'Ten knot tender'- speed predictions welcome

Discussion in 'Boat Design' started by jakeeeef, Jun 17, 2022.

  1. jakeeeef
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    jakeeeef Senior Member

    Well, it's ready for testing.

    Anyone want to have a guess what it will do?

    This is for a magazine article I'm working on, around improving the performance of a 2.4m sailboat tender that only does 4.5kn. Total budget £20 worth of epoxy and carbon cloth, I already had the zodiac and motor. ( Everything else out of skips, the board was free pickup on Facebook Marketplace). Saved from landfill. For now...

    Photos self explanatory. The one taken on the water is pre hull mods, full throttle, 4.5kn!

    Weight now all up including me- 125.4kg

    Motor- 2.2 Merc 2 stroke with restrictor plate removed, so nominal 3.3 hp. Standard prop..
    For now...

    It also answers the question I've had for a long time, "what would happen if you put an outboard on the back of a SUP/windsurf board/ surfboard etc.

    Ignore the Zodiac (hydrodynamically)! It's only there to lend it's transom and to keep the motor and me from going swimming. ( If I mounted the hull high enough). We'll see.

    The board is a Fanatic Mega Cat, a course racing board from the early 90s, if that matters. Last metre or so removed for several reasons. I chose it for it's volume and boxy rails.

    Attached Files:

  2. bajansailor
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    bajansailor Marine Surveyor

    Does the Zodiac have an inflatable floor / keel?
    What is the volume of the standard Fanatic Mega Cat before you chopped the back end off?
    Re the side profile view of the engine, I am wondering if the propeller should really be a bit deeper? It might be affected by turbulence from the board in front of it, and / or cavitation re not being immersed enough? If it is affected, then you could perhaps bevel the back end of the board to improve the water flow into the propeller (?)
    Are you taking it for a test run today or tomorrow perhaps?
    Last edited: Jun 17, 2022
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  3. jakeeeef
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    jakeeeef Senior Member

    The floor of the Zodiac was a single layer pvc skin with heavy mahogany slats inside. I took the slats out. No air floor or keel. Air keels and floors help inflatable performance quite a bit. I'm hoping to go beyond this though. The cleverest thing the makers of small inflatables have done in recent years is taking the dropstitch air floor from the inside and putting it underneath and outside. But there's issues with the back end of a dropstitch air floor- the curve at the edge of the dropstitch encourages squat. The only way to get larger ribs to go fast is to remove the tubes completely from the water when planing, and as far as smaller boats are concerned even quite good SIBs like the Zodiac Futura don't quite manage to get their tubes out of the water. They have plenty of fans, but they are quite sub optimal.

    Re mega cat. Board was 250 litres before I took the saw to it.

    Re prop height. It's a guess really at the moment. You'll note I have ended the planing surface ahead of the Zodiac transom. This is to increase SETBACK, allowing me to run the engine higher and in less turbulent water. That's the plan anyway. I've had bigger, faster Ribs that with a decent setback can run the prop hub centre near to the theoretical water level, shown in my photo by the sheet of plywood. The ability to do this reduces with smaller motors, (proportionally higher appendage drag) so I'm starting with the theoretical water level slightly above the gearbox. I agree I might have still overdone it though!

    I say theoretical water level as the water looks like it comes horizontally off the bottom of the transom of a planing hull, but as you probably know in reality, it always recovers upwards a little especially at 'only just' planing speed. The more the setback ( within reason), the more advantage it gets from this recovery, plus placing the prop well aft of the transom gives the turbulent flow off the hull that bit more time to even out before it reaches the prop so you can run the prop higher still. Jack plates don't just work by lifting the engine higher, they also, critically, move it further from the transom.
  4. kapnD
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    kapnD Senior Member

    To quote an often used phrase from another boating forum, “It looks Tippy!”
    I think what you’ve created will be in the long run, a heavily overloaded motorized paddle board that will likely not even float level at rest.
    Put a transom on the board and mount the motor there, and loose the rubber boat for best results.
  5. bajansailor
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    bajansailor Marine Surveyor

    A rather 'left field' thought, for if it does prove to be too tippy as kapnD suggests.

    If it is too tippy, how about converting it from a 'monohull' (re the board) to a catamaran by slicing the board longitudinally along the centreline and moving the two halves further outboard?
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  6. BlueBell
    Joined: May 2017
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    BlueBell . . . _ _ _ . . . _ _ _


    I don't know...




    ('Ten knot tender'- speed predictions welcome')
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  7. bajansailor
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    bajansailor Marine Surveyor

    @jakeeeef did you take it out for a test run today?
    If yes, I hope she was not too tippy?
    I heard that Europe has a heat wave - I guess the most important consideration then would be your Tilley hat and sunscreen.
    Oh, and the thousands of mobos zooming up and down Southampton Water giving you rough conditions with their wakes. :)
    (For folk from t'other side of the Atlantic, it is a bit like a maritime version of the Whacky Races on a sunny weekend in the Solent and Southampton Water).
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  8. jakeeeef
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    jakeeeef Senior Member

    It will indeed be somewhat tippy at slow speeds and when stationary. That's why it's got a whopping great inflatable tender attached above it- it should only tip so far. It will seat its rider on a lightweight version of a jockey seat (the boats usual athwartships inflatable seat turned 90 degrees and sat/ kneeled on). High tech it isn't, (but 0.5 kgs it is).
    I actually have THREE sets of smaller, matched pairs of windsurfing boards, and a cat is a more stable option, but a lot of skin drag. I've tried a few iterations and they are appreciably heavier than this route and not that fast in view of the hassle involved to set up (They require crossbeams of some sort, which add weight, complexity and always seem to interfere with errant wave tops).
    I actually think this monohull variant, if it planes, which I think it should. Will carve pleasing turns too. I do have a mechanism planned at some stage that cants twin boards for carving steering at speed, which could be fun, if a little gimmicky.
    I didn't get a chance to take it out yet. Seawork this week, so a bit busy on the day job- but I did get to go out on the SeaAir foiling RIB today, which was... interesting and a worthwhile substitute.

    Having slagged off the Zodiac Futura a bit, I should add that as well as being of a much more usable size, things like the Futura do roll up and pack down into a bag, and any fool can add a planing surface underneath an inflatable to vastly improve its performance, while conveniently forgetting about its main advantage of rolling up and going in a bag.
    However, there's not much going on with my board underneath that couldn't be largely replicated with modified dropstitch. A few of the problems of dropstitch (hydrodynamically for use at planing speeds) could be solved by bonding a flat vertical carbon/foam/carbon transom in. I've not seen it done yet . I guess it would need doing to a standard of quality that would leave the planing surface flat and without a big wobble ahead of the transom, which would mean making the transom EXACTLY the same size as the dropstitch threads. That might be quite an expensive manufacturing process.

    Re the name: 10 knot tender- that is the draft name for the magazine article I hope to write about the process. I have little reason to believe it will achieve this! I just thought it was a half decent headline. 9 Knots would mean I'd doubled its speed, which would be quite impressive in itself for a £20 outlay.

    I'm doing this one as a 'what if' and for a bit of a giggle really. It's entirely made from crap I had hanging round the house- mostly out of skips.

    The next stage could be little leeboards and a sailing rig. With the board underneath it might sail better than the usual inflatable with a little rig.

    I'm not doing it with any commercial aspirations, but I do idly wonder, if, following the demise of the popular, and still missed, Tinker Tramp, there is room in the market for a little higher performance motorised tender that converts to a planing capable sailing dinghy. Especially one that as a motorboat could plane one up with the smaller electric motors from Torqeedo and ePropulsion...
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  9. CT249
    Joined: May 2003
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    CT249 Senior Member

    Interesting idea, but I can't see it planing under sail very often if at all. You've got something that's about half as heavy again as a Laser, or about 8 times as heavy as a hybrid windsurfer board or Raceboard, with high form drag and low planing aspect ratio.

    Cutting down a longboard means you miss the good points of a longboard (the good performance at displacement speeds and the small "hump" in the shift to planing speed) and the good points of a widestyle board (ie the high aspect planing surface and the resultant good lift/drag ratio when planing. Like a hybrid windsurfer (ie RSX) you can come close to ending up with the worst of both worlds and unlike the RSX you don't have an Olympian pumping their guts out on a big rig to get it to work.
  10. jakeeeef
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    jakeeeef Senior Member

    It's mainly intended to see if I can get the dinghy over the hump with a small outboard. Yes, agreed sailing wise, but it would only have to be slightly better to sail than a Tinker Tramp or a Dinghygo though to be a success in that regard. I wasn't proposing that it would beat a Laser. There is a high performance inflatable dropstitch skiff from France called the Tiwal. It's an awful lot of money, has taken an awful lot of effort to develop and I suspect would probably also struggle to keep up with a Laser in most conditions.
    Won't mine be doing the same sort of thing as a widestyle board re planing aspect ratio? Haven't I made a higher aspect ratio planing surface by getting rid of the pintail and mounting it forward of the transom so it rides on its tail somewhat?
    I think the idea of planing under sail is going too far though. A 9m battened sail would be unmanageable on such a craft and I'm guessing it would need that to get an adult planing. I was thinking more a 4m soft sail and just having a stiffer inflatable sailboat than a Tinker Tramp or what have you- something that kids could actually learn to sail with of a summer evening rather than drift sideways.

    I take it you're not a fan of the RS:X? I've never sailed one. Actually, messing about with this MEGA Cat made me think about getting a decent raceboard or widestyle board for summer cruising here. Unfortunately this Cat was really beaten up on the rails and missing mast track and centreboard (which I believe would have been a quite nice carbon one), so I did this with it instead.

  11. Zilver
    Joined: Nov 2007
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    Zilver Junior Member

    The picture of the outboard and propellor suggests that if you would get it on a plane, the prop will be half out of the water.....
    Can you lower the motor ? Or use a long shaft ? Good luck, Hans
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