Herreshoff Marco Polo Tender Construction Detail

I am considering building the boat originally designed as the tender for the Herreshoff Marco Polo. I'm thinking this would be nice for rowing and sailing on the Connecticut River near where I live. The plans are viewable here:

http://www.duckworksmagazine.com/11/columns/austin/05/

This is basically a multi-chine pram of dory-type construction; with two lengthwise bottom strakes, with the seam on the centerline covered by a hardwood false keel.

The plans call for both the forward and the aft transoms to be braced with a knee at the centerline. Aft there is also a skeg attached to the false keel. It seems that the fastenings for the knees would go right through the seam between the bottom strakes, and this would not be good structurally.

The solutions I can think of are 1.) running screws through the seam and calling it good 2.) fastening the skeg to the false keel with screws from the top surface of the false keel into the skeg (so the heads of the screws would be covered by the bottom strakes; then fastening both knees to the false keel with screws running at an angle from vertical from the edges of the false keel, through the bottom strakes and into the knees. 3.) Adding a wider, flat piece of wood under each knee (which would add fastenings and complication).

Also, does anyone have suggestions for making/obtaining knees? (There are sawmills near me.) Do most people just cut them out of straight-grained wood, or else laminate them up? (I'm thinking that if I made laminated knees, it would be easy to make them extra wide at the bottom, and that could be a solution to the above problem.)

Thanks in advance,

Debbie

 

Debbie,

That skeg should not take much lateral force from anything and is tight in enough not likley to be struck. It would probably only require 3 screws maybe 3/4 inch going through the plywood at the seam (assuming 1/4 inch plwood construction). The hull is flat where the two planks come together, so with epoxy in the seam and fiberglass tape then the joint will be plenty strong to hold that skeg on with a few screws. You should actually epoxy the skeg on anyway so the screws are really just acting as clamps, the epoxy is actually stronger than the screws. You could remove the srews once the epoxy sets. I've always tried to remove screws as much as possible, light craft built with expoxy/glass over the wood don't need them. On larger boats I would only use bronze screws where they are to be left in. If you add the extra strength of a 'plate' under each knee that will give you more thickness to screw through, but I don't think that would be needed on this design but its not a bad idea.

You can absolutley laminate your knees from a few layers of plywood, very easy to cut out with a jig saw or small band saw. If you are painting the boat then no one will ever see the end grain of any of the plywood. You could also use hardwood from a lumber mill and make them solid, especially if you want to varnish those parts. Because those knees are relativley small you stand a good chance of getting 'cut offs' or scrap blocks of hardwood that would otherwise be much more expensive.

Good luck.

 

plywood or planking

Hi CloudDiver,

I was actually thinking in terms of what I would do if building her as designed, with all sawn planks (although I'm open to ideas).

If I was planking her with plywood, I would probably just use one wide piece for the bottom and eliminate the seam problem. (I think a seam there in plywood would be less tight than one of regular planking, so fiberglassing would be called for.

Note that the plans call for different thicknesses for the different strakes. I guess that could be done using plywood also.

One place where I *did* think plywood would be good, would be on the second floor-board, with its cutouts for bracing the feet when rowing. (The plans call for reinforcing cleats to prevent splitting.)

You wrote,

Right, epoxy along the entire joint would be stronger than a few screws, *except* for the inherent structural weakness of any adhesive joint that only works at the surface. But, yes, with adhesives doing much of the work, one can use metal fastenings sparingly.

My idea with that was just to allow fastening with screws (or through bolts) withough going through the center line.

That's an idea I didn't have, knees laminated sideways from plywood. I was thinking of knees laminated from thin strips of wood as wide as I wanted the knees to be, steam-bent into the right bend then epoxied together and trimmed. A problem would be that screws fastening them to the frames would go through the seams between laminations. (With your idea, screws fastening them to the thwarts would go into the end grain of the plywood.

Yes, BTW, a trick I've used on cabinets to hide the end grain of plywood is to apply a coat of paint, then dust with coarse sawdust and sand down before applying another coat of paint. Fills in the the rough grain very well.

I've done that, except that the grain runs straight. The original idea of hackmatack knees was that they were cut from stumps and had the grain following the bend.

I'm thinking that perhaps scraps that were scrapped in the first place because of bent grain (as from branches) would be ideal -- fi I could find them at a sawmill or elsewhere.

Greetings,

Debbie

 

Actually, despite what that text says, I'll bet there were only 7 panels, not 8. If you look at the section plan, it shows the bottom is single piece with a rubbing strake (that is the CL line, not a plank line as it would have been impossible to build a seam like that and notice that there is only one "circle 1" piece number), which would be typical for this type of construction at that time for flat prams and dorys. If the bottom had been V'd it would have had a keelson, but dead flat like that is a single piece.

See this thread at the WoodenBoat forum.

http://forum.woodenboat.com/showthread.php?146109-Building-a-Marco-Polo-Tender

 

2 or 1 ?

Hi jehardiman,

Thanks for your reply.

You wrote,

The directions in Sensible Cruising Designs (Herreshoff, 1973) do say, "...1. Bottom planks....Each plank about 10" wide with seam on centerline..." The table of offsets says that at it's greatest breadth, the bottom is actually 27" wide. I am guessing that Herreshoff did not expect builders to be able to obtain such wide stock. Obviously, if wide enough planking is available, there is no reason to use narrower strakes with a seam. "Geoff C" at the Wooden Boat Forum did build her without a seam on the bottom panel, but he was using plywood. (He did need to scarf all the planks to get sufficient length.)

With plywood, Yes, it would be very hard to make a seam like that watertight (without fiberglass and resin), but again, with plywood there would be no reason for a seam. With the material specified (7/8" pine or cedar) it would be pretty standard batten-seam construction. I think the seam would be pretty leak-proof as long as you didn't start wedging it open, which is what a centerline screw would do. I don't understand how the knees and the skeg were supposed to be fastened when Herreshoff first designed this back in the 20th century!

Greetings,

Debbie

 

Yes a glued full-batten seam would work, or a lap seam (felted and tarred) for 10" stock <shrug>. It would seem strange to me that "juniper" (atlantic white "cedar") or cypress could not be found in suitable widths. There a lot of "down east" Carolina and Core/Albemarle Sound flaties single planked with stock that wide up until the 1950's.

I would have built the original Nordic style, so right side up with the bottom curve forced by shores and braces. This makes perfect sense given the canoe type cinch nailing shown and the lapped frames. The skeg would most likely not be screwed, but use a copper "drift bolt". This is a copper rod drifted through both the knee and skeg and then peened over a washer. Many examples in Chapelle's Boatbuilding and easy to install if the hull is built upright. The stern and bow knees along with the skeg would have be the first things installed.

Edit: here is what I mean

http://www.northhouse.org/courses/courses/course.cfm/cid/21

 

Knees are fine out of solid, usually at 45 deg or the most suitable angle. I note the stem one is probably around 10 or 15 deg. Laminating them is just fine. If you use fasteners as well as glue to hold them, try and do it from the outside to inside. This is stronger in practice than the other way round, especially for the four 'corner' knees ie transom to gunwhale/inwhale. To be very neat you can laminate the outer gunwhale strip over the screws, rather than just core and plug. Same across the transom, a small strip across the top, aft side say 3mm thick. You need to plan the build order though.

As jehardiman says, these would have oringinally been fastened by copper or bronze likely square shank rovings peened over a washer or formed with a die and matching steel form. I have not long removed that type from a 1960s' composite!. This side of the Pond, we tend to call this clenched roving, similar to your cinch term.

My instinct for the skeg would be to have it be quite well tapered in cross section, so it is relatively wide where it lands on the underside of the keel. If you use epoxy and a fillet, I doubt you would need much more. This taper would give you the option to screw from the inside (not on CL) to the side of the transom knee and small bedlog type strips supporting the knee or epoxy fillets could cover the heads. Hope that makes sense!.

The 7/8" (22mm) stock seems very thick, oversized and hard to bend. She does not require more than 5 or 6mm ply for the hull plus support battens, certainly if 8 to 9mm (just under 3/8") it would be pretty bulletproof. My own take, if you want the solid wood look, would probably be to double diagonal the bottom, maybe 3 layers to a max of 15mm (5/8"), but that is to take all the curvature stress out of it. If you can heat bend the thicker stock, you may get away with jointing it fore/aft especially with biscuits or dowels. Maybe even T and G with a router, always fun on less tan perfect radii...

If you do use ply, it is good practice to cap with solid if possible. It generally helps prevent water ingress, looks neater and lasts longer. Even a small ie 2-3mm (1/8" or less) cap is sufficient

 

I wouldn't trust anything that Paul Austin has to say about anything boat related. It's not that he's a lair or anything, he just doesn't appear to have any technical or practical knowledge of design, maritime history or construction. He's a writer and has a good style too, but I also believe he's simply an avid reader and uses his new found "understandings", to generate texts that he really doesn't fully grasp. Simply, I've read enough of his stuff to avoid reading anything else at all from him, mostly because it's always full of incorrect assumptions, flaws, inaccuracies, etc., so just not worth the bother. Personally, I find it insulting he does this, attempting to push himself off as some sort of expert, on subjects that he clearly doesn't comprehend, at least in regard to someone that does. Maybe there's a place for him with novices, but little else.

As to the Polo tender, it can be handled several ways and John is correct it's a plank keel design originally, not a V bottom. This plank keel concept is often doubled up, to offer stiffness particularly in sailing hulls. I've just completed a planked keel lapstrake. The bottom plank(s) were two 1/2" lengths of plywood.

 

If you'd like to see a sailing lapstrake which does move try this link. There's a really good demonstration of ragging the main to keep the kite flying some distance in (I think about 17 min). I enjoy sailing these beasts occassionally, a lot more some years back.

http://www.youtube.com/watch?feature=player_embedded&v=EzKo8OWpUZc

That was in the tail end of Bertha btw.

 

FWIW, WoodenBoat has an article this issue (Sept/Oct) on building an Amesbury skiff. The flat bottom would be similar to the Marco Polo pram.