Kempenfelt 12-1/2

I went back to an earlier design version and tightened the bilges further with a compromise transom profile that preserves the Haven look. I also made the bilge radius more-or-less constant from transom most of the way to the stem, relieved at the ends to get the desired transom and stem profiles.

The above changes increased the KM considerably but the beam has crept up so I have reduced it back down to the original target of 4.8 feet. kM is now an accceptable 5.4' at 425 lb displacement.

Cross curves look smooth but I don't yet know how to interpret them and flooding angle is better than 44 deg solo and 42.5 deg with crew of 2, which should be safe enough even for me.

Drag is still around the 9 lb mark at 4k and does not increase significantly with a crew of two so sail area requirement is small and still an easy row at 3.5k. Drag numbers are somewhat relative since FreeShip is hardly the most sophisticated analytical tool but I use the same drag as I do with my canoes to estimate easy rowing speed so it won't be far off.

Not visible in the image is a slight deadrise, which for the planned construction method is easier to achieve than a perfectly flat bottom.

The cost of these changes to achieve my stability and esthetics objectives is the bend radius for the 4mm ply around the bilge which is 7.5", very tight, may require heat and/or moistening to obtain without breakage. However a percentage of plank breakage is more acceptable now with a slight change to the construction approach whcih allows the planks to be separate instead of one piece per side as previously planned.

Preliminary construction plan is as follows:-

Approx. 32 Transverse ply planks (about 65% identical pattern) butted and laid gunnel to keel over inwale, stringer (doubles as seat support) and keel; other stringers as required for fairing and strength. Ply ribs (all identical) over plank to plank joints, epoxied of course. Usual stem, transom and outwale arrangements and probably a rail below the transom to separate the bilge curve from the almost vertical sheer. For a car-topper I don't plan to use epoxy preg or glass.

Now I have to go back over the design to ensure I didn't lose sight of anything else like the creeping beam mentioned above!

 

Ok, at this point I can't resist making a suggestion. 7.5 inches, while it may be impressive for some things, is an extremely tight bend radius for 4mm ply being bent along the grain. My bet is you will require some shenanigans to achieve that, and this means more work. You also have all of those frame to make, and you say you'll have stringers as well.

It seems to me that for the less work you could put the ply diagonally over stringers with very few frames. This will give a much easier bend. It may not be a new and exciting construction method, but it works.

 

We'll have to see about the simplicity of build, and I'm not yet sure I will proceed anyway - I don't really NEED another boat . . .

The bend radius is indeed tight. Bending plywood is generally OK to around 80x thickness which would be around 12" for 4mm ply, then it gets tricky. It will almost certainly need heat, but the planks are only about 3' long, so a hot box is practical. Dry heat seems to work fine. I have found the ply can be bent in stages, heat will easily get it to about 9" rad then it will go to 7.5" easily - often without heat. Need to experiment to test for breakage rate and loss of strength.

I still want to play with the lines a bit more to see if I can slack off on the bend radius . . .

 

Found some information about bending plywood on this site http://schmeling.com/reference/pdf_files/plywood_bending_radius.pdf

It is reassuring that the article claims the plywood's strength is not diminished by bending. Based on that the min dry bend radius for 4mm ply would be 15 inches. I found it bent to 12" probably because it is only 3 ply, which is more flexible since the core veneer that is bending with the grain is further from the outer surface. This article suggests I can get the bend radius I want in one step.

However, I suspect there is more optimization to be done on the lines.

 

I picked up some nice aircraft style 0.8 mm ply today during my yearly trip to worship before those beautiful and unattainable* Veritas planes at Lee Valley Tools. I'll be able to make a scale model to see how the vertical chine idea turns out. I'll probably make it about 1/6 scale, that will make the ply scale to 4.8 mm full size, a little thicker than the 4 mm ply I plan to use if I build the full-sized boat. I find using slightly thicker than scale wood works well in a model, it compensates for my inherent ham-handedness. I can use card for the decks and accommodations, that will leave half the ply sheet for modelling another boat design I want to build later - that stuff's expensive.

So many boats, so little time . . .


* unattainable, that is, since my van spent 5 days in the local garage . . .

 

I've been thinking about the weight of the Kempenfelt from the car-topping perspective. In an earlier post I set a somewhat arbitrary target of 125 lb all-up weight That’s sail-ready but unloaded - not displacement! Deducting 20 lb for rig and maybe 15 lb for other removable items such as rudder, daggerboard and maybe seats leaves me with 85 lb for the car-top hull.

An independent finished weight estimate for the hull came out to 87 lb skinned with 4mm Okoume, calculated from FreeShip’s skin and deck areas plus 10% for ply ribs, 40% for lumber and and an additional 25% for seats, fittings and finish, based on other boats I built.

Currently I have a 60 lb sailboat hull that I could car-top even when I was struggling with polymialgia rheumatica. That is now gone and I am confident I can handle the extra 50% without problems. I used to be able to hoist my heavy (80 lb) fiberglass on top of a car unaided, those days are gone but I’ve learned the trick is - never lift more than half the hull at a time.

Not much other progress to report as I am currently building a model of a Wee Lassie, my next planned build on which I plan to use the construction concept in my infamous http://www.boatdesign.net/forums/wo...storation/zipper-seam-construction-18316.html thread, wich has been waiting nearly 5 years to see the light . . .

 

I have revisited and - I hope - finalized the design.

The beam remains at 4.8' and the minimum bend radius for the ply is now 12" which reduced the KM to 5.04' at the solo displacement which compares with my existing sailboat which I find very stable. The radius is virtually constant - it's hard to be exact with FreeShip - from the transom forward to about 3' from the bow when it begins to increase - necessary to define the forefoot.

It has a compromise transom shape, reminiscent of the Haven 12.5 but flattened to increase RM when heeled, necessary due to lack of lead ballast. The transom is more vertical, 30 deg instead of 45 deg to extend the LWL and also permit hanging the rudder from inboard when changing from rowing to sailing.

The centerboard of the Haven is changed to a daggerboard; I am looking at a modified trunk to allow it to pivot forward when retracting so it can tuck under the foredeck, and allow the aft of the trunk top to be closed off with a rowing seat above it, but this is just an idea at present

There will be a permanent skeg bringing the rudder pivot axis almost vertical and the rudder will follow the Haven's line above the surface but it will be a vertical fin below, matching the change from a centerboard to a daggerboard.

No water ballast is planned.

I would prefer a lug rig for its simplicity but that would completely disguise its relationship with its larger predecessors so I will probably go for a gaff sloop rig similar to the Haven's 133 sq ft rig. A straight scaling brings that to 80, a bit too large I think, I will further reduce it to around 60 sq ft and try to fit the sticks inside the hull at the same time for rowing, which I expect this hull will do rather well.

 

ancient kayaker- for what it's worth, I finally seem to have my little water ballast stability program running. see post 13 over here - http://www.boatdesign.net/forums/stability/water-ballast-hull-form-42435.html

 

Accommodations and sail rig

Accommodations:
- since the boat is only scaled down to 78% and there is still plenty of beam (4.8') I can have the same side benches as the Haven although I will need a mockup to determine the exact dimensions. The rear deck of the Haven will not work at this size however; I think it can be replaced by a transom seat, more for appearance and structural reasons than actual comfort although it should be useful for shipping the rudder from onboard when changing from rowing to sailing. Due to the flat bottom there should be no need for a cockpit floor. A rowing thwart will be needed, that could be a nuisance when sailing, I may make it removable so it can be tucked under a side seat.

Rig:
- I wish to retain the elegant gaff rig, which also allows the use of shorter sticks than the alternative marconi rig. Even when scaled down and after reducing sail area by a further 25% to allow for the reduced righting moment of a smaller, unballasted boat, the mast is still over 10' long which will not fit inside the hull. However, a slight change to the rig with a higher peak and a longer spar will allow the mast length to fit within the WLW, although a mockup will be needed on a model to confirm this. I will probably make the jib loose-footed.

Most of the initial requirements appear to have been met, but I'm not sure how to verify the ability to be righted solo after a knockdown. Maybe I can do that on the model . . .

 

Terry, I really like your boat. My only thought is that you are losing so much righting moment due to loss of beam and displacement that those little coamings become problematic. Sooner or later, I think you will want to plonk your butt up there on the side decks. Even if its just for self preservation. Can you design those coamings to be nonstructural and removable to facilitate hiking out? Or have a removable filler piece to give you 6" of butt land if you need it?

Aft decks are really nice, even if it's only 12". Just bring it out 2" in front of the transom bottom. The helmsman needs a place to stuff stuff even in a 12 1/2 footer.

 

The side decks are very narrow so I can get well off center inside the cockpit, but if I find I have to change something I am always willing to do that on my boats. My existing sailboat is getting a new rig and rudder this year, for example.

I like your ideas for the rear deck, good thinking, and I will have another look at it later - just put it on my list of things to do.

 

Summary for those new to the thread who don’t wish to wade through all the posts to date: I am trying to scale down Joel White’s Haven 12-1/2 design (2-1/2 LWL) to a smaller and lighter car topped, beach launched sailboat, the Kempenfelt 12-1/2, that’s 12-1/2 LOA and about 78% of the size of the Haven. My approach was to toss the ballast to reduce weight allowing me to also flatten out the bottom to reduce draft. This boat would, rely more on the movable warm ballast (crew). I also want to explore a construction method I currently call vertical chines.


I put the project aside in April but it continued to evolve in my unconscious. Now it’s time to come clean . . . although I have most of the scantlings and construction approach worked out and the vertical chine method looks doable, I just don’t like the design shown in the last few posts. The midships bilge looks too hard for my liking and the transom is blah. I didn’t even go ahead with the 1/6 model I had planned.

Recently I reread the article in Wooden Boat #217 on the Somes Sound 12-1/2 by John Brooks, and I noticed that the midships section is more circular - to reduce wetted surface. I found clearer station curves in another publication and realized that the section has a constant radius and this radius is carried from the transom forward for about 60% of LOA.

Well, if it works for Somes Sound I see no reason why it shouldn’t work for Kempenfelt. Scaled by 78% the radius works out to about 15" which is easily achieved working with 4 mm ply. The Somes Sound station profiles straighten out below the waterline. However, I think if I continued the fixed radius it would merge into a flat bottom at about the draft I want for the target displacement. However, I still plan to start with the Haven outline which I think is closer to the Herreshoff 12-1/2 original.

I also rethought the stability issue. The righting moment varies as the 4th power of the scale, and heeling moment of the sailing rig varies as the 3rd power. In a scaled-down boat the HM is excessive but it can be matched to the RM by scaling down the rig by a mere 8% which, for a gaff rig (closer to the original than the Somes Sound 12-1/2) should keep the sticks short enough to store inside the hull for rowing or trolling. The projected sail area is about 70 sq ft, modest enough for a 12-1/2 ft long boat. The rig may not need to be downsized by the extra 8%: if it gets built I will probably not downsize the sticks provided they will still fit within the hull so I can increase the sail size about 25% if the boat looks like it can handle it.

This new approach will yield a rather more tender boat than the earlier design but it should be more true to the original, easier to build and the sail area is modest. It should behave similarly to the Haven and other Herreshoff derivatives in the water so I don’t have to worry about odd behavior showing up when heeled.

 

Another change I am considering is to the accommodations. The Haven and other Herreshoff-inspired designs have benches along the sides of the cockpit which I assume are appropriate to a larger, ballasted vessel.

However benches give the crew little variety when it comes to balancing heeling forces especially in relatively calm conditions. A transverse seat will probably be better for this more tender boat, and it would provide a suitable place for rowing.

This boat should row easier than the earlier larger designs with its reduced weight, flat bottom, reduced wetted area, and the optimum fore-and-aft position for a rowing thwart would be close to the optimum position for the daggerboard trunk so a permanent thwart will add extra support to this highly-stressed member.

I need FreeShip for hydrostatic calculations but it is difficult to obtain a specific hull shape precisely with FreeShip, so I am going to take time off to try and develop a spreadsheet to generate offsets to import into FreeShip. I cannot get close enough using Carene2008.

 

Hmmm - I was doing some research found myself looking at Joel White's Catspaw design. An open boat but looks very similar to my current design but less bouyancy in the bow and slightly more vertical transom. The current design pursues the constant chine radius all the way to the bow, something I have been thinking about changing.

So I am somewhat conflicted: on the one hand I feel reassured about the sailing and rowing qualities, on the other I didn't really want to produce something so similar to an existing design. However, with all the thousands of designs around I suppose that's inevitable, especially as White produced the Catspaw by upsizing a Herreshoff design, and I produced the Kempenfelt by downsizing another Herreshoff design. The common heritage is bound to show. The proposed construction method is, however quite different from any other row-sail boat I have seen although related to the radius chine method found in many larger boats.

OK, were are we? I haven't quit yet, but the existing lines drawing is an approximation that I'm not entirely happy with. I am looking into making a spreadsheet program or possibly a compiled application to enable me to import the precise lines I want into FreeShip. It's going to take a while as my software skills have atrophied through years of disuse.

 

I’ve been working on the spreadsheet. If it works OK it will help design constant bilge radius hulls which is an area I am particularly interested in right now. I hope it will enable me to get precisely the hull shape I want into FreeShip for hydrostatic and hydrodynamic analysis.

It's taking a while as I haven't created a complex spreadsheet like this one for about 15 years. I got help on how to create simple text files for importing into FreeShip from RWatson at http://www.boatdesign.net/forums/boat-design/making-scale-cardboard-model-45220-5.html#post591692 - previously to that I thought FreeShip import files had to be complicated like the ones that the Carene hull design program generates.

Spreadsheet programs work differently to compilations and their graphics capabilities are limited to say the least. But I'm making some progress. If I get something useful I’ll post it in the software section.