Balsa core replacement

[Christof]

Hi all.

I was wondering if anybody has replaced his Balsa core from beneath?
Dit you use a bevel or not or did you just overlapped it? Is it strong enough?
Any thoughts...

Something like this on yt:

https://youtu.be/kg9nHiEKSK4

I have a Gulfstar 41 and yes the core is rotten.

Greetings Christof

[psk125]

We did this for the overhead of our cabin, where leaks from winch and handrail fastenings had allowed the core to rot. Going at it from the outside would have meant re-doing large sections of the very visible cabin top. We had about 130cm x 15cm on the port side, and a shorter section 100cm x 15cm to starboard. We did essentially what the video says to do, and then had a pro spray it with gelcoat to match the existing interior finish (there is no lining on our overhead). We cut through the overhead fiberglass layer (a dremel works well) and cleared the rotted core back until we got to dry, well-attached balsa and did NOT bevel the core. As the video mentions, you do have to make sure there are no air spaces - he uses thickened epoxy to fill any gaps around his core. Staggering the layers of fiberglass from small to big ties the repair into the inner layer of the hull. THAT constitutes whatever bevel might be indicated here. We did this more than 20 years ago on our boat and have not worried about it since. Rebedding the handrail fastenings also helped. Bon vent!

[Chotu]

Originally Posted by Christof

Hi all.

I was wondering if anybody has replaced his Balsa core from beneath?
Dit you use a bevel or not or did you just overlapped it? Is it strong enough?
Any thoughts...

Something like this on yt:

https://youtu.be/kg9nHiEKSK4

I have a Gulfstar 41 and yes the core is rotten.

Greetings Christof

You don’t need to bevel balsa core at all. It’s in compression. All you have to do is line it all up next to each other so that there are no air gaps.

You also don’t need to overlap. It just has to sit next to the other balsa and be of the same thickness.

[Christof]

I don’t mean to bevel the balsa core. Just the outside layer of fibreglass. Like west system says with a 12/1 ratio. In the yt video they don’t bevel it. I did some fair amount of fibreglass work by now. I love sanding it............................. So I do know a bit about it.
I’ve contacted WS US for this matter and they redirected me to WS UK. I’m from Belgium. They said to me that there is a difference between the European manual and the US version.
WS Europe says to bevel everywhere. Whereas The US doesn’t. So I went in a small discussion with them. Politely off course. As a final result they said to do as I want, they did however gave me some little advice. So I’m still in that grey zone.

That’s why I asked some opinions here.

The email from WS;

Hi Christof

Some of the information presented differs between the American Manuals and website compared to the European Manuals and website.

That is why it is important to are referencing the European site wessexresins.co.uk

Where ever possible we recommend bevelling the joint as this will provide better load transfer. Where access is restricted and there is not room to create the bevel then over laminating the rebounded skin with a generous overlap should be sufficient.

Where ever looking at structural alterations to the boats structure we always advise seeking the advice of a suitably qualified marine surveyor or engineer to regarding the work being completed.

We trust this information meets your requirements and if you require any further information please do not hesitate to contact us.

Regards,

[rslifkin]

In my mind, the bevel is critical if you want the final joint faired flush with the surrounding area, as it's the only way to get enough contact area in that situation.

If the joint isn't cosmetically important and you don't mind it being built up higher, then as long as the skin is well bonded and you fill the joint, then glass over it with lots of overlap, strength should be a non-issue, as you can get plenty of contact area and use plenty of glass to hold it together.

[Chotu]

My advice would be the same as the west system advice. Ha ha. You are not going to have a problem with bevel or lap joint. It is mostly a question of the faring and cosmetic look. I assume the underside is just the anchor locker? If so, I would just do the lap joints and be done with it. No one is going to look under there to check your work. Ha ha

[rslifkin]

Originally Posted by Chotu

My advice would be the same as the west system advice. Ha ha. You are not going to have a problem with bevel or lap joint. It is mostly a question of the faring and cosmetic look. I assume the underside is just the anchor locker? If so, I would just do the lap joints and be done with it. No one is going to look under there to check your work. Ha ha

Exactly what I was getting at. I lap-jointed my pulpit re-core, as I did it from the bottom and there's a cover panel to hide it, so I figured making it strong was more important than trying to make it pretty (and having strength be more questionable due to my limited skills).

[Christof]

That are my thoughts to. No one will see the repair. It’s the ankerlocker and parts of the side decks that need to be done. The side de ks will be covered. I need it to be strong. But I wasn’t sure a lap joint would be a good strong bounding. WS Made me doubt that...

[Christof]

Some pictures. Everyone loves pictures. I have a keel stepped mast. This is the masthole true the deck. Then a piece rotten balsa I replaced on the coach roof. And now I’m currently rebuilding my rudder.

[gonesail]

Originally Posted by Chotu

You don’t need to bevel balsa core at all. It’s in compression. All you have to do is line it all up next to each other so that there are no air gaps.

You also don’t need to overlap. It just has to sit next to the other balsa and be of the same thickness.

if you leave a layer of epoxy between the balsa squares then if there is water intrusion it will be contained.

[psk125]

Re-read the info you got from WS: " Where access is restricted and there is not room to create the bevel then over laminating the rebounded skin with a generous overlap should be sufficient.” They make the stuff and know how well it works. Wessex may be worried about cosmetics.

The WS advice (and the video) both agree that simply overlapping is fine. You are over-thinking this. KISS and go sailing! If you WANT to bevel the two layers of fiberglass cloth (5mm?) in a 12:1 ratio, you will need to sand out 6cm beyond the balsa. Wear a mask. You will still need to stagger (from small to large) the layers when you epoxy them in. Enjoy.