Need some help repairing my daggerboards...

[Sunsetslim]

Calling all woodworkers! I could really use your help:

Hit a reef coming into Punta Allen, Mexico. It was really early morning just before the sun came up. Took me 2 hours to get her off the reef. Airex is pretty amazing, she took a beating but she stayed afloat with water intrusion only from a broken through-hull. Scary.

I've got everything sorted except the daggerboard. I'm not sure the best way to scarf it. They are western red cedar, 2" thick and 10.5ft long. I broke off 3-4ft. I found a place here with good cedar planks, but I'm trying to figure out where to make the scarf...

When they are fully lowered they come out of the boat about 2" below the starboard guides. You can see the where the paint rubbed off. If I try to keep as much material as possible, the scarf is going to be right at that fulcrum. (2" thick 7:1 = 14" scarf). If I cut them off right above the waterline I'd have solid wood where it has most of it's lateral force.

The other possibility is scarfing 1" thick pieces to each side of the existing board, thereby reducing the scarf to 7". Shave it down on both sides, almost like a pencil (if it were flat).

The other thing is, I don't know how the heck I would make a good scarf in the tapered area of the board if I tried to cut as little material as possible. That looks like not only a lot of work, but near impossible to do.

Questions:

1. Should I laminate all the replacement ceder into one piece, and scarf it in?

2. Should they be cut into 1"x2"x 6' lengths and epoxied together, or left as 1"x6"x6'?

3. Where to I cut? Does it matter if the scarf is in the lateral fulcrum?

Thanks in advance!! Any help would be greatly appreciated.

[smj]

Can't give you any advice on the daggerboards, but nice Spindrift, almost bought it a few years ago. Best of luck with the repairs.


Sent from my iPhone using Cruisers Sailing Forum

[Redreuben]

You really don't want the scarfs lining up in a line across the board, think "break along dotted line"
You also don't want the join where the lateral pressure is. I would move the join up into the case and make the cut at 45 degrees and scarf in.
I would also seriously consider just making a new board, it won't be that much more work for the piece of mind.

[keepondancin]

Can you just get 11' planks and make a new dagger board. You may have fractures in the wood extending pretty far up the board. It may almost be less work than creating a good scarf joint, unless you have the appropriate tools and the patience and skills involved. My thinking is I would always wonder how strong is the joint, and did I do it in the right place. i would be concerned about any lateral force, on a joint, at a fulcrum point. Also, 7:1 may be a bit steep for a dagger board.

[Benz]

Ditto on the advice for new boards--that old one will never be the same.

[Redreuben]

Addendum;
If you do repair, consider removing some material at the maximum chord thickness and build it back up with a few layers of uni each side to span the exit point.

[Seaslug Caravan]

Originally Posted by Redreuben

Addendum;
If you do repair, consider removing some material at the maximum chord thickness and build it back up with a few layers of uni each side to span the exit point.

Exactly. Scarf cedar in and shape. Grind off material up the board at point of maximum thickness and lay uni (say 200 mm wide). Then add outside laminate . Don't get talked into complete new board. The stiffness is in the laminate. Cedar core is rot resistant, cheap easy to work and and dense enough to take the compression at hull exit .

A quick google will show similar boards being built, rather than me starting a bunfight over what is a suitable amount of uni and outer laminate.

[Cavalier MK2]

I'd just make a new one too. And if you add uni make it carbon.

[catsketcher]

If it is the stock Lock Crowther board then it will be in engineering terms a wooden board with sheathing cloth over. Lock's early boards were not really composite boards. I broke my Twiggy board, when I hit a rock, and wrapped it in unis and it was fine after.

So you can change the board from a cedar board with sheathing to a composite board. In the composite board you can use the cedar as a high density core and add unis and double bias to take the external skin loads.

Step 1 - scarf new cedar planks in. I did 8.1 or 10.1 scarfs on my boat. If possible alter the position of the scarfs. Don't cut all of the board off in a clean cut. Or at least try to stagger the cut so that the scarfs are not exactly in line. Or if this is not possible scarf the board with half the angle on one side and half on the other. The Gougeon brothers book says 8.1 is minimum and up to 12.1 for high stress areas. I reckon 10.1 is fine.

So once you have rejoined the cedar it is onto the glass.

Eg - Old board only Cedar taking load. Modulus of rupture cedar = 50MPa.

So the cedar will break at the outer fibres when the stress is 50MPa. So we have our limit stress. Then we have to put the load through some unis above and below the join. Some data from the web has the tensile strength of E glass as 324 MPa. So we can say that glass is about 6 times stronger than the cedar. I am going to go out on a limb and guess that the outermost 10mm of the section took most of the stress. (Look at a cantilever stress diagram) So you need about 2-3mm of laminate to easily take the same stress.

Taper the unis so that the load can feed in and out of the unis. Usual method is to put a piece about 600mm long, centred on the break and then add 100mm to each end for each extra piece. (Put the longest piece on first and build up from there) Ensure you have ground off enough cedar for the reinforcement.

cheers

Phil

[UNCIVILIZED]

1st, I'm glad that you & the boat are okay. Well pretty much okay on the latter. I'm guessing that this incident will make a good tale at some point, if it hasn't already. And doesn't being in a "ship wreck" rate the wearing of an earring?


As to the boards, given where the breaks are, along with what's left of them. It'll be cheaper, & easier to build new ones, than to repair these. With the benefit that you'll know that the new ones are sound, assuming they're properly built.

I say this as the only 2 possible options to "repair" these is to use a minimum 12:1 scarf. And were it me, I'd scarf in the new wood from both sides. Sandwiching the new wood together below the repair. And in order to avoid making hard spots, I'd want the scarfs on the 2 sides to be staggered, meaning in distinctly different places. Which, I'm uncertain if there's enough of the original boards/material left to be able to do this. But assuming that there is, then after the scarfing in of new wood, a bit of overkill in terms of reinforcements would be in order.

The 12:1 scarfing ratio comes straight out of the Gougeon's book. Where said figure is the minimum recommended one for repairs in critical areas. And boards definitely qualify as such, given the loads they see. Particularly in the vicinity of where they exit the trunk. As the edge of the board where it exits the trunk is the highest loaded spot on it.

One other possible, but complicated, repair option is to use a 2-layered, woodworker's finger joint to add new core material. Which would be a tricky joint to cut into the remaining bits of the board(s). And it too would need to have the 2 layers of added material bonded to the old boards at different "heights".

So with this type of repair, in effect you'd be rebuilding the lower half of the boards using a 2-layer strip plank type of construction in order to build up the core. Then machining it to shape, followed by adding the external reinforcements.

I've read a lot of good stuff on board construction via Kurt Hughes's writings. Including his book on how to build his designs. Which is packed full of info. And another resource would be www.BoatDesign.net/forums As quite a number of the members there build, & or repair boats. Along with having degrees in fields associated there with.


But again, were it me, I'd consult with someone like Kurt Hughes, & then get the recommended foam & reinforcements, & start from page 0. Which, foam glass boards would probably be easier to repair in the future should the boat have another real estate argument. Plus, foam's rot proof, & a heck of a lot easier to shape than wood. Though foam or wood is easy enough to have CNC milled to shape, after which things get glassed.

PS: It might be worth looking into a few of the automatic kick-up systems for daggerboards, & see if they'd be viable for use with the current DB & trunk design. For example, I'm fairly sure that John Shuttleworth's designed a few of them. And they could be a handy thing to have.
Which, speaking of trunks, it'd be a good time to give them a detailed inspection, inside & out too.

[Stumble]

I hate to pile on, but I have to agree, fixing these is going to be at least as difficult as replacing them, and I wouldn't trust a repair. Unless you have a sentimental attachment to the boards there just isn't enough to work with.

But before you start trying to make your own I would suggest giving Phil's Foils a call and see what commercial ones would run you. Compared to the cost of doing them yourself they really aren't that expensive.

[Cavalier MK2]

I'd be careful of making a dagger stronger than the stock one. It is better to break a board than rupture a trunk so take a page from Catsketcher and do the math if you switch to a foam core.

To help shaping a wood board I like to run a router down to the section profile every inch or two using a ball nose bit and a edge guide. The depth is easy to gauge from the plan and it is easy to get a good section. Because the ball nose bit tapers to a fine line you know you can remove material faster when it is wide. A profile sanding board cut to the foil section can take sanding paper as well as gauge if you need help fairing things out.

Naturally if you are spending for a CNC you don't need to do this but it works well anywhere. The Gougeon site has a download by J. R. Watdon on building wood foils that can be helpful.

Glueing together the blank is much faster than trying to scarf and shaping cedar is actually kind of fun. The scarf makes sense if you are stuck someplace without full length materials but it is going to be harder as well as heavier.

[Ken Z]

I think a7-1 scarf is too small i would go to a mim. 16" scarf, at a 35-45 degree angle but if good wood is avialable, build a new board, less labor and more piece of mind.

[Sunsetslim]

Mornin gents!

First of all, I'd like to thank everyone for your replies. I've read and considered all your input carefully, and decided the best option for me right now is to scarf ceder back into the boards at 10:1 at the thickest part (in the trunk above the waterline). Then profile the shape, map, and shape start routing.

I forgot to tell you guys I'm in Progreso, Mexico - supplies are pretty limited here. I can only get 8' planks, and even then we had to go through two pallets to find enough... Foam isn't really an option unless I want to wait on the hard for a month.

I would really like to make new ones out of foam, but the time and cost is prohibitive for me... I'm sure i can find a place with an anchorage and better tooling to tackle that in the future. For now, I'll do the best I can with what I've got.

I'll update this post if any of you are interested in the progress. This is my first cat, and definitely my first daggerboard build, but I've got an old salty tri-sailor taking me under his wing so between the two of us I think we've got a good shot.

I would love any further advice you can give. CF has been extremely helpful on this journey, I can't begin to thank you all enough.

[Cavalier MK2]

You could still make a new blank scarfing your 8' pieces to length and staggering the joints when you edge glue. Let us know how it works out. Having a trimaraner around is good luck!