Why the keel radius?

[Capt. Earl]

The way that I understand sandwich construction, this method incorporates glass that is considered a structual glass over a form or shape, that without it would not be sea worthy. The strength is acquired by the seperation of the laminates, but requires both the core and the glass to make it sea worthy. This will need proper engineering.

When the use of a coating glass only, such as a finishing cloth, only, the hull needs to have the ability to stand alone, if chosen to forgo anything other than a coating for resisting abrasion in most cases. The finishing glass, with the resins, will help in the cosmetic part and seals out water, as a bonus. The way that I understand it for small canoes and kayaks, the shape will help just finishing cloth become some structual part of the hull. I do not beiieve that builders use only finishing cloth in larger hulls to gain strength and rarely will decrease hull thickness and depend on this type of glass to make up for the reduction in the scantlings.

I do think a stitch and glue hull's main advantage is this method eliminates framing as a way to save weight and gain interior space that eliminates the heavy timbers and the abundance of heavy fasteners.

In some conversations there may be some innocent confusion or just some oversight with terminology. This can be seen with people in different parts of the country, and may be the case here.

[Greg Luckett]

I looked in "The Gougeoun Brothers on Boat Construction" 5th edtion for "sandwich construction". Their reference is only for deck construction, but they say that it is the same as core construction. "This uses the basic principle of separating two load-bearing skins with a core material", pg 346.

Now for my understanding of this. Essentially, S&G is the joining of two or more panel edges, while sandwich construction is the process of making the panels using a core material such as foam or balsa. I think that a rudder made this way would be considered sandwich construction. Using tape to re-enforce a joint is not "sandwich contruction" when thought of in this manner. I think that applying fiberglass cloth on all sides of panels would be a form of sandwich construction.

[boatbuilder.org]

Hi everyone,

I think I might be missing something here. Why are the terms "stitch and glue" and "sandwich construction" exclusive of each other? It would seem to me that if the plywood panels are stitched together with wire to temporarily hold them while they are filleted (glued) that pretty much seems like "stitch and glue". If those same panels are glassed on the inside and outside then the plywood would seem to me to be sandwiched between the layers of glass.

If all you do is fillet each each side and don't use glass, isn't the plywood still "sandwiched" between the fillets and at this point no longer considered stitch and glue?

Sandwich construction is using a lightweight core that would normally not be considered a structural material like balsa or more often a honeycomb type of material and adding a rigid surface on each face to end up with something that is strong yet lighweight.

The BS 1088 plywood is not an inferior core material that is, or needs to be strengthened by adding glass. What I am doing it making a plywood butt joint as strong or stronger than the wood around it.

---Joel---

[mrthethird]

I think....

Make the fillet with wood flour, knowing that it will work, glass it in, and get back to building the boat. You are probably not going to get the strongest joint possible working in a garage, during the winter, with hand tools in a hobby environment....but you will get a joint that will probably outlast the boat.

Spring will be here before you know it.

[Barry Pyeatt]

Well for what it is worth, if anything at all, we built boats using sandwich construction for the hulls and decks and bulkheads. The inner core for the hulls was 1" or thicker end grain balsa and occasionally foam. It has virtually no structural strength of its own so it was sandwiched between several layers of glass and glass mat saturated with resin. This was vacuum bagged to eliminate any air from between the layers. Together this made up a sandwich that was incredibly strong and still lightweight. Also with positive flotation properties. We made up bulkheads with two layers of plywood and foam or balsa cores and then coated with glass on each side and fillets with glass tape and colloidal silica and wood flour mixed to join them to the hulls and each other. We also stitch and glued panels with foam or balsa cores and glass on both sides and fillets on the seams.

I would call this sandwich construction. But when we needed additional strength to withstand impacts we often laminated stainless steel to the composite as well.

I wouldn't call Devlin's method of building sandwich, I would think of it more as glass encapsulation.

[Greg Luckett]

Barry,

When doing the sandwich construction like you described, how did you join the edges to other panels. I really think that S&G is a jointing technique while sandwich construction is a panel making technique.

I have used Okume BS1088 to replace the rotted core sections in decks. It works well.

[Barry Pyeatt]

Tongue and groove for bulkheads to other surfaces for the most part. Route out the foam or balsa core and set a tounge on the other surface epoxyed in place with fasteners. Then tape and fillets to the outer layers with thru fasteners. Sometimes "L" brackets set into the panels and thru bolted. Or blocks on the sole or sides of another surface and then attached with fasteners thru the panels into the blocks. We got pretty creative in some applications with interlocking interior panel joints where they were visible and needed to be "finished". Occasionally we used "I", "H", "L" or "U"channels set into top and bottom surfaces with the panels slipped into them and thru fastened.

[capt jake]

A very nice article in this month's Wooden Boat on Sam Devlin. Refering to his building technique as 'Stitch and Glue' all the way throught the article. Joel, did you get my e-mail? I was offline and out of service for a bit, might have missed it.

[boatbuilder.org]

Did not get your email, try again.

---Joel---

[capt jake]

I re-sent it.