The Pitchpoling Myth

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.

 

Dynamic trap

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

 

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.

 

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?

 

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

 

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

 

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....

 

It may be in the way its sailed

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

 

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....?

 

Pitchpoling

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/

 

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

John Shuttleworth has some stability indices for diagonal capsize as well as pitchpoling and sideways capsize.

 

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

 

A bit more to say

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 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?

 

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"....