Foil profile most tolerant of different angles of attack

Discussion in 'Boat Design' started by jakeeeef, Oct 28, 2022.

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  1. jakeeeef
    Joined: Sep 2009
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    jakeeeef Senior Member

    Hi, I'm prototyping a vastly improved LARS ( launch and recovery system), a set of launching wheels for small RIBs employing high volume beach wheels placed precisely (and adjustably) at the balance point of the boat. (Along with some other enabling technologies I'm not at liberty to discuss).
    It's a lightweight framework that uses particularly buoyant tyres so when under the boat, the setup thunks very snugly (and satisfyingly) upto the hull with its axle a metre or so forward of the transom.
    This is fortunate in that the axle and wheels assembly stays in contact with the hull once deployed allowing the boat to be intentionally run some way up the beach with a blast of engine at low speeds (I fancy military and SAR and expedition cruisers to be potential markets so getting it up the beach quickly and with no fuss in a shorebreak is one of the USPs. Or people who simply want three quarters of the usability of a sealegs rib at a tenth of the cost. (6 grand for a decent used 4.5 m rib, + approx 2 grand for my LARS kit). Unfortunately the parts count and requirement for lightweighting and likely necessity for professional fitting and tuning for each RIB hull will make it likely too expensive for the general small yacht tender market).
    Because the axle is narrower than the beam of the boat, for ease of handling ashore, it looks like it's going to be awkward to reach underneath the boat with a boathook to sink the trolley (which hinges at the bottom corner of the transom). To move the wheels into lifted on water mode, it's necessary to sink the axle past it's mid point beneath the transom (fulcrum).
    It strikes me that an inverted foil round the axle or near to the axle, and gunning the engine in deep enough water will very elegantly sink the tyres down and back past the transom for them to bob up behind the boat, ready to be grabbed with the boathook and pulled up snug to the transom where the framework clips up in place.
    This would make it a matter of seconds to both deploy and retract the wheels- critical considering SAR etc. usage.

    Still with me?

    So, this submerged foil will start it's journey with a reasonable AOA, but that AOA will increase and increase and increase all the way to 90% as the framework hinges back and down, soon completely stalled at which point the tyres' buoyancy once past the fulcrum will ensure the journey will be completed satisfactorily.

    Bearing in mind that it's all about lift and drag isn't an issue, it's an unusual foil requirement. Any ideas for a suitable foil profile? I guess a bit of 18 mm plywood just radiused across back to create an ogival foil might work fine, and would be easy to fabricate for my prototype. I imagine if I give it a decent cord, like 200 mm, once it sinks the framework much off the bottom of the boat its position below the fulcrum at the stern and its wide cord will work like a flag and despite being completely stalled will still hinge back well enough in a low tech horrendously stalled but ok fashion? Or do I need to come up with something more foil shaped?

    For obvious reasons I don't want to weight the framework to reduce its effective buoyancy, so it's always going to be light and with big beach tyres it's always going to be very buoyant when under the boat and underwater. So the foil might need to generate a good level of dunking force.
     
  2. upchurchmr
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    upchurchmr Senior Member

    Pictures if you actually want to discuss your design.
    If this is just an ad then we don't need them.

    I lost you early on with your obvious "code word" descriptions which didn't have enough to be clear.
     
  3. jakeeeef
    Joined: Sep 2009
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    jakeeeef Senior Member

    No code word descriptions intended. As my initial posting and especially the title explains, I'm simply wanting to know what sort of foil shape is most tolerant of the widest range of angles of attack.

    So if you wanted to fix a foil to the front part of a metre long buoyant framework under a boat, hinged at the lower corners of the transom, with the intention of forward movement of the boat forcing that framework to dive, against its natural buoyancy downwards and then behind the boat, what foil section would you choose and why?
    I probably gave too much incidental information by way of background. Certainly not advertising anything - a long way off that yet!

    I'm guessing it's going to be a relatively thick foil as I vaguely remember they tolerate higher AOA before stalling? Hence why a dinghy's rudder is usually (relative to cord) a bit thicker than its centreboard.

    It's almost not a foil. It's basically going to be in stall conditions most of the time. Maybe designers of commercial fishing gear etc would understand it better? Maybe it's more like the sort of thing used in trawls?
     
  4. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    I think that reversing would be more practical. Going forward, the trolley is going to hit and/or get snagged on the outboard. A foil is not necessary. A flap that hinges down with gravity will create enough resistance when going in reverse to accomplish the task.
     
  5. jakeeeef
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    jakeeeef Senior Member

    The trolley hinges at the bottom of the transom. It hinges down and underneath the outboard. It is about a metre long.
     
  6. gonzo
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    gonzo Senior Member

    You can still do it with flaps. There is no need for a foil. All you need is to create a force towards the stern. A flat plate will work. A soft sea anchor like device would also work.
     

  7. Eric Lundy
    Joined: May 2017
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    Location: Vermont

    Eric Lundy Junior Member

    I thought I read an article or forum thread about the lumpy bumps on the front of a foil like a whale's helps with higher angles of attack.
     
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