Re-drawing Whisp

[Charlie43]

Steve Redmond's 16' sharpie skiff, Whisp, is a much-loved, much-built boat, as a Goggle search for its images suggests and his report that over 5,000 plans have been sold.  
https://www.google.com/search?q=Redmond+Whisp&rlz=1C1PRFC_enUS540US548&tbm=isch&tbo=u&source=univ&sa=X&ved=0ahUKEwiGraCpxJnPAhVC4mMKHdJxB7UQsAQIOQ&biw=1600&bih=794

Touted as light-weight (70-90 pounds, depending on scantlings), fast to row (reputedly, 6 MPH), and easy to build (three, single-plane, lapped planks rather than strakes that create transverse curves), there's a lot to like about the design. But the boat really is longer than I want to haul on roof racks. Hence, I'm exploring the feasibility of building a shorter version, something in the 10'-12' range that I'd call Whisper, and I'm currently re-drawing his boat, which immediately raises a whole bunch of design questions.

By tradition and conditions of use, launching off beaches and shorelines into windy, choppy waters dictated a flat bottom and “pointy” bow, which meant the hull would “pound”, but wouldn't “slam”. Waves aren't a problem I encounter, and I really prefer the building ease of pram-bows to stems.  So if I made that change, am I really building his boat? At this stage in the design process, it's easy to draw both versions. But quite quickly a choice has to be made, so that plank shapes can be inspected and "regularized". An answer to the 'stem versus a bow transom' question depends mostly on “the look” I want and how much work I'm willing to do to achieve a look that offers me little functionality. Stated that way, the answer becomes obvious.

A second question is this. If the sides are straight, they can be constructed from a single plank rather than planks overlapped traditionally or dory-style. So, again, an answer hinges on whether to do work for the sake of looks, or for ease of construction. The sensible answer, the “right” answer, would be to not lap the planks, though a tiny argument can be made that the laps function as “stringers” that increase hull rigidity and lessen the need for frames. Frankly, I think the argument is tenuous, if not bogus. But it's not a style of construction I've attempted before, and now would be as good a time as any to explore it. So that becomes “a keeper” about his boat.

A third question is whether to reduce width proportionally as length is reduced.  The originally-drawn hull has a beam-to-length ratio of a narrow 20% compared to a typical flat-iron's of around 25% and the Salt Bay's very fat 35%. In numbers, Whisp's gunnel-to-gunnel spread is 40”, or roughly that of an Adirondack guide-boat, which means over-lapped oars and cross-handed rowing, which I prefer.  But I wouldn't want to make Whisper's beam narrower than its parent's. So that feature, too, is a keeper from the original.

Whips's rocker can likely be kept, though that will need to be confirmed with displacement tests. But its up-in-air sheer of 21.5 inches above the baseline at the stem can and should be lowered by a whole bunch due to expected conditions of use, which will be protected-water rowing, not bashing though heavy seas under sail.

The doubled gunnel will be kept, the single skeg will be swapped out for a double, and the fore and aft thwarts will be eliminated. What remains to decide are scantlings. Whether longer planks are created from shorter ones by any of the various methods, ugliness results if the boat is to be finished bright, unless the joints are numerous enough to “hide in plain sight”. Edensaw does sell 5'x10' sheets of 1/4" Occume, but for $113/sheet, to which the hassle and expense of transportation would have to be added, making the cost/sq ft about twice what locally-bought 8' Merenti would run. A lighter boat could be built by dropping down to 5mm or even 4mm, but those sizes aren't cheap. So if I'm going to be joining the side and bottom planks and using 1/4" stuff, the boat might as well be 12' instead of 10', which adds roughly 20 pounds to the hull, but picks up another 1.2 MPH in rowing speed, or fast enough I'd have to slow down when I entered 'NoWake 5 MPH' zones, or else risk being ticketed for speeding.  LOL

OK. That seems to be the essentials. Now it's just a matter of fussing with the drawings until a buildable hull emerges. Then I start marking and cutting wood.

[Charlie43]

The re-design is done, maybe a hour's work from initial idea to a buildable table of offsets. Drawing boats ain't no biggie.

[Charlie43]

Fussing with the building patterns suggests I can cut a 10' version of Whisp from two standard sheets of ply. Even assuming a high 85% utilization of material, the skin would come in at 38 pounds when a very satisfactory weight for the whole boat would be 60 pounds. Therefore, I can frame with whatever I choose, and walnut would be my preference, though I could save 40% of whatever the framing weight would turn out to be by going with cedar.

When scantlings are kept the same, walnut makes a more rigid boat than cedar, which translates to slightly greater speed. Cedar is easier to work, probably more hazardous in terms of its dust, glues equivalently, dents easily, and sucks at holding fasteners. I always use a cedar keel anyway. The doubled skegs could be cedar, as also the thwart, scupper blocks, oarlock blocks, and rubbing strips. Frames are a toss-up between walnut and cedar, because scantlings can be adjusted. Where cedar does fail to offer good service is the inwale, outwale, and seat-risers. There, I really do need walnut's rigidity and/or its hardness.  

A mixed-woods boat is harder to build in terms of coordinating colors, grains, and figure, but worth doing in terms of weight avoided. So the goal is to build a style of boat new to me at my usual 5 pounds, $15 dollars, and 10 hours per foot.  When the planning work transitions to lofting and then to building, I'll post photos.

[Charlie43]

Some of my calculations in the opening post need revision, because 'beam at the gunnels divided by length from stem to transom' merely describes the look of the boat, i.e., whether it is "skinny" or "fat", but isn't useful for estimating narrowness at the waterline.

From its drawings, the original Whisp can be seen to have a narrow 'waterline beam to waterline length' ratio of just 16.5%. That's a fleeping canoe, not a rowboat. Small wonder the boat is reported to be fast in races.  

I prefer to put 30-32 inches on the bottom at the mid-frame, because that leaves me with about 16" on the sheet of ply to cut one side plank, meaning, I can cut a boat out of a sheet and a half, or two boats out of three sheets, which is $60 bucks of "skin costs" for the bottom, two sides, bow and stern transoms of an 8' pram, plus have a stable enough boat in which to stand and cast.  I can stand and cast in narrower boats, and friends stand and cast in their canoes. But I don't like doing so.

So, as always, I'm going to have to make some choices. How much 'lateral stability' am I willing to exchange for a bit more 'hull speed'? If I were truly trying to build the fastest rowboat I could, then it wouldn't be Whisp, but something like the Drake 17. But I don't want to build longer than 10', and I want a boat that's comfortable to stand and cast from.

Currently, my drawings are putting 105 inches on the waterline when the displacement is 225 pounds. Narrowing the waterline beam by an inch or two or three isn't going have a significant effect on the waterline B/L ratio. But those changes would make the boat increasingly 'tender' as the 'center of gravity' is shifted higher, which is exactly what happens when standing to cast. My drawings already produce a boat that moves 1.5x faster than an average person walks. If that's fast enough on land, it's fast enough on water. My re-design is good enough. I'm done.