Slipper’s New Rudders

Those who have followed the misadventures I’ve had with this little boat may be relieved (and a bit amazed) to learn that now she seems to be working out. At last! You can see her huge new rudders here:

Conceived as a sort of thought experiment in simplicity and cartopping, and as a cat for those who’d like to learn a bit about the breed but who don’t have a trailer or a place to store a wide trailerable boat, Slipper was simplicity itself to build. Each hull consists of three pieces of quarter inch ply, a transom, two bulkheads and a center frame to take the pull of the shrouds, and not much more. The strakes are stitched together and taped with epoxy, the bows are pulled together and secured by an epoxy pour inside and a bit of fiberglass cloth on the outsides. Her beams are simple I-beams of ply capped with solid stringers, her center deck is ply stiffened with stringers and bolted to ledges on the I-beams. She has a cat rig– using the polytarp sail from Slider, but no jib, so only one sheet to handle.

I got into trouble this way: Slider, my much beloved 16 foot open cruising cat, goes to windward surprisingly well even without her big deep daggerboard. I thought that by duplicating her hulls on a smaller scale, I might be able to take the whole simplicity thing to new heights. I gave Slipper barndoor fixed rudders that, unlike Slider’s kick-up rudders, would require no fiddling about.

Alas, this is not the way it turned out. Slider’s rudders are fairly high aspect, and are carefully shaped into NACA 0012 profiles, so they are extremely effective. Slipper’s original barndoor rudders were long and shallow, and they did not do the job. She was hard to tack, and I’d been spoiled by Slider’s effortless tacking. Slipper didn’t go to windward very well either; in a nasty chop she’d tack back and forth, gaining a few feet on each tack, but thoroughly frustrating me.

I tried various things to improve her. I cut off the skegs she’d originally had to support those long shallow rudders, hoping for better tacking. Not the solution. I tried rudder fences to improve the rudders’ efficiency. Nope. Then I made even longer barndoor rudders, in the hope that she’d behave better with more rudder area. There was a slight improvement, but it just wasn’t good enough.

So I set aside the project for a while, and gave it some thought. After much cogitation, I concluded that since the main difference between the two boats, in hydrodynamic terms, was the rudder configuration, this was the likely culprit. This conclusion was reinforced by some recent correspondence with Dean Pacetti, who completed the first sistership. Dean is very happy with his Honu (Polynesian for “turtle”) but Honu does not tack with the same reliability as Slider. I finally got up the nerve to ask Dean if he’d put the designed NACA 0012 rudder foils on Honu (and it took some nerve, because Dean is a much more competent boatbuilder than I am.) He told me that he did not build his rudders to the designed foil profile. As far as I can tell, Honu is otherwise built to plan.

It seemed to me that maybe the difference was that Slider’s rudders were not stalling as soon in the turn, continuing to lift the boat around the corner after foils that weren’t quite as good had stalled and lost their lift.

Eventually I decided to bite the bullet and carve out some decent foils. I intend to use NACA 0012 foils on the new 20 foot cruising boat, too. These must be larger than Slider’s rudders to have the same effect. To save the effort of making two templates. I decided to build the rudders for Slipper on the same profile. This approach means that Slipper’s rudders are a little short and wide, compared to the optimal shape. Two pieces of 3/4 inch ply, for a thickness of 1.5 inches, calculates out to a NACA 0012 profile with a 12.5 inch chord. I shaped the lower 19 inches, and faired the shaped area carefully. I bolted these blades on to the stubs of the old rudders, and took the boat to the water.

The difference was day and night. Slipper was much better behaved, tacking far more reliably. On the couple of occasions she missed stays, it was much easier to shunt her on to the new tack by backing the rudders.

The big surprise was her windward ability, which was greatly improved, even though her rudders are the only foils on the boat. Unfortunately, my GPS had dead batteries, a condition I discovered only after we left the ramp, so I was unable to quantify her tacking angles. Still, I spent some time out on the bay, tacking around a fixed buoy, and leeway was obviously much lower than with the old rudders. I had no difficulty in tacking over to the weather shore, and did so several times.

I’ll have to do something about the crude manner in which the new blades are bolted to the old stocks. I plan to try other leeway prevention technologies after I’ve gotten some tracks on the GPS for comparison purposes.

I also learned that Slipper is not nearly as dry a boat as Slider, due to Slipper’s much-reduced freeboard. We crossed a couple of wakes out on the bay and shipped a couple quarts of water over the bows. I think if I were building another, I’d put foredecks and flotation between the stems and the first bulkhead, despite the extra weight. The hulls are now a bit over 50 lbs. apiece, so a couple more pounds won’t make them too hard to get onto a roof rack,

The boat is such a simple build that I believe I’ll just put free plans up on the website, if there’s any interest. I’m about to start the new boat, so I’ll have to work the drawings in as I get a little free time. Meanwhile, here’s a brief video from our expedition yesterday.