[MHml] Friction in lines and blocks

[sysfx]

Rob,

Good morning,

I've been observing the work of a group developing a 12ft entry cat for kids with a self standing mast and sleeved sail. One of the designers demonstrated that the use of two solid struts to support the mast at about boom level (google "Hobie Bravo" to see it) almost cancels compression on the main beam (=lighter structure), greatly reduces mast flexion (= lighter mast) and, since the struts' forces on the structure are mostly horizontal and outward, the tramp tension helps cancel them (=lighter structure).

A cat is certainly different from a proa, but after seeing a similar strut system in Sailrocket (speed record attempt proa) and remembering having seen them in older proas, I thought the same principle might be interesting for your boat.

All the best,
Luiz Schechter

----- Original Message ----
From: Rob Denney <harryproa at gmail.com>
To: Informed discussion of multihull issues <multihulls at steamradio.com>
Sent: Thursday, December 6, 2007 11:03:50 PM
Subject: Re: [MHml] Friction in lines and blocks

G'day,
If Roy's cruising cat with a stayed mast is the dark side, then this is a comment from way beyond the dark side ;-)

On the unstayed masts on harryproas which do not have ballestron rigs, such as my 15m solo boat, the wing mast is round where it goes through the deck.  This round is continued up 300mm/12" from the deck.  The boom fits on this part of the mast, in plastic bearings.  The boom can rotate independantly of the mast but cannot move vertically.  ie, it is self vanging.  Leech control is done with a multi purchase block along the top of the boom.  Angle of incidence of the rig is controlled by a 3:1 line from the cockpit to the end of the boom.  This line sees less load than a traveller line as there is no traveller friction to overcome.   

Much cheaper than the systems mentioned above as most of it can be built, rather than bought.  Probably lighter as well as there is no traveller, winches or huge blocks required.  Whether the deep boom is a plus or a minus I am not sure, but suspect it does not make much difference. 

Won't work on most boats, but is great on unstayed masts.

regards,

Rob

regards,

rob


On Dec 7, 2007 5:56 AM, Roy Mills < rsirfj at shaw.ca> wrote:


----- Original Message -----
From: "Claas van der Linde" < Claas.vanderLinde at gmx.ch>
To: "Informed discussion of multihull issues" <multihulls at steamradio.com>
Sent: Thursday, December 06, 2007 9:56 AM 
Subject: Re: [MHml] Friction in lines and blocks



> Ross writes:
> >In carbon the boom will save more than that - so overall a positive
> >saving of weight.
>
> That's the right attitude. On our boat modifications, replacements or
> repairs are only allowed if they result in an overall weight saving.



       A comment from the "Dark side", having seen little which has not 
been from the racing types.  My main sheet is an 8 to one set up as per
Tennant's diagram. lead through a clutch and passing by a Lewmar 24 2 speed
none selftailer. The Traveller is a 12 foot length of Harken track and 
positioned by a 3 to one tackle, also passing through a clutch located
alongside the main sheet clutch,and passing close by the same winch. All
blocks are the 3 inch Harken Big boat ball bearing jobs. TYpically I do not 
need the winch to tighten the mainsheet as much as I want for the 490 square
foot main provided I can get both hands on the sheet, if I can't, then the
winch comes in handy. I sometimes do need the winch for the traveller, but a 
lesson learned a few years ago helps a lot to minimise that too. I had
turned a mark from a broad reach to a beat, the traveller was well down, the
main sheet tight. I tried to wind in the traveller without easing the main 
and ended up breaking one of the traveller blocks, which at the time was not
a 3inch Harken. So I learned to dump the main sheet, haul in the traveller
hand over hand, then haul in the mainsheet similarly. May not be up to 
racing speed but it always seems to work fine.  One thing I have in common
with earlier posters is that when the main sheet is tight the traveller will
not ease, despite the ball bearing car, until I have eased the main, and as 
they have found it does not require much of an easing to let the traveller
move down, then the main can be hardened again. The traveller will move over
the full length of the track because the two double blocks on the boom are 
spaced about 18inches apart, something you might think about Ross.

       I don't hand hold the sheets, relying upon the fact that the main
and traveller clutches are close by the tiller in each cockpit and the sheet 
tails are stuffed, not coiled, into tail bags so should run freely.  Like
Martin I launder the sheets each winter, though I had not thought to use
fabric softener. Unlike Claas I do not restrict modifications to weight 
savers only, the 550 pounds it cost me to fit the sitting headroom 8 seater
cabin is worth every ounce, and more.


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