On 21 Oct 2004 17:01:27 -0700,
(peterMelbourneAustralia) wrote:

I am almost finished designing a 23ft proa and am aware of a new
aluminium section for sale; I would like to know if it will be strong
enough for a mast.

The proa has a mainsail around 100sqr feet (a little less). At either
end are small furling jibs for balance (30 sqr ft). The mast is round
section, 6m high, 82mm in diameter and 2.6mm thick. It is made of the
proper high grade aluminium from a professional mastmaker. It does not
have a track, the mainsail being attached to the mast via lashings.
There are 3 stays at the top, one forward, one aft and one to
windward, 90 deg to other 2. Three stays is all a proa needs!!!!

From my days sailing windrush catamarans, I am a littel worried that
the 82mm diameter might not be enough. I know circular section is not
high performance but that is not of concern. I am keen on this section
because it is new, made of high grade metal, from a professional and
failry cheap, $250 Australian.

I intend to go cruising, thus needs to be able of withstanding strong
winds. Have every intention of reefing when gets blowy. 23ft proa
weighs 200kg unladen.

Will/should 82mm diameter be sufficent?

N. Peter Evans

A 3.5 in diam round section with 0.1 inch walls sounds reasonable.

You might like to proof test the mast in your application.

Take 4.5 lb /sq ft as the wind load at 60 mph.
130 sq feet would load the structure with about 600 lbs if caught
broadside in a 60 mph gust,.
Support the mast at the foot and at the level of the stays, then
dispose the weight of 600 lbs of kids - perhaps eight or ten of 'em?
Have them sit along the mast in a representative way.
.. Does the mast retain a set after this load? It's not strong enough!

Brian Whatcott Altus OK

Brian says:

snip Mr. Wizard experiment

Does the mast retain a set after this load? It's not strong enough!


Correction, Brian - it WASN'T strong enough ;-)

Steve

I knew you'd just love this one!

:-)

B

On 23 Oct 2004 01:19:18 GMT, ospam (Stephen Baker)
wrote:

Brian says:

snip Mr. Wizard experiment

Does the mast retain a set after this load? It's not strong enough!


Correction, Brian - it WASN'T strong enough ;-)

Steve

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

Nicholas says:

Thanks for your input, all very scientific.

;-)

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.


If you can weigh the boat, each hull singly, that would help. Yopu don't have
to take it apart, just slide a scale under each hull in turn and record the
weights (yes, it can really be that simple.)
From that, and with the 50 litres of WB, a rough RM can be obtained.

Let us know...

Steve

On 23 Oct 2004 12:26:51 GMT, Stephen Baker wrote:

Nicholas says:

Thanks for your input, all very scientific.

;-)

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.


If you can weigh the boat, each hull singly, that would help. Yopu don't have
to take it apart, just slide a scale under each hull in turn and record the
weights (yes, it can really be that simple.)
From that, and with the 50 litres of WB, a rough RM can be obtained.

Let us know...

Steve

Perhaps he could take a little help of old Archimedes! (do some maths on
displaced hull volume) ;o)


Morgan O.
Wondering if the man go'na bite this time too?

On 23 Oct 2004 12:26:51 GMT, ospam (Stephen Baker)
vaguely proposed a theory
.......and in reply I say!:

remove ns from my header address to reply via email

I reckon the guy who said to talk to a spar maker had the right idea.

At least check out a google of

"righting moment" multihull

If you can weigh the boat, each hull singly, that would help. Yopu don't have
to take it apart, just slide a scale under each hull in turn and record the
weights (yes, it can really be that simple.)
From that, and with the 50 litres of WB, a rough RM can be obtained.

This is a really light boat, IIRC.....here we go..... "23ft proa
weighs 200kg unladen."

Sounds scary actually, as a cruiser.

This means that weighing the boat and using that as a righting moment
would be questionable, as the weight of crew and gear would
_significantly_ alter the system. You need to weigh in "cruising
trim".

And how far apart are the hulls, etc?

Also some proas' righting moments when the ama is to the lee will be
the floatation capability of the ama.

What you need to do is place the boat in the water and do some real
righting force tests, under loaded conditions IMO. This was not
uncommon practice as I remember it.

Also wrt multihulls. They release pressure on the mast much more by
acceleration than by heel. Their righting moment should never be
_needed_ ....joking G.

Seriously:
- 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.
- when sailing at speed on a brorad reach, and _being kept flat_ as
multihulls largely should, their high speed generates significant
apparent wind. This has to be accounted for.
- multis can actually suddenly _increase_ their righting moment as
the weather hull lifts from the water, or as the lee ama of a tri /
proa touches the water.

Also, on a multi, especially a lightweight one, the mast and stays
hold the boat together. So there are many other strains on the rigging
as the boat gets twisted about by wind and waves.

I mucked about designing, building and repairing cats. I have owned
and sailed several, including a Crowther 23' International. It weighed
maybe 450Kg unladen, and had a _heap_ more mainsail than that being
suggested. But it had a huge mast. 150mm * 100mm * 4mm, maybe. I can't
find the specs any more.
************************************************** ***
Have you noticed that people always run from what
they _need_ toward what they want?????

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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homepage: www.ncf.ca/~ag384/top.htm
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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