DN NA Class

I softwater sail, windsurf, and kiteboard.  Now someone passed along a DN to me (free is always good).  All of the parts are there but the hull needs lots of work and I have some questions.  1.  The sail number is 799.  Does this indicate an approximate built date?  Just wondering.  2.  The angle brackets that connect the hull to the plank have a one inch gap between the hull mounted bracket and the plank.  Is there a purpose for the gap?  3.  Due to leaving the hull out in the weather, most of the decking and floor plywood has delaminated and warped.  I am tearing all of that off down to the frame.  I see on the plans that the cockpit floor is a double layer with a one inch hollow space in between and spaced vertical supports.  Is this to add strength with little weight?  I am thinking about replacing it with 1/2 inch plywood.  Just working towards having it ready for next winter.  Any thoughts or suggestions are appreciated.

Can you post some pictures?  That might help people understand the details.  You can attach them to a post by clicking the "+ Additional Options..." at the lower-left of the entry window.

The sail-number is assigned to an individual, not a boat.  For example, Jane Pegel (805) is currently sailing a boat that's only a few years old.

I will suggest that the best glue to use for the rebuild is WEST-System Epoxy.

Cheers,

Geoff S.

My uncle Bob Benson sailed 778 out the Shrewsbury club in the late 60s. I sail 1313 which is a number he asked for to match name, names were big back then 1313 Double Trouble . My first sail number was 1240 some thing like that in 1968.Sold needed Beer money for college didnt need a iceboat in Daytona Beach Fl.
If the hull is that old you will need to rebuild. They were made with Recorcinal glue which was the best at the time, but not as good as epoxy,If you remove all decking that would be a good start. we use to deck them with 1/4 inch plywood nailed and glued.If  there are nails fastening deck thats a clue.the bulkheads and mast steps were put in with glue and screws. If it is that old the only thing holding it together is the screws as glue is probably crumbled.the screws are also probably corroded.I would try to remove screws and  reinstall  bulkeads and mast step with Epoxy and put on a new deck and bottom with Okume 3mmm or 1/8 plyood . rebuild floor like current plan make sure all interior blocking is reglued. I asked my Uncle a number of years back how much those hulls weighed and he commented , dont you remember it took 2 of use to carry one onto the ice. I guess thats why he brought me along. Glad he did 40 years later still building  and trying to go faster. You should try to find some local sailors to help ,get a year book call around or post on BB your location.
Good luck
Bob Rast
DN1313

The minimum allowed weight for a DN fuselage with hardware and tiller is 46 lbs.   A DN built to current plans and using epoxy is customarily 46 to 50 lbs with hardware and tiller.  The fuselage you're considering rebuilding was likely close to 75 lbs and no where near as strong as a fuselage built to current plans.

For the time and materials involved, I'd suggest you start from scratch and build a new fuselage.  It will be lighter and stronger and a good deal more fun to carry around and to sail.

I began sailing DNs in 1956 with sail number 305.    More than half a century later, still sailing a DN but for the last 20 years in a minimum weight fuselage.  I'm sure I would no longer be able to carry around my 1956 fuselage.  Having alot more fun with a modern DN fuselage.

DN 805

Thanks to all who responded.  I have the decking off.  It was glued and screwed (also a few ring shank nails).  Screws were all severely rusted.  Some of the bottom decking was masonite rather than plywood.  Bare frame weighs 35 lbs and is in very good condition so I think I will use it.  I am impressed with the craftsmanship.  I have included a picture of the gap between the hull to plank mounting hardware.  I can't think of a reason for the gap.  I would think that the angle bracket should be in direct contact with the plank.  The plans show either internal mounting holes through the floor of the cockpit or an external mounting plate in contact with the plank (sheet 2, section D).

I would think so, too.  All the older boats I've seen with that kind of mount were positioned with the bottom of the angle flush with the bottom of the sideboard (I assume there's a bolt that goes through the plank and into the fitting?).

Cheers,

Geoff S.

The older boats 1960s did pick up plank with a bolt through the plank and bracket like you show in the picture. Some might have had a aluminum block for the bolt to go through thus the space.
If rebuilding for day sailing I would recommend re gluing all bulkheads mast step etc with epoxy.You could rebuild the floor with current plan 5/8 or 3/4 strips cedar basswood or something light.Look at current plan. Also if you don't have it there should be a cable bob stay on older boats went from front to back over a angled post. newer post is removable mounts just under the mast step and attached half way up the floor area,see plans, you need blocking in floor for the newer set up or just use the old school front to rear.You could bedeck with3 mm Okkume ,$50.00 a sheet or baltic birch or find some nice 1/8 cabinet plywood for top and 1/8 luan for bottom keeping weight down.
Put everything together with epoxy,most guys use west system . They make airplanes with it so i think DN would be alright.you could have a nice day sailor.
Also you could use current flush outside plank mount brackets and stud plates on the plank.
If you intend to eventually race you will probably look for a newer hull design min weight usually available on various websites Four Lakes or the DN board. Get to work first Ice in MN next year around middle of November.
Good Luck
Rast
DN1313

Thanks for the ideas.  I will do my best to keep the weight down.  Is there much stress on the plank to hull connection?  It would seem to me that the rigging cable from the end of the plank to the mast is exerting only compressive forces at the joint, therefore I shouldn't worry too much about the strength of this connection.  Is there an advantage to having the bolts go through the cockpit floor into the plank as shown on the plans?  With respect to the bobstay, a full length cable goes from the nose all the way to the stern.  Rather than a removeable post to counteract the mast loads, the builder had fabricated a metal "V" strap with a groove filed into the metal to capture the bobstay cable.  I will want to replace this with a removeable post so that it doesn't stick out when transporting.  I will try to stick with this project now so I can avoid the "crunch time" next fall.

Boat to hull connections see stress in sheer in normal sailing.  The shrouds try to pull the plank forward.  If you hike, or turn or spin out, the hull tries to move side-ways relative to the plank.  The plank flexes due to weight and mast loads, and the bolts resist the flex (unless one side has a slot instead of a round hole).

The connection sees considerable stress when one runner hits snow, falls into a crack, hits ice, or another boat.  Then the plank is trying to spin relative to the hull.  Things break.  There are several connections in use, and all consider this potential and install a mechanical "fuse" to break before the hull or plank break.  The fuse lets the failure be benign, so a repair can be easily made.

The most common system in the US uses 3/8" studs (one or two each side) welded to a 2 x 6"plate; the plate is screwed to the plank with five #6 screws.  Another plate with 3/8" holes is mounted to the hull, also fastened with five or six #6 screws.  This second plate has multiple 3/8 holes (one side) or slots (other side, to let the plank flex) so the plank location can be adjusted fore and aft, and for the double studs, if used.  This plate used to be mounted with it sticking out to the sides, so the studs were visible outside the hull.  More usually, the plate is under the hull and the studs are accessed through holes in the bottom of the seat.   The latter became popular with the wider hulls aft, beginning in the late 1970s and was almost universal in the US by the 1990s.  There is variation, but 16 inches between stud centers is kind of the norm.  To mount the hull on the plank, nuts are put on the studs. 

In failure in a runner-stopping event, usually the screws pullout or heads break off, leaving the wood parts intact.   Over time, the stress cycles accumulate on the stud-plate welds.  I have lost studs and nearly lost studs on several occasions, so change the plates every few years.  Stress also loosens the screws in the wood, and I check these every tear-down cycle.  They need tightening several times a year, and a drop of epoxy in the hole once in a while if the wood is too worn or damaged.  (coat the screws with a bit of Vaseline when installed in wet epoxy for easy removal, otherwise you'll need a soldering iron).

In other variations, bolts are mounted straight through the plank, and the fuse is the plate fastened to the hull only.  Higher risk, as hull repairs are more likely. 

Also a fiberglass or carbon piece is sometimes found fastened to the hull with small screws, and the plank mounts to that piece (many designs and styles) which usually remains with the plank in a boat-crunching impact.  There are other variations popular in other countries.

Older DNs, like yours, use steel straps bolted to the side boards and typically through-bolted to the plank.  Strong and reliable, but if you have a runner-stopping event, you will have a broken boat.

Thanks for the thorough explanation, Ken.  If you or other people could post or send some pictures, that will help me fabricate the "fuse plate" correctly.  The original construction of the hull had two parallel one-inch boards attached to the underside of the cockpit to position the plank perpendicular to the hull prior to installing the bolts.  After reading about the shear forces involved when one runner suddenly stops, I think that this construction would certainly lead to hull damage.

See www.sarnshardware .com and shop on line, search for "plates" and browse away.

http://69.73.170.14/~sarnshar/store/products.php