Dagger board clearances

[ssor]

When I build a dagger board completely covered with fiberglass and epoxy, how much clearance is called for in the trunk, presuming that the inside of the trunk is also glassed with epoxy?

[DGentry]

I'm not a naval architect, but I've raced a number of dinghies. If you're building a racing boat, and you want it to do well, then relatively tight tolerances are called for. One design rules might well specify this size.

If you're just sailing, then looser is fine. In any case, getting your board stuck is not good. And wood swells when water gets in it. So perhaps you should plan for down the road when the glass on your board or trunk gets old and some water eventually makes it's way in . . . .

I don't think that 1/4" (6mm) is too big on either side, if max efficiency isn't your goal. Your board might move around a bit, but not if it has any pressure against it. Less? More? Might all depend on the relative size of your board.

Good luck! What are you building?

[Tom Lathrop]

This is the way I do it. I leave about 1/8 to 3/16 total width difference between the DB an the trunk. This means that both must be built carefully with the DB flat and parallel over its center section and the trunk sides must be clean and parallel also. Your glass and epoxy coating will prevent any swelling for a boat that is normally stored out of the water. A layer of felt on each inside of the trunk on bottom and top will both hold the DB in place wherever you put it in up or down position and allow it to be adjusted while underway, even hard on the wind.

This is primarily for a racing boat but works better for any boat. A sloppy gap will always be less than ideal.

[Boatmik]

Depends a bit on the size of the boat and the depth of the trunk.

Too much slop and it will bang around every time you tack - or on the mooring everytime the boat heels in the opposite direction.

Generally I agree with Tom above.

Dinghies I leave around 3mm (1/8") total extra width.

20 to 30 ft range I leave maybe 5mm (just under 1/4") total.

If I designed bigger I would probably go a little bit bigger for deeper cases.

MIK

[Andrew Craig-Bennett]

If it is for a sailing tender, err on the wide side, as you will get mud squeezing into the box sooner or later.

[mizzenman]

Tom, How do you fasten the felt int the slot? Epoxy?

[Ron Paro]

I had recently seen a suggestion to leave extra space on the aft side of the dagger board to glue in a high-density foam strip as protection for when the dagger board hits an underwater obstruction. What are other's thoughts on this? I am almost finished building a CLC Jimmy Skiff, and I will be learning to sail in this boat. http://jimmyskiff.blogspot.com

[Andrew Craig-Bennett]

When you it an underwater obstruction with a dagger board you most certainly know all about it! This is, of course, one of the two drawbacks of the daggerboard - the other being that you cannot safely sail with it part raised downwind because the exposed part may foul the boom in a gybe and thus capsize you.

[Presuming Ed]

the other being that you cannot safely sail with it part raised downwind because the exposed part may foul the boom in a gybe and thus capsize you.

Partial solution - chop off the forward corner. You can just see here:

http://www.ukmirrorsailing.com/mca/Images/demo-boat.jpg

[Ron Paro]

One of the things I liked about the CLC Jimmy Skiff was the sprit boomed rig and loose-footed sail. I believe that the dagger board can be raised partialy without fouling the sail, and definitely not the boom.

http://www.clcboats.com/images/boats/jimmy-2.jpg

http://jimmyskiff.blogspot.com

[Boatmik]

When you it an underwater obstruction with a dagger board you most certainly know all about it! This is, of course, one of the two drawbacks of the daggerboard - the other being that you cannot safely sail with it part raised downwind because the exposed part may foul the boom in a gybe and thus capsize you.

I think the danger is exaggerated here. Generally in most smaller boats there is quite a large gap between the sail and the centreboard case. Simply don't raise it so high that the boom will catch the board.

As far as hitting things ... yep you know about it ... I quite agree - but for most smaller boats even at quite a speed there is little or no damage done.

Sometimes if the back of a centrecase has inadequate gluing area to the bottom of the boat or inadequate fastenings (for those that still use them) then impact can spring the case off the floor.

Pretty dramatic - but it can easily be avoided by having a cross boat hull stiffener glued to the back of the case and the bottom of the boat extending the width of the boat.

Or a bulkhead. With sort of setup I've been in racing dinghies and really high speed crashes and have only ever done a little bit of damage to the back of the board.

Generally if you are going really fast you are heading downwind anyhow and the centreboard will be up a foot or two so much less likely that you will come a cropper anyhow.

Best wishes
Michael Storer

[Ron Paro]

...As far as hitting things ... yep you know about it ... I quite agree - but for most smaller boats even at quite a speed there is little or no damage done.

Sometimes if the back of a centrecase has inadequate gluing area to the bottom of the boat or inadequate fastenings (for those that still use them) then impact can spring the case off the floor.

Pretty dramatic - but it can easily be avoided by having a cross boat hull stiffener glued to the back of the case and the bottom of the boat extending the width of the boat...

Best wishes
Michael Storer

Thanks for the encouragement Michael! The more I read and experience, the more I appreciate the design of my little boat. Here is the dagger-board trunk (centercase). The blue tape is there because I am applying varnish...
http://bp2.blogger.com/_XuaT2XHyCMw/RmOH9uE3jhI/AAAAAAAAAS8/JpzdnYFswOw/s200/IMG_3235.JPG http://bp1.blogger.com/_XuaT2XHyCMw/RmOHgeE3jcI/AAAAAAAAASU/_eNeBecbuVo/s200/IMG_3198.JPG

http://jimmyskiff.blogspot.com

[Tom Lathrop]

Tom, How do you fasten the felt int the slot? Epoxy?

Epoxy is ok but don't overdo it. You don't want the felt to be saturated with any glue you use. On boats with a wide opening in the trunk, use outdoor carpet or more layers of felt. On many boats, the boom vang and the DB getting together on a jibe is the main problem with long DB's. One of the best things about this method is that the d**n DB stays where you put it without any other contraptions to hold it in place. Can be worth a lot of time in a race.

A piece of rubber at the aft lower end of the trunk is a good idea as was mentioned. I soak a piece of synthetic line like dacron or nylon and attach it to the leading edge of the DB which completely eliminates damage from hitting stuff. Of course, it must be faired in to the foil shape.

[ssor]

Thanks for the discussion. I am cataloging such information for a future project.

[Ben Fuller]

For really tight daggerboard fits on international canoes we used to wrap a few layers of waxed paper around a finished board then lay up a cassette on it. The cassette then lived in the rectangular slot.

If you square off the after ledge of the db trunk to keep things nice and straight when db is down you can rake the forward ledge forward which give the board a place to go if you hit something and lets you rake the board aft on reaches and runs when you have it partially up.

Top of board needs only a couple of holes and a rope handle which also acts as a stop.