Stringer / bulkhead help

I am in the process of replacing a fuel tank in an 89 Arriva 2001 originally it was just supposed to be the fuel tank and the floor but after opening her up I have a bad stringer on one side, knee braces on the transom and the front and rear fuel tank bulk heads. I could be wrong on terms as I am new to the boat repair world. I got most of the bad stuff out and am preparing to replace. I know the stringers are 3/4” laminated plywood and the bulk heads that held the fuel tank in were 1/2”. My question starts with the bulk heads. In the original form the stringers were dropped in between the bulk heads and not one piece I want to change it to keeping the stringer as a single unit and extending it to where the front stringers pick up if possible. I believe this would be a stronger design but could be wrong with little experience. My second question is if I went this route would I need to have the bulk head extend as far as it previously did or could I just fit it between the two stringers? The bulk heads look like they were only covered in csm. Lastly I can’t for the life of me figure out lamination for the replacement stringer the one is under 6” high max and the one I have to cut out still is even less than that.

 

Nothing like learning how to swim by jumping in the deep end of the pool

Been there, done that on a couple of my boats, but not a complete cut and gut as you are doing. I agree on making the stringers the full length of the hull, or thereabouts.

That hull is not very deep, so your stringers don't have much depth. You'll want both the inboard pair and outboard pair of stringers. I would extend the bulkheads out towards the chines, because the deck/floor will be better supported.

Was the floor directly attached to the stringers/bulkheads? I would assume so but thought to ask. I would use a couple of layers of 1708 cloth over the stringers/bulkheads and onto the hull as tabbing.

What material are you planning on using for the stringers and bulkheads ? Wood, HD Foam, Coosa, Divinycell, etc. . . ?

You should be able to build it back better than it was, since it did not seem so great to begin with.

 

The bulk heads were not attached to the floor. The front was touching but not attached and the rear didn’t come close to the floor it was about 6” away. The bulk heads as I call them held the fuel tank in place I wasn’t sure if they had another purpose or not structurally. So in theory I could create smaller bulk heads to help seal off “compartments” in the stringers and to help give the whole structure more rigidity. As far as replacement I am sticking with Marine plywood to keep everything consistent and that’s what is readily available near me. What weight cloth would you recommend?

 

The bulkheads stiffen up the structure to some extent. Attaching the stringers to the floor and the outer 'wing's' of the bulkheads, along with tabbing of the floor to the hull sides at the chines will provide the most rigidity to the hull.

1708 cloth ( aka 1708 BDM) is just that . . . It is a bi-axial stitched cloth of 17 oz weight. I use it on most of my glassing projects . . . Get 2 oz CSM for the irregular shaped areas and use a few layers of it.

 

I have read not to directly attach the stringers to the hull but instead to tab them in and I have also read to make a peanut butter consistency epoxy and attaching them then creating a fillet after it has set up. I have read this way will create hard spots which are prone to cause cracking. Is this true? And what is the preferred method?

 

WARNING.
Check for rot in the transom core!!!
If you have rotten sole, stringers and transverse... There is a greater than 99.9999% chance of transom rot


Stringers stiffen the hull. They are often used to support the sole(floor). Extending their hight to support the sole would not be a bad thing to do. Calculating the extra might be tricky. Tabbing to hull is good. If it becomes a"hard spot" then the hull's bottom was way too weak. Bullheads extending up the side do require a strategy for preventing hardspots.

 

From what I have been able to check the transom is good I poked around in the various mounting holes and everything was solid. The Sole was actually in good shape it was just one area that had a crack in it that had let water in that had no access to drain. you can actually see where the laminations were not done well so it allowed water to wick into the stringer. The stringer was actually solid when you got away from these areas. So with the Hull being fiberglass with no core to it am I safe to attach the stringers directly to the hull and then fiberglass them in? Also as a preventative measure should I coat the new stringer in epoxy before I install it and let it dry then start my laminations when the epoxy has cured? Being new to this I don’t want to go about it the wrong way and have to redo it again. I have a short window of good weather before it starts getting to cold for me to work and I have to wrap the boat up and put it in storage for the winter and I am hoping to get the floors in before I have to transport it.

 

As another general question how tight do you want the joints to be? Is it a tight fit like you would use in cabinet making or a bit looser so the wood can move a bit?

 

Yes . . . there are a few methods.

Peanut Butter . . . (shown in purple) to provide a fillet shape, then tabbing (Blue) over to attach the stringer. You can then glass over the entire stringer for added longevity & strength.




Hull Offset . . . 2 of the boats that I have owned use an air gap (1/2") between the stringer and the hull so that the tabbing provides some 'give' or flex. I'm not a huge fan of this approach, because the air gap provides a channel for water (if it got in there) to spread throughout the structure. But if one were to glass the underside of the stringer, or use foam to fill the gap, then it would be better.



There are probably some more methods that other folks may share . . .

 

Tight joints as in the wood pieces fitting together? You don't want it too loose to the point of being sloppy, but then again, you are not making dovetail cabinet joints either. Also, keep in mind that most boats rot from the keel upwards. As such, the bulkheads are what tend to soak up water first, and they in turn spread the moisture to the stringers at the joints. A transom acts like a bulkhead in this regard, because it is also at the keel of the boat.

How you re-build the structure will have an impact on the longevity of the boat. I like to provide a layer of fiberglass between the joints of a wood structure so moisture does not readily spread throughout the wood. So, in your case, if you put the stringers in first with glass over them, then you add the bulkheads, there will be a layer of glass at each stringer/bulkhead joint. Same thing for the transom, as you may be replacing that as mentioned. Isolate it with fiberglass from the stringers.

 

I recumend pre-sealing all wood. Especially ply!!

Wood can suck reason up leaving a weak dry bond to the FG cloth.

Count your lucky stars that you cough the rot early.

 

Blue . . . more coffee

 

before proceeding a thorough transom inspection is a must

 

foam pad is really the best, imo

cut it to shape top is a bit wider than stringer width; 45s the sides 12mm foam, pb epoxy it down

One of the big issues with a hard spot is a point.

If you square the bottom of the stringer; it creates a single point of contact on a line when in place with the hull bottom.

 

You talk about epoxy as a bedding compound and as a waterproofer, it works for both, but don't mix and match resin types, choose one and use it throughout the project.

In a rebuild like this both epoxy and polyester will work and hold up for many decades to come. Your choose on which one to use won't make as big of a difference as you may think.