Keel: To fill or NOT to fill, that is the question......

I'm in the process of rehabbing a 17-foot runabout . It's all waterlogged, and I took out all the stringers and stiffeners.

In doing that, I found that the molded hull has a keel "skeg" , if you were, a slight U-shaped dip in the hull shape to form a stiffener in the hull. That shape is repeated on the inside.

The stringer in the center was just resting for the most part on the edges of the "U", and while tabbed in, was just over the void below. The void is about 1.25" wide and 1" deep, running the length of the keel skeg.

SO- When replacing the stringer, should I round the bottom of it to fit the "U", and epoxy it in place, or fill the void, then bed the un-rounded stringer in the epoxy? --(of course tabbing it after, in both cases. ...) OR, just put the new one in like the old one was, and tab that (NO epoxy) ?

 

If you leave it open, water can run freely aft, and be drained out. For strength reasons it apparently is not needed to fill the keel, at least if there is no cracking in that area. In that case, install as it was before, perhaps with higher quality materials, and more care.

 

Fill with foam and 'glass down the stringer as planned. Conversely, you could bed the stringer in a flexible goo of some sort, then tab it in or just hard mount it and tab it in. If the stringer touches the hull, it could cause a stress riser, so it's generally preferred to support them on bedding or a foam filler and let the tabbing carry the loads..

 

After much research / discussion, reading, and thought, I think I am going to fill the keel with nat and resin , put at least one layer each of mat and cloth on the hull, bed the 2x6 stringers in 4000 (flexible but strong), epoxy all the wood, double up the floors from 3 to 6, and put the sole in. Tabbing all the way. Time to start making parts!

 

If using epoxy, the use of mat is just a waste of resin and fabric. If it was me, I'd just apply a generous bead of goo (polyurethane) and bed the wooden stringer on this, maybe use the ooze out to create a fillet along the edges too, so the 'glass can lay down nice and neat. The reason? It's easy and fast, using a caulking gun to apply it. Foam works too, but you have to shape it and all that.

 

Hi Par- thanks for the input- I am being unclear:

I have advice from a fleet operator that I should just bed the bare wood 4x6 in 4000 on the hull, as it is flexible, THEN epoxy what is left exposed. It seems to me that epoxying the 4x6, then bedding it, would be better, as the whole thing is going to be glassed and tabbed in- the epoxy would eliminate any question of water penetration, the 4000 would be the bed, and the tabbing would hold the whole thing ddown to the hull...I'm talking about glassing over the epoxied 4x6 onto the hull then tabbing all of that in also.
The mat and cloth I mention are on the hull. It's just 1/8" thick, which to me is too thin. It flexes like crazy when walking on it. I put just one layer of mat on the bow section two days ago, and it stiffened it right up. I figure mat / cloth / mat then stringers should do well.
Filling in the keel section seems good because the resin in there is cracked here and there. . I'm thinking of grinding it all out to the roving, and just making it solid.... the stringers should hopefully keep it all from flexing too much.

 

You are correct, in that you want to encapsulate the stringer in neat epoxy forst, then bed them, though why pay more for 3M-4000 when 3M-5200 is much stronger and cheaper. 3M-5200 isn't quite as pliable as 4000, but in this application either will do, so I'd go for the product with the better cost.

You can bond to existing mat if you like, but don't add mat into your epoxy laminates, unless you just want to waste epoxy (mat is like a sponge). It'll be stronger and lighter with cloth only.

 

In my continuing saga of "what to do with the stringers" , I have been doing more thinking. Probably a bad idea, but...

My intention is to make the hull stiffer, without going overboard- it's pretty flexible right now...

In the photos below I can illustrate the problem. PHOTO A shows the stern with the old stringers and floors installed. The floors were 3 feet apart, so there were just 3 along the whole length of the boat. The RED arrow is pointing to where that OAK floor was notched and screwed into the fir stringer before glassing and tabbing. The blue is where I am thinking about adding floors. The purple arrows point to the original FG stiffeners, which were halves of four-inch rounds over cardboard, tabbed to the hull. SO, if I add the floors (with limber holes against the central stringer), do I need the stiffeners? Should I make holes in the floors to accomodate the stiffeners in one piece, or leave the floors whole, and still put the stiffeners in, ....just shorter? Put 2 in each section versus one?

In PHOTO B you can see the bow area. Where the arrow is is where the 12-foot keel stringer ended, and where a 2.5 foot piece of "stiffener/stringer" was tabbed in, though NOT attached to the 2x6, It's 2" high on the stringer end, and 3/4" thick on the bow end. Pretty useless in my opinion....I'm thinking of making it full height to support the sole, but does that make the bow hull area TOO stiff? I don't understand the purpose of this piece....

PHOTO C shows the settees before removal (the black web-type stuff is the paint scheme). Those were 1/2" plywood, 8" high, and were tabbed into the hull and sole . I'm thinking of NOT replacing the bow section, and perhaps not the port side settee. I need the space and access to the bow hatch, which is almost nil in the original layout . The question here is : does not putting that all back in bring up a stiffness problem again? Or is the sole enough to keep things stiff enough.....

AND, in relation the bow section, PHOTO D shows a level ON the stringers. You can see that the sole didn't touch the stringers here at all. The sole is the thick brown layer, with the settee level denoted by the horizontal line around the hull above that.
What I would LIKE to do to get more head room in the cabin is drop the sole to the level of the stringers, or at least the level of the top of the level , tabbing it to the hull along with more stiffeners along the hull, as before. What happens then though is the sole is below the level of the chine, which gets back to a stiffening proplem again, I think. Strength too, probably......
Would putting the settees back the bow in all around help this situation?

I know this is a lot - thanks for any input. Thank you PAR for your previous comments too-

I am working outside in the cold, so nothing is going to be done until March or so. Back to thinking !

 

The longitudinal stringers will provide the bulk of hull shell stiffness. The athwart floors are probably just sole supports and not structural floors as seen in wooden boats. This will add some stiffness, but really just hold up the sole.

I don't think you need additional stiffeners, so long as the longitudinal stringers are firmly attached to the hull with sufficient tabbing.

The sole plate (plywood) well tabbed to the hull will stiffen the hull considerably. Coupled with the longitudinal stringers and a few athwart braces, you'll be fine. Lowering the sole to the tops of the stringers is fine, but you'll lose floor space rapidly in this area of the boat, because of the hull's deadrise. Again, the sole plate, tabbed in good, will be more then enough, assuming good stringers.

Don't over think this or put in too much extra material. Weight is the enemy of all small boats, so light and strong is the ticket. If you want to "beef" up areas, put more material into the tabbing, not the lumber elements.