to know what you guys think.

 

[Watermann]

A jack plate would allow for up or down movement of the motor. Not sure I saw where you're having a problem your trying to correct by raising the motor.

The anti-ventilation plate should be at to 1" above the bottom of the hull and yours looks from the pics to be right there.

The pin you're moving is the thrust bar and it allows for trim angle more so that AV plate height. What's best is that the thrust bar be all the way in the bottom hole and trim the motor upward after reaching a plane.

 

[natemeins]

I'm not interested in a jack plate. Sure, it might help determining the proper height, but then what? I will say that I'm hoping to get a trim & tilt plate for it next spring, but I'm not sure if those bolt holes are slotted or adjustable.

I'm not necessarily having any problems per se, just trying to maximize effeciency. I am noticing quite a bit of splash happening between the transom and motor at cruise- that seems detrimental to moving forward. If the cav plate is buried in the water as it is, then there's more motor in the water causing drag, the prop can't "vent" as much to allow faster revs so there's potentially slower speeds, and I'm nearly out of trim and the boat feels like it could be trimmed more at cruise. I can't run a standard prop because the prop is buried and cannot make revs. If there was a way to sneak a few more MPH at cruise and burn less fuel, or decrease the amount of splash happening at the stern I'd love to do it.

Has any Starcraft operator had this issue? Can I removed the bolts, place a block under the saddle, and use the clamp bolts only to hold it, at least long enough to test and determine proper height?

In case you were interested, this is the link to the motor height conversation:
http://www.thehulltruth.com/boating-forum/168322-outboard-motor-height.html#b

 

[Watermann]

Ok,

You don't need the blocks on top of the transom for the motor to rest on, take a look at my CMC TnT. There's nothing that rests on top of the transom and nothing is needed for the OB besides the mounting bolts.

 

[natemeins]

That's my plan next year is to install the PT-130 unit. Since it looks like you're installing yours, have you had an issue with your steer tube with the 6" setback? Mine, like many of these older Starcrafts, the steer tube is not attached to the tilt tube on the motor, rather it mounts with a bracket attached to the inside of the transom.

 

[Watermann]

I've been using the PT130 for 3 years now, super solid unit. No problems with my steering although I have the cable that goes in the tilt tube.