Thwart Knees

Each of the thwarts have “knees” – L-shaped pieces that help to secure the thwart to the hull.  Some builders don’t feel that thwart knees are necessary in glued-lapstrake construction, arguing that the cleats should provide plenty of support.  But they are shown on the drawings for Tammie Norrie, and I’ll use them.

The thwart knees are shown on the plans to be sawn from lumber.  This is a fairly simple and certainly very functional method of making knees.  But we can fix that.  Rule Number 3 says “Simpler is best, except when complicated looks really cool.”  Thwart knees are one of those details that set a traditional boat apart from their modern counterparts, so they deserve a little attention.  One of my reference books is “Building Classic Small Craft” by the late John Gardner.  In one of the chapters Gardner discusses making thwart knees from the crook or branch of a tree.  If a crook wasn’t available, the knees could be laminated.  I’ve got a nice live oak tree in the front yard which has a number of branches that would make some fine knees.  But getting the wife’s permission to cut down the tree for use as boat parts ain’t gonna happen, and even if so, I’m not patient enough to air-dry the lumber.  So that leaves me with laminating the knees.

Rule 3 notwithstanding, making laminated knees isn’t really that complicated, but it is time consuming.  Figure on at least three weekends making a pair of knees for both of the thwarts.  But a lot of that time is letting stuff dry, so you can be working on other projects in the meantime.

Start off by determining the angle of the knees.  I used a scrap block held against the inside edge of the sheerstrake, just below the inwale, and picked off the angle with a bevel gage.

2018 01 27 Picking up the angle.

The angle should be the same for the port and starboard knee on the same thwart, but will be different for the knees on the forward thwart than those for the aft thwart.

2018 01 27 Laying out the pattern.

With the angles determined, lay out the pattern for the knee on some scrap plywood, cut to shape, and test fit on the boat.

2018 01 27 Plywood knee template.

One leg of the knee will be screwed to the thwart, the other leg will be screwed to a block that fits under the inwale and is glued to the sheerstrake.  The knee will be molded 5/8″, and when installed will be flush with the inner edge of the inwale.

The knees are going to be laminated from five strips of 1/8″ thick ash.  Ash was chosen because it will take a bend (with some help) and will be plenty strong.  When varnished, it will also contrast nicely with the cherry I used to make the thwarts.  Use the plywood pattern to make a male and female mold from not-cheap lumber.  Once the mold is made, the strips can be pre-bent prior to epoxying. If you try to bend the strips dry, they will probably crack on you, so I soaked the strips in hot water for about an hour and then drove the strips into the molds.

2018 01 27 Drying the strips.

Let the strips dry for a week.  When removed from the molds, the strips will have a “memory” that will make it much easier to epoxy.  While waiting, make the blocks that will be glued to the sheerstrake.  I put a curved edge on the bottom of them just because it looks nicer than a square edge.

2018 02 04 Hull blocks.
2018 02 04 Strips being “buttered up” with epoxy.

The photo above shows the strips for two knees getting epoxied.  The epoxy is messy and slippery, so having the strips pre-bent to fit the mold makes this operation a lot easier.

2018 02 04 Epoxy curing in the molds.

Finally the epoxied strips are placed back into the mold, and set aside to dry.  At this time you’re also glad that you remembered to seal the edges of the molds with either clear plastic tape or waxed paper.

2018 02 10 Knees removed from molds.

I let the epoxy cure for a week before removing the knees from the molds.  I wanted to make absolutely sure that the epoxy was completely cured.  After being removed from the molds, go over the knees with a rasp and coarse sandpaper to remove any globs of epoxy, and trim the ends of the knees to final dimensions.

2018 02 10 Gluing on mounting blocks on sheerstrake.

With the knees cut to their final shape, test fit and determine the postion of the mounting blocks, then epoxy these onto the hull.  Let cure overnight.

2018 02 11 Knees installed on forward thwart
2018 02 11 Knees mounted on aft thwart.

The knees are secured with silicon bronze screws; no glue.  I want to be able to remove them in the future.  Right now the knees are bare wood, as are the thwarts.  Both will get six coats of varnish later.  The lower photo also shows that I’ve got some not-cheap wood laid out for the side benches; that’s the next project.

 

3 thoughts on “Thwart Knees”

  1. Well, at least you can work in the garage…. I would have to be outside on the yard deck… 33 deg out there rat now, and I don’t care to work as in Nome, AK…..Bill Beach just told me it is snowing at Lake Palestine. Looking good my man, spring is coming so I hope you can have her ready for Lake Striker……

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    1. I hear ya, Bill, the space heater in the garage felt really good this morning. Seems like it’s been a cold and wet winter this year.
      I suspect I’ve got another year before Tammie Norrie is ready to splash, but Wee Lass is ready for a trip to Striker when we get some good weather.

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      1. Sounds good Al. we had 30 this morning and suppose to be 29 or so in the morning. Brrrrrrr. My Kerosun heater in the loft is getting a workout. And kerosene at 2.50 a gallon ! When I was a kid it was .20 cents a gallon….. lolol Thinking on selling my Sparrow 16 because I just don’t have time to use it. Making too many sails for Duckworks.

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