Alternate wood to Sitka?

[Sunfish1909]

  I know this question has been asked before somewhere on this site, but my searches come back with minimal answers. What other species is very close to the properties of sitka without the price tag  of sitka. Can poplar be used for the inner construction and basswood be used for the sideboards? I can not find any length, locally, for basswood over twelve feet long. Can a sideboard be scarfed? If so where, fore or aft? Can a complete boat perform if made of all poplar? Has anyone used other species and what types. Any comments and wisdom here will be greatly appreciated.

[Ken Smith]

Wood discussions: Go to the opening page (www.DNAmerica.org) and click on the archive selection, choose the "collective wisdom" top choice and look through the index for discussions.

The closest wood properties to Sitka spruce that is readily available in North America are Alaska Cedar (heavier in most boards), Poplar (slightly heavier, slightly lower properties most boards) and Basswood (lighter and lower properties).  See a Forest Products Handbook for average properties and values.  Successful boats have been made of western red cedar, redwood, pine, white spruce, Douglas fir, mahogany, tropical woods, and northern fir.  I am sure other species as well.  You just cannot beat S.Spruce for the combination of strength, split resistance, gluing properties, good looks and weight! 

If the wood properties for the species you choose are are inferior to spruce, then make the side boards thicker than minimum to compensate.  Red cedar and redwood tend to pull surface materials off or are more prone to splitting than spruce, so two layers of thinner boards (possibly with glass between) will make a more durable side board.  Of course, Jeff Kent makes his boats of a very light core wood and adds the strength with glass reinforcement on the outside, also legal.  I have a 1983 hull that uses a two layer side board, sitka outside and cedar inside, that is still very strong.  Consider two layers of two species, like poplar and basswood, or maybe pine and cedar.  Building this way lets you use narrower boards and to spread your lap joints.

Of course you can scarf.  It adds weight and labor.  The boat is supported at the bow and at the plank, the rig tension tries to pull the bow and stern up and drive the mast step through the boat.  Your weight adds to the stresses in the hull in the same directions.  Side loads at the mast step (and the shroud loads on the plank) twist the hull between the mast step and plank.  So the mast step forward needs to be the strongest, resisting bending the bow up and dealing with spreading the mast step loads.  Next the forward part of the cockpit has to deal with the twist and the transition from a box to a U section at the front bulkhead.  The middle third has to deal with your weight and the twist, holding the sideboards from expanding outward in response to the stress and passing load into the plank.  Fingerboards help especially in the front half of the cockpit.  So... The best place for a scarf is in the aft third, but a two layer side board with two scarfs (both aft but not lined up) would be better yet. 

Lots of alternatives, many of which will work well, and even the worst combination still gets you sailing, and having fun!
 

[Bob Gray]

  I've built about 14 DN's out of basswood and have never had one fail under normal use (collisions, jumping  over a pressure ridge and almost making it etc aren't normal use). I build medium height  hulls and make the sides just over min thickness. I've sailed my boat in 20 mph gusty winds without the bobstay pin (you can't cure stupid) with no ill effects. There are a good number of basswood boats up where I sail and no problems. If you build a medium width boat with a minimum length hull and make a fairly wide stern block, 12 ft. of good wood will suffice.
 Sitka is stronger and definitely more attractive, but basswood is plenty strong enough,  is easier to get and cheaper. I think yellow poplar would also be a good choice.
                                            Bob

[KB [us5219]]

My brother in law made his daughters Opti DN out of clear select pine which is available at Lowes or Home Depot.    that boat is holding up good even with grownups ripping around in it.  plank lengths not available long enough for full size DN without doing a scarf.

[Sunfish1909]

The choice for my sideboards will be basswood. Thanks everyone for the generous replies.  They idea of adding glass to the sideboard face is an interesting tactic. Since it is legal, and the quote in Ken Smith's reply states it was placed on the outside face, what ounce cloth should be used? Is the glass added to the outside only so it will not hinder the wood to wood bonds with epoxy on the interior of the hull? Is it easier to add glass to the outside since it can be done at the end of the construction of the hull with a continuous sheet (looks and strength). Could glass be added to the interior only to preserve the natural look of the outside of the sideboard? Are there more stresses in the outside skin area of a sideboard than the interior?________Pete.

[Bob Gray]

  Adding glass works just fine. If I were adding glass I'd put it on the outside and use 4 oz.. 6 oz. is obviously stronger but the added weight of the epoxy to fill it is considerable. Jan Gougeon once told me to build a boat out of western red cedar (which is very expensive now) , resaw the boards and put a layer of glass in between  the boards and epoxy them back together. It was a very strong, light boat but a lot of extra work.  Bob

[Geoff Sobering]

On the subject of adding glass, I believe it is fairly common these days to add a layer of vertically oriented unidirectional glass to the inside of the sideboard from the seatback forward to the knee.  This helps to keep the grain from splitting as the top of the sideboards try and flex outward under load.

Also, don't underestimate the structural benefit of full-length max. cross-section grab rails.  They add quite a bit of material right were it helps the most to strengthen the sideboards.

[Bob Gray]

  Since it's too snowy to sail my boat I might as well talk about building one.  I agree totally with Geoff  about grab rails although I taper the front section for looks (pure vanity). Here are a couple of other things you can do. Instead of ending the fore deck at the cockpit bulkhead and the aft deck at the seat back, extend the plywood an inch or two aft and forward on  the top of the side boards. I believe this helps some to distribute the load at these somewhat weaker points. If you are worried about side board splitting you can epoxy a piece of your 1/8" deck material on the outside of the hull over the area by the plank attach fittings. I generally make the piece about 15" long and 3" wide. It actually looks good. There is one more thing that won't hurt and that is to butt the two pieces of bottom ply under where the cockpit knee will be. If you put a wide floor listing over this seam and later the knee over that, it kind of ties the whole mess together. Building a hull is fun, good luck and enjoy. Bob

[Ken Smith]

Boy, I gotta disagree some with Bob.

Butt joints are both heavier and weaker than scarf joints.  Plus the stuff glued to the bottom (listings) have to be adjusted in way of the plywood backing piece. The joint should be as far aft as possible to get it as little load as possible, 8 feet from the bow.  Plus at this location, the bottom is wider, so the same force is spread over a wider area, minimizing stress in the joint.  There was a recent object lesson in MN about but joints near the front of the boat: a side board split repair and new bottom was required.

Scarf joints are easy:  stack the wood to be joined plus another layer on top and bottom, end the boards in a step pattern the distance you want the scarf to cover (think through what side you want to show on each piece).  I use 1 inch for 1/8 inch ply.  Use a belt sander and turn the steps into a ramp.

Adding wood to the outside as suggested by Bob is ok, but the inside surfaces need vertical fibers.  The class plans call for vertically oriented grain plywood inside on the sideboards inside, but Uni on the sideboards as described by Geoff is stronger.  Basswood is soft and subject to dings.  Dings show on the outside.  Epoxied glass is hard.  Glass on the outside surface will make the basswood more durable.   Light glass there is almost invisible if done well, but still put the uni fibers vertically from the plank plates to the knee on the inside.  I don't think you need this outside glass for strength, but make the sideboard and structural bulkheads thickness a bit above minimum.  Glass the inside (at least the uni-directional vertical stuff) before doing the bottom.  You want the fibers to cover the whole side board.  If this is all the glass you decide to add to the inside, it is not bent when shaping the sideboard, so can be applied as soon as the sideboard is shaped.

[ROBERT RAST]

concerning the plywood placement on the bottom of the hull Ive seen a number of people put the seam 1. under mast, step not advisable Ive seen side boards crack here under mast as a result.If you look at the plan at 8' 3" aprox is your blocking for the bobstay cable .instead of a small block I ran a strip about 2/1/2"  to 3" wide across the width of the hull for the 8 ft plywood seam.. The stem blocking is 9"   start the 8 ft sheet about 3" from the bow and  8ft will land in middle of full length stringer.Put a small piece of scrap to finish bow.The bow blocking is solid so the plywood seam here has no effect. Less waste on plywood ,stronger bottom hull.

DN1313

[Paul Goodwin - US 46]

Why add a 3" wide piece of wood the full width of the hull just to have something to butt join the plywood on.  Other than this, it adds no strength to the hull, just weight.

I'm with Ken, the scarf joint is the only way to go.  They take a little practice to get right, so try making a few with some scrap plywood first.  I prefer 12:1 (Ken's 1" long scarf on 1/8" plywood gices 8:1, a 12:1 joint requires a 1-1/2" long scarf).

I also prefer to use a really sharp block plane rather than a belt sander, and have learned to cut the scarf on each side individually.  A nice advantage here is that once you get good at it, you can scarf the deck or bottom skin in place when doing a repair.  A proper 12:1 scarf will give you a joint with the same strength as the original plywood. Wooden plane builders rely on 12:1 scarfs to hold their plane together - don't even bother talking to them about butt joints, you'll look foolish.

On the deck I run the foredeck (and aft) a couple inches onto the cockpit sides, then scarf a thin strip to fit in between.  It's not needed for strength, but it sure is beautiful.  Once the art of cutting scarfs is mastered, it becomes an insignificant task, enjoyable actually.

[Bob Gray]

  I've helped build several fishing drift boats  and am very familiar with scarfing plywood. I know it's a strong, light method of joining wood. On my hulls I like doing it the easier basically as strong way. I place a 1 1/2" piece of wood the full width of the hull  that is 86" from the front of the hull. I choose this  spot because this is where I wanted the cockpit knee. Between the cross piece, the cockpit floor and the full width knee structure, you have an extremely strong joint that is EASY. I do scarf the top decks where they meet the cockpit sides cap strips.
   Basswood is 20% softer then sitka. To protect it while I'm building the boat, I coat the outside of the side boards with epoxy that I cut 25% with acetone or MEK (finish sand the boards first). After the boat is done I put a full strength coat of epoxy on the outside of the boat.
   

[Paul Goodwin - US 46]

Protecting the outside of the boat ...

Here is a little known tidbit - fiberglass weighs less than epoxy!!!

You can use this to advantage.  Properly applying a layer of 4oz fiberglass cloth to the hullside will result in a thinner coat of resin than coating with epoxy alone.  This is because it is very difficult to control the thickness of an epoxy coat, whereas the 4oz cloth allows a very controlled coating thickness.

Just apply the cloth to the bare wood, and wet it out in place.  Squeegee thoroughly to remove all excess epoxy - you should be able to see the weave of the cloth distinctly.  Let this first coat partially cure (it should be just barely tacky), then recoat with a thin layer of epoxy, rolled then brushed.

After it cures, sand this layer lightly, enough to barely expose the cloth over the whole surface.  Apply another thin coat of epoxy, as thin as you can manage.  Sand this last epoxy coat until it's smooth and apply the finish coat.  The idea is to just barely fill the weave of the cloth, so the final thickness is only slightly more than the thickness of the glass layer.

If you're using 4oz glass cloth the resulting layer will be only 0.005-0.0010" thick, is mostly made up of glass (which is lighter than epoxy), and still has the added protection of the fiberglass.   A win-win method.