The Pitchpoling Myth

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, 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?

 

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

 

Sarcasm will kill the discussion.

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 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.

1. 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.)
2. 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'.
3. 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?

 

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

 

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 enjoy Richard Woods' harmless sarcasm.

 

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.

 

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.

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.

 

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 on stability:

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.

 

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?

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.

 

Great post RHough

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 ...

 

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.

Lol, geeks might draw the thing, but there is always someone who 'knows what the plane can take'.