Converted displacement sailboat - stability

Discussion in 'Stability' started by Icebreaker, Apr 30, 2011.

  1. Icebreaker
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    Icebreaker Junior Member

    At the end of WWII a Sea Rescue boat was designed along the lines of the Norwegian Sea Rescue boats which followed Colin Archer's successful 'double-ender' design. This boat was a 65ft (20M) Steel 65 ton, rigged as a Ketch on a displacement hull - launched in 1948, she sailed the Baltic Sea for some 25 years, involved in hundreds of rescues, often in very bad weather, recognized with many awards for her crew.

    pix#1 Here she is as a model of the boat, rigged :-
    pix#2 Here is a picture of her steaming without sails up :-

    Then in 1974 she was converted to a power driven Sea Rescue 'light ice breaker' when the Masts Rigging Sails, Single Engine and Pilothouse were removed, and a new Aluminium Pilot house installed, 2 engines installed driving a single propeller, It is presumed that as a sailing vessel she required extraordinary sailing skills to attend rescues where people were in the sea, where boats were foundering etc.. By converting to her to power enabled her to maneuver more easily and safely. It is also presumed that as her area of operations were confined to the Baltic Sea, she managed the conditions without a problem for the next 10 years.

    Pix#3 Here is a picture of her as a powered Sea Rescue light icebreaker :-

    Recently she was brought to the Iberian Peninsula via the North Sea and the Bay of Biscay. She rolled the whole way like a pig in mud ! Never out of control, but exceedingly tiring with the potential for crew injuries.
    Have considered adding a shoe to the keel. However, believe that the addition of 3 "steadying" sails is a better option. This would mean the addition of fore and back stays + two shrouds to the existing 30ft utilities mast.

    Pix#4 Here is a picture of the sails hanked on to the stays.All told 275 sq ft.

    When the sails are made what be the objective - regarding the stabilizing and propulsion ratio in the shape. The sailmaker tells me that we can have whatever shape we like. That is the dilemma !
    Help, comments, ideas.

    PS just noted that there is a specific forum for 'Stability' topics - Thanks for moving it.
     

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    Last edited: Apr 30, 2011
  2. Eric Sponberg
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    Eric Sponberg Senior Member

    You are going to get practically nothing in sailing performance with the sails configured as you have shown, simply due to their overall shapes (triangles) and their positions on the boat. Certainly this applies to the middle and after sails. There is more hope for the jib (forward sail). Actually, you could put some camber into the jib (forward sail) and you may get a bit of drive out of it. The other two would best be made pretty flat.

    A thought comes to mind--will these be mounted on furlers? That would greatly ease their deployment--wind them out and wind them in. In that case, there would need to be some shape built into the sails to accommodated the roller furling mechanism.

    Good luck, I hope your new sails improve the ride; they should.

    Eric
     
  3. Icebreaker
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    Icebreaker Junior Member

    Eric, thanks very much - your suggestions confirm initial thinking.
    Just to add some info regarding the need for the sails - the vessel is being prepared for passage across the North - North Atlantic; from Scotland to Newfoundland via Iceland and Greenland.
    Here is a sketch of the foresail that needs some shape to provide a little propulsion in addition to countering the roll. Because the sail is so different to a jib found on the average sloop - its leading edge presents like that found in a lateen sail -- angled only 32° above the horizontal.

    Which way should the panels run ? a) parallel with the foot or
    b) parallel with the luff? At 90° to the luff?

    The camber expressed as a percentage of the sail chord - %?

    Maximum camber expressed as a percentage - %?

    Weight,, would 140g/ft2 woven dacron be adequate ?

    Obviously this sail is not going propel the boat by itself - it is hoped however to assist first & foremost in countering the roll and as a bonus provide enough drive to improve fuel consumption, measurably.
     

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    Last edited: May 3, 2011
  4. Icebreaker
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    Icebreaker Junior Member

    A comparative foresail sail design can be found in Ted Brewer's Grand Banks 33 (a very popular power boat to be found in every ocean)
    The GB's foresail is very similar to the design proposed for the Icebreaker's steadying sail in terms of size and shape.
    These sail additions have made all the difference to this Grand Bank's success as a blue water cruiser.
     

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    Last edited: May 2, 2011
  5. Eric Sponberg
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    Eric Sponberg Senior Member

    Your sailmaker can confirm answers to these questions perhaps better than I. Like yacht design, sailmaking is a combination of art and science. In my experience, jibs are usually laid out with panels perpendicular to the foot and the leech as shown in the Ted Brewer drawing. The seam between the two sections is perpendicular to the luff. Your sailmaker will make the decision on how to lay out the panels.

    Yes, the camber is usually expressed as a percentage of the sail chord, and your sailmaker can advise on the best amount.

    For fabric weight, again, consult your sailmaker. This should be a heavy duty sail for the harsh conditions it will be in. There are many types of fabrics with different properties and weights, and I don't have intimate knowledge of each of them. Sail weights are usually given in ounces per running yard (36" x 28.5") or in grams per square meter. However, your weight of 140 grams/ft^2 is equivalent to 35 oz/yd. sail fabric, which is way overkill. You wouldn't be able to easily handle it--it would be like cardboard. Maximum sailcloth weight, as I recall, is about 13.5 oz/yd., so the basic cut, before patches and reinforcements, will not be heavier than that. For the size of the jib as you show it, I would be surprised if it was much more than 9 or 10 oz/yd.

    Eric
     
  6. JRMacGregor
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    JRMacGregor Junior Member

    Your boat maybe rolled violently because she has excessive stability/stiffness. This stiffness comes from having the centre of gravity low down. It could be low down because when the aluminium deckhouse and low down engines were added they did not compensate for the removal of the high up weight of the original masts (I have not checked the sums).

    It is quite common for high stability to be combined with poor seakeeping/motions.

    If this is the case for your vessel, then adding more weight around the keel would make the problem worse. The ship would become stiffer, roll period shorter and accelerations greater (accelerations are proportional to the rolling period squared).

    So I agree with the advice already received - adding steadying sails would be the safest bet to improve the roll motion (would damp it out).

    If the vessel had sufficient stability to carry taller masts and sails, then the ship could become even less stiff and would have better motions. This would need calculations to confirm.

    Incidentally, it may be that the vessel roll motions were OK for the short seas in the Baltic. Perhaps her natural roll period is longer than the Baltic waves, and so they do not excite resonant rolling even if rather high. But when exposed to the longer seas in the North Sea and Biscay perhaps the wave period co-incided with the ship natural roll period – giving resonance even in low waves.

    Big bilge keels are another way to damp rolling if you cannot do anything about the basic stiffness of the hull and weights.
     
  7. Icebreaker
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    Icebreaker Junior Member

    Hello JR ,,
    Many thanks for your thoughts - they coincide !!!
    Where in Scotland you from ?
    I am 3 west miles of Elgin -- spent time on fishing boats out of Lossie Mouth.
    Richard
     

  8. JRMacGregor
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    JRMacGregor Junior Member

    Hi Richard, I am from UL !

    Not so young that I can't remember the steadying sails on the East Coast herring drifters (INS and points east), although that was mainly to keep their head up to the net !
     
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