Fishing kayak

Discussion in 'Boatbuilding' started by ShagRock, Jan 4, 2009.

  1. Guest625101138

    Guest625101138 Previous Member

    In terms of performance the Hobie is quite poor compared with a hull like mine or Ian's. They are good for about half the speed of my hull. The flappers are not very efficient compared with a propeller. Data I have indicate they achieve about 35% efficiency in cruising mode and lift to 50% in a sprint although the latter does not achieve much because the hull drag kicks in once over 8kph.

    The actual speed can be quite deceptive because most tend to relate big waves to high speed. I see waves as wasted energy. The attached photo shows a Hobie tandem competing in a 100m sprint. It averaged 10.2kph for the distance.

    This video shows my boat with a late model engine fitted just looping the lake at 12kph:
    http://www.boatdesign.net/forums/at...581d1225663272-pedal-powered-boats-jn_150.wmv
    It takes a minute or two to download this clip - almost 5Mb.

    The Hobies are durable and I believe reasonable value for money but they are slugs compared with most other paddled or pedalled boats apart from short sit-ons. Most people who buy them do not have experience in other boats and some get disappointed when they find they cannot keep pace with a small kayak.

    Rick W
     

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  2. ancient kayaker
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    ancient kayaker aka Terry Haines

    Rick: I see what you mean about (hobie) inefficiency, white water all over the place and a big wake with roostertail to boot! Big contrast with your boat's minimal wake. I had the same experience with my sailing kayak which is a Crittur, 9.3 ft long, BWL 30" and shaped like a sharpened bathtub. Under sail in a goodly breeze I had to keep my elbows tucked in or the bow wave would soak them, it was halfway up up to shoulder height. Almost as stable as a dock though. Molded hulls designs are compromises, not made for speed.

    Not sure if I mentioned it in this thread, but I did not use an ama for the sailing kayak, I used a 1.5 sq ft Bruce foil on that, and it was very effective for cancelling heeling moment once I had the outrigger length and tilt angle right. Monster for leeway though.
     
  3. ShagRock
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    ShagRock Junior Member

    Rick - that video was great and I certainly know of no way someone's going to catch you on a calm day like that!:) Is that a home-made propeller system? Also, I meant to ask earlier if styrene pipe can be bent to shape using heat?

    Ak - Ive seen many types of sails on canoe/kayak. The lateen sail has traditionally been the choice for local fishermen in southeast Asia - easy to take down and out of the way while fishing. But I have seen balanced lug and other rigs used there as well.

    I decided to build one with 4mm marine plywood and chine logs (stringers). I'm going to use dual amas designed to provide flotation to match displacement - but not sure yet how to calculate that? Since I will be using a sail rig, I plan to install the arms just ahead of the mast. There are so many different designs being used for the ama itself. I'm thinking of placing them more forward so as to give more fishing space for rod use at the center and rear of canoe. What defines acceptable placement of amas with respect to lift, buoyancy, drag etc?

    Finally, what do you guys think of using a double-ended kayak paddle in an oarlock at the stern? Will it serve adequately as a rudder? I've found it's handy for turning the boat side to side or keeping it in line while drift fishing. I just leave it in the oarlock while attending the rod (like a traditional sculling oar:).

    Newfie
     
  4. Guest625101138

    Guest625101138 Previous Member

    Most of it. The frame, seat, crankshaft, prop shaft and propeller are all home fabrications. The gearbox is an industrial item while the chainring, sprocket and chain are bicycle components.

    I have never used styrene pipe. Polystyrene is a thrmoplastic so can be formed with heat but if it is expanded then it will take a while to heat soak to soften it.

    Rick
     
  5. ancient kayaker
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    ancient kayaker aka Terry Haines

    On a kayak even more than a larger craft it is important to be able to lower the sail rig quickly and easily from inside the boat if you are planning long trips. Another issue is keeping the CoE (center of effort) down to limit the heeling moment.

    My first kayak sail was a square sail which rolled up over its spar and was hoisted with the paddle but was very small at 8 sq ft. The last sail I had was triangular and folded umbrella style in seconds but it was still small at 15 sq ft and short on performance. I am planning a 25 sq ft lateen on my next experimental effort but have not yet figured how to lower it easily and pack it away for paddling. That may be easier with amas however

    That is the construction I use for my simple lightweight canoes. Simple box shaped “3-plank” hulls like pirogues practically build themselves; you can glue the chine logs and inwales to the sheer planks before bending them over one or two molds; this makes the planks stronger and easier to handle and gives a good fitting joint which allows me to use Titebond III. You can glue the bottom directly to the sheer chine logs, then a sit-on-top kayak could have the deck added at this stage. My canoes are open 5-plankers; after bending the sheers as above I glue chine logs around the bottom plank, bend it over the mold(s) and plane bevels for the garboards; it is more difficult to get a good joint here so I use epoxy. For a more complex hull a proper jig with station and stem molds and stringers is indicated.

    Design depends on the kind of performance you are aiming for, screaming along on one ama or more sedate progress. For the screaming mode one ama and its outrigger must support the entire weight and you might wish to hike out on a trapeze, net or platform. For sedate mode, think along the lines of:
    ama displacement X outrigger length > wind pressure X sail area X CoE

    Having the amas well forward is usual for a kayak to facilitate paddling as well as fishing. That drives sail CoE placement which again is usually well forward. This is not optimal for sailing as it usually puts the sail too far ahead of the CoG. Note that a lateen sail with its CoE somewhat aft of the amas may have more potential for a capsize: this is less of a problem for me as I use Bruce foils which are easier to retract than amas.

    In my experience using a paddle as a rudder is not effective and I tend to run out of arms even though I’m not fishing! It is difficult to turn the paddle to a sufficient angle to get enough steering effort especially at slow speed. A foot operated rudder worked better, unless you are planning on pedal power of course.
     
  6. ShagRock
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    ShagRock Junior Member

    Rick - that's very creative work on your kayak with all the home-built parts!

    AK wrote:
    The akas do come in handy for this purpose. The favorite for many fishermen is the unstayed tacking crab claw, which comes down real easy when you're ready to fish and easy to set up again.

    AK wrote
    I guess one way of getting a more rounded shape in a hull would be to rip strips from plywood sheets and then strip-plank on ribs? Do you see any disadvantages to this method?

    AK wrote
    Another way to free up fishing space is to use a single aka (crossbeam). By the way, what do think of the placement of the outriggers in this picture re it's impact on sailing and stability? Also for theoretical discussion, what effect would 'tacking amas" (like skidoo skis that shift attitude with the 'humps and bumps' of uneven trails) potentially have on sailing capacity.
    [​IMG]

    As you suggest AK, a foot operated rudder is practical and convenient,especially if you are fishing too.

    Newfie
     
  7. ancient kayaker
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    ancient kayaker aka Terry Haines

    For a rounded hull I’d prefer to use cedar strips which bend both ways. Ply objects to bending within it’s own plane. Unless the planks are very narrow they must be cut accurately as in the stitch-and-glue method or cut to fit by building over an accurate jig.
     
    Last edited: Jan 26, 2009
  8. ShagRock
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    ShagRock Junior Member

    Ak - as you note, plywood is more difficult to 'twist' to fit, so I guess that's why it is more applicable to hard chine hulls. I was actually planning to use battens and install ribs first (more of an east coast method), in which case the narrow plywood strips should bend enough to tack to the ribs before gluing seams. However, since we're on this topic, you mentioned 5 plank method. If one was using 10 planks to 'approximate' a round hull (more smooth chine hull), would not the plywood planks bend to fit on stringers just as well as gluing them to chine logs and then bending them?

    Since cedar is getting more expensive here and less quality, would western spruce be a reasonable alternative? I ask this because eastern spruce is widely used for round bilge hulls in small boats in Atlantic Canada?

    By the way, the 'tacking amas' was bit of an offbeat question, but what do think theoretically?
     
  9. ancient kayaker
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    ancient kayaker aka Terry Haines

    Ply is about 50% denser than cedar so for similar weight you might use 2/3 thickness, say 4 mm ply instead of 6 mm (1/4") cedar. Ply strips or planks must be very narrow to bend, you would need much more than 10, or planks must be cut accurately to the development. If you use a great many narrow strips with battens you will end up with a stripper with a ply skin, it’s not worth it, and it would be very difficult to build. Not sure about using spruce. Makes more sense to me to reduce number of planks. A bought design for stitching-and-gluing should have plank developments or you could download FreeShip which will provide plank developments.

    I didn’t pick up on the tacking amas first time I read you post: it should do the same thing as an idea of mine-
    http://www.boatdesign.net/forums/sailboats/articulated-sailboat-24598.html
    -which would use Bruce foils instead of amas; for my design I plan to mount the outriggers and mast on a turntable; using a lateen sail theoretically results in the sail automatically taking the wind on the other surface when the turntable is rotated. It leaves the hull unrotated of course, but as the sail/mast/amas move along the new heading it should fall into line with the aid of a fixed skeg.
     
  10. ShagRock
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    ShagRock Junior Member

    Well AK, that poses some some interesting issues. One would seem to be a force choice into either strip-plank with wood, or stitch and glue plywood. That's a decision I'll have to make. However, in some situations one could use a combination of both. I say this because some Pacific islanders actually employ both , even today, i.e. hollowing out a log to get the below waterline round raked hull and continuing with mortised ribs and plywood for the upper sections. If your planks are same width as 4mm plywood, then both can be used; especially if the thing is going to be painted.

    Take your idea of a 'turntable' for masts and amas. I've never seen such a thing but it sounds innovative. Is this your own creative design? Are you going to build without an engineer's plan? Just curious;) Also, have you ever seen amas made out of a discarded foam-sandwiched surf board and would such a thing work?

    Maybe I should be specific re the 'close to' rounded hull on a sailing canoe. If a beam measure at waterline was say 24" at midship and the depth of displacement below the water line was say 6" at the keel line, what number of plywood strakes would be "reasonable" to achieve said 'close to' round bottom, without pushing the plywood into 'fracture mode'?

    Newfie
     
    Last edited: Jan 27, 2009
  11. ancient kayaker
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    ancient kayaker aka Terry Haines

    Certainly cedar-strip and stitch-and-glue with plywood are the most popular construction methods today, but you still have plywood plus chine logs. Hybrid construction is also a possibility which can give the boat rounded below underwater shape with the simplicity of ply from the waterline up; the interface between strip and ply may be a challenge. Do you need/want the rounded underwater shape that much? There is not a great deal of difference in performance as Rick noted in post #14, and I suspect it is more of an issue with monohull design where the stability has to come from beam and hull shape rather than the amas. Rick say his boat is a "pure speed machine" and it looks it, but his first picture showed a flat bottomed hull, very easy to build and you might not notice much difference in performance.

    The 'turntable' for masts and foils is my own design, of course there's little new under the Sun so someone may have already tried it, but if so I haven't found any information on the attempt. Being an engineer, although long retired, I am incapable of building without a plan but it has to be tailored to fit the kayak so I will build it more or less around the boat. There's a bit of science in the thing but I try to get out of old habits these days.

    I haven’t seen amas using bits from foam-sandwiched surf boards but I imagine it would work rather well. The hard part would be the joint with the outrigger.

    It’s not the beam and draft that determine the number of bottom planks required so much as how ‘round’ you want it to be. Using cedar strips allows you to round off the corners between the strips to achieve a round hull. You can’t do that with ply or you may remove the outer veneer which is responsible for much of its strength. If the amount of thickness you are prepared to remove is T from a strip of width W on a hull of radius R then:

    W = sqrt (8RT)

    For a half-round hull bottom 24" beam, using 1/4" thick cedar strips and restricting thickness loss to 0.05" allows you to use 2" wide strips, but with 4 mm ply you should not take off more than 0.01" or 20% of veneer thickness so plank width is limited to 1", requiring 38 planks. Handling 38 ply planks 4 mm thick and 1" wide is going to be far more difficult than 19 cedar strips 1/4 x 2".

    If cedar is costly, I know of no reason why a cheaper wood cannot be substituted provided it is reasonably light and straight-grained, fairly knot free or with small tight knots, glues well and is easy to work. The major cost in cedar strip construction is the milling of the strip edges, if you can do that yourself cost is reduced by an order of magnitude. That being said, I still think a simple rectangular-section hull will be adequate and easier, faster and cheaper to build, especially if the amas are made using the same construction method.
     
  12. ShagRock
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    ShagRock Junior Member

    AK - I suspected you were an engineer and it shows in your well reasoned response - just excellent! Thanks so much for the math based analysis regarding wood, plywood and relation to build shape. AK wrote
    . Yes, and the key is simply faring the wood strip to match the plywood thickness.

    AK wrote
    Perhaps Rick might comment on the capacity of his craft to sail. The Boracay paraw has quite the knife edge, deep V shape with a ratio of L:W of 20:1, but it will flop over without amas to support it - different type of race machine. But you do make an excellent point with respect to 'just how rounded' the lower hull needs to be. The amas allow one to have a more rounded hull, aka Hawaiian, Polynesian, and Micronesian sailing canoes. The other factor is depth, which can be referenced by drawing a circle and moving a ruler from the horizontal diameter downward and observing what happens to the shape at the bottom.

    AK wrote
    . Key point made!

    AK wrote
    Stated like only an engineer can, and most helpful. This certainly opens up the third option you refer to as 'hybrid construction'!

    I'll re-check the cedar option at a specialty cedar store as opposed to big box store, but I could use spruce anyway. The cedar could look good matched with South America mahogany plywood. I could also use the ply for decking. I'm hoping to have a 'track saw' by the time I start this project which should make stripping my own planks easier. I don't know if you or others have used one for boat building purposes?

    The winds of indecision are starting to abate!
    Newfie
     
  13. ancient kayaker
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    ancient kayaker aka Terry Haines

    I must admit I hadn't heard of a track saw. I'll pass on some tips that might help.

    My method for cutting up a large sheet of thin ply is to use a 3-1/2" circular saw (the "precision saw" from Canadian Tire in my case). I lay the ply on a cutting table that is topped with a sheet of 1" insulating foam to protect the blade and the table.

    I use a regular 7-1/4" circular saw for stripping planks, also on top of the cutting table, with a thin kerf finish quality blade (cost as much as the saw!) and a home made guide which extends at least a foot either end of the saw foot plate to ensure the start and end of the cut remain straight. I fit a ply plate under the foot plate of the saw and drop the blade through it while running to get a zero clearance slot.

    I prefer a hand saw to a table saw for cutting long planks as the workshop has to be more than twice as long as the plank with a table saw, something I cannot always guarantee. A hand saw is not quite as accurate but I can easily keep thickness variations below 1/64" which is adequate for most purposes.

    Good luck! Don't forget to post the pictures.
     

  14. ancient kayaker
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    ancient kayaker aka Terry Haines

    Newfie: I can't get those elegant boats of Rick's out of my mind! A hard chine "square" hull is much easier to build than a half-round bottomed hull. Here is my take on the construction of a compromise hull with quarter-round chines. It would be really easy to build, pretty and maybe just a hair faster than a square hull due to reduced turbulence.

    Basically it just consists of cutting square and rectangular section strips off a plank of clean wood of a split resistant kind such as straight-grained poplar and gluing them to ply planks in the order shown in the figure. The chine would be of constant radius, and each stem foot would turn out with a rather nice elliptical profile and a sharp entry.
     

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