The Pitchpoling Myth

[Richard Woods]

I read it everywhere: If you make a catamaran wider it will pitch pole

Now where is the evidence to back that up? It seems to me that this is
something someone wrote once years ago and since then everyone has just
blindly repeated the dogma.

Probably they do so because at first sight it sounds logical. If a catamaran
is made wider it becomes more stable sideways. Thus proportionately it must
become less stable fore and aft, all other factors being equal.

But A) is that true? B) how many (besides me) have tried making a catamaran
wider to see what happened?

The original writer was, I suspect, a promoter of early narrow English boats
(like Prout and Sailcraft) worried about newer, wider designs. So it is
ironic that one of the first pitchpoles was of a very narrow, low freeboard
Prout 27 in Germany.

In practice catamarans tend to capsize diagonally, not cartwheel end over
end. Indeed if they did go end over end then obviously the hull spacing would be irrelevant.

So it is the diagonal distance from windward stern to lee bow that is
important. Clearly then, as a boat is made wider this distance increases and
so it becomes more stable overall.

My 24ft Strider design has a 22ft WL and normal hull CL spacing of 10.6ft
giving an overall beam of about 14ft (so when it was designed over 25 years
ago it was considered wide). In 1986 I built an experimental Strider with a 14ft CL spacing.

Compared to it's WL length that is wide! In fact it looks scarily so on paper, still wide in the boatyard but looks great on the water.

A number of these extra wide versions have been built since then. None have
pitchpoled or capsized. Indeed I have always thought that these wider boats
sailed better and were more stable than the narrower ones.

I guess if there was any truth in the rumour that wide boats pitchpole then
catamarans would gradually be getting narrower. Instead they are getting
wider. Even the last generation of Prouts were wider than earlier versions.

So I say it again.

It is a myth to say that just making a catamaran wider means it will pitchpole.
There are many more important factors that determine whether a boat will
pitchpole or not than just the hull spacing.

Any sensible (ie constructive) comments are welcome!

Richard Woods of Woods Designs

www.sailingcatamarans.com

[Crag Cay]

It seems to me that this is something someone wrote once years ago and since then everyone has just blindly repeated the dogma.
As this is the origin of so much 'wisdom' within sailing, I can't see any reason why it wouldn't be the source of the pitchpoling theory.

It would seem the force couple that causes a diagonal pitch pole wouldn't be hard to replicate with a tethered cat. I would be amazed if a wider boat was easier to upset.

[RHough]

I read it everywhere: If you make a catamaran wider it will pitch pole

Now where is the evidence to back that up? It seems to me that this is
something someone wrote once years ago and since then everyone has just
blindly repeated the dogma.

Probably they do so because at first sight it sounds logical. If a catamaran
is made wider it becomes more stable sideways. Thus proportionately it must
become less stable fore and aft, all other factors being equal.



Just making it wider won't cause it to pitchpole.

Just shooting from the hip, it is the ratio of force needed to pitch pole (or as you more correctly state capsize diagonally forward) to the force needed to capsize to leeward. The transverse RM is limited by width. If the size of the rig is increased to take advantage of the higher RM the available force is closer to the force needed to sail the leeward bow under and trip the boat.

A 8x20 boat has a 8 ft righting arm, and a 21.5 ft diag arm. About a 2.6:1 safety margin.

A 16x20 boat has a 16 ft righting arm and a 25.6 ft diag arm. A 1.6:1 safety margin.

If the rig remains the same you get a 20% increase in pitchpole resistance, and double the lateral capsize resistance.

However, the extra separation also doubles the rotational inertia around the lee bow. The 20% increase in the diagonal arm does not compensate for the added inertia.

If you go ahead and put a bigger rig on the boat, you have a combination of a smaller safety margin and have done nothing to compensate for the added inertia. The boat will be more likely to pitchpole with the greater hull spacing.

The big offshore tris are close to square, the lack of longitudinal stability and the increased chance of pitch pole are limiting factors when the rotational inertia is considered.

Going from 10.6 x 22 to 14 x 22 is not so radical. The righting arm is 32% greater, the diag in 6% greater, the ratio of righting arm to daig arm is 20% lower. If the rig is not large enough to make lateral capsize a concern, the simple change in beam should not make pitchpole a concern. If the rig was made bigger to use the 32% extra RM, the leeward bow would get buried more easily and pitchpole becomes more likely.

Does that make sense?

[terhohalme]

You are right. It is a myth. Making catamaran wider doesn't make her more sentitive to pich pooling, if keeping the sail area as it was.

[RHough]

You are right. It is a myth. Making catamaran wider doesn't make her more sentitive to pich pooling, if keeping the sail area as it was.

Which begs the question, if the rig is the right size for the beam and RM, what is gained by increasing the beam? Greater pod/deck/tramp area?

[terhohalme]

Yep, the world is relative.

I'd like to think, the rig must be the right size for both the transversal RM and the longitudinal RM. Perhaps having the transversal and longitudinal inertia (second moment of waterplane) almost the same? This leads to LWL/BCB = 2, about. (LWL = length waterline, BCB = beam between centerlines or bouyancies)

[Richard Woods]

Which begs the question, if the rig is the right size for the beam and RM, what is gained by increasing the beam? Greater pod/deck/tramp area?

Also a big reduction in drag from wave interference between the hulls. Also less slamming under the bridgedeck.

The wide Strider is 24ft long, 22ft on WL and 17ft overall beam. So it is wider than many (older) trimarans. Wider than the original Telstar 26 for example.

Richard Woods

[Alan M.]

So I say it again.

It is a myth to say that just making a catamaran wider means it will pitchpole.
There are many more important factors that determine whether a boat will
pitchpole or not than just the hull spacing.

Any sensible (ie constructive) comments are welcome!

Richard Woods of Woods Designs

www.sailingcatamarans.com

If you only make the cat wider, and don't change anything else, it is no more likely to pitchpole. But if you use the extra righting moment and specify a bigger rig, (which many designers do) that's a different story.

Probably a better way of looking at it is, for any given beam, a longer boat is less likely to pitchpole.

[bill broome]

surely bow configuration plays a role? a spoon bow like a surf ski would tend to skid out from a pitch pole. this question is just an extreme form of the 'diving' question that is inherent in multihull design.

if a wider boat has a bigger rig, it has a bigger driving force tending to bury the lee bow. so one must balance the various objectives in the usual way that makes boat design interesting, and frustrating.

[RHough]

Also a big reduction in drag from wave interference between the hulls. Also less slamming under the bridgedeck.

The wide Strider is 24ft long, 22ft on WL and 17ft overall beam. So it is wider than many (older) trimarans. Wider than the original Telstar 26 for example.

Richard Woods

I notice that the Strider Turbo has the wider beam and also an increase in sail area from 25 m^2 to 34 m^2 ... did the height of the CE change also? That is a 36% increase in area, and only a 32% increase in RM, correct? If the CE height increased also, then the heeling moment increased more than the 36% greater sail area.

How does the boat handle with the 34 m^2 rig on the narrow beam? ;) What windspeed does it take to get a hull up? That would reduce wave interaction to zero. ;)

I would think that a boat that does not fly a hull regularly also is a low pitchpole risk.

[Petros]

Richard,

I think you are correct, depending on what of the many variables you are changing. The first time I ran across this idea was on some NA's web site explaining that in their new design high-speed cat it had the "ideal" ratio of length to beam of 2 to 1 (40 foot long to 20 foot wide). I could not figure out what they were talking about, it did not make any sense there was any such thing.

I went over in my mind all the variables and could not see any inherent advantage to this "ideal" 2:1 L/B. I figured it was just marketing of their new design. Later I have seen it explained elsewhere on this forum that to take advantage of the wider beam you would want a larger sail, but then the forward driving force tends to push the lee bow into the water causing a pitch pole. But this would vary widely with both rig design (how high the aerodynamic center is on the sail plan), the L/D ratio of the rig and the bow and forward hull design (resistance to diving). Most racing cats have really narrow bows, but ISTM a clever designer could put a lot of flair in the bows so the water normally sees a narrow bow, but as pushed down it develop more righting moment to resist pitch poling when pushed hard.

Merely making the beam wider will increase stablity, but it will also add weight. If you do not need it any wider for the given sail plan, why add the extra width? IT seems to me that it just comes down the the amount of safe margin you are willing to tolerant.

I do not think there is any such thing as an "ideal" length to beam ratio, it just depends on what the designers intent is and what kind of trade offs the designer is willing to make. And I am sure there are as many opions about that as there are designers out there.

[terhohalme]

Richard
Is there any data of the accident of Prout 27 or the boat in internet? I found very lite data of Prout 27.

[SheetWise]

I read it everywhere: If you make a catamaran wider it will pitch pole

This does make sense (intuitively) if you take it to extremes -- for example, if beam was 3x or 4x the length.

I mention this only as an exercise -- at what point do you believe that beam to length has exceeded safe limits? Is there a maximum? Do you believe there's an optimum ratio?

Curious minds want to know ...

[Richard Woods]

The Prout 27 was the Haxted Argo (spelling?) running into the Elbe against the tide in the mid 1960's.

Very well reported at the time and also for the next 20 years.

Try the AYRS publications for more details, or any pre 1980 book on cruising catamarans.

Richard Woods

www.sailingcatamarans.com

[catsketcher]

Hello all

I think the case is as Rhough said, wider boats usually get more sail. However it is pretty important not to make the boat too wide because of feedback to the sailors.

I had a Twiggy tri in my 20s which was almost square. It had a pretty powerful rig for a 31 footer- especially with its thin bows. Working, reaching or broad reaching it was very stable and I would be very happy with its ability to carry sail. If I was very powered up in these points of sail and then went square then she would go very bow down.

I once got hit by a squall in Sydney Harbour. I was on my own and within a minute I went from having fun to - Oh my God - type sailing. My dinghy and mono background had me bear away to a square while I tried to work out what to do. The bow depressed so far as to scoop up every chop and my deck was almost green with water. I was very concerned and realised pretty quickly that this wasn't working. When I rounded up the load went onto the float and the bows popped up and off she went happily.

In my view it is important that a mulit have LESS sideways stability than fore and aft and diagonal. This is very tricky to work out as real fore and aft stability is a product of both lever arms and the drag of an immersed bow. However I would not like to sail on a boat that had an assymetric stability profile.

The biggest reason is that sailors can do something about easing sheets and reducing sail when the multi lifts a hull upwind or reaching. Downwind - when the bow digs in all they can do is hope they didn't put up too much sail and cross their fingers. So don't go too wide - what that is is up to you but I think your boat should be less stable reaching and working than on a square run.

cheers

Phil Thompson

[Spiv]

I had the same problem that Phil had when I had a Nacra 16 Square.
I could always control it on a beat or a reach, but as the wind freshened past 20/23kn (as it always does here off Fremantle), there was nothing I could do and it ended up in a pitchpole and had to be rescued.
I ended up not racing in fresh winds as I could not even jibe even though I could always tack.

However here we must stop and make a distinction between overpowered cats and well powered ones.
My 12.5m cruising cat was meant to be 6.5m wide, but I had the design re-engineered and widened it to 7.5m; however, I did not increase the sail area.
Since this boat had a conservative sail area, I never felt in danger of pitchpoling.

[ropf]

If you increase the side force the forward force also increases. This is what you want to go faster. So if you have a wider beam and one hull is flying then not only the other hull sinks but the lee bow sinks even more.

Now we have some kind of dynamic trap. There is a point where more pitch makes higher drag and the hull is braking - while the rigs moment of inertia of drops the lee bow down further - braking more - dropping more - flatch!

A hullshape with very fine bows and wide flat sterns favors this behavior. While there shoud be a proper ratio of stability to each direction it can't be expressed alone by beam/length ratio.

regards ropf

[sandy daugherty]

Mr. Woods is right again. What has been left unsaid by the wide-beam nay-sayers is that no boat would be built wider without increasing the mast height. Pitchpoling is a beachcat phenomena, and cruising cats have a LONG way to go to reach those proportions. I contend that any cruising cat that pitchpoled just happened to be pointing in that direction when overwhelming rollover conditions arrived. They would have gone over backward if they had been pointing the other way, and so would any other tall floating object under a certain size.

[BHOFM]

This is a bit OT, but I saw some kids beach a Hobie on a
down wind and when it hit the beach, it went right over
on its nose!
I think you call that,, STUPID?

[yipster]

mitchlett dont give pitchpoling but is a hulls and spacing program to check

[Alan M.]

Also a big reduction in drag from wave interference between the hulls. Also less slamming under the bridgedeck.

The wide Strider is 24ft long, 22ft on WL and 17ft overall beam. So it is wider than many (older) trimarans. Wider than the original Telstar 26 for example.

Richard Woods

My understanding is that going wider would cause MORE slamming or slapping under the bridgedeck, if clearance isn't increased. Not from the bow waves, which are virtually non-existant anyway, but from just plain wind waves.

From what I've seen, the boat's own waves don't really meet until somewhere behind the boat, and these are mainly stern waves.

http://www.youtube.com/watch?v=xg0ar4WFkA4

[masalai]

I am a little confused here, I am sure some of the "European High Performance" racing cats are close to LOA = BOA? - - - - - In smaller stuff I have PUSHED HARD, (various "beach" cats), a risk of pitch-polling is present when the leeward bow gets driven hard into the water..... Good buoyancy in the bow sections reduces the risk of calamity, as does judicious sail handling.... and counter-balancing by the crew....

[catsketcher]

Hello all

I am interested in Masalai's point about the Lake Garda cats being almost square. The Physics part of me has me thinking of vectors from the CE of the rig. In a very fast cat, especially on a lake, you would tend to sail angles downhill like a skiff or beach cat.

The vector from the rig in these sailing conditions is going to be 45 degrees forward or thereabouts. Furthermore th apparent is going to be coming from 90degrees. This allows the crew to determine rig loading by altering sail trim, easing sheets and the like. In skiffs the helm steers all over the place keeping the hull under the rig. I see F18s doing the same thing. It places all the vector of the rig over one bow but a confident skipper can work the height of the lee bow with helm and by jib sheet tension. Hobie 16 crews need to learn how to do this quickly - lose the bow - ease sheet. All within the realms of the proficient crew.

Off shore and in big cats I see a different problem. It is very tricky to steer across big waves and so you tend to steer down them in a squall or when conditions get up. This brings the boat to a square run. Easing sheets if the bows go down will do little in these situations to help raise the bows. Rob Denney will smugly weathercock his rig but those with large normal rigs can do nothing when the bows go down but pray. A flare gun to the kite may help.

Tripping along close to the edge is safer reaching than running as the crew are in control. If the bows dig in when square then stability is totally dependent on boat design - low drag bows when immersed (not user friendly), high flotation (still keeping low drag) reasonably modest sterns and low CG will help.

I did ask Nigel Irens about this when I interviewed him about B and Q for an aussie mag. (It was a great excuse to talk to an idol) I asked him if there was a magic number he used for diagonal and fore and aft stability. He said that as the drag of the bow has such a huge impact on pitchpoling that there were no magic numbers or ratios.

So in my humble opinion I would still worry about taking a square boat offshore. Cats don't make you behave well like a mono does. Even a tri tells you it needs less sail by depressing a float. I like Richards boats but I wouldn't like a square cat as it wouldn't tell me it needed less sail until it was on a square run and that is too late.

A forum member said that he hadn't heard of cats pitchpoling. We have had a few. Pumpkin Eater went over its bows in the early 80s and it was pretty narrow. It had a big kite up and had a large wave at the same time that caused it to flip. Gary Martin flipped his Grainger Tusk in another Gladstone pretty soon after the start over the fine bows. Both boats had lots of sail up and other boats close by that didn't flip. A lightwave 38 flipped on a bar over its bows too. The design has pretty wide sterns too. If you go up to the QMYC and ask for pitchpoling stories you will hear of more.

cheers

Phil Thompson

[masalai]

catsketcher, you sound like a mono sailer?, cat sailing demands you FORGET all your mono sailing habits and guidelines. - - How can I define the differences - something like writing with a quill and doing a cut&paste on a computer - if you know what you are doing, in a cat you can go BLOODY FAST in a cat consistently and for long distances. - - - One slight lapse of attention and you are in one huge mess....?

[Doug Lord]

I hope that this adds a bit to the discussion on pitchpoling cats. The Stealth F16-one of the first production F 16's uses t-foil rudders and they say it allows the boat to be pushed much harder than w/o them.
http://www.formula16.org/content/view/26/48/lang,en/

[tspeer]

To the notion that cats don't pitchpole, may I offer the counter-example of Catsass in the 2006 Swiftsure race (www.pacificfog.net/photographs/)? (For once there was a photographer in the right place at the right time!)

John Shuttleworth has some stability indices (http://www.john-shuttleworth.com/Articles/NESTalk.html)for diagonal capsize as well as pitchpoling and sideways capsize.

[Alan M.]

Did someone suggest that cats don't pitchpole? I must have missed that.

[catsketcher]

Thanks Tom for the link to John Shuttleworth's page. In his Q and A section he states that he likes the boat to have greater diagonal stability than transverse. He works this out using hydrostatics fomr the computer and adjusts hull volume and distribution to suit.

This is probably an argument about semantics more than anything. I think that the way a square Garda cat is sailed would be similar to a skiff or Tornado. It would be tacked downhill. This gets the drive vector at about 45 degrees off the bow or so. In small cats with the apparent exactly on the beam(which is where you put in tacking downwind) you can alter trim to keep the bow above water level. In small cats you ease the jib to let the bow lift - crews constantly trim the jib - on to go fast ease when the bow gets too deep.

Out on the ocean you can't do this so I think the boat has to be less than square. The vectors on a square run are stright forward and there is no way to rasie the bow of an overpressed boat by sail trim. This means the boat must have extra stability forward than sideways - narrower than square.

As toi Masalai's point that you should foget everything for multis I must disagree. In bloody big waves the last thing I want to do is to take them at an angle. Straight down and trust in high bows is my motto. Getting close to a broach with one rudder in the air is the last thing I want.

Cheers

Phil Thompson

[Richard Atkin]

Richard Woods says that widening the hull spacing reduces bridgedeck slamming. But I too have read the opposite. I have read that a cat with a length/beam ratio of 50% will suffer less from waves building under the bridgedeck. Who to believe?

Boats have been around for a long time. I think the science needs to be clarified with proper controlled tests, without human nature getting in the way!! Some sailors will sail more safely when the boat feels more vulnerable, and will make claims that a certain design is good...or bad....based on personal opinion.

Do we need to build gigantic wave machines and wind machines to find the real answers?

[masalai]

I feel that "science" as in mathematics and engineering theory, must still be tempered by practical experience & common sense. Apply all the knowledge and information available simultaneously to come up with a "compromise" that one feels will meet the current needs....

Those who rely solely on the constraints of a narrow discipline will find failure when exposed to external variabilities of the "real world"....

[Richard Woods]

No doubt you have read my comments on bridgedeck slamming on my FAQ's page. But if you haven't here is a brief extract.

"Much of this slamming is self inflicted. Imagine two hulls close together pitching into a wave. The water they displace has to go somewhere, and it piles up just as the bridgedeck sails over it. Clearly a wider hull spacing will turn a narrow high peaked mountain of displaced water into a low flat molehill. A wide knuckle and flared hull will also help reduce the size of the induced wave. "

Doesn't that make sense?? Who is it who says that a narrower spacing reduces slamming?

I guess I need to sail catamarans a bit more, after all I've only sailed them 60,000 miles in the last 30 years. Then I'll be in a better position to judge who is right and who is wrong.

However this thread isn't about bridgedeck slamming, but about pitchpoling. So I'll leave it here for now

Richard Woods of Woods Designs

www.sailingcatamarans.com

[Richard Atkin]

Richard, I have read that wider hull spacing increases the chances of slamming from several different websites...but I can't find or remember those sites now....dammit. I just Googled the term 'bridgedeck slamming'...but now I get different results. I won't go on about it. As you say, this is about pitchpoling.

From your experience, do you accept the main argument presented in this thread ie. greater hull spacing increases the risk of pitchpoling if you increase the sail area to match the greater righting moment?

[Richard Woods]

I have been waiting to see how the discussion pans out before making another post. I hope to write it and post it before I fly to the UK on Tuesday

But for now something to think about

If you make a boat SHORTER but keep the beam and sail area the same, how does its stability change?

Richard Woods of Woods Designs

www.sailingcatamarans.com

[RHough]

No doubt you have read my comments on bridgedeck slamming on my FAQ's page. But if you haven't here is a brief extract.

"Much of this slamming is self inflicted. Imagine two hulls close together pitching into a wave. The water they displace has to go somewhere, and it piles up just as the bridgedeck sails over it. Clearly a wider hull spacing will turn a narrow high peaked mountain of displaced water into a low flat molehill. A wide knuckle and flared hull will also help reduce the size of the induced wave. "

Doesn't that make sense?? Who is it who says that a narrower spacing reduces slamming?

I guess I need to sail catamarans a bit more, after all I've only sailed them 60,000 miles in the last 30 years. Then I'll be in a better position to judge who is right and who is wrong.

However this thread isn't about bridgedeck slamming, but about pitchpoling. So I'll leave it here for now

Richard Woods of Woods Designs

www.sailingcatamarans.com

Sarcasm will kill the discussion. :P

I've not sailed 60,000 miles in catamarans so I am no longer qualified to offer an opinion.

Fine, we can take Woods Designs of the short list.

It is very obvious that bridge height not hull spacing is the major factor in slamming.

Cheers

[sail.scow]

I feel that "science" as in mathematics and engineering theory, must still be tempered by practical experience & common sense.
I read some where that science is founded on observation. The language of science gives a common tool to share our observations. Like for example Ohms law. Georg Ohm observed something previously unreported, and his name becomes attributed to the effect.

Thinking about sailing cats pitch poling.
Normally it is one bow going under first. If both bows go under at the same time, then a mono hull would bury its bow also. But that is very rare. The force on both vessels would have to be close to the centreline. (Like running in large seas, but I want to use the effects of the rig.)
So if this force is acting off the centreline, that gives us the broach in a mono hull. But cats aren't known for broaching. They bury the lee bow instead, and 'pitch pole'.
If I imagine the lee hull of a cat trying to broach in the same manner as a mono hull, forces are working against it. Drive through the CP imparts the pitching moment around the CB. The CB moves forward to equalise it, until it runs out of buoyancy in the bows. The beginning of the end.Beam and pitch poling is linked.

Increasing the beam moves the CP to leeward, while at the same time CB moves farther to windward (when sailing). The combined forces of drive and heel are originating from a point that is leeward and above the CB.
This pitches the bow down.
Once the combined pitching moment has crossed to leeward of the hull in the water, the boat can't 'round up'.

Is that a reasonable take on it?

[masalai]

Make a pair of models and see how they compare? - I don't know conclusively?

[Bruce Woods]

No doubt you have read my comments on bridgedeck slamming on my FAQ's page. But if you haven't here is a brief extract.

"Much of this slamming is self inflicted. Imagine two hulls close together pitching into a wave. The water they displace has to go somewhere, and it piles up just as the bridgedeck sails over it. Clearly a wider hull spacing will turn a narrow high peaked mountain of displaced water into a low flat molehill. A wide knuckle and flared hull will also help reduce the size of the induced wave. "

Doesn't that make sense?? Who is it who says that a narrower spacing reduces slamming?

I guess I need to sail catamarans a bit more, after all I've only sailed them 60,000 miles in the last 30 years. Then I'll be in a better position to judge who is right and who is wrong.

However this thread isn't about bridgedeck slamming, but about pitchpoling. So I'll leave it here for now

Richard Woods of Woods Designs

www.sailingcatamarans.com



Well Richard, most of this is simplistic crap. Fine for flat water, but add waves and what happens?. I'll help. As the hull spacing widens, for the worst case scenario of beam on seas ,the boats bridge deck clearancs must increase to prevent contact (at speed it becomes slamming). Otherwise a boat with a bridgedeck clearance of say 600 mil's is going to be limited to opperating in waves of less than this height if wide enough, or it'll get a pounding.

Hence we see the ratio of tunnel width to wing clearance expressed as around 18%.

[Richard Atkin]

Sarcasm will kill the discussion. :P

I've not sailed 60,000 miles in catamarans so I am no longer qualified to offer an opinion.

Fine, we can take Woods Designs of the short list.

It is very obvious that bridge height not hull spacing is the major factor in slamming.

Cheers

I enjoy Richard Woods' harmless sarcasm.

[sail.scow]

If you make a boat SHORTER but keep the beam and sail area the same, how does its stability change?
I missed this page when reading this thread, and this is an excellent question.

Increasing beam or reducing length causes the same effect.
+beam=combined drive and heel moving towards the centreline.
-length=combined drive and heel moving towards the bow.

How about using a Hobie 16'?
Double the beam and halve the length.
The biggest problem is pitching. Heel is not an issue.

Regarding increased sail area and it's relationship to increased beam.

Higher aspect ratio means CP higher above deck, and more area gives more force. More force on a longer lever.
Lower aspect ratio means CP further from the CL, closer to the windward hull, a good thing. And more force but on a shorter lever.
So increasing beam and sail area but lowering aspect ratio would still increase pitching moment because of the increase of total force through the CP. But lowering the height of the CP by reducing aspect ratio would contribute to longitudinal stability... and the increased beam would be handy while reaching. So we lose something and gain something...

Depending on what the requirements of the cat are, displacement is a handy thing to throw into the mix.

[Richard Atkin]

Everyone seems to be so sure of themselves.

hmmmmmm......

I guess the best person to believe would be the one who has the most experience at pitchpoling on a wide variety of cats.:D:D

What's that?? Nobody has much experience with that? OK...back to the giant wave machine testing laboratory.


Boeing put wing tips on their 747. The highly experienced pilots complained about the change in the plane's handling. The geeky engineers with no flying experience left the pilots with two options: shut up or get another job. The computer geeks won the war.

Unfortunately computer predictions for pitchpoling don't seem to be good enough. I think we need a World War 3 to push naval technology a little faster. We need Richard Woods (the equivalent to a very experienced Boeing pilot) to sail a variety of boats (including his own designs) in a man-made ocean inside a giant dome.

We can then complete our understanding of sailboats the way we did with aeroplanes about 50 years ago.

[RHough]

I enjoy Richard Woods' harmless sarcasm.

That makes one of us. ;)

Richard always includes a link to his business, he portrays himself as a professional. He has boats to sell.

Would YOU buy from him? I don't tolerate smart assed sarcasm from people that are trying to sell me something, perhaps your standards are different?

If wanted to state the obvious, I'd say that if his Strider Design is better at wider hull spacing, and that both drag and bridgedeck slamming are reduced in the wider version, that he missed with the first design. :)

John Shuttleworth (http://www.john-shuttleworth.com/default.html)on stability:

Question.
Do you calculate the diagonal stability for your designs, and what do you give to boat owners to inform them about the stability of their boat and when to reduce sail?

Answer.
I do these calculations for my designs. If you look at Fig 1 in my article on Multihull Design Considerations for Seaworthiness, the diagram shows how the diagonal stability is calculated from the hull form. I ensure that the diagonal stability is higher than the sideways stability in this way. I can do this by adjusting bow volume at the design stage.

I define diagonal stability as the point when the whole displacement is taken by one hull with the deck at the point of burying. The downwind stability is the point when both hulls are taking the whole displacement of the boat, with the bows about to bury.

In a heavier cruising design like the Aerorig 52 the diagonal stability will be in the order of 15 % higher than the sideways stability, and the directly downwind stability will be 23 % higher than the sideways stability.

In a cruiser racer like the Shuttle 40, where the displacement is lower, and the boat is likely to be sailed closer to the stability limit, the diagonal stability is 38% higher than the sideways stability and the downwind stability is 28% higher than the sideways stability.

These calculations are done for when the bow is about to go under. If the bow does go under, then the hull drag goes up, and there is a component of downforce on the deck right at the bow. This means that the stability will decrease much more sharply when the bow buries on the diagonal and downwind, than for the sideways stability where bow burying is not a problem. So the 15% and 23% extra stability represent a reasonable reserve to account for the bow burying effect.

Therefore I give the sideways stability figures for various sail combinations to my clients, and I know that that includes a safety factor for diagonal and downwind sailing, to account for bow burying. I also give a guide to how much that stability should be reduced as the wind strength increases and the effect of waves is taken into account.

Here a desgner gives some good numbers. Note that in the higher performance designs the safety margin is higher than for a cruiser.

More good information from the same designer.
Multihull Design Considerations for Seaworthiness.
By John Shuttleworth. (http://www.john-shuttleworth.com/Articles/NESTalk.html#Ref2)

[RHough]

Boeing put wing tips on their 747. The highly experienced pilots complained about the change in the plane's handling. The geeky engineers with no flying experience left the pilots with two options: shut up or get another job. The computer geeks won the war.


Maybe things like fuel economy are more important that how the hired help feels about driving? :)

Since when does how a bus handles dictate design?

747's are buses. They burn something like 1,900 gallons an hour. If the new wing makes it handle differently but saves $400 per hour in operating costs, so what? Maybe the poor pilots are willing to take a $400 per hour pay cut to get the handling back? :D

Damn straight the geeks won.

It *is* pretty simple ...

A capsize (in any direction) happens when applied force exceeds resistance to that force. A capsize forward or pitchpole happens when the drag goes up (with speed) and the force driving force combine to create a bow down pitching moment that exceeds the longitudinal righting moment. If you decrease the bow down moment or increase the righting moment the speed that the boat becomes unstable is higher. If you reduce the bow down pitch moment to zero, there is no limiting speed.

The idea that a modern Cat will just pitchpole or cartwheel or capsize without warning is absurd.

The way to reduce the risk of these events is to put better sailors in control.

Just like your precious pilots need to know the limits of the aircraft they fly, sailors need to know the limits of the boats they sail. Pilots can put aeroplanes into attitudes that the aeroplane cannot recover from, in some cases their senses are not good enough to prevent it, they have to rely on instruments that some geek built to save their sorry asses. ;) There are warning systems and "Do Not Exceed" limits for airspeed and G loading. There are loading and stability tables to help prevent bubba from putting all the engine blocks in the tail and the cheese puffs under the wing ... on and on it goes.

Most of the people I've talked to feel that sailing vessel design is every bit as challenging if not more challenging that aircraft design. The situation we have is more complex machines being designed and sailed by less qualified people.

The safety warning system on a sailboat is how brown your shorts are if you manage to survive your mistake ... or the mistake of some backyard designer with a pot of glue, a stack of lumber, and a dream. (I am NOT implying that RW is one of these).

It should be simple enough to calculate a safe "Do Not Exceed" speed for a catamaran design on various points of sail. This would be placed next to the speedometer, just as the deviation card is posted by the compass.

On the other hand, people had figured out how to build boats that could sail across oceans and sail up wind long before Mr. Bernoulli had any principles. :)

They just didn't do it very well or very safely.

[Richard Atkin]

Great post RHough :D

Just for the record, I think Richard Woods' trailable Wizard is an awesome design. (nothing to do with pitchpoling....sorry)

[RHough]

Great post RHough :D

Just for the record, I think Richard Woods' trailable Wizard is an awesome design. (nothing to do with pitchpoling....sorry)

Thank you.

If you want to prove that yacht design is harder than aircraft design, just hand someone a sheet of paper and give them these choices; build an aeroplane or a boat that can sail upwind. Any idiot can build a paper aeroplane ... :D

[sail.scow]

A capsize (in any direction) happens when applied force exceeds resistance to that force. A capsize forward or pitchpole happens when the drag goes up (with speed) and the force driving force combine to create a bow down pitching moment that exceeds the longitudinal righting moment. If you decrease the bow down moment or increase the righting moment the speed that the boat becomes unstable is higher. If you reduce the bow down pitch moment to zero, there is no limiting speed.

The idea that a modern Cat will just pitchpole or cartwheel or capsize without warning is absurd.
So the interesting thing is blending those forces into a vessel that can meet different criteria.

A large light weight racer needs the beam for stability, because it is so light. Resistance of hulls can be low because planing is normally the priority of the design. Pitch resistance is secondary to speed. Long full bows don't make much sense for speed, only safety. The helm is the first defence against pitch poling.

A large displacement cruiser needs the sail area to get it moving because it is so heavy. Hulls can be closer together and still provide adequate resistance to heel. The cruiser is not too worried about pitching because it is unlikely to raise a hull, therefore both bows can be drawn to cope with the bury. A cruiser would be vulnerable to sea state rather than aerodynamic forces.

Beach cats are really half proa half cat. Hiking out and flying a hull on each tack is rather proa. Planing under a spinnaker that generates lift is the neat solution to speed and pitching during a run.

Probably the most finnicky thing to draw would be a small cruising cat. They normally can carry enough sail to lift a hull or bury a lee bow. Their displacement is normally not enough to prevent either. They have the risks of a lightweight racer and the performance of a displacement cruiser. Who would draw one of these and reduce the sail plan for safety's sake? Performance is the main drawcard for cat people, followed by comfort.(Delicate speed freaks...?)

Modern cats are various, and so are the rationalisations to pitch poling for each of them. The right tool for the right job, and the crew being prepared for the particular design characteristics of their vessel. Cruisers can cleat the sheets and read a book in the saloon, whereas the racer must clutch the sheets for their watch.
There are warning systems and "Do Not Exceed" limits for airspeed and G loading. There are loading and stability tables to help prevent bubba from putting all the engine blocks in the tail and the cheese puffs under the wing ... on and on it goes.
Lol, geeks might draw the thing, but there is always someone who 'knows what the plane can take'.

[ropf]

Hello,

in my mind the main topic is interesting enough to stay on it. Dependent of the kind of cat the pitchpoiling appears completely different. If we talk in the same time about old style narrow cats, modern crusing cats, slightly wider modern performance cats, overpowered beachcats and the nearly square lago di garda racers - this is total misleading.

It's even not enough to express the stability in single numbers for different directions. We need plots of the righting moments vs pitch and heel angles - compared for different length/beam ratios. Once we got some basic clarify here we may disscuss the influence of different wind directions, wave interaction, crew trim, hull shapes, dynamic effects...

If somebody provides a functional model (freehsip) of a typical cruising cat hull I will do the calculations and some graphical represenatation.

regards
ropf

[sail.scow]

The topic gets the brain thinking. It would be interesting to see some of the theory quantified.

Personally I am happy just thinking it through, and I hope the other contributors don't mind my musings. Sort of thinking out loud.

How about transferring the moment from the CP to the leeward hull somewhere? It would work out like placing an equivalent weight onto the deck.
Like using a Hobie, and placing a weight on it.
A static representation of the moment.
SA, wind speed, educated guess at the CP
estimation of total force at CP times the height of the CP above the float or hull where we want to put the weight=Nm
transfer that moment down vertically from the CP to the height of float or tramp
that moment divided by gravity=kg.There is a weight we can place, on a spot on the hull or tramp. If CP is over the water, then it's another step to move the moment inboard until it lands on the vessel.

a horizontal line from CB to CP with a point 'g' where we want to place the weight on that line

distance CB-g divided by CP-CB, times kg
Any thoughts on that?

[masalai]

sail.scow, said: "So the interesting thing is blending those forces into a vessel that can meet different criteria."

The prime design problem is rooted firmly in the intelligence (lack thereof) of the skipper & crew in allowing / causing a capsize (unless their mission was "destructive testing).... :D:D:D:D:D

[rayaldridge]

Well Richard, most of this is simplistic crap. Fine for flat water, but add waves and what happens?. I'll help. As the hull spacing widens, for the worst case scenario of beam on seas ,the boats bridge deck clearancs must increase to prevent contact (at speed it becomes slamming). Otherwise a boat with a bridgedeck clearance of say 600 mil's is going to be limited to opperating in waves of less than this height if wide enough, or it'll get a pounding.

Hence we see the ratio of tunnel width to wing clearance expressed as around 18%.

I'm not sure I buy this. Here's the problem for me: as wave height increases, so does length. A wave high enough to pound is so long from crest to crest that it ordinarily isn't a matter of the two hulls being on either side of a breaking crest, and besides, the boat would have to be running parallel to the crests in order for this to happen.

However, anyone who's sailed on a cat with narrow hull spacing can probably remember seeing the water humping up between the hulls, even in flat conditions, if the boat is going along pretty fast. We used to own an old Wharram, which had fairly narrow spacing, and slat decks, and you could see the phenomenon Richard alludes to-- water was forced up as the hull waves collided.

It may be that this problem is primarily associated with older cats with the spacing that used to be considered acceptable. There may be a point when increasing beam does affect pounding negatively, but as a general rule, I think the closer the hulls, the higher the clearance should be.

Ray

http://slidercat.com

[rayaldridge]

Probably the most finnicky thing to draw would be a small cruising cat. They normally can carry enough sail to lift a hull or bury a lee bow. Their displacement is normally not enough to prevent either. They have the risks of a lightweight racer and the performance of a displacement cruiser. Who would draw one of these and reduce the sail plan for safety's sake?

Me?

http://slidercat.com/blog/wordpress/wp-content/gallery/slider1/sailjul5.jpg

Ray

http://slidercat.com

[Spiv]

Gregor Tarjan has probably more miles on cats than all of us put together, as well as Americas Cup racing. He is a USCG licensed captain and member of SNAME.
In his great book "Catamarans, every sailor's guide" he has a section on Diagonal Stability & Beam-to-length Ratio that I have scanned for you all to read.

You will see that he agrees with Richard Woods wave interference factor, but not wave slamming.

Rubbishing anyone on this forum does not conciliate to discussion, I have great respect for Richard Wood, he has made a lot of worthwhile contributions on many threads on this forum, as well as to the world of Catamarans in general and I certainly appreciate the time he spends posting here.

We all sell something, even if just our credibility and I would buy a design from Richard.

[rayaldridge]

The Tarjan book is a really good one, especially if you're in the market for a good big cat.

I reviewed the book for Living Aboard, and later posted it to Slider's website:

http://slidercat.com/blog/wordpress/?page_id=8#rev5

Ray

http://slidercat.com

[Richard Woods]

I apologise for my sarcasm of a couple of days ago, sorry to have caused offense.

Rhough. I always read your messages as they are sensible. That is why some time ago I asked what boat you sailed and where

Richard Woods of Woods Designs

www.sailingcatamarans.com

[Alan M.]

I'm not sure I buy this. Here's the problem for me: as wave height increases, so does length. A wave high enough to pound is so long from crest to crest that it ordinarily isn't a matter of the two hulls being on either side of a breaking crest, and besides, the boat would have to be running parallel to the crests in order for this to happen.

However, anyone who's sailed on a cat with narrow hull spacing can probably remember seeing the water humping up between the hulls, even in flat conditions, if the boat is going along pretty fast. We used to own an old Wharram, which had fairly narrow spacing, and slat decks, and you could see the phenomenon Richard alludes to-- water was forced up as the hull waves collided.

It may be that this problem is primarily associated with older cats with the spacing that used to be considered acceptable. There may be a point when increasing beam does affect pounding negatively, but as a general rule, I think the closer the hulls, the higher the clearance should be.

Ray

http://slidercat.com

I've only sailed on half a dozen or so cats, and all of them would have had a LWL/BWL of better than 10:1, so I can only base my experience on that. But in my experience the cats I've been on hardly produced any bow waves at all, even at high speeds. (which is why cats can sail at much better than theoretical hull speed.)

Here there's a video of a relatively narrow beamed 55' powercat at 30 knots. There is footage taken under the bridgedeck. http://www.alluramarine.com.au/

As for already existing waves hitting the bridgedeck, it seem fairly obvious to me that for a fixed bridgedeck clearance a wider beam would allow a less steep wave to hit. If you had 500mm clearance and hulls 4m apart, a wave gradient of 1 in 4 would touch. If the hulls were 6m apart a gradient of 1 in 6 would touch.

[RHough]

I apologise for my sarcasm of a couple of days ago, sorry to have caused offense.

Rhough. I always read your messages as they are sensible. That is why some time ago I asked what boat you sailed and where

Richard Woods of Woods Designs

www.sailingcatamarans.com

No worries mate. It just seemed out of character for you. Your wealth of experience is something we all benefit from.

I started sailing after I helped my father built a Penguin, then raced Stars and what ever I could get a crew place on, A-Scows, and got my first multi experience racing a Couger Cat ... horizioned the fleet in light air then they left us for dead when the breeze filled in. :(
In my 20's I sailed Lasers and crewed on monos.
There was the lost decade married to a non-sailer ... so a bit of crewing and a Bic sailbord to keep me sane.
Now I have a "Classic" Catalina 30 ... that no matter what you do to her is a 5-6 knot shitbox ... 12.4 knots in 40 true is *NOT* fun on that boat ... I'm back to having a Laser and a Hobie 18 for sailing, got a tatse of real mutli saiing racing a Corsair Sprint 750 in Mexico ... 18+ knots in a 24ft boat *was* fun!

Cheers,

Randy

[Richard Atkin]

I just read all that stuff posted by Spiv. (nevermind any copyright laws Spiv :D:D). Maybe Gregor Tarjan does know what he's talking about, but he didn't explain much in those pages. Maybe I read it too fast.
Tarjan says that if you keep the same clearance but widen the beam, you end up with the wrong beam to clearance ratio which will lead to slamming. No explanation.

Alan M seems to explain it a bit further, but to be honest, I just don't get it.
He says " If you had 500mm clearance and hulls 4m apart, a wave gradient of 1 in 4 would touch. If the hulls were 6m apart a gradient of 1 in 6 would touch."
I don't understand that.

Apologies to Richard Woods for wandering off the main topic....could we include both topics in this thread? cos slamming and pitching are so important to hull spacing.

[Spiv]

I just read all that stuff posted by Spiv. (nevermind any copyright laws Spiv :D:D). Maybe Gregor Tarjan does know what he's talking about, but he didn't explain much in those pages. Maybe I read it too fast.
Tarjan says that if you keep the same clearance but widen the beam, you end up with the wrong beam to clearance ratio which will lead to slamming. No explanation. Richard,
I am sure Gregor is only to happy about me giving a plug on his great book; some of the readers of this thread might even end up buying his book....

Another very experienced author is Charles Kanter with over 100,000 miles of sailing, delivering, surveying and critiquing 1000+ boats. In his book "Cruising Catamaran Communique" he talks at lenght about slamming, pitching and LBR.

He says a LWL/BMAX of 2 makes a 'Balanced" cat, diverting from that ancient Polynesian rule can create more problem that it is trying to cure.
He says wave interference is reduced, but wave pounding is increased (same as Tarjan), so you must disproportionally increase bridgedeck clearance.
He also says that the biggest problem is that the transverse stability can eventually be compromised, leading to burying the lee bow.

I have given this some thought and realised that even if one builds a cat with larger beam and does not increase the sail area, he could run into some problem:
Immagin you are going with 70 degree apparent wind of 25knots.
The wide beams allows you to carry more canvas than if you had a narrower beam, you are happy with your high speed and feel safe and secure.:):)
Now your route has to change and the same apparent wind comes from 120 degrees and you start burying your lee bow.:(:(

So, I would say, make your cat as wide as practical, but keep it in the 39~42% BCL/LWL ratio advocated by Tarjan. (where BCL= Beam between Center Lines and LWL= Length Water Line)
Additionally, make the bridgedeck as high as possible 14~18% of BCL, using the higher ratio for larger boats.

[CT 249]

catsketcher, you sound like a mono sailer?, cat sailing demands you FORGET all your mono sailing habits and guidelines. - - How can I define the differences - something like writing with a quill and doing a cut&paste on a computer - if you know what you are doing, in a cat you can go BLOODY FAST in a cat consistently and for long distances. - - - One slight lapse of attention and you are in one huge mess....?

Excuse me Masa, how are you so sure that you have more multi experience than other people here?

[Doug Lord]

Has anyone sailed a Stealth F16 with rudder t-foils or another cat similarly equipped? Can you discuss the improvement in handling or lack thereof from the use of these foils on a catamaran?

[Richard Woods]

That is an interesting point. I haven't sailed a Stealth but nearly 30 years ago I was working for Derek Kelsall and we designed a trimaran with foils on the rudder and daggerboard. In its first major sail it capsized in a pitchpole. We understood that was because the rudder foil provided too much lift as it surfed down a big wave.

However when I raced radio controlled multihulls we found that they NEEDED a foil on the rudder to hold the stern down or they would pitchpole very easily.

So not all boats behave the same

Richard Woods of Woods Designs

www.sailingcatamarans.com

[Alan M.]

I just read all that stuff posted by Spiv. (nevermind any copyright laws Spiv :D:D). Maybe Gregor Tarjan does know what he's talking about, but he didn't explain much in those pages. Maybe I read it too fast.
Tarjan says that if you keep the same clearance but widen the beam, you end up with the wrong beam to clearance ratio which will lead to slamming. No explanation.

Alan M seems to explain it a bit further, but to be honest, I just don't get it.
He says " If you had 500mm clearance and hulls 4m apart, a wave gradient of 1 in 4 would touch. If the hulls were 6m apart a gradient of 1 in 6 would touch."
I don't understand that.

Apologies to Richard Woods for wandering off the main topic....could we include both topics in this thread? cos slamming and pitching are so important to hull spacing.

A gradient of 1 in 4 means that it requires 4 metres of length to produce a rise of 1 metre, whereas with 1 in 6 it takes 6 metres. So 1 in 6 is less steep. So for the same clearance it requires a less steep wave to reach the bridgedeck with the wider hull spacing

[sail.scow]

A handy sketch of transverse stability.
The sail is square and set from a yard.
The blue dot is the CP.
The moment originating from CP can be directly transferred onto the 'line of transverse stability' now.

[sail.scow]

Move the yard to the drawn mast next.

Then make the sail a leg o' mutton mainsail.

That blue line originating from the CP can be transferred to deck level.

That blue line is the direction of the moment.

So if moment on that line is 500kg@ 20m, then it is 1000kg@ 10m.

But it is only in the direction of total force at the CP.

[waynemarlow]

Has anyone sailed a Stealth F16 with rudder t-foils or another cat similarly equipped? Can you discuss the improvement in handling or lack thereof from the use of these foils on a catamaran?

Got to confess I own a Stealth although I'm not sure I have enough experiance of big cats to comment. From a Stealth perspective the T foils are very very good at stabilising in pitch movement, certainly it makes their modest 16ft seem much longer. I have sailed with both ordinary rudders and T foils on the same boat and would say using the T foils are a step in the right direction in that all pitch movements are much dampened and thus allows the Gorilla on the back more time to do something about the impending splash.

There are several photo's of a Stealth getting deeper and deeper on the leeward hull, until the hull is almost fully submerged but almost horizontally uniform along the water line such is the power of the T foils to stabilise. There is a very good video on the Stealth Marine website which shows the T foils at work.

I think also the flat bottomed design of the boat also prevents the boat pitch poling as one normally has the Spinnaker up ( our spinnakers are now so flat as to be more hooter than spinnaker ) on the longest pole allowed which lifts the bow and lets the flat bottoms do there stuff.

These 16ft rocket ships which only weigh 100kgs all up really are very different than the cats generally talked about on this forum, if you can get a chance to hitch a ride on a F16 style cat then give them a go, they really are a great adrenaline rush with really good boat handling manners, ie you can do some really crazy fast sailing but still feel totally in control.

[Doug Lord]

Great comments! Thanks Wayne. Do you have any serious problems with weed on the t-foils?

[Richard Atkin]

OK Alan, so what you are saying is this? I was trying to imagine a more complicated picture.

Cheers

[waynemarlow]

Great comments! Thanks Wayne. Do you have any serious problems with weed on the t-foils?

Surprisingly little as the T foil itself is a very rounded semi circular shape, only a direct wrap around the central rudder may pose a problem. To be honest the areas I have been sailing are pretty weed free so I may not have seen the worst possible conditions.

To sort of retiterate my previous statement, the T foils do " dampen " the pitch movement really well and its a rare occurance to see the Stealths pitchpole despite the very large area they sail with.

[Alan M.]

OK Alan, so what you are saying is this? I was trying to imagine a more complicated picture.

Cheers

Yes, that's what I was saying. Your diagrams show it much better than my words did.

[Richard Atkin]

The Pitchpoling Myth

I am too impatient to wait for Richard Woods to make his conclusion, so I will make it for him....

The Pitchpoling Myth has been busted...and it has not been busted.

There are no golden rules. One rule will always affect another. I think we can all agree that yacht design is all about finding the right balance.

Now everyone can relax and get back to what they were doing.

bye

[masalai]

But, but but, I was waiting my chance to pontificate too - Oh well.... another thread prematurely wound down?....

[Richard Atkin]

Stoke it up Mas :D The fire fizzled out ages ago.

[masalai]

I havent found a myss to pitch my pole yet and the only boats that go upside down were caused by the "skipper"..... guns don't kill people the fool using it does - on that basis of anal ysis by the NRA, then people who capsize their boats should be given a gun?

[Richard Atkin]

extreme anal ysis, Mas. :)