Bulkhead Scarf

[Freerider]

I have to replace my main bulkhead and I'll need to scarf 2 pieces together because the bulkhead is wider than 4'. The original bulkhead separated where the 2 pieces joined but it wasn't a proper 12:1 scarf.

In the picture below you can see that the builder used the 4' piece starting in the centre of the boat, then joined a small piece which was then tabbed to the hull. The fact that the small piece was joined to the side of hull and the larger 4' piece was bolted to the liner and tabbed to the bottom of hull (I know i hate the bolt on too) it resulted with the 2 piece pulling apart.

Now maybe if the builder had used a proper 12:1 scarf this wouldn't have happened. However, I'm wondering if I should move the location of the scarf to the centre of the boat which would allow the largest piece of the bulkhead to be both tabbed to the hull, plus bolted to the deck liner. The down side to this is that now the scarf is really long and will run from the bilge to the deck.

What do you think? Copy the builder but instead use a proper 12:1 scarf or move the scarf to the opposite side of the 4' piece to the centre of the boat?

note: There's no chain plate attachment on this bulkhead.

[Tony B]

Photos with arrows would be a good idea.
I am not sure about what you are talking about.
Are you talking about 2 pieces of 1/4" plywood?
Are you using the word Scarf? instead of Butt Block?

[virginia boy]

Are you talking about scabbing a piece of wood to make it one piece?

[Freerider]

The photo's are just to show how the bulkhead split, its a photo of each side.

I'll be removing this entire bulkhead and replacing it, but because its wider than a 4ft piece of plywood I'll need to scarf 2 pieces together; I won't be using a butt block.


Attached photo is of a sister (at the office right now). I'll be replacing the main port bulkhead shown in this photo.

[Freerider]

Originally Posted by virginia boy

Are you talking about scabbing a piece of wood to make it one piece?

I'm not familiar with the term scab. As I've researched it seems that 'scarf' is the correct term, not sure though.

If I could find a piece of 5' x 8' marine teak plywood I wouldn't have to worry about this, but i can't seem to find 5' wide plywood.

[minaret]

Originally Posted by Freerider

I have to replace my main bulkhead and I'll need to scarf 2 pieces together because the bulkhead is wider than 4'. The original bulkhead separated where the 2 pieces joined but it wasn't a proper 12:1 scarf.

In the picture below you can see that the builder used the 4' piece starting in the centre of the boat, then joined a small piece which was then tabbed to the hull. The fact that the small piece was joined to the side of hull and the larger 4' piece was bolted to the liner and tabbed to the bottom of hull (I know i hate the bolt on too) it resulted with the 2 piece pulling apart.

Now maybe if the builder had used a proper 12:1 scarf this wouldn't have happened. However, I'm wondering if I should move the location of the scarf to the centre of the boat which would allow the largest piece of the bulkhead to be both tabbed to the hull, plus bolted to the deck liner. The down side to this is that now the scarf is really long and will run from the bilge to the deck.

What do you think? Copy the builder but instead use a proper 12:1 scarf or move the scarf to the opposite side of the 4' piece to the centre of the boat?

note: There's no chain plate attachment on this bulkhead.

Option B-move the scarf- is what I would do. A 10-1 scarf would do fine for this, no need to go to 12-1. If you are scarfing with marine grade and veneering over afterwards, you can dish out the scarf joint and lay a fiberglas taper over it as well, fairing flat before veneering over. This will give you a scarf which will not fail. If you can glass the seam it probably doesnt matter where you land the scarf, but better safe than sorry.

[minaret]

Originally Posted by Freerider

I'm not familiar with the term scab. As I've researched it seems that 'scarf' is the correct term, not sure though.

If I could find a piece of 5' x 8' marine teak plywood I wouldn't have to worry about this, but i can't seem to find 5' wide plywood.

I get 5'x10' marine grade here in Seattle, not too far for you, but it is not pre-veneered in teak. You would have to veneer yourself if you went that route, I don't think I've seen teak faced ply in 5'x10'. And yes, your terminology is correct, it is the others here who are confused. A "scab" is never done on a boat, that is a house carpentry term. Butt blocks yes, scab no.

[virginia boy]

I'm not qualified to offer an opinion, but I'm intrigued and I have a question

I clearly understand the advantage of using a scarf joint, but don't understand the benefit (in this case) of centering the joint between two same size pieces?

I have a similar issue with a transverse bulkhead so I'm anxiously awaiting the pros responses.

[minaret]

Originally Posted by virginia boy

I'm not qualified to offer an opinion, but I'm intrigued and I have a question

I clearly understand the advantage of using a scarf joint, but don't understand the benefit (in this case) of centering the joint between two same size pieces?

I have a similar issue with a transverse bulkhead so I'm anxiously awaiting the pros responses.

What he's saying is that his bulkhead was improperly joined at the factory. The short piece was attached to the hull by tabbing, the long piece was not. Therefore all the load was on the joint, which wasn't even a scarf. Of course it failed. If he moves the joint, then the area where the tabbing stops and the bulkhead picks up a lot of load from "bridging" will be solid ply instead of a joint. So in this case moving the joint is a good thing because of how this particular boat was built. In your case it may not matter.

[virginia boy]

Originally Posted by minaret

What he's saying is that his bulkhead was improperly joined at the factory. The short piece was attached to the hull by tabbing, the long piece was not. Therefore all the load was on the joint, which wasn't even a scarf. Of course it failed. If he moves the joint, then the area where the tabbing stops and the bulkhead picks up a lot of load from "bridging" will be solid ply instead of a joint. So in this case moving the joint is a good thing because of how this particular boat was built. In your case it may not matter.

Thanks

[Blue Stocking]

Depending on the finished thickness of the bulkhead, you could build it up of 2 laminations of different widths with staggered joint positions.
I did this with Bluestocking's portside, main cabin bulkhead which was 5 1/2 ft wide, 8 1/2 ft high, and 1" thick.
Much easier to handle smaller pieces when inside the boat.

[Bluemansailor]

Freerider- I'm doing the same thing to my boat also the portside. What Blue Stocking recommends is a great way of doing it, I'll be doing mine in 2 even sized pieces of 3/4" and then putting a 1/4"plywood with veneer piece over it which will hide the scarf joint.

Just make sure you use a long enough scarf- the recommend ratio is 8:1. If you have the wood 10:1 is fine but nothing under 8:1.

[Freerider]

Originally Posted by Blue Stocking

Depending on the finished thickness of the bulkhead, you could build it up of 2 laminations of different widths with staggered joint positions.
I did this with Bluestocking's portside, main cabin bulkhead which was 5 1/2 ft wide, 8 1/2 ft high, and 1" thick.
Much easier to handle smaller pieces when inside the boat.

I'll be using 3/4" Mahogany ply.

What were the approximates widths of the 2 pieces? Were they similar in width or was 1 piece wider than the other?

[Blue Stocking]

You can laminate with epoxy, 2 sheets of 3/8" to get your 3/4".
Determine your max width of bulkhead and just lay out your cuts so that there is 6 to 9" of stagger in the vertical butt joints when the sheets are laid one on top of the other.
This will allow you a convenient first piece to tab strongly in place along its hull-contact side.
Screw and glue the second sheet, hull contact piece in place. Continue to build out the bulkhead using the stagger to fasten the sheets in place.
Make sure your vertical datum is well established so that your door opening, or corner location is accurate for re-assembly of trim pieces.

[Cheechako]

I like the idea of using two 3/8 thick pieces glued together with the joint in different places.
or...I might make a rabbet joint the full depth of the table saw blade I have, overlapping the two pieces. Then glue and clamp flat out side of the boat (can you get that into the boat?) No reason a 4" overlap joint with proper glue shouldnt be as strong as any other part of the plywood is there?