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Thread: GIS sail tweaking
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7th December 2012, 05:40 PM #511Senior Member
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MIK: From your description it looks like you mean to round near the chine... I was more talking about making the rear section of the bottom a bit bended, so that together with the rocker, it will create a more round shape while the boat is in level. You wrote: "you have to imagine the boat heeled"
path2991.png
I made a quick picture of what I mean. To the left is how a transom might look like, and to the right how the water will be touching the bottom because of the rocker and the bended bottom near the transom. This makes it more streamlined which would help in at least low speeds. I was curious of how it would affect the boat in high speeds, as that is what the goat is aiming for.
But the more I think about it, the more difficult it appear to be to achieve this with just plywood, as you can not bend it to such a shape easily without making several cuts into it.
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7th December 2012, 09:36 PM #512
This stuff does my head in, but I'm still interested. One thing that might happen is the waterline length may reduce when heeled? Not a problem for dinghies that are sailed flat though.
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8th December 2012, 12:08 AM #513
I think the waterline length argument is well overdrawn. Upwind non trapeze boats are usually the most efficient about a knot below hullspeed (in Laser, NS14 or Goat size) so that the waterline length variations don't really effect much.
For boats that plane upwind - they are well above the hullspeed - so waterline length is not hugely critical.
For the example of the rounded transom ... you have a long waterline length along the keel and less parallel but a foot or so out from the keel - how fast should the boat be sailing?
The reality is it is about volume distribution and how much the boat squats down into its wave train. Boats with more volume in the ends - dinghies - or yachts that have long overhangs that "extend the waterline" really have ways of reducing the amount they squat down into the trough of the wavetrain when it is under the mid body of the boat. It's not so much about waterline length I would say ... it is about volume and the distribution of the volume.
If waterline length was so critical - we would see a huge surge of speed one way or another when the forefoot comes out pitching over a wave. Just don't happen.
i think I'm being grumpy
MIK
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8th December 2012, 12:16 AM #514
It is not to bad to make a shape like this as it is a bit like turning up the corner of a page of paper.
The normal reason to do it is to prevent the corners of the transom from dragging deep in the water when the boat is heeled. It is one of the "reasons" that Northern hemisphere designers or maybe just "talk" gave for persisting with round bilge designs. One of the "traditional" reasons given.
So the reason for the feature is about heel.
But the bad effect from it is at higher speed when it means that there is excess rocker at the back end of the boat except along the keel line which is unaffected.
MIK
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8th December 2012, 11:45 AM #515
Yes yes yes and yes - pics. Maybe I need to move this to a sailmaking "thread" (seriously - we don't really have one dealing with that specifically.
MIK
I did make a new sailmaking thread and moved the posts there - Sailmaking 101 for Goat Island Skiff, Beth
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8th December 2012, 11:34 PM #516
And so I did.
There was a discussion between Warmbeer, davlafont and others about sail design elements - do you want them moved to the new thread or not.
What do you think. Also if you really don't like me messing with the location by making a sailmaking thread - I ca move the posts back.
MIK
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9th December 2012, 01:30 AM #517Rusty Member
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I think that's a smart idea, having a different thread. Heading there now!
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20th January 2013, 02:52 AM #518Rusty Member
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limiting traveller
I can't fully visualize how and when limiting the traveller is beneficial. All I've got so far is restricted traveller = more twist, unrestricted traveller = less twist and flatter leech on heavy sheeting. I'm thinking sheeting pressure and horizontal versus vertical vectors. More horizontal means less vertical, so the boom can rise, creating more twist, and a looser leech. But, doesn't this also change the relative sheeting of the body of the sail when thought of as horizontally parallel chords from foot to head, in that less vertical vector (restricted traveller) will mean the foot has a flatter chord relative to a body chord compared to traveller position with a greater vertical vector which would have the parallel chords more closely matching? Can someone explain this, and under what conditions you would use it? How do you calibrate it for your sail and or the given sailing conditions?
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20th January 2013, 06:37 AM #519
GIS sail tweaking
Travelers, leach tension and twist was extensively discussed some months ago but don't ask me which thread! There may even have been diagrams and pictures.
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6th February 2013, 12:27 PM #520Senior Member
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I think what you say above is pretty much correct, though I get a little lost by the bit I've highlighted in blue. Traveller position is all about getting the twist right.
I think you absolutely need to have three or four tell-tails up the leech. You want to get the sail twist so they are all flying back nicely when on the wind. If the tell-tails are flicking around the back the leech is too tight/stalling. So trim the boom to about 10 degrees and sail on the wind so the lower tell-tail is streaming back. If the upper leech tell tails are flicking onto the leeward side of the sail when the lower ones are flying nicely and lower sail shape is good, you need to let the upper leech out a bit, so you bring the traveler position in a bit/ restrict the traveler travel a bit.
If the converse is the case, then sheeting position needs to be further out, pulling the traveller as tight as possible will help.
I have my traveller quite far back (across immediately in front of the transom corner knees). My traveller is pulled very tight and is restricted so my sheeting position is about 6 inches in from the gunwale when on the wind. (a little more on the port side, a little less on the starboard side to account for the boom being on the port side of the mast.) I feel that lets me get all the leech tell-tails flying when I'm on the wind better than otherwise. It also has the advantage of stopping the block beating up the gunwale at that point.
Lots of different sails/sail cuts/yard stiffnesses and other variables make it hard to say what settings will work best for a given rig, but I think the principle of always having and looking at the tell-tails and trying to get/keep them flying is the story. Also being able to set up the rig exactly the same each time and track and analyze changes you have made. It's no use if you change something on one excursion, note that it helps, then go out next time and find it doesn't work as you inadvertently changed something else when you rigged it the next time.
Ian
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