Displacement powercat single 40hp

Discussion in 'Boat Design' started by Saqa, Oct 8, 2014.

  1. Saqa
    Joined: Oct 2013
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    Saqa Senior Member

    What does hobby horsing mean? Something to do with rearing up? Would the foils and tabs on his boat help with that?
     
  2. whitepointer23

    whitepointer23 Previous Member

    markham single engine
     

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  3. Turnpoint
    Joined: Jun 2011
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    Turnpoint Junior Member

    The boat without foils would start to hobby horse in steep chop from 2-3 feet. It would not get really bad until the chop was very steep and 3' or more... Ie stacked up by current. But any small boat in these conditions would be bad. My natural inclination at first was to slow down to 14 knots or so in that I was used to driving aluminum work skiffs that would knock your fillings lose in those types of waves. It took me a little time to realize the boat was happier driving at full speed through that stuff. It has a wave piercing bow shape and takes some power to drive through that chop... Slow seems to give too much time for the waves to lift the bows and start to build up a resonance with the waves. But with foils the ride is dramatically different and any pitching motions are dampened out. now you can head at a largish power boat wake at full speed and the boat slices through with out hardly a quiver. New passengers will instinctively brace themselves for the impact but the wake will most times be hardly noticeable.

    Before we put in foils I thought the benefits would be mostly for efficiency/performance ... The added complexity would not be worth it but now I am sold on their benifits with the greatest benefit being to ride quality.

    i can't send any good pictures because I am away from my real computer but here is one with the boat on a mooring. I can send better ones in a couple days.
     

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  4. Turnpoint
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    Turnpoint Junior Member

    Here is a picture of a similar set of foils that we put on a power tri. The turbicled leading edge was to stop ventilation in these surface piercing foils. The foils on the cat are the same shape just without the turbicles because in the cat they are not surface piercing and do not have a tendency to ventilate. I don't have any pictures of those on my phone...
     

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  5. groper
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    groper Senior Member

    Great!

    The sea keeping improvements were the exact reason I was considering them. There is a lot of research published which goes back quite some time, the mechanics and characteristics of the foil assisted catamran are well understood these days. The main issues I wanted to solve was the ability to retract them in shallow water for beaching and to prevent marine growth.

    Looking at yours, I can only assume you have mounted the foils in a traditional type of curved dagger board case? I've got a few more questions if you don't mind... How much angle of attack did you give them and how did find the optimum? You said you had trim tabs on the rear foils... I assume these are adjustable so that you may adjust the balance of the foil lift with varying center of gravity conditions as people and load move around on the boat? Would you do anything differently next time?
    Cheers :)
     
  6. erik818
    Joined: Feb 2007
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    erik818 Senior Member

    Saqa,
    I've done some performance calculations on 3 tonnes / 10 m catamarans (the approximate size I'm interrested in.) 40 hp would give me 14 knots, which people with experience of real world cats consider optimistic. Anyway, my conclusion is that a 10 m displacement hull is a bad idea if you want to go faster than 15 knots or so. You have to lift the hulls out of the water by planing or by using foils to achieve 25 knots with acceptable economy.

    I don't understand your problem to find small outboards with power tilt and electric start, at least if you consider 4-strokes. Tohatsu seems to have them available down to 10 hp (I own a 25 hp myself). If you really need high cruising speeds you also benefit from the improved fuel economy with a 4-stroke, but you have to provide the money up front for savings later on. Two engines give you redundancy and eliminate the need for a spare limp-home engine.
     
  7. Turnpoint
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    Turnpoint Junior Member

    Hi Groper,
    THe main foils are mounted in a trunk. There is a top sliding bearing to allow adjustment of the angle of attack. The first couple days the foils were in I played with all the foils but now they are set to around 2 deg of angle of attack. You are correct that the aft outboard mounted foils trim tabs were to help trim the boat but also to allow some adjustment to try to find the optimum performance at cruise. Currently I have these at full trim tab down which is we're the boat is happy. That is not an efficient way to get lift aft and so my next step will be to remount the aft foils with another deg of angle of attack to try to get those foils happy while the trim tab is at the neutral position. What I would do different next time is to mount the main foils further aft and closer to the center of gravity of the boat so that the main foils which are deeper and more eficient are taking more of the load.
     
  8. Saqa
    Joined: Oct 2013
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    Saqa Senior Member

    Fellas
    Does anyone have any information on the Americas Cup camera boat "Cambria". I came across some info in another thread here where I learnt the boats name but there is very lil info that I can find on the net. Seems like she been built on modified extreme 40 hulls whatever they are. Extreme 40 page has very lil on the design of the boats they run. Would really appreciate any links to discussions or resources on such hulls please
     
  9. groper
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    groper Senior Member

    They are simply a 40ft racing sailing cat, not all that dissimilar to many other racing cats. There is not all that much to know about them other than what you can see. Very long, slender hulls , a small amount of rocker, very low volume inside which can't be used for anything other than storage of gear or fuel tanks etc. And they are built extremely lightweight. A bloke standing at each end could lift them. They are most likely a foam core with carbon epoxy skin material, fabric weight would be approx 450gsm. If done in eglass, similar scantlings could be used as the stiffness requirement is not high dictated by small panel areas. Thin ply of approx 4mm could also be used, with a thin glass sheath similar to what you already did with your last boat. Plenty of similar hulls are built like this which end up only slightly heavier. .. the key to the whole thing is displacement to length ratio, very light boat, with a long waterline gives maximum efficiency. ..
     
  10. waikikin
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    waikikin Senior Member

  11. Saqa
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    Saqa Senior Member

    Thanks for the info Groper

    Wikikin, awesome find. Here is hoping she still around when I am ready to invest. But pretty sad too, someones unfinished dream
     
  12. Saqa
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    Saqa Senior Member

  13. Nick_Sinev
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    Nick_Sinev Junior Member

    Saga, could you please start a separate topic on fishing?
     
  14. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Nice fish. I have never caught one. I know there is some angler confusion between that one and the Red Bass, which is reputed to carry ciguatera.
     

  15. Saqa
    Joined: Oct 2013
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    Saqa Senior Member

    Yeah, I know a few people who have eaten red bass, we call it bati damu. Not pretty! The tiri damu was delish though :)

    Nick, this whole topic is about creating the means of catching this fish and and associates in comfort and style mate ;)

    Does anyone know how aluminum performs in a displacement cat?
     
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