CRACKS DEVELOPING... WHAT TO DO?

[Rob Blackburn]

Have some shots here of the cracks that have started in our boat....

http://homepage.mac.com/blackburns/Expeditions/PhotoAlbum129.html

It is the cross thwart that holds the mizzen up. It started on the starboard side and slowly moved along. I poured some epoxy in thinking it might hold it until the winter and could look at it. It did not and continued. Now it has started on the other side.

The crack looks to me as if the timber is slowly swelling but there is no way that water can get in..?... There are no cracks under the thwart, inside the seats or on the timber blocks holding the thwart.

It all seems to be coming from the ends of the thwart. So.. how to fix it.

To take the thwart off would be a major and what is to say the problem is not going to come back. Has anyone else seen this? Is it the working of the boat or the thwart or twisting of the hull or moisture in the wood? Who is to say?

Rather than rebuild it I wondered whether I could not take the fittings off the thwart, sand it back, and laminate another piece the whole width of the boat back on top of it and onto the tops of the seats and screw through to timber under the ply in the storage lockers.? This would spread it across the whole thang as you guys would say hopefully would go away for ever.

Any thoughts?

Rob

[Designer]

Rob,

Due to nature of solid wood endgrain butting up against plywood you must be getting slight movement. I am wondering if you cut a slot with a skilsaw about 3mm wide for about 1/2 the depth of the thwart and filled it with a flexible adhesive like 3M 5200 or Sikaflex to allow for that movement.

While epoxy is the best waterproofing material that we have, it is not 100% vapour proof.

Because the mizzen puts a lot of load into that area I first suspected the connection between the thwart and the blocks but as there is no indication of cracks around and under them I would go with the caulk.

I once inspected a CS17 that the owner decided to eliminate the side seating and run the thwart to the sides of the boat. The screws alone were not enough to carry the stress of the mizzen and the ply had fractured on the outside of the boat around the thwart both port and starboard.

[Brent]

Rob, If I interpret correctly the thwart on your boat is ply and is integral with the seat top? The crack is at the join where the thwart meets the seat side? This would be a high stress area for sure. Let me know if I am reading this correctly.

The idea of laminating over the thwart might help, but before going with that I would consider cutting out the thwart and replacing it with a new thwart made of 2-3 cm thick wood epoxy coated resting on stringers at the seat sides. The thwart is to be bolted into the stringers. Dont try to get a smooth epoxy filled joint between thwart and seat top, allow a small gap between seat top and thwart. This arrangement allows a little play without cracks forming up.

The thward sides will press into the seat sides, which per the plans are backed up with bulkheads at exactly that location so to further distribute the large thwart forces (from supporting the mizzen) into the hull. The thwart needs to be wide so that the forces are distributed over a broad area, and not overloading any single point - at least 25 cm wide - mine is about 11 inches.

[Mike Vacanti]

It looks to me to be only a cosmetic problem. I agree with Graham, there is always going to be a small amount of movement on the interface between the endgrain of the thwart and the side of the seat. I suspect that if the end of the thwart had never been painted with the same color paint as the seat that you never even would have noticed the slight movement.

[mustangermatt]

Is this area built to plans? Where does the "side bulkhead" under the seat meet the thwart? Is it possible the thwart is causing the unsupported portion of the seat ply to deflect?

[Tom Lathrop]

Listen to Graham. You cannot expect to have a wide piece of flatsawn wood not move (or at least try to) with environmental changes. There needs to be a gap there to hide this movement. Same is true of every piece of solid wood furniture in your house.

[Joe Nelson CS#35]

I've been thinking about installing a butt block on both sides of the thwart...about 4"wide, centered so that 2" covers the thwart and 2" rests on the seat top of the side bench. Glued and counter sink screwed to both surfaces with a nice round over of the top edges.

Please let me know if this is NOT a good idea. Seems that it should cover the joint and keep the thwart from working loose. Thinking about using some left over VG fir. I've got 1/2" decks but you should be able to get to the underside to add a block to screw into behind the 1/4" ply seat top. The 4" wide block will have a lot more glue surface area than the end grain of the thwart.

[Rob Blackburn]

Sorry to be so long coming back but it is a long weekend here and was away in the sticks. (no boat)

The thwart is not made of ply but Jarrah a rather hard hardwood. Eric reckons he followed the plans to the letter. It sits on blocks screwed to the vertical faces of the side seats and it is itself screwed to the blocks and epoxied too.

I guess the idea of cutting the groove and putting some sika in is one to try and Eric favours that idea. I get the feeling that the thwart has swollen at the ends due to the water getting in despite the epoxy. I guess you could build it with the thwart going inside the seat and then being bolted etc to the inside and having sika on the join into the seat so that there is no loss of structural integrity ie it is fixed further in and no reason for the water to get into the end.

ie all strength giving elements are inside, flexible sealing on the outside.

Let you know how it goes.

Rob

[Rob Blackburn]

Here is the finished job... Eric had some fancy router which cuts a 3mm trench. When we looked inside the wood etc is all dry and the glue etc all looked fine so bogged it up with Sika..

Photo attached...