Water-ballasted multihulls

Discussion in 'Boat Design' started by xarax, Feb 11, 2005.

  1. Tim B
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    Tim B Senior Member

    could I suggest that someone times and GPS tracks the two manouvres at some point, I'd be interested to see the results.

    Tim B.
  2. s v ugly sister
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    s v ugly sister Junior Member

    Orma 60 Trimaran - Water Ballast Capability

    My ORMA 60 Trimaran project boat - (ex-WATERWORLD sailing version) - has 6 water ballast hull sections in each 57 foot ama - the 1993 build agreement specifications between Universal Studios and Jeanneau / Lagoon calls for water ballasting hull sections in the amas & a filling & emptying system - I believe this type of ballasting would be useful in long offshore races when a tri would be on one tack for a long time - ORMA 60 class rules allow use of water ballasting in the amas under certain conditions - the 12 water ballast hull sections all had some water in them when I bought the boat from Universal Studios in March of 2004 - movie scenes from WATERWORLD show the tri as being seriously loaded down & really laboring in the fast sailing scenes - the boat never flew the main hull in the movie - I believe that was because of the weight of: the onboard props - crew down below sailing the boat - & perhaps water ballast - (perhaps a lot of water ballast) - - - the WATERWORLD sisteship - (transformer version) - is being re-built in California for racing - mine will be re-built in Florida for fast cruising - the two racing sisterships - (ex-LAKOTA & ex-PRIMAGAZ) are both for sale in europe at this time. Dale Miami
  3. sharpii2
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    sharpii2 Senior Member

    baby bird single outrigger

    Hi everyone.

    Just thought I would join the discussion. When I get bored or depressed, I usually play around with design concepts. Recently, for a gag, I sketched a single outrigger. By single outrigger, I mean a multi that has only one float, like a proa, but sails with it on both the windward and the leeward side. I thought the idea was crazy. Such a boat could never be safe. With the float to leeward, there should be plenty of stability. But what about the float to windward? Surely the light float to windward would offer little resistance to capsize. Then I thought 'hey. Who says the float has to be light?'.

    I then got the idea of making the float as long as the boat but only half as wide and half as deep. This would give it one fourth the displacement of the main hull.
    And it would be ballasted accordingly. I then did my stability calcs as if for a catamaran with the same beam but with the displacement equal to two of the floats, fully loaded. I found that it could handle a reasonable sail area. Nothing like a catamaran of the same beam and weight, but enough to move it through the water. Thus was born Baby Bird. It is:

    20ft long,
    13ft wide, and has a
    4ft wide main hull and a
    2ft wide float, displaces
    2500lbs total. and has
    140sft sail rig.

    As you can see, the sail area is modest by multi standards (and mono standards too), but is so only when the main hull is fully loaded to its 900lbs of capacity. Also, the main hull is too wide for good multi performance. It will probably be stuck in the single digits for top speed. Even so, it has its vertues.

    One, is that it is nowhere as sensitive to over loading as a typical multi. The main hull has enough footprint to have a decent imersion rate.

    Two, being overloaded adds nothing to its righting moment. This may be seen as bad, but I think it is good, because multis have to be designed to take the maximum anticipated loads. An overloaded multi can risk structural failure if pressed too hard under sail. That is because the stability goes up proportionately as the weight goes up. Surprisingly, this does not seem to be the case with most monos. One that I did stability calcs for had its stability go up almost not at all as the displacement was nearly doubled. Most multi designers always seem to be warning their clients not to over load. Maybe this is the reason.

    And three, it can be simpler and cheaper to build. You only have to design to the force needed to submerge the float. (which will probably be greater than its displacement-probably less than half the main hull's empty displacement)
    There is no ballast keel to construct, as with a mono of similer performance expectations, and the whole thing can be much easier to dismantle for transport and storage (probably an all day project, but one that can be done with little or no outside help). Also, all of the acommodations will be in the main hull. In fact, the outrigger beam and float can be thought of as a very deep, bulbed, fin keel, but one with the bolts where you can actually see them.

    For a sail rig, I got creative. I went for a low aspect ratio. I did this so the boat could carry its sail in windier conditions (up to force 6) . I chose a boomed lateen sail with equal spars. I did this for three reasons:

    1.) so I could get the sail down in a hurry if I had to.
    2.) So I could have a shorter mast. Even with the small boats I have had, getting the mast up and down without damaging the step or the rigging has been more hassle (Can't do on windy days) than I care to put up with.
    3.) For a minimum of complexity. (that's after the boom and yard are engineered, of course). It seems that many people in the small sailboat community are taking a liking for this type of sail for this reason. 'Performance' to them is getting out on the water as quick as possible, not being the fastest once out there. 'Racing' is for like boats with like capabilities only.

    To keep the sail from chafing against the mast, I decided to go with a much heavier bipod mast arrangement. This would not be practical on most typical rigs because the heavy mast would be too tall and would cause stability concerns. The masts will be side by side and joined at the top. They would somewhat resemble a hangmans gallows. Heavy, especialy at the top, but their 10.5ft height would, I hope, not do too much harm. The two 20ft spars would be slung between them.

    I guess the only reason I brought this up on this thread is because a single outrigger, if it was ever raced (probably not its strong point), could benifit greatly from water ballast. Here, it would not have to be shifted, but merely loaded and off loaded. Such an enterprise would make for some spectaculer crashes if the timing was ever off. To this, I'll simply point out that a single outrigger, like a proa, is much easier to right than a typical tri or cat.

    Hope I haven't bored you.

  4. rob denney
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    rob denney Senior Member


    Neither insane, nor a waste of energy. The (racing) shunting boat luffs to head to wind, losing no energy at all. To then turn the boat onto the wind on the other tack is a function of crew moving forward, sheeting on the aft sail and luffing hard with 2 rudders. Definitely less crew energy than tacking. I have shunted a 25' proa single handed dead upwind between marina arms and in narrow channels. It would be hard work doing this on say a Tornado, and probably impossible on a Hobie. The chances of a fluffed tack are also much higher on the cat.

    Arguably less waste of wind energy as well. However, this is hypothetical at the moment, as we have not seriously tried shunting like this, and nor has anyone else to the best of my knowledge.

    I will be sailing the boat in a couple of weeks and we hope to have a video on a 15m high jetty to record the maneuver. Keep an eye on the web page for the results. If you have access to video or gps tracks of cats tacking, I would very much like to see them for comparison sake. I can track it on my gps, but there is no time or speed readout recorded. Still, maybe combining this with the video, we can get some meaningful results.

    Regarding water ballast, it is hard to think of a more energy sapping method of stopping a boat falling over, particularly if you are racing and trim the ballast to the gusts/puffs. Whether a cat is faster or slower is a moot point at this stage. Theoretically, for a given sail area, the proa can have a longer waterline for the same weight and righting moment, which should overcome any losses in tacking/gybing.

    We had our first race a month ago and have barely been out since. I am pretty sure that if we had swapped boats with the guys who won the race, they would have thrashed us by an even bigger margin. Again, time will tell. The racing program starts in earnest in April in Perth (Australia) and continues through the northern summer in Europe. By the end of that, we should have a much better idea of what works and what doesn't. Again, watch the web page for updates.


  5. Steve Gray
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    Steve Gray Junior Member

    Water-ballasted multi-hulls

    Any views on the 1970s 'cool-tubes' idea used on 'Outward Leg', a trimaran? The amas and main hull had thru-tubes mounted along their length through which seawater flowed unhindered when submerged, but if the hull lifted the seawater was trapped long enough to weight it back down again...
  6. Doug Lord

    Doug Lord Guest


    Sounds like a lot of extra weight for the tube structure plus a lot of wetted surface. Do you know the history of the boat?
  7. Steve Gray
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    Steve Gray Junior Member

    Outward Leg was Tristan Jones' tri that was featured in, I think, three of his books. The tubes were about seven feet long and 9" in diameter. The design was by Leo Surtees (from Oz), so it couldn't be all bad!
  8. Doug Lord

    Doug Lord Guest


    Much shorter tubes than I imagined from your first post. Any word of how it did in competition?
  9. Steve Gray
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    Steve Gray Junior Member

    Cool tubes

    It didn't compete, but it apparently performed fairly well. Look at the pic:
    http://larryhaftl.com/images/SI07.jpg (and maybe the rest of the article). Looks like the tubes were only on the centre hull, not the amas/sponsons/whatevers... stv
  10. tspeer
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    tspeer Senior Member

    I always thought the cool tubes were bogus. The only make sense to me if you use inconsistent weight/buoyancy bookkeeping. It all depends on where you draw your control volume. In one case, the control volume goes along the skin, excluding both the weight of the water in the tube but also the buoyancy of the volume in the tube. In the other case, the tube is virtually capped, and both the tube water's weight and its buoyancy are included "inside" the boat.

    Jones included the weight of the water when the tube was in the air, but didn't include the weight of the water when the tube was submerged. But to do it right, you have to include both in a consistent way.

    I'm quite sure of one thing, though, and that is the cool tubes definitely added drag! Once he broke them off, he sailed just fine without them.
  11. CT 249
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    CT 249 Senior Member

    Can the ballasted catamaran carry more sail area than a proa? For an equivalent all-up weight isn't the proa beamier and therefore has comparable stability?

    There is a loss of energy in shunting perhaps, but there's also surely a loss of energy in dragging water around much of the time. If you dump the water you may encounter the fact that in very light boats there is also a loss of energy in tacking, as you have to turn into the breeze and waves and have very little momentum. That's why an 11' 28kg Moth can be out-tacked by a 17' C85 kg International Canoe in light airs; the Moth just stops as it goes through the tack.

    Cats are not the fastest thing to tack anyway; you have to slew two hulls 90 degrees through the water, and often re-start from a fairly low speed which is a problem with the little foils inflicted on many cruiser/racer cats.
  12. Skippy
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    Skippy Senior Member

    Tom, he didn't include the volume inside the leeward tube as flotation, did he? The windward tube is closed, forcing the ama to bear the weight of the water, whereas the leeward tube is open. I don't see a problem with it. If the leeward tube were closed, the weight of the water would be exactly canceled by the effective increase in submerged volume. The only difference I see is the drag of the hull or ama with a hole running through it, vs. the one with the hole filled and closed. If leeward water ballast is going to be below the surface anyway, then why not let it flow freely if that decreases drag?
  13. tspeer
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    tspeer Senior Member

    I'd be willing to bet the drag is less with the tubes capped and faired than with them open. Less wetted surface and people are always forgetting that a sailboat does not travel where it's pointed - it makes leeway. I've no data to support this, but my expectation is a torpedo-shaped body will have less drag at an angle of attack than a long open tube of the same diameter.
  14. Vulgivagus
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    Vulgivagus New Member

    I've read with interest all your views on water ballast to improve the capsize moment [is "improve" the right concept?] and leaving aside the mechanical equipment required [not feasible in an emengency without a crew/forward planning for transferring mass] it seems that the "cool tubes" on the ama's are in fact the "best" solution so far. I intend building my final boat - trimaran with "chinese junk" rigging and unstayed main and foremast and am seeking the most evolved form of water ballast/cool tube.
    I note that its' been some time since a message was last posted on this thread but I'm hoping for a positive response either here or to my email address. See ya!

  15. rwatson
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    rwatson Senior Member

    One thought I had - why is it necessary to pump water from one cat hull to another?

    Surely you would just dump it from the unwanted hull, and flood the windward hull? If there was any hull speed, simple venturies would work for both processes. If there wasnt any hull speed, you wouldnt need the ballast.

    Of course you could flood both hulls for a ballasted cat in the face of really bad weather.
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