Subject: [harryproa] Re: leepods - con
From: Mike Crawford
Date: 2/10/2006, 6:33 PM

<<If you are actually talking about a wave that is as wide as the boat is wide and the mast is tall, and extremely severely sloped...>>

  I think you've boiled the disagreement down to its essence.  What are the conditions for which we're theorizing?

A)  Racing

  If you're going to be racing or just horsing around, there's are lots of good arguments for the leepod.  It will let you fly the windward hull with more safety and less stress.  Since you're going to be on the sheets at all times, you should be able to dump the wind when the leepod warns you that it's time to back off.

  Some would say that the additional weight won't be worth it over the entire length of a course, since flying a hull is mainly realistic on a reach.  You could also argue that there's little reason to fly the hull because it will only save you 10% to 20% in drag, and strategy and sail handling will be more important than the drag savings.

  I'm not going to argue for or against these points.  I'll just simply go along with the idea that a leepod could be a good asset when going for max speed on an extended reach.

B) Cruising

  On the other hand, if you're going to be making a passage, then it really is time to think of extreme situations.  How extreme?  That's a matter of what you can afford to do given weight and cost limitations.

  Of course, I'm an engineer, and tend to first think of designing for extremes before designing for average conditions.  That's why I wear a seatbelt, even though my car has airbags.  The airbags will be fine in most conditions, but in the conditions they don't work, I'm not willing to pay the price for skipping the belt.

  As long as we're going to debate capsizing and self-recovery, we might as well deal with the event of a complete knockdown, whether due to tall, steep waves, an unexpected gust, poor seamanship, or all three.  After all, if you actually do go over in the middle of nowhere, there's little solace to be taken from the idea that the capsize wasn't very likely.  Regardless of whether or not other boats would have failed.

  A Harry or Visionarry, with a sealed mast, will be just about the only cruising multihull you'll be able to recover from a full knockdown.  This is because there's nothing leeward of the mast, and it provides enough flotation to stay at 90 degrees, or even a bit past.  Once the windward hull weathercocks downwind, wind and wave action alone may be enough for self righting.  If not, you could use a kite or a sea anchor to pull the windward hull back down.  No complex righting or flooding maneuvers required.

  That's pretty significant if there's no one around to help you right the boat.

  A schooner rig will be even more stable, and the additional masthead flotation would increase the margin of safety.  Some argue for gas-assisted flotation, others fixed.  Regardless of the flotation used, the question is:  can we afford this margin of safety given its impact on performance, cost, and weight?  Given that any  flotation system is going to add less than $10,000 to the price of the boat, and the weight will be negligible, the answer is yes.

  My worry about the leepod is that the higher the leeward hull floats in a capsize, the more past 90 degrees the windward hull will heel, and the more heeling moment it will create.  This will require more mast buoyancy and make it harder to right after a knockdown.  If there's enough flotation in the pod to really prevent going past 70 degrees in a good gust, there might be enough to raise the whole boat.  Then righting the boat would be more like trying to right a cat or a tri, which is different than trying to right a proa lying flat at 90.  Also, anything to leeward of the masts can potentially add more heeling moment if the boat goes past 90.

  Once the extreme is reached, I'm a bit leary of anything that could make recovery harder rather than easier.


  With that said, you could probably design a leepod: 1) with enough flotation to add some useful righting moment at 40 to 50 degrees, but not so much that it will make recovery difficult, and  2) high enough to provide righting moment even at 100 degrees, but low enough to still come into play at a useful angle of heel.

  Then my arguments against the pod would be largely moot.

       - Mike

proaconstrictor wrote:
> > "My objection is the use of the lee pod as I believe

> > Right at the point where a PP without one might become stable in
> the
> > knock down position, it is doing it's work.  Somewhat similar to
> what
> > ballast does in the same situatiom.
> This is where we have a difference of opinion
> On a side of a wave by the time the leepod STARTS to work the boat
> pretty close to 90 degrees, not when it has any significant
> By the time it provides significant bouyancy you are probably past
> dead center and by the time the rig slows you down you're over and
> the leepod is actually preventing the return.
> This is my interpretation of the mathematics.

The rig never slows you down, the leepod does all the righting in
combination with the CG.  You need to scale this realistically.  If
you are actually talking about a wave that is as wide as the boat is
wide and the mast is tall, and extremely severely sloped, and is
there for a period long enough for the capsize to occur, because
there is a timing element also, then I'm not objecting, every boat
has it's limitations, but this sounds like a situation where it isn't
so much the leepod that is a failure as the sea conditions are
savage, and a number of boats might be at risk.


> >
> > "In breaking seas, if hit from the
> > side by a breaking wave it could dig in and flip the boat."
> >
> > Not heard of that happening, doesn't seem to be a serious
> objection.
> > Reminds me of the Wharram/Boon drawing of a tri perched ona 
> rbeaking
> > wave about to get rolled over.  It might happen, but it either
> > doesn't or they have the sea anchor out, one hopes the literature
> is
> > not full of that kind of capsize.
> There are not many leepod boats out there in Bass Strait
> My experience of breaking seas over many years in pretty nasty
> conditions coupled with my experience surfing all sorts of craft
> (including many capsizes in craft that weren't meant to be surfed)
> suggests it is a real possibility.
> This could be tested by making a scale model of a PP with a leepod
> and taking it down to a beach with a small to moderate surf. I am
> sufficiently convinced by the mathematics and my own experience to
> think that a leepod is at best useless, except in flat seas and at
> worst a trigger for a full capsize

> If my reasoning doesn't make sense to you , so be it, but at least
> is out there for people to at least think about.

It's not as though this is an untried technology.  It's been around
for 40 years, and quite a few ocean crossings and a fair amount of
cruising has been undertaken.  Boats have been knocked down and
righted.  Including the amusing case of one of Russ' friends falling
asleep on his watch and wacking up to find the boat happily loaping
along in the pod immersed condition.

Math doesn't get us very far, because one has to be able to design
the boat in the first place beforeo n can run the numbers.  You can
do the math and get the wrong result just because you don't get the
design, which by your own admission you don't.  It is perfectly clear
that there is a pod of some size say 300 feet on a 30 foot bat that
makes lee capsize impossible, OK trim it back till it is a little
more manageable and you feel comfy sailing it all over the Atlantic,
and across the Pacific over a period of 25 years.  Now run some
numbers.  I'm not trying to be insulting but I have seen far to many
failures of numbers to take much stock in the situation simply
because the actual working principles are not understood. 

Absolute security is an illusion.  I can't tell you what boat is
right for the Bass strights, probably every type of boat has been
thrashed there are one time or another.  Local boat designs vary
because not every type is as suited to every environment.

> is that the lee pod does not prevent capsize on the side of a
> moderately steep wave and encourages the boat to go way past 90 in
> this situation, preventing its return back to 90. Wouldn't it be
> better to be stabilised at 90 degrees in that situation?

This is the fantasy diagram example.  Phill Weld's 60 Gulf Streamer
was capsized by a rogue wave.  Bad stuff happens.  One can always
draw a picture of the point of no return in any boat.  This kind of
weather is only survivable in a multihull with a para anchor out. 
You still have to get the boat way past 90 degrees to capsize it.  As
a mater of degrees, the boat has to be orders of magnatude more
rolled over than the capsize trigger point for a cat or tri.  That
said, I haven't heard of any situations in whcih proas dealt with
huge nasty conditions with a para anchor.  A lot of these guys just
don't talk about what they have done so the data is not deep.

> I am sorry if my explanation is not clear enough as the mathematics
> are clear enough for me.
> Regards,
> Robert

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