Stringer help

I have just removed the deck and stringers from an old Avon Searider, which was a good thing, as I found rot throughout the stringers and deck, and I am in the process of replacing them.

My question: The original stringers were .5" plywood wrapped in what looks to be about 1/16" fiberglass on each side. I would rather not use wood again, and I'm considering using closed cell polyurethane foam (about 2" thick for the stringers and the deck support.

Does anyone see any issues with this? Am I getting myself into a structural issue by changing the stiffness or thickness of the stringers?

The hull is very solid, even after the stringers and deck have been removed.

 

That would work if you go thicker on the fiberglass to compesante for the lack of stiffness of the foam. It will still not be as stiff as a wood core, but should be strong enough if you can make sure it is well bonded to the old surface.

I would stay with wood, but I would use solid doug fir, white oak or mahogany, (not plywood) and seal it up well. these woods have good rot resistance.

Realize moisture intrusion will occur with either wood or foam core, and eventually rot it out either way. You need to regularly inspect and perhaps reseal the stringers, and all other wood, with varnish every few years or more.

 

Petros,

Thank you. I'll take a look at some solid wood core vise the closed cell foam. I appreciate the comments.

One question, I understand from the guys that manufacture the foam that water will not affect the foam. Do you have a different experience? I've used closed cell foam on a floating dock, and it seems to last forever (my apologies to the environment!)...

 

Take some time to read a thread that was posted here some time ago. I posted some pictures on this thread and they may give you an idea of what you can do with fiberglass and closed cell foam rather than 1/2" plywood for your stringers.

http://www.boatdesign.net/forums/fiberglass-composite-boat-building/stringer-deck-repair-29459.html

Try using the search function as there are numerous posts from people regarding stringer repair or replacement. As you can imagine this is a very popular issue!

Good Luck and if you have any other questions (which you will) just post.

MIA

 

Yes we have.

Foam can get waterlogged. The dock does not see the cyclic loads a boat has to handle. There the foam is a valid solution.

Wood can be completely encapsulated. If that was not true, we never could have built long lasting wood / epoxy boats. (which, as time told us, outlast the GRP boats)

As it is so often, it all depends....

The cheap ply, which was used (most probably) in your boat was never a good choice for this application. But cheap....When you replace it with a solid timber, as Petros recommended, and encapsulate with epoxy***, you will have a rock solid repair which lasts longer than the original.

Some sort of maintenance and control of these areas are required though. But that is valid in any case.

Using foam means you have to beef up the covering GRP layers substantially, which is a cost issue. (and it consumes a bit of space which probably is not there)
Done right, and the same care taken as with the wooden core, there is nothing wrong with it.
But if you have water ingress, even very little, the foam is gone in short time and the surrounding GRP structure gets weak too.

I have had this debate with PAR on several occasions here and in private conversation. We found no final consensus.

So, my personal advice still goes for a well executed wooden structure.

***you have to use Epoxy instead of poly! Don´t let you talk into anything else. Poly does not stick proper to anything, and worst to fully cured poly!
In that case PAR and I were always in concensus.

Regards
Richard

 

Richard,

Thank you for the post. You make a solid case for wood. While I'm still on the fence regarding the wood vs foam (still need to do some reading), I can certainly assure you that I will only be using West Systems, as this is what I'm used to.

While I'm not rich or loaded by any means, I try not to make cost the first variable when choosing a solution. If I have too, I'll save my money, and wait until I can afford the right thing.

Having said that, I'll look into a solid wood option as you and Petros suggested. I need to look into the Par forum, just to get the other side.

Either way, I'm having fun...

Christian

 

Thanks, MIA.

I will be asking questions, for sure. I will check out the PAR posts. Still haven't decided.

 

It will be hard to find all the posts by PAr on this topic, try to search for "transom repair"

"Raka" And "System Three" provide as good or better quality and are cheaper than "West".

If repair time is not your main concern, laminate the wood. A scantling 2x6 is much stiffer when done from 3 planks 2x2 laminated. Put one coat of neat epoxy on the laminated frame before you lay up the glass cover. You avoid a starving coat doing that.

Ask more questions when you are ready.

Regards
Richard

 

You mentioned it was an old AVON and it sounds as this was the stock piece, right? What is the life expectancy of this panel?
I'd go with 2 1/4" marine plywood pieces with a piece of presoaked mat in between, then pressurize till cure, then laminate top and bottom, use a better grade resin (isopthalic) and you might double the life span of the original while adding strength. The less it will flex the less likely water will crawl in there.
2" of foam even of high density will need very good bonding and substantial laminates to stay in tact. It is bound to delaminate with the first hop or impact (someone dropping a heavy object on it).

 

zerogara -

Thanks for the input. The boat is a 1994, so it's at least lasted 16 yrs. it probably could have gone a couple more, but the wood was getting pretty bad. It didn't look that way until I finally got into the deck and stringers, and saw how soft the wood was.

You make a good point about the (de)lamination. I guess I saw how bad the wood looked, and thought it wood be better to go with foam vice wood. BUT, your point is good.

Do you have any experience with Baltek? would that be as good as marine grade plywood?

 

Naah wait a minute guys, you are discussing two fundamentally different structures here! Originally, there was a ply member acting as the main load carrying beam. The glass and whatever chemical was there to keep it bonded to the skin. The bending modulus of that ply strip, including a part of the skin, is what counts "strengthwise" in this case.

The alternative with a foam core is requiring that the glass is dimensioned to take the bending loads; the foam (if covered all sides) is ONLY performing as a shaping member during layup of the glass! This means that if you go for the foam (or cardboard pipe or...) for creating the beam shape, you must check the strength of the resulting hollow glass beam against the strength requirements. In this case 1/16" of glass certainly won't be enough! On the other hand, what is inside does not matter, it may even rot, as long as the grp is ok!

 

Do not use anything but marine grade ply!
(or laminated solid timber)

Do not go the foam way!
It is hard to achieve a proper bond and even distribution of the loads with the new glass structure.

Do not use isophtalic resin, use Epoxy!
All polyester resins don´t stick well to wood (and anything else), you invite the next problem using them. This s not a 50m² repair where the price difference would make a difference. And even if, it would not be worth to save money at the wrong end.

Regards
Richard

 

Another Option

You could always use Seacast's Self-leveling formula for your stringers


It will never rot, plus it is water proof and water resistant.
It also has plenty of strength.


www.transomrepair.com

 

Here we go again, Seacast is a core material and can't be used in this application, without additional laminate and tabbing upgrades. Baeckmo has mentioned it and I'll follow, the current laminate isn't bearing the load, the wooden stringer was, which was tabbed to the hull shell. This is a structural wooden member, attached to the hull with plastic tabs.

If you switch to foam or decide to use a product like Seacast, you'll have to make the laminate load bearing (which is currently isn't). This will require some costly foam or pour in Seacast goo and a lot more 'glass then what was previously used.

Now, lets look at why it only lasted 16 years. Most builders don't saturate the wooden elements before tabbing. They tab the wooden parts to the hull shell and literally hope for the best, which in my experience is about what you got. Some are better then others, but generally 15 to 20 years and you've got soft soles, transom rot, mushy stringers, etc.

I don't know if you've priced out closed cell, high density foam or Seacast, but they ain't cheap, plus you still have to triple or quadruple the laminate and tabbing thickness to make these methods work. On the other hand, you could use encapsulated wood (as everyone has been trying to talk you into), which means you use the same amount of 'glass and tabbing as before, just replace the wood.

Your call Projectavon2010, but the easiest and cheaper way is with epoxy and wood. If you're going to use wood, consider softwoods, not hardwoods. If it was me, I would switch from plywood to solid wood (as has also been suggested). I'd use a Douglas fir decking material from Lowe's/Depot. They sell this stuff in a 1x6 tongue and groove board. Rip the tongue off on a table saw and you've got good looking, solid stock.

It would be helpful if you could post some pictures. I ask because it sounds like these aren't actual bottom stringers, but part of the sole support structure, which is a different load animal all together.

 

There you go, a practical, durable and economical solution offered to you by an acknowledged expert. I'd hike it to Home Depot, pick up said material, order some epoxy and get it done. Leave the structural foam to High Modulus and other pro fabricators working under ultra controlled conditions.

Chuck