Aft Mast help please

Discussion in 'Multihulls' started by Fanie, Jan 1, 2009.

  1. Fanie
    Joined: Oct 2007
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    Location: Colonial "Sick Africa"

    Fanie Fanie

    Hello everyone,

    I went sailing with some mods to my aft mast rig. You valued comments please.

    Some general comments and some pictures -
    The sail is 12m^2, the mast is 6m, I should have had an 8m mast for this size boat.
    The foot of the sail is a bit long
    The wind was really up, I couldn't handle the sail properly, so I tied it aft just off centre where I sat and left it there. Sorry, arms too thin :rolleyes: I got pulled out of the hull onto the trampoline a few times.

    The sail have a lot of force. My friend taking the pictures suggested a longer mast and sail with a shorter mast foot ?

    I was doing about 20km/hr in the pictures.

    Too much weight aft and with the sail's lift I had a lot of transom wake. I went and sat in the centre a few times. You can feel the boat accellerate.

    The mast was very nice, it's stiff enough. It is very easy to put up and take down, as is the sail. It fiurls easily and if partly furled it works very well to come into shore at half throttle.

    It was my first go using a daggerboard. Brilliant, I can reach a target for a change. It seemed the faster one go the less drift you get also. I was impressed. I could also tack to wind properly which was nice for a change :D

    The 2hp motor was excellent, 8km/hr. Used it a few times. The bracket was a bit low.



    I am aware of all the flaws in the boat, the weight aft, the drag on the transom, the operator...

    What I'm really after is your opinion and comments on the sail.
     

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  2. Fanie
    Joined: Oct 2007
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    Location: Colonial "Sick Africa"

    Fanie Fanie

    We later fitted the windrifer's sail, but it wasn't the same. It's about 8m^2 and I was a bit disappointed, there was less wind and it didn't feel as responsive as the aft mast sail.

    One thing, with the aft mast you have to tack downwind as well, there is no boom to keep the sail at 90 degrees, so one runs at a slight angle off dead downwind
     

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  3. TeddyDiver
    Joined: Dec 2007
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    Location: Finland/Norway

    TeddyDiver Gollywobbler

    While waiting the experts to wake up... A couple of blocks to help sheeting, and more tension on the stay perhaps.. Anyway looks a lot of fun:D Congrats!
     
  4. Fanie
    Joined: Oct 2007
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    Location: Colonial "Sick Africa"

    Fanie Fanie

    Hi Teddy,

    TBH, I didn't expect that much force from the wind. I did mount two plastic cleats both sides (the ones with the toothed cam) for holding the rope but they proved poor quality. The rope came out a couple of times and I can see how one can get gung or properly lashed that way :D

    The stay is ok (I think) I didn't want to make it too taunt, there's about 20kg in idle position. The mast gives a little, but not much.

    I may look at using a small winch and an auto tacker would be nice.

    I can now better imagine the forces from a 50m^2 sail ;)
     
  5. Alik
    Joined: Jul 2003
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    Location: Thailand

    Alik Senior Member

    Finally someone built this 'aft mast' prototype and tested. This is what should be done before trying to sell the concept, bravo Fanie!

    Others had written thousands of posts here on 'aft mast rig', published dozens of articles in magazines - big marketing of unproven idea. Should be tested first, at least on dinghy!
     
  6. Fanie
    Joined: Oct 2007
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    Fanie Fanie

    Thanks Alik, but there are other boats that have such rigs already, but very little information is available unfortunately.

    Quite interesting on the two pics with the hoby in the picture is the sail shape my sail have - without battens or a boom.
     
  7. Alik
    Joined: Jul 2003
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    Location: Thailand

    Alik Senior Member

    We have done the design of 40-footer with aft mast recently, should be launched soon. I tried to push the customer to conventional rig, but he is too impressed by writings of 'aft mast dreamers'. Let's see, I am sure aft mast rig is only good for downwind sailing.

    PS Our 40' cat could be converted to normal rig any time.
     
  8. Fanie
    Joined: Oct 2007
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    Fanie Fanie

    I can assure you I can sail to wind as well as any of the other boats.

    Also despite my mast (and leading edge) being smaller in relation, my friend's little tri is not that much faster. I'm not sure the hobies are faster.

    The sail makes a lot of power, and lift. With the right refinement (which I hope to get some help with here) I'm sure it's going to be hard to beat. Keep in mind the jib to use with such a mainsail would be inside the main sail and should add to the confusion I mean performance :D
     
  9. Doug Lord

    Doug Lord Guest

    Fanie, that is one cool looking little tri. Have you posted the details anywhere yet-or would you post them here?
     
  10. Alik
    Joined: Jul 2003
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    Location: Thailand

    Alik Senior Member

    Fanie, yes, but slack of stay is considerable. Also wide low-aspect ratio genoa is not easy to control - these two factors only decrease windward performance.

    Another problem we met is load from mast to bridgedeck on catamaran, but it is not the case for Your trimaran.
     
  11. Fanie
    Joined: Oct 2007
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    Location: Colonial "Sick Africa"

    Fanie Fanie

  12. Alik
    Joined: Jul 2003
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    Location: Thailand

    Alik Senior Member

    Fanie, this is great that You test it.
    We will have test results for 40' in few months, but I already expect some problems with this concept. I mean it works, but not better than conventional rig.
     
  13. Fanie
    Joined: Oct 2007
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    Fanie Fanie

    I wish I can be there to help with the testing !!

    The sail handles a bit different from the bermuda, but I think you are going to be surprized. Have you any pictures / dimentions of the sail / mast / boat ?
     
  14. Fanie
    Joined: Oct 2007
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    Location: Colonial "Sick Africa"

    Fanie Fanie

    Any comments on the sail and it's setup ?

    Any comments on the pictures and the sail's behaviour ?
     

  15. deepsix
    Joined: Dec 2007
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    Location: SA

    deepsix Senior Member

    Hi Fanie

    Im no expert on sail shapes but hopefully this will get some conversation started. The angle of the pictures makes it quite difficult to evaluate the sail shape. I would suggest that you put the sail up in the garden on a day that there is some wind. Lie on the deck of the boat and take a photo directly upwards in the center of the sail, something like this.

    dp Rumours Hvy-Med 1 006.jpg

    Please also take a photo of the sail on the ground along the luff. I would like to get an idea of how much hollow has been cut into the luff of the sail(luff curve).

    If we look at this very simplistically we need to evaluate the angle of the sail relative to the boat and also the section shape of the sail.

    Angle of the sail to the centerline
    What is the angle of a line through the tack and clew relative to the centerline of the boat, I think that it should be as narrow as possible. Some of the more experienced guys around here will hopefull explain this in more detail. The hull shape and daggerboard section come into play here.

    How does this angle vary up the sail(twist). Your sail should only have enough twist to account for the wind sheer, unlike bermuda rigs which must also account for the interaction with the mainsail. I would suggest minimal twist. Twist is controlled by moving the position that the sail sheets to forwards or backwards. Move you turning block forward for less twist.

    Section shape of the sail

    camber.gif

    Multihulls tend to be light and powerfull so they usually have flat sails.

    The depth or draft of the sail is measured as a percentage of the chord length. Camber(depth) to chord ratios of ~10% is the ballpark you are aiming for. The amount of depth is controlled by the amount of luff curve cut into the sail and the amount of sag in the forestay. Try and keep your forestay tight to keep the sail flatter. This is difficult with the aft mast setup.

    The position of maximum depth(draft) should be ~40-45% of the chord length(forward of center). This is mostly cut into the sail but can be adjusted with the luff tension. More luff tension = draft further forward = less heeling force and more driving force.

    You can download software from the UK-hasley website called accuMeasure which will allow you to evaluate the section shape and twist of your sail from photographs of it. It will give you the draft and the position of draft.

    JibSplines.jpeg
    Example of output from AccuMeasure

    You can also look at the Saildesign yahoo group. There are some good sail makers on that board which can give you an idea of what sail shapes you need.

    You should consider adding two small blocks on the clew of your sail and rigging up a 2:1 purchase system on each sheet. It will approximatly halve the sheet load. This is what I use on my boat which has a 12sqm jib and the righting moment is probably quite similar too. The purchase makes the sail much easier to handle, I use a 6mm jib sheet and I can handle it fine in 25kts of breeze.
     
    Last edited: Jan 2, 2009
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