Foil assisted multihull design

Discussion in 'Boat Design' started by groper, Sep 29, 2013.

  1. Turnpoint
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    Turnpoint Junior Member

    Hi Groper,
    I have been working on a couple light weight foil assisted multi hulls. I don't think a foil that spans the hulls will work at all for your boat... Those are more appropriate for the split hull catamaran hull shape because of foil immersion depth. On your boat, if you put a horizontal foil between the hulls the foil would be too close to the water surface and would underperform.

    Have you thought about two retractable curved foil? It would exit the bottom of the boat at about 45 deg to the surface.. Curving to be horizontal to the surface near its tip. The advantage is that you can retract it, the foil can operate in deeper water, you get some dihedral to help with roll stability, and you can have a size it to give a high aspect ratio foil with a nice elliptical tip.
     
  2. sottorf
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    sottorf member

    using your weights with the required speeds of 15-20 knots, the Froude number is borderline for application of foils. To make it work you will need to use a tandem foil system i.e. two foils spanning the tunnel of almost equal size. If you go with a standard HYSUCAT system you will likely get around 10% speed improvement in full load.
     
  3. sottorf
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    sottorf member

    Not true! HYSUCAT foils work almost equally well on symmetrical and asymmetrical hulls. On symmetrical hulls it is only the foil-hull attachment which is more complicated. The foil immersion is not determined by the hull shape but is a function of the weight and speed of the boat and must be chosen appropriately.

    A retractable foil like you describe will not give sufficient lift at the speeds in question here. Such foils only work on very light weight craft; not to mention more complicated to build and more prone to damage from debris strikes. I would not recommend it for this application.
     
  4. Timothy
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    Timothy Senior Member

    I know practically nothing about foils and this is probably a stupid question, but why is the rear foil necessary? If the center of gravity is behind the foil and that foil at a certain speed is lifting , say, 90% of displacement then if the aft surfaces of the hulls' bottoms are flat, would they not support the other 10% like a planing hull? And as the foil lifts the boat would not that lift be limited relative to proximity of the foil to the surface? I thought that the rudder foils on the AC 72 s were set for neutral lift for minimum drag, and that there main function was to produce down force to appose the force of the rig and the resulting bow down attitude and negative angle of attack of the main foil. I don't see how this is applicable to a power boat.
     
  5. Turnpoint
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    Turnpoint Junior Member

    Scottorf,
    Hold on a second. I don't design Hysucat's for a living but it stands to reason that hull shape DOES have a big effect on foil effectiveness. Take a look at the quick drawing I made...attached. The drawing is pretty crude but should show a problem that can arise with a foil stretched between lightweight catamaran hulls. The drawing shows two catamarans... one with a split tunnel and the other with U shaped hull sections. Note that all hulls drawn have the same beam. Also, the displacement is the same on both cats. The big differenceis that the split tunnel has 73% more foil immersion. As the foil approaches the surface of the water its effectiveness is greatly diminished. A foil approaching the surface can also ventilate causing a loss of all lift/crash/porpoising. I don't know if Hyscat type foils have been installed successfully on a lightweight U shaped catamaran hull. (I would be very interested to know of examples and the increase in performance they achieved). But, there seems to be a reason the good hyscat designs use the split tunnel hull shape.
     

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  6. sottorf
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    sottorf member

    The rear foil is necessary for pitch stability much like on an aircraft. Without the rear foil, the vessel will pivot on the main foil and become unstable very quickly in waves. For the same reason it is not really possible to carry 90% of weight on the foils. Depending on the hull shape, one has to allow the hull to carry a certain amount of the weight on the hull for a stabilizing effect.
     
  7. sottorf
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    sottorf member

    Turnpoint,
    At rest the immersion will be different - agreed. However AT SPEED - all other factors being equal - the immersion of the foil will be similar for the two hulls for the same foil loading. There will be small changes depending on how much lift the hulls generate (depending on their shape) but in my experience this does not have a big effect on the foil immersion if it is only a question of symmetry or asymmetry of the hulls because lift is so sensitive to immersion when the foil operates less than 1 chord length below the surface - common for HYSUCATs. The equilibrium immersion stays quite constant as a result.

    In your sketch, you show a very deep-vee hull and an round-bilge hull. Round bilge hulls tend to generate suction forces at speed countering the foil lift. the same foil design could therefore not be used on such ah hull compared to a deep-vee planing hull.

    For the symmetrical hull the foil should still be located at keel depth in order to lift the hull sufficiently out of the water. Locating the foil above the keel is one of the worst mistakes one can make. The foil will be jumping in and out of the water in waves and will give the vessel atrocious seakeeping. Locating the foil below the keel works in some situations but from a practical standpoint has some limitations - increased draft, risk of damage, special cradles required on land etc.

    Regarding a foil approaching the surface ventilating/porpoising etc. Yes this can happen and that is where one needs to balance hull and foil lift. Too much lift on the foil and yes there will be porpoising in waves when the foil ventilates. With reduced lift the hull will stabilize the vessel should the foil ventilate. In actual fact HYSUCAT foils ventilate all the time and craft often ride the crests of waves without any instability.

    Unfortunately there is very little technical literature on this subject around. You can try and find this report from David Taylor Model Basin where this problem was touched on for monohulls with foils:

    Karafiath, G. (1974). Performance of NSRDC Model 5184 Configured as a Partial Hydrofoil Supported Planing Craft and a Comparison with a Powering Prediction Technique. Bethesda, Maryland.
     
  8. Timothy
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    Timothy Senior Member

    I was thinking that if the depth of the foil is any way proportional to its lift then as the foil rises its lift and there fore its ability to lift the hull is limited by the free surface effect ,and or ventilation. Because the hull sterns are weighted to the surface as CG is aft of the foil ,pitch angle is limited to the travel of the foil. I look at it as a teeter totter with one end fixed to the ground and the fulcrum being lowered and raised within imposed limits. I see the hull sterns acting like the tail of a kite. Again I know nothing about foils but I understand that "Banque Populaire" did not have lifting foils on the rudders and that the curved foil on the leeward ama carried 70% of the boats displacement and pitching was not a problem. Am I looking at this the wrong way?
     
  9. Turnpoint
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    Turnpoint Junior Member

    Thanks for the great response Scottorf,
    I'm still a little confused when you say that the two hull shapes will work similarly. Please see the attached drawing. In this I show a foiling waterline where each hull shape is getting foil assist. 70% of the hull displacement is being carried by the foils. The difference between the two hull shapes seems to be even worse as the foils develop lift. You can see that the U shaped hull's main foil will now be 4 3/16 from the surface of the water (within a cord length) where as the deep V still has 9 1/8" immersion. I take it that the Hyscat design relies on the diminished lift of the foil as it approaches the water to maintain a consistent ride height in a sea. However, if you look at the U shaped hull in my drawing, I do not think the foil would be efficient carrying 70% of the displacement... seems like 50% of displacement would be more realistic in this example (just a guess here). Also, seems like a foil assist design that has deeper foils ( does not rely on loss of lift as the foils approach the surface) would be more efficient.... as long as you could maintain a stable ride.

    Timothy,
    I think Banque Populaire had different design elements... For those sail boats lifting the bows was a good thing in that a pitchpole was the most likely disaster that could end there day. So the foils reduced wetted surface but also allowed them to push the boats harder without risk of pitchpole. Also, the aft sections of the amas were broader and flatter to compensate for the foil lift.
     

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  10. groper
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    groper Senior Member

    Sottorf,

    Interesting...

    15-20kts was the minimum speed...

    The current plan for installed power should see the boat without foils to reach 27kts. With foils added, probably considerably more than that... BUT, in rough weather, i doubt the boat will run at high speed and therefore it should maintain good performance at 15kts - sorry if that wasnt clear. When i say maintain good performance, i mean similar resistance and fuel economy to the boat without foils.

    Shallow running foils are all im considering right now, deeply immersed foils (retractable included) are more complex to build, reduce the boats running draft - the boat runs in shallow areas, and ability to use normal propulsion systems by not lifting completely clear of the water. The hysucat system is very simple and the low draft is very appealing for my application.
     
  11. sottorf
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    sottorf member

    yes as the foils rises its lift reduces. The foil is designed not for maximum lift at deep submergence but to have the correct amount of lift when it is shallowly immersed. THe foil cannot be designed in isolation from the hull. It requires and iterative process to come up with the correct size and angle of attack that results in the right amount of lift and the right running trim angle for the boat.
    The pivot point depends very much on the location of the foil to the LCG. If the foil is very close to the LCG then the whole vessel tends to pivot on the foil. If the foil is far forward of the LCG then it tends to pivot on the transom much like a planing hull. You are correct there are some limits of motion. The foil will never submerge very deeply as foil increases lift as it submerges and also the hull picks up buoyancy. However normally deeper immersion of the foil is also associated with reduction in trim of the vessel as it pitches down which in turn reduces the angle of attack on the foil. The interaction is quite complicated.

    I have not really looked into the dynamics and stability of sailing multihulls with foils. But taking a look at the pictures of this boat on Google, I would say the stability is provided by keeping one Ama in the water at all times. Secondly the thrust force from the sails acts very high above the CG and creates a bow down moment. I therefore dont think that pitch stability is a problem. The main problem is to prevent the bow from digging into a wave. This is probably controlled by varying the submergence of the foil as it seems to be retractable.

    Maybe somebody else in this forum can shed light on how that system works?
     
  12. sottorf
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    sottorf member

    If the boat is running 27 knots w/o foils you can expect a good speed improvement with a foil system. I expect around 30% increase if you do it right and get the right props pitched for this increase in speed.

    Fully agree, keep it simple - fixed foils with no moving parts. otherwise it will be a headache to maintain and get working.
     
  13. sottorf
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    sottorf member

    Turnpoint,
    in general a HYSUCAT will only carry 70% lift on the foils in very calm water otherwise it tends to be 40-60% range.

    Looking at your drawing, the foil on the two hulls are not immersed the same amount below the surface. Assuming they are the same, the deeper one will be generating more lift (it is incorrect to assume that they will have equal lift because the foil + hull has risen an equal amount from the static floating condition). So in the sketch the U-shaped hull has a foil which is generating more lift and the hull has higher buoyancy so it is going to keep rising until it reaches equilibrium and that equilibrium will be pretty close to where the deep-vee hull is shown to be in equilibrium.

    Thanks for the info on the sailing cats - it sounds like a really complicated design problem.
     
  14. Richard Woods
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    Richard Woods Woods Designs

    If the foils offer 50% lessening of displacement then the boat will be say 250mm higher out of the water when foiling. So either the stern lifts to match or the boat trims by the stern

    In either event how do you plan to adjust your engines/props? Given that you aren't building a full planing boat, nor fitting big engines

    Just curious. I won't fit foils on my Skoota powercat because of all the weed, never mind debris in the water in the PNW. I was out sailing a couple of days ago and passed a deadhead that was missing its flag (the markerstick was still attached, that's how I saw it). Just visible at 6 knots, impossible to see until too late at 25.

    Deadheads have ripped off propellors from ferries round here. Not sure what it would do to a foil, while weed will be impossible to remove from between the hulls

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    My opinion would be that at 15 knots, foils won't help you, but will create drag. Your best plan would be to leave things as they are imo, your boat at "considerably more than 27 knots" would be a very scary conveyance, not to mention the problem you have with propping the engines to run like a hare with foils, but still run properly backed off at 15 knots in lumpy conditions. I put a foil under a 21' tunnel hull one time and it didn't kick in at 15 knots to any noticeable extent, 25 knots maybe, but the speed gain was not great by any means.
     
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