Would a keel strip add much drag to a fast SOF kayak?/

Discussion in 'Boat Design' started by mitchgrunes, Apr 29, 2021.

  1. mitchgrunes
    Joined: Jul 2020
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    mitchgrunes Senior Member

    Some people add thin keel strips to the centerline of their sea kayaks, to reduce damage when launching or landing on rocky beaches, or when there is surf. E.g.,


    Other possibilities are duct tape, or resin impregmated fabrics, like polyester, fiberglass or Kevlar.

    Applying a Keel Strip - Repairing a Composite Kayak ~ SeaKayaker.org https://seakayaker.org/applying-keel-strip

    Reviews say strip durability results vary.

    What fractional increase in drag force is to be expected?

    How would that change if only the front part of my boat has the strip? (In which case I would probably launch facing backwards. :()

    Assume my boat will be a fast lightly built SOF sea kayak. About 16” x 15’ or 16’, full length waterline (which increases wear while landing and launching), but a little thinner than 16” at the waterline. Displacement hull. 25 - 35 pounds, plus about 160-170 pounds for me and gear. I have to experiment to know how unstable I can stand – maybe 5-8 pounds of passive stability? – but the ends will have a somewhat V=shaped cross section, and the center will be relatively flat for maybe half that width, approximately half oblate cylindrical in between, but in flat segments to match the stringers. Not sure whether I will need stabilization chines. No rudder or skeg.

    Assume the strip will be about 1/16" - 1/8” thick by 4” wide. I don’t know whether the edges of the width can be feathered, to make them less abrupt – would that matter much?

    BTW, how do people figure out things like this? Is there free design software that can model drag, including this type of small scale shape change?

    And would such a thin keel strip affect handling - e.g., would it track any better?
  2. kerosene
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    kerosene Senior Member

    It will probably not be noticeable in any way. Bit more abrasion resistant but that is about it.

    I understood that we are talking about single strip of fiberglass tape. It will not be 1/8” barely 1/16”. And yes you can sort of feather. Either by sanding, this will cut into the fibers, or by using filler.
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  3. Milehog
    Joined: Aug 2006
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    Milehog Clever Quip

    1. < ignore that number, I don't know how I put it there and can't make it go away. Look at gaffers tape. Fairly tough but easy and clean to remove. Helicopter tape. I got it for my origami ORU kayaks but haven't used it yet so can't say if it leaves a mess when removed. Mine is 8-mil urethane. Commonly used for rock chip and scratch protection.
    As far as software I suspect empirical data will be easier to collect and more accurate.
    Now, are you really that fast?
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  4. Waterwitch
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    Waterwitch Senior Member

    Considering an advanced surfski has a 19" beam and a OC-1 outrigger canoe has an 16.5" beam it will be quite impressive if you can balance in a kayak with the 16" beam you are considering. Those boats mentioned are also 5 feet longer than your planned
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  5. Kayakmarathon
    Joined: Sep 2014
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    Kayakmarathon Senior Member

    I am concerned about your safety. A kayak with a 16" beam and no rudder to be used in surf is for experts only. Any water that enters the hull through pin holes will make the kayak difficult to control. Before you even build this kayak, try paddling a wildwater (14' 9" long) kayak designed within the past 25 years, or an Olympic sprint kayak (17') such as the Struer Hunter, Struer Tiger, Struer Cleaver, Wenonah Orion, or Gaybo Jaguar.

    Launching stern-first into surf is a bad idea. Maximum power (as when forward launching) is needed to punch through. Even in light surf, launching stern-first means you will have to eventually turn and get hit broadside by waves. You would need to have the ability to balance on two basket balls stacked one on top another without letting your feet or hands touch anything. This is incredibly difficult.

    I am being blunt because I've paddled at the elite level, experienced extreme conditions, written my own boat building software, and built several boats. I don't want your dream to become a nightmare. It sounds like you are trying to learn 3 things: design, build, and paddle. If you want to challenge yourself and enjoy some struggle, I would recommend you build a 14' to 16' long kayak from a kit or table of offsets from a classic hull.

    If you want a skin on frame style kayak that has some scuff resistance, instead of dacron or nylon use 4 Oz fiberglass cloth. Place the first layer lengthwise and give it a little tension. Using a disposable foam roller, lightly coat the first layer with epoxy resin. Don't worry about pinholes because the amount of resin should be light enough to keep the cloth stiff after the tension has been removed. Let cure for 24 hours. Add the second layer, but it should run at a 45 degree bias to the first layer. Each piece of fabric should overlap 2-4". This time make sure the resin fills pinholes. The third layer runs a continuous piece, full length. The fourth layer is 45 degree bias, same as the 2nd layer. The fifth layer is like the third layer, one piece full length.

    To try to lay up layers 2 through 5 in one session. This will minimize resin since the excess of the previous layer gets absorbed as the next layer is being applied. Do all cloth cutting before mixing and applying resin. Place layer 5 cloth on a table first, then 4, 3, 2, and 1, so they will be built in the correct order. If you want to reinforce the keel, cut 6" wide pieces as long as the width of the cloth, and overlap their ends by 2-4 inches. Put them between layers 1 and 2, as well as layers 3 and 4. Don't use Kevlar because it will not sand smooth. The deck needs only 3 layers. The middle layer is at a 45 degree bias.
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  6. mitchgrunes
    Joined: Jul 2020
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    mitchgrunes Senior Member

    No! I need an efficient boat to keep up with stronger paddlers.

    To the other kind respondents: While I'm not an expert by any definition, I've paddled a variety of whitewater and sea kayaks since about 1980.

    I indeed had trouble briefly trying a wildwater boat with "wings", perhaps because my skills and knowledge weren't up to it at the time. I would love to try one again, but I don't know anyone who has one.

    I've more recently had trouble briefly paddling racing surf skis, like the Epic 18. I could keep them upright if I went very slow and ignored the rudder, but I was constantly fighting for stability. That's why I want a somewhat flat center cross section, and I want something my knees can grab onto and control.

    I briefly paddled someone else's 16"x16' Greenland kayak a little, and loved it, though the waves were small. I've paddled my 19"x19' Greenland rolling boat built for someone bigger than me, a lot, once in 6-6.5' storm waves - more difficult conditions than I would ordinarily choose. It handled a lot better than my 21-3/4" x 17'6" Current Designs Caribou (S?), whose bow bounces in the waves because I don't weigh nearly enough.

    I don't love the SOF for different reasons - the deck is too low to the water for on-water re-entry and probably isn't strong enough for a T-rescue - like many SOF Greenland boats it is a roll-or-die boat. It rolls easily, but that still makes me nervous. I've done a few re-enter and rolls in near-flatwater, but am afraid to try that under difficult conditions.

    I hope 15' or 16' isn't too short if I have a full length waterline, and don't try to go over 5 knots. That will be a longer waterline than either the SOF or the Caribou, because the upswept bow and stern loses me 6 or 7' at each end in the SOF, and 3 or 4' at each end in the Caribou.

    Because I can roll, I'm less concerned with stability in my proposed design than with sea sickness. I once got sea sickness paddling a very old design 4 meter slalom boat (Phoenix Cascade) in 2-3' beam chop on the Chesapeake Bay. Perhaps it had something to do with the rounded cross section. I had paddled the boat for many years in class 2 - low class 3 whitewater without that reaction - but I have gotten car sick when someone else was driving, and I've gotten airsick during stormy weather.

    I'm not up to building with fiberglass and epoxy at this time. Plus it is hard to do quick design trials that way - SOF is easier to modify, so people say.
    Last edited: Apr 30, 2021
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