DN NA Class

Over the past 20 years I've only built 2 DN hulls and am now building a third.  The first was a Becker hull, with a 'keel' and longitudinal listings.  The (heavy) hull I built last year had longitudinal listings too, and balsa core between the listings.  I'm trying again, with a goal of something closer to minimum weight.

The Goodwin plans and the website PowerPoint build show transverse listings, which seems to be the norm.  Is there a reason, apart from habit?

Is orientation related to veneer orientation of the plywood, and so, the puncture resistance?

Do tranverse listings improve torsional stiffness (if it even matters?)  It's tempting to say that longitudinal listings improve longitudinal bending stiffness, but they probably only drag the neutral axis lower and increase compressive stresses on the cockpit rails.

Do transverse listings improve transverse strength in way of the plank mounting hardware?

Certainly, installing a few longituninal listings at narrow spacing is waaay less picky than all those transvers ones, but if there is good reason to go transverse (aside from 'because it's always been that way) then the time spent is worth while.

Anyone care to offer an opinion, or some history?

I have no insight on the history or stress analysis of the listings, but A.31 allows the use of foam-core in the floor (although I believe you still need to comply with A.17 "cockpit floor skin must be on top of listings." (i.e. two or more).

I've never convinced myself that foam is lighter that Sitka listings. I think it's close (esp. if you use low-density foam and do a good job preping the surface so you don't end up with a lot of epoxy in the open cells at the surface).

FWIW, here's a photo of a cockpit floor we're about to glue up. There are actually four transverse listings (one at the knees, two at the bobstay blocking, and one at the back edge. The foam is the lowest density core-foam I could easily get (I think it's Divinycell H45 - 3lb/cu ft):
(http://iceboat.org/photos/nordhaus/11-6-2012/2012-11-06%2021.09.18_screen.jpg) (http://iceboat.org/photos/nordhaus/11-6-2012/2012-11-06%2021.09.18_screen.jpg)
(click for a larger photo)

Geoff,

Thank you - Yes, solid material has got to be better than listings in either direction, assuming you can offset any weight disadvantage elsewhere.  It will also make for a more robust cockpit sole, less likely to develop holes from awkward feet.

I was talking to a friend today who is also building a hull and he is going to try to get some foam tomorrow - if it's in stock we'll split it, but if not I'll press on with listings, which means I'll have to make up my mind on orientation.

(PS - on an unrelated matter:  How do I not be a 'newbe' on the bulletin board - I've been an IDNIYRA member since about 1982.  It must be in settings somewhere?)

I like it because there's no compartment between the skins to get water into (ex. if I spike a hole with my shoe).

The best place I've found to get foam in small quantities is:
   http://www.aircraftspruce.com/catalog/cmpages/divinycellfoam.php
and:
   http://aircraftproducts.wicksaircraft.com/viewitems/aircraft-foams/divinycell-foam (scroll down)

Cheers,

Geoff S.

There are a number of boats in our area that have been sailing for years with foam inbetween the floor plywood skins. The foam of choice has been good old blue board ( now its pink) available at any building supply store and it's feather lite.

Bob,

Here too - the late Dick Vine first used 'blue board' locally about 15, if not 20 years ago, although the problem with foam sourced from building supply centres is that you end up with a thicker sole than you might otherwise prefer.  We've found 3/8 Divinycel locally in stock, so I will go with a mixed listing/foam core sole of minimum thickness (for max cockpit depth).  I will attach a photo when I'm there ...

Warren

Chris up at GTIYC put together a nice series of videos on the construction of his ice boat last year.  #12-14 show the listing/foam configuration Bob Gray was talking about.

http://www.gtiyc.org/chrisiceboat.htm

I said I'd attach a photo, although I'm not 'really there' yet.  It's my wife's birthday, but she was out long enough for me to do some dry-fitting.

I will be using Divinycel between the seatback and the knees.  The cockpit sole will extend slightly aft of the seatback base, without core, and will have listings instead of core forward of the knees.  Those forward listings are longitudinal because I will glue the 'ladder rungs' to the cockpit sole, thus dividing that forward area of the cockpit sole into a series of stiffened cells.  I didn't see much point in a foam core there because that area is unlikely to see cleat damage.  I haven't yet thickness-planed or cut out the materials for the plank-plate doublers yet.

The listings are left-over strips from a strip mast I built in the early 1990s. Obviously, I am building the boat on top of the bottom rather than bending the sideplanks around forms - I laid up the sideplanks as a three-layer lamination to make them manageable.

Update:  I did some weighing with my wife's kitchen scales.  The big piece of core weighs 7 oz. and the four longitudinal listings in the for'd part of the sole weighs 2 oz.  I cut out a piece of Divinycel to replace those four listings and it also weighed two oz.  So I've replaced those for listings with foam core.  Of course, the foam core uses much more resin than listings will, but I think the robustness of a cored cockpit sole is worth it.

The best advice I know of about bonding to foam comes from "The Gougeon Brothers
on Boat Construction" section on foam-core (p. 99):
http://www.westsystem.com/ss/assets/HowTo-Publications/GougeonBook%20061205.pdf#page=113

I've used a very stiff mixture of WEST 410 Microlight filler to fill the open surface cells in foam.

Seeing how people are back to building boats....maybe this will be useful:

https://ice.idniyra.org/image/tid/28

Some random comments;
1) I've built boats with foam core, and with listings in the cockpit floor.  I personally like the listings, probably for nostalgic reasons.  I get into a Zen state when cutting listings, and I don't think I pend much more time than cutting foam.
2) I've never had a serious hole in the plywood floor between listings, other than a spike hole or two (a few that looked like they went through, I added a drop of epoxy to seal off).
3) An exception - I had one boat where spike holes caused a problem with water inside the cockpit floor, the floor rotted and had to be replaced.  However I left that boat out all summer under a cover, and didn't realize the cover leaked - the cockpit filled with water and sat all summer.  I must add... that boat had a foam core in the floor (Gatorboard).
4) If you start with 3/8" core, I would be surprised if you get to the 5/8" minimum floor thickness required by Spec A.17
5) Don't worry about torsional stiffness in the cockpit floor, spend your time on something else.
6) Transverse listings provide a stiffer cockpit floor than longitudinal listings (more correctly called stringers).  Stiffer in this sense means less floor deflection under the skipper, not stiffer as in the fuselage bends less.  Now that I put this in writing, a softer cockpit floor may not  be a bad thing...

Paul,

Thanks for your comments.  Actually, I did have problems with puncturing the floor on the Becker boat (with longitudinals) so it does speak to transverse listings being better.  The 'spike punctures' weakened the plywood until it failed an fairly big bits broke into the hollow sole.  I had to cut out and replace the sole - an experience not to repeat.

Warren

My teeth are getting long. Meaning old.  Two lessons:
My first DN had a foam core. It soaked up water from who-knows-where, got heavy and "punky" and replaced.
Foam absorbs water.  Old foam breaks down, especially the cheap, light, styrofoams.

Transverse listings.  My one and only water excursion in a DN left a sloshing sound inside.  It took many small holes to drain it. But the boat is still in service.  A keel or longitidudinal piece would have doubled the trouble.

I too had two hulls sit full of water all summer when a trailer leaked.  They required new bottoms. This was not a hard job with a router.  Had they been foam, more work with chisels.

Listings are easy, light, and cedar for lighter weight, at least in my hulls.  Plus, I can be at minimum thickness.  Or any thickness..

Ken

Only crappy foam absorbs water.

I wouldn't use anything other than core material intended for marine applications (Divinycell, Airex, etc.).
They are available in lower densities than cheap home-center stuff, too.

Cheers,

Geoff S.

Don't want to get into a long argument, but all foam absorbs water. Low permeability foams are better, but bonding to plywood decks is not easy or perfect, and water trapped in the voids is bad.  Rot inducing. Wood skinned foam core structure is what you are making. Water absorption rating in my world would be "fair."

Of course, water exclusion in voids between stringers in our application is also "fair."

Yes, you can use foam and it has advantages. And disadvantages. Most of the disadvantages dont apply to iceboats.  (impact, high temperature, flexing damage) Some might. (long term soaking, bonding voids in panels, service life, cost, labor to install)  The choice is yours.

For more than you would ever want to know:

http://www.marinecomposites.com/PDF_Files/g_composite_materials.pdf

           Cores are late in the document.


http://www.tech.plym.ac.uk/sme/mats324/mats324a10%20core.htm

http://www.tech.plym.ac.uk/sme/MATS324/PowerPoint/MATS324A10%20core.ppt

Holy Hannah Ken that first pdf makes my little brain spin.  The first question I have from a quick skim is ... Will using "Weft Inserted Warp Knit Laid in Warp" fabric allow me to hit Warp Speed faster and easier?  I must read this more closely.....