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 Post subject: Diamond wires setting
PostPosted: Mon Aug 24, 2015 6:35 pm 
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How do you gauge how tight to make the diamond wires on my h18 ?


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PostPosted: Tue Aug 25, 2015 4:48 pm 
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I hate this topic. :roll: Many of the top sailors have different opinions with varying results. I like mine tight so the mast doesn't bend much on the minor but others keep theirs very loose.

Some will say tight for light wind and loose for big wind and others just leave it alone (Medium tension) and beat everyone.

Unless your sailing perfectly and are just trying to catch the last guy on the course, then I would say set it so that you can almost push the diamond wires to the mast about 2 feet up the bottom of the wires and leave it.

Good luck and if you make an adjustment that allows you to catch that last guy.........let us know. :mrgreen:

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H18 '85
H18 '89 "Knotty Passion"
H20 '96 "20/20 Vision"
Fleet 259 Central Coast California


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PostPosted: Wed Aug 26, 2015 5:46 am 
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Not to disagree with you wscotterwin, but changing the diamond wire tension has seemed to be a major factor in being able to keep power and shape in my main, or to flatten and reduce power. The interaction between the diamond wires, mast rotator and downhaul cannot be understated, in my opinion. I'm not sure how I'd be able to change the powering more effectively if I didn't adjust diamond wire tension!

The convention that I've heard and adhere to is:

tighter for light wind/heavy waves/heavy crew - more shape & power
looser for heavy wind/calm seas/lightweight crew - less shape & power, more speed

Now if the topic was spreader rake, I'd agree with you - useless adjustment! :lol:

The way you measure the tension is like wscotterwin said - try pressing the diamond wires to the side of the mast, both at the same time. If you can get the to touch the mast with a moderate amount of pressure at 12" up from the lower anchor point, that's about your "maximum tension" setting. 36" up is about your "minimum tension" setting. the 2' mark is pretty much your 50% compromise. I'd suggest you play with it and decide on your own if it's a worthwhile adjustment.

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PostPosted: Wed Aug 26, 2015 5:47 am 
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I set mine snug and leave them the same for all conditions. By snug I mean that there is no perceptible looseness in the wires, but not so tight that they are inducing any pre-bend. I think they can touch the mast at about 1 foot up from the bolt with moderate to heavy force.

My opinion is that you want to keep as much power down low in the rig because the 18 is a relatively under-powered boat. Plus the comptip is so flexible that you can do all the bending you need with downhaul tension to bend the comptip and flatten the top of the sail.

sm


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PostPosted: Wed Aug 26, 2015 8:24 am 
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SabresfortheCup wrote:
tighter for light wind/heavy waves/heavy crew - more shape & power
looser for heavy wind/calm seas/lightweight crew - less shape & power, more speed


Let's think about that......

Tight for more shape? I don't think so. Tight would prevent the mast from bending on the minor and you would have less shape and have the ability to flatten the sail out more with down haul. Flat = more efficient sail and less drag through the air as winds pick up........but, as winds tend to over power the boat, only a little bend in you mast can come from the down haul (little on the 18 on the Major access) and alternative methods of depowering have to be looked at (Mast rotator, Traveler, sheet...in that order).

Loose for less shape? I don't think so. Loose will cause the mast to bend on the minor axis (Sideways) and make the sail appear to be fuller and create more drag (less efficient) and slows the boat, but the increased shape may keep more power for waves.

In my humble opinion, When you loosen the diamond wires, you leave speed on the beach before you enter the race. I really prefer to depower my boat other ways without using the mast. Maybe if I KNOW it's going to be over 20 Knots I would consider loosing the wires but only in extreme situations. But even then, for me to take the time to loosen the diamond wires, I would have to be chasing someone really important (Sohn, Marshack, etc. etc.) :o

Foot note: I destroyed a new sail (Main) in the 2014 Hobie NACs by over using the down haul. The luff of a dacron sail just can't take that much pressure. Was it worth it? As far as I am concerned, in big events, bring it all and leave it there........ Yes. :mrgreen:

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H18 '85
H18 '89 "Knotty Passion"
H20 '96 "20/20 Vision"
Fleet 259 Central Coast California


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PostPosted: Wed Aug 26, 2015 9:48 am 
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Scott, agreed - tighter = less 'shape'

Ours are snug (a bit tighter than SRM's) for different reasons.
We run a square top (not a good idea), and therefore run with mast rotator all the way out to get the mast to bend a bit.
Then I can crank on the mainsheets to get a tight leech.

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SOLD 1989 Hobie SX18 Sail # 1947 "In Theory..."
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PostPosted: Wed Aug 26, 2015 3:44 pm 
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I don't mean to disagree with you wscotterwin, but the information I posted does adhere to at least one school of thought.

The rationale behind the tighter diamond wires for more shape is as follows:
_______________________________________________________________________________
The shape of the sail does not come from bending the mast, but from the cut of the sail. The luff of the sail is specifically cut with a "curve" to it, just as each panel is cut wider in the center than at the luff and the leech. If you unroll your main on the ground, the sail will lay relatively flat and the luff will appear to curve aft rather than having a straight edge perpendicular to the bottom of the sail. However, when the sail is hoisted up a straight mast, the curved shape of the luff will result in the excess material in the center falling off to one side or the other, building shape into the sail.

When the diamond wires are tight, the mast bends less and the sail maintains it's shape. When the diamond wires are loose, the tension in the downhaul and mainsheet bend the mast on its minor axis more readily, allowing the mast to bend to the curved shape of the luff and allowing the sail to flatten out.
More mast bend = flatter sail

Note that, with the exception of rigs carrying extreme spreader rake and extreme diamond wire tension to pre-bend the mast on its major axis, the mast will ALWAYS bend on the minor/weaker axis rather than the major axis (you can never keep the forces purely in line with the major axis, especially when rotating the mast, so the mast always bends in the direction of the minor axis). Rotating the mast allows for the mainsheet and downhaul tension to bend the mast more readily, but tighter diamond wires reduce the effect of mast rotation on mast bend, and looser diamond wires enhance the effect. Mast rotation is of course always desirable to an extent in order to ensure a smooth entry of the mainsail and encourage attached flow over the low pressure (leeward side) of the mainsail. Mast rotation also ensures that any mast bend on the minor axis, to whatever extent it is present, will bend the mast mostly forward rather than sideways into the slot.

I agree with you on when more/less shape is required. The key to remember is more shape = more lift AND more drag - aka, more power, but less overall speed. Hence why an airliner deploys flaps on takeoff and landing - the flaps create a fuller shape to the wing and more lift at slower speeds, but also create more drag, and they keep the plane from going faster. When flying at altitude, retracting flaps reduces the drag and allows the plane to go faster, but also reduces the lift, so the plane can (and must) go faster to generate the same amount of lift to stay airborne.
____________________________________________________________________________

I'm mostly reiterating here the information from "Catamaran Racing for the 90's" (Chapter 7: Strings to Pull, pg. 79 - 85), but it is information which corresponds with my own experience on the water.

Clearly not all Cat sailors agree with the information presented above, but that is one school of thought. I actually hadn't heard of anyone disagreeing with that theory, but most of my experience with other cat sailors has to this point been soley through this forum and cat sailing books, admittedly written decades ago. Certainly interesting to hear that not everyone rigs their boat quite the same way!


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PostPosted: Wed Aug 26, 2015 10:43 pm 
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Sailshape is a combination of the sail cut and the bending of the mast. For the purposes of this thread I figure we're talking about a stock cut Hobie 18 sail or Hobie 18 sail clone, so sail cut is negligible.

There are two schools of thought on the 18.

One is the older style where you sail with a loose rig to depower, so that the mast can bend to fit the luff curve of the sail and produce a flat sail, depowering the boat. The bend is produced by the minor access by allowing excessive mast rotation (90*) The rig is powered up by tensioning the diamond wires to an appropriate setting for your weight, and the conditions you sail in. For my typical 350lbs of combined crew weight in 12-20knts of breeze I'm with SRM with diamonds tight enough to touch about a foot, maybe 1.5' up the mast from the bolt.

The other school is the use of the major axis of the mast to bend in line with the sail by cranking on the downhaul. Wscotterwin has an 8:1 Miracle 20 style setup on his boat that allows him to do this, at the expense of his main. The Hobie 18 was designed around the older school of thought, and does not have sufficient strength in the luff of the sail to regularly tension the luff hard enough to bend the mast on the major axis. Newer boats use this tequnique because it is much easier to switch from low power to high power, by just cranking the downhaul rather than adjusting diamond wire tension. When using this technique, you can tension the rig for appropriate power and have the ability to adjust as you're sailing.

In both cases mast bend is also induced by the tension from the leech applied by the mainsheet. A 7:1 or 8:1 mainsheet system can help with that.

I've been racing and placing well for a few years now and am starting to get back into testing this out. The only concrete information I have is that if you can touch the wires 3' up the mast and you're sailing with 300+lbs on board, you should have at least 25-35knts of wind or you're dumping too much power.

Ultimately for me what it comes down to is that under the conditions that I sail in, I'm not looking to depower much. If I am, I'm into survival mode, furling the jib or moving cars all the way back, traveling out, centering the rotator, etc is more than enough to get me home. When I'm working with moth farts, I want all I can get.

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Tom
Fleet 259, Central Coast CA
H18 ('81)
H18 ('85)
H20 ('97)
H18 ('78)


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PostPosted: Thu Aug 27, 2015 7:09 am 
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Well said, Tom.

Put me in the old school camp. Tried the new school and couldn't make it work.

At around 300 lbs total crew weight, we start getting overpowered around 17-18 kts. Loosening the diamond wires works to de-power for us. It is a decision you are stuck with for at least one race. We carry the tools necessary to adjust on the water (and often do). Better to have the mast set for the lulls and use other methods to deal with the puffs.

Jim


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PostPosted: Thu Aug 27, 2015 10:04 am 
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Thanks Tom & Jim, I didn't realize that wscotterwin and John are trying to bend their masts on the major axis, or that the 7:1 and 8:1 mainsheet & downhaul systems were more effective at bending on the major axis. I had thought that without using aft raked spreaders and very tight diamond wires to pre-bend the mast (which doesn't seem to work on the 18), it just doesn't work. Even with my 5:1 downhaul, it seems that without pre-bend in the mast, it'll always prefer to bend in the minor axis. I think your diamond wires would need to be tighter than all hell to strengthen the minor axis enough for the mast to prefer to bend in the major axis, especially once you add in some rotation!

I'm often soloing at 160 lbs, so I end up having to play with my diamond wires more often.

My boat has cylindrical nylon? locking "nuts," so it's a little easier to adjust (no tools needed), but the downside is that they do occasionally loosen up on their own.


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PostPosted: Fri Aug 28, 2015 10:46 am 
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See why I don't like this topic. :lol:

My number one rule. Don't leave the speed on the beach. Learn to depower on the water with the tools available.

Does anyone want to start a thread on batten tension? That's my next least favorite topic.....

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H18 '85
H18 '89 "Knotty Passion"
H20 '96 "20/20 Vision"
Fleet 259 Central Coast California


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PostPosted: Fri Aug 28, 2015 11:08 pm 
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wscotterwin wrote:
See why I don't like this topic. :lol:

My number one rule. Don't leave the speed on the beach. Learn to depower on the water with the tools available.

Does anyone want to start a thread on batten tension? That's my next least favorite topic.....


The tighter the better right? ;)

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Tom
Fleet 259, Central Coast CA
H18 ('81)
H18 ('85)
H20 ('97)
H18 ('78)


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