X Dimension with Sportmaster and Extensionboxes Twin Engines
 

Hello, i think a lot about my drive System:

Formula 311 deep V with 2x 8.2L HO
Neutral Imco Extensionboxen
Sportmaster Short -2"
Propshaft 4,9" under hull
​​X-dimension 15,6"

I have measured all today. Finding the X is not easy with box installed..
The boat runs good, have no proplems to get i
on plane.

Only one proplem is, when i trim a little bit, the boat begins to porpoise a lot. at Wot is nothing. Only at cruising speed
When the water tank in front is empty, i dont can trim.

Is my propshaft to deep?
I have test a lot of propellers, but the proplem is the same.
Bravo 28 lab, Bravo 26lab, Revolution 4 25,
only now the signature sl5 five blade is a little better.

I have read, a 12" setback, put the drive 1" deeper.
Also when i have now 15,6" ×2" shorty, is 17,6". But real only 16,6"? To deep?

What you think?

Thanks
https://cimg2.ibsrv.net/gimg/www.off...5c4da70767.jpg
https://cimg5.ibsrv.net/gimg/www.off...8a2ed8b098.jpg
https://cimg9.ibsrv.net/gimg/www.off...7a8deb7f37.jpg

It might be the "improvements" you made that is causing the problem. Did you change props? If you didn't have the problems before the I'm inclined to blame it on the sportsmaster drive and/or extension boxes you added. The drag from a standard lower unit would be more than a sportsmaster and, thus, would react better to trim at cruise speed. If the drive has less drag and is a bit higher now than when it was original then it would require less trim so when you trim it at all during cruise it starts to porpoise.

I've had two older twin engine boats now and under both boats I have had luck resolving porpoise by trying as best as possible to get back to the original setup, same weight, same props, same standard drives, etc. Anything that has changed and could impact the weight balance or anything outdrive related try to go back to the original setup unless of course your trying to fix something that was universally considered to be a problem by all right out of the box when new. It doesn't take much to throw off the balance.