aerofoil cross beam generates lift on catamran..

Discussion in 'Boat Design' started by assycat, May 13, 2012.

  1. assycat

    assycat Previous Member

    here is an aerofoil section as a cross beam to join the ama's of a cat...note the lift generated which theoretically acts l.ike an airplane wing and should lift the hulls in lighter airs and get the boat on plane easier and to give a proper angle of attack for planing speeds...i wonder if it is too much lift which could cause a reverse pitchpole in heavy airs when close to the wind like a hydroplane catching air and lifting off, spinning backwards???
    it certainly looks feasible??! all honeycomb combined with core cell construction...
     

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  2. Silver Raven
    Joined: Oct 2011
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    Silver Raven Senior Member

    Gooday 'assycat' - great idea - it worked fine thanks - has been used on cats - both 40's & 59's as far back as 1967. Oh & the idea - put together very carefully with the right hull & deck designs will work for a long time into the future.

    Hey Canuck - y'all have a good time over there on the 'St L' now that the water has changed from ice. Ciao, james (ex-pat Canuck)
     
  3. philSweet
    Joined: May 2008
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    philSweet Senior Member

    I think you would want to be very careful with the placement.

    1. It's a WIG and you need to consider the wake interaction. Oracle's AC boat wasn't entirely sure about the thing if I remember correctly. They were planning on using one at the extreme aft end, I assume to modify wake as much as to lift the boat. So maybe it depended on wind speed. It was a bolt on affair. I think Oracle's could also be adjusted panel by panel; so lift on the leeside/low pressure/ fast windspeed side only???? It might also reduce induced areo drag off the bottom of the sails. Maybe TSpeer will comment.

    2. Placed at the bow or stern, it might effectively increase waterline length perhaps change the effective prismatic coefficient of the hulls. I don't see it working as well placed midship.

    3. Doesn't a low pressure area above the deck aft of the mast suck the boat backwards? Why not use the mainsail's high pressure side to pressurize the underside of the deck? Ie., use the water surface as an endplate and not the deck.

    I think the foil needs to do more than simply provide a bit of centerline lift to earn its keep.
     
  4. assycat

    assycat Previous Member

    ahoy James! aka Silver Raven.

    your post has both good news and bad news for me- first the good- it might actually work..
    the bad: I actually thought i might have thought of something original...only to find out this has been tried...grrr...

    yes its beautiful now--i just live for spring--but sure dont miss winters anymore..im getting too old for winters...seems as a kid i loved it--now , i just count every day so that i can get in swimming/sailing and enjoy the beauty of the waters here...

    where were you located here?..how bout down under?..whats your 20?

    thanks for the post Mate!
     
  5. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    One other thought. As sketched, centerline lift reduces righting moment. You have to add some weight hiking out to get it back. So if you add five pounds to the boat to build the wing and you get 8 pounds of lift, you haven't really gained three pounds because you will need another pound of weight hiking out to get the RM back. Spray drag and sneezing are two more troubles to deal with.
     
  6. assycat

    assycat Previous Member

    Hi Phil--you sure know your stuff...more than i do. I know what a WIG is--but didnt think it fell into that category--but now that i think of it--i guess it certainly would-- I dont know about the other factors you mentioned since i persoannly have never seen one like this- so my guess is the only real way to test in a wind tunnel perhaps build one...

    the placement for me actually was twofold--one to create some lift in lighter airs...helping to reduce wetted surface and creating lift, but also to act as a strong support beam for the two hulls. one that could be walked one. it acts as the trampoline in a way. i can lounge on it while not in use...

    im not sure also about the technical idea of the water as an endplate --would you mind explaining abit more about that--like i siad im no expert--just a dabbler...

    great post!
     
  7. assycat

    assycat Previous Member

    Ill try not to sneeze....if i spread the hulls out further- would this increase the stability?..and the righting moment? i dont know if i menrtioned--the boat is 14 ft wide--but could be 17. and it is LOA of 32 ft. nboth hulls are stepped 3 inches at about .33333 of the waterline length...i hear what your saying...i have wondered if it is worth anything at all--but what i thought was--it maybe couldnt hurt and it makes a strong support beam for the two hulls?...just thinking...i love the idea of planing hulls of displacement ones...:)
     
  8. philSweet
    Joined: May 2008
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    philSweet Senior Member

    Sneezing is when the boat pitches down or heaves down and spray is blown out the front of the tunnel. It's annoying and one normally tries to avoid throwing water forward.
    I won't deny that at one very particular design point you can probably set something up. Sailrocket is using a bit of a WIG sticking out from the bottom of the the wing. Landspeed boats use downthrust beams to gain RM, but they don't make waves. It's just when you figure that you carry it 100% of the time but it only helps 20% of the time that you have problems.
     

  9. assycat

    assycat Previous Member

    Phil--do you mean like when hobby horsing??...i wondered why the crossbeams on a cat dont bury in heavy weather? on a big swell?..it would act like a big dive plane forcing the boat under until forwartd motion stopped?.. when i was 15 i built a 10 ft hydro. it was a glen-l design... it had a very flat nose section forward. if the water got choppy on the lake and i was not careful -she would bury the bow--i did this twice in that boat at lower speeds and let me tell you its not an experience i want to repeat. thought me and the boat were going to become submarines very quickly...so whats the rule of thumb on clearance for the cross members over a breaking wave? how high should that wing be off the water to be safe?--thanks again for the definition--again--never heard that term --but i did suspect it was a nautical one when you used it in that context in the first post...
     
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