Launch of my car toppable planing cat

Discussion in 'Sailboats' started by alyne, Aug 22, 2005.

  1. alyne
    Joined: Aug 2003
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    Location: Somerset UK

    alyne Junior Member

    Hi All

    I have finally launched my planing beach cat I have working on over the last 18 months on and off. Here are some pictures (don't laugh I'm trained in IT not boat design :) ):The Pink Onion

    I was trying to achieve the following charecteristics in my design:
    a) Car toppable - partly achieved, it doesn't pack up as cleanly as I had anticipated
    b) Quick and simple to rig - achieved, first rig up time in 23 mins.
    c) Prove whether a 12 foot cat with planing hulls can give an exhilarating ride - achieved, it does - in my biased view :D
    d) power the boat with a standard unstayed windsurfer rig - achieved, certainly seemed to provide enough power with an 8 metre rig
    e) Single rudder and no centre board for simplicity. Use of hard chines on the sides to provide lateral resistance - hmmmm terrible weather helm and hard to tack


    My first sail was in a steady force 3 on an estuary (this boat is not designed for sailing in choppy sea). From that initial sail, I have learnt much about what works well and what doesn't about my design. Here are the key things:

    1.) The concept of windsurfer style planing hulls works. She got up on the plane on a reach and certainly gave the local windsurfers a run for their money (may be a little exageration here)
    2) Windsurfer rigs have lots of potential for small projects like mine
    3) A little prior knowledge can go a long way. In my case an understanding of boat balance (Centre of effort and Centre of lateral Resistance) would have served me very well when deciding on the position of the mast :(
    4) Terrible weather helm as soon as I try feathering up into the wind - good balance when on a reach.
    5) She is a pig to tack (hardly surprising I suppose)

    So, I now need to try to address the handling issues and would appreciate a little advice.

    I am planning to substantially increase the size of the rudder to hopefully improve the weather helm problem and the difficulty of tacking. Do you think this will help a little? If not then would the next step be to move the mast forward? I want to try to avoid a centre plate if possible

    My other question is about rocker. I have a little rocker in the front and none aft. The hull shape seems to get up on the plane nicely, but would it be better to introduce a little rocker aft?

    Anyway, I have had fun!

    Andy Lyne
     
  2. casavecchia
    Joined: Jan 2004
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    casavecchia Senior Member

    new boat

    A boat has been built, not only dreamed of.
    Such a pity no one seem to notice.
    Double cheers, Alyne!
    Putting rudders on the transoms will help but I think you need boards.
    Try leeboards on the exterior side of the hulls. No need to make holes in your hulls for them and it's easy to find the right position.
    Try two different location on the two hulls and see what works best.
    Marco.
     
  3. dougfrolich
    Joined: Nov 2002
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    dougfrolich Senior Member

    Fun boat!! Ply with rudder(fwrd) and mast(aft) rake to help relive the helm, boards will definatly help with tacking, and helm too. Why not just jibe around on a plane the whole time and forget about tacking? Don't add rocker to the tail. Fast is Fun!
     
  4. alyne
    Joined: Aug 2003
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    Location: Somerset UK

    alyne Junior Member

    Thanks Marco and Doug for your feedback.
    I have received similar advice on the windsurfing forums about two rudders, I am going to try a couple of other things first, as it sits up on the plane so easily I am hesitant to add any more drag than I have to.

    I am going to make one change at a time. First (as it's easy) I'm going to increase the size of the existing rudder. Next I was going to try moving the mast foot forward 6 inches....do you think this may help?

    Doug, if I am reading you right you are suggesting keeping the tail with no rocker is a good idea for a planing shape such as mine?
     
  5. dougfrolich
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    dougfrolich Senior Member

  6. gggGuest
    Joined: Feb 2005
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    Location: UK

    gggGuest ...

    One thing to be aware of. when you say you have weather helm do you just mean the rudder is tugging hard at you, or do you mean that you have to have the helm pulled well off the centreline to get the boat to go straight. They are different issues. If the tiller is on the centreline when you're going straight then you might be able to try just balancing the rudder - angle the blade forward slightly so its under the pivot line to rdeuce the load.

    Weather helm going to windward is a little suprising - I think I'd have expected lee helm with that layout. Might be a lot to do with how immersed the bows are. See if the balance changes if you sink the transoms some.

    Making the rudder bigger could be an error. It would be an interesting execrise to minimise the rudder so its as small as it can be and still steer. If you're going to be plying with size a lot I'd recommend making a daggerboard rudder stock, then you can slide the blade up and down and experiment with different areas.
     
  7. alyne
    Joined: Aug 2003
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    alyne Junior Member

     
  8. gggGuest
    Joined: Feb 2005
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    gggGuest ...

    Yep, they can be. When you think of weather helm you think of some unblanced boat heeling 35 degrees with the helm clutching the tiller to his chest in a desperate attempt to stop it rounding up,and vast quantities of drag and distrubance fromn the tiller. All the load is coming from all the turn the rudder is doing to counteract the hull pushing the other way. This is of course a BAD THING!

    However nowadays in at least some of the development dinghy classes its fashionable to distribute the side force between daggerboard and rudder. The thinking is tht you're dragging all that wetted surface through the water anyway, it might as well provide some side force while its at it. That way the daggerboard is smaller and further forward.
    Now in this case the rudder blade should be exactly in line with the daggerboard so both foils have the same angle of attack to the water flow. There's going to be a pull on the rudder blade which is proportional to the amount of the total side force the rig is taking up, but its purely caused by the side load,and the rudder isn't fighting the hull trying to steer the other way.
    Well it depends whether its a problem, like the top para, or a feature like the rest. If the load is coming because the rudder is taking the vast majority of the sideforce, as it might with your desgn, yet the tiller is on the centreline, then balancing the blade will just reduce the percentage of the side orce you feel, so that's a good way to run. You can tell thi s because, althouugh letting go the tiller will still make the boat round up, it won't do so until the tiller is on the lee side of centreline.
     
  9. LP
    Joined: Jul 2005
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    LP Flying Boatman

    To weather or not to weather.

    To get a feel for what's going on, and it probably goes without saying, but is the weather helm in bringing the craft throught the eye of the wind or is it after coming through and trying to accelarate on a reaching course?

    Followed by a general question, when planning, Are you on both ammas(sic) planning or do you get up on a single amma?

    In other regards, I think you've got a wonderful craft on your hands. It's small and fast. It's unique. You will get the quicks ironed out and be the envy of both powered and unpowered craft.

    It's a cool project.
     
  10. alyne
    Joined: Aug 2003
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    alyne Junior Member

    Eureka!

    Hi ggGuest and learpilote

    I have only planed with both hulls in the water, it hasn't been windy enough to try lifting a hull but I suspect things will slow down considerable as soon as the hulls arn't flat.

    Thank you for the explaination gggGuest. I have been misunderstanding weatherhelm! I took the pink onion out for her secon sail today, unfortunately not very windy but enough to try out what you recommended.

    You are right, although I am having to tug the rudder towards me when feathering up and on a close reach, I notice that to actually steer a straight line, the tiller is ever so slightly leaward of centre line. As soon as I put the rudder in line with the boat it starts to sail very slightly to leaward.

    So the force I have been experiencing is due to lateral resistance.

    I have extended the tiller arm to give more leverage so I did not notice the force on the rudder so much, also it was much less windy than the last sail.

    Also, I can now tack, steer and gybe quite easily. All I do is sit right back on the rear beam, raising the bows and she spins round fairly easiliy on a tack, at least in a gentle force 2, probably a different story in a good puff.

    Next step, I am going to balance the rudder by bringing part of it forward of the pintles, not sure how much, I'll probably put about a third of it in front.

    I am not sure about what to do to try to correct the fact that the tiller needs to be slightly leawards of centre to steer a straight line. This is obviously ineficcient. Any suggestions appreciated.

    Thanks Again!
     
  11. gggGuest
    Joined: Feb 2005
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    gggGuest ...

    OK, so what you've actually got is slight lee helm, but the rudder side load means it behaves like weather helm. A third sounds like a lot of balance, I'd start with less, say about 3cm. You could try raking the mast back to reduce the lee helm effect. A daggerbard would sort this out of course, but the object of the exercise is to avoid that. Too much mast rake and the rudder will be less effective I suspect, so its question of establishing a good compromise.

    Of course if you get to be flying a hull all the time what you'd want to do would be to angle the hulls outward so that the lee hull is flat while the windward one is clear of the water. Suggest that sort of thing is a project for the next boat though! The hot racing sailors in things like the A Class sit out to leeward to fly a hull offwind in light airs!

    As for misunderstanding weatherhelm, nearly everyone does in that respect. I'm amazed that some of themore traditionally minded haven't told me what nonsense I'm talking...
     
  12. casavecchia
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    casavecchia Senior Member

    Hallo Alyne,
    simply lash onto the exterior side of a hull a piece of board roughly shaped in an airfoil section about halfway between mast and rudder and 0,6 mt deeper than the hull itself and see what happens.
    This should give you interesting information.
    Cheers,
    Marco.
     
  13. terabika
    Joined: Aug 2005
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    terabika Junior Member

    you might also experiment with getting a bit of your rudder surface area forward of the pivot point. This creates a balanced rudder and shall steer much better.ideallly, the pivot point should be just forward of the point of max thickness but if you are using flat surface, a pivot 20-25% back of the leading edge will help greatly. GODD JOB!!! nice to see it !!! I love seeing people do their own thing :)
     
  14. alyne
    Joined: Aug 2003
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    alyne Junior Member

    Thanks marco and Terabika

    I have now balanced the rudder....was going to try a test sail today but no wind :(

    Marco, I think I am going to be able to get away without dagger boards. This boat wasn't really designed for max upwind performance, more for reaching up and down. As such I am trying to avoid any wetted surface area I can.
     

  15. mojounwin
    Joined: Dec 2004
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    mojounwin Junior Member

    I love the boat, the concept is excellent. Good, cheap, do it yourself fun. Looks fast aswell

    You should sell plans

    Mike
     
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