Side Boards

[Bob Gray]

 A friend of mine is going to build a DN hull this summer. In our discussion I told him that the transition from the front structure to the cockpit is one of the weaker areas of the hull and suggested that he build it with full length grab rails. Even though I've never done it, I also suggested that he take some 1/8" decking material and glue it on the insides of the side boards (full width) from about 5 or 6 inches in front of the cockpit bulkhead to 6 inches aft of the bulkhead. I believe this would strengthen this area considerably with a minimal gain in weight.  Do any experienced builders out there think this is a sound idea or just a waste of time.
                                                                                        Bob
 

[Geoff Sobering]

Bob, what kind of stress and strain are you thinking the plywood would help resist?

My main concern would be reducing the bond between the bulkhead(s) and the sideboards by introducing the plywood.  Instead of one (WEST epoxy) joint, you are now relying on bond between the solid-wood and plywood, as well as the bonds between the plys inside the plywood.

I'd think that local fiberglass reinforcement would be a better alternative.  If you're concerned about cross-grain splitting, you could apply unidirectional vertically.  If it's fore-aft forces you want to resist uni horizontally would help.  Tabbing the bulkhead to the sideboard would help that bond.

Another alternative would be to add a layer of the same wood as the sideboard (essentially locally thickening the sideboard) in the region you want to reinforce.  You could taper the extra layer to avoid a stress concentration where is ends.

[Ken Smith]

The weakness here is actually the stress riser as the stress "flows" from the deck spanning the width of the hull to the top of the side boards at the front of teh cockpit.  In Jan's original plans, there were no grab rails there, and the plywood deck would sometimes crack near that corner.  Other plans provide more area to absorb that stress, and the cracking problem is rare.  Adding knees or foot braces to help hold the sideboards vertical also reduces deck stresses here.

Grab rails either inside the side boards (Goodwin plans and Ron's clone) or outside provides most of the additional area, and these should continue forward of this joint.  This wood adds a lot to the stiffness of the side boards and the rigidity of the joint at the forward cockpit bulkhead.   Personally, I also put a small 1/2 by 1/2 by about 2 inch piece beneath the deck at this corner, set as a diagonal between the bulkhead and sideboard also glued to the deck, where it is out of sight.  Honestly, that may be overkill, but I once had to repair this deck in that area, and will eliminate the possibility with 2 ounces of wood and glue.

I have never seen a sideboard with a fore-aft split at this area, which would need some vertical grain (or glass) to cure.  So adding plywood or glass here appears unnecessary to me.  The vertical grain is needed in way of the plank connection, however.  Besides, I put wood corner fillets behind this bulkhead as the plans show, ant that is vertical grain.  The only boat I saw with structural problems near there used epoxy filets instead.  Heavier and less strength, imho.

For the hull to make minimum weight, all these little details and additions to cure "maybe" problems are bad.  Every piece you leave out is saved weight. The Goodwin plans provide sufficient structure for a reliable boat.  Unless there is a good reason, sticking to the plans is always good advice.


Regards,

[4695]

The smartest thing we did when building my Cedar hull was to clad the sideboards (inside and outside) with 8 oz glass/epoxy after planing the boards to Min thickness and prior to bending them...  and btw, cedar/glass epoxy is probably both stronger and as light as bare sitka spruce.

The hull has been in one very bad accident, where the other guy spun out on a downwind at warp speed and hit (t-boned) me so hard it snapped both runner and plank prior (we're both lucky to be alive, presumably eating up a lot of inertia) to hitting the hull.

One new plank and a set of runners later, it's still a great and light hull, ~ 47 pounds.

[Bob Gray]

 Several years ago I built a cedar strip kayak. It 1/4" cedar with 4 oz. glass over it. It's extremely light and ungodly strong. I'm sure a cedar iceboat that's glassed is as strong as it gets and when you get down to it, it's actually a fiberglass composite structure with a wood core.
                                                                            Bob

[Sunfish1909]

4695,
When you glassed your sideboards before bending, did you bend them after they cured? Did you just roughen up the bonding areas with like 60 grit where bulkheads and stringers would connect? This sounds like something I may pursue. Your hull surviving a "T-boning" sounds very convincing and a hull that is certainly "car-top worthy". Thanks.

[4695]

Well here's the basic process we populated.

1.  source the best straight grain quarter sawn (cedar?) boards you can find, of suitable heighth and width, and memory serves, they shouldn't weigh much over 15 pounds in this condition.
2.  plane to minimum thickness (5/8")
3.  glass outsides of side boards and let kick
4.  glass insides of side boards and let kick (peel ply is your friend here)
5.  shape sideboard profile
6.  cut slot in aft wi bandsaw for bend
7.  glass slot and bend sideboards on form
8.  begin assembly process

while there's about two hours of extra time in the front end of the building process, it also makes the finished areas of the sideboards almost impervious to those little shop dings you're sure to spend hours cleaning up on in the final finish process.   

good luck,

4695

[Sunfish1909]

Thanks for the how to.

[DN5135]

Following along on this subject, could any one give me an idea of the typical side board weight for a minimum weight boat?  I have 2 under construction using birch veneer ply decks, sitka spruce internal members and poplar side boards. I was planning on applying one layer of glass as well.

I'd love to know what the typical sitka side board weight would be and how much a layer of 9 oz West glass and resin would add.

It's not like I'll be a light weight out there, I weigh 250! but I'd like to make a reasonably competitive hull. I am building a mid length hull with a side profile similar to the Sherry clone I own.

Thanks