tanzer22.org / 2104 / Dinghy / Oars

For those not able to find the article online (there doesn't appear to be a definitive webpage for it), I have included a copy of the article on my website.

The text below discusses my process of building the oars, as well as where I diverged from the original plans.

Marking and Cutting

The oars are 6' long, which is close to appropriate for a dinghy of this size. They're laminated from 1x6 pine. In contrast to many oar designs, these ones laminate the loom (shaft) from three pieces. More common is to have the loom extend into the blade, and then glue other pieces of wood to the side of the loom to make the blade.

One oar has been cut out and clamped together. The second shows the cutting lines on the wood. A single cut on the bandsaw along the long taper cuts each of the loom pieces to shape, matching the taper of the loom.

This was my first epoxying operation with my then-new venturi vacuum system. Here we are preparing the vacuum bag. The long edges have been pre-sealed, so that when the oar is inserted, we only have a 12" seam to close off.

Laminating

The loom of the oar is laminated using epoxy. Small dowels protrude from both sides of the main workpiece to align the three pieces of wood.

Vacuum Bagging

After epoxying, the oar is inserted into the vacuum bag and the bag is sealed. Note that there is no pink or blue or black nylon fabric visible. At the time, I didn't know how critical a nylon release fabric was. As a result, we ended up with a VERY fuzzy oar by the time the epoxy had cured!

Learn from my mistake -- always use nylon release fabric, and remember to put it closest to the workpiece, with the felt on the outside. I've almost made that mistake a couple of times too.

With the bag closed, the vacuum hose is attached and the venturi is turned on. The principle of the venturi is that it uses compressed air blowing past a small opening to create a vacuum. This means it is very noisy, compared to a vacuum pump. In photo 2, I'm opening the valve on the 5 gallon air tank, here being used as an auxiliary vacuum reservoir. It greatly improves the cycle time of the vacuum system.

Workspace

My Dad built a fume tent in the basement to enable working with stinky chemicals, like paint and epoxy, without smelling up the whole house.

It's an 8' x 8' wooden frame suspended from the rafters, with a bathroom fan exhausting the air when epoxy is curing, blowing it out a 3" duct outside the house.

The top and sides are made of plastic vapour barrier, with the bottom edges weighed down by old 1/2" nylon docklines. The sides can be hoisted up for access to the laundry when there's no open epoxy or paint.

Two other views of the oar inside the tent. The venturi and 5 gallon tank are visible in the first photo.

Release Fabric

On the second oar, we learned from our lesson and used nylon release fabric to prevent the felt breather fabric from being permanently glued to the oar.

Shaping

With the oar laminated, the next step was to taper the loom in the opposite dimension. We clamped a long straightedge to the loom to mark the cut, which was done on the bandsaw.

Steps not pictured here are shaping of the loom and blades. This was done roughly with the bandsaw, and finer work was done using a drawknife followed by a 1" bench-mounted belt sander.

I also found that drywall screening was very useful for sanding the looms. It's long enough to be able to pull back and forth, and the open mesh prevents it from loading up with sawdust.

Fiberglassing Blade Edges

With the oar shaped to size, I fiberglassed the edges and tip of the blades. The oars are made out of pine, which is a fairly soft wood. I hope that the fiberglass will prevent most dings in the oar blades.

The fiberglass is held in place with 3M 77 spray adhesive. Clear packing tape is placed around the oar loom, where the vacuum bag will seal around the loom. This was much easier than making the bag large enough to hold the entire oar.

The oar blades inside the vacuum bag. The blue fabric is nylon release fabric, yellow is felt breather fabric.

Oar "Lock"

Here I have confirmation that the oars are the correct length. I purchased an M-shaped oar "lock" to keep the oars from growing legs when not in use.

Oar Leathers

One of the drawbacks to living in an age of synthetic fibers is that natural fibers are very difficult to acquire. The "cotton" braid available at my local big-box hardware store was cored with "mixed fibers", whatever that may mean. Well, it does mean that it won't be possible to epoxy it.

As a result, I ended up re-coring 60' of cotton braid with two strands of real cotton twine. The angel hair in the first two photos is the core that I removed from the cotton braid. Photos 3 and 4 show the new core, and the bunched-up cover ready to accept the new core.

With the cotton braid ready for use, I started application of the leathers. The edge of the leathers is marked with blue tape. Next, ravel about 2" of the braid, and start wrapping the oar. The wraps lock the end in place.

The slack is gradually worked out of the wraps, the end is pulled tight (with pliers if needed), and then cut off flush.

With the leathers in place, the next step is to tie Turk's Head knots at the ends of the leathers. These serve as an index point when rowing, as well as to keep the oarlock in position when the oars are stowed.

I tied all these knots as 5-lead by 3-bight knots. This is the most common turk's head knot, and instructions are easily found on the web. The knots on the right of these two photos are tripled (the end is passed through the knot a total of three times), and are complete. The knots on the left are doubled, as a base for a second, larger knot.

Once the knots have been tied and the ends cut off, the oarlock is covered with masking tape, and the knots are epoxied to lock them in place.

The next step is to tie another 5x3 turk's head (tripled) on top of the doubled knot mentioned above.

With the knots tied and epoxied, we move on to varnish to protect the epoxy from ultraviolet degradation due to sunlight.

At the same time as varnishing the leathers, the rest of the oars are varnished as well. All that remains is to install the handgrips.

Oury Handgrips

Common wisdom about oar handles says that they should be left unfinished, to maximize grip on the handle. I figured that if the goal was maximum grip, I should adapt a product from the industry that has perfected this - mountain bikes.

The handgrips I selected are called Ourys. I installed them using compressed air, which is much more effective than hair gel. The gel makes it possible to install the rubber grip, but often doesn't dry, leaving the grip loose on the oar.

The trick with compressed air is to insert the nozzle of a blow gun at the open end of the rubber grip. The air expands the grip just enough to make it loose on the oar, while the air is flowing. Once the nozzle is removed, the grip is now very securely installed.

Oarlocks

On a boat as small as the Minipaw, one design decision often has a large effect on another. As a result of routing a cove into the mahogany gunnel, there was not sufficient wood to properly install the oarlock sockets.

Combined with the fact that I didn't know exactly where I was going to install the sockets, a creative solution was called for. I installed four "rowlock doublers" made of 1/4" plywood on the inside of the boat. These provided additional wood thickness for installing the screws of the oarlock sockets.

Thin kerfs were cut into the back face of the plywood to enable them to follow the curve of the hull more easily.

This is the view from outside the hull. The red circles highlight the alignment triangles I used, as when installing the seats. With the doublers higher than the gunnel, I was later able to cut them exactly flush with the top of the gunnel. This was much easier than trying to align them while epoxying.

The Minipaw plans specify an optional sculling oarlock, centered on the stern transom. To keep it out of the way, I cut a groove into the mahogany transom stiffener, and epoxied a stainless steel tube in place.

After the tube was in place, I applied three layers of fiberglass across the opening. This used the same principle as vacuum bagging (hence the colourful materials), but used closed-cell foam, wood, and clamps to apply the pressure.