Old Nick ) writes:

- they load a mast very high in a puff, because they are not
supposed to heel more than a very small amount. IIRC (and it's been a
while) and extra 20% (?) or more needs to be added to the mast/stay
strain for a multi over a mono.

good point. a mulithull behaves more like the solid ground mentioned in an
earlier post than like a boat. what you usually see for boats is a graph
of righting moment against angle of heel. at some point there is a
maxiumum righting moment.

imagine the cross section of a catamaran. one hull has to be lifted out of
the water at some distance from the sail, making for quite a bit
of leverage for the sail to overcome. as soon as the raised hull leaves the
water it loses all bouyancy and becomes a dead weight for the sail to lift
at the end of the lever. teh fulcrum is teh hull which is still in the water.

If I remember correctly, according to TF Jones catamarans don't heel more
than 5 deg or so. they still roll with the swells so they don't stay flat,
but they don't heel much at all. I'd guess it's almost like being on a raft.



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William R. Watt ) writes:

.... what you usually see for boats is a graph
of righting moment against angle of heel. at some point there is a
maxiumum righting moment.

I think it should be clarified that the graph you see in the texts is an
abstraction and is not what happens on the water. On the water there are
more forces involved, especially on a mulithull. Take the extreme case of
a raft. One side is being raised through air while the other side is being
immersed in water. It's easy to push air aside but not water. As the raft
rotates there is a lot of turbulant drag around the side being pushed
through the water. There is a righting moment, but its not quite what is
calculated in the texts.
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I am a bit overwhelmed by the amount of information from this thread.

I was kinda hoping for something like 'my 14ft cat had a similar mast,
so 82mm diam should be OK'.


Yes the proa is light. As a comparison Rob Denney's Elementary proa (1
person in cabin) weighs 110kg unladen. Proas tend to be long narrow
and light, disadvatnage is that thy do not have much space or carry a
lot of cargo.

The proa is a pacific proa, meaning that the outrigger (weighs 25kg
without ballast) is always to windward. At this point feel tempted to
give the mast a go becasue it is so cheap (is new and proper grade,
not junk), worse comes to worse loose $250 mast. As a comparison some
dingies with similar sail area have mast diam of 60mm in aluminium.


N. Peter Evans

On 24 Oct 2004 16:11:22 -0700,
(peterMelbourneAustralia) vaguely proposed a theory
.......and in reply I say!:

remove ns from my header address to reply via email
Are you making your own sail? I still reckon a spar maker should be
asked, and as someone said, he will ask right back at ya.

Have you even checked out similar proas?

Good luck.

I am a bit overwhelmed by the amount of information from this thread.

I was kinda hoping for something like 'my 14ft cat had a similar mast,
so 82mm diam should be OK'.

You yourself had a cat. _Is_ 82mm OK?


Yes the proa is light. As a comparison Rob Denney's Elementary proa (1
person in cabin) weighs 110kg unladen. Proas tend to be long narrow
and light, disadvatnage is that thy do not have much space or carry a
lot of cargo.

The proa is a pacific proa, meaning that the outrigger (weighs 25kg

without ballast)

what can I say?

25kg 2 metres from the main hull would have _negligible_ righting
moment. At 6 metres, it starts to matter. How much ballast? How wide?

is always to windward. At this point feel tempted to
give the mast a go becasue it is so cheap (is new and proper grade,
not junk), worse comes to worse loose $250 mast. As a comparison some
dingies with similar sail area have mast diam of 60mm in aluminium.

A dinghy is at best a poor comparison.

1) multihull
- different stabilites, different bending forces on the mast due
only to rigging, not wind
2) cruising = at least 20-30% extra fudge factor.
3) not many dighies expect to be out in a 45 knot + gale,

************************************************** ***
Have you noticed that people always run from what
they _need_ toward what they want?????

Mt last post in this thread I think.

The outrigger is 15ft long, weighs 25kg unladen, but has 50kg of water
ballast. The outrigger is 4m from the main hull and always to the lee
hull. Yes crew can go out on the tramp towards the outrigger for more
righting moment, but unlikley in very strong winds due to safety.

The mainsail is 85sq feet. I have a jib, which is smaller, approx 60
sqr feet, but am thinking of cutting it up to make even smaller jibs
at far ends to balance the rig.

My feeling is that the windrush 12 cat I sailed had a rig that was
strong enough in huge blows and went like the wind. So if I new what
that was feel that similar would be fine. Rob Denney uses unstayed
carbon masts. My question has been posted on a site that he looks at
regularly but he has not posted. Dont want to annoy him again with
another email.

What I will do is look at rigs of small cats (closest to my 170kg
proa), and see what mast sections they are using. 14ft and 12 ft cats
seem the best comparisons for they go out in huge blows without undue
drama.

N. Peter Evans

peterMelbourneAustralia ) writes:

.... The outrigger is 4m from the main hull and always to the lee
hull. Yes crew can go out on the tramp towards the outrigger for more
righting moment, but unlikley in very strong winds due to safety.

traditionally crew hike to windward.
not a slave to tradition myself.

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On 24 Oct 2004 14:52:10 GMT, (William R.
Watt) vaguely proposed a theory
.......and in reply I say!:

remove ns from my header address to reply via email


William R. Watt ) writes:

... what you usually see for boats is a graph
of righting moment against angle of heel. at some point there is a
maxiumum righting moment.


I think it should be clarified that the graph you see in the texts is an
abstraction and is not what happens on the water.

I think one I saw was real case.

On the water there are
more forces involved, especially on a mulithull. Take the extreme case of
a raft. One side is being raised through air while the other side is being
immersed in water. It's easy to push air aside but not water. As the raft
rotates there is a lot of turbulant drag around the side being pushed
through the water. There is a righting moment, but its not quite what is
calculated in the texts.

A cat is actually harder to tip than a barge, because weight, and more
importantly, buoyancy are at the extremes. With a barge, the water is
still pushing up under the middle of the vessel when it is quite
heeled. In fact it almost becomes a triangle as it tips.
************************************************** ***
Have you noticed that people always run from what
they _need_ toward what they want?????