Tweak dinghy build notes - my first boat!


My first boat build

B. Mathews

7/4/19 - 7/31/19

Greensboro, North Carolina, USA

First time in water 7/30/19

Background

I’ve thought about building a boat off and on for a very long time …

I grew up on the water in Florida, lots of puttering around in rowboats and power boats. Water skiing, surfing, canoeing etc. I did a lot of windsurfing in the late 80’s and 90’s. But I never tried building a boat.

We moved to Greensboro North Carolina in March 2019 and they have several nice lakes very nearby. At one of the lakes they offer sailboat rentals for $15 so in May I tried out one of their boats, an Aqua Finn, similar to a Sunfish. I had a great time and did it again in June and also signed up for the Learn to Sail class in July. I should mention that I am retired so I have plenty of free time.

Please understand that I am a hack woodworker. I’ve never done any fine woodworking. I’ve never made a miter joint or a dovetail. I have used deck screws to knock something together. I can operate a hand saw, a circular saw, an electric drill, and a hammer. My only goal with this project is to make something functional. I am not at all concerned with how it looks. This will be built to a “sub-workboat” finish.

I scanned many websites about boat building trying to decide which one I should try. I downloaded some free plans. Good drawings but not a lot of instructions.

I decided to pay for some plans assuming they’d come with complete instructions. There are many websites with boat plans for sale, but I have found that I really like www.duckworks.com.

What to build? Canoe? Kayak? Pram? Dinghy? Sailboat?

My criteria:

The “Tweak” dinghy design by Alex Bogdanov seemed to meet all the criteria so I bought it for $40.00.

https://www.duckworks.com/product-p/ab-tweak.htm

Materials

The Tweak is a “stitch and tape” type of boat design. The curved sides and bottom are created by cutting out panel shapes from plywood and then bending the plywood and “stitching” the panels together with wire or zip ties. Then typically a paste made of epoxy and thickening filler is applied to the seams where the plywood pieces meet. Then a 3” to 4” wide “tape” of fiberglass cloth is placed over the seams and epoxy is used to adhere the tape to the surface.

I acquired two Kindle books:

“Ultrasimple Boat Building: 18 Plywood Boats Anyone Can Build” by Gavin Atkin

https://www.amazon.com/Ultrasimple-Boat-Building-Plywood-Anyone-ebook/dp/B00140FXZM/ref=sr_1_2?crid=2T2HLXABYF1AH&keywords=ultra+simple+boat+building&qid=1564599512&s=gateway&sprefix=ultra+simple+boat+%2Caps%2C381&sr=8-2

Small Boat Building: A practical - step by step guide - to building Plywood Boats

https://www.amazon.com/Small-Boat-Building-practical-building-ebook/dp/B07HLKZ8LQ/ref=sr_1_4?crid=2T2HLXABYF1AH&keywords=ultra+simple+boat+building&qid=1564599512&s=gateway&sprefix=ultra+simple+boat+%2Caps%2C381&sr=8-4

Mr. Atkin describes an alternative to epoxy and fiberglass tape. Modern polyurethane construction adhesives can be used instead of epoxy and drywall fiber tape can be used instead of fiberglass tape.

Figure 1 - some of the polyurethane construction glues I tried on the Tweak build, all from my local Lowe’s building supply store

Figure 2 - Drywall fiber tape from local building supply store

I also used Titebond III wood glue for making the center frame and a few other areas.

WOOD

I am fortunate to have a local source of marine plywood 30 minutes drive from my home, “The Hardwood Store of North Carolina”

https://www.hardwoodstore.com/hardwood-plywood

I purchased two sheets of 3mm Okoume plywood. I had them cut it into four 2’ x 8’ pieces that I could fit into my Prius.  Later, I realized I would need another sheet so I purchased a sheet of ¼” Baltic Birch. Marine ply at these thicknesses was about $50 per sheet.

Frame and cleat lumber was cheap 1x2 from Lowes. I also bought a couple pieces of nicer 3/4  x 2 for the “rub rail” (gunnel? Gunwale? Outwale? Still not 100% sure of the terminology).

I discovered a couple of hardwood boards that had been stored under my deck so I pulled them out and used one of them for the quarter knees. I am also in the process of using that wood to create some oarlock cleats and I will also use it for the skegs. Maybe it is walnut? I really have no idea.

Epoxy

Waterproofing and reinforcing homemade boats is done primarily with marine epoxy and fiberglass cloth. I purchased my epoxy from Total Boat through Amazon: ~$125 for a one gallon kit (free shipping) that includes pumps, gloves, and mixing containers.

https://www.amazon.com/gp/product/B00HR85152/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

   And a one quart kit:

https://www.amazon.com/gp/product/B06XH5CVFR/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

Fiberglass

Because I was using such thin plywood (3mm) I decided to use a fairly heavy fiberglass cloth to strengthen the hull. I found 9 oz cloth on Amazon for $26.99 for 3 yds of 50 inch wide cloth.

https://www.amazon.com/gp/product/B06XC62T74/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

In retrospect 4 oz cloth would probably have been fine. 9 oz is a little difficult to work around corners.

Materials - Misc & tools

I found brass wood screws at Lowe’s and used a few of those.

I purchased wooden paint stirrers that looked like big tongue depressors and used them to stir epoxy, spread epoxy, and also to spread glue.

I purchased a palm sander recently for a home improvement project and it has proven very useful for the boat build.

https://www.harborfreight.com/14-sheet-orbital-palm-sander-61311.html

Nitrile gloves are useful to keep the expoxy and glues off your skin.

https://www.harborfreight.com/5-mil-nitrile-powder-free-gloves-100-pc-large-68497.html

I used my son’s circular saw to cut out the panel parts from the plywood. Eventually though I did purchase a jigsaw from Walmart ($18.73)

https://www.walmart.com/ip/Hyper-Tough-3-5-Amp-Jig-Saw-JS55G1B/199349530?wmlspartner=wlpa&selectedSellerId=0&wl13=1498&adid=22222222227000000000&wl0=&wl1=g&wl2=c&wl3=42423897272&wl4=pla-51320962143&wl5=9009652&wl6=&wl7=&wl8=&wl9=pla&wl10=8175035&wl11=local&wl12=199349530&veh=sem&gclid=CjwKCAjw-ITqBRB7EiwAZ1c5Uy971fbhCRZLV84TWOdE5k-6C-I2NPyUFl0KI2GWaAlQLF0BKdmQPhoCVbEQAvD_BwE

(walmart has ridiculously long urls :(

I bought some small stainless steel nails but I have not used them yet.

I’m sure I am missing a few other materials which I will add later when I think of them.

I used a hand plane and other material removal tools like a Stanley Surform rasp and a box knife quite a bit. A few times it was easier or quicker to just use a hand saw instead of a power saw. All good things to have …

Blue painters tape so as to not leave any residue on the wood.

Zip ties, a pack of 100

Cordless electric drill is almost a necessity. They are cheap and incredibly useful. You should have one anyway. ~$20

https://www.harborfreight.com/power-tools/drills-drivers/cordless-drills-drivers/18-volt-38-in-cordless-drilldriver-kit-with-keyless-chuck-21-clutch-settings-62873.html

The build …

I purchased and downloaded the plans on the 4th of July. I declared my independence and decided to become a boat builder!

My plywood was in 2’x8’ pieces so I could bring it home in my Prius. I brought the 2’ x 8’ pieces of plywood into the house as it was much more comfortable to do the panel marking in air conditioned comfort. Summer in central NC is no joke, mid to high 90s every day that week.

Figure 3 - measuring and marking out the panels

Fig 4 - using a batten to draw a curve

The plans indicate x-y coordinates for the corners and edges of straight panels, but for curves they just give the coordinates of a few points along the desired curve. Those points are plotted and then tacks are inserted at those points. Something long thin and flexible (a flexible piece of plastic molding from Lowes)  is gently bent around the tacks to create the curve and then the curved line can be traced with a pencil on the plywood.

Panels are then carefully cut with a hand saw or a power saw. I tried a hand saw but I’m impatient and switched to a circular saw with the blade depth carefully adjusted to just barely cut through the thickness of the plywood. I generally tried to cut outside my pencil line by a bit and then shaved off the excess with a hand plane.

Fig 5 - panel cutting with circular saw and trimming with hand plane

I next made the central frame and added a stiffener to the transom. Cheap 1x2 lumber and Titebond III glue.

Now is when I started “building a boat”. It’s exciting to see it start coming together!.

The central frame is attached to the sides with zip ties.

And then the bow and stern transoms are attached. You drill holes in each piece and then use zip ties to “stitch” them together. A cordless electric drill is extremely useful for this.

Next the bottom pieces need to be attached. The instructions called for adding zip ties every 6 inches or so along all the edges.  However, I have seen some articles that show people using duct tape to hold the pieces together. I decided to use a few zip ties near the corners but then use tape for the rest. Fewer holes drilled, fewer zip ties to cut. I used blue painters tape because it does not leave any sticky residue when you remove it.

We’re up to July 10th now.

Now it is time to turn it over and start applying construction glue to the seams.

I used Loctite PL Premium 8X for the aft section. It is very thick and difficult to squeeze out of the tube. However, it cured up nice and hard and it is very strong. I used large tongue depressors to spread the glue into nice fillets. When I ran out of 8X I switched to Gorilla Glue. It is a nice texture, easier to squeeze out of the tube, about like cake frosting or tooth paste. Unfortunately it cures to a flexible texture, like silicone seal, not good for sanding. Then I switched to Loctite PL Premium 3X. It is not as thick as 8X. Unfortunately, it bubbles a bit as it cures. Once all seams were sealed and filleted I gave it some time to cure. Then I applied drywall tape to all seams and “wetted” it with more PL glue. I switched to Loctite PL 375 because it is the cheapest, about $2.50 a 9 oz. tube vs. $9 for PL 8X and $5 for PL 3X.

This photo shows the gunwale being glued and clamped on. I don’t have a large number of clamps so I also used some screws to hold the gunwale in place. It also shows the aft seat parts dry-fitted in place. The instructions recommended ¾” x 1-½” for the gunwale, but all I could find was ¾” x 2”. I could not get it to bend vertically at all so I ripped it down to ¾” x 1-½” and then it would bend in the vertical direction a bit, just enough.

July 12th: Now it is time to turn the boat over and work on the bottom. I took this opportunity to pick it up and weigh it. It was 26 pounds! I was quite pleased since achieving a light weight was one of my goals. That’s why I selected 3mm Okoume plywood.

 

I guess I am now a boat builder!

With the boat flipped upside down I now removed the painters tape, clipped off the zip tie heads, and started smoothing the seams with my palm sander. I filled holes and gaps with either wood putty (stuff I found at WalMart Plastiwood) and/or PL glue. I did not bother with taping the seams on the outside because I was going to cover the entire bottom with 9 oz fiberglass fabric.

Tape removed and zip ties cut off flush. Gaps filled and corners rounded.

I applied a coat of epoxy to the entire bottom at this stage, thinking that it would be a good idea to have the wood sealed first before applying the fiberglass cloth. I suspect now that was unnecessary. This was my first time working with epoxy and fiberglass cloth and I probably should have read more instructions on the right way to do it. It ultimately worked out ok, not pretty but it’s done. Don’t lay a large drape of fiberglass cloth over uncured epoxy. The uncured epoxy will partially wet portions of the fabric and then it won’t absorb the actual wetting epoxy properly. Also, if you are not 100% sure you have enough epoxy to do the entire job, then only glass portions. Lastly, a drawback of the heavy 9 oz. cloth (at least the type I had) is that it doesn’t like going around corners. It likes flat areas and open(shallow?) angles. It’s hard to explain, but I will use 4 oz. next time.

My wife helped me drape the fiberglass cloth over the hull.

I poured drizzles of epoxy onto the cloth to “wet” it in. I spread the epoxy with a large tongue depressor (small paint stirrer?) to work it into the fabric and avoid any air bubbles (still ended up with some).

Wetting the fiberglass fabric with epoxy (you should _always_ wear gloves!)

Fiberglass is amazing stuff. I should have done more reading on the proper ways to use it and find out tips and tricks. I still will, but I fumbled through and made it work. The concept is fairly simple. Epoxy is plastic, but it is relatively weak plastic until it is reinforced, similar to using rebar with concrete. Combining epoxy with the glass fiber makes a strong light “composite” material.

At this point construction slowed down a bit. I had some other things going on, including taking a “Learn to Sail” class at a city lake here in Greensboro.

http://laketownsendyachtclub.com/activities/AdultLearnToSail.asp

The epoxy cured nicely but I ran out before getting all the fabric wetted in. I found a small quantity at a hobby shop and was able to finish a few days later. However, there were some bad spots, air bubbles, drips, all on the sides. The glassing on the bottom panels was ok.

Eventually I took a good bit of time to cut away the bad spots or rasp them down and I sanded the entire bottom.

A dust mask or respirator MUST be used when sanding fiberglass. Also long sleeves and gloves with tape where they meet. Keep that stuff off your skin.

After sanding I waited a day then hosed off all the dust.

Now we jump to July 29th. I have installed the seats and applied a coat of epoxy to all interior and exterior surfaces. I have done some fairing, smoothed rough spots and added some cool corner knees up front. My aft seat does the same thing as a corner knee so I didn’t bother those in the back. I decided to take some scraps of fiberglass cloth and add a second layer along the center of the hull (on the outside) since that will get a lot of wear and tear going in and out of the water. I added some on the front transom corners too. Note that I made the seats into watertight floatation chambers. I will cut holes for screw-out deck plates / inspection ports to allow access to the chambers for drying out.

https://www.duckworks.com/product-p/sd-337140-parent.htm

Almost ready for first on-water try out!

I spent a bit of time on these quarter knees, getting the bevel right, making a nice curved shape etc. They would look better up at the gunwale, but I’m too impatient for that. I wanted to be able to put a screw in through plywood to hold them in place for gluing.

Cool quarter knee

I built some “ghetto” pvc oars, really cheap using huge zip ties to attach scrap plywood blades to slots cut into 1 ½” pvc pipe. They worked.

I took it for a test row on July 30th at Lake Townsend Marina, Greensboro NC.

It floats! No leaks. Stern rides high. It desperately needs a skeg. The plans call for one, but I decided to try it out first. I have an idea ….

First water try out. It floats!

So back at home I start working on the skeg problem. The plans call for a 25” long skeg that is 6” deep. I have two ideas I want to try.

  1. A deeper skeg. The stern rides so high a 6” skeg will barely touch the water. I think I need 8” to 10” deep.
  2. A dual skeg approach. Two skegs set apart will act to hold the boat up flat when it is on dry ground. Being a v-bottom boat it leans when you set it on the ground. Also two skegs should provide even more straight-line stability. It is such a short wide boat it will turn in circles very easily with no skeg.

I designed and installed two very deep “ shark fin” skegs today August 1st.

Dual “shark fin” deep skegs should improve rowing performance

I am very happy with how this went. I learned a lot and I now have a boat I can take to the lake whenever I want.

I really want a sailboat so my next build will be an 11’ row/sail boat designed by Jim Michalak called “Piccup Squared”.  

https://www.duckworks.com/product-p/jm-piccupsquared.htm

It’s supposed to be an easy hull to build, very square, no fiberglass required, so I can focus my attention on getting the sail rigging done. I will also build a framework to add to my motorcycle trailer so I can haul boats on it. With this one I will be a little more careful so I end up with a nicer looking craft. I have ordered the plans already. ‘Can’t wait to get them.

If you’re thinking about building a boat I wholeheartedly encourage you to just give it a try. Start with something small and simple. You might like it or you might not, but you won’t know for sure until you try.

Best of luck!

-Brian


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