Transom Construction Question

[Scott Rosen]

I'm contemplating the Catspaw Dinghy for my next project. Instead of study plans, I got a copy of the How to Build the Catspaw Dinghy book from WB.

The plans call for the transom to be constructed of three mohogany planks, edge fastened with splines and glue. It's a flat transom (i.e., not radiused), so there's no bending and the plans don't call for a transom frame. So far so good. It should be easy to shape the transom from good flat stock without a frame.

At the hood ends, the planks are lapped over the transom and fastened with wood screws through the planks into the end grain of the transom. That could be a weak link, don't you think? It seems to me that fasteners into the end grain are not going to hold very well. There is a moderate amount of curve to the hull, so there will be tension on those joints, pulling the planks away from the transom. With the sailing rig, there will be additional twisting forces that will bear on the transom to plank joint.

Chapelle says that small boat transoms should have frames, and that the hood ends should be fastened to the frame and then additional fasteners into the transom. That makes sense to me and seems like good practice.

On the other hand, Joel White is a good designer and must have known what he was doing when he created the construction plans.

What do you think?

[WFK]

Scott;

It seems now with glued lap construction that coupled with epoxy, it'll be plenty strong, however, that being said, when you flip your boat over you could easily add a frame or listing in each corner for added strength. I think If I was going to use an outboard, I'd add not only this but a piece as well where the transom meets the bottom.

Bill

[Art Read]

Scott... The raked, flat transom on my project, (quite small actually...) is one solid piece of mahagony. The plans drawing doesn't show ANY framing other than a transom knee. For the same reasons you cited, I added "fashion pieces" on both sides and along the deck edge as well. Offers a lot more "meat" for fastening the hood ends and the decking, rubrail, etc... The weight added is negligable but it was VERY "interesting" trying to bend in those side pieces. Some pretty odd geometry going on there. Might be why they were omitted on the plan in the first place? After busting my third attempt at steam bending 'em in, I finally gave in an laminated some up. They're the only laminations on the whole boat.

This shot shows the laminations being beveled in place on the mould before the transom went on...

http://www.imagestation.com/picture/sraid16/p8e95a1ef780339b7be3a4d560073089a/fdef50b4.jpg

(I take that back... The plans DO show the framing. Just on the profile view though. That's why I wound up with all half inch stock for my keel bolts instead of the mostly three quarter that I really needed... Relying on memory! ;) )

[ 10-19-2002, 12:47 PM: Message edited by: Art Read ]

[cherokee]

scott- i'll add that in small boats if the transom is hardwood like w.oak or jatoba or such that if the pilot holes are sized properly screws hold very well.i comptemplated using jatoba left over from my folkboat restoration for the transom on the acorn skiff i built for a patron. i was planning to rout away large nicely shaped sections of the inner face to 5/16" from the 13/16" to cut weight. i then found well priced spanish cedar for the boat and used it instead for all the trim and transom. but it IS glued and nailed. i think if i was reliying souly on the fastener and the plank lands with a lot of bend and spring it'd be superior to have the screws in cross grain.it seems to me a screw in tension set in end grain will break down the wood fibre over time, and hull racking won't help. i'm not familiar with the white catspaw-sounds like it is modified to glued lap. if so go light and save weight.for what it's worth,in john leather's clinker boatbuilding he nails his transom laps into what looks to be a hond.mahog transom which is only med hard at best. hope this helps....the catspaw is sure a beautiful choice for a boat project, have fun with it! john

[Bruce Taylor]

WFK -- Catspaw is usually built in carvel, not glued lap.

Scott -- Glancing at the plans, I see no reason not to fix a frame to the transom, if that would give you peace of mind. You'd have to notch the stern knee around it, of course, and I presume the frame would have to be bevelled along with the transom. I'd be inclined to bend an oversized frame flat to the transom's inside face, bed it, temporarily affix it with screws, and then bevel it.

However...if I were building, I'd probably trust Joel White's design.

[ 10-19-2002, 04:10 PM: Message edited by: Bruce Taylor ]

[holzbt]

Scott,
There is not as much strain on those hood ends as you are imagining. Remember that the transom is held to the keel with a transom knee and at the sheer with quarter knees.The transom is less likely to split without the frame as the transom and planking are swelling in the same direction. Use long thin screws as opposed to shorter fatter ones when fastening into end grain and they will hold fine.

[Jack Heinlen]

Scott,

I agree with holbzt. While it could cause problems restraining the transom across its width, I just think the added framing isn't necessary in such a small boat. The quarter knees add a great deal of stiffness.

I wonder about the use of splines in the glue up. Anyone else think they are un-necessary?

Jack

[holzbt]

The splines aid in lining things up during the gluing and they are nice to have if the glue eventually fails. A lot more water can leak through an open seam than through a splined one. Of course if your glue joint is perfect and the glue never fails you don't need the spline.

[Jack Heinlen]

Hi holzbt,

I see your point about a spline keeping the water out of a broken glue joint, but might it also make a pocket for water to collect in? I'm not that familiar with current practice in small boat construction, but most cabinetmakers don't use a spline in panel construction. I guess a few buscuits have become popular for alignment in some cases. It has been awhile since I've been in the shop.

I would think a well made joint, glued with epoxy--being careful not to starve the joint--would be darn near bulletproof in either case.

Wonder what Greg Roessel recommends? Any other pros care to chime in?

Jack

[Thad]

Being planked carvel (thicker planks) screws through the hood ends can be angled into the transom without cutting through the plank with your countersink. However the angle of attack from the keel through the turn of the bilge is such that a transom frame would help a lot in fastening with confidence. I have not had problems with my Columbia lifeboat dinghys without frames but I would recommend them although Nat Herreshoff obviously didn't think them necessary.

[ 10-20-2002, 05:33 AM: Message edited by: Thad ]

[On Vacation]

We normally have put a rib -frame in small craft and regular planked skiffs against the transom that is screwed in place to transom and the planking is screwed to it and the transom boards end grain.

[ 10-20-2002, 09:44 PM: Message edited by: oyster ]

[J. Dillon]

Scott,

If it was up to me I'm make her strong as possible considering a boat is not a static thing like a piece of furniture. Boats are subjuct to all sorts of strains, racking, torsion :( compression etc., as well as what the elements throw at you... Add the pieces and sleep better, :D What's another pound of two. ;) Your not going to carry it. It's like the security a second anchor in a blow gives you. smile.gif

JD

JD