Fiberglass tape size for a plywood rowing shell

Discussion in 'Boat Design' started by waterbear, Jan 31, 2022.

  1. waterbear
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    waterbear Senior Member

    I have plans for an open water rowing shell called "Dawn Treader" by Joe Dobler. Dobler is long gone so I unfortunately can't ask him questions. It's unclear if the plans are complete or not, however there is no included information on sheathing the boat or taping the seams. It is specified that the chines are taped but that is all.

    Here are the specs:

    20ft x 30in
    4mm plywood chined hull
    1/16th inch plywood deck
    bulkheads every approximately 26 inches
    1/2 inch spruce stringer at hull-deck interface

    My question is what should I tape and sheath this with? Should there be a fillet and if so, how large? What about attaching the bulkheads to the hull?

    Comments and suggestions are greatly appreciated.

    Last edited: Feb 1, 2022
  2. Rumars
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    Rumars Senior Member

    Resin = epoxy, fillets = structural filler, radius = whatever makes the fiberglass lay smooth over the corner, tape = 6 or 9oz, sheat = 6oz
    Look up similar constructions with 4mm ply, for example from CLC for guidance.
    kerosene, Ilan Voyager and fallguy like this.
  3. Tops
    Joined: Aug 2021
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    Tops Senior Member

    Looks like a fun project!
    Agree that CLC methods are good guidance.
    Also recommend NOT to finish raw wood or ply exclusively with epoxy, use fiberglass cloth on the the plywood and varnish or paint all surfaces for added UV protection. I have a ply boat that need serious re-work because I did not use enough cloth and paint.
  4. messabout
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    messabout Senior Member

    I like it. The boat is a sensible compromise between an all out rowing shell and a more friendly boat such as an Adirondike or 'gentleman's" rowing boat. That it uses the trapeze bottom makes construction much less labor intensive than a stripper. Very little more wetted surface than a stripper and also just a bit more stable than a fully round chine type.

    I would use a single layer of six ounce tape on the outside of the intersections. After some careful and laborious feathering of that tape I would reckon to use a whole bottom covering, up to the second chine, of 4 ounce or even 6 ounce glass-epoxy. Inside, the radius of the fillets will vary with the angles of the parts. Obviously the forward sections will have a thinner fillet than the ones near mid length. In either case I would like to have a minimum of one inch fillet of diminishingly thinner epoxy goop, spread on the upper and lower parts of the seams. the mid part, or the fillet that is deepest, will vary with the angle of the joining parts. The fillets followed by a covering layer of 6 ounce glass tape.
    That is one of the more tedious parts of the build. You need to have the tape in place before the fillet goo begins to fully gel. It would be most helpful to use Peel-ply on both the inside and outside where the joint work is being done.

    For joint work of this type you can use ordinary packaging tape to take the place of commercially sold Peel-ply. It is the tape that is commonly used for cardboard boxes. 3M calls it packaging tape. There are other brands. It is made of some sort of plastic, usually transparent, and sticky on one side. It is reasonably cheap, conveniently two inches in width, and comes on rolls. To defeat the sticky side use talcum powder. Put the non sticky side down on the gooey glass and use a soft squeegee to press the tape into place as well as to remove excess resin from the green laminate. If you have not used this or a similar method before, then I urge you to experiment on some test strips of scrap before doing the boat itself. If this is not a familiar routine, then also experiment with some filleting. That can be kind of tricky. Use some scrapers like a credit card or thin plastic with a rounded corner. Leaning the card or boards at various degrees will produce different radii of the fillet. When cured the epoxy will release the packaging tape with no problem. You then get a smooth finish that will save lots and lots of sanding.

    Go for it. I really like the boat.
  5. waterbear
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    waterbear Senior Member

    Rumars & Tops, thanks for the CLC suggestion. I looked at the CLC oxford mk1 shell and it has 9oz x what looks like 3inch tape on the inside, no tape on the outside and is sheathed in 4oz cloth. No tape on the deck joint or glass on the deck. One builder reduced tape to 6oz and sheathing to 3.25oz with CLC's blessing. There is a newer oxford shell mk2 but I couldn't find any details on it's construction.

    It's worth noting the oxford shell is much narrower than the dawn treader and is much less complex with just two bulkheads and far fewer structural elements.

    Tops, this boat will probably be built in Joubert okoume and stored inside so I'm not worried about checking.

    Messabout, thanks for all the suggestions. I will try your packing tape peel ply method. Last time I built a boat I did a lot of weave filling and that was no fun.
  6. wet feet
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    wet feet Senior Member

    Unless the water in the region where the boat will be used is full of floating debris I don't see any real purpose in glassing the whole of the hull.If the designer had thought it necessary,he would have stipulated the amount to be applied somewhere on the plans.This might put me at odds with the bulk of the other posters on this thread but I have reasons and a little history on my side.Weight seems to be ultra critical on rowing shells which seems to be the driving force behind so much carbon finding it's way into them and keeping the weight down will be appreciated when carrying the boat to the water.I have seen quite a few rowing shells carried and the rowers seem to carry them overhead,with the boat inverted to be absolutely certain that no water is within.Given the very fine entry of the hull you are unlikely to hit any debris square to the hull skin and the glass tape at the stem will give a modest amount of protection.

    As the OP has noted,filling the weave and fairing the surface is no fun and it does nothing to keep the weight down.I would point out that around 70,000 Mirror dinghies have been built (mostly by amateurs) with just 2 inch tape on the inside and outside of the joints and these boats are subject to sailing loads as well as enduring a less pampered life than a dry stored rowing shell.I would encourage using epoxy instead of the polyester that the Mirrors used and I would suggest adding a bevel on the outside of the hull only and extending 1 1/2" from the edge of the panels and about 1mm deep at the chines.This will place most of the tape below the panel surface and if a small amount is sanded off as it fades into the panel,the whole fairing job becomes much easier.I think the builder should proceed as he sees fit and after all,sanding filler gives a form of exercise too,just not in such scenic surroundings as the rowing.
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  7. fallguy
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    fallguy Senior Member

    I'd glass that with 6 oz tapes inside only and 6 oz glass. If the bottom is going to see abusive wear, sand, rocks, oysters, I'd go to 9 ounce or double the 6 to over the 1st chine; likely double 6 for economics...some depends on if u want super light; then I'd drop to 4 oz and double the 4 on the bottom

    I'd probably skip exterior tapes and lean toward doing the entire boat in 6 oz, personally.

    After glass, weave fill at 2 oz epoxy per yard, plus 4 oz roller loss allowance, 2 coats should do it..

  8. waterbear
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    waterbear Senior Member

    Wet feet, indeed keeping the boat light is critical. I won't be racing, but at the same time I don't want to ruin an elegant design like this by overbuilding. Dobler was an aeronautical engineer at North American Aviation (boat design was just a hobby), so I imagine he was pretty savvy at designing light weight structures. Also, I agree "keeping the weight down will be appreciated when carrying the boat to the water." As a side benefit, my ego will be happy when I can brag about how light my boat is, since I certainly will not be able to brag about my rowing pace.

    Btw, even in this 40 year old design there is some carbon fiber, you can see it at the top of the drawing near the corners of the cockpit. This is specified as "graphite fibers about 2in x 24in," again with no weight mentioned.

    This is an excellent point. I did a little digging and found the mirror dinghy utilizes 3/16 plywood, and according to some random forumites elsewhere it uses just 38 or 40mm tape, so even narrower than 2 inches.

    Something jogged my memory and I looked up Dave Carnell's old site in the wayback machine (Epoxy Knowhow Dave did some empirical testing and found that a butt joint of one layer of 6oz tape inside and out on 1/4 inch plywood fails at the plywood, not the glass. He also states "For angle joints such as chines in -1/4" plywood a 1-1/2" fiberglass strip laid over a -1/4" radius epoxy fillet on the inside and a 1-1/2" strip on the rounded outside edge gives a joint that breaks by pulling the plywood apart." It's likely he was using fir and these results may not hold for something like meranti or birch.

    Fallguy, thanks for the recommendations. This seems in line with other stuff I am reading. I'm not sure if I will sheathe the outside or just tape it. If I do sheath (sheathe?) it I will do either 4oz or 6oz cloth and avoid abrading the bottom. I really want to avoid the paranoid "make it last forever" build mentality that leads to overbuilt, overweight boats.

    I think at this point it's pretty clear that a 1.5 inch strip of 6oz tape inside and out is more than adequate for 4mm plywood. If we extrapolate Dave's results, it seems even a 1 inch strip of 4oz tape inside and out could be enough for a boat like dawn treader. Of course doing so would only save a pound or two....
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