Subject: [harryproa] Re: leepods - pro
From: Mike Crawford
Date: 2/10/2006, 7:22 PM

<<Boats have been knocked down and righted.  Including the amusing case of one of Russ' friends falling asleep on his watch and waking up to find the boat happily loping along in the pod immersed condition.>>

  Good point.  I'm not impressed by arguments that boats are safe because they have made ocean crossings.  Safe miles at sea are often more a product of good seamanship and good luck than actual boat safety.  But actual knockdowns and recoveries say something.

  For the record, I've been a fan of the "Dutch Proa" ever since I first saw the web site, and would like to see more people work on the concept.  I think it's neat that you can have a training wheel which will increase righting moment to give you time to loosen the sheets.  As Han Biljard points out, this removes the temptation to push the boat past its limits because you slow down as soon as that leepod starts creating drag.  What a great combination.

  I'd want a relatively short aka length, though, and a low displacement.  This would provide some additional righting moment which serves as a warning, progressively getting buried further and further as you push the boat too hard.  This would be a lot better than the 150% buoyancy amas we see on some trimarans, which will raise the whole boat and make a capsize much more dangerous.

  If the leepod is integral to the leeward hull, such as on Jzerro, then it's not inconceivable that it would fit these criteria.

  The challenges of a leepod on a harryproa would then be twofold:

A) Structure and complexity

   The leeward hull in a harryproa has a lot of stress going through it, and you'll need some solid structure if you're going to want to put storage or accommodations inside the leepod.  This is not impossible, but it adds complexity to what is a very simple (and easy to construct) design.

B) Increased requirement for righting moment

  A Jzerro design is great for a leepod because it has most of its weight in the leeward hull.  Even with water ballast in the windward hull, there's not a whole lot of weight up there in the event of a knockdown, and it is angled in such a way as to contribute to righting even when the mast hits the water.

  The harryproa has more than half of its weight to windward, though, and with standing headroom and a cockpit there, that weight is a lot higher up.  Between the additional weight and its angle to the leeward hull, this will create a lot more heeling moment than a Jzerro design if the boat goes past 90 degrees due to sea action or wind pressure on the upturned hull.

  It's this heeling moment that the leepod would have to counter, and that's different than with Russ Brown's boats.


  With enough design time and experimentation it's probably possible to overcome both of these challenges.  The resulting design would get away from Rob's criteria of maximum speed, accommodations, and righting moment for a minimum of weight, structure and complexity, but no one every said that every single one of Rob's criteria is sacred.

  I think the key is in getting data on how a leepod would affect a weight-to-windward proa once it's in a capsize condition of 90+ degrees heel.  If there were some solid calculations or real-world tests of this combination, I believe a number of people would consider adding leepods to their designs.

        - 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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