How much less efficient would a planing hull sea kayak be?

Discussion in 'Hydrodynamics and Aerodynamics' started by mitchgrunes, Jul 11, 2020.

  1. fallguy
    Joined: Dec 2016
    Posts: 4,459
    Likes: 782, Points: 123, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    how far can he go?
     
    mitchgrunes likes this.
  2. Tiny Turnip
    Joined: Mar 2008
    Posts: 760
    Likes: 193, Points: 43, Legacy Rep: 743
    Location: Huddersfield, UK

    Tiny Turnip Senior Member

    Across the English Channel - Gossamer Albatross and Gossamer Condor, designed by Paul McCready. The story is fascinating ...
     
    mitchgrunes, Will Gilmore and fallguy like this.
  3. mitchgrunes
    Joined: Jul 2020
    Posts: 57
    Likes: 13, Points: 8
    Location: Maryland

    mitchgrunes Junior Member

    I was aware of both the human powered flying machines (one of which I saw at the National Air and Space museum), and the human powered hydrofoils like Flyak. Both are designed for elite-level athletes, and are therefore irrelevant to me.

    I only need an efficient boat to paddle with people who are somewhat stronger and more fit - and am more likely to paddle with groups that only paddle 3 - 5 knots (mostly 3-4), for up to 8-12 hours, with many stops and slowdowns. It is quite likely that my current sea kayaks are already longer than is optimal at those speed - especially since a lot of their length, at my weight (I'm 5'4", overweight at 149-150 pounds), is out of the water due to rocker (by which I mean lengthwise curvature).

    My faster boat is 19' long, but 7 or 8' are out of the water at my weight. It is 19" wide, including near the waterline, which is definitely wider than is efficient. The front deck is too low to let my knees bend much, which means that with my limited flexibility, I can't lean forwards, leading to an inefficient paddling style. It's also a 30 pound coated-nylon-skin-on-wood-frame design, and is not strong or durable enough to do much bottom scraping, or to land on a gravel beach, or to assist another boater by giving them a T-rescue (i.e., to empty their boat by rocking it across my front deck). And I can't re-enter it from on top, without flooding the boat. Basically it's the wrong boat for me.

    AFAICT, none of the commercially available off-the-shelf kayaks are "right" for me. They are all meant for bigger people, and/or are wide, slow, heavy boats designed for flatwater stability. Except for the race boats that are unstable , built too weakly, and are probably too long to be efficient at the speeds I paddle. If I had tools and better tool skills, I would make my own, as some people I know have done. It would probably take several attempts to get everything right.

    I guess I was looking for a boat that could turn and play like a whitewater playboat, but could move at reasonable speed efficiently like a performance sea kayak.

    Perhaps any boat design is a compromise. The fact that most whitewater playboats turn efficiently leaning into turns is probably true in part not just because they are designed to skim over surf waves, but maybe because they are short, flat, much wider-than-tall, and have relatively sharp edges. That's great for play, but they are not meant to go with minimum effort for long distances. Maybe no boat can do both efficiently, and it is pointless to look for a boat that does everything well.

    The truth is, most sea kayakers I've paddled with don't go places where they need strong sharp turns. I should probably just ignore turns.

    It's like looking for a vehicle that is reliable, easy to load long kayaks onto, has high road clearance, good snow handling, has a full size spare tire, can be serviced by backwoods mechanics, carries lots of gear, is easy to park, and is great for car camping. That's too many things. I tried, and ended up with a silly vehicle that doesn't do any of those things especially well.
     
    Last edited: Jul 17, 2020
  4. mitchgrunes
    Joined: Jul 2020
    Posts: 57
    Likes: 13, Points: 8
    Location: Maryland

    mitchgrunes Junior Member

    The only slalom boat I've paddled (a Phoenix Cascade) was from your era, but you might find modern slalom boats intriguing. I don't know how they do it, but as of several years ago, they generate almost no bow or stern wake at several knots, in flatwater - maybe a few mm high.
     
  5. fallguy
    Joined: Dec 2016
    Posts: 4,459
    Likes: 782, Points: 123, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    "Perhaps any boat is a compromise"

    Fact
     
    mitchgrunes likes this.
  6. Rumars
    Joined: Mar 2013
    Posts: 1,100
    Likes: 480, Points: 83, Legacy Rep: 39
    Location: Germany

    Rumars Senior Member

    The so called "planing" shapes have nothing to do with "planing" and everything to do with surfing. Planing is impossible under human power simply because we can not generate enough horsepower for our own weight. Planing is irrelevant for kayaks because enough of them exceed the L:B ratio of ~8:1 and Froude ceases to be a problem.

    Everything is a compromise. If you choose a flat bottom boat without much rocker that can surf waves the downside is increased wetted surface, and greater stability. Top speed in calm conditions will suffer (or you will not be able to sustain it for long) and maneuverability will be poor even with leaning and special strokes. With rocker you gain maneuverability and loose top speed.
    Boats are either adapted to a specific environment or a happy compromise that can go everywhere and exceed at nothing.
    It's similar with weight. A HDPE boat can be dragged over rocks but it will not be light. A very light boat is usually vulnerable to abrasion (even sand not only rocks) and puncture.
    Making a stiff boat is possible even with SOF, if yours is not, it was definitely not designed for your weight. From your description I suspect it is one of those designs with a rockerless central tube keel that get their rocker by the paddlers weight. Good for ultimate weight and simplicity, bad for versatility.

    The boat you choose based on prevailing conditions and live with the downsides the rest of the time. If you want to experiment make some prototypes in SOF by using cheap wood and PVC tarp. Don't be concerned about weight or durability, those aspects you can tackle after you have a good design.
     
    mitchgrunes and Doug Halsey like this.
  7. Will Gilmore
    Joined: Aug 2017
    Posts: 500
    Likes: 207, Points: 43
    Location: Littleton, nh

    Will Gilmore Senior Member

    @Rumars, for those of us who are uninitiated, could you please explain the initials you used in the post above. I don't know what SOF or HDPE or L:B ratios are.

    Thanks.

    -Will (Dragonfly)
     
    mitchgrunes and Tiny Turnip like this.
  8. Tiny Turnip
    Joined: Mar 2008
    Posts: 760
    Likes: 193, Points: 43, Legacy Rep: 743
    Location: Huddersfield, UK

    Tiny Turnip Senior Member

    Skin On Frame, High Density Polythene, Length:Breadth
     
    mitchgrunes, Will Gilmore and Rumars like this.
  9. Rumars
    Joined: Mar 2013
    Posts: 1,100
    Likes: 480, Points: 83, Legacy Rep: 39
    Location: Germany

    Rumars Senior Member

    Skin on frame (SOF), boat has a skeleton and is covered with a flexible fabric. The name comes from the original eskimo kayaks made from lashed driftwood and covered with sealskin. Today we have wood, metal and carbon skeletons, lashed, glued or demountable and the skins are nylon or polyester with different coatings (polyurethane, hypalon, PVC, etc.). There are also SOF/inflatable hybrids, some more SOF (the air bladders only tension the skin) and some more inflatable (the air chambers keep the boat from collapsing and the frame is for shaping).

    HDPE is a form of polyethylene, the rigid non transparent kind like the kitchen chopping board. One can make boats out of it, from small kayaks to pretty big ones. Some are rotomolded in a form and some are welded. The material is lighter then a fiberglass/resin matrix by itself but not as stiff so you have to use thicker skins. The advantage is that this thicker skins of plastic are basicly bulletproof regarding abrasion. Scratches can simply be filled by welding.

    Lenght to Breadth ratio (L:B or L/B), expressed either overall or at the waterline is one parameter that determines how a boat behaves. For example the boat described by mitchgrunes 19 feet long and 19 inch wide but with a waterline of only 11 or 12ft long. It is easier in metric so we have 5.79m:0.48m=12:1 overall, but 3.35m:0.48m=6.9:1 or 3.65m:0.48m=7.6:1 at the waterline. Those numbers of course imply that the boat is actually 19 inches wide at the waterline, some boats have flare (the topsides are not at 90° to the keel) so the overall breadth is bigger then the waterline breadth.
     
  10. mitchgrunes
    Joined: Jul 2020
    Posts: 57
    Likes: 13, Points: 8
    Location: Maryland

    mitchgrunes Junior Member

    Nothing is completely rockproof, and some rocks are sharper and harder than others. I destroyed a cross linked polyethylene kayak by scratching against a rock in whitewater. :)

    My SOF is not "rockerless", or flat bottomed. If it were rockerless, it would have a full length waterline. Instead, it has a highly curved rocker. Nonetheless, it is lighter, faster and paddles with less effort than most off-the-shelf plastic and fiberglass sea kayaks, which are mostly 24-36" wide or so, and are often 65-85 pounds, but it is not optimal for my purposes.

    I'm not an engineer, but as far as I have been able to determine, stretching a material that is strong in tension (the skin) over a material that is strong under compression (the frame) is a fairly lightweight way to create a somewhat stiff, somewhat strong structure. That is how SOFs are made.

    Genuinely traditional Greenland boats are made of materials available at semi-arctic latitudes - seal skin (or skin from other sea mammals), small amounts of driftwood, bone, and sewed and tied together by ligament. Mine was only meant to look traditional: Urethane coated nylon, wood from a hardware store, thread and (I think) fishing line.

    Longer boats are not always easier to paddle in the groups I paddle with, because length increases wetted surface drag. If the Guillemot website is right, at 4 mph, about 10' is optimal, and at 6 mph, about 16' is optimal. So my SOF has about the right waterline length, at least if it had a thinner waterline - but the ends that are out of the water don't need to be there, and it should be thinner. Someone who has has built a few kayaks told me that strongly rockered boats have two main advantages - they land on beaches and slightly elevated surfaces more gently (the Greenland boats often landed on on ice sheets), and they make for a drier ride in waves. I don't worry about the latter - paddling is a watersport.
     
    Last edited: Jul 19, 2020
  11. mitchgrunes
    Joined: Jul 2020
    Posts: 57
    Likes: 13, Points: 8
    Location: Maryland

    mitchgrunes Junior Member

    This is off-forum-topic, but can you suggest a single concise source for beginning boat makers with little tool experience, and no engineering background? I also need to keep my tools simple, cheap and compact (e.g., jig saw or hacksaw instead of band saw). And I need to understand how to get the dimensions right for my body.

    I've gathered together a bunch of links, forums and other sources (Kayak and Canoe related links http://mgrunes.com/boating.html#bowo), and there are weekend stitch&glue boat classes offered by various people, that cost more than store-bought boats (plus people tell me they spend many months finishing what they build in those workshops). Plus, they all seem to pertain to expensive kits and designs that create large, heavy, bulky boats that service the same market as the off-the-shelf consumer beginning kayakers market.

    I need a simple source to start learning.

    You mention a "rockerless central tube keel". If I understand right, it sounds like very a simple idea - but I guess you need a very strong skin. I wonder if a typical tarp would be strong enough for my 150 pounds. I guess you need a lot of tension to get a reasonable shape...

    Is there a way to create a reasonably tight skin using PVC tarp?

    A rather dated source on composite construction I found but haven't read much of yet (Charlie Walbridge, Boatbuilder's Manual) indicated that (wet) epoxy is highly toxic and carcinogenic. I've read that a lot of composite whitewater boat builders died of it decades ago, so I guess that meant you needed extreme measures to be safe. Is that true using modern marine epoxies? I've seen videos that showed people casually using West System Epoxy without obvious precautions.
     
  12. Rumars
    Joined: Mar 2013
    Posts: 1,100
    Likes: 480, Points: 83, Legacy Rep: 39
    Location: Germany

    Rumars Senior Member

    mitchgrunes and Will Gilmore like this.
  13. Will Gilmore
    Joined: Aug 2017
    Posts: 500
    Likes: 207, Points: 43
    Location: Littleton, nh

    Will Gilmore Senior Member

    A frame, like what you are talking about, should be easily built with only hand tools. You aren't talking about shaping large timbers or big bulkheads or fitting complex panels. A few light frames and stringers notched in. A little glue, some hand plane work and sanding.

    Also, check out this construction method I came across. I haven't had the opportunity to try it yet, but it looks promising for marine applications.

    Just recycled plastic bottles, a heat gun and your imagination. For modeling purposes, at least. You could strengthen the joints by using multiple layers.

    -Will (Dragonfly)
     
    Tiny Turnip likes this.
  14. mitchgrunes
    Joined: Jul 2020
    Posts: 57
    Likes: 13, Points: 8
    Location: Maryland

    mitchgrunes Junior Member

    Thanks! I'm looking for something simpler, that doesn't assume as extensive woodworking knowledge, but will ask and look elsewhere.
     

  15. BlueBell
    Joined: May 2017
    Posts: 1,604
    Likes: 415, Points: 83
    Location: Victoria BC Canada

    BlueBell "Whatever..."

    Skin on frame is the way to learn, without power tools.
    Pallet wrap is cheap and extremely effective for covering SOF.
    I've built entire boats in hours for less than $100CAN including paddles!
     
    mitchgrunes and Will Gilmore like this.
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.