You kinda hit the head of the nail yourself. "So far" is the key word, and 21' cats are still easier than big heavy offshore vees and cats to move. Especially the one running a high x and 800 hp is on borrowed time. 2 inches below will still create problems down the line, but it's at the same time a height where you go from submerged to surfacing props. Now if they ran 100% surface ie. 1-3 inches above like you do with SSM, ASD, BPM ect. drives on cats, I'd bet you anything both prop and drive would be done by now.

you're right ,and I will !! And if it works on my hull I convert the second one I have in to XR by Max workx

so it will last ,I no one whit 800 hp on it whit there mods

Keep spare parts on hand, 800hp tears XRs apart like nobodys business. The gears in the Max Works, IMCO SC, Teague drive ect. are still just stock XR gears, nothing more.

I have a short X, right about 3" below the pad. I always run labbed props, i know they are ticking time bombs, but they just run so much better. I have a 725-750hp 598ci N/A. I do get alot of air time, but am always careful on reentry.

The props always crack and then tear from the trailing edge of the blade, midway down. I have had one crack, one with a blade completely gone, and one with a blade hanging off. no damage to the drive other than a small knick where the blade hit the cavitation plate.

Philm, good information about the prop failures. I was wondering what the failure modes were.

Now you guys are going to be jealous. I just got an email from a friend with a 21' Daytona with 682 hp. He said he's had no drive failures (Imco SC), and no prop failures even though the prop shaft is only 2" below the hull. His buddy with 800 hp, and prop shaft only 1/2" below the hull has been running the same prop for 9 years! They also do full throttle hole shots.

As an engineer, I have been trying to figure out what the magic formula is. The only thing I could come up with is 1) light boat so not heavily loaded most of the time, and 2) prop rarely leaves the water (they only do lake and delta boating).

If anyone else has some theories, I'd love to hear them.

Michael

The magic formula is the amount of/lack of weight its pushing and the smooth water.

I think my freind who owned a 42 Fountain poker run with a very high x dim and stock 525efi's threw blades off 2 or 3 props in less than 50 hrs.

Thry also have been seen behind a few Cig 20's with standoff boxes and they seem to run real well in those, whats the Hull your rigging with the BH drive?

And the ability to throttle correctly

Your probably gonna want a set of these for that

It s a 21 ft v hull from the UK called phantom 21 normally a outboard which I converted to a inboard whit a stroked ls 2
waiting to get in soon . here a few pics ,