38KV

Quick question: As far as I remember, the common transom angle is 12 deg.
Am I right?

Dave M

That sounds about right.

open87

iTrader: (2)

13-16 degrees per mercruiser service book

Zanie

It's totall builder dependant.
My boat is 14
IMCO builds their extension boxes for a 13.5 transom angle
Best off to measure it rather than take someones word forit.
Even if you call the builder, they might give you the wrong answer.

Falcon

Mercruiser engine drive combinations are designed for a level install, compared to the keel, with a 13 degree transom angle. If you install a Mercruiser drive on 15 degree transom and do a proper engine alignment, the engine will be at 2 degrees compared to the keel. Some boat companies bump up from 13 degrees to give more trim under on the drive. Think of your drive/engine install as tied completely to the transom. If you tip the transom, the engine and drive go with it. Extension boxes don't need to be designed for any specific transom angle, they just move the transom mounting surface straight back, parallel to the existing transom. Any questions!?

rookie2

yes question about boxes do they help with performance, or longevety of outdrives and why would you put boxes on looking now at boat with boxes thank you for your help

Falcon

Boxes "may" help performance. Talk with others who have experimented with your brand and model boat. To get extra performance it may be necessary to raise the drive and also the engine, unless you use the Stellings style box with an internal driveline to allow some offset or use the Bravo short Sportmaster gearcase. The reliability or longevity shouldn't change for the better or worse.

38KV

Thanks guys! I better measure it.

Zanie

Um..Yea, it's not quite that simple. It would be if the transom angle was ZERO but its not. Say we design an extension box for a zero degree transom angle.
With a zero degree transom angle an extension box would look like a perfectly square box as viewed from the the side). With the mounting holes directly across from eachother. If you bolted that on a 13 degree transom you would bury the drives very deep in the water.
Therefore an extension box must be designed with a certain transom angle in mind. With parallel mounting surfaces.
get it?

Falcon

[QUOTE]Therefore an extension box must be designed with a certain transom angle in mind. With parallel mounting surfaces.

Uh, I get it, I just didn't go through the full explanation and I shouldn't have really said "straight back", a poor explanation. I think you get it too, except that you are saying they are designed for a certain transom angle when what you meant was the angle that is designed into the drive. The 2 surfaces must be parallel. All boxes should be designed for a drive/engine centerline 77 degrees to the mounting surface. Boxes are not designed for specific transom angles. They are designed for the drive/engine to transom mounting angle, which on Mercruisers is 13 degrees. You can run a box on any angle transom (of course, within reason), it just affects your x-dimension calculation more. Thats the reason the x-dimension charts are different between standard and ITS Bravo's, except at 13 degrees.