Dinghy heeling moment (again)

Discussion in 'Sailboats' started by revintage, Nov 7, 2016.

  1. revintage
    Joined: Nov 2016
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    Location: Sweden

    revintage Senior Member

    Hi,
    I recently bought an old 49er lacking among other things, its wings. Decided to rebuild it to single handed no trapeze using only a cut down main and a 29er kite. I have one main cut down by 1m at the bottom, actually for the old Flibustier, that forces me to cut the lower mast(done) and adding a jib for balance summing up to about 14sqm. Planning to cut the leech on another main meaning the mast could be uncut, giving a high aspect ratio sail of 11sqm. Having three mains, three jibs and two masts makes it possible the experiment a little. See pics.

    In an attempt to find out the design wind on a few common skiffs and dinghies I got a little puzzled as I didn´t get the numbers that usually is mentioned on the internet. Using Bernouilli´s formula the figures got totally f-cked up. Using the 0,0053*v^2 (lbs/sqft) formula and changing it to kg/sqm makes sense but result is not up to the earlier mentioned figures. I would be grateful if anyone could check the simple spreadsheet (metric) for any faults.

    Lars
     

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  2. gggGuest
    Joined: Feb 2005
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    gggGuest ...

    Can't help you with your sums, I tend to just say X has about the right amount of rag, I'll have little bit more/little bit less.

    But I dunno, this feels like a journey too far to me. Seems to me that you're going to end up with a big and relatively heavy boat that has an enormous amount of wetted area for the amount of sail you can carry.

    But as for comparisons with other boats, as you should know, being in Sweden, an International Canoe has 10sqm of sail and phenomenal amounts of righting moment, albeit on a narrow hull. 14sqm on main and jib, on a boat that uses a kite to get downwind feels like a hell of a lot. An RS600 which is a modern single sail two hander has I think 12.5m2, a contender 10.8, and arguably as single sail boats there's a tendency to be oversailed upwind in order to get down the reaches. The RS700 has 12.7m2, the Musto Skiff 11m2 . On the whole 10-11 sounds about the right sort of area to me...
     
  3. revintage
    Joined: Nov 2016
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    revintage Senior Member

    Hi,

    I agree with you, actually my versions are only ideas so nothing is set. As I am not a young person anymore, trapeze and slide is not so tempting.

    Even if the figures in the spreadsheet aren´t 100% correct they are good enough for comparision between the listed boats. Added a new one with hopefully better accuracy. Still wonder why the design wind, even in the "0,0053" version is higher than expected?

    As I am not a sailmaker it costs quite a lot to make changes. I am quite tempted to go for the 11sqm without cutting the mast even it has a high CoE. It will then be easy to run the standard 49er rigging when you are two on the boat.
     

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  4. revintage
    Joined: Nov 2016
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    revintage Senior Member

    Realized the swedish version of Excel isn´t compatible with the english so a pdf might help you to read.
     

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  5. CT249
    Joined: May 2003
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    CT249 Senior Member

    I thought those figures actually looked low for "design wind", if you mean by that the conditions in which the rig is fully powered up (but not depowered) and the crew is fully hiking.

    If you are not as young and nimble as you'd like to be, the most significant issue may not be when the boat is sailing in a straight line, but when you make a mistake such as moving too slowly through a tack or gybe. Once you lose the ability to luff or bear away to keep the boat in balance, sometimes sheer righting moment is the only thing that will save you. You could feel this issue in the 79er prototype, for example; if you had an older club-level crew who wasn't used to getting across the boat fast enough in gybes you didn't have the sheer power to just hike the boat flat like you do in a 'real skiff'.

    It's hard to get accurate data on rig weights and even harder to calculate the drag of flogging sails, but the problem could be an example of the issues of relying on calculations rather than modelling off boats that have been proven to work in action with a crew of similar ability and agility.
     
  6. revintage
    Joined: Nov 2016
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    revintage Senior Member

    "I thought those figures actually looked low for "design wind", if you mean by that the conditions in which the rig is fully powered up (but not depowered) and the crew is fully hiking."

    Actually those figures are somewhat high for Tasar compared to where I have seen Bethwaite mention 11-12knots(5,5-6 m/s). Also 49er figures look somewhat high. Note I use metric system in the table, so design wind is in m/s, not knots.

    Wether correct or not the figures are interesting for comparison. To lessen heeling moment a self tacking jib plus main must be the way to go, even when single handed, as CoE can be kept lower, even with high aspect ratio sails. Unfortunately this means higher costs when modifying existing sails. My initial idea was to only cut the leech of the mainsail to keep costs down and move CoE forward, but I doubt this will work as planned.

    As you mention straight line sailing might not be the issue. Keeping beam more narrow should help a slower crew.
     
  7. CT249
    Joined: May 2003
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    CT249 Senior Member

    D'oh. I should have realised that was M/S! I'm Australian - we use metric units for just about everything BUT wind speed.

    The issue of beam and crew speed is interesting. A narrower overall beam relative to waterline beam will help when you meet a sudden lull, but it really hurts when you need maximum righting moment to get the boat flat again. You also don't have to go out to windward all the way in gusty winds.

    I recall the lack of simple righting moment as being an issue with one of the prototype 59ers and with a skiff-type 15 footer I used to singlehand. The 59er prototype comparison may be relevant because it seems to be the closest boat to yours in conception.

    Personally I prefer the wider beam because where I sail capsizing to windward in a lull is less of a problem than capsizing to leeward in a gust, but obviously it depends on a lot of personal and geographical factors. When all is said and done, perhaps the single greatest design factor of a skiff (which is what you seem to be looking at here) is that it has higher righting moment than a dinghy.

    Have you looked at the small high wind rigs that the Aussie Skiffs use? They may provide some interesting information. Googling 16 Foot Skiffs may give you an idea of their rigs, and you can see some of the small rigs in action on Youtube videos. Of course, being specialist high-wind rigs they are not directly comparable, and the Skiffs can really struggle when they get caught out with the wrong rig up.
     

  8. revintage
    Joined: Nov 2016
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    revintage Senior Member

    Have considered my project and come to the point where I am thinking of adding a small bulb in the ballpark of 40kg at the end of the centerboard. Seems it works on the VX-One, so it should also work on this one.

    Would also be a relief for a slow, grumpy, old man ;-) . Single handed with a 29er kite that I happen to have lying around.

    As I have all the other parts including a good set of sails(except original wings) it will be easy to restore the 49er to original.
     

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