Mast lowering construction

Discussion in 'Boat Design' started by Jan Oostdam, Mar 23, 2008.

  1. Jan Oostdam
    Joined: Mar 2008
    Posts: 2
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    Location: Netherlands

    Jan Oostdam New Member

    Dear people,

    Via via, I found your address and I think your organisation will be the right one to help me out. If not one of your members is able to answer my question, you probably will be able to point to the right source of information.

    What is my question?
    Shortly I bought an Albin Ballad (Danish built, production nr. 2004 made in 1982).We have no intention to use the boat for racing. My brother and I are still busy refitting the boat. It's still on the yard.
    We are Dutchman and live in the centre of the country. That means at the border of one of the main branches of the German Rein flowing through Holland.
    Mostly sailing the Ballad on the IJsselmeer and on the waters between the islands in the South West (province of Zeeland), I have to migrate through a lot of canals.
    That’s no point; the Ballad has a good auxiliary but..... our canals have lots of bridges that cannot or will not open for a boat with a mast height of 11 m above water level.
    So the only way out is to use the “Standing mast route”. That means a trip of about 150 km through cities and densely populated areas during the night.
    Discussing the difficulty by e-mail with a mastmaker he said;” I will calculate for you the possibility of cutting the mast at caochroof height, closing the gap with polyester and roving, leaving the under part as a strut and constructing a hinging mast foot at the top of the caochroof.
    For this calculation however he needs to know the mast cross-section profile (number and make) and weight of the mast.
    He also said to need the ‘RM’. As far as I understand, this “r m’ means ‘righting moment’ (A boat that heels easily when hit by a blow needs a ‘thinner’ mast than a very stable boat that will hardly heel.)
    My question:
    Do you know some one who I can address for these data?

    Many thanks in advance,

    Jan & Bart Oostdam
     
  2. FAST FRED
    Joined: Oct 2002
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    Discussing the difficulty by e-mail with a mastmaker he said;” I will calculate for you the possibility of cutting the mast at caochroof height, closing the gap with polyester and roving, leaving the under part as a strut and constructing a hinging mast foot at the top of the caochroof.

    This is simply known as a Tabernacle and should be in any design book.

    A far simpler method we have used is to obtain an aluminum flag pole lowering base of suitable size..

    These will have a geared crank that operates a large curved gear , and the mast is simply cranked up or down . The forestay will need to be removable as will the 2 forward lower shrouds.

    The most simple method I have ever seen was to hinge the mast with a tabernacle mounted on deck, but the foot of the intact hollow mast would be partially lead filled , and a pair of deck doors would allow the mast base to come up to deck level.

    I believe this was called a lutchet.

    Might be hard to keep the foredeck area dry , but at least the stick still stepped in the keel, no hinge in the middle!

    FF
     
  3. Jan Oostdam
    Joined: Mar 2008
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    Location: Netherlands

    Jan Oostdam New Member

    Fast Fred, you are a credit to your name.
    Thanks for all the effort. Where did you find a mast maker on Easter day?
    Let me first tell you that the solution with the lutchet (?) is the normal solution for all traditional Dutch freighters (max. about 80 tons) as this allows lowering of the mast very fast.
    These boats (still sailing for pleasure purposes) called ‘tjalk’, ‘aak’, ‘westlander’ etc, etc, have rigs that are very sophisticated in this sense that only the skipper and the deckhand (or his wife or son) are on board and are able to handle the boat the two of them, sailing through the canals. When - in the old days - they had to sail into town to deliver goods, they had to pass large numbers of bridges. Already in this time, not all of these bridges opened or could be opened.
    Back to my problem:
    The Albin Ballad is a sea going racing yacht from the ‘70ties: LOA 9,14cm, waterline length is 690 cm, width 2,95 cm, draft 155 cm, displacement 3300 Kg and a ballast weight of 1550 Kg. Mast top 12,4 m above water level.
    The construction of a hinging mastfoot is not the main problem. I know the demands.
    My real problem is to be describes as follows:
    When a compressed, slender pole is “cast-in” or “fixed” at the underside (almost the situation with the Ballad mast: one fixed hinging point at the keel (keel stepped mast) and one hinge where the mast passes through the coach roof, at a relative short distance above it (1,71 m) Total length 12,80 m.
    This situation creates a totally different “slenderness factor” compared to a mast, hinging at the top of the caochroof.
    In other words: A single, compressed prismatic pole has a “slenderness length” equal to “L“
    A pole of the same length and cross section, “cast in” or with a “fixed end” at the bottom end and a hinge at the top, has a “slenderness length” of ½ V2 * L = 0,701 L. The ‘length’ of the Ballad mast however will – after being cut at coach roof height – be only (12,80-1,71): 1280 = 0,86 but this is not decisive. Although the mast of a boat with spreaders is no single compressed prismatic pole, (in fact in the original position there are 4 hinges (one at the top, one at spreader height, one at deck height and one at the keel) and after placing a hinging construction at the coach roof there are still three. In fact, cutting the mast at coach roof height is equal to removing the fixed end at the bottom.
    The core question is: Is the danger of buckling increasing or not?

    We hope you or someone else is able to help us out.

    Regards,

    Jan and Bart Oostdam
     
  4. Guest625101138

    Guest625101138 Previous Member

    Jan
    Stepping the mast in a tabernacle will increase tendency to buckle as the base can no longer carry moment in the longitudinal direction. Whether it will buckle is another matter. You could work through the engineering involved using this site:
    http://www.tech.plym.ac.uk/sme/desnotes/buckling.htm
    There is a calculator linked to the page that you can use to determine how close you are to buckling.

    I would replace the mast bottom with steel or aluminium pole with a welded-on flat plate and gussets under the coach roof to mate to a similar flat plate that supports the pivot above the coach roof. You also need to fit a spacer on the pin so the sides of the mast do not crumple at the pin.

    Rick W.
     

  5. Brent Swain
    Joined: Mar 2002
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    Location: British Columbia

    Brent Swain Member

    A good set of 1 inch sch 40 stainless stanchions at the pivot point of the mast let you lash your lower shrouds onto them and control the sideways movement of your mast while raising or lowering it, making the proccess much easier and safer. Welding a brace at 45 degrees from the pivot point prevents any loads from bending the stanchion.
    Brent
     
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