Thanks guys for your input I will keep in touch have a great weekend!

K, I'm all stock, but re-doing my rear transom and want to know if I'm decent ball park for prop depth.

1986 Sonic 36SS with stepped hull.
415 HP 502 MPI with TRS drives
Set at between 4" and 5" depth.
Labbed 25p Mirage plus props.

Does this sound ok???

1988 32' Hydra Cat, twin 700's, 12" Stellings boxes at neutral, Imco Sc's with -2 shorties. Started at 2 3/8" above. Got up on plane no problem but very loose in the rear over 85mph. Down right hairy over 100mph. best speed of 106. Spaced drives down 1/2" at a time. Settled at 1 1/2" down, 7/8" above bottom of hull and picked up 9mph for best of 115mph so far. Running 36P Bravo 1's and will get up on the rev limiters at 6K but takes a minute. Gonna try regearing the lowers to 1:36's and a different prop pitch. Still playing with 20% throttle left to go so hoping for 125mph or so in the near future?

Shorty lower Drive
 

I have a 1991 292Sr1 Formula with 502s running 26B1 5150rpm getting 77-80 all trimmed out.Thinking if I put shorter lower unit on should be better.
1.Can a imco shorty bolt up to a Bravo drive?
2.How short should i go?
3.What will be my increase in speed.
4. What will it cost????

Thanks for your advise....

Cool, my old thread is still alive. There is a ton of good info on this thread. First thing you need is to find out how deep your drives are.

I'm going to revive this thread because it has a lot of good info in it.

My Boat is a '89 Formula 292 w/ 650 hp 509's
I'm running 2' Sportmasters, with long skegs, 28p Bravo I Labbed props (they are a true 28p, they are 30 castings)
ITS Boxes.
I'm 5" below the hull.

My best was 80 mph @ 4800 with the hatch cover off and 100 gallons of fuel.
Hatch Cover on I lost 4 mph and 200 RPMs with the 28's and fumes for fuel.
Hatch Cover on with Bravo 26's (stock) 78 MPH 5150 RPMs and 60 gallons of fuel.

This weekend I'm going to try some labbed 27 mirage plus props. I'll report back. I'm also going to start trimming off the excess skeg. They are at least 3" to long, but I'm going to start by removing 2".

endeavour32, is the hatch choking off air circulation ?

would that screw w/ the afr ?

What's that hatch weigh?

I need more airflow into the engine compartment. Donzi has a slick set up on the sides of the new ZX hatch. This winter I'm going to replicate them and put them on my hatch to increase the air flow. The one thing I don't want are large racing scoops taking over my hatch. What I think it boils down to, is hot air vs cooler air. I'm going to try cracking my hatch and see what happens. I just need to make sure don't crack it too much or it might come flying off.

I know when I was a kid I would always move the engine box forward a bit on my dad's boat and I could always gain a few mph. I think it's pretty common with boats that have a relatively sealed up bilge, it's just more noticeable with larger engines. When I bought my engines, Tinkerer said he experienced the same thing when the engines were in his Scarab.

Lesson here is, if you want more speed, make sure your engines are getting plenty of cool air. It's a pretty cheap way to gain some speed. As you can see by my testing I lost 4 mph from that, and that alone!

Mine would fade about 200 rpms after about 30 seconds of wot...most of the problem was the the heat emanating off of them heating up the air in the engine compartment.

I built a fresh air intake system for the engines that only allowed the to draw air from the outside, and problem solved.

I also had to add additional ducting and blowers on the inlet side to turn the air in the engine compartment, since the engines aren't pulling in air anymore.

I also made different outside air inlet grills that had alot more open area than the different sized ovals that were in the stock pieces.