Stringer rebuild - fibreglass powerboat

Hi all,

i have spent a bit of time going through some old posts and searching for info on this topic but I have some questions relating to the method of rebuild ie can I strengthen my existing rotted stringers by overlaying uniaxial fibres and woven roving such that I essentially end up with thicker fibreglass stringers where the old plywood is simply providing a form for the new stringers. I understand there will be a weight penalty for doing this but I don’t think this would be excessive. Also I guess odd have to make certain I could get good adhesion on the existing glass tabbing of the wooden stringers to the hull. To ensure this I could grind out the existing tabbing to ensure I’m bonding to the hull and then have a fairly big fillet at the base of the stringers where they meet the hull and taper the tabbing width on the hull do the thickness is less further from the stringers base. Is there anything else I need to consider/be concerned with? The reason for all of this is I don’t want to spend the time grinding out the entire hull and remaking new stringers out of plywood.

If I was to do this how can I determine an appropriate thickness in additional fibreglass to replace the strength lost from the rotted timber? I haven’t yet worked out how thick the old plywood was but I eventually will and can then calculate how much glass and resin I’ll need. Given the stringers are longitudinal supports I figured uniaxial fibreglass would be the best way to increase strength without having to add too much additional weight in glass to replace the wood? Or should I also be using some roving?

The other question I had was what about rebuilding on the existing trailer and doing each side one at a time? I believe my boat has 2 stringers either side of the keel and one down the centre along the keel. So I’m thinking I could remove the floor one side at a time as I fibreglass over the existing stringers.

The boat is made of polyester resin so I was planning to use either vinylester or polyester resin for the fibreglassing.

 

Welcome to the group.

One stringer at a time.
On the trailer is fine if the boat hasn't deformed.
Leaving as much of the original in place will help greatly in preventing hull worpage.
You don't have to remove much of the old tabbing.
Poly on poly repairs are acceptable.
I would use 1708 fabric.
You want csm between each layer when using poly.

Good luck

Ps
Check the transom for rot. It frequently Roy's at the same pase as the stringers.

 

Thanks Blueknarr, appreciate the response. By 1708 I guess you mean double bias +/-45deg fibre orientation. I’m in aus so I’ll find the gsm equivalent which I think would be 600gsm. How many layers do you think would be required if the stringers are 19mm ply? What would be the right calc to ensure that I’m making the structure as strong as with just the plywood? I’m terms of laying up I was thinking I’d do the tabbing all the way to the top of the stringer and then back down to the hull on both sides of the stringer and then a “cap” layer draped over the top of the stringer and then repeat this for the number of layers required.

Yes transom needs replacing also! Was just going to do that from the outside whilst I’d prefer to go from the inside I don’t want to cut and remove the top deck at the rear.

 

Are the existing stringers showing signs of weakness other than the core rot? If not, they could be fine as is. The core is often not structurally significant in the first place.
I’ve had some success with removing just one side of the glass to replace rotten stringers. Removing the wood while Leaving one side in really helps retain the hulls shape.
Go vinylester if you can, it handles very similarly to polyester, but is much stronger, and better for waterproofing.

 

How can anyone offer you any advice? You don't even mention the size of the boat or it's power.

 

Fair point! It’s a 5.3m Haines signature (530F) with a 115hp outboard motor.

 

You said the stringers are 19mm ply.
How tall?
How thick is the existing FG coverings?

 

I’m guessing the existing stringers are either 19mm or 21mm I haven’t ripped the floor up yet as I want to have a solid plan before starting any work. I’ve taken a rough measurement off the gusset coming up off the main stringers either side of the keel and onto the transom. The external thickness of that member is about 25mm so the FG thickness will be about 2-3mm depending on the plywood thickness. The main stringers are roughly 200mm high and I believe there will be another stringers either side of the main stringers which will be shorter. It’s a deep vee hull with 22.5deg deadrise.

 

Does the above additional info change anything stated on previous responses?

 

The easiest way to do the repair is to remove the cap and hog out the rot and replace with plywood and then repair with 3 layers of 1708. This, of course, depends on the extent of the rot, ease of removal, etc.

The alternative to leave the rotten wood and use as a former is less straightforward. But I can frame it a bit.

Based on my experience only; no engineering analysis, 3 layers of 1708 bent in a tophat shape are pretty flimsy. And a piece of 3/4" plywood is not. So, this is not very scientific, but I would probably want about 6 layers of glass to go over each stringer and down onto the hull. This is very hard to do well as there are too many directions and even a pro would end up with air entrainment without vacuum. 6 layers is also going to heat; so vac is not a panacea.

But a general guidance would be to lay 6 pieces of glass 1708 on each side of each stringer. The first piece would start at the top edge, then go onto the hull 6-7"". Each successive piece would recede 1" on the top and one inch on the bottom. Then after that cures; grind it smoothish with 40 grit and cap the top of the stringers with a single piece about 16" wide and decrease each piece by about 2" and apply 6 pieces as caps.

Always longest first to avoid air entrainment. As you see, the glass method is also a lot of work.

Typically, with a rotten transom, the work is far easier to manage from inside; even if you must lift a cap. This is because finishing is so much more work and the inside of most transoms does NOT require much finishing.

I might catch some flack for the 6 layers of glass, but I'm using my experience as a guide. I'd also use epoxy for thd repair, but that's just me. The bonds will be super strong as long as you make a good 40 grit key surface.

 

Thanks fallguy, appreciate the input. I guess I could remove the inside plywood using a handheld battery chainsaw/pruner and a chisel/screwdriver to pry out whatever timber is left inside the cavity and then carefully use a power file belt sander to key the inside of the original tabbing and the bottom of the hull before replacing the timber sections.

Regarding the transom I’ve given it a lot of thought and I must admit I’d much rather do the replacement for the inside also because I could double skin the entire transom not just the middle section where the outboard bolts to. However I don’t want to remove the entire cap for risk of distortion of the hull and removing just the back section will be a lot more complex repair than the finishing on the transom which is a flat surface. I have been toying with the idea of doing the repair from the inside and recapping and paying a peep to do the aesthetic work on the cap..

 

The timber gets bedded in resin. If you decide the path; ask for more advice.

 

If I had to remove all,or part,of a bonded on transom pad my tool of choice would be an oscillating multi-tool with a diamond or carbide blade.It would make less mess than the alternatives .Some pictures would have helped this thread as we might have been able to discern how high the water was in the boat.I am guessing that the cause of the rot was an accumulation of water in the hull during a period of neglect.Before chopping too much of the transom pad away it would be worth drilling a few holes to determine if there is rot present and how far it extends.My suspicion is that a good bonding job would have made it unlikely for too much rot to have occurred and if the bonding was porous or skimped,the rot would probably not have extended more than about 70mm above the waterline of the big puddle.If no rot is evident,just fill the holes and carry on with the other aspects of the job.If the boat was laid up with polyester,you will be able to repair using polyester since the resin didn't rot and epoxy is several times the cost.

 

The rot in the transom is likely caused by quite a few fittings which were attached by drilling into the transom and not correctly sealing afterwards. There may have been an accumulation of water in the bilge at some point and this would have added to it as even the drain hole drilled by the manufacturer was not sealed the plywood was just left exposed.