Monday, February 6, 2012

Low-Buck Dinghy

Over at Dock 6, Brian set out to design and build himself a beautiful stitch 'n glue dinghy, with design targets of:
  • under 25 hours total working time
  • a $250 budget
Did he make those targets?  Read on...
Here's one from the vault- originally published on Anything Sailing back on 2009, this is the tale of "Chirp."

Alright, get your minds out of the gutter- SWMBO has already made all the Viagra jokes and vastly amused herself when I laid out my plan. (Okay, I guess i set myself up with that last sentence. BTW, anyone know of a good place to buy AA batteries? We seem to be running short around the house.)

*ahem*  Be that as it may, after pricing inflatables at the boat show, thinking about the pros and cons of inflating/deflating, storage, actual use, etc., I have decided to spend the rest of this winter, or at least the non-drinking portion of my spare time, building a dinghy.

My goals are thus:
  • Multi-purpose-  sail, row, outboard propulsion.
  • Short- I don't want the length of my dinghy to exceed the 8' beam of my boat. I plan to build davits to accommodate it while at dock, to keep the marina nazis happy, but will tow it while underway.
  • Weight- I want to stay under 75 lbs. for the basic hull- engine and sail rig additional.

I am figuring on a 3'8" beam, 7'6" LOA, 1/4" thick hull, 3/4" thick transom, 14" freeboard. Doing the math, I think I am looking at about 1 and a 1/2 sheets of 1/4", so I should be able to meet my weight projection, and if i do the math right, I am looking at a capacity of roughly 450 lbs with 6" of freeboard.

The plan is to be able to get this bad boy built in under 25 hours total working time, on a $250 budget. Here's how it's breaking down so far:

Hour 1: Alright, I got on the tools today and started making sawdust. Scoured my scrap lumber stash and found a likely victim to be made into a transom, a 3' x 3' scrap of 3/8" ply. I lofted my transom plan, cut the carcass, and ripped the offcuts into 3 " sections which I laminated to the carcass, clamping and weighting the bejesus out of it to give me a 3/4" thick transom, suitable for a small outboard.

Hours 2 and 3: Grabbed a motley assortment of castoffs and built a strongback. Laid a 4x8 sheet of 1/4" luan on the 'back, and lofted the sides of the hull. I have decided to go with a traditional bow for a couple of reasons- I think it may tow better behind the boat while underway, and it also means one less piece to lay out and cut. Laid out and cut the frames for spreading and shaping the hull.

Hour 4: Cut stem piece and very, very simple temporary frames, then cursed, sweated, grunted, pushed, pulled, nudged, wiggled, and screwed the chines, frames, stem, and transom together:

After pouring some more coffee, I will figure out the rocker, shim the strongback and form the bottom of the hull.

The chines are secured to the transom and stem with #6 wood screws and Gorilla Glue. The frames are temporary and are only screwed in place. Once all the pieces are cut and fitted, then the whole mess will be stitched and glued together, epoxy filleted, sanded, inside seams fiberglassed, and I will probably cloth the whole outer hull, just for durability's sake.

Total cost so far:
  • 2 sheets of 1/4" luan @ 14.70/sheet = $29.40
  • 3 8' lengths of 2x2 @ 4.30/ length = $12.90
  • 1 bottle of Gorilla Glue $5.95
  • handfull of screws @ 3.75/lb. $1.00 (approx.)
  • Assorted scraps and off cuts from other projects: $0

So, so far I am $49.25 into this fiasco. If I didn't have scraps for the strongback and the transom, figure another $20 or so.

Hour 4.5: Shimmed the second 4 x 8 sheet on the strongback to conform to the rocker of the chines. This sheet is the bottom of the hull.

Once I had the shims in the right place and there was no daylight visible between the chines and the bottom of the hull, I traced around the hull, then grabbed my jigsaw, closed my eyes and pulled the trigger. Voila-

Here's a selection of the tools used sitting in the bottom of the dinghy:

I am NOT a woodworker. My tool selection for messing around with tree-based construction is barely at the home handyman level, but so far I have not run into any real issues. The biggest problem I have is space- my workshop is really designed for building engines and general automotive futzing- I am working in a space that is approximately 7 x 10, with sheets of 4 x8 ply. Space is so tight I have to leave the room to change drill bits. To cut the hull once it was traced, I had to shuffle the chines to port, cut the starboard side, shuffle the chines to starboard and cut the port side. Now that the big pieces are cut, I've got a little extra room whicgh will speed the process and lower the frustration level. okay, coffee is ready, time to get back to it.

Hours 5- 7: Tied the bottom and chines together. Instead of the traditional tied-wire method of stitching, I decided to make use of the big-ass grande size tub of zip-ties I bought at Costco 8 years ago and have never used. Then I laid in the forwardmost frame, and spent a frustrating hour and an alarming amount of wood cutting the breasthook and quarter knees. the compound bevel angles damn near made me give up. I did prevail however, by remembering the mantra, "filler will cover it up, filler will cover it up."

I am surpised at how quickly the basic hull took shape. I am further surpirsed that i have gotten this far with all major and minor appendages and digits still intact. Now that the major cutting and fitting is out of the way, i am sort of at a standstill untill i can pick up some West System supplies and start messing around with harmful chemicals. I gotta say, so far it has been pretty damn enjoyable, and i am kind of kicking myself for not picking up enough treestock to build two or three at one whack. Aside from workspace limitations, the material lends itself to mutliple simultaneous builds. I could have lofted and cut three or four hulls at the same time, with judicious use of clamps. Of course, I have no use for three or four dinghies, but this small indoor mid-winter boat-building stuff can quickly get addictive.

BTW, I just ran the numbers through a hull design program i downloaded. If this program is to be believed, I am looking at a payload of nearly 1000 lbs. before she swamps. That is a lot of beer, and beats almost all other dinghies of similar length i have looked at.

Hours 7 - 11: Tore out the breasthook and started again. Rather than trying to get an acceptable result with 3/4" ply, I decided to carve some cleats and install a small spreader frame, then use 1/4" ply for the breasthook itself.

Rethought my thinking, and realized there was a lot I could (and should) get done before pumping epoxy. I built the mid and aft frames, and installed them: Here's the bow frame I installed yesterday and today's midframe:

To be continued...

No comments:

Post a Comment

Related Posts Plugin for WordPress, Blogger...