Hi we are running a Vee bottom with a SSM #5 drive. Its quite a deep V race boat.
Currently there is a 21 inch standoff box built into the back of the boat that the drive is mounted on...The boat without the standoff box is 28 feet. The standoff box is a few inches above the bottom of the boat

The center of the drive is about 1/2 inch below the bottom of the Vee and the #5 drive has a 2" spacer added.
Me thinks the drive might be a bit low as when you really get up the throttle in the 5800 rpm mark the back of the boat lifts too much.
Me also thinks the 21" box is too long and should be cut to 12-14 inches.

Advice please, thanks so much...Jas

a #5 is a surfacing drive so it does not like being below the bottom. with an extension box you generally go up 1" for every 12" of setback. based on that it sounds like you need to remove the 2" spacer and see what it does.

Do you think 21 inches is too far back, I was thinking if cutting it off and making it 14 inches..
Jason

I would not cut it yet. the easiest thing to try first is take out the spacer. if you do not have the correct studs you can have some aluminum bushings made to take up the space on the stud (pull the spacer, put the drive up, install a two inch bushing, the washer and nut) and try it. the longer extension box will create more leverage so who knows, the boat may like it. try it first without the spacer and see how the boat reacts so you at least know what direction is fixing the problem. most boxes are 12" but there are plenty of boats running longer ones. need to first see what the boat likes and dislikes to find the changes needed so start with the simple to do things first.