Fairey Atalanta Stability Question

Discussion in 'Boat Design' started by RHP, Oct 22, 2009.

  1. RHP
    Joined: Nov 2005
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    RHP Senior Member

    Gents, I come to you for advise on a classic yacht the 'Fairey Atalanta', two main models were built in the UK in the cold moulded multi veneer method in the 1950´s and 60´s, the more numerous A26 and the A31.

    The yacht has twin retractable keels either side of the centre line, narrow beam and less sail than a modern rig would have. The main numbers for the Atalanta 31 are:

    LOA 31´ (9.45m)
    Max beam 8´3" 2.51m
    Freeboard fwd 4' 1.22m
    Freeboard aft 2'3" 0.7m
    Draft keels up 2' 0.61m
    Draft keels down 7' 2.13m
    total weight of keels 2,120lbs 962kgs for the two
    Designed load displacement 8,000lbs 3,628kgs
    Sails, main 192ft2, Genoa 275ft2
    Height of sail plan over sheerline 37' 11.28m

    The yacht can be seen in the attachment including separate shots of the keels dropped and retracted.

    The A26 was the original design and the A31 follows her hull shape and keel concept.

    I´m ointerested to know how in your opinions the double retractable keel system compares to a modern single swing keel yacht in terms of stability, angle of capsize etc.. and self recovery and righting ability? If the Atalanta goes on her side after a gybe for example will she self right? How she would behave if she went past 90 degrees? I believe there is a locking mechanism to stop the keeps retracting under gravity though they do bounce up if an obsticle is struck.

    Her slight beam compared to modern yachts of the same length, hinders or improves her safety aspect combined with her twin keels? To a large extent her hull resembles 18 foot centreboard open racing daysailors of the time.

    In modern terms her sail area is small, is this limited by the design dynamics of the hull or that designers were more conservative 50 years ago due to sail handling technology being behind what we have today? I´m also surprised to see no one has plumbed for a retractable bowsprit over the years.

    I´m curious as I´d like the challenge of owning, sailing and maintaining an A31.

    Cheers
    Richard
     

    Attached Files:

  2. apex1

    apex1 Guest

    Hi Richard,

    you know the owners club:
    http://www.atalantaowners.org.uk/

    Do´nt know anything about the performance of their sailing boats, but the motorboats have had a very good reputation for performance and quality.
    Being a former aircraft builder, I guess they had a unequalled sense for quality.

    Regards
    Richard
     
  3. RHP
    Joined: Nov 2005
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    RHP Senior Member

    Hi Richard, yes I am also interrogating them over there with the same questions!

    Received one very informative response thus far:

    "Lets start off with stability - and the Atalanta 26' in particular!

    The boat is self righting to 90 degrees of heel with both keels FULLY RAISED.
    They are totally self righting when the keels are fully lowered.
    The keel clamping mechanism holds the keels in place when inverted - as long as the keel bolts have been tightened and the mechanism is in working order.

    It is also worth mentioning that the Atalanta will float on her side with both the main washboards removed, and the forehatch open. At 90 degrees heel all of these openings are clear of the water. Of course ventilators which have been added after construction could let water in if they are not near the centre of the boat.

    It is worth adding that the beam was limited by the maximum legal width when towing - anything wider would not be allowed on the road behind a 4 x 4 - it would need a lorry to tow it. I suppose that the Atalanta was also the first floating caravan, and as such is still one of the largest trailer sailers available. However she is unique in that she is also renowned for her seaworthiness!

    There is a photo of the prototype on her side, being tested - and I will scan a copy and attach it to this thread in due course.

    As far as I am aware all the boats we are interested in the A31, Titania and Fulmar - are all designed and constructed to the same safety criteria - so the comments above should all apply equally to them.

    As far as comparisons with modern boats - it is difficult to say. I should doubt if it is economically viable to calculate the righting moments - in the way which it would be required to satisfy the modern recreational boat directives. However I suspect that the Atalanta would easily satisfy all modern requirements.

    The rig is an integral part of the design - in that if you change it you alter the stability, and all of the other considerations. So the rig will be a compromise - not so big as to endanger the boat in a heavy weather, as big as can be safely accomodated for light airs, as light as possible and as strong as possible.

    Like most serious cruising boats Atalantas are slow in light airs. It needs a good force 3 to get beyond 4 knots, but when the wind gets up they will beat to windward at 5 to 6 knots.

    If you have the guts to hang on to the spinnaker (in a strong enough wind) the Atalanta is supposed to plane - just like a firefly. I have read something about this in an old Bulletin, and it seemed to suggest speeds of 13 knots.

    It didnt take many years before the masthead rig was added to the Atalanta - on a slightly modified standard mast - and of course that helps in lighter winds.

    I know Denny Desouter of PBO used to advocate adding bowsprits to small cruising boats - and I have followed his advice on a previous boat. I have als seen a photo of a Titania with such an adition. Unfortunately this boat seemed to have had so much added to it that it filled through the cockpit drains - and I dont know if the bowsprit would have been a help or part of the problem!

    However I think that a bowsprit is unlikely to add anything positive to the performance of an Atalanta. You would be better off removing kit if you wanted to improve performance. These are light weight boats and need to be kept light. If adding a bowsprit you would need to be careful that it was well supported - the decks are only 3 layers of veneer thick!

    I am an advocate of leaving the boat largely as it was designed and built. It is far too easy to destroy the performance or safety of an Atalanta when trying to improve it! "
     
  4. Crag Cay
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    Crag Cay Senior Member

    Just for the sake of accuracy, they weren't cold moulded, but rather 'hot moulded' in autoclaves.
     

  5. RHP
    Joined: Nov 2005
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    RHP Senior Member

    Yikes you´re right !!







    (but after 50 years does it make a difference? :eek: )
     
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