Prop Depth

[bjkaboom]

Hi..! I have a few questions.
Can anyone give a "rookie" a prop starting depth from the bottom of the hull to the center of prop shaft for a 12 ft. glass Grand Prix Sport C with a M40C Tohatsu and a 11.5 x 18 three blade SS cleaver?? The transom is 18 inches high and presently the jack plate is set 2 1/2 inches above the top of the transom. With the Short Shaft M40C Tohatsu the centerline of the prop shaft is 2 5/8 inches below the bottom of the transom.
I'm having a problem trimming out porpoising at top speed. The boat appears to be riding water just behind to cock pit seat when I do eliminate the porpoising and my top end falls off about 5 mph. Should the boat be out of the water more and riding closer to the transom?
I have not GPS'd it yet. The on board speedo indicates 55 into the wind and 60 with a tail wind and some small chop. I just got a tach to install and will get her out soon as the ice is off the water and the temps warm.
I'm aware of height checkers for the C's and D's....but not aware of any tool for the Sport C. This is my first Sport C hull...so consider me a real dummy.
Last of all............will anyone loan out any of their Sport C props that I can test?
I hope someone will give me some help. I know of no one to call on. Thanks in advance if you can help!:)
Regards,
Bernie

[MN1]

Bernie,
On my boat we started with the prop shaft center line at 5/8" above the bottom of the sponson and I think we raised it alittle but will be going back down to about 3/4" above the bottom. Some boats in Sport C are alot higher than mine though. The farther back your transom set back is the higher you need to raise the motor.
The boat should be riding at close to a 3 degree angle of attack. So maybe about 6" to 12" of the back of the sponson touching the water.
To get rid of the porpoising there many things to try. One could be propellers.
Try one with low rake ( transom lift ) and some with higher rake ( bow lift ). Another thing to try would be shifting weight in the boat ( foward or aft ).
I'm sure Ron Hill could help with info on props. In the Mid West some of the Sport C guys use props from Jim Merten. For motor work most use Mike Wienant (sp). Both of these people can help with boat setup too.
I know most people use the jack plate in Sport C, but why? Because the transom was too short because it was used as an SST 45 boat. Some of the transom set backs I see in Sport C are really long.
I guess it takes alot of test time to find what works for your boat. And don't be afraid to try something new. If you do what everyone else does, your boat won't be any faster than theirs.
Mark Nelson

[TABARARACING]

Sport C's have a tendency to purpose if drivers are heavy... How much do you weigh? I bet you like driving it in choppy water...
Brian