Building an Aluminum Two Piece Dinghy

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Over a beer or three, Scott Banerott and I had discussed that our two piece nesting dinghy, 'Little Utopia', based on Danny Greene's "Chameleon" design , very closely matched what Scott had considered to be the ultimate fishing dinghy in his book, The Cruiser's Guide to Fishing. The major difference was that I had built it using the 'stitch and glue' technique of glass over plywood instead of aluminum that Scott preferred. Having already spent many hours building 'Little Utopia', I had little urge to make another dink and simply had another drink instead.

A year later, we found out that Scott and Wendy weren't the only people who had admired our dink, after it was stolen. In an effort to ease the pain, I decided to build the same dinghy, but out of aluminum and incorporate several design changes that I had thought of after 'Little Utopia' was built from fiberglass and wood.

I have welded before, but never extensively, and mostly with oxygen-acetylene on steel. I had little prior successful experience welding aluminum, or had I ever done MIG (correctly, GMAW, gas metal arc welding), or TIG (GTAW, gas tungsten arc welding) welders (Photo 1).

The lack of experience isn't as critical as it may seem, as the majority of the construction time was spent fabricating pieces to fit. I spent about an hour learning how to set up the MIG welder to do tack welds and 'stitches'. I had an expert do the critical structural welds using TIG for the root weld and MIG for fillets (Photo 2). Total welding time was less than 10% of the 160 hours that I spend building the boat.

Yes, 160 hours. The 'Chameleon' is a great design, but it isn't a simple one and frankly is more of a boat rather than a simple dinghy. Producing the bare hull, including welding was only about 40 hours of work.

With the exception of a welding power source, all of the tools used were the same hand tools used for a wood boat. These were:

5" L-head sander-grinder	Electric Drill
Jig Saw	Router
Random Orbit Sander	Quarter Sheet Finishing Sander
Scribe	Rafter square
Quick Square	Bezel
Block plane	Tape measure
Metal Yardstick
And about a billion clamps of all shapes and sizes, excluding the three that I needed and haven't been invented yet (Photo 3).

The scantlings of the boat were from 2.5mm 5034-H3 alloy for the bottom plates and all bulkheads, transoms, and stiffeners. The sides and floatation chambers were made from 2.0mm 5034-H3. All of the extruded parts (square tube, parts of the flotation chamber, center weld strip, and the skeg mounts were all 3mm thick 6061-T651. Even though harder to work with, I chose to use 5325 filler rod to ensure compatibility with sea water.

As I had access to a plasma cutter (sweet!) to cut the flat stock, I lofted the plans first onto the cheapest 1/4" plywood I could find with a decent surface, cut the plywood and smoothed with the block plane. I then used the ply as a template for cutting with a plasma cutter. Even if you do not use a plasma cutter, it is far easier to do the lofting on the plywood, as you can drive nails into the ply and you cannot (easily) do the same directly on the metal. On some of the parts, I just scribed directly onto the aluminum and cut the pieces with a beefy jig saw. Final shaping was done with the grinder and 24 grit paper. Although unadvoidable in places, using a grinding wheel on any soft metal is a poor idea, as the pores of the stone will clog and glaze, possibly causing the stone to overheat and explode. You have been warned! Instead of drilling and stitching the hull panels, I tack welded thin pieces of aluminum to the mating edges of the hull panels to hold the edges together when they were spread to accept the insertion of the thwart ship panels (forward transom, locker bulkhead, mating bulkheads and stern transom). The plan was to tighten the panels together by levering the thin aluminum outward away from the hull. This didn't work as well as I thought it may. I did try a test section tack welding aluminum fencing tie wire and this worked much better (Photo 4).

Also, with perfect hindsight, fabricating the reinforcing pieces of square tubing on the thwart ship pieces would have been much faster and easier had I cut and welded them in place BEFORE assembling the hull (Photo 5).

The inwales and outwales were made from a local wood, tava, which enhanced the appearance of the boat, but round tube sliced to slip over the upper side would have worked just as well and would have been less work (Photo 6).

The fenders were made from four pieces of refrigeration tubing covered with tarp material. The covers are held on via drawstrings on the bottom and tie downs on the top (Photo 7).

The fishing pole holders (formed by bending flat strap around a 2" pipe, secured with a single clamp), the fiddles on the forward hold and after buoyancy tanks, and the combo coffee cup/beer can holders were all modifications to the first Chameleon that I had constructed (Photo 8 and 9).

The anchor roller assembly may appear to be glitz but it was heavily used in Rin Tin Twin's predecessor. Being able to easily set or recover the ship's anchor with the dink without damaging a hard dink is priceless and impossible in a deflatable dinghy. Because of the high bows on Rin Tin Twin, to recover the anchor, I stand on the aft lip of the cargo compartment, grab the rode by the bow roller, then I sit down keeping my legs straight. The leverage, with the counterweight of the engine on the stern, has always led to success (Photo 10).

On the underside of the dink, I made a few more modifications to the basic design for impact resistance with reef landings. I welded a piece of 25mm equal angle channel to protect the center seam. Flat strap was bent around the wooden skegs, then through bolted to 'roots' welded to the bottom of 25mm equal angle channel. The thought here was that the skegs are sacrificial and that the wood will break in lieu of the bottom of the dinghy being stove in (Photo 11). I also used the same bolts to attach the anodic zincs.

Our satisfaction with the finished product is high. We cruise with a 3.5 hp outboard at 4.5 knots, with a top speed 6.5 knots. With 7-foot oars, it rows like a dream come true. The final weight is 2/3 that of glass over plywood, for ½ the price and 20% less work (Photo 13).

Photo 1-WeldingWelding in the bow transom to the bottom plates. Please note the colour: Inert gas welding produces a tremendous amount of UV light and the 'fresh' metal acts as a mirror. It is very important, especially for fair skinned people, to be aware of the probability of sunburn. Also, especially after the bottom seam is welded, argon is heavier than, and displaces, oxygen. If you do not ventilate well, you will die from lack of oxygen. If you are bearded, as I am, its also a great way to get a nice, even trim with the bottom of the helmet!
	Photo 2-TIG (GTAW) WeldingSam TIG welding exterior seams AFTER the seams were tack welded. together with the MIG.
Photo 3-Clamps In ActionOne store bought, the others weird, homemade clamps. Wire on the top, ¼" threaded rod through tack welded tabs on the bottom. Getting the bow transom to fit was the hardest part of assembling the dink. Since the bottom of the bow transom comes to a point, it was difficult to get a proper weld without the tip of the point 'going away' from the heat.
	Photo 4-Welding Stitch Tabs to Bottom Plate Before Expanding Panels The design is too large to cut from a single sheet of either plywood or aluminum, Danny Greene's loftings call for a total of 4 pieces to create the bottom. In aluminum, it is far easier to just put in a couple of tacks between the forward and aft bottom sections. As mentioned above, in retrospec I would have used aluminum fence wire here and just twisted the pieces together.
Photo 5-It's Impossible To Weld On The Underside Of The Bottom Horizontal Square Tube. What I should have done on the bulkheads is welded the square tube stiffeners in FIRST, then assembled them together. The gusset that I am working on here is to transfer the load under power to the bottom of the dinghy instead of just the stern section. Also seen is a stiffener that is run from the stern section mating bulkhead to the aft transom to prevent 'oil canning' of the plates over the long section.
	Photo 6-Tava Inwales and Outwales (The Vailima Bottle Has Solvent In It, Not Beer. Drinking Beer and Using Power Tools Results In A Rapid Reduction of Surplus Fingers). To fit the inwales and outwales, I first clamped the outwales in place. I then drilled from the inside of the dink into the outwales with a pilot bit for a #8x3/4 self tapping screw. Next, I pulled the screws and enlarged the holes in the aluminum to be a clearance hole for the #8 screws. I then used 3M 5200 adhesive to glue the outwales into place. After a week end to cure, I pulled the screws and clamped the inwales into place. Then from the outside, I drilled a #10 pilot hole first thru the outwale and aluminum into the inwale (but not so deep as to penetrate the inwale). I then pulled the inwale off and counterbored and counter sunk the outwale and aluminum to a #10. I then applied the 5200 to the inwale and using clamps, 'walked' #10 screws into the inwale. Finally, I bunged the holes with plugs cut from scraps of tava.
Photo 7-Gunwale Covers Refrigeration Insulation (Port) Is Covered With Tarp Material (Starboard). I used aluminum wire (from the hot spray treatment plant feeders) to make 'U' shaped pieces to tie the outer drawstrings of the covers to. It may look better than having a lacing thru the topsides, but I do not ascribe to the 'toilet paper' strength theory. If you haven't heard of this, the theory is that since as weak as toilet paper is, it never, ever tears on the perforations. Thus, to improve strength, metal should be perforated. Humbug! I say!
	Photo 8-Rod Holders Port and Starboard Foam Filled Floatation Chambers Before Wood Covers Installed Port, Midships, and Starboard. The rod and bottle holders are two issues that Scott and I had discussed at length as essential for the ultimate fishing dinghy. To create the chambers, I welded 25mm 'L' channel to the top of the chamber plates. I then measured my angles and welded the two sections together. Lastly, I fitted the curve of the hull to the bottom of the chamber. Lastly, they were welded in place to the sides and stern of the dink, but only tacked to the bottom. The theory is that without drainage, the chamber (and the foam) will eventually fill with water. If they can drain, no problem. There is enough inherent floatation in the shape of the stern section that I can stand in it to bolt the bow and stern sections together after they have been launched into the water.
Photo 9-Mandatory Combination Coffee Cup/Beer Can Holder/Knees The fiddles are great for keeping small items from the bilges. There is a common 'trick' used by shiprights using a drawing compass to spile a plank or to fit a curve. It makes a job like this easy if you can get someone to show you how its done.
	Photo 10-Inside Bracing of Bow RollerYou can see from photo 6 the heavy bracing in the bow section. The crosshatch of 25mm square tube gives the bow tremendous strength. I actually pushed a Chinese Long-Liner fishing boat away from 'Shangri-La' after it hit us because of the re-inforcing. We also loose some storage space in the bow section from the floatation chambers that I created with plywood bulkheads (seen laying down here), however the dink can be completely swamped with the engine mounted, and I can jump in it and bail it dry with a 5 gallon pickle bucket.
Photo 11-Skegs and Center Seam ProtectionWe routinely beach the dink on non-sand beaches. The outer 'shoe' is to save some wear on the wood. As all wood in the water gets wormy in the tropics, it is understood that the skegs have a limited life. The handles makes it easier to launch the dink from deck and also to secure to the deck. Sacrifical anodes were also attached here as the bolts do not penetrate the skins. The most expensive material cost in building the dink was the anti-fouling system for aluminum. It, of course, promptly failed! If RTT didn't spend so much time in the water, I would have just put on a boot stripe for looks and left the 'coating system' on the shelf!
	Photo 12-Mandantory ChristeningWe debated this caption between "Why Waste Money on Champagne?" And "Everyone is a critic". On a more serious note, you can see the longitudinal stiffener. In photo 8, you can see an athwartships stiffener running from just under where the rowlocks will be mounted. This is to prevent any 'oil canning' of the plates.
Photo 13-Rin Tin Twin Floats Again!The seats were built by making a perimeter of 2x2 treated pine, then glueing plywood skins onto the top and bottom. This also gives additional floatation to the dinghy. In the bow seat, there is also a 4x2 'doubler', which acts as the root for the mast. In addition, the aft side of the forward bulkhead has two u-bolts (home-made from 3/8" threaded rod to support the mast.)

Final Thoughts

The day of the launching of 'Rin Tin Twin' a two piece boat, by chance coincided with the American Samoa Flag Day, celebrating ceding the Island of Tutuila to the US. A key event is the racing of fautasi. Fautasi in Samoan language literally means make one. In this application (racing), the design of the boats is based on bolting the stern of two whaleboats together, stern to stern.

Note the bowman pushing the bows during the turn	The Finish Line Sprint

In the nation of Samoa (ex-Western Samoa) when it was a Commonwealth Colony, it was used as an attack gun boat by placing a machine gun in the middle!

Since the boat now had two bows and was powered by many strong Samoans, they could row up very quickly (over 20 knots!), attacking the Commonwealth troops, turn around on the thwarts, move the rudder to the 'other pointy end', then retreat before the troops could react.

Samoa is now an independent, democratic nation.

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