stringer layout

I have spent last 3 hours searching this forum in hopes of not bothering anybody with this. I give up. My son & I rebuilding what we think is old 14' Ashcraft. I will attatch pictures as soon as I figure out how. Anyway, the typical stringer/floor/transom rot as expected. We have completed demolition & new transom was tabbed in couple days ago. It has gone smoothly so far, looking very strong. We are beginning to contemplate design of stringers & have some questions on design. The original design had 3 stringers; one in the recessed keel that ran from stern to bow, flanked by an 8' stringer on either side, about 12" from keel. There were no bulkheads. It is important to note that there was virtually no bilge in the original design. Approx height of center stringer was 6"-7", side stringers maybe 3", with the floor coming within a 1/4" of resting on hull at the sides.This boat is semi-v with very flat bottom.We had planned from beginning to duplicate original design adding a bulkhead at each 4' plywood joint, but my son threw me a curve ball last nite suggesting we eliminate center stringer in keel, just use 2 outer stringers, & maybe add a few additional bulkheads. Anyone want to opine on his idea? Can't think of any advantages but can't think of any disadvantages.(Then again, can't think at all; I just want the itching to stop!The joy of fiberglass!) Any websites that might provide plans for layout would help as well. Many thanks in advance.

 

here are a couple photos of hull

 

I do not know if I understand well what you intend to do, I think you try to delete a logitudinal item and compensate by adding transverse elements. If so, is not correct: the role of girders, securing longitudinal strength of hull, can never be developed by a transverse element. I think the main girder should be maintained always and in any case, depending on the thickness of the bottom and the width to the knuckle, see if you can eliminate lateral girder, sespecially in the bow of the boat.

 

You have three primary pieces of longitudinal structure in this boat. The centreline girder, and two side girders. These take the global bending loads from stem to stern. Typically, global loads will be due to the distribution of displacement, the weight of items you install, and dynamic loads due to vessel operations.

Structurally, it's always nice if you can make this girder the full height between hull and deck, however, this isn't usually possible on the centreline, but it is possible at the sides. And that is why you find chine logs and sheer clamps (they're not just there to let you glue the sides to something).

Now, this means that you have something which resembles a beam with two flanges and a thin web. The next pieces of structure are the transverse frames. And their purpose is not only to hold the hull to the correct section shape, but also to prevent the (now very thin) sides buckling by reducing the panel size. The frames also help the hull to take lateral stresses, like cornering, or rigging, or point loads like people stepping on the gunwhale.

After that, are the stringers. These are longitudinal again, but serve to reduce a panel's effective size, (therefore increasing the panel stiffness), and spread local point loads (like slamming) into the rest of the structure. This prevents fatigue problems, and generally stiffens up the boat which improves the handling. It also allows you to use a thinner skin.

That's the theory, but unfortunately, producing the most efficient structure in terms of weight costs a lot of time, and therefore money. Consequently, many lightly loaded boats are moulded without much internal structure at all, just relying on the hull skin to take the stresses. Whilst this works, you get big problems once the skin starts to degrade.

In summary, I'd effectively build up a framework of the main longitudinals, frames and stringers, then use the existing hull skin to keep the water out. The framework could be primarily foam formers with uni-directional glass on top and woven or csm on the sides used to hold it all together. Make sure you clean and cut back to good laminate, though, or the new structure will just pop off when it's needed most.

Best of luck,

Tim B.

 

Really bad idea..

In this hull the large center keel piece is the backbone of the boat. A 7 or 8 inch tall keel has a very good section inertia when combined with the v of the hull. The big keel, combined with the floor makes a strong I beam that is what keeps the boat from bending. This keel transmits vertical forward impact loads to the bow where they are reacted by the sides. If you don't have that piece in there and try to replace it with a couple of bulkheads, you will have three boat sections that will flex inbetween and that will result in a wavy bottom that you certainly don't want in a planing hull. Back in the planing surface, if you don't have that backbone the hull will develop hook and bend upward and it will bury the nose when you are running....

The layup schedule of the hull was designed to work in concert with that beam, and if it isn't there there's no telling what the hull is going to do if it isn't in there.

 

Basics !!

Is this to be a small power boat ? what kind of horse power ??
The keel (stringer ) is the most important one to keep in the boat But i would be more inclined to add 2 stringers to the out sides instead of the one like it had !! The srtingers are doing a dual purpose job !!, one they are stiffening the hull bottom and second strengthening the floor when its in place
Because the boat is older and not new i suggest the 2 strings each side to help strengthen the botton and make the panel sizes smaller so the hull bottom will not bend and flex as much as i possibly has been when its loaded and going through th water !!.
The outer stringer can be simply glass covered plywood on edge with a 2mm thick by 30mm wide timber on the top edge to screw and glue your floor into
The stringer would be glassed to the hull with a doulbe bias tape each side to allow flexing without breaking .

 

A close up of the stringer tops shows the sole of the liner was bonded (not very well apparently) to the stringers, which would tie everything together nicely. It doesn't appear to be an engineered structure, just a heavy layup with some localized supports/stiffeners to make things easy during assembly.

I've worked on countless boats built like this and the hull shell can almost stand alone, without the liner (again assuming the laminate is heavy). If the speeds are kept below 30 MPH, you'll be fine just bonding some new stringers in place and plywood to their tops, tabbing this into the liner (or better yet the hull shell itself).

You don't need additional structure, though it would be a good idea to reinstall the stringers, where they used to live. If you had a core sample or make, model and year, we'd be in a better position to offer technical advise, as the the value of those stringers, but my guess, based on what I see, is they're only to support the sole and of course, plane patch loading at speed. If the laminate is as heavy as I suspect, you don't need much, more then some lengths of plywood or 1 by stock, well tabbed to the hull shell to restore integrity.

Since you've cut away the liner, rather then remove it, you'll have some additional difficulties during reassembly and tabbing the new sole back down. If it was me, I'd level the hull side to side, so I could employ a laser to mark the sole location, then I'd make new stringers up to that line. Next I would cut the liner up, away from the sole and bond a new plywood sole to the stringer tops, transom face and hull shell sides with a few layers of biax. Lastly I'd reconnect the liner to the new sole with more biax, fair and paint o suit.

The plywood sole will keep the stringers minding themselves and if enough tabbing is utilized, no future issues. Naturally, you'll want to sheath the sole with some cloth to fully protect it, 6 ounce or heavier will do. 10 or 12 ounce will leave a nice texture, if you don't fill the weave for good wet foot traction.

 

All great advice. We will be attacking this part of the project soon. Family death so must travel a bit. All projects on temporary hold. My son & I are grateful for all input. Par-my dad lives up your way in Tavares. Next time we get up to see him we will try to say hello & thanks in person. Again, many thanks. Will post updates & photos as we progress. Oh & tunnels-we plan on putting one of my 2 evinrude 48spl on this boat. we do alot of ocean fishing, have 5 boats total, 14' polarcraft aluminum, 14' answer, 22' chris craft, 17' sea way, & this we think is 14' ashcraft.

 

Its a pitty but a all glass top hat frame would be ideal for what you have and not to difficult to make it all out of glass then will never have to worry about it ever again !! My old boat is 1975 and has a glass top hat with glass floors when i cut the last bit of wood out (the transom) i looked in side and it was as good as the day it was built !!
All glass framing is a little more work but a lot less worry for ever more and a day !!

 

Tavares is only a few miles from here. Give me a call when you're back in town.