Catamaran science.^

[srm]

Its difficult to generalise: which design of monohull verses which catamaran? In either case are we considering cruising or racing designs and how are they being sailed?

I seem to remember reading that the early IOR rule vessels had a propensity to capsize as the rating rules penalised ballast in the proper place - as low as possible. Very many years ago YM published an article about stability. An irate reader wrote in (remember - letters) complaining that his new modern design boat had a worse stability rating than a much older design and build. He presumably felt that newer should equal better.

Likewise catamarans come in all sorts of hull shapes and sizes. A typical catamaran curve of static stability tends to show the angle of vanishing stability around 45 degrees. Yet I have experienced a cruising catamaran at 45 degrees with both hulls in the water and surfing sideways as a breaking wave passed under it. It had V shaped hulls and no boards or keel. I know the angle as it had a clinometer on the cockpit bulkhead (previous owner was a senior naval officer) that was right in front of me as I ducked and hung on expecting the wave to break over us.

Which brings up the difference between static stability, where one can draw nice graphs and calculate angles of vanishing stability, and dynamic stability which is what matters at sea and is difficult to calculate as the varying dynamic forces of wind and waves have to be considered.

Should the windward dagger board be down to act as drag and therefore stability leaving the leward dagger but up. Certainly in my opinion if the leeward dagger board was down with the windward dagger board up, then the cat will trip over its dagger board.

When I was actively interested in catamarans (I have owned two, both sedate cruising designs) I had the impression from a number of sources that it was good practice to have the leeward board up to prevent tripping.

 

[BurnitBlue]

The resistance to capsize is essentially the area beneath the portion of the heeling moment curve above the x axis. A quick look on google shows some graphs of both cats and monos, with the cat being a narrow pointy looking peak compared with the long undulating curve of a monohull. It seems that the angle of vanquishing stability of cats might well be less than 90 degrees, especially if the CoG is above the CoB when upright.

So I’m not convinced that a cat has more resistance to capsize than a monohull. Without real figures it’s hard to be definitive but I think I’d rather be on a monohull than a cat if the C word becomes imminent.

Yes i saw those graphs and it appeared that initially the cat had great resistance to capsize. My experience was that there comes a point with a lifted hull that is very sharp and over she goes. Getting to that point of no return must be quite difficult because not many big cat capsizes are reported. Charterers would die, the cat market would disappear.

There are definitely hydromatic ideas that can assist a hull not to lift. Angled dagger boards for instance acting like the foils on racing cars holding down the hull. Or, perhaps the reverse of the foil keels that raise the modern foil racers.

Developement has just begun and I suspect big changes will be seen soon.

 

[boomerangben]

Yes i saw those graphs and it appeared that initially the cat had great resistance to capsize. My experience was that there comes a point with a lifted hull that is very sharp and over she goes. Getting to that point of no return must be quite difficult because not many big cat capsizes are reported. Charterers would die, the cat market would disappear.

There are definitely hydromatic ideas that can assist a hull not to lift. Angled dagger boards for instance acting like the foils on racing cars holding down the hull. Or, perhaps the reverse of the foil keels that raise the modern foil racers.

Developement has just begun and I suspect big changes will be seen soon.

A catamaran has excellent initial stability but don’t let that be confused with resistance to capsize. And since capsize is really only a risk in a dynamic situation, very 3 dimensional situation where hydrodynamic appendages might well work against you, I would suggest that increasing the energy required to capsize a cat has to be the primary goal, for cruising cats anyway. What racers do might be very different

 

[boomerangben]

I'm following this with interest but isn't another factor that the centre of lateral resistance can move sideways on a cat, as one hull lifts and the opposite one digs in? I don't mean when flying a hull but just normal heeling, which of course is slight but not negligible in a cat. So rather than being more or less on the centreline of the vessel, the reaction to lateral forces imposed on the centreline by the rig moves to leeward as the wind increases, thus increasing the lever arm? Ultimately, of course, that can result in a cat "tripping" over the leeward hull.

I shoudl say that I am not a catamaran sailor, and this is thinking out loud!

I’m thinking out loud too!!

Is it tripping or hydrodynamic lift on the board adding to the capsize moment?

Moreover at higher angles of heel, the windage and waves acting on the underside of the bridge deck might make a substantial contribution to overturning moment

 

[srm]

Moreover at higher angles of heel, the windage and waves acting on the underside of the bridge deck might make a substantial contribution to overturning moment

Which was one of James Wharram's reasons for slatted bridge decks.

 

[Chiara’s slave]

Which was one of James Wharram's reasons for slatted bridge decks.

We like a well perforated trampoline.

 

[Gsailor]

Why will a motorboat or other monohull sailing vessel or otherwise capsize actually easier than a catamaran of the same beam. Or will it. A bit baffled by the science. Google has diagrams of the various types but no discussion with comparison using words instead of numbers.

This leads to optimum beam/length ratio against capsize or pitch-pole. Some cats are not far off having close to square shape where beam is not far short of the LOA.

I have just returned to catamaran sailing after my last monohull.

Oh boy - can of worms opened there. Having sailed both, I will read all of this thread before posting and even then I will think twice before advancing my thoughts.

 

[Gsailor]

Which was one of James Wharram's reasons for slatted bridge decks.

It would be a brave person to criticise his designs ( that he sells for cheap as chips) and his first tiny (very very basic) cat that he sailed across the pond with his two (TWO) girlfriends !

Then there is Richard Woods - another great designer AND sailor.

Could be quite an interesting discussion if everyone is polite.

Then there are a lot of modern you tubers who have 1000’s of miles under their keels in monohull sand catamarans...

 

[Chiara’s slave]

The odd thing with multis, counter intuitive to mono sailors. The features that give multis sparkling performance mostly make them safer. Obviously, having a racing rig and massive rotating wing mast is not on that feature list. Wharram got the rig right for cruising cats, though almost any other reasonably modest rig will do too. Theres another cat thread running, about Woods Striders. That’s a lightweight 24ft cat, as cheap as any boat, pretty much, that can sail to windward better than any other 24ft cruiser, do 12-15kn in flat water, not just surfing, sleeps 4, draws a foot, beachable, trailable, why it isn’t an everyday sight is beyond me.

 

[Bajansailor]

The Ramform Titan seismic survey vessel has a Length / Beam ratio of approx 1.5, and it is a monohull -
Ship RAMFORM TITAN (Research/Survey Vessel) Registered in Norway - Vessel details, Current position and Voyage information - IMO 9629885, MMSI 258070000, Call Sign LARX8

 

[AntarcticPilot]

The Ramform Titan seismic survey vessel has a Length / Beam ratio of approx 1.5, and it is a monohull -
Ship RAMFORM TITAN (Research/Survey Vessel) Registered in Norway - Vessel details, Current position and Voyage information - IMO 9629885, MMSI 258070000, Call Sign LARX8

Also see this: Russian monitor Novgorod - Wikipedia

 

[boomerangben]

The Ramform Titan seismic survey vessel has a Length / Beam ratio of approx 1.5, and it is a monohull -
Ship RAMFORM TITAN (Research/Survey Vessel) Registered in Norway - Vessel details, Current position and Voyage information - IMO 9629885, MMSI 258070000, Call Sign LARX8

And the sister, the Ramform Banff which was an FPSO used to leap around like a mad thing. Its sea keeping was appalling. Which probably says it was extremely stable

 

[srm]

Why will a motorboat or other monohull sailing vessel or otherwise capsize actually easier than a catamaran of the same beam.

Going right back to the original question:
According to tank testing results of scale models (Oceanography and Seamanship by Willian G VanDorn 1974)

"No vessel (tested) capsized or pitch-poled in single none breaking waves.
All vessels tested occasionally capsized or pitch-poled in single breaking waves when the wave height approximated their beam or length overall, respectively.
In capsizing a vessel was characteristically caught in the curl of the horizontally breaking vortex, becoming, as it were part and parcel of the breaker."
(All the models were monohulls).

So on the face of it likelyhood of capsize is a function of breaking wave height =/< than the beam of the vessel. So a catamaran should be less likely to capsize in a given sea state than a monohull of similar length. However, as we discussed hull form and sail configuration will also have an influence, especially on a catamaran.

In my post #21 my catamaran survived a breaking wave with a height that was probably near its beam by sliding sideways in the breaking water until the wave passed underneath. A monohull with deep keel or catamaran with board down may well have tripped over its keel. The keel/board would be held in dense water as the hull was subject to the impact of the breaking crest.

 

[BurnitBlue]

Going right back to the original question:
According to tank testing results of scale models (Oceanography and Seamanship by Willian G VanDorn 1974)

"No vessel (tested) capsized or pitch-poled in single none breaking waves.
All vessels tested occasionally capsized or pitch-poled in single breaking waves when the wave height approximated their beam or length overall, respectively.
In capsizing a vessel was characteristically caught in the curl of the horizontally breaking vortex, becoming, as it were part and parcel of the breaker."
(All the models were monohulls).

So on the face of it likelyhood of capsize is a function of breaking wave height =/< than the beam of the vessel. So a catamaran should be less likely to capsize in a given sea state than a monohull of similar length. However, as we discussed hull form and sail configuration will also have an influence, especially on a catamaran.

In my post #21 my catamaran survived a breaking wave with a height that was probably near its beam by sliding sideways in the breaking water until the wave passed underneath. A monohull with deep keel or catamaran with board down may well have tripped over its keel. The keel/board would be held in dense water as the hull was subject to the impact of the breaking crest.

That is very interesting. Breaking waves must be a factor in the dynamics. However, they can be easily overlooked like I did because even white horses are basically breaking waves so we kind of get used to them until they build up to dangerous size. In fact i wonder if the definition of dangerous being a dimesion dependant on the beam of the boat is accurate or a guess based on just tank testing. Every boat must have its very own dangerous breaking wave. So many variables like depth of water etc.Thanks for that info. Something to ponder on.

 

[srm]

Every boat must have its very own dangerous breaking wave. So many variables like depth of water etc.

So I made a Jordan Series Drogue for my small Prout, with the intention of running before anything that looked threatening. Fortunately have never put it to the test.

Its interesting that the designer of the Heavenly Twins catamarans, Pat Patterson, recommended lying a hull beam on and streaming warps in extreme conditions. That was until a HT capsized around 100 miles NE of Shetland while using this tactic.

 

[Chiara’s slave]

So I made a Jordan Series Drogue for my small Prout, with the intention of running before anything that looked threatening. Fortunately have never put it to the test.

Its interesting that the designer of the Heavenly Twins catamarans, Pat Patterson, recommended lying a hull beam on and streaming warps in extreme conditions. That was until a HT capsized around 100 miles NE of Shetland while using this tactic.

It’s accepted wisdom these days that running, or at least broad reaching, is the survival technique. We’ve done it on our boat, the racier the boat, the more helm input required. Steering through the waves. We hit just under 20kn with just a 3 reefed main.

 

[srm]

We hit just under 20kn with just a 3 reefed main.

Out of interest, what was your speed relative to the wave field?

 

[Chiara’s slave]

Out of interest, what was your speed relative to the wave field?

Substantially higher. It requires a lot of concentration, and maybe the Beach Boys on the sound system. If there had been any more wind, we’d have had to drop the main completely and show just a corner of the jib, and do it much more slowly.

 

[BurnitBlue]

Substantially higher. It requires a lot of concentration, and maybe the Beach Boys on the sound system. If there had been any more wind, we’d have had to drop the main completely and show just a corner of the jib, and do it much more slowly.

I have heard that running fast downwind could keep the catamaran inside the storm system longer than if it was allowed to pass on by. That, i believe, depends on the quadrant the boat is in.and the direction of the storm system.

Without weather forcasts it may be difficult to work out speed tactics and whether to angle to port or starboard during the run.

 

[Chiara’s slave]

I have heard that running fast downwind could keep the catamaran inside the storm system longer than if it was allowed to pass on by. That, i believe, depends on the quadrant the boat is in.and the direction of the storm system.

Without weather forcasts it may be difficult to work out speed tactics and whether to angle to port or starboard during the run.

Luckily we all have weather forecasts now. Otherwise, you could indeed prolong your stay in the weather system by being on the wrong gybe. Gybe prevention is the other reason not to dead run of course.