Radically Different Yacht Keel - "Loop Keel"

Discussion in 'Sailboats' started by Bad Mac, Mar 9, 2007.

  1. rayk
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    rayk Senior Member

    Just a couple of questions...

    Ballast ratios for the same hull? Are they different?
    Less ballast on shorter keel isnt healthy for static stability.

    The increased dynamic stability underway, and drag. What is the relationship? I am thinking of the flaps on the back of the keels. And boat speed. Added mass looks like drag to me.

    Re the biplane theory, generating a lot of lift from short, easily engineered wingspans was useful when aero engines weighed hundreds of pounds for fifty horsepower. Great lift for a lot of drag.
    Have a look at the development of biplane wings, and their individual aspect ratio increased in line with speed and engine power until drag, not lift, was the devil.

    The rigging strains transmitted through the circle keel?
    I am thinking of this thing needing major reinforcing on both sides of the hull and for mast compression.
    Put this thing on the hard and watch the shrouds go slack.

    My favourite, the lee shore.

    The biplane theory as animated is a crafty conclusion.

    In fact I started off this post trying to be reasonable, but after going through the website a few times, it is just not that attractive an idea.

    In fact it is pretty shallow.

    I am closing some browser tabs and using my brain for something else.

    I have been accused of being a Luddite before, but you wont ever find me admiring the emperors new clothes.







     
  2. Bad Mac
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    Bad Mac Engineer

    Hi Rayk,

    Ballast ratios for the same hull? Are they different?
    Less ballast on shorter keel isnt healthy for static stability.


    The lasers that we used as test craft had identical mass and COG and hence the same static stability curve. You can either use the loop keel in the same configuration as a fin and get better sailing performance/same static stability or reduce the draft for equivalent sailing and lower static stability. It simply depends upon what your design criteria is.


    The increased dynamic stability underway, and drag. What is the relationship? I am thinking of the flaps on the back of the keels. And boat speed. Added mass looks like drag to me.

    Added Mass does add some drag, but it is in a very 'slippery' form. Consequently the drag for 100kg of added mass is much much less than if you actually added 100kg to the yacht, but the effect in terms of inertia is identical. If sailing upright the drag rise is minor compared to the increase in apparent mass. When heeled the drag rise from dynamic righting is more significant, but this has to be compared against the extra power generated. Hence, after about 14 degrees of heel you are winning overall - power increase is greater than drag rise.

    Added Mass was heavily studied in both Airship and Submarine Design. We are going to try and put a couple of good papers on the subject up on our website as it seems that this could do with some additional explanation.

    Re the biplane theory, generating a lot of lift from short, easily engineered wingspans was useful when aero engines weighed hundreds of pounds for fifty horsepower. Great lift for a lot of drag.
    Have a look at the development of biplane wings, and their individual aspect ratio increased in line with speed and engine power until drag, not lift, was the devil.


    Sorry but you are simply not correct on this point. There is an excellent paper on this subject (non planar wings) by Professor Kroo at Stanford University.

    http://aero.stanford.edu/Reports/VKI_nonplanar_Kroo.pdf


    The rigging strains transmitted through the circle keel?
    I am thinking of this thing needing major reinforcing on both sides of the hull and for mast compression.
    Put this thing on the hard and watch the shrouds go slack
    .

    There are a number of ways of solving that problem, for example a bulkhead at that point would immediately remove the problem.


    James Macnaghten
     
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  3. Doug Lord

    Doug Lord Guest

    ------------------
    What an absurd, ridiculous, rude and uninformed
    comment!
     
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  4. Chris Ostlind

    Chris Ostlind Previous Member

    The Pot Calling the Kettle...

    Doug... So, let's put this to rest once and for all.... just which one of you is the blackest?
     
  5. Jon Howes
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    Jon Howes Insomniac- sleep? Wassat?

    Chris, Doug,

    James has told me to be on my best behaviour (sound of biting lip very hard...) so lets all keep the party polite? Although philosphically, the idea that "everyone is entitled to their view" is flawed in several respects it does, unfortunately, enjoy much popular respect.

    On the other hand a good row can be immensely enjoyable so perhaps we should have a special shouting at each other thread?:D

    Jon.
     
  6. longliner45
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    longliner45 Senior Member

    the way I see it ,,,if boats are built strong ,,the loop keel should work ,being attached to a strong chine rather than keel,,the chine on my boat is 5 inch white oak about 12 inch wide at points ,,I just dont see why this wont work,longliner
     
  7. tspeer
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    tspeer Senior Member

    I think the V-shaped keel is a worthwhile concept, but I think I'd be inclined to use different terminology and different lift & drag bookkeeping to account for it.

    "Added mass" refers to the apparent increase in mass of the boat when being accelerated. For example, the typical sea-keeping equation of motion for the sway (sideways) direction looks like

    (m+A22)*Y_ddot + A24*Roll_ddot + A26*Yaw_ddot +
    B22*Y_dot + B24*Roll_dot + B26*Yaw_dot = Fh2 + Y_aero

    Y_ddot = sideways acceleration
    Y_dot = sideways velocity
    Roll_ddot = roll acceleration
    Roll_dot = roll rate
    Yaw_ddot = yaw acceleration
    Yaw_dot = yaw rate
    m = mass of boat

    A22, A24, and A26 are the added mass coefficients. B22, B24, and B26 are the damping coefficients. Fh2 is the exciting force from the waves and Y_aero is the aerodynamic side force from the sails.

    My point is, that when sailing along at a constant speed in flat water, Y_ddot, Roll_ddot, Roll_dot, Yaw_ddot, Yaw_dot and Fh2 are all zero, and the sideways velocity takes on a value (leeway) that balances the side force from the sails. The apparent mass terms have no effect.

    The change in the height of the water's surface is just a consequence of the increased pressure on one side of the foil compared to the other. All sailboats have the surface of the water raised on the leeward side and depressed on the windward side of the lifting surface or body compared to the zero lift case. I see this on my trimaran - there's a visible difference in the water coming off the stern of the ama when going to windward. indicating that the ama is carrying some lift.

    It's really a bookkeeping issue. You could measure the change in elevation of the water's surface everywhere and that would account for the change in pressure at the surface, which you could relate back to the keel geometry. You'd need to also get the change in pressure and the water's velocity for all the other sides of the volume of water enclosing the boat, too, in order to get the total forces.

    Or you can calculate the pressure and sheer stress on the wetted surface of the keel, bulb, and hull to also get the total forces and moments. Which approach you choose really depends on which is the easier way to accomplish the engineering objectives. For the purposes of calculating the performance, I'd go with the latter. If I wanted to separate wave drag from viscous drag in a tow tank, I might go with the former.

    But I don't see anything new in the way of hydrodynamic princples here between a V keel that pulls down on the windward side from an inclined daggerboard on a multihull that lifts up on the leeward side.

    None of which in any way takes away from the merits of the configuration. I think it has some definite promise. I've considered a V configuration myself, although in the context of an inverted "V" rigid wing sail rig instead of a keel. Using a panel code to do a parametric study of the geometry, I found much the same sorts of things claimed for this keel - less induced drag and less heeling moment for the same span. I also considered the effects of differential trim of the two panels - equivalent to adding or subtracing circulation to both.
     
  8. CT 249
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    CT 249 Senior Member

    My apologies for mis-reading your post in such a way. I tend to read this forum when I can't sleep and I wasn't doing very well in basic comprehension at 4am!

    Yes, Lasers certainly don't handle very well in a breeze when heeled.

    Thank you for the reply and again I apologise for mis-reading your post which was clearly written.
     
  9. Ceilidh
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    Ceilidh New Member

    Many Thanks

    Dear Mr. Howes and Mr. MacNaghten,

    May I please ask a neophyte's question?

    If your loop keel can induce a low-drag ballasting effect on a hull heeled 15+ degrees, and if it can be trimmed for minimal impact on an upright hull, could it be used without the heavy ballast bulb in something like an inland lake scow? These boats are typically heeled 15+ degrees when going to weather, but are often hiked flat(ter) when planing downwind -- it would seem that an unballasted loop keel might be a nice way of increasing upwind power without losing too much on the way back down, and perhaps it might allow an overall beam (and thus weight) reduction?....

    Could it effectively work on such a craft?

    And on a practical note, is there any ready way of retracting an unballasted loop keel? Could the keel be split at the bottom to allow retraction gullwing or daggerboard style (for launching and trailering, etc.)? Or would any sort of gap down there cause problems?

    I guess what I'm daydreaming about is something like a Melges 17 with the normally straight pivoting leeboards removed and a pair of semi-circular (in front view) dagger/leeboards slotted down through the (modified) trunks. It'd be nice not to have to fuss with raising / lowering leeboards on every tack, and if the loop keel's ballasting effect could allow a singlehander to carry the sort of sailplan that the (2-person) Melges 17 carries -- well, that'd be a very fun little rocketship!

    Thanks very much in advance for any of your thoughts (even if it's just to tell me this is a silly idea), and thank you doubly for the fascinating and insightful posts you've made on this forum. All the best to both of you in your projects and business, and may you please have a great 2007. :)

    - Ceilidh
     
  10. Jon Howes
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    Jon Howes Insomniac- sleep? Wassat?

    Tom,

    You have most of what we were trying to achieve within your post:

    Added mass; Yes, in terms of boat handling this is primarily a way of increasing the energy required to cause a disturbance, just like an increase in the physical displacement of the vessel. This increases the resistance to accelerations and therefore resistance to upset. Being an added mass term it is not the samevalue however in the x, y and z directions.

    The change in water surface level to leeward is well known. What the loop keel does is raises the water level to windward within the loop, this is not seen on current, conventional boats and, in lay terms, is a good way of describing the travelling water ballast that this creates.

    The added mass here is contained within the circulation system formed by the limbs of the keel, the hull between the two limbs and the elevated water surface (ie, increased water pressure in the loop between the hull bottom that has broken the water surface and the weather limb at the waterline, a closed circulation system and therefore of very low loss). This rise in level is a direct function of the added mass contained in the circulation system although the actual raised water portion extends both ahead of, and behind the keel. Of course this is caused by the pressure differential from outside the loop foil to inside, however, as is very well known, this is exactly equivalent to a circulation system bound to the foil added to a linear flow.

    The difference between a dagger board pushing up to leeward and a loop keel pulling down to windward is the closed loop nature of the loop keel. The lifting daggerboard comes with a significant trailing vortex system associated with the creation of this force, and hence a vortex drag associated with the righting moment, the loop keel does not. Generation of righting moment with the loop keel does create a wave system and this is directly related to the added mass (think of a vortex doublet, the flow field around it and its relation to a moving body's flow field).


    We are not the first V keel proposal (or even the first V keel patent) what we have done is to exploit the characteristics of a closed loop vortex system to create a low loss way of changing the characteristics of a boat at will from light to heavy displacement (or vice versa) while simultaneously generating a dynamic righting moment. As these two primary effects are both due to the same thing, namely, the angling of the zero lift surface to create a circulation loop, they are inextricably linked. We have also expoited the lifting of the weather limb to vary the longitudinal position of the yachts hydrodynamic centre by sweeping the keel although this was secondary to the added mass effects. The variation does not need a flap system; If coupled to a hull that drops by the bow on heeling this is equivalent to adding flap. In this case the keel could be configured to provide a minimal, or zero, upright drag penalty but to become effective only when the yacht heels.


    "Or you can calculate the pressure and sheer stress on the wetted surface of the keel, bulb, and hull to also get the total forces and moments. Which approach you choose really depends on which is the easier way to accomplish the engineering objectives. For the purposes of calculating the performance, I'd go with the latter. If I wanted to separate wave drag from viscous drag in a tow tank, I might go with the former."

    This is exactly what we do. It does not give a full picture however as we found early on that if the leading and trailing foil stagnation points indicate that the exit and entry areas of the loop are not equal drag can rise sharply. (this also occurs with flap deflection if flaps are fitted although when flaps are deflected it is generally because conditions demand it and the drag rise is not a primary concern). The water surface level changes can be estimated fairly accurately by calculating the pressure field ahead and behind the loop and replacing these with static head at the waterplane, (needs vortex lattice methods at the very least for this). The other complication is that this flow system is additional to the flow system around the hull, if not taken into account the result is less than ideal.

    We also found that we got the best results with foils with a 50% datum cp location and symetrical pressure distribution about the 50% chord point. The design process goes something like this:

    1: Estimate the chord distribution for minimum induced drag when resisting leeway (vortex lattice optimiser), a compromise for all heel angles, 15 degrees is a good place to start.

    2: Define camber distribution around the loop to ensure that, without leeway incidence added (ie, foils must be at zero geometric incidence for equal inlet and exit planes) the circulation will be constant around the loop. (vortex continuity dictates that the hull will finish the job).

    3: Now super-impose this configuration on the curved flow path around the hull. (needs care as this also means locally scaling the limbs for the same circulation in accelerated flow).

    CT249;

    No problem, I often type tired as well!


    Ceilidh;

    "If your loop keel can induce a low-drag ballasting effect on a hull heeled 15+ degrees, and if it can be trimmed for minimal impact on an upright hull, could it be used without the heavy ballast bulb in something like an inland lake scow? These boats are typically heeled 15+ degrees when going to weather, but are often hiked flat(ter) when planing downwind -- it would seem that an unballasted loop keel might be a nice way of increasing upwind power without losing too much on the way back down, and perhaps it might allow an overall beam (and thus weight) reduction?...."

    This is something that we have considered. Given the way that boats like this are used we considered increasing the beam of the keel beyond the hull and reducing the vee angle of the limbs as this would give much more powerful righting. If flap equipped the resersing the flaps would possibly allow the loop keel to transition into a hydrofoilf downwind although this is as far as we got with the idea.


    We have thought about various retracting methods in this context and, although possible, none have been very pretty which is why we have not looked into it any further as yet.

    So, no, it is not silly, just needs a bit more work.

    Vega;

    Sorry for not responding to your post earlier, We actually tested seven configurations in all, (more details under "history" on our web site) including the basic fin keel stake boat. We sailed all versions of the laser exept for the reverse-camber loop which we only tank tested to prove a point about added mass (in this case that it could be negative as well as positive).

    We sailed our final swept version and this version is the subject of the sailing trials reported in the April 2007 Yachting World article.

    You are correct that this allows more sail to be carried if desired.

    The righting moment does increase with the square of the speed as it is a dynamic effect although the apparent mass is there all the time, it just gains momentum as speed builds. The damping effect of this mass is therefore always present (Tom, I knoew this is not really the same as damping but "constant energy to excite a given perturbation" is less accessible).

    When upright the drag of the basic boat is increased by about 3% from our tests. The drag rise is more significant when heeled but the increase in sail carrying power gives an overall benefit past about 15 degrees of heel, ie, if you add sail to hit 15 degrees of heel in a light wind you are likely to be faster than a conventional boat.


    Jon.
     
  11. longliner45
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    longliner45 Senior Member

    in really good wind ,on the right hull ,,,,,could it be used as a foil also ,,even with the ballast?
     
  12. tspeer
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    tspeer Senior Member

    Even a closed lifting surface, like a loop, leaves a vortex wake. It's a consequence of the finite span of the loop. The loop may have less induced drag than a planar surface of the same span producing the same net lift, but there's still a trailing vortex wake.

    But your keel isn't even a closed loop in operation. The photos show the windward panel coming out of the water, which opens the loop, making it equivalent to a surface-piercing "V" hydrofoil.

    I should think flaps would be quite useful in optimizing the loading between the two surfaces. The optimum lift will be obtained when the induced velocity ("downwash" for a wing) is uniform along the span and proportional to the cosine of the "dihedral angle", which in this case would be the vertical, since you're trying to produce side force from the keel. So when the boat is upright, the two panels would be similarly loaded, but as the boat heels, the lift should be shifted from the leeward panel to the windward panel, since the leeward panel is more inclined to the flow. (see Munk, Max M., "The Minimum Induced Drag Of Aerofoils", NACA Report No. 121, 1923)

    To some extent this happens automatically because of the reduction in angle of attack of the leeward panel. But flaps would be useful in fine-tuning the distribution of lift. Right now, you're picking one trailing edge angle through the choice of camber of the surface. But due to the need for symmetry, you're stuck with the same camber on both surfaces when the boat is heeled, whether that's the best choice or not. Flaps could be used differentialy to both shift the relative loading and to put the lift coefficient in the section's drag bucket.

    The loading from the NACA a=1 camber line meets these criteria. At its design condition. Since it produces a uniform chordwise loading, it will produce the least variation in the pressure distribution associated with the selected thickness distribution. It is also easy to scale this camber shape for the desired design lift coefficient. So it would make a good choice as a furst cut.

    So long as the lift is held constant, I don't see why the chordwise pressure distribution would have any effect on the induced drag at all. Even for a loop, it's the spanwise lift distribution that counts.

    I would consider the pressure distribution on both sides, with the pressures due to the thickness factored in, to design section shape. Then extract the camber line from the section if you wanted to approximate it with a thin vortex lattice for the 3D calculation.

    An alternative approach would be to design the total circulation distribution about the loop, in the actual operating condition, using Munk's criterion of constant sidewash distribution. This spreadsheet uses this technique for a single inclined surface, including surface effects using either the zero Froude number linear approximation or the infinite Froude number linear approximation. It would be straightforward to modify it to handle both V panels, or three panels to form a complete loop. No hull effects, though.

    But it would be interesting to see how the circulation distributions and induced drag predictions compare for the case of an isolated keel.
     
  13. Jon Howes
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    Jon Howes Insomniac- sleep? Wassat?

    Tom,

    Thanks for your detailed response.

    "Even a closed lifting surface, like a loop, leaves a vortex wake."

    I am not sure that my answer will prove very digestible... apologies for that.

    Perhaps I was not clear enough in my previous answer: Of course a lift producing ring wing/boxwing in a continuum produces a vortex wake, minimising this is the essence of the problem of non-planar wing design. Most early theoretical analyses used a lifting line approximation and, although very close to the truth in most cases, the simplifying Treftz plane assumption, that the projection of the wing onto an x-y plane is exactly the same as the real 3D problem is only an approximation and a full 3D optimisation is needed to fully solve this problem. The vortex wake, however, is not eliminated, merely minimised. For example, a full 3D optimisation of a simple, no winglet, rectangular wing of of modest aspect ratio (about 5 or so), by the time it has been relaxed onto the flow surface arising from an optimisiation like this ends up as a fairly curvaceous surface with a span efficiency slightly better than that from an elliptical span loading, of course, by the time this optimisation is concluded the wing is also highly non-planar. Note that all non-planar wings produce the lowest value of Di/L^2 with non-elliptical span loadings. This is also fundamental to non-planar wing design. Biplanes and multiplanes are a slight exception but are actually arrays of planar wings (cascades in effect). A modern winglet, for example, approximately cuts off the elliptical distibution by taking it around a corner at each wing tip.

    Your comment was actually regarding the righting force from the loop keel and this is a little different to operation in a continuum. You comment that the circulation system is no longer a closed loop due to the water not filling the loop. The increase in water level within the loop is actually the continuation of the bound vortex. Not at the surface, obviously, but the level differential between the outside flow and the inside flow provides the continuous path, rather difficult to describe so let me try:

    If you sketch the load distribution on the loop (initially ignoring leeway resisting forces... more later) this is a set of arrows pushing outward from the centre of the submerged loop. If you project the outside loop water level inside the loop and intersect this with the hull the portion between the loop and the hull is a little under water. The static head above this line is an exact continuation of the pressure differential on the weather limb of the foil and is exactly equivalent to a continuation of the bound vortex. This pressure obviously continues over the hull surface down to the leeward limb of the keel and this closed loop force system is exactly equivalent to a closed loop bound vortex. The result is a standing wave travelling with the boat, which, on its own, does not shed vorticity into the wake although there is a wave system. The couple produced by this mass of water contained in the stading wave and the static bouyancy of the heeled hull is the dynamic righting moment. This may get semantic as one persons wave system due to force creation is anothers vortex drag in disguise! A case of equivalent ways of viewing the same problem.

    Leeway resistance does produce vortex drag since the keel is now similar to any other non-planar wing, in this case with the water surface as an approximate reflection plane. Minimisation of this is the optimisation process referred to in the first paragraph (a fun excercise... being the numerical solution of an array of quintic equations with circulation as the variable with the objective of minimising Di/L^2). As the leeway resistance is equivalnet to a superimposed vortex system on the basic loop, this is proportional to leeway and therefore chord distribution, the design approach therefore is to define this chord distribution for the optimum induced drag loading and then set the zero-leeway constant circulation by changing the camber around the loop.

    My quote:
    "we found early on that if the leading and trailing foil stagnation points indicate that the exit and entry areas of the loop are not equal drag can rise sharply. (this also occurs with flap deflection if flaps are fitted although when flaps are deflected it is generally because conditions demand it and the drag rise is not a primary concern)... "

    This was not an induced drag issue, it was a wave drag problem caused by the proximity of the water surface. A convergent path results in a need to accelerate the flow in the loop from front to back, a divergent path, to retard it. In each case, this results in a within-loop water level different to that caused by the bound vorticity and resolution of this level imbalance created an additional wave system. We first found this in testing and it caused a lot of thinking to take place as the answer was non-obvious. The solution was foils for which the stagnation points would be close to constant, this meant fairly sharp leading edges and zero geometric incidence. The angle of the zero lift surface of course follows all the usual thin aerofoil theory rules and is a convergent cone. This problem would not arise in a continuum, but boats work at an interface and it was a very important discovery made during development in terms of minimisation of wave drag.

    The NACA a=1 camber line must be one of the most useless things in aviation! It assumes constant chord loading and makes a mess of the trailing edge flow, that is why the better NACA sections do not use it. I was concerned about the sharp LE as this can be a separation trap. Sweep helped to avoid this but the main thing resolving this was that I designed a family of sections with an elliptical chordwise load distribution, ie, not the extreme camber gradients found on the a=1 line at LE and TE. The use of an elliptical chordal loading rather than some other form with modest LE and TE loading was primarily to try for the minimum energy waves when surface piercing. The foils actually run exceptionally cleanly with no visible spray, to the point where the Wolfson unit staff were surprised as surface piecing foils that do this seem to be very rare indeed.

    "So long as the lift is held constant, I don't see why the chordwise pressure distribution would have any effect on the induced drag at all. Even for a loop, it's the spanwise lift distribution that counts."

    Circulation per unit incidence for a given flow is solely a function of chord. Knowing the circulation distribution for min' drag while resisting leeway therefore dictates the chord distribution and the circulation distribution around the loop. This is not much different to the joined tip aircraft wing concept design techniques in which the chord distribution gives the circulation per unit incidence around the wing array which may then be modified by a superimposed ring vortex system, achieved by camber variation only, to produce a dragless pitch-positive couple for stability purposes. An excellent text covering some of this is "Wing Design" by R T Jones (very dense, more in one paragraph than is often found in enitire papers.... Expensive but well worth finding a second hand copy).

    Jon
     
  14. bhnautika
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    bhnautika Senior Member

    Jon have you looked at the work done by Hans Von Schertel in 1936 on surface piercing hoop foils.
     

  15. Bad Mac
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    Bad Mac Engineer

    Pictures of a Rotating Loop

    We've been asked a few questions about a rotating loop keel, so here are some pics of what a one might look like. It is obviously dependent upon geometry - in this configuration it is designed so that the craft will plane, hence the roots of the foil are further under the hull than normal.

    You can see that the craft benefits from almost the same static righting moment, but with a significant additional benefit from dynamic righting.

    Some obvious advantages of this configuration:

    There is no need for two asymmetric daggerboards to resist leeway.

    If there is a failure of the control system it should be possible to lock the loop in place with a simple safety clamp.

    Access and controls can be kept above the waterline. In the event of a major failure there is reduced risk of flooding the vessel.

    James
     

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