Tips on Dagger Board Trunk Construction?

Today and recent decades, monocoque construction is the norm. Easier for mass producers to work from repetitive use molds. In one off, a mold is extra work and expense.
in the late 50's I built my first plywood racing dinghys. Fiberglass wasn't available to home builders.
Dagger board cases and centerboard trunks, were fabricated in two halves from plywood. They were bolted together including gaskets and a shim (just thicker than the board). The each side of bottom of the case had a "log", thick piece of wood that was bedded and bolted to keelson.
The advantage of this system was, you could dissassemble, clean, and repaint inside of cases/trunks.
i'm not proposing you return to the "good old days".
Just offering the insights how once was done. Maybe it'll inspire some innovative solution to your problems.
All the best to you, may none of your problems be insoluble.

 

a crashbox is similar to the automatic kickup daggerboard case i previously posted. But without the trigger and bungee cords.
Observe the piece of pvc pipe at top of case. It absorbs and deflects under shock load, allowing board to disengage trigger and jump up.
A crash box usually has shock absorbing material top and bottom. Forward up top, aft in bottom of case.
It prevents the shock of collission being directly transmitted to case.
A broken board is terribly inconvenient. A broken case is catastrophe.

 

except from www.sailingcatamarans.com

The alternative is to profile only the below water portion, leaving the rest rectangular. This means the box is also rectangular, so the board can be made after the box. True, there is some extra turbulence when the board is half raised, but you can negate much of that by fitting a profiled cover plate on the outer side. Also I've found that there are less chance of leaks with a rectangular box and more important, weed, twigs etc are less likely to get between board and case and jam the board.

Many people suggest building a "crash box" to absorb any grounding shock. But be warned! I made one once years ago as an experiment on a dinghy. I ran aground and the board moved back into the crash box, exactly as planned. But then jammed - not part of the plan!! I had to capsize to free the board so I could raise it. Hardly practical on a cruising catamaran!

 

One thing that I did not like was that the centerboard casing was flush with the underside of the hull - wont do that next time

Next time I will ensure that the casing sticks OUT AT LEAST 10 mm
it will be much easier to make very strong
flush is not nice

 

Wow, now this thread is really going somewhere. Thank you for the great advice. That's just what I was looking for - some general ideas on how these dagger board trunks are usually put together, so I could better interpret the drawings I have in front of me.

A lot more complicated than I was thinking, originally. No wonder so many production boats use LARs. Much cheaper to put in.

 

Can't you just have the lines cross through the hull above the water line?

 

Hey Hoyt, I was thinking about leaks from the deck, into the hull where the line(s) come out.

Maybe it would be simpler and hence better to just bring an uphaul up out of the trunk to a turning block and lead it aft through a bunch of fairleads on the deck- The jib sheets will have to be done that way anyhow, and since the line would be attached to the board at about mid point, the board could be brought up clear out of the water. All you'd have to do is figure out how to keep from squishing the line between the trunk sides and the board.

I am guessing that a down-haul would not be necessary, since the weight of the board would do all the work when lowering it?

 

hey barnacle,
about those "profiled cover plates" what materials should be used, and how attached? I have been wondering about this. Also you maybe should fill the extra area inside the trunk that is not taken up by the board, to displace any water that might enter. How best to do it? My trunks are already installed, and have rounded ends, but the boards will still have a lot of play.

My plans also call for crash blocks. maybe they would be less likely to jam if the trailing edge of the board was blunt and flat at the contact point?

 

Charly,

After discussing this with some other KHSD builders, most of them plan to make the trunk just a little bit larger than the board, using the top deck/guide plate thing to hold it in place. Then, they put a notch in that plate to run the lines all at deck level. Much better than holes in the deck.

Or, alternatively, you can create a small channel in your dagger board trunk to hold that line and keep the tolerances closer. The channel is probably a very good way to do it.

I plan to start on these trunks tomorrow.

 

How about a rectangle trunk with a delrin insert at the hull opening which is tapered to 45deg, and the insert is fixed with5200. The delrin makes moving the board while loaded easier. The top of the board should have a delrin collar around a HD foam collar. Wax the board, insert and fill trunk with expanding high density epoxy foam. In the event of a collision, hopefully the board breaks, if not, the bottom delrin insert pulls out absorbing some of the load, and the foam the rest. Hopefully no damage to the hull.

 

Agreed, idkfa. Delrin is a wonderful thing. How is it on abrasion resistance? With fiberglass rubbing on it all the time?

The boats we are building (Kurt Hughes) have dagger board crash blocks, which are foam aft of the board. These provide the same impact absorbing qualities you are talking about, but Delrin is a great low friction material. My rudder bushings are delrin, thanks to somebody turning me on to that stuff.

 

Delrin has it defects. Hard and prone to point loading the foil. Lift keelers are using Thordon.


From Ron Holland yacht design.......

" The pad material was researched and an elastomeric material called Thordon has been chosen. "

"The manufactures provided us with enthusiastic technical support and proof of the materials unusual “memory” properties in returning to its original shape following the application of high local loading, rather like a
very dense sponge. "

" This material will spread out local unevenness of load due to wear, marine growth or
slight shape variations in the keel surface. "

“Bedding in” and bending of the keel foil will also cause slight
unevenness of pressure distribution in the side pad areas."


Inspection hatches on the keel trunk are common for servicing

 

One other detail while we're on the subject - be careful if building to a design that calls for rectangular section trunks with daggerboards that have rectangular section'ed heads with foil sectioned wetted lengths. When partially retracted you are putting a serious point-load on the daggerboard at its point of max-thickness as it contacts the edge of the trunk at the hull. Be sure to have high density material in the core to handle this scenario.

 

GOOD Point!

 

I like the idea of the delrin insert at the bottom of the trunk and was actually thinking along that line a while back. I have one of those white plastic cutting boards for the kitchen lying around, and was thinking of doing the sametoype of chamfered insert, with something like 5200 for adhesive, but only at the forward and side parts of the board, with a "crashblock" fashioned for the trailing edge of the board, and all this faired onto the bottom of the hull. Then, if the rest of the area inside the trunk that is below the waterline was filled with a closed cell foam, that would keep the water out. You could still leave the area around and above the crashblock open so you could replace it if necessary, accessing it through an inspection port. What does everyone think of this idea? And how would you make it all slick enough? Am I crazy?