Cat hull Hydrofoil

Discussion in 'Multihulls' started by Fanie, Jan 11, 2008.

  1. Fanie
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    Fanie Fanie

    Hi Guys !

    Is there any reason why a hydrofoil built into a cat's hull won't stop it from diving into the water and thus prevent pitch-poling ?

    As per the attached dwg, if the hull gets submerged at speed the foil would force the hull upwards thus prevent getting forced under the water. The cat can now look like a hammerhead shark ;) and even more speed may well be possible. On smaller hulls it may also make for a nice handle. It would also reduce water spray a lot.
     

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  2. Richard Atkin
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    Richard Atkin atn_atkin@hotmail.com

    I thought of that too. I would want the foils to be high enough so as to not interfere with light air performance in choppy sea. The problem with that, is that the foils start to act as a 'cure' rather than a 'preventative'. But then I don't know how far a bow will bury before it starts to slow the boat down too much... and pitchpole. Perhaps even a small bury at high speed is all it takes??? Foils act too late??
    I guess a lot of people would be concerned about damaging the foils and so would rather just go slow and forget the foils.
     
  3. Richard Atkin
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    Richard Atkin atn_atkin@hotmail.com

    I would like foils that are positioned nice and low....but can be lifted out of the water during light winds. Too flimsy though?
     
  4. deepsix
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    deepsix Senior Member

    Why would a hydrofoil be more effective than simply increasing reserve buoyancy in the bows?

    Recent thinking is along the lines of keeping the drag of the submerged hull low so that it does not trip up when submerged, look at the A-class cats. I would design a very long and narrow bow with high freeboard. This way smaller waves will be pierced without slowing the boat, but the high reserve buoyancy will lift the bow if completely submerged.

    A hydrofoil may be usefull on a short boat with a tall rig, but I would rather build a longer hull under the same rig rather than a hydrofoil because you get the added bonus of longer waterline length and a narrower hull. Beach cats, lowrider moths and I14 dinghies have successfully developed hydrofoils for the purpose you describe. The foil needs to be placed behind the pitch axis to acheive any pitch damping and the deeper the better, this means it goes on the rudder.

    Your foil is drawn on the bow above the waterline and infront of the pitch axis. This means that as you pitchpole the foil will be submerged and will generate lift. If the boat continues to rotate, the angle of attack on the foil decreases and the foil produces less righting force. Eventually the foil will only contribute drag and a may even generate a force pushing the bow down. If the foil is behind the pitch axis, the righting force will increase as the boat rotates until the foil stalls.
     
    Last edited: Jan 12, 2008
  5. Fanie
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    Fanie Fanie

    Hi Richard, sorry I missed you. You NZ guys go to bed much too early ;)

    I know the picture I uploaded doesn't look very elegant but you should see the paint job :D

    Wanted to know firstly if there is any reason why this wouldn't work.

    Also I like wide decks, so if the foil would prevent hull burroughing then it may well be possible to shape the upper hull to be wide on top looking like a mini landing strip.

    Also some of the boat designers suggest widening the hull rapidly above the waterline fore and aft (where the hull is narrow) would oppose pitchpoling, prevent burrouhing as well as increase space in the hulls fore and aft.

    If nothing else the space gained could be used for floatation. One advantage I can already think of is it provides space in the bow where the petrol generator can live in.

    Implementing this could have a lot of space advantages especially in a small hull like my 10m Boxy Fisher. Open deck cats could benefit hugely from having wide upper front decks.

    I would think the foils should be placed well above the waterline. They should work only when conditions plays up and under more extreme conditions. Placing them lower would result in too much bumping and water slapping. They would have to be more sturdy since thee would have to be strong enough to lift the hull.
     
  6. Fanie
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    Fanie Fanie

    Deepsix, the widening of the hulls would prevent pushing the hull under water, but it would be boyancy only. The idea of the foil shapes is more for when you are sailing fast and down a wave, instead of sailing right into the wall of water the foil should 'ski' right up while still maintaining some speed. Just sailing into a wall of water should have an immediate stopping force on the vessel.
     
  7. deepsix
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    deepsix Senior Member

    Reserve buoyancy is the amount of buoyancy above the waterline. Flaring the hull as you describe is one way of achieving reserve buoyancy. The problem with flare is that if you manage to get it into the water there is alot of drag, the boat slows down and is more likley to pitchpole. This is how the older cats were designed, it worked well but things have progressed.

    Modern thinking is also to have buoyancy to lift the bows, but it is distributed differently. Modern boats tend to have long narrow bows with high freeboard. The bows are shaped so that if they submerge they dont slow the boat down and are less likely to trip up. Think about it if you are doing 20kts downwind in 30kts of wind and you hit a wave and your speed drops to 10kts, the wind speed has doubled. It all contributes to the pitchpole. Modern bows are narrow so they can cut through the waves, and the high freeboard gives the same effect as the flair you are describing, it is also buoyancy, it is just more effeciently shaped to slow the boat less. The high freeboard also gives good bridge deck clearance.

    If you want to go fast you need to go straight through the waves, not over them. The concept is called "wave piercing". It is generally a racing idea, but if you look at the modern cruisers bows are getting narrower look at the gunboat, also read up on the hull design of the lightspeed 32. Obviously im not suggesting you go to the low bouyancy extremes of the racing boats, but the idea of not slowing down so much when you hit a wave is important.

    I saw the "hydrofoils/ski's" you speak of on some monohulls in the early 90's. They were called "bullhorns". They have not returned for over 15 years so my guess is that they dont work. No other boats need them, and your boat is not that different, so why should they be necessary?
     
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  8. Trevlyns
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    Trevlyns Senior Citizen/Member

    Fanie

    I would be wary of placing “heavy” items like generators in the ends. This will add momentum to a potential pitch particularly in a lightweight cat.

    My interest lies in Proas which behave differently from cats in that they don’t tack but are shunted. Because of this, keel rocker can greatly be reduced, pushing up the prismatic coefficient and adding fullness to the ends whilst still retaining a slim hull. Have a look at some of Rob Denny’s designs – very flat keel lines.

    Lekker bly!
     
  9. Richard Atkin
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    Richard Atkin atn_atkin@hotmail.com

    Sorry Fanie my browser can't read your attachment. Weird. I was just going by your description.

    Everything that deepsix says sounds believable to me. I have always thought that big wide flaring must act like a brake if the bow is going 'down'.
    Lift is more effective than buoyancy. Perhaps if the foils are shaped like scoops, they would be less likely to fly down to the bottom of the ocean.

    I don't believe that a cruising cat should be designed to be a full-on wave piercer. There are no second chances. Just don't go too fast.

    You should check out my thread again: Trying to design my own cat

    You will see a magnificent example of what deepsix is talking about

    - Richard
     
  10. Fanie
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    Fanie Fanie

    Richard, the dwg is the nose of a cat's hull with a hydrofoil angled and horizontally near it's top and it sticks out both sides, hence looking like a hammerhead shark. When the hull is pushed under water at speed the water pushing against the bottom of the foils force the nose of the hull upwards, so the hull would ski on the hydrofoil but won't go under. Ok you can paint the eyes on the thing... would really look cool too.

    I was watching some sailing video's on youtube where there was a lot of cats pichpoling due to the leeway hull's burrouging and hence capsizing in high wind - spectacularly. The thought came to add some hydrofoils on the hull to force them up out of the water so they cannot burrough so deep that they would capsize. It seems that if the leeway hull's could be prevented to burrough too deep the cat may be blowed away, but won't capsize.

    If it could work on a small scale it could work on a larger scale too. I think we made it through the 90's and I don't plan to go back. A LOT has changed since then. Come to think of it... if I do have to go back I would know where to catch more fish... just go streight to the spots :D

    Ahem... almost got carried away there :D

    Another article I read showed some boats sliding of the side of a big wave while running and in the bottom they stick their noses in the wall of water. (sounds like the nosey neighbour) Maybe it isn't a problem with larger cats, but implimenting such a feature could save the day. just a thought.
     
  11. Fanie
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    Fanie Fanie

    Oh, the generator. Trev, the genny would be a small one, just for charging a battery maybe, or run a PC so I can go on the net :rolleyes: while fishing and maybe ask dumb questions here on the forum like 'big wave comming, what must I do ?'. There doesn't seem to be anywhere else to put the thing than in a hatch in one of the bows. Maybe closer to the time I would find a better space for it.
     
  12. masalai
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    masalai masalai

    Fanie, Don't you ever sleep? Are you like Yipster who sends his avatar to observe everyone? check "Where's Yipster" thread, - straight from his web=site. I would love a moving avatar but I am too ugley & would scare the **** out of any observer :D

    I'm going crosseyed trying to learn Delftship and peering into one of those horrid hi-gloss lcd screens on my note. **** it is designed for a vein woman to check her ? - ? face? I would like to kick the arse of the ******** marketing exec who thought up this "crystal-bright" crap screen.

    Imagine this sitting well off to the side in a room with a dark curtain backdrop to stop reflections from obscuring the important stuff on the screen & not reflected by it........ Marketing exec must be woman of narcissist.

    & you think you've got problems :D :D :p
     
  13. Trevlyns
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    Trevlyns Senior Citizen/Member

    Hey Fanie

    On a catamaran, you even weigh the ruddy tooth brushes for optimum placement for yacht balance.
     
  14. masalai
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    masalai masalai

    and the toothpicks. It also pays to weigh the crew/passengers to ensure when being weighed off the do not leave any crap (literally) behind... :D

    You want anal you got anal :p
     

  15. Richard Atkin
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    Richard Atkin atn_atkin@hotmail.com

    Fanie, quote "Another article I read showed some boats sliding of the side of a big wave while running and in the bottom they stick their noses in the wall of water. (sounds like the nosey neighbour) Maybe it isn't a problem with larger cats, but implimenting such a feature could save the day. just a thought."

    I bet it is a big problem with cruising cats (love that term). With all the extra weight, and fast hulls, they are just asking to become submarines.

    I think deepsix sums it up well though.
     
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