I have to put a new coupler in my Thor and was thinking about installing a extension box. I have a lot of prop slip 14% for a cat. do you think if I use the strait back box it will kill the boat or would this work out ok. Any opions would be great. I know you are suppose to go up with a box but in my case I was thinking of a 1/2 spacer without the box. I think the box may make the drive to low but wanted some opinons, BTW the drive is a xr sportmaster -2 drive.

If you are looking at a straight 12" box, it would be like dropping the prop 3". That's a lot of drive in the water. Where does the prop shaft sit, in relation to the bottom of the sponsons.




Darrell.

I am already 1/2" below the bottom. I guess that idea is over with.

Now, you could do the IMCO box with a 3" rise.

dont want to rerig the engine mounts

I thought moving straight back 12 inches equates to dropping the drive 1 inch. Using a 3 inch raise box would be like raising raising the prop shaft 2 inches because of the setback????

This is a old post but I just thought we could get the facts strait on this issue. I have heard a -3 raised Imco would actually raise the propshaft about 2 1/8", information was given that a standard would lower by 3". This does not add up?

GETTING FACTS STRAIGHT !!!!!

a 3 inch lift box,,lift the propshaft 3 inches.

BUT,,since u set it back 10 inches it like raising it 2.1/8

on a neutral set back its 0 ,,,but it feels like u lowered it 1 inch.

RULE OF THUMB,,,,,for each 12 inches back u need to go up 1 inch !!!!!!!!!!!!!

Reason ,,,,the water breaks @ the hull not @ the box !!!!!!!!

I hope this makes sense ?!

Yep, thats what I thought. Not good to have the facts incorrect on a post.