Hey boat experts, this is my first boat and I need some help!

I have a 1975 Lund trihull with a 1990 85hp Suzuki outboard.  I have what seems to be a twofold problem.  For starters, I had the boat out on the water for about 7 hours or so the other day.  When I took it out of the lake and pulled the drain plug, about 10 gallons of water came out (this is a 16-foot boat).  When I was on the water and got it up to plane, the boat would porpoise badly.  I figured since I was the only person in the boat, the porpoising was probably due to the water sloshing around under the deck.  Does that sound right?  The boat doesn't have any noticable holes in it other than some screw hole penetrations in the back and the previous owner caulked those.  Under the boat, the gelcoat is worn off all along the centerline and you can see the fiberglass, but it's smooth looking fiberglass.  I figured that most of the water must have been seeping through the worn off gelcoat where the fiberglass is exposed.  Does that sound right?

Second problem.  The boat putts around just fine, but when I open the throttle and the boat planes, the motor revs high and the boat slows way down, almost to a stop with the motor still revving high.  If I back off the throttle and then punch it forward again, the boat picks up speed, planes again, but then the same things happens.  Every once in awhile, I can get the boat to plane and maintain speed, but it porpoises so badly (because of the excessive bilge water, I think) that I have to back off the throttle again.

Assuming I can solve the leak problem and stop the porpoising, what do I do about the problem with the motor?  Right now, I have an aluminum prop on the motor that is 13" in diameter with a 19" pitch.  The motor gets about 5500 RPM wide open.  I have another prop that is 12.75" with a 24" pitch that I haven't tried yet.  I doubt the larger prop pitch will help because it seems like a transmission problem, but then again, I don't know.  Is the prop loose?  How do I check?  Or should I just take it to a mechanic?  I would like to fix it myself so I can learn, but this might be over my head.  Thanks for any comments or suggestions.

The over-revving sounds like a hub problem with the prop.  Props probably slipping on the hub. You could stick your other prop on to prove that, though a 24 pitch prop is way too much for that motor.

As for the water.  Plug the bilge drain in the bottom of the boat and then begin filling it with water.  Areas to look for are under the boat obviously and take a close look at the livewell thru hull fitting, bilge drain and any other water inputs or outlets that go from inside the boat to outside. Often the silicone sealant wears out and water gets in that way.  If you find nothing that way you'll need to seal off the livewell system and start filling it completely with water to see if you've got LW plumbing leaking.  

OK, I discovered something that may be causing the mystery prop issue.  I was looking at the boat after I lowered the motor all the way down and noticed that the alignment of the prop shaft is not horizontal when the motor is in the fully down position.  The prop shaft was pointed slightly upward (which would be toward the surface of the water).  That would explain why the boat does fine at low speed, but the prop doesn't grab after the boat planes at high speed.  When I open the throttle, the boat digs into the water fine because when the bow lifts up, the prop angle goes down into the water.  If I'm not mistaken, when the boat planes, it rises a little higher in the water.  If the prop is then closer to the surface, I would think that if it is angled upward slightly it's just pushing water up to the surface and not pushing it horizontally like it should.  Anyway, just a theory.

Put aside my theory for a minute.  My outboard has power tilt.  When I lower it until it stops, the lower unit rests against a thick metal bar.  I noticed that on each side of the lower unit there are two semi-circular cut outs that look like they would rest against some round bars that go through the motor mounting bracket.  Those cut outs never make it to those bars though because the thick metal bar behind the center of the lower unit stops it.  Is there a way that I can adjust it so the motor comes all the way down to 90 degrees?

I bent over the little piece of metal that was keeping the motor from going all the way down.  Now it goes all the way down.  I'll take it on the water today to see if that fixed it.

I took it out to the lake and was able to drop the motor all the way down after I bent that little piece of metal out of the way.  I roared like a champ!  Operator error was my problem (as is often the case).  That little trihull with the 85 hp 2 stroke is a quick hunk of junk!