Recreational Rowing Shell

This site has some good stuff on strip building. The process would be the same for your boat.

http://www.guillemot-kayaks.com/Building/StripBuilt/index.html

Gary

 

Thanks Gary - you've helped me surprise myself - it's done exactly as I thought!!

Can plywood be used in place of solid cedar etc?
And what hods the strips in place, once the staples are pulled?

 

"Then, how is the outside of the hull faired?"

Sandpaper and elbow-grease, me boyo....

Enjoy. ;-)

 

I’ve seen many different woods used for this type of construction. In North America, cedar is used because of its stability and workability. Too much expansion and contraction can cause a delam. The strips are edge glued; when you pull the staples everything is fairly solid.

 

Will -

As mentioned in the previous post, edge gluing holds the strips together after the staples are pulled. For canoes and dry-sailed boats, i've used Titebond II, a highly water resistant aliphatic resin glue (wood glue) that can be used for exterior wood projects. It air dries, is inexpensive, and very safe. The entire hull is clad in epoxy saturated cloth, of course. I understand polyurethane glues are another option for edge gluing the strips, and that it has better gap filling capabilities than aliphatic resin adhesives while still avoiding the mess and tedium of mixing numerous small batches of epoxy and fillers.

If the hull got holed and water sat in the core long enough to weaken the glueline, you've got far bigger problems than adhesive failure. Which, by the way is a major reason I'd stay away from filling the entire immersed volume of the hull with styrofoam, urethane , or any other low-density foam - just a place for water to sit and cause rot and delamination where you can't find it. If you want flotation, go for multiple sealed chambers with decent inspection port access and maybe use airbags or foam sheets you can remove.

The typical strips would be ripped from a plain-sawn board milled to the thickness you want the finished width of the strips to be. If you're board is 3/4" thick, you'll end up ripping off nominal 3/4" x 1/4" strips which will now have quarter sawn grain along their 3/4" faces. Quartersawn wood typically exhibits 1/2 the shrinkage across its face that plain sawn wood has.. This results in a more stable hull, with less likelihod of "print-through" the fiberglass cloth and painted finish a few months after you've finished your boat!

If you can live with a painted hull, you can avoid most if not all interior sanding by bedding your fiberglass cloth into a thin layer (1/16" max) of lightweight epoxy fairing mix that you screed into the hull interior just before laying the cloth. The convex curvature of the outside is much easier to sand, but spot fairing ridges and divots speeds things along here, too.

I've built strip planked canoes over male moulds, and it works fine. I built the electric launch in a female mould, which was great because it allowed me to fair and glass the interior hull surface, then install all my bulkeads, inwales, and horizontal panels prior to removing the moulds. I had a totally rigid structure when I rolled the boat, and the nastiest work (on the inside) was out of the way first. The only disadvantage was you can't work strips into the bow easily. Solved that by putting a female staion mould maybe 10" behind the the stem, and angled to the nominal stem angle and ended strip planking there. A bulkead was fitted there. and the final stem piece was a light cedar block fiited to the front of that bulkhead. The block was trimmed to match the stem line in profile, and faired into the rest of the planking on the exterior.

regarding strip planking your hull, narrower strips at the turn of the bilge will mean smaller facets at the plank joints, so less sanding. Narrower strips are also easier to edgeset if necessary. There are numerous schemes for planking the hull, but I'd probably start at the bilge and test a couple planks dry in to find a nice curve that minimizes edge set, then work up to the gunwale and down to the keel.

If you want to maintain topsides planks parallel to the gunwale, measure down equal distances from the gunwale to the bilge along mould stations and see if you still have a plank that will run through those points at the bilge easily. If not, you could always plank from the gunwale down until it becomes too difficult, scribe in a new fair plank line, rip to the line with a skilsaw set to your plank thickness, and then start planking again. Check out "The Gougeoun brothers on Wooden Boat Construction" for info on all your wood epoxy composite options.

My rough guess is for a boat of your size with 1/4" cedar (or an equivalent species at ~ 29 lbs/ cubic foot density) planking, 6 oz glass cloth in and out, and a full deck pan of 3/16"-1/4" ply with light framing and a couple 1/4" bulkheads would be on the order of 60-70 lbs. That's a wild guess, and doesn't include a rowing rig and sliding seats and hardware. You might save 15-20 lbs going with a pvc foam core, thin vacuum bagged skins, and meticulous workmanship, but wheterthat's worth it is up to you.

 

To figure out the weight add:
The planking--multiply the surface area times the thickness times the density of the wood.
The framing--same thing.
The fiberglass--surface area times weight of cloth times 1.6(to account for resin)
Fasteners--weight a box of them and figure how many you'll use
Paint--weight the can minus 20% for solvents, divide the surface to paint by paint coverage.

If you use a styrofoam block for the bottom, it will save the weight of one fiberglass laminate on the inside. Also all the framing for the bottom and cockpit sole.

 

hmm... you've all given me a great deal to think about - not the least of which is what type of timber to use and where to source it. I asked about ply for the strips as I'd be able to buy the sheets in the appropriate thickness - but now I realise it's unlikely that I could get sheets long enough to do the length in one strip.
My 1st thought would be to use King Billy Pine. Dunno if you guys get that sort of stuff over there - it's light, easy to work and often used in boats building in these here parts. Doubt whether cedar is available here...I'll have to check it all out.

As for the weights, I know how to work them out - just don't know the various densities - but then neither do you as nobody knows what type of timber I'll be using!!:!: (Gonzo thanks for the approx grp density...it's a good start...)

 

Forget the plywood, use whatever they use down there for light weight boat building. Butt joints are fine. I use short pieces because they are cheaper. Unless your building for an art exhibit paint it and nobody will know. There is no highly skilled woodworking required.

Gary

 

...should read "no skill required". Period.

I'm off up the coast for a few days - hope you all have a great weekend!!! And thanks again for the help - you can bet I'll be calling on your expaert assistance again very soon!!

 

That's a beautiful hull. May I suggest a possible solution to your dilemma?

You're trying to keep the weight down and the build a simple as possible while maintaining the sexy and slippery curves of that rendering. Foam construction is great for simple and curvy, but as mentioned, there are too many negative attributes (low puncture resistance, filler weight, delamination).

I recommend building your hull on frames approximated to your surface and planked with thick (10mm-12mm) balsa. The interior of the planks would meet the frames square and the exterior could be faired to your beautiful shape with planing and sanding.

The frames too could be made from balsa. You could laminate one or both sides with fiberglass, Kevlar, or 2mm-3mm plywood (best).

You could cut external form station frames from 1/4" ply to check your curves a each station while sanding. Do not use end-grain-only balsa as this will not provide much longitudinal rigidity unless you also laminate the interior. just use planks. Get some great balsa info here:

http://www.zimsweb.com/balsa/

An external layer (or two) of kevlar would make your hull nearly inpregnable (you can bounce a hammer off a balsa laminated sandwich and only damage the paint). Seal the grain inside with epoxy and fill sections with poured polyurethane. You may want to laminate a section of decking inside too.

Balsa is cheap here and probably even cheaper there because so much of it comes from Malaysia/Indonesia/S.E. Asia. Balsa is easy to work with, strong (for its weight), very light weight, available in a variety of densities (strengths), cheap, and NATURAL (ala non-toxic!).

I starting to sound like a balsa salesman but, in fact, I'm working on a light-weight flats fishing boat that uses balsa throughout to keep weight down and thus needing much less power and fuel.

Bill Raymond

 

I checked out the website; it's awesome. The information about balsa is very thorough. Also they have links to suppliers, which makes all the difference. It wouldn't be the first time I hear about a wonder material and find out they only sell it in Timbuktu.

 

Be very cautious about using balsa wood as a core material in long grain orientation as opposed to end grain.

In typical wood strip composite boat construction, the lighter pine or cedar strips provide a moderate weight core with moderate side-grain compression strength. The primary bending strength in tension and compression is provided by the wood fibers nearest the inner and outer faces of the strip, along its length. The fiberglas cloth inside and out provides the cross grain and torsional strength, as well as increasing abrasion and puncture resistance. As a matter of fact, a better orientation for bidirectional glass cloth on wood strip hulls is at +/- 45 degrees rather than the 0/90 degree orientation we typically use. That's why you see newer designs using the +/- 45 degree stitch-bonded biaxial cloths.

When a section of the hull gets hit by a wave or plunges underwater, the inner face of the hull laminate is typically has an increase in tension, while the outer face has increased compression. End grain balsa has high sheer strength perpendicular to the grain, so the core won't experience interlaminar shear. Ever see an overloaded floor joist in a house, where it has large splits along the grain, right near the center of the board? That'd be long grain balsa core in your boat.

When the hull experiences a point load , like hitting a rock, a core weak in compression perpendicualr to the face will crush (assuming the thin skins that don't dissipate the energy of the impact). Long grain balsa loses here, too. Depending on density, it has much lower compression strength than a typical 4#/cubic foot rigid PVC foam core! 2#/cubic foot extruded polystyrene (styrofoam) is much worse!

Hot racing boats, especially multihulls, use PVC foam or end grain balsa cores and carbon fiber skins with fiber orientation tailored to loads, with global loads typically being fore-aft. The foam or balsa keeps the the skins separated and prevents them from buckling. In wood strip composite, the strips usually supply most of the fore-aft strength while also serving as the core.

 

Yes, Chris, all true. But, I believe the intent was to not use foam or the whole molding and filling process associated with it.

This is no "hot racing boat," it's a light rowing/sculling hull. He may have occasion to bump into hard objects, but at a top speed of what? 5mph?

I believe long grain balsa planks will provide plenty of compression resistance for this application. End grain balsa does have a much higher compression strength but, it very forgiving in the long grain orietation as well. Balsa has a tendency to "spring back." Go ahead and hit a 1/2" thick piece of plain balsa with a hammer. It will absorb well and leave a shallow dent. Now hit PVC or urethane and you get dust and a deep divot. Foam gets crushed and you have instant delam.

Glues and epoxies adhere exceptionally well to balsa which will minimize delamination problems. The curve of the hull lend nicely to a very rigid structure and two layers of kevlar (oriented on the 45 as you mentioned) will make this a tough little boat.

I would recommend building and testing some samples. Build several small scale models using methods you are considering. You don't have to duplicate your final hull design but, the models should be nearly identical to properly compare - perhaps a simple half cylinder about 1 meter long. For each model, time the construction, rate the finished product, list the materials, and calculate the cost. Then decide which method best suits you.

 

I had thought of using balsa, but gave the idea away because of the problems of water migration within the core. Even end-grain is susceptible to this over time (having repaired the balsa-cored deck of my own boat, nobody will convince me otherwise...)

Going full-circle - back to my preferred material, closed-cell foam - I spoke to a few people and they recommended using 6mm 80 kg/m^3 Corecell with 2 layers of 200 gsm cloth. Now allowing for the fact that I'm not a very experienced fibreglasser (so expect my layup to be a little resin-rich) I wind up with a surface area weight of 2.7 kg/m^2 (with 2 layers of cloth outside, 1 layer inside). This gives a total hull and structure mass of approx 27kg (excluding the sliding seat and hardware). Which is in the ball-park for my target weight.

What do you think? Would this be tough enough?

 

Will-

Without doing any math...The recommendation of 80 kg/m^3 Corecell sounds about right. Most hulls and decks for small boats use this, or maybe even 60 kg/m^3 in areas less subject to impact than a hull bottom. 6 mm may be fine, but many boat designs I've looked at with options for both wood or PVC foam construction tend to use a slightly thicker foam core than if it was made of softwood. 2 layers of 200 gsm cloth sounds close to 2 layers of 6 oz boat and tooling cloth, so I'd say that sounds right.

wtraymond is right - build some samples. You may not need to build models of the boat, but make up some strips and small squares (maybe 12" x 12" ) of your possible hull laminates. Use a sample of 6 mm cedar or light pine quartersawn strips w/ 1 layer 200 gsm cloth each side as your control, and then try foam and long grain balsa laminates too at 6mm and 8 mm thick.

Test the bending strengths of the strips clamped to the edge of a bench with weights hanging from them, noting weights and deflection until they fail. Try something similar with the panel samples with supported opposing edges. Whack 'em with a hammer to look at impact damage. You'll get a sense of what works structurally as well as how to handle the material.