Dory Skiff Transom

[DJM]

Hello all,
I have been reading this forum for a few months, and have just registered. I am very impressed with the amount of knowledge exhibited by the members here.

After many months of considering a lot of designs, I have decided to build the Marblehead Dory Skiff. I am building it from the plans available from Mystic Seaport. It is very similar the 14' Chamberlain Dory Skiff in John Gardners book, so I am using his building description to help out. This is my first boat project, although I am a fairly experienced woodworker. I will be constructing it with plywood as Gardner recommends, but will use the glued lapstrake method to join the planks instead of the glass tape Gardner shows.

One thing that mystifies me is how a solid wood transom can be joined to the plywood sides without wood movement causing problems. I have both Ian Oughtreds book, and the John Brooks lapstrake book, and they both show boats where a solid transom with grain running horizontal is epoxied directly to the plywood sides. Since the transom should change size (vertically) with moisture changes, and the plywood should not, how can this work without something giving?

Also, Gardner recommends 3/8" fir ply, but there have been some recent posts suggesting that 1/4" ply would be adequate. My local lumberyard carries both 1/4" and 3/8" inch hydrotech brand marine plywood so I can use either. Any thoughts on that? I am thinking I could use the 3/8" for the garboards and the 1/4" for the upper strakes. That way, the weight is kept lower, and the extra strength will be useful when pulling the boat up onto the beach. I am planning on making the bottom out of cypress, but will laminate it using veneers running alternate directions.

[Denman James]

That's a good question, and I can't say that I personally have a definitive answer, being somewhat of a neophyte builder myself.

I did attend a great workshop at a WoodenBoat show two years back that talked about the differences between modern epoxy based building and traditional building.

The explanation was basically that in tradtional building, the wood is expected to swell and move, and the design needs to accomidate that. In modern methods, the wood needs to be fully encapsulated to avoid wood movement, since the stresses developed would be "fatal" to joints and the structure. The presenter then went on to say that either method was fine, but MIXING the two methods was pretty much verboten. I think that the question you raise implies that the whole boat would need to be encapsulated to avoid water absorption, and thus avoid the problem you mention of the transom growing on the planking.

[Denman James]

That's a good question, and I can't say that I personally have a definitive answer, being somewhat of a neophyte builder myself.

I did attend a great workshop at a WoodenBoat show two years back that talked about the differences between modern epoxy based building and traditional building.

The explanation was basically that in tradtional building, the wood is expected to swell and move, and the design needs to accomidate that. In modern methods, the wood needs to be fully encapsulated to avoid wood movement, since the stresses developed would be "fatal" to joints and the structure. The presenter then went on to say that either method was fine, but MIXING the two methods was pretty much verboten. I think that the question you raise implies that the whole boat would need to be encapsulated to avoid water absorption, and thus avoid the problem you mention of the transom growing on the planking.

[DJM]

Thanks for the responses.

I wasn't suggesting mixing strake types, but I know Gardner reccomends doing so for one of his dories, can't remember which one. He specs plywood for the garboard, and solid wood for the upper strakes. This should work, because there is little change in length of either ply or solid wood.

It seems like a solid wood transom might move differently from solid wood strakes also, but maybe the fastening of the strakes to each other have enough give to accomodate the move differential?

I had another look at Ian Oughtreds book last night, and although he does not address the question directly, he has a couple of pictures that clearly show a solid ( probably mahogony ) transom, with plywood strakes being glued to it.

I am sure many members have built Ians designs, can we hear from someone who has built using a solid transom and ply strakes?

Dave Mundy

[Bob Smalser]

Use a real stable wood in quartersawn flavor and you won't have a problem with a small transom.

The key is stability, and that's why H. Mahog combined with q-sawing usually get the nod. That, and not allowing the surface coatings to deteriorate.

In turn, flatsawn Doug Fir, Yellow Pine, or something else that moves a whole lot seasonally, will likely result in a cracked transom....and sometimes with stable cedar planks, let alone plywood.

That said, if the transom boards are drifted together like the boatbuilding gods intended, then cracks are no big deal in a painted transom, either, as the drifts hold everything in place....just add some cotton and putty to keep out the water and row on.

[ 09-16-2005, 01:05 PM: Message edited by: Bob Smalser ]

[DJM]

Thanks Bob,
I am making my transom out of quartersawn Mahogany,
but I haven't drifted it. I am sure there must be something in WB building and repair on how to do that. Too bad I have so much trouble with the search. It seems to me that the search used to work pretty good.

Dave.

[Bob Smalser]

Originally posted by DJM:
Thanks Bob,
...but I haven't drifted it. I am sure there must be something in WB building and repair on how to do that. Too bad I have so much trouble with the search. It seems to me that the search used to work pretty good.

Dave.Not a transom, but the same techniques:

http://www.woodenboat-ubb.com/c gi-bin/UBB/ultimatebb.cgi?ubb=get_topic;f=1;t=010983 (http://www.woodenboat-ubb.com/cgi-bin/UBB/ultimatebb.cgi?ubb=get_topic;f=1;t=010983)

Use silicon bronze, not brass....brass isn't very stiff in comparison. Plus it can corrode.

You don't have to pin the boards together for edge gluing like I did - mahog won't move around like oak will. Just lay them up, alternating any cups if riftsawn, and glue them with epoxy or resorcinol.

Drill and drive the drifts after the glue cures, bedding the drifts in lead or copper paint which also acts as a lube. Make sure the holes are sized fairly tight (but not as tight as some books call for or you'll split the wood) and notching the drifts with a chisel helps prevent them from withdrawing with seasonal movement. And knock off the sharp corners of the end going in.

Hide the drift ends if you like with mahogany plugs just like you would with screw countersinks.

The traditional way of doing it used white lead between instead of glue, but only because they didn't have good waterproof glue...and those old boats often gapped some over time, requiring more white lead and squeezing the assembly back together every few years.

And I'm not trying to criticize Gardner or Chappelle, but driving 3/4"+ galvanized steel drifts into 8/4 stock is a very different critter than driving 3/8" bronze drifts into 4/4 stock. Using a 3/8" bellhanger bit for a 3/8" Alaska Copper or McMaster Carr bronze drift will work just fine. Driving a 3/8" rod into a 5/16" hole in soft, 4/4 stock will likely split it.

[ 09-16-2005, 06:34 PM: Message edited by: Bob Smalser ]

[L.W. Baxter]

Just to be clear, the drifting is only in case you can't find a wide enough board to do it in one piece, right? I don't know the Marblehead dory skiff, but I imagine it has a fairly small, knuckled transom?

If you're going to use honduras mahogany, you might find a big piece that will do your transom and thwarts and so forth, all nicely matched.

[Bob Smalser]

Wide, single boards are usually the first ones to split, and are always weaker and more prone to warp than an alternating cup layup. Doable, but riskier.

Even when I have wide stock, I use three boards for a small transom.

If I used a single board, I'd certainly drift it so as to both stiffen it and retain its structural integrity should it split some day.

And when I say alternate the cups, that applies to riftsawn wood, too where feasible. It isn't hypercritical like it is with flatsawn stock, but mixing up the grain makes the layup stronger and less prone to warp:

http://pic3.picturetrail.com/VOL12/1104763/6791366/85653808.jpg

[ 09-16-2005, 07:20 PM: Message edited by: Bob Smalser ]

[dmede]

You mentioned H.Mahog, how does White Oak do? And what about P. Mahog?

Are the drifts threaded or smooth?

Once the Fiddelhead is done I will be facing this same issue with one of Atkins small outboard skiffs.

dave

[Bob Smalser]

Originally posted by dmede:
...how does White Oak do? And what about P. Mahog?

Are the drifts threaded or smooth?

Real awful, close to awful, and smooth.

I'd spring for H. Mahog with plywood planks...or just make a plywood transom.

Plywood doesn't move at all to speak of, and with glued laps you have zero joint flex compared to rivets. The only risk-free woods over an expanse much more than 12" are VG cedar or VG H. Mahog. I wouldn't trust lumberyard P. Mahog for much of anything unless I wuz sure of the species.

[ 09-16-2005, 07:31 PM: Message edited by: Bob Smalser ]

[Dave Fleming]

I'm gonna put a pennies worth of comments in here now.

As Bob has said drifts are the best way of keeping the transom boards together along with adhesives.

Drifts should be tight BUT not so tight that they will possibly split the boards.

'barbing' the drifts with a cold chisel and drilling off vertical holes for them are two well known techniques. With the drifts off vertical or off right angles to the joints there will be less tendency for the boards to come apart. The barbs well, that should be self explanatory.

Use good wood, True Mahagony, is fine.
Don't forget the transom holds the boat together at the aft end.

[Cosmo Lengro]

;)

[DJM]

Wow,
Thanks for all the great information! So, to sum it up, the way the solid wood transom works with glued lap is to use a real stable wood to minimize movement, and further stabilize it by drifting bronze pins through it.

Bob, that Gunning Dory looks fantastic. I am sure I will be referrng to your posts more to see what will apply to the Marblehead Dory Skiff.

Dave

[yorgie]

Sorry to piggyback onto this thread but it's been informative for me.

I might be building a red cedar lapstrake sailing dingy this winter.The transom is fairly broad and I can mill my own douglas-fir and yellow cedar.Which of these two woods would you recomend or should I spring for the mahogany?

Thanks,Chris