In reply to Tom and Felix,
Atlantic White cedar is not all that much heavier than RWC. Nick Schade lists
NWC at the same specific gravity as RWC (0.32 relative to water, (Figure 3-20
page 37) Bram, he also lists RWC as 20lb per cubic ft (Figure 3-22, page 39).
The two numbers are in agreement. The data for AWC shows that it is slightly
more elastic and slightly more prone to dents, but, I doubt that it will be
significant. Anyone disagree with that? I plan to use it for an accent strip or
two. The cedars are the lightest and strongest cores for this type of
application. Redwood is also just a smidge heavier than RWC for those looking
for a different accent material with other desirable qualities. Alternatively
endgrain balsa sheets could be used as core to build the hull on a male mold if
the hull was to be painted. (Sorry, but I had to mention that Bram :-0 ) The
last idea would be a lot more work, make a boat too light and might not be
legal since rules in consideration would require new core materials to be
sanctioned by the ISSA technical committee as "in the interest of the class".
The real interesting question is, should you decrease the thickness of the
strips? IMHO don't do it. I can tell you that making the boat stiff is a very
good thing. Bram has gone to lengths to ensure the scantlings for the Swift are
(more than) adequate. The reason we are going to the effort of using a
structurally resilient core like cedar is to allow the boat to be, and remain,
stiff throughout its (long) life. There are (performance) dinghies which do not
hold rig tension past 1 season, sometimes less, (This I know to be true, for I
have one). Interestingly, they may not be cheaper or less heavy relative to a
Swift. This seriously impacts their performance and results in steep drops in
used boat prices. (Bram took a chainsaw to one skiff hull that was not worth
selling, I hope I can find a sucker when my Swift is finished, I do not posses
a chain saw).
If you are not interested in WHY core thickness is important skip this
paragraph.
My engineer buddy, Ian Glasgow, sent me this explanation. Strength and
stiffness of the hull is exponentially dependent on the thickness of the core.
If a strip of the laid up hull was considered a beam, the stiffness of the
beam, in bending, is calculated from the equivalent flexural rigidity, (EI)eq.
Using the parallel axis theorem (EI)eq = (Ef.b.(t^3)/6) + Ec.b.(c^3)/12 +
Ef.b.t.(d^2)/2 where the beam width is b, the core thickness c, and the face
thickness, t. The beam thickness is d where d = c+2t; Ef and Ec are elastic
modulus of the skin and core respectively. For a core with good shear rigidity
the third term is substantially larger than the first two. Therefore decreasing
skin thickness has less effect on stiffness than a corresponding decrease in
the core thickness.
Putting things in perspective - The minimum hull weight of the Swift is
generous. A well constructed boat, where the builder has paid close attention
to the glass to resin ratio, will be probably be underweight! The class rules
will include all up weight as well as Lamboley style swing tests which measure
center of (vertical and horizontal) gravity and the Mass Moment of Inertia of
the boat as a pendulum.. So you will not just be able to fill the deepest part
of a light hull with resin (lead is illegal) and have a hull that can compete
in National events. I believe the relevant rule (in consideration) will
penalize any boat failing the swing test by bringing it to the nominal minimum
weight/MMI (rather than the minimum minus variation).
So what can and should be done?
If you are using the 6oz external glass layer you will save a lot of weight
anyway over the 9oz alternative. Make sure you don't use plain weave s-glass.
The 8 harness satin S-glass style has an inherent increase in modulus and the
decreased millage of this style, most importantly, will help you get the
glass/resin ratio higher, thereby decreasing weight. Keep in mind that for a
"very good" hand lay-up, the resin component will be ~ 60% of the skin weight
and the skins will be 75% of the core weight or 43% of the "all up hull
weight". Also be aware that the 8 harness satin is harder to use than plain
weave, so have practical minded helpers at hand on the day. 9oz 8HS is said to
be a bitch! (pardon my Australian strine) You might consider vacuum bagging
the external skin to eliminate trapped air pockets and increase glass to resin.
It's a lot of extra work! If you do this on the inside surfaces be sure to use
the peal ply designed to leave a smooth surface so that it will not hold a
water film. (See Bram's water absorption tests).
If you are planning using other heavier accent materials think of using
veneers. I'm not really recommending it for the Swift but one cold molded 11ft
skiff I owned (all brightwork) needed nonskid in certain spots. I used an
unfinished teak veneer, glued down with epoxy, instead of peel-off-stick-on
sandpaper style nonskid. It worked out very nice, just enough friction for me
when wet the pores were raised and maintained the beautiful wood appearance of
the boat with grey weathered teak accents.
If your boat is under weight you will be adding permanent ugly lead chunks to
the deck where they can be admired and they may need to be placed
asymmetrically (fore and aft) to pass the Lamboley test
Finally, weighing and swinging the boat will most likely be done with only the
gear attached to the hull that is a royal pain to remove. Save as much weight
as you like on your blades, rudder cassette, tiller, extensions, rigging,
sheets, spars (except spin pole) and sails. Loosing a few pounds from the mast
will obviate any advantage you can get by designing within the Lamboley limits.
If you are fat like me, loose weight. Your spare tire is the only weight you
will be allowed to adjust during a regatta. But within reason, as Martha would
say about movable ballast, "It's a good thing!".
This post is already long I'll post again soon about the milling of strips.
Ciao
Greg
Swift Solo USA 009 - Rose
49er USA023 (for sale by the way :-) )
PS my opinions only. I'd be glad to debate here on the server.
----- Original Message -----
From: tom owen
To: BDally6107@xxxxxxx
Cc: Felix Schliebitz ; Greg Ryan
Sent: Friday, September 26, 2003 8:34 AM
Subject: Tom & Felix in RI
Good morning Bram,
we have a question for you regarding the cedar strips:
obviously there is a larger supply of good quality atlantic white cedar this
side of the USA, but we know that it is more dense than red.
would it be possible to use smaller strips of white cedar, so overall weight is
the same as red, while still maintaining the stiffness and strength required?
is there a particular reason why you used red cedar for the strips?
can you recommend any good suppliers of red cedar in the NE? seems a long way
to ship good quality red cedar from the west coast.
we plan to mill our own strips.
Greg - where did you get your red cedar from? did you mill it or buy in strips?
any help greatly appreciated.
many thanks,
Tom.
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