On Sun, 24 Oct 2004 07:13:04 +0700, Egis/CORE wrote:

On Sat, 23 Oct 2004 16:58:02 GMT, Morgan Ohlson
wrote:

On 23 Oct 2004 16:01:42 GMT, Stephen Baker wrote:

you know little.

I don't like people to give faulty advice anyhow. I just liked to give a
hint to PeterM.A about that.

..but I will not fall to your standards of attacking...

...explained this way...

If you have a rig in solid concrete, which would matter the most to the rig?
A) wind speed?
B) the momentum of the solid concrete?

take care... hope you understand something new now!



Morgan O.

You are absolutely right. If the rig were mounted in concrete the
important force would result from wind velocity and sail area.

However -- we are discussing a rig installed on a BOAT and the
important force is righting moment.

i.e., in the case you are discussing the mast is fixed and therefore
the effective area of the sail is constant. In the case of a spar
mounted on a movable base, i.e., a boat, the spar moves and therefore
the effective area of the sail changes with changes in wind velocity,
thus the important figure is the force opposing the spar movement, the
righting moment.

Most people who have any knowledge of boats understand this fact
instinctively.

1'st... I undertand exactly what you are saying...

But tech /nature doesn't stop there. There is more to it. Especially that
part comes into account in this case, a 2 hull vessell.

You are stuck in what sometimes goes under the label "psychic prisons".


Morgan O.

On Sun, 24 Oct 2004 11:20:44 GMT, Morgan Ohlson
wrote:

On Sun, 24 Oct 2004 07:13:04 +0700, Egis/CORE wrote:

On Sat, 23 Oct 2004 16:58:02 GMT, Morgan Ohlson
wrote:

On 23 Oct 2004 16:01:42 GMT, Stephen Baker wrote:

you know little.

I don't like people to give faulty advice anyhow. I just liked to give a
hint to PeterM.A about that.

..but I will not fall to your standards of attacking...

...explained this way...

If you have a rig in solid concrete, which would matter the most to the rig?
A) wind speed?
B) the momentum of the solid concrete?

take care... hope you understand something new now!



Morgan O.

You are absolutely right. If the rig were mounted in concrete the
important force would result from wind velocity and sail area.

However -- we are discussing a rig installed on a BOAT and the
important force is righting moment.

i.e., in the case you are discussing the mast is fixed and therefore
the effective area of the sail is constant. In the case of a spar
mounted on a movable base, i.e., a boat, the spar moves and therefore
the effective area of the sail changes with changes in wind velocity,
thus the important figure is the force opposing the spar movement, the
righting moment.

Most people who have any knowledge of boats understand this fact
instinctively.

1'st... I undertand exactly what you are saying...

But tech /nature doesn't stop there. There is more to it. Especially that
part comes into account in this case, a 2 hull vessell.

You are stuck in what sometimes goes under the label "psychic prisons".


Morgan O.


Sorry old buddy but whether the boat has one hull or many the
calculation is the same since the force is the same - the force
necessary to heel the vessel, or to phrase it another way, the force
the vessel exerts in attempting to stay upright. the Righting Moment,
in other words.

Certainly the force necessary to heel a multi-hull is higher then that
necessary to heel a mono-hull but that doesn't change the fact that
the force applied to the mast is exactly the same in each case -- the
force necessary to heel the vessel against the opposing force of the
vessel trying to remain upright -- the Righting Moment.

To stay in the real world, if you approach a mast manufacturer with
the intent of designing a mast you will be asked for the righting
moment, usually RM30, i.e., Righting Moment at 30 degrees. If you
approach a marine engineer/boat designer for information regarding the
strength of a mast you will be asked for the RM30.

In short, no matter what you think, the rest of the world firmly
believes that the strength of a mast is directly dependent on the
forces opposing it, i.e., the force the vessel can exert against the
mast, the righting moment in other words.

Now it appears that either (1) you are wrong; or, (2) the rest of the
world is wrong. Take your pick.





Cheers,

Bruce
(k4556atinetdotcodotth)

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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Egis/CORE ) writes:

Sorry old buddy but whether the boat has one hull or many the
calculation is the same since the force is the same - the force
necessary to heel the vessel, or to phrase it another way, the force
the vessel exerts in attempting to stay upright. the Righting Moment,
in other words.

why hasn't anyone simply stated Newton's law?
for every force acting on a body at rest there is an equal and opposite force.
whether you measure the acting force or the reacting force they are equal.
in many cases, like the heeling of a boat, it's easier to calcualte the
reacting force that it is to calculate the acting force.
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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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On Sun, 24 Oct 2004 20:20:53 +0700, Egis/CORE wrote:

On Sun, 24 Oct 2004 11:20:44 GMT, Morgan Ohlson
wrote:

On Sun, 24 Oct 2004 07:13:04 +0700, Egis/CORE wrote:

On Sat, 23 Oct 2004 16:58:02 GMT, Morgan Ohlson
wrote:

On 23 Oct 2004 16:01:42 GMT, Stephen Baker wrote:

you know little.

I don't like people to give faulty advice anyhow. I just liked to give a
hint to PeterM.A about that.

..but I will not fall to your standards of attacking...

...explained this way...

If you have a rig in solid concrete, which would matter the most to the rig?
A) wind speed?
B) the momentum of the solid concrete?

take care... hope you understand something new now!



Morgan O.

You are absolutely right. If the rig were mounted in concrete the
important force would result from wind velocity and sail area.

However -- we are discussing a rig installed on a BOAT and the
important force is righting moment.

i.e., in the case you are discussing the mast is fixed and therefore
the effective area of the sail is constant. In the case of a spar
mounted on a movable base, i.e., a boat, the spar moves and therefore
the effective area of the sail changes with changes in wind velocity,
thus the important figure is the force opposing the spar movement, the
righting moment.

Most people who have any knowledge of boats understand this fact
instinctively.

1'st... I undertand exactly what you are saying...

But tech /nature doesn't stop there. There is more to it. Especially that
part comes into account in this case, a 2 hull vessell.

You are stuck in what sometimes goes under the label "psychic prisons".


Morgan O.


Sorry old buddy but whether the boat has one hull or many the
calculation is the same since the force is the same - the force
necessary to heel the vessel, or to phrase it another way, the force
the vessel exerts in attempting to stay upright. the Righting Moment,
in other words.

Perhaps ordinary engineering isn't good enough for boat historians.

Think...
Inert mass
Roll resistans
Non capsizeing vessells
Heavy ghusts

....and you get quite different scenarios.

It's quite alright to use old rules of thumb... but know their limits!


Morgan O.

Morgan O says:

Non capsizeing vessells

No such thing ;-)

Some boats will always right themselves, but there is NO boat that is
"non-capsizing".

Steve

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?????

On 23 Oct 2004 00:40:24 -0700,
(peterMelbourneAustralia) wrote:

Thanks for your input, all very scientific. Having the mast tested
using weights has some logic to it.

The boat is a proa, a multihull. The side says (there is only 1) is 4m
from the mast. Mast is 6m high. Righting moment could be rather high
as I was hoping for 50kg of water ballast in teh outrigger at 4m to
windward. 50L water ballast tank need not be full I guess.

The mast comes from a professional mast/spar maker.

The boat is a proa. Main hull is 23 long, 2 ft wide, boat weigh
unladen is less than 200kg. Not intending to break speed records.

I was hoping to do a comparison with a boat like a Hobie 14 cat. I
wonder what their mast dimsnsions woudl be, cannot recall the mast
section details of the windrush 12 cat that I used to sail, for some
reason never got around to measuring it. Feel that a hobie 14 - 12 has
simialr sail area/righting moment, is stayed out wide as per proa. So
what is good for that would be good for me.

N. Peter Evans


Here's another way to compare and contrast your selection with
comparable masts. Look at the sections shown by this aluminum mast
company - [Dwyer Mast Co]

http://www.dwyermast.com/families.asp?cat1ID=20&cat1Name=Masts

Notice that the lighter sections come in several shortish lengths and
the beefier sections come in correspondingly longer lengths.

This gives you an idea of the preferred section for a given length.
A proa will be stiffer in roll than a regular rig, so you would want
to compare a section where your desired length is the shorter of the
mast length offerings for a given section.

Short masts (as fitted to dinghies) are not typically let out in
stiff blows, so this is another factor to bear in mind.

Still, this exercise should reinforce your judgment.
I am disappointed that the other respondents in general gave you
every suggestion short of anything actually helpful.

I have never seen so many different way of saying,
"Ooh, mast section selection is awfully difficult and mathematical,
and far too difficult for a regular person to consider...."

I hope you will realise sooner or later, that the basis of many mast
selections is in fact "cut and try" and "It worked on that and the
other so it should be OK on this..."

Regards

Brian W