Bow deck flex--the continuing saga General Forum Forums

I will  chip in here and recommend if you can get it reasonably to re-core with foam. Two reasons. First  and least important is weight. Isolated areas re-cored are not going to affect the balance of the boat drastically. Second and more importantly is insulation. The foam is a significant insulator of the boat from heat and cold. That deck area is right over the V-Berth which is a major living space. Other than that marine ply is a perfectly adequate and in some ways structurally stronger replacement material. Unless you are missing stock stringers under the decks I'm not sure that you need to reinforce the deck below with additional ones. Your issues now are due to either or both of water penetration and delamination or heat and age degradation of the foam over a period of 30+ years. The basic design is sound. I would just repair it to that standard there.  I have thought about re-enforcing with a stringer that the bowsprit ties to to re-enforce that but have decided that might not be such a good idea. Most stuff like that is designed to a working strength but also designed so that if something catastrophic happens that it can fail in such a way that pieces come off the boat without ripping the boat in half 🙂

Just a few idle thoughts on the subject. in all honesty the range of what works is inc readably broad with lots of grey areas to debate what is better or worse within that range.

6in x 6in? that would be good for backing plates but something that small wouldn't go good as replacement core. If I had a spot so small that that would replace the core then I would just inject some epoxy maybe and ignore it till it was bigger. Small pieces pieced together will create potential points for it to bend and flex which will create cracks sooner than later and you will have issues again. I think when angela did it on Valkyr, each soft spot was replaced by one sheet the size needed. for example the cockpit on starboard got a piece about 5 ft by 3.5 or so. doing it all in one piece or large pieces if it is an area that large maintains strength. The farther to the edge of the deck you can get the edge of the new coring the better as there is not a break or seam in the core that would tend to bend down under weight.

As to the strength of the foam.. well it isn't that strong so to say. . Plywood is much stronger 🙂 however as a core it isn't depending on the foams bending strength. It is the skin of fiberglass on the top and bottom as well as the slight curve of the deck that makes it strong. Basically your taking a beam and turning it into an ibeam which is magnitudes of order stronger than a basic beam. My brain is a little fuzzed so my explanation isn't the greatest.  Just think of it as engineering over brute force 🙂 There are some good explanations on the web with a simple google search for cored deck strength or ibeam strength. You probably ran into some better ones already in your search on cruisers forum..

There are also a bewildering array of different design and strength foam cores nowadays. a light weight foam core might be described a a 2lb foam or a stronger one as a 8 lb foam.. sorta like your foam mattress on your bed will be rated by lbs per square foot. The lighter weight stuff is less long lasting and not as stiff, the higher the poundage the stiffer and stronger it is. Vast oversimplification I'm sure, but I'm not an expert on the subject so you are only getting "my best" not "the best" advice you could probably get 🙂

I'm willing to bet  you could call one of the manufactures of marine foam core and describe what your doing and how thick the existing core is and they could give you 3 or 4 perfectly adequate options in a couple minutes. You would want to mention that you are doing a hand lay up and not vacuum bagging it as I believe that might make a difference on what exact product or style of product they would recommend.