racer cruiser catamaran lifting foils

Discussion in 'Multihulls' started by bushsailor, Nov 15, 2013.

  1. bushsailor
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    bushsailor Junior Member

    Thinking of installing curved lifting foils to a cruiser racer catamaran
    (think farrier trimaran type foils.)
    Cat details 4 ton sailing weight
    40' long.
    Good performance in breeze downwind 20 to 23knots sustained speed.
    Foils (off another boat) generate 1200kg lift at 20 knots for 45 kg drag.
    Questions:
    Will foils lift drag reduction of the boat overcome foil drag of 45kg
    We know that foils stabilise the boat and make it "feel" longer however will actual pitch dampening performance improve boat speed.
    I know there must be a reason, but why do foils always curve in and not outwards. (so you get a combination of DSS and foil assist)
    We know that foils contribute little upwind (150kg lift and 12kg drag) however would pitch control make them worthwhile once boatspeed gets to 12 knots. (boat pitches upwind up till it starts to fly a hull.)
    Are there any examples of this. (has it been done before)
     
  2. Doug Lord
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    Doug Lord Flight Ready

    ===========
    1) The chances are that properly installed foils would overcome drag at a certain speed and above.
    2) Pitch dampening is one of the proven benefits of DSS foils and other foil assist applications-definitely improves speed and ride comfort.
    3) DSS foils go horizontally outboard so have minimal effects from leeway, but a curved foil like the Farrier foils you mention or other curved foils run into a big problem with drag if curved outboard. The foil experiences leeway and that causes high and low pressure on the same side of the foil. At the "crossover" point there is tremendous drag. Greg Ketterman discovered this in a "real life" experiment with a Hobie Trifoiler prototype when he tried this very thing and found the boat significantly slower with foils pointed outboard.
    ======
    There are numerous examples of foil assist improving boat speed and handling in catamarans: GC32 which has used foil assist until now when they are converting to full flying, NACRA 20, NACRA 17, most every A class cat and some C class cats, the Catana(?) 59 cruising catamaran and more.

    Pictures,L to R- 1-rough illustration of a foil curved outboard. At the point marked zero(the crossover point) where I was thinking there might be no induced drag, Tom Speer point out that there would be massive drag as the two lifting forces-vertical and lateral meet. Due to leeway the upper portion of the foil has high pressure on it whereas the lower top portion of the foil has low pressure, 2-Catana 59 cruising cat with curved foils,3) the Catana foil,4) Farrier foil and trunk-note that top of trunk is longer than bottom of trunk to allow for foil rake to asjust the angle of incidence of the vertical lifting portion of the foil.
     

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  3. Baltic Bandit

    Baltic Bandit Previous Member

    Well the answer is a touch more complicated

    first off, you need to be certain your trunks are designed for 1200# of lift.

    Secondly, for there to be 45# (and its gotta be ft lbs/sec or some similar unit of force) of drag reduction, you essentially need to be lifting the hull enough to reduce skin friction by that amount.

    this has nothing to do with "proper installation" and everything to do with your hull shape and waterline loading characteristics.

    the one thing doug is right about is why the foils are curved inwards and not outwards.

    there are other pragmatic aspects of being able to use the visual edge of your hull as a visual guide to hitting things as well as by putting them inboard the pressure wave of the bow protects them better from debriss
     
  4. bushsailor
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    bushsailor Junior Member

    thank you both for replying and thank you for not sniping at each other(much), a refreshing change.
    We are setting the foils up so that bottom is horizontal at 6 degree of heel and I doubt the foils will contribute much to prevent leeway in this situation. We have already got large daggerboards.
    We actually do not want the foil to prevent leeway when running.
    Trunks will be plenty strong enough, we have installed these same foils in other boats. We actually build the trunks larger than the foil and use special plastic bushes to take the load. Then to adjust and fine tune the foils we can simply make another bush and bolt it on.
    One of the reasons I was asking about the outboard foil was that it would pull the boat to leeward, which when running would be a real bonus, as well as increasing form stability. however I now understand why.
    We know it takes 45kg of extra force to drag the foil through the water at 20 knots when the foil is working. I am sure the 1200kg of lift will more than offset the drag but how do you prove it. I know this because when we put an extra 10 people on the boat performance is probably halved.
    I very much doubt the curved foils on the Catana would do anything more than be used as a marketing tool.
     
  5. Baltic Bandit

    Baltic Bandit Previous Member

    I've not sniped Bush... I've been attacked and responded. I've been attacked again in another thread as well.

    the way you prove the offset is by looking at your immersed lines and all up sailing weight. you can do a rough calc by hand of course but to do a full on calc you need the cad model and a hull modeling tool that easily calcs immersed surface area.

    Essentially your hull drag is going to be driven by immersed surface area (other factors apply but that's the dominant force here) So if the boards generate enough lift to reduce your wetted surface area by that much drag (you can look up the EQs on Wikipediia for a ballpark) then you are well off

    Its not clear that you want a board to generate lift to leeward going downwind. that would give you lee helm. and lee helm is drag. Ideally you want to be able to balance your sailplan and CLR so that your helm is neutral downwind. That usually means some lateral resistance in the board. After all if you want to generate Apparent Wind, you need some lateral resistance.
     
  6. catsketcher
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    catsketcher Senior Member

    I know of two largish cruiser/racer multis that have had foils fitted. One is a tri and the other a cat. The tri found them to make the boat more stable at speed and the cat owner is not incredibly happy with them - they seem to make more drag than they reduce on both boats.

    I would say stay well clear on a cruising cat. If you are really going to build or modify a cat then almost always you are better served putting your time and money into

    -reducing weight - LED lights, better fridge means you need half battery weight, reduce dinghy size and install watermaker and other ideas.

    -stern extension - every one has one or wants one - unless their boat was built underweight or built to carry load. Making the boat 1m longer will cost little more but make the boat faster and pitch less. And it makes the stern steps a nicer spot to sit.

    - put big prodder on and get nice big screecher and kite

    Plus all of these will give you better resale.

    As an aside I think that it never works to have the foils pull a boat sideways. Gybing boards only ever seemed to work when they stopped the hulls making leeway - not pushing them to windward. The Gougeons did a fair bit of work on this as did a lot of FD, 505 and 470 sailors - hulls have reduced drag when going straight. Plus if you get pulled downwind you reduce your apparent anyway.

    But why foils on a cruiser? You will need rudder T foils to get stability so you are going to pull around 4 foils in all winds for a benefit in the top range. If you made a Moth slower and raced in all winds then an old lowrider Moth would probably beat one that was slow in the light and medium breezes and flew in the heavier air. Things that fly should be light and I don't think that a 40ft 4 tonne cat can be called light by foiling standards. Top this off with weed and barnacles in the case and I think the ability to get the micro finish that seems so important in the Farrier and ORMA 60 foils (have a look at their crazy sections) of the Moths (they use CAD to mill the sections) will be achieved in a cat with its cases permanently immersed. I have been known to be wrong but I think the gains will be minimal and the problems large



    cheers

    Phil
     
  7. Baltic Bandit

    Baltic Bandit Previous Member

    Actually gybing boards work not by reducing lee-way but by reducing hull drag by letting the hull cant more in the CMG direction. So even there its an issue of drag reduction.

    the point about reducing #1200 of gear aboard is a good one. Do you notice that your boat sails worse when you have 6 more people on board (5 people with weekend luggage)? I doubt it.
     
  8. bushsailor
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    bushsailor Junior Member

    Thanks for replying.
    The idea is certainly not to make the boat foiling but to lift the leeward hull (or to reduce its displacement) by 1200kg with the added benefit of reduced pitch. (Pitch dampening is the biggest benefit)
    The foils will only be used for racing although on some delivery's we do push the boat if we have a few crew on board. 70 nm in 4 hours has been achieved.
    I was going to install t foils on the rudders first but this is a lot more work than the curved foils due to new rudders, new kick up assemblies, and strengthened transom.
    By the way we simply lift the foils clear of the water when not in use, they only weigh 10kg each. I was also thinking of making the hulls longer and boat wider but probably easier to build a new boat!!!
    I was going to install them now and test them on a 400nm cruise down the coast and a race back but will think about it for a bit longer, maybe in the new year.
    A more accurate description may be racer cruiser.
     
  9. Baltic Bandit

    Baltic Bandit Previous Member

    Well what you might consider doing is putting in one board for your trip down the coast. And take the time in different wind and wave conditions to tack and pull up the other board and compare your speed and VMG.

    Catsketcher may well turn out to be right.
     
  10. Doug Lord
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    Doug Lord Flight Ready

    -------
    Why don't you run your foil ideas past someone like Mark Pivac? I think he is in Walliston, WA( suburb of Perth ) and designed and built Spitfire and ,as I understand it, worked with Brett Burvil on the first Moth to win a race on foils.

    You could PM him here: http://forums.sailinganarchy.com/index.php?showuser=43756
    This is his company: http://www.bdg.com.au/

    Spitfire under sail:
     

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  11. Baltic Bandit

    Baltic Bandit Previous Member

    Remember to use Doug's name when you do this. I'm sure it will open doors for you
     
  12. Graham Murray
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    Graham Murray Junior Member

    Mr. BushSailor, why not contact this person, I'm a client of his and I can say that the process has been fascinating.

    Martin is a gentle man of prodigious intellect who will explain stuff in a way that
    an enthusiast like me (you?) can understand.

    Even if what he has designed for me never gets built, for the education alone, it has been worth it.

    There is nothing wrong with a lot of the free advice on boatdesign but a lesson that I seem to continually relearn is that you get what you pay for......

    Cheers,
    Graham.


    http://www.catsailingnews.com/2010/05/cs-interview-martin-fischer.html
     
  13. Doug Lord
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    Doug Lord Flight Ready

    ===============
    Martin Fischer is absolutely top notch and has helped and inspired me a great deal. Here is a thread about his 40' trimaran:

    http://www.boatdesign.net/forums/multihulls/jessica-rabbit-40-foil-equipped-trimaran-46495.html

    He is a member here...... http://www.boatdesign.net/forums/multihulls/mayfly-class-catamaran-fischer-39616.html
     
  14. catsketcher
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    catsketcher Senior Member

    Thanks for the link Doug - Jessica certainly looks like a nice boat.

    On driving back home I wondered about foils on normal (what is normal?) cruising cats. You can't use an AC foil because it will get weed on it and it would have lots of drag so the curved foils seem obvious but there may be a wrinkle that Bushy will have to ask an expert about (Graham's concept I endorse wholeheartedly).

    The AC cats pulled up their windward boards because of a rule but it did use a crew man to do it and the case design must have been very good (Sean Langman's ORMA has very nice cases too). So the only board in the water is the leeward one.

    In waves the boat may get thrown to leeward hard (usually in the trough). If you still have your curved windward board down then the windward board will develop downforce, or at least some of it will in line with Doug's drawings above.

    Most non AC cats leave both boards down going to windward so one could find yourself having a windward hull foil assisted and then slammed down with negative lift as soon as leeway reaches a certain amount. This could be uncomfortable.

    Tri never have this problem as the dihedral keeps the windward foil well out of the water. Certainly many tris from Kelsall, Newick, Irens, VPLP use foils but there may be some hidden reasons why cats are harder to design foil packages for. The AC cats didn't get this problem as they only every had three foils in the water (leeward board and rudders)

    This effect of having the windward hull pull down was remarked upon as a good safety feature with Seawind 24 owners. They would slip sideways in a gust and stay flat. Their boards are straight but highly canted.

    PS - If you would like to talk to the owner of the cat with foils in Sydney take a paddle around Lilli Pilli. The cat is a modified Shockwave 37 that the owner did a heap of work on. It may not be what you are going to do at all but there may be some lessons for you there.

    cheers

    Phil
     

  15. bushsailor
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    bushsailor Junior Member

    I would love to talk to the guy in Sydney. I will do that when I get there.
    Yes Jessica Rabbit is a beautiful boat and a different league to anything in Australia and especially my boat, but the real performance boost to her was the t foil rudders, again mainly as pitch control. And yes I think he is one of the lead designers in the world, prepared to have a go at the leading edge stuff.
    Upwind you are correct, need to pick up the windward board, mainly because it is trying to give you lee helm. Downwind you can leave both down, I have used the foils on a tri a fair bit.
     
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