Device to reduce rounding up of short, fat sailing cruisers

Discussion in 'Boat Design' started by jakeeeef, May 24, 2023.

  1. jakeeeef
    Joined: Sep 2009
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    jakeeeef Senior Member

    Down at the sailing club early this morning to do some gelcoat repairs and as always when looking at the transom of my boat, my mind turns to potential interceptor and trim tab experiments to improve my boat's attrocious performance.

    One of the (many) curses of small, but relatively voluminous sailing boats (ie., Mini cruisers like my Swift 18), is their readiness to round up in a gust and eventually broach. Mine's particularly bad as it is spacious internally (4 proper berths in an 18 ft cruiser!), But this need for volume in the middle gives it an awful lot of tuck at the stern.

    It strikes me that a slow speed, high lift foil, shaped to roughly follow the upper two chines of the boat, or in the case of a normal boat, curved to roughly match the curve of the transom.....

    Pause for breath ...

    ...could provide a useful redirection of forces.

    Now I'm going a lot here on what a broach FEELS like, which may very well be a highly incomplete understanding of what is actually happening.

    What a rounding up and broach feels like to me is the leeward quarter of the boat essentially falling into its own wave trough. Which because of the shape of the stern, and the shape of a hull speed wave trough causes a sucking down of the stern, rotating the boat in its own wave trough, and a large turning upwind effect that can only be countered (pun semi intended) , by a large rudder input which is rather costly to forward progress. The rudder becomes less effective at higher heel angles until it stalls entirely.

    When my boat is sailing on its ear, the toerail as it reaches the transom is often touching the forward face of the stern wave.

    Of course, one solution is to carry a lot of the beam of the boat from amidships through to the stern, then put twin rudders on it to allow for the greater levering out effect at high heel angles. But I don't want to rebuild the back of my boat into a 1990s Open 60!

    So, I'm wondering if foils mounted on the quarters, that would be largely out of the water when the boat is sailing flat in light airs, might take this huge, steep stern wave you see during a rounding up and convert it into:

    1) Lift at the stern quarter only when you really need it.
    2) A component of forward drive (this foil is using a fairly steep wave face meaning the angles would be very beneficial for creating a forward driving lift vector).
    3) little or no drag penalty in light airs.
    4) A force countering heel, keeping the boat more upright and the sails producing better forward drive.

    Re 3), as the foils would be tweakable and adjustable for AOA from a curved foil mast mounted in brackets on the transom, they could swing up entirely out of the water in light airs.

    I have very mocked the rough foil position up with the only roughly correct sized bit of wood I could find for the attached photos. If I try this, I would use a proper shaped foil, not a scaffold plank! probably based initially on the big, low speed 80cm windfoils, which produce circa 100kgs of lift at about 3 knots.

    Re. Distance from the hull. I think I recall from biplanes, as long as the lower foil is mounted behind and a chord width below the foil above it (the hull), then its lift generation is unaffected by the foil above?

    Am I missing something or could this actually work? Anyone tried it?

    Attached Files:

  2. skaraborgcraft
    Joined: Dec 2020
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    skaraborgcraft Senior Member

    Take a look at the optional twin daggerboards used on the Setka 5M boats. They help with tracking down wind. Your boat was designed from the get-go as a trailer sailer. Any boat with as little hull and keel area is going to face the same issues. Personally I would increase the size of the rudder as a first step.
  3. gonzo
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    gonzo Senior Member

    The outboard rudder is probably ventilating.
  4. MalSmith
    Joined: May 2004
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    MalSmith Ignorant boat designer

    I think this is an interesting idea. I'm assuming you mean that the foil would have a positive angle of attack relative to the waterline, but a negative angle of attack relative to the aft face of the stern wave, with the idea being to recover some of the energy use to create the stern wave. Have I got that right? Positioning and angle of attack would be crucial. You might at least recover enough energy to negate the added drag of the foil. I'm not sure it would do a lot for the broaching problem though, as the more the boat heels, the less effective the energy recovery and the positive angle of attack relative to the waterline/hull centreline would only increase the rounding up moment. It depends on how the foil is configured. Can you do a sketch?

    Edit: I just realized that the foil could be on the forward face of the sternwave, having negative angle of attack relative to the hull, and positive angle of attack relative to the face of the sternwave. That would be better.

    Edit 2: In the first case above, initially it may prevent broaching, but as the boat heels it starts to cause broaching. In the second case, the reverse is true.

    If you really want to stop the broaching, I would suggest putting a T foil on the bottom of the rudder. I've used them on model yachts to good effect, although primarily to stop nosediving (which I guess on your boat is not a problem). However, I also found that they would prevent broaching, and would negate the weather helm upwind, when heeled. The foils I used were negatively cambered and had a neutral or slightly negative angle of attack.
  5. CT249
    Joined: May 2003
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    CT249 Senior Member

    Why blame the boat? The Swift 18 has not got a particularly wide stern for a trailerable yacht of its size, and most ex-owners on the internet speak of them with a lot of enthusiasm.

    With respect, a boat like the Swift 18 shouldn't be sailing "on its ear" with the toerail down so low that it's almost at water level. If you have the rudder hard over and the boat heeled to that extent, it's the equivalent of driving an early Golf GTI at a curve with the accelerator flat out and then blaming the car because it loses control.

    What it your mainsail trim like? What is your vang tension and sheeting angle?

    As an analogy, look at your local fleet of Lasers in a strong wind. You'll probably see the guys at the back heeling and having lots of trouble with weather helm and round ups. You will also see (if it's a good fleet) the guys at the front of the fleet sailing the same boat flat and with a nicely balanced helm.
  6. jehardiman
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    jehardiman Senior Member

    Yeah, modern hulls like to be sailed flat. Not much past the CCA boats of the late 1960's like to be sailed rail down.

    Edit, note that "sailed flat" is relative...many modern twin keel/canting keel/twin rudder boats (like IMOCAs or Classe 40's) have a port/stbd heel they are designed to be sailed at.
    bajansailor likes this.
  7. gonzo
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    gonzo Senior Member

    The best device would be good seamanship.
  8. Clarkey
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    Clarkey Senior Member

    The Swift 18 was designed to the 'micro cupper' rules - not really meant to be sailed on it's ear and most likely to perform well with a chunk or two of rail meat powering it up.

  9. peter radclyffe
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    peter radclyffe Senior Member

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