Bulkhead repair suggestions Gemini 105M

[tharch]

Hi all

We got the mast down and removed the chainplates. The chainplates seems to be fine. Before buying the boat, surveyor said there may be some possible decay on the starboard side of the bulkhead. I was glad to see that there is no decay that side. However, surprise was on the port side. It looked fine visually but as soon as i poked with a Phillips screw driver it just crumbled. I will attach a picture to show the soft areas. I included the startboard side which shows some old repair and seems to be pretty strong.


Port side, I included 3 images, 2 showing damage and one is my plan on how to repair it.


I outlined the area in a red polygon, to replace. I can cut that out and make a replacement panel with marine ply, of the same size. Then cut that panel along the margines of the green and blue overlay so that I can insert the green bit first through the overhanging lip of the roof and then slot in the blu e overlay bit. If I don't do that, I don't think I will have enough flexibility with half inch marine ply to put that whole piece in the slot with the overhanging fiberglass lip from the roof.


Does my plan sound like a reasonable idea?


Thanks in advance

[coopec43]

Hi

What sort of timber is the bulkhead made of?

I would want to replace a lot more than just the area where it is decayed. I'd remove a (say) 12in wide strip down to where it attaches to the floor (the beam going from side to side of the boat)

Then I'd epoxy the join of the existing part of the bulkhead to the new section and then finish it off by epoxying X2 layers of glass cloth on each side of the bulkhead. (The glass cloth should have plenty of overlap - 4 inches(?)

Of course the new section would have to be re-glassed to the hull

[tharch]

Hi coopec43
It is apparently cherry laminate and it does not appear to be glassed to the hull. It seems to insert in to a slot created by a fiberglass lip

[coopec43]

Originally Posted by tharch

Hi coopec43
It is apparently cherry laminate and it does not appear to be glassed to the hull. It seems to insert in to a slot created by a fiberglass lip

Well that doesn't sound too good! You are going to have to fix it and fix it strongly because your bulkhead/hull joints have failed!

The bulkhead is supposed to attach to the hull very, very soundly. (Mine are radiused (see drawing below) and I used Airex) In addition I used a hole saw to cut about eight 2 in. holes around the perimeter of each bulkhead, tapered the edge of each hole so that the fiberglass on each side of the bulkhead became joined

[psk125]

It does not look like we're dealing with a 40' RTW cruiser here. If the whole bulkhead is set into a slot or lip in a fiberglass "rib" that is bonded to the hull, would 'glassing this section of the bulkhead in create a hard spot that might cause problems by concentrating forces in one place too much? How big is this boat?

[coopec43]

Originally Posted by psk125

It does not look like we're dealing with a 40' RTW cruiser here. If the whole bulkhead is set into a slot or lip in a fiberglass "rib" that is bonded to the hull, would 'glassing this section of the bulkhead in create a hard spot that might cause problems by concentrating forces in one place too much? How big is this boat?

Well I disagree.

The bulkheads are part of the yacht structure and make the hull/deck stiff. (A bit like diagonal bracing in a timber house)

I'd like to know more about the yacht too.

[Spot]

Originally Posted by psk125

How big is this boat?

From the OP's post title Bulkhead repair suggestions Gemini 105M

https://sailboatdata.com/sailboat/gemini-105m/





Hull Type: Catamaran Twin Cbrd.
Rigging Type: Masthead Sloop
LOA: 33.50 ft / 10.21 m
LWL: 31.75 ft / 9.68 m
S.A. (reported): 510.00 ft² / 47.38 m²
Beam: 14.00 ft / 4.27 m
Displacement: 8,000.00 lb / 3,629 kg


@tharch , Could we get a couple photos to see the fiberglassing/slot and more of the 'lay of the land' around the repair area (not so zoomed in)?

[Fore and Aft]

Coopec it's not that uncommon to have bulkheads slotting into moulded fiberglass interior liner grooves. Then there's nothing more than Sikaflex etc holding it in place.
The biggest issue here I can see is that the repair (remove rotten plywood, epoxy glue in new plywood then fiberglass join) is that the repair won't be invisible. Do you paint the whole bulkhead or what are the other options?
Cheers

[coopec43]

Originally Posted by Spot

From the OP's post title Bulkhead repair suggestions Gemini 105M

https://sailboatdata.com/sailboat/gemini-105m/


Hull Type: Catamaran Twin Cbrd.
Rigging Type: Masthead Sloop
LOA: 33.50 ft / 10.21 m
LWL: 31.75 ft / 9.68 m
S.A. (reported): 510.00 ft² / 47.38 m²
Beam: 14.00 ft / 4.27 m
Displacement: 8,000.00 lb / 3,629 kg

@tharch , Could we get a couple photos to see the fiberglassing/slot and more of the 'lay of the land' around the repair area (not so zoomed in)?

Thanks Spot (I must be going blind)

I haven't worked on multi hulls but I know they are light and structurally sophisticated.

There's quite a few threads in regard to Bulkhead/hull joints on multihulls. Maybe it is a common problem

https://www.google.com.au/search?q=f...t=gws-wiz-serp





Multihull Bulkhead Refit Advice


Boat Design Net
https://www.boatdesign.net › ... › Design › Multihulls

1 Mar 2021 — I am looking for advice on how best to refill these gaps (see poor quality photos enclosed) between the bulkhead and the hull to facilitate ...

Structural panel bulkhead joining question. - Boat ...

15 posts 26 Dec 2012

Re-tabbing Bulkheads on a Production Catamaran

11 posts 28 June 2019

Question on Glassing bulkhead “seams” - Boat Design Net

6 posts 16 Mar 2021

[coopec43]

Originally Posted by Fore and Aft

Coopec it's not that uncommon to have bulkheads slotting into moulded fiberglass interior liner grooves. Then there's nothing more than Sikaflex etc holding it in place.
The biggest issue here I can see is that the repair (remove rotten plywood, epoxy glue in new plywood then fiberglass join) is that the repair won't be invisible. Do you paint the whole bulkhead or what are the other options?
Cheers

You can't see one chain-plate in my yacht as they are all inside cupboards, inside hanging lockers, inside the toilet. (Mind you they are all easily accessible for inspection)

Maybe use veneer (What was it? Cherry laminate?)

[coopec43]

Pre-purchase survey Gemini 105Mc

https://www.google.com.au/url?sa=t&source=web&rct=j&opi=89978449&url=https://www.cruisersforum.com/forums/f48/pre-purchase-survey-gemini-105mc-1393
1.html&ved=2ahUKEwjivr_JmZeGAxVl1jgGHcqCDdwQFnoECB 8QAQ&usg=AOvVaw1Gl8h7XCwI6OuL1JI0b57-






https://www.catamaransite.com/refere...ni-105-review/

[tharch]

Originally Posted by Spot

From the OP's post title Bulkhead repair suggestions Gemini 105M

https://sailboatdata.com/sailboat/gemini-105m/





Hull Type: Catamaran Twin Cbrd.
Rigging Type: Masthead Sloop
LOA: 33.50 ft / 10.21 m
LWL: 31.75 ft / 9.68 m
S.A. (reported): 510.00 ft² / 47.38 m²
Beam: 14.00 ft / 4.27 m
Displacement: 8,000.00 lb / 3,629 kg


@tharch , Could we get a couple photos to see the fiberglassing/slot and more of the 'lay of the land' around the repair area (not so zoomed in)?

Here it is, sorry, a bit far out

[coopec43]

amarinesurveyor
Registered User




Join Date: Jan 2008
Location: Jupiter, FL
Posts: 169


Check these things that I have found on a number of the Gemini's that I've looked at:
Look for movement or shifting at the main bulkhead (the one below the mast and separating the main cabin from the forward cabin). It is most evident at the doors to the forward cabin and the head.
Check the foredeck for buckling upward, where the baby stay attaches just forward of the sloping windows in the forward cabin - the backing plate on the underside of the deck is not very big.
Check the chain plate attachment for the aft lower shrouds for cracking or signs of weakness. These are accessed behind the seat backs of the settee from the galley and the nav table.
Check the Sonic drive in the up and down positions and make sure it locks and unlocks easily.
Check the steering system to the rudders and the Sonic drive closely - this always seems a bit sloppy to me, but I guess it works.
Check the plywood bulkheads in the foredeck lockers for rot.
Be very very careful with the propane refrigeration and instant water heater if fitted.
And then check everything else that you would in any survey.
I just gave away all my secrets! Good luck in your purchase.
Brian



https://www.cruisersforum.com/forums...5mc-13931.html

[tharch]

Originally Posted by Fore and Aft

Coopec it's not that uncommon to have bulkheads slotting into moulded fiberglass interior liner grooves. Then there's nothing more than Sikaflex etc holding it in place.
The biggest issue here I can see is that the repair (remove rotten plywood, epoxy glue in new plywood then fiberglass join) is that the repair won't be invisible. Do you paint the whole bulkhead or what are the other options?
Cheers

Hi Fore and Aft
I am thinking of either painting or adding a small hanging locker on either side

[tharch]

Any further suggestions? Especially, my plan of making a replacement in 2 halves?

Thanks