I'm in the process of planning a hollow wood board. At this point I'm researching & planning the construction method. As yet not sure of actual type/style/dims etc. (just finished a 6'1" mini/midi/simms foamblank/polyester resin construction yesterday so might replicate that in wood).
A quick bit of background re: my woodworking skills etc. I'm a trade certified carpenter/joiner although I no longer work in that industry so all my woodwork is for my own pleasure. I use very flexible Finnish Birch plywood (from Finland) in 1mm & 1.5mm thicknesses which I use to construct the shells for custom built bespoke snare drums which is my other hobby (quite profitable too! lol) I have always used Gorilla glue to laminate up my drum shells & I see its being used a lot in hollow wood board construction. I love the stuff. Its so strong. Wear latex gloves tho.
My thoughts are running towards cladding the deck & hull of the wood board with the 1.5mm Finnish Birch plywood (as it bends in both directions, laterally & longitudinally, very easily) & then adding a 0.5mm veneer or strips of some exotic timber/s or mix of both to get an aesthetically interesting finish (I have a load of Sapelle, some Zebrano & English Light Oak which has fantastic Medullary Rays across the grain) which would then be glassed in the traditional manner of 6 & 4oz on the deck & 4oz on the hull.
I'm thinking that would give a reasonably strong skin to both sides which would only be approx 3mm thick (with the glass) therefore keeping the weight far less than say laminating up 1/4" or 6mm thick boards for skins as seems to be the most common practice.
Any thoughts/critiscisms/suggestions from you guys would be welcomed & appreciated.
You work with the stuff, so you should be the best judge of the thickness / strength / weight combination that will work best for your needs.
My only comment is to state the obvious: the thinner the skins, the more you have to rely on the supporting framework, and the closer the spacing required. The pic below is 5" o.c., you'd have to go closer, or maybe glass the inside as well. Might try glassing a few pieces to test the strength after glassing, before deciding.
I was previously thinking about doing something similar (not that I have any experience working with hollow wood construction). One idea that came to mind was the idea of using that NL-20 cork or coremat or innegra or something similar between the interior and exterior, to thicken the skin without adding as much weight.
Yeah I guess you're right. As I work with the stuff regularly I know how it performs in several applications. It's quite strong when curved & under stress so I think it might be ok on a domed deck.
I'll take on your advice & cut a test length of centre spline & several dummy ribs which I'll then try at various centres (maybe 6" o.c. & 5" o.c. & 4" o.c.) glue on the ply, lam on a 4 & 6oz cloth & a hotcoat & when cured give it the old 201 pound (me) 'jump' test & see how it copes.
May I ask if your rib construction method is lighter than the more common 'ply with forstner bit holes airplane wing' method which seem to be set at wider than your 5" o.c.?
The other idea I'm having is: I have access thru a mate who owns a hobby shop to a quantity of good quality balsa in 4" wide planks. so maybe I should use that.
Any thoughts on what thickness would be the optimum for a deck & hull skin. I can get it in 4mm, 5mm, 6mm & 8mm thicknesses. Once again I guess the 8mm would need less ribs so we're back on the weight to strength ratio roundabout again. More tests maybe?
gdaddy yes thats a thought too. I'll do some research into those materials. Thanks for the suggestion.
Thanks again & any other suggestions, thoughts etc will, again, be greatly appreciated.
I generally go for about 3/16" thickness on my skins, after sanding. So I build them with 1/4" thick strips, then I have plenty to work with when sanding. So I'm leaning toward 5 or 6 mm. Balsa is lighter than the redwood I use, but I think the strength is comparable. Some builders use spruce, and it will span much further than redwood.
I don't glass the inside of my skins also, but some builders do. I build mine in place (on the board). Others build a flat skin, then bend / clamp / force it over the framework. If you did that, and glassed the inside, then you could go with a thinner skin.
The construction method I used wasn't done because of being lighter than ribs with cutouts per se, I did it to eliminate the need to pre-plan the exact shape of each individual rib or cross-section. With my method, I made and rough-shaped the rails and added the stringer, so then I could add supports wherever I wanted.
hey there - about 8-inch / 200mm rib spacing works well with 5mm decks, glassed single 4oz
have done a couple of boards with 10-inch ribs and used a bit of extra support under the deck, and double glassed the first one (2x 4oz) and probably do 1x 2oz and 1x 4oz on the current one - the designer of those boards (posts as boco-rio on here) uses spruce, whereas i used vintage close grained pine and paulownia respectively. i've also just finished a mini simmons, with pine decks (B+Q 3x2, ripped to 6mm), glued up then planed to about 4 - 5mm on 200mm rib spacings - single 4oz glassed and feels rock solid and was nice and light, just went a bit daft with the fill coats.
all my board have been done by gluing up a few planks then fitting to the ribs - kind of like single planking but in threes and fours
use the materials you have to hand and have experience of - that way you will find your way = good luck!
Hi again Huck,
Thanks for your reply. I see the logic in your skin thickness theory so I'll go with that idea. Was thinking I would go with glassing the insides of the skins or at least the deck if not the hull as the deck usually gets a beating (stood on, knee-ed etc.) I have several metres of very fine weave 2oz. which should do the insides very nicely.
I also was leaning towards the idea of building the skin onto the frame rather than building the flat skin then clamping & gluing to the contours of the frame, not knowing that you use the building of the skin on the frame method which I have to say makes more sense to me. Less effort required to bend a 4" wide board in 2 directions at the same time than say, a 20" wide board.
Thanks for your explanation of your frame construction method, which I also gotta say makes real sense to me. Get your rails right (which gives you your plan shape/outline absolutley spot on) & then build the inner frame to suit. Simple but brilliant!
Thanks again, all very helpful.
Hey there blue_pig,
Thanks for your input. I'm going to try a few test runs with all the stuff I've got to hand as you suggest.
I live & work at a secondary school which gives me access to a very well appointed woodworkshop & I have a supply of Radiata/Monterey pine in all sizes at my disposal. I've also got a large lot of lovely old & very well seasoned 12" x 1" Red Mahogany which was once shelving. I've grabbed it every time an old cupboard or shelves have been discarded from the main school building which was built in 1859. I intend to rip some of it down ( the mahogany that is, not the school) & mix it in (4" balsa planks with 1" mahogany stringers or whatever)
I'm also now thinking I'll test run using the 1.5mm Finnish Birch ply I mentioned in my original post, first over the frame & then laminate the mahogany strips & 5mm balsa planks (one plank at a time) over that. I've always believed the theory that laminates are stronger than solid timber.
I'll post pics. of my tests & hope for some feedback from you guys.
I would like to use the method that Huck refers to (and Paul):
“Others build a flat skin, then bend / clamp / force it over the framework.”
However, I have a couple questions.
- Do you agree that it might be easier to skin over the rails first, then do the deck and bottom. Won’t it be easy to form the deck over the ribs except when I get to the rail? I just need a ballpark feeling for whether I should envision only deck and bottom, bent and clamped, or should I think of three stages here? Can I bend the deck all the way over the rail down to the bottom? Will 6mm redwood do this? Will 3mm redwood do this? I don’t yet have a feeling for the bendability of my deck skin depending on the wood and thickness. It also may depend on the grain structure of the wood.
- Also, I would like to reinforce the rail from inside (sort-of an inside lamination after gluing over the framework). So doing the rail first will make this easier.
- So I guess I’ll be taking paper, or cardboard and gluing the redwood strips to make the deck skin and bottom skin, and possibly doing the rail with extra strips separately. I mention the paper because I would think it makes the strips easier to bond together. Then I peel the paper off after it dries for some number of hours.
I’m trying to avoid bead and cove or a separate rail-structure approach for my first board - just want to keep it simple. The above seems simple - just a whole lot of clamps and bendable wood to keep the shape while drying, even though the board will not be as strong as a more-sophisticated method.
I haven’t yet decided whether or not I will put a “stringer” along the rail, probably not.
I’m not so much worried about the above questions just curious about people’s opinion. I’ve already printed out my cross-sections (18 of them), profile, and outline. But I’m not in a hurry as I do not have any wood yet to get going on this. I realize that I need a good strength to weight ratio (ideally) so woods such as redwood, northern white cedar, paulownia, and balsa are some good choices. And people use marine ply and poplar sometimes. I’m not a huge fan of poplar but I can see that it has some slight advantages over a typical piece of pine. So eventually, I’ll have enough wood but I’m not yet sure where I’ll get it. I have seen a list of suppliers on this site though so I’ll certainly look into that list.
I DO trust my table saw to cut wood strips though so if I need to use more than one type of strip for my first board, it’s ok. If I get desperate, I’ll just start cutting strips and gluing them into a deck. The important thing for this board is that I get it done some time in the next year, that’s all.
By the way, kudos to Huck for using a clever technique to acheive strength to weight on his cross-sections. Most people do NOT use that method!
Sorry, I forgot to mention Huck’s picture - he DOES do the bottom and rails first.
I see you mentioned your in a Secondary school - have you got access to a laser cutter there? If so i’d recommend using that for your ribs - it will enable you to produce structures quickly and more precise than by hand - where are you based? I’m UK and teach Design in a secondary school and have a laser at my disposal that I’d be happy to cut materials with for you. The thinner the better - I frequently cut ply with it. Wouldnt cost much to post if you are UK based.