Unstayed masts. Yet another thread...

Discussion in 'Sailboats' started by tom_burton, Nov 17, 2013.

  1. tom_burton
    Joined: Oct 2013
    Posts: 3
    Likes: 0, Points: 1, Legacy Rep: 10
    Location: Illinois

    tom_burton New Member

    I was wondering if there are any guidelines on designing unstayed masts. For example given a certain sail area and displacement of the boat and beam etc. what would be the appropriate mast diameter. I am looking to do this mast and aluminum. It is a small boat only 16 feet long and about 1250 pounds displacement. Gaff rigged.
     
  2. bpw
    Joined: May 2012
    Posts: 290
    Likes: 4, Points: 18, Legacy Rep: 34
    Location: Cruising

    bpw Senior Member

    Haslers book on junk rig has some rule of thumb numbers for free-standing junk rigs, loads should be similar on a gaff. Though higher if you have a jib.

    For a boat that small easiest would be to look around at similar boats. I bet if you build something that "looks right" you will be just fine.

    Eric Sponberg has some good articles on his site about the engineering of unstayed rigs if you want a more technical look.
     
  3. gonzo
    Joined: Aug 2002
    Posts: 13,609
    Likes: 382, Points: 93, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    Is that the displacement with how much load?
     
  4. groper
    Joined: Jun 2011
    Posts: 2,467
    Likes: 121, Points: 73, Legacy Rep: 693
    Location: australia

    groper Senior Member

    The load definition is simple... you use the max righting moment of the yacht as the maximum load - because any higher and the boat starts to capsize and limits the load at this point.

    Its a cantilever post model, bending model.

    Some allow an extra 10% bending load to account for the forward drive and sideways force, the combined vector is about 10% more than a simple sideways force.

    Dont forget buckling analysis for thin walled sections. Rule of thumb is approx 3% wall thickness to total section diameter or span to account for this. Better to do FEA instead tho. Highest load will be at the top support bearing at deck level. You need to ensure there is enough bury or stub length to limit the load on the lower mast section to that above it aswell.

    Structure may be deflection driven - check allowable deflection limit to keep sail shape as per design including bending.

    Then you design it with a safety factor of 2-3 depending on how conservative you are. Sponberg and others like to use a SF of 3, but i have seen others which are much more weight sensitive/performance oriented done at SF = 2.25 when done in carbon fibre. These SF allows some margin for fatigue, microcracking, longevity yada yada...

    Theres a start for you at least.

    I doubt youll find a good result using aluminium. You cant taper the section towards the top, so you have an inefficient strength to weight scenario. Aluminium is probably too flexible to make it work well also, as its modulus of elasticity is quite low. Bottom line is it will be a rather heavy mast to get the job done if aluminium is used... the high modulus of carbon fibre, and the ability to taper the laminate really is the only sensible choice for unstayed masts - which is why you dont see them in aluminum....
     
  5. gonzo
    Joined: Aug 2002
    Posts: 13,609
    Likes: 382, Points: 93, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    On a 16 foot boat, the crew will have a huge influence on the righting moment. You need to take into consideration the sailing style. For example, will they hike out, use a trapeze or sit inside.
     
  6. foxy
    Joined: Aug 2009
    Posts: 25
    Likes: 1, Points: 0, Legacy Rep: 20
    Location: Florida

    foxy Junior Member

    For unstayed masts, you really need to consider the moment of the boat going downwind and stuffing the bow under in the rig design, not just the heeling force of upwind sailing. It is still a simple lever for design purposes.

    The boat being mentioned is fairly close to the Marshall Sandpiper in size & weight. They use a 4" diameter spun tapered aluminum light pole. Wall thickness is about 5/32". Might be a little overkill, but its worked for 40+ years.
     
  7. Eric Sponberg
    Joined: Dec 2001
    Posts: 2,002
    Likes: 205, Points: 73, Legacy Rep: 2917
    Location: On board Corroboree

    Eric Sponberg Senior Member

    Actually, in my experience and design studies, stuffing the bow does not really load up the mast that much. That is because the forces causing the boat to go fast enough to catch up with the wave ahead have more to do with gravity pulling the boat down the face of the wave it's on than it does with wind making it sail that fast. Once the boat hits the wave ahead there will be some inertia and wind load that will load up the mast, but that is momentary because the boat is usually pretty directionally unstable in that situation and she quickly broaches, thereby relieving the mast load. In over 35 years, I have never heard of any failures of any free-standing rigs caused by wave stuffing and pitch poling. It doesn't really happen.

    There is a mix of things here that are worth comment: First, for boats less than 30' long or so, aluminum is a very good material for a mast. Mr. Burton says his boat is only 16' long and weighs 1250 pounds--an aluminum tube of constant diameter is actually a very cheap and easy solution. (He and I have been in private contact, by the way, wherein I gave him some guidelines on checking what size of tube he needs.) Aluminum is heavier than carbon fiber, but tapered aluminum poles are available and have been used as sailboat masts for decades. Aluminum and carbon fiber laminates made with standard modulus carbon fiber have similar moduli of elasticity. Depending on the lay-up and quality of construction, most carbon fiber masts will have a modulus of elasticity on the order of 7x10^6 psi to 11x10^6 psi, whereas aluminum has a modulus of elasticity of 10x10^6 psi. Certainly, on larger boats, say 40' and above, carbon fiber comes into advantage over aluminum because of the ability to taper both diameter and wall thickness, which together with the lighter weight of carbon fiber helps to tailor the mast shape, weight and spar performance to clear advantage over aluminum. And when you consider that you don't need the weight and expense of heavy rigging and all the fitting that they entail, the cost for a free-standing rig at this size can be less than a comparable stayed rig.

    Eric
     
  8. foxy
    Joined: Aug 2009
    Posts: 25
    Likes: 1, Points: 0, Legacy Rep: 20
    Location: Florida

    foxy Junior Member

    Eric,

    I appreciate that most of the time, the upwind righting moment is all you need to consider, but..........

    I lost the main mast on my cat ketch in about 1985 while running wing and wing in 30+ on Buzzards bay. The main (forward) was by the lee and we were surfing along quite nicely when we hit a set of steep waves from the current coming out of the canal against the wind. With the main by the lee, the bow was driven down and the boat did not round up. The boat slammed hard several times and the mast came down.

    Granted I probably should have been reefed, but the wind had built up over time and we having a good ride and in complete control up until then.
     
  9. Eric Sponberg
    Joined: Dec 2001
    Posts: 2,002
    Likes: 205, Points: 73, Legacy Rep: 2917
    Location: On board Corroboree

    Eric Sponberg Senior Member

    Foxy, Well, that's the first time I have ever heard of that happening. It seems from your description that the situation was more of extremely hard pitching in short squarish waves than sluicing down large rollers, which is the picture I had in my mind from your original statement. What model of boat did you have?

    Eric
     
  10. foxy
    Joined: Aug 2009
    Posts: 25
    Likes: 1, Points: 0, Legacy Rep: 20
    Location: Florida

    foxy Junior Member

    The boat was a 24 foot one-off that I designed and built for myself. Mast was a 5 inch diameter light pole and I used to pull the boat down with it to clean the bottom quite regularly so I thought it would probably never break. Its always the situations that you don't think of and plan for that get you.

    I've heard of several Non-such cats that broke aluminum masts, but never heard exactly how. Perhaps fatigue? I know my deck collar used to work loose from the boat pitching on its mooring. Probably the forestay on the gaff rigged catboats stopped that more that did much for sailing loads.
     

  11. tom_burton
    Joined: Oct 2013
    Posts: 3
    Likes: 0, Points: 1, Legacy Rep: 10
    Location: Illinois

    tom_burton New Member

    Thank you all very much for your responses. I always enjoy a lively discussion in these forums!!
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.