Any benefit to rotating a non-wing / non-teardrop mast?

Discussion in 'Hydrodynamics and Aerodynamics' started by pmudesign, Dec 5, 2018.

  1. pmudesign
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    pmudesign Marine Simulation

    I've recently acquired a Kurt Hughes Trikala 19 trimaran. This was a one off prototype and the builder strayed from the original design in a number of areas, including the rig. The mast is larger and heavier than Kurt's design calls for and appears to be more appropriate for a larger monohull. It's 4"x5.5", un-tapered, and semi elliptical / D-shaped in cross section. I'm torn between making the rotation system functional (as per Kurt's design, it is currently difficult to rotate) or make the base fixed.

    If the mast were "tear-drop" shaped or "wing" shaped I wouldn't hesitate to get the rotation system functional, but my understanding is that "D" shaped masts were designed not to be rotated and are more forgiving of various wind angles.

    What's your advice?
     
  2. JamesG123
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    JamesG123 Senior Member

    Unless you are racing it doesn't make a difference. Not worth the added complication and maintenance for the minor reduction in drag. Streamlined masts are a marketting gimick on 99% of boats and operators IMO.

    Yes, AFAIK D-shaped masts are supposed to be fixed directional fore-aft anyway, With the flat facing aft for the mainsail track.
     
  3. messabout
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    messabout Senior Member

    Whether you rotate the mast is not the important part. What is plenty useful is to arrange things so that the luff of the sail is on the leeward edge of whatever is supporting the sail. A round mast with a sail track ought to be capable of rotating so as to put the luff on the lee side on either tack. . The difference in driving force is easily felt or measured when beating or reaching.

    Your mast dimensions seem pretty generous for a 19 foot boat. .................Is the trailing edge of the D shape flat and 4 inches wide?

    If the sail track or groove is in the middle of the flat space then I think that arranging for limited rotation is worth the hassle. If the mast has shrouds and stays they will have to be left rather loose and floppy. That is not unusual on certain small boat types, especially racer types.

    All this depends on how important boat speed is to you. If the non rotatable mast and sail bring the boat to near maximum potential velocity then rotating or other trickery is not very important. If on the other hand the boat can use more power or you are itching to get the last bit of speed, then the rotating system might be worth the bother.............With all this commentary I am presuming that you have a regulation medium or high peaked Bermudan rig.
     
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  4. pmudesign
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    pmudesign Marine Simulation

    The design called for a beach cat style rig. The builder used a mast that is larger diameter and shorter. At some point I may replace the mast but for now I'll make do with what I have. The mast base is set up to rotate manually with lines and cam cleats. Assuming I keep the rotating ability I will simplify this with an arm mounted to the base of the mast that will be rotated by the boom (such as is typical on beach cats).
    20181121_105916.jpg
     
    Last edited: Dec 6, 2018
  5. messabout
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    messabout Senior Member

    not to be too critical of the layout here, BUT ...............How will the mast be able to rotate if the heel has a square plug as in the picture. Does the square plug rotate with respect to the mast? Surely the socket in which the square rests is not hourglass shaped????? to accomodate rotation..............Diamond stays on a mast that big used on a 19 footer seems to me as bigtime overkill.

    The diamond spreaders appear to be swept.......to affect mast bend in a blow or ???

    How much total sail area are we dealing with here??
     
  6. pmudesign
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    pmudesign Marine Simulation

    Like I said in my original post, the builder used a much larger and heavier mast than the design calls for. I don't have any photos of the mast base, but it appears to be a non-rotating mast base that has been modified to rotate. The boat is almost 20 years old and roughly 20 others were built just like it. If I get an opportunity to replace the mast I'll be sure to restore the entire rig as close to the designer's intentions as I can.
     
  7. upchurchmr
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    upchurchmr Senior Member

    Could we see pictures of the boat?
    Who built the 20 boats?
    I wasn't aware this boat was series produced 20 years ago.
    Would you say where you found the boat?

    USA is not very specific.
    I always found that design interesting.
     
  8. pmudesign
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    pmudesign Marine Simulation

    I had a hard time tracking down info on the boat. What little I know I learned from previous owners and through conversations with Kurt Hughes. The boat I have was built in Spain and, as far as I know, was a prototype for a small production run, all built in Spain. The company failed not long after. I bought this boat in North Carolina but it was previously in Georgia and then Florida before that. I know of one other still in use in Georgia and apparently there are still a few in Europe.

    I've read that the builder had no previous boat building experience, and from what I see in the construction of my boat, that's readily apparent. The mast base is probably a good example of this. Rather than implementing a rotating base similar to beach cats or Corsair tri's, they modified a fixed, hinged base to rotate. Unfortunately I don't have any good photos of the base, but hopefully you can figure it out based on the photos I've posted.

    mast base 1.jpg mast base 2.jpg
     
    Last edited: Dec 8, 2018
  9. pmudesign
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    pmudesign Marine Simulation

    Here's what the rig is supposed to look like. Instead, my mast is substantially larger in diameter (and heavier), about 1 foot shorter, not tapered, and (as already observed) with swept spreaders.

    trikala-19-trimaran-charleston-draw-sketch-e1519254406462.jpg
     
  10. upchurchmr
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    upchurchmr Senior Member

    Thanks for the history.
    I was looking for pictures of the whole boat.

    You might think about a Hobie 16 rig if you want to use a typical rotating rig - it is just a little bit larger than the main/jib shown in the drawing.
    They can be found for cheap usually.
     
  11. pmudesign
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    pmudesign Marine Simulation

    Kurt suggested a Prindle (16 or 18?) mast. Although this mast is heavier and shorter than I'd like, it's otherwise in good shape so I'll keep it for now.
     

  12. upchurchmr
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    upchurchmr Senior Member

    About the same as a Hobie 16.
     
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