Pitchpoling - Designing it out

I agree! Reducing tendency.

 

Having forward sections designed like Sodebo and Idec seem to be the latest trend in design to deter pitchpoling. And since Idec went around the world safely in record time they must have helped.

 

Old Sailor asked for my take.

I cannot view the video right now as I only have dial up internet in my island house in British Columbia, so cannot really comment.

However I can comment on the T foils on rudders.

In the late 1970's I was working for Derek Kelsall on one of his OSTAR boats and we designed a rudder foil that in theory would reduce pitchpoling (as others have described above)

Maybe it does work in flat water (it certainly does in dinghies like Moths and I-14's). It is also essential on RC multihulls as they pitchpole almost immediately without a rudder foil (or at least they did when I last "sailed" one)

But we found they weren't so successful when sailing offshore as after sailing less than 1000 miles Derek's tri pitchpoled when sailing fast under spi.

We decided it was because it was sailing in big waves and the angle of attack became the opposite of what was intended so the foil gave positive lift rather than holding the stern down.

As old sailor says. It's a bit more complicated than it seems.

You won't find anything on foils on my website, but don't let that stop you looking!

best wishes,

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Alternatively, on smaller multihulls, the advent of square top sails with shorter booms enables the mast (Rig) to be set further back in the boat and the board(s) further back in the hull(s), to balance the rig.

This alone would reduce the pitchpole tendancy.

 

Hydrofoils aside, other efforts would include longer floats on trimarans, more bouyancy forward (increased prismatic), more stem overhang (Wharram), bow bulbs(Crowther), higher bow topsides, rounded foredecks, deflectors, wave piercing bow shapes and combinations of some or all of these.

Since he has singular experience of racing in multihulls, both on his own designs and other multihulls, I would like to hear Richards views on prevention of multihull pitchpoling, especially since his racing designs appear to have an exceptionally good record in this regard.

 

I wrote my previous message, above, late last night. Here is my second reply, written this morning when I am more awake.

I haven't ever put the bows under on any of my designs. The nearest I have ever got to pitchpoling was when racing a Norseman 43 in the Capetown to Rio race. We were about 1000 miles offshore at the time so it was all a bit scary. You can read about it on my website in the Race Reports section of my Articles Pages.

A few weeks ago I sent this video link of the trimaran Groupama 2 to a friend for his comments.

www.youtube.com/watch?v=8veVWVYRVv8

He is one of the worlds experts on stability, and certainly knows more than probably anyone reading/contributing to this forum. Note he is a Naval Architect, not a yacht designer, so doesn't actually design boats himself.

This is what he said having viewed it.

"Ref the trimaran video, very interesting. From the airborne shot it looks as
though the L/Bcrs is about 1.0. I strongly suspect that the calculated
"safe" wind speeds would be driven by pitch stability rather than roll.

Looking at the video a second time, you can see that even in smooth water
the pitch motion is very slow, suggesting that the pitch stiffness (GML) is
very low, which could easily contribute to a pitchpole. The reason that the
boat sails so bow up is that the users are aware of this, and that even a
slightly aft CG generates a strong trim angle. Watch carefully and you can
se the trim vary between well bow-up and slightly bow down without any
obvious external influence causing it - again symptomatic of marginal pitch
stability.

Instructive."

For comparison I also sent him this video clip from my website (see my Videos page for the original) showing similar trimarans that behaved differently.

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

Hope this helps

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

T-foils

Very interesting stuff, Richard-thanks! Here is an article about an experimental retractable t-foil on the stb ama of Gitana-an Orma 60:
http://www.thedailysail.com/ism/art...3063EA9D9F80257385007488FE?OpenDocument&Login
----
Also, there is some interesting stuff on catsailor.com(http://www.catsailor.com/forums/showflat.php?Cat=0&Number=85473&page=0&fpart=all&vc=1) about t-foils used on the "Stealth" beachcat.
Pix from Gitana(and the Daily Sail):

 

T-Foils

Here are some comments from John Pierce Of Stealth Marine-the picture is of Darryl Barrett's swept t-foils:
"I will add my experiences to this debate.

We first tried the T-foil rudders 5 years ago, it was immediately obvious that they completely altered the sailing characteristics of the boat.

This is not one of those changes that you make and you are not sure whether anything actually changes, I say again the boat is totally different with T-foil rudders.

Next we had to check whether or not the difference was fast. Like Darryl we carried out 2 boat tuning with identical boats except for the rudders. In no circumstances was the non T- foil rudder equipped boat any faster.

In any kind of wave pattern whatever the wind strength the T-foils were quicker both upwind and downwind and in winds > 12 knots the T-foils where again quicker.

We also tried them on our formula 18 HT boat and the effect was just as marked.

And more recently the current Sptfire European Champion purchased a set and tested them he also confirms my/Darryls comments

I have no idea how much drag the T-foils create, and quite frankly I don't care, catmarans go quicker with them on so whatever the figure it is less than the drag of a normally equipped hull travelling through the water.

We did no tank testing (I think you will find that not much tank testing time is bought by any beach catamaran manufacturer since it is cheaper to build 2 boats and do the job properly on the sea).

As to the weed question, obviously a dagger rudder is harder to clear than a kick up rudder, just as a dagger board is harder than a centreboard. Perhaps if your water is very weedy these are not for you.

My credentials for carrying out these tests are that I was a full time member of the British sailing Team racing Tornado, my ISAF World ranking got to 9th, I was paid to sail by the Royal Yachting Association and my job from 1997 to 2000 consisted of 9-5 most weeks 2 boat tuning, with 8 -10 regattas a year thrown in, I have done thousands of hours of this work.

I would entirely echo Darryls findings and since we are the only people I know of who have done this and we both completely agree the chances are that we are not mistaken.

There is one further effect that is also noticable although I didn't pick up on it for ages, and that is that not only is the boat smoother through the waves but it is also smoother through gusts. Put simply the T-foil equipped boat lifts a hull more slowly when the gust hits, we think that this is because as the gust hits and the rig drives harder, it tries to push the bows down, of course the T-foils resist this, and the windward t-foil obviously adds load to the windward hull slowing down the hull rise and so squirting the boat forwards."

--------------------
John Pierce

stealthmarine@tiscali.co.uk

 

Richard, that vidio of Groupama 2 was very instructive.

I am no expert on this---but would you say that a higher prismatic on the mainhull would have reduced, or even prevented that slow pitching motion???

 

Hulls that have better flotation up front (wider) would resist pitch poling but that would mean they would have more drag in all sailing conditions, not the fastest design. It would also give a rough rid in chop. It might be possible to put a lot of flair in the hull so it was slim profile in the water during normal conditions, but when driven down it would have more flotation. But I suspect this would be a slower design in chop and not ride as well in big seas.

Slim and low drag amas are not going to offer a lot of resistance to pitchpoling when driven below the surface. A small foil on the top of the bow that generates lift when driven under might stop a pitch pole, but there would be a sudden increase in drag when the foil hit the water slowing you down. It might also increase the risk of fowling the jib sheet or other rigging.

I suspect it could be done, and likely is in cruising tris and cats, but it would not be suitable for a racing design. It is, like everything else, a design compromise that trades one characteristic off for another.

There was a good thread on this several months ago with great pics (page two) and a good discussion about the forces involved. it is at:

http://www.boatdesign.net/forums/showthread.php?t=20534&page=2&highlight=sheet release