Water Ballast in a Multihull

36 is a cat

Gday Watson

The Mac 36 was a cat - two hulls and could go on a trailer. Only one ever made it out to Oz and I only ever saw that as an ad. 36ft long and 4 ft wide hulls would seem very narrow.

Stopped production in the early eighties I think.

Cheers

Phil

 

That's a good point, but it occurs to me that when you need it, the boat is heavier with the water than it would be with the extra beam. This is, I think, because you require more weight with a shorter lever arm. If the extra beam adds a hundred pounds to the boat, how much more than a hundred pounds of water do you have to add to get the same effect at a narrower beam with less leverage?

Additionally, wider beam adds comfort and deck space to a boat.

In order to handle the stresses of heavy water tanks, scantlings must be heavier on the water-ballasted boat. There are safety concerns as well... what happens with a water-ballasted boat that gybes unexpectedly in strong winds. A water ballast system adds complexity to a boat and it's another system to maintain. And in order to handle the stresses of the extra weight when needed, the beams will have to be stouter and heavier than for a boat without water ballast.

Have there been any successful water-ballasted cats built for cruising? I'm not sure you could call the G32 a success, though it did very well in light-air racing. Someone's trying to sell one over in Pensacola, I think, that was rebuilt as a wide-beam cat.

 

One should probably observe the typical limits of racing cats at max beam with respect...

Orange II, which currently holds the crewed record for circumnavigation with a length of the hull: 36.8 m / 120.7 ft and a beam of... 18 m / 60 ft. Length

Clearly, within the limits of existing cat design, there is a length to beam ratio for big time, ocean going, high performance cats at a 2:1 ratio of length-to-beam in order to deal with all the conditions one must encounter along the way.

I see this as a good base line for design purposes.

If one wishes to exceed this ratio, in either direction, then perhaps a specific set of conditions are being called into play. Along with a specific set of experiences while sailing and another design process is appropriate.

There will always be the realities of too wide of a boat making for a tendency to pitchpole in excess of what one might consider "a norm". You can go wider, of course, but it would be prudent to recognize that wider boats will have their own specified tendencies.

From that perspective, perhaps water-ballasting scenarios would have a real benefit.

My take: the R33 and the G32, could have been much better boats had they been built with the observance that most sailors are not "racers" from the experiential, nor attitudinal perspective. The everyday rec. dude with a high perf. multihull is not going to be able to assume the issues present with a reduced beam, high perf. cat, without having to go through a decent sailing history with the boat, showing a reduced sail area until things are properly sorted. Perhaps the Gougeon's observed this tendency and made note of it in their presentation, I don't know.

Clearly, there is a sweet spot for beam and all the ballasting strategies available, before one settles on a length-to-beam ratio for a given application.

 

You'd only use the water ballast when there is a lot of wind though, so the extra weight isn't such a penalty. Also, wider boats need heavier scantlings too, due to their greater righting moment. And there is more boat, so more weight.

I'm not sure how you would define a successful cruising boat. The Oram "Mango" design had the option of water ballast. I know 2 of them were built with the tanks. It was a very simple system, there were no pumps, just some valves.

One of them raced quite successfully, and is now off cruising SE Asia.

 

Water Ballast

Here is the location of Tom Speers original comment: http://www.boatdesign.net/forums/boat-design/water-ballasted-multihulls-6504.html
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I also remember that several of the ORMA 60 tri's used water ballast in the main hull to help with pitch stability.
And here is Toms comment:
"The Spitfire hydrofoil catamaran uses water ballast. When I asked Mark Pivac why he went with water ballast instead of using hydrofoils to create all the righting moment (ala Rave), he said, "Why lift the same load twice?" And, of course, he makes perfect sense.

Water ballast isn't needed until there's enough heeling moment that the lee hull or foil is supporting the whole craft. After that point, one can use down-force on the windward hydrofoil or an equivalent amount of water ballast in the windward hull. If you elect to use water ballast, the lift on the lee hydrofoil has to increase by an equal amount, which will incur a drag penalty. If you elect to use hydrodynamic down-force, the lee hydrofoil still has to oppose the down-force and incur the same drag penalty. But in addition, the windward hydrofoil has its own drag penalty. Hence, Pivac's comment about lifting the same load twice - once for the leeward foil and once for the windward foil.

However, for a high-speed craft more sail area is not necessarily a good thing. So it may be better to back off on the heeling moment than to add weight so as to sustain a greater moment. For example, I have two different sized sails for my landyacht. Provided there's enough wind to move the yacht, the small sail is invariably faster than the large sail. It's only in the light winds that the large sail pays off.

I think the real question regarding water ballast for a multihull is, "Is the extra sail area worth it, or would a lighter boat with less sail area be faster?" In principle you can get any righting moment you want from a mulithull just by making the platform wider. But you have to think of stability in two-dimensional terms with a multihull instead of just roll stability. So when the beam is great enough, pitch or diagonal stability become the limiting factor. Of course, water ballast can be used aft as well as forward. But you're submerging the lee hull even more so you need reserve buoyancy to cover the water ballast, too. You get into a vicious circle that may not be buying you much additional performance.

The real problem with multihull stabilty is the downwind trap. And the most promising solution for that issue is new types of sails, not water ballast."
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Tom Speer

 

Water Ballast

I have been sailing my Raptor with the water ballast system I installed for a while now. It has worked very well to the point I removed my Bruce foil and hardware as I was not using it anymore when I went sailing. The roller furling sail combined with the water ballast lets me sail in any wind conditions in complete control. I also found out I was 1 -1.5 knots faster on a starboard tack using the water ballast versus using the foil.

As you can see by this video in 16 -18 MPH winds and my first time sailing the boat using the water ballast only it sails pretty well. I never had to lean as shown in the video but I was extra carefull that day as the water was very cold.

http://www.youtube.com/watch?v=gHR7mg5LBz0

Jan Gougeon's new boat also has water ballast and when properly designed work very well.

The Gougeon G32 won many races on the great lakes and I wish I would of bought it instead of my Corsair F24 as I probably would still have it today.

Overall I am very happy with the changes I did on my Raptor and it is one of the most comfortable boats I have ever sailed on.

 

aaaah - now I understand, first time I ever heard of it, Thanks