Bowsprit design - Resolving forces

Discussion in 'Sailboats' started by ODL, Aug 9, 2010.

  1. ODL
    Joined: Aug 2010
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    ODL New Member

    There is a length of pre-fab carbon pole on the boat I hope to buy.

    I'd like to determine if it is suitable to be used as a bowsprit for an Asymmetric sail of approx. 440 sqr feet.

    I think the first step is to resolve the forces I could expect under loading into their horizontal and vertical components.

    The shear strength of the pole would compare to the vertical loading, the compression/buckling strength to the horizontal.

    I recognise that the vertical component will deflect from the centreline as the sail loads up and the luff falls off.

    Any other things to consider at this stage? Any ideas as to how I could estimate the forces in the tack line/halyard?

    Regards, ODL
     
  2. latestarter
    Joined: Jul 2010
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    latestarter Senior Member

    I see you have just joined.
    You will get more and better replies if you provide sketches and more details.

    Before being able to analyze your problem there are a lot of factors to take into consideration. If you are unable to obtain accurate figures for all of them it would not give a relevant result. I am assuming the pole is unstayed and acts like a cantilever.

    How far will the pole extend beyond its support. Is it circular. Does it taper. Do you have or can you calculate the section modulus and cross sectional area. What are the allowable stresses and modulus of elasticity for the carbon used.

    When you say horizontal and vertical components, as this is a cantilever it would be clearer to use the terms axial ( along the pole) and radial ( at right angles to the pole).

    You have not mentioned the bending moment, which is likely to produce the biggest stress.


    Estimating the forces in the tack line and halyard is beyond my experience.

    From the above I am doubtful that you will be able to come up with a meaningful result.
    In view of the number of unknowns I think your best move, given you live in Southampton with so many boats nearby, is to see if you can find boats with similar arrangements to your idea and see if it is comparable.
    What would be the consequences if it failed.
     
  3. fastwave
    Joined: Jan 2007
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    fastwave Senior Member

    In your case I would take the breaking strength of the tack line and add a safety factor on top. The line is more stretchy than carbon so it can absorb more shock loading, so u need to put quite a bit extra.
    That would be the easiest way apart from putting a load cell on it and sailing for a while.
     
  4. foxy
    Joined: Aug 2009
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    Location: Florida

    foxy Junior Member

    I know this sounds simplistic, but I have never had a sail maker give me a better number and I have never had a sprit break when designing to these tack loads.

    A quick rule of thum.
    Tack load in kg = area in square meters X 5 or
    Tack Load in lb = area in square feet

    The assumption is that you will not be able to carry the chute in apparent wind ranges that will exceed this load.

    The second method is
    Tack load in kg = area in M^2 X wind velocity squared X .02104

    Using this formula, you will equal the rule of thumb in about 16 kts apparent.
     
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  5. Paul B

    Paul B Previous Member

    Thanks for that info. We had a similar discussion a couple of years ago on this thread when I was putting a pole on my boat. None of the calculations anyone gave me seemed to make sense.

    FYI: Looking at your rule of thumb I would mention the Melges 24 uses a Harken Carbo 40 for the tack line. It has a SWL of 485 lbs (breaking 1620). The kite on the Melges is 670 sq ft. So either they are exceeding the SWL on the block or your rule is a bit conservative.
     
  6. foxy
    Joined: Aug 2009
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    foxy Junior Member

    I would say that the working load stated for the block was probably based on distortion of the bearings rather than breaking load of the block. So a normal published SWL of BS/2 would be 810 lbs and over our rule of thumb.

    The Melges would only exceed the block rating on occasion and would probably never break the block. Its an inshore race boat where people will be replacing lines, sails and distorted blocks all the time. Just part of the game and likely considered when specing blocks for the boat.

    If you are specing gear for an offshore racer, I would expect a more concervative approach. I have never had a pole/sprit break, so we are probably on the conservative side, but probably not by a lot.
     
  7. Paul B

    Paul B Previous Member

    I suspect you are probably right. My boat is about the same dspl as the Melges, similar RM, and about the same size kite. Most of the time I can adjust the tack line by hand (it is held by a single Harken 150 Cam Matic). If the load was 485 or 670 pounds I don't think I would be able to do that!

    On the other hand, I've never tried to adjust the tack when we are power reaching with the rudder right on the verge of stall.
     
  8. bruceb
    Joined: Nov 2008
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    bruceb Senior Member

    pole size

    I use a melges 24 chute on my 24' tri and the 500 lbs range at the tip seems about right, maybe a little more power reaching. ( my righting moment is higher) Similar boats are using 3" carbon poles, mine is smaller but I have to use side stays as well as the kicker. We once broke the pole on an Ultimate 30 "power reaching"- it broke to the side and messed up the whole day, once it gets bent too far, the tack line helps compress and break it, and it happens quickly, way too fast to release things. Make it strong :) Bruce
     
  9. foxy
    Joined: Aug 2009
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    foxy Junior Member

    Just remember, you are engineering for the peak load which you will only see on rare occasions. The rule of thumb was derived for heavier boats that will not get up and plane.

    For a Melges or similar, you would probably be better off to compute the apparent wind speed for different sailing angles/ & true wind velocities and use the formula to estimate your loads.
     
  10. bruceb
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    bruceb Senior Member

    real world

    Hopefully, you are engineering for the un-expected puff that hits you when are already surfing at 14 kts. You don't want to lose the sprit before you can turn down. Formulas only work when you have good numbers to enter, and small fast boats that depend on crew ballast are hard to quantify. B
     
  11. foxy
    Joined: Aug 2009
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    foxy Junior Member

    You have to think of how the boat is used.

    The situation I think about most is the crew that is trying to sail high and carry the sail to a mark when they should have dropped it. Kite collapses and then fills with a bang. You want to survive that in an offshore racer, but most sport boat guys sailing around the cans would balk at the extra weight and may be willing to loose an ocasional pole.

    When I sailed Stars loosing rigs was just part of the game and most of the serious guys always had a spare rig on the trailer.
     

  12. bruceb
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    bruceb Senior Member

    test it

    Test your carbon pole. Lash or clamp your potential tube to something solid (I often use a trailer frame) and attach about an eight to one tackle to the other end and then pull. Stay clear! A decent 8/1 should give you around 550-600 lbs and you can see how much the pole bends. There are more scientific ways, but if you already have the pole, just test it. B
     
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