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Mike Kane
6/1/2023
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Just throwing this out there… Flat water paddling gets really boring and the mind begins to drift. On one such occasion I began to think about hull surface area and wondered if I could replace my under stern rudder with a rudder that simultaneously provided lift. Not a full blown foil to lift the entire boat out of water, but just 2 inches of the real hull to reduce total surface area. Not sure if that would fly but food for thought. Bonus points for calculations on 10 pound lift at 6 miles per hour require a foil with this much surface area

Comments

• Per Ason Wouldn't it just seesaw the whole thing and dip the fore deeper?
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    • Shaun Hulley Per Ason this is what I thought too… lift needs to be evenly distributed otherwise it will just upset things, and in this case lift too far rearward could cause the bow to be buried… now, if OP decided to put a foil below the cockpit, that could be interesting…
    • Gary Cziko Per I don’t think so. A normal hull displaces a volume of water whose mass is equal to the mass of the boat plus paddler. But with part of the mass supported by the foil, there should be less water displaced and less wetted area, both of which will reduce drag. With your boat floating without you, lift the the stern. Does the bow sink farther into the water when you do? Makes no sense to me that it would, although of course the boat will be inclined more forward which in itself might be disruptive. But overall drag should be less.
      • Reply * Per Ason Gary Cziko I understand your reasoning but intuitively, I'd say that my bow would sink farther into the water if I lift it at the stern. Of course, science doesn't care about what's intuitive to me, so I probably have to try it next time 😅 Imagine you are at the beach and floating in the water, flat on your back. If I would suddenly lift your feet, wouldn't your head dip down under the surface?
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          • Gary Cziko Per Ason A floating human body is more complicated than a floating hull as the legs are typically more dense than water (they tend to sink) and the upper body (with inflated lungs) is less dense than water (it tends to float). So let’s stick with the boat hull that is buoyant from stern to bow. If you lift the stern of your surfski just out of the water while it is floating in the water, I doubt very much that you will see the bow sinking further into the water. I am travelling now in Spain so I can’t try it myself. But anyone else could and let us know what they find.
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            • Per Ason Gary Cziko I would say that that argument works against you. I agree that the upper body is more buoyant, and I am convinced that it would dip down anyway. Doesn't a surfski typically have more volume and therefore buoyancy at the bow? I see a similarity to a human body there… What would happen if you'd over exaggerate and lift the stern like 30 degrees, would the bow still not dip? Or does some other factors come into play then? For the record, I'm not saying you are wrong, I'm just discussing.
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  • Keith Wieland I like the way this guy thinks. Lateral stability would be challenging for foils with surf skis.
  • Peter Finlay Just fit an electric motor. Lie back and enjoy.
  • Joe White Nikolai Rogich i thought this was your post at first
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      • Mike Kane Author, Joe White Hahaha, Definitely fits his line of thinking.
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          • Nikolai Rogich Mike Kane in all seriousness, the foil is the future of this sport as far as I can tell. Just makes so much sense as far as wetted surface area is concerned. Obviously will take forever to be allowed in sanctioned races etc but look at airplanes (aka air-foils) and how much faster they can travel compared to terrestrial vehicles.
            • Reply
              • Gary Cziko Nikolai Rogich It’s not just wetted surface area. More hull in the water means more water has to be pushed out of the way for the boat to move. I suspect that causes more drag than the friction of the wetted surface.
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                • Nikolai Rogich Gary Cziko yep exactly. Lots of promise for foiling
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                    • Gary Cziko Nikolai Rogich Andy Toro, Greg Barton’s Hungarian Olympic coach, has told me that such foil experiments were made with the rudders of rowing shells. It didn’t work out for some reason, but I can’t remember why!
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                        • Nikolai Rogich Gary Cziko oh interesting. Would like to know more
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                          • Gary Cziko Nikolai Rogich It might have been the lift of the foil reducing the effectiveness of the rudder for steering. I’ll try to find out.
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  • Michael Jose Hydrofoil Kayak (Foilkayak)
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  • Paraka Mitchell Been around for ages😎
    K1 vs. K4 flyak- kajak 27km/h HQ
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    • Tony Brown Paraka Mitchell I wonder if it'll take off?
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      • Paraka Mitchell Tony Brown ahh no. Paddle length is longer due to elevation of boat and cadence is a lot higher to keep it up on the foils
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  • Oliver Barnicoat It’s been done
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Flyak hydrofoil kayak “he’s flying!”

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  • Shawn Burke Reducing the wetted area reduces friction drag. But drag comprises three components, and at all but the lowest speed is dominated by wave-making drag and form drag. You'll still have those no matter what – see my article at the link – it's like death and taxes. Since you only want _some_ lift, and not a hydrofoil that lifts the entire hull out of the water (as other commenters have noted and linked), the supporting supporting strut / rudder will add a lot of drag itself. Monohull America's Cup sailboats did some nifty things with winged keels back in the 80s but I think the hydrodynamics are different than for us (running under sail, running heeled, very different Reynolds Number). https://thescienceofpaddling.net/part-36-total-drag Part 36: Total Drag THESCIENCEOFPADDLING.NET
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      • William Glamore Shawn Burke this is very impressive. I’m a hydraulics professor and love this explanation. Thanks.
      • Reply
        • Shawn Burke Thanks, William, that article was fun to write! (If you enjoyed that my book, The Science of Paddling, went on sale today. 😉 )
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  • Russel Breyer But what point?
  • Dave Faulkner Hmm, interesting?
  • Rick Giannini I was told there would be no math
  • Mike Kane Author Think trim tabs
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Boating Tip: Trim Up for a Smoother Ride

• Reply
  • Mike Kane Author, Here’s the experiment: enter a flat, slow, moving river that has a surface flow rate at 7 mph. Tie the nose of the surfski to a force gauge that measures drag on the boat. sit in the boat in the paddling position. compare the force gauge value for different weight paddlers, boat length as well as the benefit or lack of benefit of a lifting foil attached to the rudder location.
  • Mark Sundin Easy fix mate, get out on the ocean and you'll stop thinking about things like this…!🤣
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      • Mike Kane Author, Mark Sundin Sadly, I’m landlocked. 😩
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  • Adam KingtonClay Lift at the rear would mean nose diving, greater surface area at the bow isn’t ideal, nor is an increased chance of pearling
  • Kristin Kronsnoble i listen to music