29' Marinette transom rebuild

[jgdyer]

We cut the transom open on October 19 to reveal 3" of waterlogged marine plywood motorboard ... As you can see from the shop photo, TIG MIG and Plasmarc machines are at the ready ...

I'd like some help picking my design for the rebuild ...

Propulsion is twin Suzuki DF175 4-stroke outboards ...
Kobelt electronic controls and hydraulic steering ...
Vessel weight approximately 7200 pounds with propulsion package ...
Construction aluminum - 5/32" bottom 1/8" sides
See boat dimensions in attached drawing ...

I have 3 candidate designs ...

1) A straight full width 30" hull extension. See the drawing. Option gives max bouyancy. Deadrise at transom goes to zero degrees due to convergence of chine and keel lines. Keel extension aims at countering sideslip.

2) An abbreviated 30" hull extension. See the photo with the Honda motor. Less bouyancy than (1). We are able to maintain the 3 degree deadrise at original transom to the new terminus on the pod, since we no longer have to follow the chine angle. Skip the keel extension.

3) A bracket, such as the D & D marine model in the photo with the Evinrude motors. Less bouyancy still. When under way, the pod is out of the water. Vendors claim water in the prop zone is laminar, less aerated, giving better performance and efficiency.

I recognize each choice will have its own effect on center of bouyancy. However, weight transfer experiments conducted while still powered by a single OMC Sea Drive suggest little influence on boat handling. That said, I want to be certain the vessel handles well in rough water and, in particular, in following seas.

Comments appreciated!

[Jarek]

John,
I replied to your message about a year ago, when you first asked for advice.
I do not see anything wrong, in principle, with any of the three options you put forth. The main thing, if you want to put twin 250 outboards on that boat, is to get rid of any wood from the transom area(wood should not be stuck to aluminum - corrosion), and install a full 1/4" thick well braced, welded in transom.
As you have seen in my website I do have experience with brackets for twin outboards.
Let me know if you want my help.

[jgdyer]

Thanks Jarek .. I'm still not sure you saw the post I was lamenting over ... See http://www.boatdesign.net/forums/showthread.php?t=12782 ... But I appreciate the feedback and yes, I could use some help with the decision process ...

I do intend to remove the wood motorplate and build back with all aluminum ... My intentions for any of the options are to weld in 1/4" plate with bracing at the site of the original transom and build out from there ... See the hull extension drawing at the beginning of this thread for a variant of this idea.

My principle dillemma today is: Which option gives me the best mix of function and seakeeping?

Option 3 leaves the current running surface unaltered .. Eliminates some risk but also benefits I might see from options 1 & 2 ... That is, new clean running surface at the stern ..

Downsides to 1 & 2: Screw it up and I could end up with a bad angle on the new bottom, hurting more than helping ..

Downside to 3: A bouyancy shortfall that invites wallowing in a following sea, with water coming over the stern ...


Should we discuss on the phone?

[jgdyer]

I have been partial to a full hull extension ...

However, I'm not too excited about following the lines to a flat bottom at the new transom ...

Today I started doodling with the idea of stopping the deadrise declination at the current transom and carrying the bottom line aft at 3 degrees deadrise constant ...

I would cheat the chine line by cropping the aft outside corners of the bottom panels and filling the space between bottom and side with a triangle ..

Drawing attached ...

[jgdyer]

Many thanks to member Jimboat ... After a consultation, we decided that option 4 is the safest approach ... Option 4 it is ..