Stiffening aft section of sailboat

Merry X-mas to everyone! I’m new here but think I’ve come to the right place with my problem.

The boat

The boat we’re talking about is a 1971 Tyler Offshore 8m sailboat (27 ft), hull design by Van de Stadt and more or less identical to the Van de Stadt Bries. Hull thickness between 6 and 11mm from what we’ve seen until now. Generally, in good shape. Only minor delamination of the sandwich deck due to water damage and age in the foredeck; no area of concern. Gelcoat has seen better days, but we’ve repainted the aftdeck section with new gelcoat two years ago (and fixed all cracks).

The boat is osmosis-free as far as we can tell. Apart from the rudder, but you can drink beer at anchor without having rudder, so we’re fine with that The previous owners told us about a damage in the aft section of the boat but as of now we haven’t seen any structural problems so far as a result of said accident. Only the section where the backstay is attached seems to have a lot of stress cracks in the “toerail” but we tried to fix it and added a big SS backing plate to the backstay to spread the load (didn’t have any). That should do for now.

The owners and their knowledge
My wife is refitting the boat and I’m assisting and doing engine and electrical work. She’s the one with the sander and the epoxy. We’ve done gelcoat work, have closed a transducer hole below the water line, restored a hatch, fixed sandwich deck parts and added backing plates, fixed the mess the previous owners did, and the usual add a thingy here and lay some fiberglass mat there. We’re no professional boat builders but we’ve done our fair share of work and always try to do it as proper as possible.

The plan
We plan to add wind steering. In addition, we will build a cockpit arch. Due to the small size of the boat, the need for solar panels / wind generator for long cruises and the placement of the backstay it might be necessary to extend the arch a bit back and support it at the stern. For longer cruises there will be an outboard engine fitted at the back that might also be used as a backup engine, should the main engine fail (which needs replacement anyway).

The problem
The stern of the boat (sorry I’m not native English speaker; it isn’t called transom on a sailboat, is it?) is vertical, approx. 1.5m wide and approx. 0.8m high.


The problem: it seems to flex. The previous owners have stored the outboard engine for the dinghy on a mount, and there is also a transom ladder. We’re not at all concerned with the current state in it's current use case. We have removed all the various wooden and aluminium backing plates in order to fix several holes that have been closed years ago... by adding a plug and some silicone, just above the water line. A mess. The laminate below is in good shape, no obvious damage. But I guess with two previous owners having kids hanging off the ladder and outboard engine mount throughout adolescence, an outboard engine hanging there the rest of the time, and parents being not really skinny either… after 50 years the laminate might have given in a bit.

For how we use the boat now no problem. The previous owner covered more or less the entire section with 2cm thick wood as backing plate (various types, nowhere near marine grade; not even attached properly). You see: the problem doesn’t seem to be new, but hasn’t been properly fixed. I fear with the added weight and force of a wind vane, outboard engine and maybe even other equipment / support structures problems might arise in heavy weather when we take the boat to open waters.

The solution?
That’s my main question. Our idea is to stiffen / strengthen the rear section from the inside. We are rebuilding the aft cockpit locker anyway: new gas cabinet, removal of the water bladder and adding different compartments for storing our stuff. Therefore we have now easy access because it's empty.

Adding some layers of laminate might help a bit (the wind vane and ladder will get proper backing plates anyway). I wonder if some sort of stringer-type structures might help more; maybe even with adding a chainplate-type backing plate that runs down the transom (or is attached to a support structure) for the backstay to completely eliminate any potential risks coming from the damaged backstay area. But our main concern is to stiffen and strengthen the rear section / transom.

How should I approach it? I’ve found plenty of information on transom strengthening on motorboats but I can’t really translate it to our situation where we have a big vertical surface made out of 6mm laminate that flexes. Ideally I would stiffen the section and provide it with some rigid support to the rest of the hull.


I'm open to suggestions and ideas on how to tackle this problem the proper way.
Cheers from Switzerland, Marc.

 

Turn it into a sandwich panel. Grind the inside face of the transom to glass, then install a foam or plywood, then laminate over it. The question is always how thick the core, what type, how much laminate on the inside. My answers are, min 12mm thick, 19mm better, pvc, san, or pur foam, min. 80kg/m^3, a minimum of 1mm of laminate on the inside.

 

Since it is a flat surface, plywood reinforcement is the easiest. Foam will stiffen it too, but is not very good for point loads like bolts and screws.

 

Something like coosaboard would suit, plywood cheaper, the hardest part keeping it firmly pressed together while it sets up, which might mean that just adding more glass could be the easiest, after all it isn't that weight sensitive. You could glue on a few suitable stiffeners with a hot glue gun and glass over the lot to stiffen things up.

 

You have a boat with a good pedigree.Van de Stadt designs are very well regarded and Tyler set the standards for GRP moulding in England.For clarification,the upright part of the stern is referred to as the transom in English and it is this part that I am a little curious about.Is it flat or curved?Flat would make it quite easy to add some reinforcement and curved makes it a little more challenging.Simply adding a bit more laminate would be quite simple,as would incorporating a piece of coremat and then a bit more glass.For a ladder and arch you would need local pads and they would be a lot easier to add than a full ply panel covering the whole area.I have added reinforcement to a transom by vacuum bagging the core in place,but I have done lots of vacuum bagging and this sort of thing isn't ideal as a learning project.When you arrive at the point of adding the hardware you could drill the bolt holes and then use bolts to hold plywood pads in place while the bonding paste cures.Then you could remove the bolts and bond over the pads with some glass.

 

I can see a lot of existing holes. You can add screws where the holes are too.

 

Others have already suggested some good methods of stiffening/reinforcing the transom.
I might also suggest bonding in a few gussets that would stiffen the transition between transom and hull,, alternately, laying in a wedge/triangle shaped foam strip, then glassing over it.
Some scheme that will transfer some loading from a flat transom into the curved hull.

 

Well, first welcome to the forum Marc and Merry Christmas. For the record your English is fine.

I did a transom reinforcement some years ago. Larger that what you face but the problem was the same. My inboard boat was not designed for much stress on the transom. It was just a thin laminate very similar to what you have there. Designed to keep the water out but not much else.

If your transom area is actually flat you'll have a bit easier time of it when doing a reinforcement. I'd proceed as follows:

1. Get all of that amateur reinforcement and odd wood out of there. A little heat perhaps, scrapers, chisels, careful grinding.

2. The inside of the transom is probably not very smooth, you want a good bond between the reinforcement and the transom. I would find some reasonably heavy fiberglass for this job. We have a product here called 1708 double bias stitch mat. 24oz. Double Bias Stitchmat http://www.mertons.com/Reinforcements/double_stitch.html this type of mat would work well for you

3.You'll want to mix some epoxy resin (slow hardener, you need time to work) and add some silica to thicken it somewhat so that it doesn't run down the vertical surface. Find a good local fiberglass supplier and they will help you with this if you haven't done work like this before. It's a messy job. You'll want to have plenty of good quality disposable gloves, old clothes, old shoes and some PPE.

Coat the inside of the transom with some thickened epoxy. After it begins to harden but no longer than about 12 hours later I'd add a layer of the 1708 mat to the transom. Dry fit the cloth first. When you laminate fiberglass before it cures fully you get a "chemical bond" where the second and subsequent layers of the laminate actually interlink at a molecular level with the previous lamination. Much stronger than a "mechanical bond" which is what you get if you wait. You'll use a tool like this: http://mertons.com/img_content/tools-laminatingrollers.jpg

4. Add your first 1/4 inch piece (6 or 7mm in the civilized world ) of good marine plywood.

I'd use Okoume. It's light, durable and will make a lasting repair. BEFORE you install the plywood, coat it with plain, unthinned and unthickened resin three times. Just right out of the containers with no additives. Pay particular attention to the edges. This will seal the ply permanently. Do this on a workbench after you have cut the ply to fit.

Mix up a batch of thickened resin and apply to the transom and to the plywood.

You will need to draw the panel down to the inside of the transom. The idea is to have no voids in the ply transom interface. There are a couple of ways to do this. You can drill some small holes through the transom from the outside and use wood screws to draw the panel in. Once the panel has cured you simply remove the screws and patch the holes. If you have something inside the boat to push against you can use a small hydraulic ram or a mechanical device (I've used a simple car scissors jack).

Once the first panel is in, I'd install a second panel. Apply the 1708 mat to the first panel, let it set up a bit, then install the second panel just as you did the first.

Once the second panel has set up you can cover it with one more piece of 1708. Then apply more resin to fill the weave and smooth the inside of the repair. If you take your time you'll have a nice smooth inside surface ready for a nice coat of paint. Especially since you'll be spending time in that area you'll want a finish with no sharp fiberglass edges. It will also be easier to keep clean if it's well finished.

From the outside of the boat you'll end up with the original skin, 1708, 6mm panel, 1708, 6mm panel, 1708, paint. It will be solid as a rock. When you're finished you'll have a transom that's about 20 mm thick and solid.

Don't forget to use plenty of resin around the perimeter and smooth/fair where your repair meets the topsides. You can use some fiberglass tape or scrap 1708 to tab/blend the transom reinforcement to the topsides.

How about some reading material. Here's a good resource. It's in English but I have no doubt you'll be fine.

About the System Three Epoxy Book https://www.systemthree.com/blogs/epoxy-files/83509508-about-the-system-three-epoxy-book?gclid=Cj0KCQiAlZH_BRCgARIsAAZHSBkNpNVa26i41P23zSenRwkiWQfHaJqsgyUQ5PMlvahoIKTpvrItsCoaAgLgEALw_wcB


Good luck and Merry Christmas.

MIA

 

Yes, the boat has good pedigree. That’s why we decided to invest the time and money to improve / fix where necessary.

The transom is not flat, it is slightly curved (vertically and horizontally).

There have been local backing plates on more or less all the appliances. Some of them better than others, most of them rather large. But it still flexed. That’s why we want more than just a local reinforcement.

We’ve done a lot of polyester and epoxy work but never vacuum. We definitely want to master that skill but I agree that that’s not the right time nor the right project for it.

Thank you for your help. We have the basics covered as we’re quite familiar with laminating work, that’s not the issue. My question was more targeted towards a suiting solution in general (e.g., sandwich vs. stringers). Sorry if that came out wrong in my first post.


Our first idea was in fact to just use the Okoume leftovers from last year’s refit work; I tend to buy way more material than I need, and it always paid off! But then I never calculated proper sandwich sizing and I’m not familiar with the benefits of different materials. The only thing I know is that Coosa is expensive as f., and that the money saved by using wood translates in the equal amount added in weight. Weight is not a concern on our boat, but then again… if there is a better option I’m always interested. Adding two layers of Okoume adds about 15-20kg, without all the other stuff we’re building in the cockpit locker. And every kg of weight I can save (reasonably) I can carry in water, fuel or food. But again: saving weight is not the main requirement, stiffness / strength is. It’s just that I’m curious.


The idea with screws to hold the plates in place is brilliant, thank you. I would have tried to push form the inside but that would prove rather hard the way the locker is designed. We’re going to close all the big holes, so the outside section is due for a nice fairing and paint job anyway…


As I can tell from all your inputs a sandwich laminate is favoured over other approaches like stringers / bulkhead-like structures?

Is it a problem – when using Okoumee – that I cannot fit a plate covering the entire section due to access limitations? I have to have smaller pieces that fit through the hatch. Does orientation of the pieces make any difference? (vertical vs. horizontal)


Is it enough to just lay some glass around the edges to the hull / topsides? Or is it necessary to extend the sandwich / build some gussets? And if so, how should the gussets be sized?


I don’t plan to make it overly scientific. My intuition would be to add wood and mat on the transom, extend the mat 20cm on every side, and maybe add 1-2 gussets from what’s left and be good with it. It’ll be better than before anyway. But I also like to understand how I can optimize it and improve my intuition a bit… so I’m not adding more than I need / miss on substantial improvements now that everything is easily accessible. It’s the same with sandwich constructions: depending on the type of force and core used adding more laminate won’t necessarily help. I want to avoid those types of mistakes, that’s why I’m not just following my amateur intuition but like to ask what’s the proper way of doing it.


My wife did by the way already remove all the amateur reinforcements and sand the transom and surroundings flat. But we’ll add some filler anyway. We plan on using -45/+45 and 0/90 double bias stitched mats (approx. 450g/m2) as we’re having some leftovers from last years projects. We got accustomed to prime every surface we work on with epoxy primer. The primer we use is very thin and soaks wood beautifully. We feel it also improves working on old laminate a lot.

And now back to the boat, time to remove the fuel tank and sand some more…

 

I wouldn't use Ockoume/gaboon ply as it is classified as non-durable,if you do go ahead with it you must take great care to seal any holes so that moisture can't get in.It's main advantage is comparatively light weight.I would also have a few concerns about mixing epoxy and polyester resins in a laminate.Having a transom that curves in both directions complicates things as plywood doesn't like compound curves.You may be able to add a foam stringer or two-perhaps 30mm square with a good corner radius-make sure water can run off them- and add ply pads that can be bonded in after covering the stringers with glass.the pads should be a reasonably good fit to the stringers and the glass carried over the stringers.Simply reducing the size of the unsupported area with some stringers will help and using pads to spread the load will give further resistance to deflection,without requiring too much extra weight in the ends of the boat.I'm sure you know that keeping weight out of the ends of a boat is a good policy.

 

Sounds like you're on your way pistonfields. Since you're going to be attaching hardware back there I did want to mention the epoxy grommet trick. You're familiar? I'll post a picture below.

 

The core does not have to be in one piece, you can puzzle it together from all your ply scrap. It's not different then using contour cut foam or balsa, just a bunch of little squares. Installing is by a heavy coat of epoxy thickened with structural filler (microfibres or chopped glass), making sure there are no voids. Screws are fine for creating the necessary pressure, coat them with something (wax, vaseline, etc.) so they release cleanly (otherwise you need to heat them with a soldering iron), and use a washer to spred the load. There is no need for two layers of ply, just use the thickness you want, the strenght is in the inside glass. The transom to hull/deck transition you fillet and tape.
If you really want you can add a central knee, from the transom to the hull bottom. The deck performs the same function for the hull sides, no knees needed there.
All permanent fasteners going trough the transom need to go trough epoxy rings to protect the ply from water.

The core method is preferred with transoms because it creates a smooth inner surface that allows complete freedom for positioning hardware. A grid of stringers makes an irregular surface and you have to watch where the mounting holes land, thereby restricting where you can mount things.

What is the current arrangement for the backstay chainplate?

 

yes I'm familiar with it and that's how we do it currently on other projects.

What is the current arrangement for the backstay chainplate?
It isn't a chainplate, it's just a U-bolt. We've added a backing plate to it. It didn't even have that. I guess that's why we had stress cracks in that section of the deck / edge of the deck.

 

If you are rebuilding the area anyway it would make sense to go to a composite chainplate. Or, if you want to stay metal, a strap bolted to the transom. A backing plate under the U-bolt does not solve the fundamental problem, wich is that the backstay is only attached to the deck.

 

Small update: we have so far closed all holes and redone the old patches, reinforced the transom with several layers of mats, added some unidirectional mats in an A-shape manner under the backstay and run them down the transom to spread the load, and have now finally added the first 12mm of ply. We had to cut it to pieces approx. 12cm wide to follow the compound curvature of the transom as close as possible.

The tip with running wood screws from the outside to draw the panels in was gold! Pics will follow when everything is done.