Reducing air draft calculations

Discussion in 'Boat Design' started by Manateeman, Oct 18, 2019.

  1. Manateeman
    Joined: Oct 2019
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    Location: St. Mary’s Georgia

    Manateeman Junior Member

    I would appreciate assistance with my efforts to reduce the mast height on our sailboat. We fully appreciate this will reduce performance. Particulars on the original design can be found on Ted Brewers web page, design 262, aluminum 60’ sail ketch Shenanigan.
    We wish to reduce the air draft to 64 feet.
    LOA 60’ LWL 56’ beam17’ twin keel draft 6’ , displacement 65,000, ballast 21,000 prismatic .545
    1 degree heel 6,100. Righting moment at 30 degrees, 61,122 ? Also find a notation righting moment 153,275lbs on a drawing. Main mast as drawn is 14.75 x 7.75 with moms 153 x 65. We believe VCG at LWL and Center of Buoyancy is -3.3’ not certain on these two.
    We have acquired a Pacific Spar mast in excellent condition. It is 63.5’ . Oval 7 x 10 with moms of 58 x 30.9. Originally deck stepped but we plan to keel step it. Two spreaders, two jibs similar to original design. It’s a bit light but we are a bit old.
    I don’t want to move the spreaders. The lowers match our chain plate beam. Continuous rigging.
    Cap to upper spreader is 20’ , cap to lower spreader is 40.5’ , lower spreader to heel is 22.8’.
    The mast heel will be a foot below LWL.
    We added a permanent 10 x 12 Bimini over the cockpit, a bit of weight to the boat as well.
    Help with advice and calculations would be deeply appreciated.
     
  2. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    Ummm, Manatee, it would have been helpful if you could have provided a link for people, rather than tell them to go and look for Shenanigans....
    Anyway, I went looking, and found it.
    Ted Brewer Yacht Design https://www.tedbrewer.com/sail_aluminum/shenanigan.htm
    What is your current air draft?
    Your mast is currently deck stepped; you mention that you plan to keel step the new 63.5' mast, where the keel step is 1' below the LWL.
    So the top of the mast should be 62.5' above the waterline, excluding appendages like VHF aerial, tricolour navigator light etc.
    So you should be under 64' air draft ok.
    Re how the original mast is deck stepped, might it be easier to simply chop of a section of the base of the new mast to reduce the air draft to under 64', rather than having the complexity of getting rid of the compression pillar inside the saloon, and cutting a hole in the cabin top for the mast to go through?
    And if you go down this route, you could as well keep the original mast, and simply cut a section off the bottom of it instead?

    Edit : I do realise that if you cut a section off the bottom of the mast you will have to raise the gooseneck for the boom to maintain the same height, and that this could be complicated on aluminium masts, especially if you have a special area on the track for inserting the slides. It might help if you could post a photo of your gooseneck arrangement?
     
    Last edited: Oct 18, 2019
  3. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    Why convert from deck stepped to keep stepped?

    What do you want to calculate? Any reason not to use the same rigging wire size as originally used?
     
  4. Manateeman
    Joined: Oct 2019
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    Location: St. Mary’s Georgia

    Manateeman Junior Member

    Thank you for the reply. We have never had any mast in the boat. The deck mast collar was welded when we purchased the empty hull and deck. All the chain plates were present for both main and mizzenmast.
    Sorry I didn’t know how to add a link. Thank you for doing it.
    If I add a compression post to the deck level, I understand I loose three point fixity. I would also have to cut the spar.
    I guess my question should be rephrased. I know the Pacific spar is a bit light but I was looking for advice on how to reduce the sail plan...sail area, to take a lower air draft into account. For example, the original boom is very long.
    We have two roller fullers for the headsails. We are quite old and not performance sailors. I just feel unsafe without some some option to sail and to stabilize the boat while motor sailing.
    Kind regards.
     
  5. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Ah ok that is clearer now - sorry for my misunderstandings.
    Yes, it would be more sensible in the circumstances described to go for a keel stepped mast.
    It sounds like you will still have to raise the height of the gooseneck fitting for the boom on the mast as it was originally deck stepped.
    Is this feasible?
    If I scale off the sail plan drawing in the link -
    https://www.tedbrewer.com/sail_aluminum/images/Shenanigan---sailplan.gif
    Then I get a designed air draft of approx 76'.
    And you would like to reduce it to 64' - that is 12' less at the top.
    Or approximately half way down to the top spreaders shown in the drawing.
    You mention that the boom is 'very long' - I estimate that the main boom shown in the drawing is about 21' long. Is yours much longer than this?
    The mainsheet is not going to the end of the boom (as shown in the sail plan), so it might be possible to remove the end fitting on the boom and shorten it if necessary?
    You could maintain the same aspect ratio for the mainsail as before if you shortened the boom to about 17' or 18'.
    I presume that the main and mizzen sails are fairly conventional with battens? Ie you do not have in-mast roller reefing?
    Or maybe even fully battened, with 'stackpak' stowage bags for containing the sails when they are lowered?
    Fully battened sails would be a bit heavier than sails with conventional battens, hence a bit more effort to hoist.
    So you then want to look at reducing friction in the system where possible.
    I have heard good reports re the Tides Marine mainsail tracks - a catamaran here has this system, and swears by it.
    SailTrack and Slide System Overview | Tides Marine https://www.tidesmarine.com/sailtrack/intst_overview.php
     
  6. Manateeman
    Joined: Oct 2019
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    Location: St. Mary’s Georgia

    Manateeman Junior Member

    Greetings. We have never had any spar in this boat. We acquired a 64 & 1/2 ‘ long Pacific Spar in excellent condition. It was deck stepped on another boat. We want to keel step it on a one foot high compression post on our keel. The heel will still be 15” under the LWL. The lower spreaders will now be 6’ closer to the deck. I really don’t want to move either set of spreaders as I feel this would only weaken the spar. I would really appreciate help calculating how much sail area this spar could carry comfortably. The spar was set up for a staysail and the spreaders are not just strong, but fit our existing chain plate dimensions perfectly at 6 feet length. The spar came with two Pro-Furls .
    We recognize the spar moments are light, but are interested in the collective knowledge here to put forth alternative rigging or sail solutions. I don’t want to open a Pandora’s box by uttering the word wishbone, but perhaps collective knowledge could share thoughts on such.
    We can TIG weld thus fabrication is not the issue.
    Lastly, we are old and not inclined to push the boat to her potential as an offshore vessel.
    Thank you in advance for any assistance.
    Kindest regards, Mark the Manatee
     
  7. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    Probably not a good idea unless the top of the "compression post" has sufficient transverse and longitudinal support. The bottom of the mast will have pushing down and will also be pushing transversely and longitudinally. The combination of the mast and compression post will fail by buckling at the joint between the mast and post.

    Have you considered paid consultation with someone who is knowledgable about sail rig design?
     
  8. Manateeman
    Joined: Oct 2019
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    Location: St. Mary’s Georgia

    Manateeman Junior Member

    Thank you for the reply. Yes we are aware of the loads and can assure you this mini compression post will have adequate strength.
    As to professional assistance...having the benefit of the collective knowledge of all the members here, certainly exceeds a single opinion. I very much look forward to hearing from members. Again, thank you.
     
  9. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    I am still a bit confused (but that is not difficult to do, don't worry).
    It sounds like you are confident in your ability to design and fabricate the 12" high compression post for the mast foot to sit on in the hull.
    You mention that the hull draft including the bilge keels is 6', and the mast heel will be 15" below the LWL.
    Scaling off the side profile drawing, the hull depth below the LWL amidships seems to be about 3', and maybe about 2'6" (guessing) in way of the main mast foot.
    This kind of suggests that your compression post will be resting on some hull structure maybe about 3" deep (or on the hull bottom shell even) - are you confident that this existing structure is strong enough to take the compressive loads that will be imposed by the mast?

    The lower spreaders to the heel of the mast is 22.8' (lets call it 23'), and you mention that the lower spreaders will be 6' closer to the deck when the mast is stepped on the compression post. So the spreaders will be 17' above the deck.
    The maximum hull beam is 17', and you mention that the spreader length of 6' matches the hull beam perfectly in way of the main mast.
    The mast width is (I think) 8", so the hull beam there is about 12'8" so a half beam of 6'4".
    Will the lower shrouds be taken to chainplates on the gunwhales, or are they set further inboard?
    If they are taken to the gunwhales, then the angle of the lowers to horizontal is approx 70 degrees.
    Re calculating how much sail area the spar can carry comfortably, I think that you can confidently say that it will be a lot more than what you will actually be able to carry, if you take into consideration that the mast was originally designed to be deck stepped, and you will be keel stepping it instead. Hence no need to worry about this aspect.

    Re how the spar came with two Pro-Furl systems, I presume that you will be able to shorten the length of them by the required amounts?
    You mention 'continuous rigging' - does the mast have wire stays as well, or do you have to source / make up all new standing rigging?
    Do you still have to source / purchase a boom? This is not clear. If you have a boom already, how long is it?

    As you can see, I am still quite baffled (and no doubt many others on here are as well).
     
  10. Manateeman
    Joined: Oct 2019
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    Manateeman Junior Member

    Hi. Thanks for the reply. Sorry for the delay. The answer to your first question. The hull has a vertical keel plate...1” thick ? and then another 1/2” x 10” horizontal plate on top forming a tee. The chain plates are set inside the sheer so the measurement from the mast to the chain plates is 6’. The entire structure in the galley area...the chainplates, the mast base and the deck, is massive. It was designed for a tall and powerful main mast and rig.
    The interesting question to me is the position of the spreaders on the Pacific Spar. You are correct. The distance from the deck to the lower set of spreaders will decrease by 6-7 feet. On the spar, the measurement from heel to the lower spreaders is 22’ 10” so that if we keel step the spar, we end up with 16 - 17’ above deck.
    Between spreaders the distance is 21’ 1&1/2”. Upper spreaders to cap is 20’ 5”
    Total mast length is 64’ 4&1/2”.
    I think spar designers use different rules for setting the position of the spreaders. The spacing on the Pacific Spar looks normal to me. About 1/3 but less as you go up the mast. There are formulas in the book “Principles of Yacht Design” by Larson, Eliasson and Orych, which is the book I use most. I guess placement of the boom and all the centers of the sails will influence this decision. I’ve been told to stop overthinking this and to consider that moving the spreaders involves more drilling and TIG welding which might cause more harm than good.
    When I wrote continuous rigging, I meant the rig has one shroud going from cap to deck and another going from under the upper spreader all the way or continuously, to the deck.
    I think for a conventional boom, I’d use the same proportions as the original design. I know this changes the center of effort but again, we are just too old to deal with the big rig.
    I’m trying to look at options to eliminate the main boom. There has been a lot of criticism of wishbone rigs but Bruce King used this rig for his own boat. One can fly a mule or even a mizzenmast spinnaker. A roller furler from the mizzenmast to the base of the main mast gives you a number of options to fill the space above. What I’ve found difficult to unearth are designs for wishbones which can be lowered and design details of wishbone attachments.
    Again my sincere thanks for all who have taken the time to reply.
    Mark the manatee.
     
  11. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Re spreader locations, I would agree that you are probably 'overthinking' this - your rig will be smaller than what the mast was designed to do (re it will be keel stepped rather than deck stepped), and this will have quite a significant effect.
    I would have thought that so long as your rigging wires are tensioned properly, it shouldn't matter if the heights of the two sets of spreaders are not divided into exactly equal thirds above deck level - I am sure that a bit of leeway here is allowable.
    Re your two cap shrouds, I would call the one going from under the upper spreader an intermediate cap shroud.
    I see on the sail plan in the link above that this shroud is taken to a chain plate at deck level a bit further aft than the chainplate for the aft lower. I guess that this is to 'do away' with the need for running backstays for the staysail.
    I would agree re main boom length - just reduce the length in proportion to the height, so that the aspect ratio of the sail stays the same.

    Re a wishbone, the main mast appears to have twin backstays with the mizzen mast stayed independently of the main mast (ie there is no triatic stay between the tops of the masts).
    Trouble is, these backstays would get in the way of a wishbone wouldn't they?
    Or are you thinking of having a triatic stay instead of the twin backstays?
    .
     
  12. Manateeman
    Joined: Oct 2019
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    Location: St. Mary’s Georgia

    Manateeman Junior Member

    Hi. Thanks again. The two main shrouds are the cap shroud : running from the top of the mast to the deck. The intermediate shroud: running from just underneath the upper spreaders to the deck only inches from the cap shroud.
    I was taught the cap shroud is the forward most even though only 2” seperate them at the deck.
    The mast has longitudinal support from two runners: at top, just under the upper spreaders where the stay sail attaches.
    I built a cutter with a staysail and it had two full length aft stays and two runners at the staysail / mast junction.
    This was a single spreader rig and the runner set up completely stopped mast pumping and I never worried about loosing the mast if I lost one of the back stays. In my opinion, two backstays are worth the windage and cost. When the going gets really bad, turn on a nice big diesel, set the staysail and tighten the runners and smash on ahead.
    My wife has two endearing expressions. “ I love my turbo diesel.” and “ If it’s not Dutch...it’s not much .”
    She’ Irish American . She kissed the sailmaker who cut our staysail.
    Sticks, strings and rags. Why bother...gentlemen do not sail to windward. LOL
    The official boat boy of the good ship oystercatcher.
    Mark the manatee
     
  13. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Sorry, I keep on forgetting that your mast is not anything like the mast shown in Ted Brewer's drawing.
    I agree, having twin backstays along with twin running backstays is good peace of mind!

    What about the mizzen mast - have you sourced one yet? If you are still looking, will it be 'cut down' in the same proportion as the main mast, re the height above the aft cabin? I guess it will have to be stepped on the cabin top, as otherwise it comes down in the middle of the aft cabin (although that could be a good hand hold in rough weather).
     
  14. Manateeman
    Joined: Oct 2019
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    Location: St. Mary’s Georgia

    Manateeman Junior Member

    Hi. Thanks again. I have two spars which I own which might work. The mizzenmast must be deck steppedbas the compression post is at the foot of my bed and so far...I have dry toes.
    The question becomes simply what rags am I going to fly off this stick. The larger the area, the stronger the spar needs to be. I’m intrigued by the wishbone rig idea. No main boom. Nice roller furl from the mizzenmast. Wishbone or maybe no wishbone...a large mizzenmast spinnaker ...Steve Dashow did this because he had a good distance between spars.
    I need to look at a lot of older designs to fill the space and a lot of new technology for the hardware.
    I love TIG welding aluminum. With the new inverter welders, you can control the heat effected zone and if you have a loot of time, you get results which could not be achieved only a few years ago. I don’t think carbon fiber wishbones are the only answer. Lowering the air draft might not result in a slow boat and would be a great option for really old boatbuilders and owners. I only hoped that the discussion might recalculate designs.
    For example...the safety factor in the strength of masts...is this science or not. Race boats push the limits but maybe my grandfather was right...one rig for winter, one for summer.
    Is it possible to have a summer rig which is simply a “splice extension” and extra high tech dyneema rigging and a couple of light sails? Add more ballast in the winter.
    Two hundred years ago, fishermen who worked every day in both ideal and pretty nasty winter weather without any
    engines...they faced the reality wind force changed with season and the answer was simply to adapt.
    One does not find a lot of new applications of new technology to this solution.
    Again, sincere thanks for your thoughtful and kind replys
    Mark the manatee
     

  15. Manateeman
    Joined: Oct 2019
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    Location: St. Mary’s Georgia

    Manateeman Junior Member

    Greetings all. I began this thread in the hope to open design discussions on options for larger vessel owners who wish for more manageable sails and shorter spars. A discussion of options.
    Apologies to those who have given very sound advice. I failed to properly describe the “calculations” which only confused everyone. Please permit me to begin again. I’ll not repeat my particular boat design because it is only an example...and perhaps a bad one.
    First. I built a Bruce King design cutter and just loved the boat. I’ll get into what changes we made, but my point is simple. We liked the performance and sea kindly behavior of the boat under motor and staysail in unpleasant seas. I happened upon his design for a wishbone ketch (Chanty) and noticed it had a wishbone. I had only seen “ wishbones” when I worked for a well known builder in Rhode Island. This Wishbone was way up in the air. Recently, I searched the history of this spar and was very pleased to find the following reference and discussion. The Wishbone Rig. https://www.ayrs.org/repository/AYRS//.pdf
    Well, here is the issue. Why would such a highly skilled designer choose this rig for his personal yacht. I’m not going to go carbon for my main spars, but perhaps I ought to consider wishbones as the answer to my desire for a more manageable winter rig. I sincerely welcome your thoughts. Please be polite.
    Mark the manatee
     
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