Shorty Drives, how do you know what height for max speed??
 

Wondering, what height an I/O drive should be for max speed?

Not an easy thing to play with, like an outboard with a jack plate.

Also, what is the x dimension, is that cavitation plate relative to bottom of hull, or drive shaft to hull?

Played with drive height for 4 weeks..... I found there is a very fine line and 1/2" increments can make a big difference. read this thread first. If you have any questions I would be happy to share what I learned. http://www.offshoreonly.com/forums/d...ng-advise.html

X-dimension is crankshaft centerline to bottom of hull.

I am thinking that if you are pulling a 28" prop with a stock 454 Mag, your X is already pretty close to optimum.

I think it is as well, it runs amazing. (I think I could have run a 30 Bravo Lab, but never had the chance to try)

But I'm looking at moving up to a newer boat. And since it won't need the transom rebuilt, I won't be able to re-mount the drive heigher, and will have to go with a shorty.

?? Why don't performance boats come with a high mounted drive in the first place?? I found almost no disadvantage, and huge improvment.

Thanks for the info and link, great thread, very interesting.

Its amazing how much fiddling around is needed to get a boat to run very well. And with an I/O it's so much more work than an outboard.

Why did you not go with stand off boxes, as some are adjustable, as well, I would assume, they would caviate less getting on plane since they would be deeper in the water??

(Not sure when, when not to use them, just thinking that might improve your situation)

The boat is already equipped with ITS. It is basically a 6" setback box with integrated hydraulic steering. The prop shaft depth was 4" below the transom in front of the drive with standard length drives. That is pretty aggressive to begin with. I am at 3 1/2" with my current set up. I have had it as high as 2 1/2". JMO but I was more comfortable using 4 blades props over 5 blade to put less stress on the drives, which are already the weak link in my setup. I'm sure I could have gotten the boat to work equally as well from a performance stand point with the prop centerline at 2 1/2" below the transom with a 5 blade prop, but drive life would have been sacrificed.

As I have the boat setup now with the prop centerline 3 1/2" below the transom, the boat planes well and has the best top speed that I have seen. If I go any higher I won't be able to get the boat on plane with the 4 blade props and if I go lower I lose a little speed. There is a fine line that defines being "just right". And that "just right" depth is going to be different with different props. Additionally every setup and every prop will make the boat feel differently. There will certainly be compromises each.