Aftmast rigs???

Discussion in 'Sailboats' started by jdardozzi, May 28, 2002.

  1. Alik
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    Alik Senior Member

    To stop it, a) just stop sending Your useless comments to the builder b) stop commenting any of our projects on the web. And then stay relaxed dreaming about Your virtual boats.
     
  2. Fanie
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    Fanie Fanie

    Brian and Alik,

    Neither of you can win this augmenting, and it's trashing the thread. Every one is waiting for you two to finish so we can continue discussion.

    How about it - please.
     
  3. peterAustralia
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    peterAustralia Senior Member

    Just a small point that no one has mentioned previously

    I have noticed on many of these aft mast rigs that there is not a lot of horizontal separation between the mast base and the rear stay.

    Say for example, that the boat was designed from the very beginning to accept this rig, then perhaps a stern that slopes rearwards would be more suitable, perhaps something along the double ended designs of Wharram cats.

    The advantage as I see it is,
    for the same forestay tension, the compressive loads on the mast would be less. Note of course that backstay tension is the same as rearstay tension (I assume so, otherwise the mast tip would be bending).

    I know that modern catamarans often have the transom sloping forward so as to reduce weight and windage for a given waterline length. If the transom sloped aft, as per Wharram cats, and the transom was built strongly (as it has to be on Wharram cats to accept the rudder loads) then the compressive loads on the masts or bipod would be less. This in turn would allow for slightly lighter and smaller masts, with less resulting windage.

    I am able to do the maths if desired, it is just high school statics.

    The other issue, that I do not really understand, is that often the aft mast boats seem to have the mast raked forward.

    How is the forward mast rake determined, is it by sail center of effort?

    I guess in my own mind, I am thinking of more bipod on cats and trimarans. It seems that such an arrangement on a double ended craft, would have quite low loads in mast compression, so that by increasing the distance between mast base and rear stay position, compressive loads are reduced.

    Note that this is not a linear relatiionship, double the horizontal distance, half the load... nothing like that. but looking at some of these diagrams, my estimate, prior to doing the maths, is that using a stongly built aft sloping transom (or double ender) would reduce compressive loads by about ten percent.

    I can do the maths if anyone wants.......
     
  4. brian eiland
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    brian eiland Senior Member

    I agree to that...thank you Fanie for pointing that out.

    Peter Austrailia, I'll get back to you on those questions, but it might not be till Monday as I may have to go out of town tomorrow, Sunday
     
  5. Fanie
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    Fanie Fanie

    Hi Peter,

    What I did was to cad the sail up and rotated the mast at different angles. For me the best position was the mast vertical, and it has the biggest sail area then. Imo there is not much down pressure, the weight of the mast, but there is a foreward pull from the forestay like a fishing rod. I had the mast supported on it's foot and the upper part supported against the deck of the hull, no stays, so you want a stiff mast.

    As I see it, the sail force has a foreward component and an upwards component. Light vessels will benefit but heavy ones won't have much of an advantage from the lift. The lift also keeps the boat from heeling as much as other sails would. On my friend's windrider I could sink the ama, with the little tri the ama could not even get the whole length wet.

    When running I often got the impression that enough wind could pick you up and carry you over the water instead of sailing over it. Never happened though but I do need to lose a bit of weight :D
     
  6. MikeJohns
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    MikeJohns Senior Member

    AKA the backstay angle.

    That's why the mast and it's supports will be heavier.

    Fanie's setup is probably where the tradeoffs may give some benefit. On a larger heavier boat I have some very severe doubts ( Sorry Brian :)).

    Brian
    You would have calculated the rig loads, mast compression stay tensions surely?

    The vertical lift is one tradeoff for such a long lazy luff. There is a loss of drive which is instead trying to lift ( it won't by much no matter how much weight Fanie loses).
     
  7. Fanie
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    Fanie Fanie

    Mike, the lift will of course always make a difference. I think there would be a relationship between the hull's shape/drag and the lift. If a mast is not free standing then it will have to be supported properly - obviously. I think Brian's sketches was just ideas at the time, and probably not intended for production as is. A bit like drawing a boat and not putting the motor where it can function - the idea may well be to just indicate it's pressence as a possibility.

    The taller the mast the less the relative vertical lift - and the more foreward force. So it may well be that the vertical lift stays about the same for any size sail foot length, but by lengthening the mast the foreward force increase.

    There were no tradeoff's made with my rig, I haven't lost any weight yet ;)
    Bloody gravity. Makes everything work but it also buggers everything up.
    God's sense of humour perhaps. Let me give them something they cannot resolve...

    Something else I noticed. When you sail to wind the sail dumps aft. When you run the sail dumps down which would also contribute to lift as well as foreward force.

    I asked before, if someone knows, by how much does the mast in front of the luff affect the pointing ability ?
     
  8. Alik
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    Alik Senior Member

    With mast sloped fwd, measures should be taken to transfer the load to hull structure. Aft cabin buklhead is vertical and usually has big door openings; it is no efficient to take the loads. We provided cross-beam at bridgedeck that forms door coaming, and the mast is stepped on that beam. Anyway there is some bending as its section modulus can not be competed with mast bulkhead of conventional rig arrangement. I would say with this scheme the whole boat is more flexible compared with conventional cat.

    Brian's rig has never been engineered. Period. If You ask this question to him, he would post another two pages of text based on thoughts and internet links, but without numbers or load diagram.
     
  9. Alik
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    Alik Senior Member

    C.Marchai has the graphs in his books.
     
  10. Fanie
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    Fanie Fanie

    Another possibility I was thinking about. Since you can quite easily use electronics to auto steer, it may well be possible to adjust the sailing speed with an electric furling roller. If you're fishing for instance and you want to trawl at a specific speed, the furling roller and winch can now control the sailing speed.

    I wonder how well this would hold up in a storm. If Manie wasn't completely lying about the pointing ability it may be possible to sail into the storm at a narrow angle, possibly even at a negative speed if the steering reverse steer and the mast assist in lagging the stern. You won't have to fiddle with drogues and sea anchors and one less thing you must have.
     
  11. Fanie
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    Fanie Fanie

    Thanks Alik, but unfortunately I don't have the book. Any rough indications what it says ? 2 - 5 degrees ?

    And stop stabbing will you. Each time you do that another two pages gets wasted.
    Albert Einstein was a theoretical physicist, philosopher and author who is widely regarded as one of the most influential and best known scientists and intellectuals of all time... If he got everything just right then there wouldn't have been anything left for us to develop.
     
  12. Alik
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    Alik Senior Member

    I have that book in office; can check on Tuesday.

    Besides Einstein was a unique genius, he was publishing his research in scientific magazines. This is completely different level and those are publications peer-reviewed and accepted in scientific community. Posts in the Internet are not, anyone can play as self-named expert :)

    It is important that thread readers get correct information on what is real-life experience with aft-mast rig, and that decision to build such boat is a gamble for their own money. Unfortunately I met few guys who got wrong impression from reading convincing posts of 'professor' :D
     
  13. Fanie
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    Fanie Fanie

    If you have a 45m^2 sail, then if I remember right the force in 20km/hr winds is like 7kg/sq meter, resulting in a total of 315kg, devide by 3 for the 3 tie points ~ 105kg.

    A conventional mast would be bearing a larger load since it carries the full sail, the boom assisting a bit though.

    The force in the structure from the mast head should be the 105kg times the lever ratio length of the lower support vs the above deck mast for a free standing mast.
     
  14. Fanie
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    Fanie Fanie

    I'm actually trying to tel you something. Come on man, have a bit of class.
     

  15. Alik
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    Alik Senior Member

    In brief - it is wrong; sail is not supported by 3 tie points; pressure is not evenly distributed, etc. etc. And this is not the way to calculate the load on the mast.

    If one is interested, I suggest to look at Larsson/Eliasson 'Principles of Yacht Design', they have general method of rig dimensioning there, with sample. There are also other publications, requiring different levels of background - from Skene and Herreshoff to Claughton's 'Yacht Design' series and new Fossati's 'Aero-hydrodynamics'.
     
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