stability and drop keel sailing boats

Discussion in 'Boat Design' started by howard easton, Oct 2, 2004.

  1. howard easton
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    howard easton New Member

    I need some guidance from the collective wisdom of this site!

    I am in the market for a new drop keel sailing yacht in the 35 ft region - all the bilge keel yacht makers in the UK have gone bust so this is the only way I can get a boat suitable for our drying creeks.

    the choice is between a french ovni and a british southerly, and what is worrying me is the stability curves. the ovni has an avs of 107 degrees and a max righting moment of 3.5 tonne meters at 50deg. stix 30.8 By contrast, the southerly claims an avs of 160deg, a max righting moment of 6.5 tonne meters at 67deg and a stix of 57.

    easy to judge, you might say. but I am not at all sure that I am looking at numbers derived in the same way. the southerly looks too good for a drop keeler. sure the keel is a tonne or so of iron and goes down 7 foot whilst the ovni is an aluminium center board but even then I wonder about the numbers.

    can someone explain exactly how you get the avs, what it includes, and what massage a builder might be able to apply. Does it take into account form stability or is it simply a c of g calculation? is a stix of 30.8 OK for crossing biscay, for example? and what about an avs of 107 deg - has anyone on this board ever put a mast into the water?
     
  2. moggy41
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    moggy41 New Member

    did you ever get an answer?
     
  3. howard easton
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    howard easton New Member

    no, I didnt. got the feeling that AVS is a bit of a moveable feast, a calculation from the design drawings taking into account the heeling test. but some include the effect of topsides, others dont. and nobody I talked to seemed to have a real technical answer to the capsize mechanism anyway, since it seems to be as much an issue of grip on the water and energy transfer from waves as it is one of avs.

    in other words, the ovni with the centerboard raised might avoid a capsize by sliding sideways whilst a deep fin with a higher avs might trip up over its keel. all theoretical of course since how can you compare real boats in the real world?
     
  4. markr
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    markr New Member

    Howard

    the thing that puzzles me is the comparrison between the Southerly 35RS and the Southerly 110 - the STIX on the 35RS is 30.8 and on the 110 (from posts near the beginning of this thread) has a STIX of over 55 ! The two boats are identical - with slightly different interiors, so why are the STIX values so different ?

    I am also interested in both these boats - from a comfort point of view as well as performance and layout - the Ovni is a bigger boat, but my wife loves the raised saloon on the 35RS, and of course the Ovni is cheaper as well. I am still unsure as to the comfort levels of either of these compared to say a Malo 37 (or an older 36 - I am not hung up on having a new boat and am not made of money)

    any thoughts ?

    Mark
     

  5. howard easton
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    howard easton New Member

    STIX includes all sorts of factors like the downflooding angle, and the deck moulding of the 35RS is different so these factors might well be different. However I think I remember having a chat with a very helpful guy in the Northshore design office and subsequently with a naval architect about the very high avs of the 110. Rather than me quote half remembered info, its best you also have a chat with the designers and get the correct info first hand.

    Personally, I felt more confident about the design of the Southerly. I did some simple maths on heeling angles against wind load on the sails, and proved to myself that the boat would be stiffer than the Ovni. I felt more comfortable with ballast hanging below the hull rather than just being in it, and I did wonder how well the ovni would fare with keel raised but that large rudder trailing out backwards- for example in manoeuvering into shallow water.

    I liked a lot of the deck design of the Ovni which I thought showed that the designer / builder was a sailor. Nice strong leads for the ropes, winches in the right place etc. But after a working life in the metals industry I was uneasy about painted ally - it is notoriously difficult to get paint to stick , and it very easily bubbles after any damage. Any dings on the ally hull would easily show and be very difficult to hide / repair. Probably impossible to make an invisiblke repair which is quite easy in grp. Had no doubt at all that the Ovni would require regular maintenance of hull and deck paint.

    As against that, the layout of the deck saloon on the RS wasnt good IMHO - just imagine having a couple over to dinner and 4 or 5 of you trying to eat at that table. However thats less of an issue - you can now have the 35RS rig on the 110 interior, and Northshore woodwork is fabulous and every bit as good as the Swedes.

    We couldnt decide. In the end we bought a 50k Moody 336 bilge as an interim, I use it for a bit of club racing as well as cruising, it bounces off things and I'm not bothered. Lot more relaxed than I would be with a new Ovni or Southerly and for that reason alone I think we will stay with the Moody.

    P.S. Have a look at the Feeling 36 lift keel. Its definitely not "just another cheap French boat" and is well designed for shallow draft use and looks reasonably well constructed. Doesnt have the kudos of the Swedes which cost twice as much but it isnt half the boat by any means.
     
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