I have a 1984, kevlar, 38' flat deck that I have notched the transom 12" ( with all the materials that I got from the factory). My reasoning behind the notch was to update the "old" hull to the way the newer 38's were with a notch, but before the new 2-step models came out.
The boat has Stellings set-back boxes and #5 drives. Motors are 572ci with PSI blowers. Dyno results were 1107hp ea. with 7.5 compression and a mild street roller cam.
On a "good" air day, it will run a little over 100mph. My problem is at cruise it tends to porpoise until I bring the drives in and lower the tabs, then it will level out.
High speed running ( above 70mph ), the boat is stable, with no bow steer.
Does anyone have any suggestions to alleviate this situation?
I could add weight in the bow, but this would just make the boat heavier.
If anyone is thinking about using PSI blowers on their motors, give me a call. I have 2 spare Sterling manifolds and 1 fresh PSI blower. I would be happy to assist anyone in doing a pr. of motors.
I built the motors that are in the boat now and did all the rigging. I know every inch of those older 38's, so I could be of some help.

I have actually owned a number of Cigarettes and the one consistent factor in all the boats is the slight porpoising sensation at slower/cruising speeds. This sensation is further enhanced when there is more weight towards the back of the boat.

My best friend's boat is a 1999 Top Gun (non-step) with 1000SC's and #6 drives. His boat porpoises when cruising at anything under 60mph. He can get rid of this if he uses some tab but that slows down the boat. My previous boat was a 1994 Top Gun with the same setup but with the addition of extension boxes and #6 drives. The theory being that the further back/higher the drives were the faster you would go. WRONG! The weight way back there caused the boat to porpoise like crazy at anything less than 60mph. You would have to use extreme tab to keep the nose down. Both boats run incredibly at the top end but this is not where you spend most of your time.

The main reason for this porpoising effect (from what I have heard) is that most 38' since the beginning have a slight arch or rocker in the hull. If you look under your boat and follow the hull line it will be bowed in the middle a little. This keeps the nose nice and level when the boats have Bravo drives and standard horsepower. They cruise like a freight train - nice, level, and sturdy.

In your case ... your boat is suffering from the weight factor as described above. You have the Speedmasters hanging way out back which is causing the boat to porpoise. Since you notched your transom you negated the purpose of the boxes. The boxes were originally there to get the drives further back into cleaner water. The notch is now doing that and the boxes are having much less effect.

My suggestion is to remove the boxes and bolt the drives straight onto the transom. Don't worry about drive height at this point. I suspect that it won't affect your top speed by much. See if it alleviates the porpoising. You don't ever want to add weight if possible. That defeats the purpose.

Trust me ... remove the boxes and I bet the boat will run much better.

This may be a stupid question, but what props are you running? I am not sure what is available for the #5 drive, but don't they make a prop which produces more transom lift? Also, what direction are the props spinning? In or out? This can also affect transom and/or bow lift.

Jeffrey has a hell of alot more experience with this particular boat htan I do...DJD has a flat deck and may be able to offer some suggestions---mine are general and may or may not help...you might try them first before pulling those boxes off.

I'm running a pr. of Lab-finished (read-very sharp) 17.5x31 4bbl.that I'm turning "in". I've tried turning them out before I put the box's on, but didn't like the way the boat behaved around the dock.
I'm not going to take the box's off, after having gone to all the work and expense of putting them on. I will put up with a moderate porpoise.
The "attitude" of the boat and overall performance is outstanding, so I guess I will live with it.
Any thoughts about extending the lower strakes ( the pr. that stops @ the race fairing ) to try to get more stern lift?

RMPRam,
Unfortunately it seems that the best set up for top speed is not great for cruising. However, as a top end junkie I find the tradeoffs are worth it. Jeff, I respectfully submit that the additional leverage from the notch and the boxes is not all bad. I have found that the farther back you go, the higher up you can mount the drive. The steering & handling is improved as is the top speed. Also in rough water the drives remain connected for that much longer. A #6 bolted on the transom puts the prop in the same place as a #5 with a box. If you have a boat with a 6 and notched transom your set up is about the same as the 5 with a box.

My vintage 36 will porpoise up to about 60, and after that settles right down. I agree that running the tabs down is not a great solution. The best thing I ever did to my boat was the addition of a ballast tank in the nose. It was like buying a new boat. Porpoise solved, my max speed in rough water probably went up 15 MPH. I will say that my boat is an X race, staggered engine with no cabin.

Is your boat staggered or side by side? How high are the prop shaft centerlines in relation to the bottom? Have you tried 18" props?

I agree with Mark. I may add some additional weight in the bow. A water tank, that is securely fastened, would probably be of great assistance. The only problem that I forsee, is if I add sufficient weight to keep the porpoising down, I dont want to add the problem of "bow steer". There has to be a fine line with the amt. of water I need to do do both.
The motors are side by side, on 35" centers. I believe, the propshaft is 1" above the bottom. I'm running 3" of spacers now, which allow me to get the boat on plane a LOT easier. I had the drives, w/o spacers, and getting it on plane was an ordeal. Rocking from side to side with the motors running 4000-5000rpm, was the norm with spacerless drives.
The only props that I have ( and have tried ), are the 17.5x31 4bbl's that I have. What would a set of 18" do for me??? And what pitch?? Note: the drives have a 1.5 ratio.

I need to correct an earlier post. I stated that I was turning the drives "in" when in reality they are turning "out".
I was told several years ago by Cigarette that turning the drives in would give me 4-5mph, which it didn't. The top end was the same and its manners around the dock were less than desirable. Although I have'nt tried turning them in with the #5's w/box's.
Jeffrey, You and I met @ Team Archer several years ago when you were doing your #6 project and I also sold you some props, remember?
Thanks to all that have read and responded.

It might be worth switching the rotation to see what happens. It seems to make more of a difference with the higher x dimensions.

As far as the water, I have 80 gallons and can fill or empty the tank while under way. Bow steering is not only uncomfortable but dangerous.

18" props will do a better job of carrying the nose and will also help to get on plane. Figure on dropping about 200 RPM for the 1/2" in diameter.