Reverse rocker in a small canoe?

Discussion in 'Boat Design' started by kengrome, Jun 17, 2008.

  1. kengrome
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    kengrome Senior Member

    I'm familiar with the removable skegs on some surfboards, so yes I think I know the kind of slotted skeg you're referring to. For this boat I'm thinking of a curved slotted skeg attachment, with the curve following the aft end of the boat at the waterline.

    I don't want to put the skeg(s) beneath the hull because they will only increase the boat's draft and drag on obstructions. I prefer a slotted attachment at the lower portion of the stern, so the skeg can trail behind the hull rather than be located beneath it:

    [​IMG]


    Putting something similar on the front of the boat is just too bizarre for me to think about at this stage of the game. On the other hand, a mini-daggerboard in the bow might be a 'good thing' in conjunction with an aft skeg like the one shown here, especially in windy conditions. With only this aft skeg I'm imagining the boat will weathercock to leeward rather insistently.

    In calm conditions with no daggerboard deployed it might even spin around on its tail when you try to turn it. I'm not sure if this is a good or a bad thing ...
     
  2. Knut Sand
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    Knut Sand Senior Member

    Hey...That doesn't need to stop the discussion does it?

    Esteggs had a comment redarding "bow down", I just found a working brain cell; If you take a wing of a plane, moving forward in the air... The lift is on the aft backslope of the wing, Now turn that upside down, place it in the water the max "wingspan" will be in the middle of the boat, the... well I'll call it minuslift, will be in the front half of the boat, pulling the front of the boat down, in addition to the lift mentioned by Esteggs....

    diwebb, commented that we would have a wave to "ride" on, that's correct, but I'm not an experienced paddeler, I alter speed, I lean to starboard, port, left leg forward, under my arse, standing on my knees, all these movements change the speed and operation of the canoe. And any change of speed, will force eiter to climb up or fall down from that wave, wasting energi. Come to think of it... I changed the batteries on the scale at the bathroom, I do have some energy to loose, I think.... Maybe I should borrow a canoe again....

    Kengrome; Skeg? nice idea, but I would be a bit worried during landings, I normally ground the thing, moving weight aft, giving a hard stroke right just before hitting shore, then I stand up, and walk dry on shore.... Probably not by the book, but I just hate wet feets, and I always have duct tape or electrician tape in the luggage, and I'm not worried for an hour or two in the garage....

    Rudder, the one you linked to will require to keep the legs straight out, like in a kajak..... then i suddenly remembered; I have forgotten to ask what kind of oar are you thinking here? What about a (narrow) kajakoar? A tiny keel line will then do the job? If you shall have one oar, what about considering to make the shape different from starboard/ port, so that without paddeling, the hull takes a right turn, of course, then lefthanded people would need the canoe to be identical over the bow/ end, if i get my explanation through here...

    But Confucius would have a quote that fitted here....
     
  3. rwatson
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    rwatson Senior Member

    All this talk of the hydrodynamic effect on an eight ft hull that would barely go over 6 knots at 'full throttle' seems to be making a mountain out of a molehill.
    It would not be the first boat to have a 'bulbous bow', but as for having a significant effect on paddling - I have strong doubts.
     
  4. kengrome
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    kengrome Senior Member

  5. EStaggs
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    EStaggs Senior Member

    Ken, anything forward intended to hold the path of the boat will have to be countered by something larger aft. Ask anyone with a deep forefooted boat about running along in waves under power. You can constantly feel the boat trying to move against the skeg.

    With an appendage forward, it will bite and spin with every tiny amount of pressure on either side, especially those incurred by a paddler's stroke.

    Think of a tournament ski boat. They are pretty flat bottomed, typically V drive, with a meaty rudder. They will slide, skip, and have ultimately bad manners UNLESS you add small skegs (some call them skid fins) amidships. This gives the rudder something press against in the water (for lack of a better explanation). Equal and opposite reactions, as it were.

    E
     
  6. ancient kayaker
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    ancient kayaker aka Terry Haines

    Having designed, built and paddled several lightweight single paddler canoes the subject of this thread is close to my heart. The first few of my boats were subsequently sawn up and discarded; experience is a great teacher, so here's mine for what it's worth.

    There is a considerable body of knowledge for small canoes; they are often called "lost pond" boats. I assume you will sit on the bottom and use a double-bladed paddle. Unless, that is, you are a midget. Any attempt to kneel will get you a dunking, so don't practise J strokes ...

    Stability is in short supply on such small boats. My first canoe had a vee bottom and suffered from poor stability, bouyancy low down on the centerline is bad news when it comes to stability as it raises the boat (and the CoG) in the water but does not raise the metacentric height. I ripped out the bottom and installed a flat one, better but no cigar. The next boat was also flat bottomed but broader in the beam. Big surprise, it was stable but felt tippy and unsafe; even with a snug-fitting seat and foot braces to keep me firmly centered I was not comfortable in it. The next one was a gem, bottom just wide enough to accommodate my butt, seat one inch above the bottom, garboards out to meet the sheerplanks just below waterline, all ply, a classic five-planker. I fed the Wee Lassie design (a classic lost ponder) into design software and got a metacentric height of 1.15 ft, my five-planker is 1.13; this is probably the minimum you should aim for in a boat intended for an adult. Next time I will try more rocker (zero on Wee Lassie, 0.8 on the five planker) to see it it improves stability by reducing CoG; I would not expect reverse rocker to improve stability.

    Some other notes. My boats have been in the range 10 to 12 feet; I am wondering why you want your one so short. I've seen rowing prams as short as 7 ft; a canoe that short may be slow and spinny. If the design driver is light weight, mine are made from 3 mm and 4 mm marine ply and come out to well under 20 lb fully finished and furnished; it is possible to get down to 15 lb. You originally planned on ply construction; it is much more pleasant than working in glass-fiber and the plug has to be perfect. If you changed your mind I can describe my method which produces a nice-looking boat with less than 40 hours work. Tumblehome will reduce final stability; my boats are around 27 inches beam without tumblehome with about 5 inches of freeboard; using a double-bladed paddle I have no problems with the paddle hitting the gunnels. Tumblehome is a handy technique on larger, wider multi-seat canoes with oodles of freeboard but I don't think you need it. I use a simple keel, 0.7 inch square wood full-length, gives good directional stability. Adding a skeg on a new design is probably best done after testing it in the water, another reason for choosing wood construction. In my experience most large canoes want to turn downwind which would need a forward skeg and most small canoes want to turn upwind and would benefit from an aft skeg. A slight tendency to turn upwind is OK as it allows the paddler to concentrate on power rather than steering.

    Hope some of that helps!
     
    Last edited: Jun 22, 2008
  7. kengrome
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    kengrome Senior Member

    Hello ancient kayaker,

    All of it helps, thanks very much!

    My current thinking is to eventually mold these boats and sell them commercially, assuming I can come up with a shape that works ... especially a shape that works well *and* looks cute and very much like a canoe. This is why I want all the curves. A one-off boat is always easier built in plywood, but for a production boat a mold makes sense, not only for fast and efficient production but also because there's no wood in it (since Americans seem to have lost much of their appreciation for wood in their boats over the past few decades anyways).

    The super-short length is for lightweight and easy transport, and also because I enjoy being challenged to design the shortest practical canoe in the world. What I mean by 'practical' here is that a person can store it in his/her small apartment, carry it to and from the beach without any help (and without a luggage rack) and the boat will take care of its paddler and behave well in the water -- instead of feeling too unstable or actually launching the paddler into swimming mode unexpectedly if the boat should happen to get into a few waves or breezes.

    If this boat doesn't 'glide' like longer canoes that's fine because it is really meant to be a local explorer or gunkholing boat, not a long distance traveler. The term 'lost pond boat' seems appropriate here because this boat should be just big enough for one average (or smaller) person to carry almost effortlessly from his/her car, bicycle, skateboard, etc. to the water then get an hour or two of calm paddling and exploring in -- very near where he/she launches the boat -- before going back home and calling it a day. It's not a camping boat or an overnighter, but for one person carrying little more than a windbreaker, a camera or a fishing pole, a couple of granola bars and a bottle of water, it might be a great little boat when used for its design purpose.

    You kept the garboard planks 'just below the waterline' which I suspect is ideal for smooth and efficient glide since it prevents the upper chine from getting into the water and creating unnecessary turbulence. Do you feel that keeping the upper chine above the water line also helped in terms of this boat's feeling of initial stability? Or do you think secondary (final) stability was improved by this feature more than initial stability?

    It is interesting that the metacentric height in your boat happened to be 1.13 ft because my first version was almost exactly the same at 4 inches draft and 175 pounds displacement as shown in the hydrostatic report attached to my first post in this thread. What do you think of those numbers? Based on your experience are they any good or is my first version "way off" in terms of being a decent boat?

    In my latest version which has a shallow vee bottom -- and which I widened from 28 to 35 inches just to see what would happen to the numbers -- my metacentric height has gone all the way up to 2.889 ft at only 3 inches draft but with lower displacement of only 161 pounds. Yet at 4 inches draft and a substantially increased displacement of 235 pounds the metacentric height is still quite high at 2.004 ft.

    Naturally with such a broad beam this latest version will probably need a lot of tumblehome which as you've mentioned reduces secondary stability. But the wider a one-person boat the less secondary stability it should needs anyways, so this short, wide version may actually make more sense than the narrower 28 inch wide version I was previously considering. What do you think?

    It seems like it might. I've added rocker to my recent software models too, although I suspect this feature is best evaluated during real world testing.

    Neither do I, in fact I think it will make it worse, so I have decided to forget about it and go with some normal rocker. How much to use is another question, but the boat is far from being built yet anyways so I have plenty of time to consider this and maybe try out a few models. Do you have any thoughts on how much rocker might be appropriate for such a short canoe?

    Exactly! Double paddles will make it easier to keep the boat wiggling along in a more or less straight line, but reducing the wiggling back and forth with each paddle stroke is my primary design goal ... or at least it is something I cannot seem to ignore or forget about since I think it will be the biggest potential 'problem' to overcome when the boat is only 7 feet long to begin with.

    I will still move ahead in my concept for this particular boat, but that doesn't mean I'm foolish enough to ignore your experience! I would be honored to have you describe your method -- right here in this thread if you're willing to post it here. I think we can all learn something from others if we keep an open mind, and I would very much enjoy learning how your method produces the nice looking boat you've described. You're welcome (and encouraged) to post pictures of your boat here too if you feel like it ... :)

    Do you sit on the bottom of your boats or do you kneel down or sit on your heels? How tall are you? I think the taller you are, the less likely you are to hit the gunwales when paddling. I'm only 5'9" so I will probably appreciate a bit more tumblehome than taller paddlers.

    If I ever get to the point of building this boat, I want to design it for a removable skeg. I know a skeg will produce a different behavior than a full length keel but my preference in terms of shallow draft is the skeg if I have to sue one or the other, assuming it works reasonably well of course, since it preserves the boat's shallower draft which I think is a key feature for such a short, slow boat.

    Thanks again for the great input ancient kayaker, it's a pleasure to have you here!

    :)
     
  8. ancient kayaker
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    ancient kayaker aka Terry Haines

    My pleasure, I get a lot of help from this site and it's great to be able to "pay it forward".

    Now I understand your objective I can understand your thinking. There's quite a lot of small plastic kayaks being sold at low prices, mostly around the ten foot length, my first was 9ft 4 in and with a 30inch waterline beam she is more stable than seems possible; her downside compared to my home-built boats is weight and lack of cargo storage. The shortest commercial boats I've seen were whitewater kayaks at a competition, I didn't have the nerve to stop one of the competitors so I could measure the boat but they looked about 7 to 8 ft so your concept is doable. However, I don't agree with you that Americans have lost their taste for the wooden boat, there are many wood boats being sold as kits or finished but many are costly.

    Some caveats about "going commercial". Can you compete on price with the rotomolders? In the small canoe category you will also be competing with folding boats like Pelican and Yakka ... but as you say you enjoy a challenge. If your use of tumblehome forces you to sacrifice the plug after each build your costs are going to rise and your productivity drop. You may have to aim at a niche market. As an eternal teenager trapped inside a senior body I aimed at a boat light enough for arthritic hands to carry a few hundred yards from a car park to the waterside and still have a decent performance in the water, and be easy to get in and out of without getting dunked because my legs don't bend enough these days. That's a good example of niche marketing, the geriatric canoe ... but I digress. I have had several indications of interest, not sure I want to build for $$ but I am now thinking about kits and/or plans, so I'm looking to further simplifying the build.

    I think the term "lost pond boat" comes from the adventures of Geaorge Washington Sears who wrote magazine articles of his canoing adventures under the name "Nessmuk", there's stuff on the Net about him and the Wee lassie is a direct descendant of his boats which were intended for "a few days on the water and back to the lodge" trips, but they're great for a day on the water just like your concept. I sit on the bottom of my boats and use a double-bladed paddle like kayakers, and I am 5'11", 190 lb. A shorter person with a lower CoG should be able to use a slightly narrower boat with slightly less freeboard, but commercial boats have to acommodate a wider range of paddlers than a custom boat.

    For my "classic five planker" the chine between the garboards and sheer planks is entirely below the waterline to maximise waterline beam and therefore initial stability. From that chine the sheer planks go straight up to the gunnel, no more chines. The garboards are twisted as they approach the stems for a clean entry and exit and the stems are as vertical as is compatible with my construction method in order to maximise the waterline length. Any secondary stability I got was not planned for but gratefully accepted, it's not huge but enough to relax with.

    The keys to stability are waterline beam (cube law) and low CoG which benefits from increased draft, a fraction of an inch has a noticeable effect. Your metacentric height and draft numbers sound reasonable based on my experience but I am not a marine architect. As a retired electronics and robotics engineer I can now practise the build it and toss it into the water approach but that's a reaction to decades of disciplined analysis and design reviews ... it's OK for a small wood boat that can be built two or three weeks when failures can be cut down to fit in a garbage bag but perhaps not quite the way to go for fiberglass. Based on the cube law (only an approximation as other factors can come into play) increasing waterline beam 25% should double the stability but note that a wider boat is harder to paddle and a boat that is very wide in relation to its length may be hard to keep on course; paddling a coracle requires a lot of skill. My first kayak is about 30 inches wide at the waterline and the side deck starts an inch or so above the water and slopes up to the cockpit coaming, but I still tap the deck with the paddle occasionally. I don't have any many thoughts about the design of such a short canoe as it is way outside of my experience. I suggest you troll the net; observation is almost as good a teacher as experience and a lot cheaper. Boats are complicated things and different designs behave in different and sometimes surprising ways. You may want to consider using wood construction to evaluate designs in the water. My building method is described below but it is optimised for five plank designs and you may want to use more planks in the prototypes to closer approximate the smooth hull that you are aiming for, therefore stitch and glue may suit your purposes better.

    My building method is very simple; I prefer not to use non-biodegradable materials like glass and I also minimise or eliminate epoxy. For a five planker (2 sheer planks, 2 garboards, bottom plank) I start with the sheers using 3 mm marine ply. A 12+ ft long piece of 3 mm ply of 9 inches width reduced to less than 6 inches at the center is very fragile and need reinforcement so I glue on the inwales and chine logs at this time, together with stem cheeks along the inside of the stems so the planks look "framed" from the inside. The cheeks are bevelled to half the angle between the sheer planks measured at the stems. This angle is computed using a somewhat complex spreadsheet so I can get it right first time but it can be estimated from a drawing and adjusted to fit. The boat is assembled upside-down. The two sheer assemblies are bent over one or two ply molds and the sheers are pulled in to meet the ply inner stems; I drill through the stems for two thin dowels to keep things in line as the glue is slippery, then glue, tape and clamp the stems together. The method so far is very similar to some flat bottomed canoe and pirogue designs. The inner stems extend above the sheer assemblies to support the bottom and are thickened with cheek pieces to provide extra glue area to accept the garboards. The bottom is also reinforced with chine logs along each edge; and bends over the mold(s) and is glued and screwed where it meets the tops of the inner stems. Now comes the tricky bit. The edges of the bottom and sheers have to be beveled to provide a glueing surface for the garboards, also the cheek pieces on the inner stems. This I do with a hand-held power plane modified as follows: the baseplate behind the cutter is replaced by a piece of ply of the same thickness which extends out on both sides of the cutter by a couple of inches more than the width of the garboards. This means the tool can be rested across the edges of the bottom and sheer planks to get the bevel at the correct angle, especially at the ends. I cut enough width for a good bevel, no need to cut down the full thickness of the plank and chine log, 3 to 4 times the ply thickness is enough. If the power plane won't reach everywhere I use a low angle block plane to finish off. Now I dry fit the garboards and check the fit of the bevel; using a thickened epoxy glue here means a perfect fit is not needed but a lousy joint will look er ... lousy. I drill tiny holes for temporary screws through the garboard into the chine logs about every 6 inches; this takes some care and a small drilling jig to avoid missing the log. Finally the garboard is glued and screwed. Later the screws can come out, at least for my boats where the only metal is the painter ring, and I plug the holes with short dowels. After cleaning up the garboard edges with a router I can flip her over and add the outwales and thwarts. I have a new design on the board which has no twist in the garboards; this results in a constant bevel angle so I will be able to rout it while the bottom and sheer planks are still flat on the work bench; provided it is good in the water this will save the hassle cutting the garboard bevels. I am not sure there is anything new in this, it was put together from stuff I read plus experience over a few boats and just works for me.

    Good luck!
     

  9. Petros
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    Petros Senior Member

    with many miles in canoes and kayaks, and having built quite a few too, I would say that hull will feel like you are pushing a shoe box through the water. It will be a PIA to keep in a strait line, will want to turn around with each paddle stroke, will be blown about in the wind and small waves.

    My suggestion is to stretch it out a bit to at least 9 or 10 feet, keep it under 24 inches wide, add a fixed skag as far back as possible. A simple skag can be held on with two bungy cord straps until you are happy with skag size and location.

    Good luck
     
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