Kempenfelt 12-1/2

Don’t worry if you’ve never heard of the Kempenfelt 12-1/2 - it hasn’t been designed yet, but I’ve named it after a Lake Simcoe bay in memory of the 12-1/2 LWL Buzzards Bay Boats - please read on . . .

Last year there was a post in the Fast Rowboat thread http://www.boatdesign.net/forums/boat-design/designing-fast-rowboat-14250-82.html#post466601by DickT, who asked in anyone has modeled a boat design with vertical chines (radius chine construction) instead of the more customary longitudinal ones. We kicked the can a couple of times for a few threads but I was looking for a suitable canoe design at the time, and it isn’t a suitable idea for narrow boats since the bend radius required for ply would result in very soft bilges and an unstable boat. Perhaps a rowboat or small sailboat would be more appropriate, but after creating one or two concept designs I ran out of the necessary inspiration to pursue the idea further.

Fast forward to a couple of days ago: I picked up a copy of issue #217 (Nov/Dec 2010) of the WoodenBoat magazine, and reread the article by Mike O’Brien on the Somes Sound 12-1/2 designed by John Brooks. The article compared it to its venerable (and venerated) ancestor the Herreshoff 12-1/2, and other offspring such as the Haven 12-1/2 (Joel White), Bolger’s plywood 12-1/2 and the closely related Buzzards Bay 12-1/2 and 14.

There is little difference between them, at least above the waterline, clearly several great designers tried and were unable to do more than make minor adjustments to an outstanding design, except for Bolger, and I don’t think he was too enthusiastic about the ply version.

Those three things have been rolling around in my mind ever since. The 12-1/2, ply and vertical chines. There’s nothing for it but to resume the earlier effort. But there’s no way I want or can use a boat that’s 12-1/2 foot at the waterline, too big to car-top and I don’t want to bother with a trailer.

On the other hand, a 12-1/2 foot LOA would be ideal. So what would happen if I scaled the Haven, with its relatively light displacement of 1400 lb, down to 12-1/2 LOA which is 78% of the original LOA? This is the design target that I would like to explore in this thread. The Haven seems to most suitable starting point of the 5 designs illustrated in the article, because it has one of the smallest displacements and is available as a study plan.

First, displacement would come down to 1400 x 0.8^3 or 667 lb. That seems a bit heavy for a car topper, even less crew weight allowance 425 lb for crew of two it leaves 240 lb which is about twice what I would feel able to handle. I think I can build a boat of that size in ply to a 125 lb target if the displacement could be reduced.

80% of the original beam of 6' is still adequate at 4.8' so that can stay as scaled, and the lines above the waterline scale nicely to a shape that can be built with radius chine construction based on my recent investigations - or glued ply lapstrake for the less adventurous.

The obvious way to reduce displacement is flatten the bottom; the above-the-water looks of the original 12-1/2's can still be preserved although there will be some loss of righting moment from the lack of a ballasted keel, and a centerboard with a little ballast will help replace some of that. It will be sensitive to crew weight which will make it twitchy for solo sailing, so water ballast may be an idea. Draft with board up would be around 4" at a guess with a full-width flat bottom, but a hard chine may not be a good idea; exposing the bottom would mess up the esthetics and reduce righting moment and maybe spoil the sailing characteristics, so a more gradual transition and a narrower flat bottom plank seems called for.

While the study plan of the Haven 12-1/2 is on its way to me, any suggestions and comments for or against are welcome as always. At this point I am looking for inputs on the lines, then I'll move on to construction, sail rig, accommodation and so forth.

 

Kempenfelt 12-1/2 SOR

Well, the Haven 12-1/2 study plan arrived, bit of a disappointment as it is really just a magazine article with no lines or offsets (which I expected in a study plan) although a surprising amount of construction data (which I didn't expect) so to understand it properly I will have to print out and measure the pretty pictures. It's going to take a little longer . . . however one thing is immediately clear - the Haven's construction is a challenge for an amateur builder.

In the meantime I really should create a SOR for the Kempenfelt 12-1/2; this is the SOR for the design only:

Similar appearance to Haven 12 - 1/2 above the waterline
LOA 12.5' (3.8 m)
Accomodation for 2 or 3
Ability to sail on or off the wind
Backup propulsion using oars
Rig can be lowered to clear low bridges
Added: spars to store inside cockpit if possible without interfering with rowing
Transom to accommodate electric trolling outboard
Day-sailing use with provision for easy beach-or ramp launching
Capable of single-handled sailing and car-topping
Primarily for use on rivers, small lakes and sheltered waters
Stabiity more important than performance
Able to be righted by solo sailor after a knockdown
Simple amateur-suitable construction

 

I would be very surprised if a set of study plans included offsets.Too many unscrupulous people are likely to build from such a set of plans rather than paying for the complete version.Good luck with the 12 1/2 project,I for one will be watching with interest.

 

Good point: while for an amateur such as myself the full-size part drawings, other construction details and perhaps the instructions are the important parts, an unscrupulous pro could easily get those close enough from observation and experience. Tricky for an amateur though. The construction details are of little interest as my thinking on construction is heading in an entirely different direction - exploring boat construction methods is a hobby of mine.

The lines are only needed as a pointer to get "the look" from the waterline up, but I can't hope to get as close to Herreshoff as White and Brooks did on a smaller boat. Together with the information in the WoodenBoat magazine article, I think I can get what I need from the study plans. I will likely use the sheer in plan and profile, also the stem as close as I can get with plywood. I want to get the transom much closer to the vertical and disguising that change will take work. As it is the transom would waste space in such a small boat and make it impossible to hang the rudder from inside when changing from oars to sail. I was considering changing the flat bottom to a shallow Vee to look better when the boat is inverted for car-topping but that would require floors and floorboards and lose valuable internal height so I will keep the flat floor.

I am finishing up a solo canoe with mixed ply and strip construction and laminated ribs - an interesting experiment with constant radius chine design. I also did some experiments in an attempt to adapt Kurt Hughes cylinder molding technique to canoe and kayak construction which - while unsuccessful - suggested other construction methods; but that is getting ahead of the game. Lines first . . .

 

First Cut at Lines

Prelim sheer plan and stations for 12.5' LOA design superimposed on image of Haven 12-1/2. The curve of the Haven's sheer plan is close to a circular arc which is shown for information only.

I am testing the idea of using constant chine radius. Midships is a good match and the transom is closer than I expected, but it requires some experiments on ply bending characteristics to see how good a match can be achieved in the bow. I haven't addressed the profile or transom angle yet.

If you want a copy of the Haven 12/1/2 study plan it is available at a very low price as a pdf file from http://www.woodenboat.com/wbstore/index.php?main_page=product_info&cPath=1_68&products_id=233

 

Line design is progressing. I applied 2/3 scale to Haven hull in the horizontal dimensions and 3/4 vertically; external appearance of Kempenfelt in the water should be similar to the haven with allowance for the smaller boat size.

At this point I expect draft to be at 2/3 the depth of the Haven cockpit floor which in Kempenfelt becomes the hull bottom. The seat edge height is about 10" with about 1" slope to the back edge and a coaming height about 9.5" above that, which should be reasonably comfortable, but later I'll need a mockup of the accommodation to verify this and other dimensions.

 

Prelim. Lines

Rough Vn.1 lines of Kempenfelt with 2/3 scale of Haven superimposed in Blue. Transom has errors and station 4 - which is actually at X - 2.67 due to 2/3 scale - has not been determined yet. The other stations assume constant chine radius from station 7 to stern. Waterline is still tentative.

 

Vn.2 lines of Kempenfelt transom and station identification corrected. The stations have been corrected by applying a constant chine radius and varying the arc length as required.

The hull has slightly more volume forward and less astern than the original 2/3 scale Haven with the bottom flattened, I hope this will accommodate a more forward solo crew location where the higher coaming is more comfortable. However all of this, as well as the waterline is preliminary. This is as far as I can take it using this sketching procedure, I will try to replicate it in FreeShip.

 

I'm not sure what you hope to gain by using vertical chines. Building this thing is still going to be a lot of work.

 

Combined with a constant chine radius design, the idea has some potential advantages over the conventional longitudinal chines for a one-off build, like a simplified building form, less waste and plank developments mostly identical. The planks laid across instead of along the hull will be shorter, wider, easier to handle and deliver a closer approximation to a rounded bilge hull. It's not new but as far as I know it is used only on larger boats around the curve of the bilge http://www.boatbuilding.com/article.php/radiuschineplywood/print

After reading DickT's post - per post #1 above - I started looking for a suitable canoe design so I could try out the idea on a small boat. I couldn't find a suitable canoe design at the time - although I have one now - so I looked around for a nice looking sail or rowboat I could implement in this manner.

I have to admit I mostly want to see how it works out; I would like another sailboat but don't really need one. Not really . . .

Building this design would be a lot easier than either the Haven or its Buzzards Bay predecessor by Herreshoff with their multiplicity of steamed ribs and Carvel construction. Also far lighter, hopefully with true car-topper capability. I'm not sure I will actually go ahead with a build as I have a lot of boats planned already; more likely I will create a model about 1/10 - 1/5 scale to identify any serious bugs, but who knows . . .

 

FreeShip design of the Kempenfelt 12-1/2

Burning the midnight oil here but I've fought the good fight with FreeShip and have what looks to be a reasonable design. Certainly in these images it has the "look" . . . IMHO anyway!

Draft 4/6" at 360 lb (solo), KM = 4.6', resistance at 4k: 8 to 9 lb, 3.5K at 5 lb (rowing)
Draft 6.2" at 585 lb (crew of 2)

ps., let's call that Vn. 3

 

Ooops - forgot to look at the stability! I'm not trying to design a dock here but the KM is too low. I didn't realise just how much RM is lost in a flattie without a wet transom.

I'm not prepared to flatten the transom bottom, it would be a different boat, but the bilges are pretty slack and can probably be harder and I can add a bit more beam without spoiling the esthetics.

On the plus side the cross-curves look nice, stability rises nicely at the higher heeling angles and flooding angle is great despite the narrow side decks.

 

Version 4

Significant results from changing the transom profile, which I didn't want to do. Bilges are only slightly firmer in this version (Vn4) but because of the transom redesign the waterline beam has increased approaching the stern and as a result KM is up 28% to a respectable 5.9'

She should handle a modest 70 sq ft with that, which should have an adequate performance as the drag is very low 8.8 lb at 4k. She should also row nicely.

But I doo mourn the loss of the previous transom profile. Not really what I wanted. I want to see what I can do in other areas . . .

 

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Don't settle for what you don't really want, AK. You can get it the way you want it by compromising(if need be) in some other area of the design.
Good Luck.....

 

Now there's a man after my own heart! Like PAR says, boat design has always been about compromise . . .