Overlapping New Stringers With Old Stringers

Boat Specs:
LOA: 27 feet
LWL: 24 feet
Beam: 9 feet
Weight: 7-8000ish lbs
Hull Type: Deep-V / Planing
Engine: Cummins 6BT 210HP
Gear: ZF-63IV (v-drive)
Prop: 22 x 20 3B Michigan 1.75" bore
Primary Use: Trolling for fish at 10 knots

Hi,

Trying to get some ideas on how I should overlap my new stringers with my old stringers.

My boat was basically made almost completely with 1/2 marine plywood and fiberglass.

I gutted the back 8-feet of the boat in order to convert it to a single diesel / tunnel drive.



The new engine stringers I installed are 8'6" long and made of 10" x 1.75" microlam LVL beams.

I am trying to correctly tie these in to the existing 1/2 plywood stringers that run from the hull to the deck.

The old stringers and the new stringers meet at the bulkhead represented by the blue line in this pic:



Everything forward of the blue line is old plywood stringers, everything behind the blue line are the new LVL beam stringers.







What is the best way to tie the new stringers into the old stringers?

The forward fish box is insulated, so obviously I am going to have to cut a bunch of the insulation out that way I can tie into the existing plywood. I will cut a hole through the bulkhead that way I can put overlapping wood. I am thinking I should use 3/4" marine plywood to bring the old stringers out to the same level as the new stringers. Once the stringer faces are on the same plane, I can then I can just glass more plywood pieces on the insides of the existing stringers. Would this be an acceptable practice? Is there a better material?

How long should the overall overlap be if I am using doubled or tripled up pieces of 10-inch tall marine ply?

 

The boat is pretty much all made with 1/2" plywood that runs from the hull to the deck. When I installed the stringers, I basically tabbed the stringers in where they meet the forward bulkhead like this:



I did a couple layers of 1.5oz mat and then 3 layers of biaxial in all those places on both stringers.

I am starting to think that is not enough, especially with how close the v-drive is to the bulkhead.

I was expecting there to be a new bulkhead put in a few feet aft of the existing bulkhead which I could run more plywood between like this pic here:



I will still probably do this still, but now the plywood will extend to the back side of the engine box in order to match up with the new bulkhead (since it will now be placed behind the engine). Is it sufficient to do just the green box in option #2?

I am basically trying to avoid having to do THIS:



The fish box is all insulated and beautiful and I'd rather not tear into it if I don't have to. I don't see why I can't just extend the plywood back over the engine stringers, instead of extending the engine stringers over the plywood. It's basically doing the same thing, right?

I like having the v-drive as close to the bulkhead as possible because it's giving me an 8-degree prop shaft angle.

 

I thought of one more option. I think it is the best one so far with what I have going on already:



I am going to buy another 20' microlam LVL beam, cut it into four 5-foot pieces and glass it in heavy like you see in the pic above. Sure, it will add some weight and be a lot of work, but I don't want to skimp on this piece of the backbone of the boat. I am trying to build the boat strong enough so that one day I could plug in a 370HP Cummins 6BTA if I wanted to instead of just the 210HP 6BT I have now. A little overkill but I'd rather err on that side of things with things like that.

This will allow me to tie into the existing stringers with a 5-foot overlap. It is the strongest reasonable arrangement I can possibly think of...

 

Will boxing everything in aft with plywood and adding transverse support be enough? I can even sandwich in a new bulk head over the existing bulkhead that will overlap the new stringers. This will help distribute the load greatly.

*** Disclaimer: This is all for educational purposes and will not be used in real life!