diamond tension & spreader rake

I do have some information on it:

57mm spreader rake, 810mm spreader width, 39 tension on the diamond wire, measured with

classic

Loose gauge, model A.

This is for an Alter cup blade, so aluminium mast and Glaser sails.

I thought Matt sails with 64mm spreader rake, 840mm width and also 39-40 tension.
I wanted a little less prebent, because we're a bit heavier.

To be honest, i hardly ever change my diamond settings. Just by applying enough mainsheet tension the sail becomes pretty flat if needed. Cunningham will also help a bit.

Geert

Terry, they do it to induce mast pre-bend (front to rear, not side to side). More pre-bend creates a flatter main sail all the time, ie. you cannot make it as full again unless you un-do what you did. But if you know it's going to be light, or if you know it's will be very windy, you can adjust accordingly, more or less prebend.

From poor memory, diamond tension affects the middle of the sail.
(Top & bottom of sail can be adjusted by other means which is another thread)
going half up the mast I would want about 15% of draft in marginal trap conditions,
down to 8% for double trap conditions. (%Draft = depth of sail / Chord)
which has to coordinated with leech tension, and fined tuned with downhaul & mast rotation.

I would start with the fullest sail possible(loose diamonds wires), then flatten it out, more each day,
until the fine tune controls give you a workable range (to keep the hull down) Two boat testing helps.
(Many sailors start the other way, with a flat sail and make it fuller)

Perhaps, someone has the time to explain the affects of spread rake on dual axis mast bend.

We have been re-adjusting our AC Blade. We are light weight at 265-270 and have the spreaders as wide and as back just about as far as they will go. In light air we run 39 and in heavy 41. This is working great for us. The drive is there but not being over powered. Also allows for the downhaul to be most effective. Any ways hope that helps.

Geert, Joanna,

Thanks for the information. We'll try to get into it at Carnac.

Did you enjoy the Alter Cup?

Here are the settings that we are using on a couple of Alter Cup Blades with the Glaser sails.

Ed's Mast
Spreader Rake - 2 & 5/8

Spreader bar length from bolt nearest the mast to diamond wire - 17

Wire to wire at spreaders - 33.5

Prebend - 1 & 1/8

John's Mast
Spreader Rake - 4.75

Spreader bar length from bolt nearest nast to diamond wire - 17.5

Wire to wire at spreaders - 32

Prebend - 1.5

We just started racing so we'll see how it goes. Ed

What are crew weight of each of you and do you race predominantely uni or sloop

An extra almost 1/2" is a lot given the shape of this mast. Watch the luff track as you get a lot of prebend as it will get very wavy.

Also you need to match the sail to the prebend. Too much or to little and you get a discontinuity in the shape around the top of the diamonds. Also look at the sail before messing around too much. If you can apply down haul and sheet pressure and get a flat sail, any additional pre-bend is doing nothing for you except either messing up your shape or just lessening the range of conditions it works well in. It is not possible to get flatter than flat.

Crew Weight

I am 180 and John is 200lbs ?. We have three AC Blades racing uni here weekly, and one T 4.9 but he doesn't count in this comparison. So far I'm very happy with my rig. I have yet to get out in breeze above 16 with it and if I'm unable to settle the boat down at that point I'll dial in a little more.

We just wrapped up a regatta this weekend where we had John's and my boat on the same course. I was uni and John had Noah the supercrew weighing in at 100lbs. Conditions consisted of some lighter 6 kt stuff with the majority being 8 to 12 with puffs into maybe the 16 kt range. I was able to de-power in all conditions and the boat ran real good. I was also able to power up when it came time to shift gears. John may have been lacking some power in the lighter breeze. Hopefully he'll chime in with his thoughts.

Just to make sure we're comparing the same things...

How are you measuring prebend? I've done it with a string from the top of the mast to the bottom, held tight, measuring the greatest deviation from the edge of the mast to the string.

I think it needs to be measured at the spreaders- but that should be the point of the most deviation anyway, right?

Eric. Yes. Laying the mast on it's side I'm using the halyard pulled nice and tight to the mast track at the very top and very bottom to measure the deepedt point to the mast track, usually found at the spreaders. Ed

We have been playing with the tension some, will have to get the exact measurments, but know the diamond wire tension around 40-41 has been working best lately.
Any one know what prebend the Glazer sails are set for?

Yep, I'm re-opening the topic...

I have measured a lot of the diamond wires, but I still have difficulties to compare to the measurements above for example.

Question 1: how to measure spreader rake? And doesn't rake change when adjusting the length of the arms?
Question 2: what is meant with prebend here? Distance from sailtrack to wire width or distance between sailtrack and chord from tip to foot of the mast?

These are the measurements I did:

- Diamondwire tension: 25 (gage in attachement).
- Distance from sailtrack to wire width: 800 mm
- Length of spreader from diamond wire to bolt at the mast: 48mm
- Spreader width (wire to wire): 945mm
- Mast prebend (distance between sailtrack and chord): 12mm

What is useful info, what isn't and what do I still need?

Would be great to finally have a good comparison to start from.

Gill

IIRC on the Capricorn (nearly identical mast profile) 25 is the setting used for very light winds, in high winds its somewhere in the 30-35 range.

Spreader rake is usually around 50mm, maybe 60+ for light crew and/or heavy winds, this is measure by laying a batten across the wires at the spreaders and measure the distance to the mast track (i hope you meant 80mm and not 800mm <img src=

alt=

/> ).
Rake is indeed adjusted by change the length of the arms (make sure they are at identical lengths).

Hope this helps.

Tony,

Thank you for the information.
I think still one thing is missing, the spreader arms can still be moved more backwards of forward, and still have the same rake.
I guess you also measure the distance between the 2 arms at the wires?

Gill

I always found it better to measure it to the mast track. I have come across several masts (production boats) where the spreader fixings were not symmetrical and they needed compensating for in the adjustment. Rake is then measured between a straight edge across the diamond wires and the mast track, with prebend measured as previously stated as the chord depth from a straight line between top and bottom of the mast track and the mast.
If you place two or three marks on your mainsail back from the luff at spreader height using 'recommended prebend settings' for different conditions, these can then be 'sightlined' to set the prebend with the mast in-situ.

Cheshirecatman

Cheshirecatman,

I don't really understand how to measure rake as you explain ... Could you explain differently?

Gill

Gilo,
I'm no expert either but AFAIK this is the only way you can measure spreader rake.
Place a batten on the wires and measure the distance to the mast.
IMHO the spreaders should only be set once according to your combined crew weight, you do not want to adjust it every day basically because its a PITA to adjust. <img src=

alt=

/>
You should then adjust diamond-wire tension to suit the conditions (depower), and bring a tool along so you can change it on the water.

Tony_F18s diagram correctly shows the rake measurement/setting, but issues can occur when trying to make sure each side is the same. You can get the spreader tube lengths equal but unless the fitting to the mast was spot on it can leave the set-up unbalanced. I have seen them as much as 8mm out on the mast which can make a big difference at the wire end. Set your spreaders as if each side was identical, then do a check measurement from the wire to the luff groove on each side. Compensate as necessary.
I wonder how many people have diligently checked their tube lengths without checking/noticing the mounts were not quite the same?

Cheshirecatman

If you measure the way Tony says you are missing something in my opinion.
Different combinations of spreader arm length and angle can result in a same distance from chord to mast.

Therefor I would suggest to measure:
- Spreader arm length (from mast to wire)
- Distance from chord to mast
- tension on diamond wires
- prebend in the mast (result of the above)
(and ofcourse windspeed and crewweight).
For easy comparison Spreader arm width is also handy.

Any one interested in a comparison?
Send it to me and I'll gather the info in an excell file.

(The Capricorn has only a couple of lines on diamond settings but very understandable and interesting ones).

Gill

It's true you could have the same rake measurement for different length spreaders at different angles but I have never seen this setup on a cat, only on dinghy shrouds. Most cats have either one fixed and one adjustable spreader tube per side, or a rigid blade type (fixed or angle adjustable - not length). Do any cats 'tune' with different length spreaders?

The principles of setting up a prebend rig are explained here http://www.hurricane59.com/rigging_sailing.html (sec 1-4) and can be applied to other cats. Obviously the settings will be different from class to class, but also within a class/formula if different sections are used or if there is a distinct variance in mast stiffness. However the tuning principles should be the same.

Cheshirecatman

Ed,

Any updates on where you guys have dialed in your settings?