Design of PDRacer #572 – Looking for an Ultralight
By Bryan Young
Before I write anything else, I need to make it clear that I really don’t know what I’m doing when it comes to boat design, construction, or sailing. I am a hack. I make plenty of mistakes – mostly because I try to reinvent the wheel, work too fast and don’t take the time to think things through thoroughly. I designed #572 knowing full well that it might break apart on the water, perform poorly, or fail in some yet unsuspected manner.
Having said that, I am having a lot of fun toying with design and trying out different construction techniques. Hull #572 is my fifth PDRacer, and my eighth home-built boat. Here’s a brief summary of my previous builds:
Hull #359: 16” hull with full-length side airboxes, a pivoting off-center centerboard, and flying a 59 sq ft leg-o-mutton sail. Constructed using 5.2 mm luan underlayment, 3/4” pine chine logs, and Titebond II glue. Overall, we’ve been very pleased with this hull. It is relatively lightweight (~85 lbs), sails well, and has gotten a lot of use in three sailing seasons. We knew it wouldn’t last forever, and it is currently falling apart due to delamination of the cheap plywood. I’m sure she would have lasted longer if we had glassed the chines. I’m currently converting #359 to a skin-on-frame PDRacer. More on that in a future article.
Hull #490 and 491: Built almost identical to #359, but using 1/4” exterior BC pine instead of luan. These hulls are holding up well (though the ply is checking, or splitting, in some places on the stained and varnished hull). The main drawback to these hulls is their weight: each weighs over 120 lbs (with centerboard). The extra weight makes it very difficult to transport and launch the boats, so build lighter unless you are planning on launching and retrieving off of a trailer all of the time.
Hull #504: We just recently launched this one. I built #504 using three sheets of 5.2 mm luan and a sheet of 2” thick pink Styrofoam. I started off by gluing two sheets of luan to the foam to create a ply-foam-ply laminate. From that 4’x8’ sheet, I cut out the sides, transoms, and deck for the hull. The third sheet of luan was used for the bottom. The idea here was to create a lightweight boat that was very quick to build (no chine logs to cut and fasten). I think the general idea is sound, but my implementation is poor. I did a poor job of cutting the laminated sheet (it’s hard to cut a 2.5” thick board), which lead to all sorts of construction difficulties, delays, and additional weight. In addition to my poor construction, I’ve had to deal with the poor quality luan. In fact, I had to repair the bottom of the boat before we even sailed it once. I do not expect this hull to last long, and will not be overly surprised if it breaks apart on the water.
After seeing #359 delaminate this season, I decided to sand down and fiberglass the chines on #504 – everyone says to do this to prolong the life of a boat. Now I see why. It’s not that hard to do – so do it! By the way – it’s much easier to do this BEFORE you paint the boat.
After building these four hulls, I knew that I wanted an ultralight PDRacer – hopefully one that I can carry over one shoulder from the trailer to the water’s edge. I would like to get the total weight under 70 lbs. In my quest for a lightweight hull, I considered the following options:
1. Traditional chine log construction.
2. Ply-foam composite similar to #504.
3. Fiberglass-foam composite. This could be built similar to #504, by covering both sides of a 4’x8’ sheet of foam with fiberglass and epoxy. The result could be strong, lightweight, and essentially rot proof.
4. Skin-on-Frame (SOF). I built three SOF tandem kayaks earlier this year (Chuckanut 15s designed by Dave Gentry, gentrycustomboats.com). The construction is easy, the hulls are light and graceful, and the boats are super light (45 lbs each). I’ve put a lot of thought into how a SOF PDracer might be constructed. The boxy shape of a PDRacer doesn’t lend itself to this technique. I’ve got an idea how to approach it and am testing this idea with my re-build of #359. 5. Stitch and Glue. I’ve read a lot about S&G construction recently, including several of Dynamite Payson’s "Instant Boat" books.
After weighing my options (almost literally – I did a lot of rough weight calculations, making a lot of crude assumptions), I opted for a stitch and glue. I decided that the glass-foam composite was too expensive for experimentation, and that skin-on-frame just won’t work all that well for the PDRacer hull form. I had so many difficulties with #504 that I didn’t want to go there again. Stitch and glue takes full advantage of the plywood skin to make a light, strong hull. The building technique is, in theory, easy and straightforward.
I decided to build #572 using 1/4" UltraPly XL underlayment, which has become the cheap ply of choice for many amateur boatbuilders. I would make the interior fillets with Bondo Hair (disgusting name), a polyester resin with long strands of fiberglass in the mix. I did a quick test to see whether the Bondo Hair would work. I joined two scraps of UltraPly with a thick fillet of Bondo Hair. The result was not pretty, but in 20 minutes the fillet was quite strong. I couldn’t break the joint by hand when trying to pull it open (like opening a book). I was able to fold it closed with some effort – but it was the ply that failed, not the fillet. Strong enough. With fiberglass on the outside of the joint, I figure this will be plenty strong for a small boat.
My goal was to build #572 with two sheet of UltraPly for the hull – I’d use some left-over marine ply for the foils and rudder stock and some 1” thick cedar decking boards for miscellaneous framing. I had a tough time getting the transoms and sides from one sheet of plywood, but found an arrangement that gives a nice, curved shear. I opted for a leeboard (to avoid the weight and complexity of a centerboard case) and nixed the airboxes. I’m sick of dealing with water in my airboxes. Somehow it always finds a way in, and I always have to check and shopvac out the boxes after every sail. I’m sure this will be the death of my other PDRacers. Instead of building in airboxes, I am going to cut foam inserts to tuck under the deck on the inside of the side panels.
The hull is 15” high at the bow and tapers to a 12” stern (measured from the lowest point of the hull). This design has very little freeboard and will be a bit unforgiving, but we will only be sailing on protected inland lakes and always sail two or more boats at a time.
I started building #572 on July 2nd. I had the hull 3D within three hours or so. The Bondo Hair work was messy, smelly, and ugly…but it does give near-instant gratification. I used small pieces of wood as cleats to hold the panels together until the Bondo set up, then removed the cleats and filled in those areas with Bondo Hair.
I fiberglassed the chines with one layer of 6 oz., 2” tape and polyester resin (not epoxy). Again, this work was smelly. It was extremely hot when I did this work, and the polyester set up way too fast for me to do a good job of the glassing. Still, the work went quickly and the end result wasn’t awful.
I glassed the bottom of the boat with 6 oz. cloth and polyester resin. I had a near-disaster here – I ran out of hardener on my last dose of polyester resin. I put it on anyways (with about ½ the recommended hardener). The resin didn’t set up completely for several days. I left the hull out in the sun, and I think the UV rays and the summer heat finished it off.
I was disappointed to find that the S&G hull was quite flexible. The mast partner added quite a bit of stiffness, so I fastened a similar cross-piece near the stern and one two more near the bow. I will add aft and fore-decks using 8 oz. polyester cloth (borrowed from my SOF building experience).
After my initial spurt of effort on this boat, I have only had small bits of time to work on it. I completed #504 and started my re-build of #359, and have been sailing when possible. Now that the weather has cooled down, and with the 2011 Worlds coming up, I’m beginning to turn my attention to this boat again. I’m using epoxy + microballoons to fair the hull and will start painting soon. More later...