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H18 SX diamond tension #186963
08/04/09 03:32 PM
08/04/09 03:32 PM
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Graham, NC
WindyHillF20 Offline OP
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I am trying to get an idea of proper diamond tension for the 18SX. I am aware of the settings for a standard 18 but have never seen the settings for the SX. I will be running a squaretop and would think the tension should be rather high. I do have a loos(sp?) gauge to measure by. Any ideas?

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Re: H18 SX diamond tension [Re: WindyHillF20] #186976
08/04/09 04:35 PM
08/04/09 04:35 PM
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Tornado Offline
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Sailmaker rec's?


Mike Dobbs
Tornado CAN 99 "Full Tilt"
Re: H18 SX diamond tension [Re: Tornado] #186989
08/04/09 05:25 PM
08/04/09 05:25 PM
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Graham, NC
WindyHillF20 Offline OP
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Sailmaker isn't sure about proper diamond tension. I think it will become trial and error to get it right.

Re: H18 SX diamond tension [Re: WindyHillF20] #186993
08/04/09 06:09 PM
08/04/09 06:09 PM
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scooby_simon Offline
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Originally Posted by WindyHillF20
Sailmaker isn't sure about proper diamond tension. I think it will become trial and error to get it right.


Best way as it will depend on

1, usualy wind conditions
2, usual wave conditions
3, Crew weight
4, spreader angle
5, luff curve and shape of mainsail.



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Re: H18 SX diamond tension [Re: scooby_simon] #186994
08/04/09 06:22 PM
08/04/09 06:22 PM
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White Bear Lake, MN
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Duh, the sail-maker should know the beginning pre-bend sweet-spot for his own sail. Then you dial it in further for skipper/crew weight, etc.

Good luck & sail fast, Tom G

Re: H18 SX diamond tension [Re: h17racer] #187005
08/04/09 08:23 PM
08/04/09 08:23 PM
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Graham, NC
WindyHillF20 Offline OP
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Maybe the better question is should the diamonds be loose to let the rig flex off or tight to keep the sail full. And maybe I didn't state that correctly but hopefully you understand what I'm asking. The Tiger and Fox have tight diamonds, TheMightyHobie18 runs them loose. Even with tight diamonds the comptip is going to bend off with the squaretop. Pentex vs Dacron on the same mast, tighter for pentex?

Re: H18 SX diamond tension [Re: WindyHillF20] #187011
08/04/09 08:56 PM
08/04/09 08:56 PM
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White Bear Lake, MN
h17racer Offline
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This excerpt from a mast/sail design manual may better elaborate.....

• Sail Shape & Boatspeed — There are several reasons to use prebend. The main benefit of prebend is to
“match” the mast to the luff curve of your mainsail. If the prebend and luff curve are in sync, then the mast/sail combination will have a wider wind range potential. Net net is more speed. Another benefit of prebend is a more stable mast.

The most important use of prebend is to match your mast to your mainsail. If you sail in a class that has an optimum rake and prebend, your sailmaker will construct a mainsail with a luff curve and an overall design to match this mast curve. If a main is built with too much luff curve, the sail will be too full in medium- to heavy-air conditions and cannot be depowered. Assuming your mainsail is built with the proper luff curve to optimize the speed and versatility of the sail, your prebend should be adjusted to best match the sail. For instance, if the lower portion of the main is too flat in medium air, try reducing prebend by any of the methods discussed earlier. Working closely with your sailmaker to “match” the sail and mast bend will mean more speed on the
racecourse. To have consistently good boatspeed through a wide wind range, you must shift gears to change sail shapes to suit the wind and wave conditions. If your sail and mast are in sync, the sail shape transition is almost automatic simply with downhaul/outhaul tension. If your sail and mast are not in concert, you will grind your way
through the wind range with less boatspeed. Bottom line: spend time getting your prebend right. The results are dramatic on the racecourse.

Sail fast, Tom G

Re: H18 SX diamond tension [Re: h17racer] #187062
08/05/09 12:32 PM
08/05/09 12:32 PM
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Prebend only applies to...prebend rigs...the Hobie runs the old "soft" rig...spreaders are not swept aft enough to make diamond tension bend the mast in the long axis (that's what a prebend rig does).

So, you adjust diamonds to control/limit bending in the short axis.

Get Rick's book...Cat Sailing for the '90's (available on this site's webstore). There is a detailed section on soft & prebend rigs and how to set them up.
Any newer books/video's will not cover the old soft rig setups...they are so 1960-1980's :-)


Mike Dobbs
Tornado CAN 99 "Full Tilt"
Re: H18 SX diamond tension [Re: Tornado] #187077
08/05/09 02:37 PM
08/05/09 02:37 PM
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Graham, NC
WindyHillF20 Offline OP
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actually some pre-bend can be induced in the TheMightyHobie18 rig. It takes some unorthodox methods but is possible. There is a limit though. Is it spreader rake or wire attachment point that really effects bend? Pre-bend rigs seem to have the lower attachment on the front of the mast, soft rigs are mounted on the side.

Re: H18 SX diamond tension [Re: WindyHillF20] #187199
08/06/09 12:04 PM
08/06/09 12:04 PM
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Adding prebend is done by sweeping the spreaders aft and cranking on diamond tension. Older soft rigs did not have much spreader rake adjustment if any. Adding prebend to a TheMightyHobie18 will mean you'll need a custom cut mainsail and there not be class legal any more.

Placement of the diamond attachment points is not that relevant to generating prebend. There have been many iterations of the attachment points. Currently, have a single turnbuckle on the front of the mast is in fashion...makes it easier to quickly adjust tension on both cables at the same time. The carbon T mast runs the lower diamonds into the mast with the adjuster mounted in the mast base...a large single adjustment nut sticks out the bottom of the base.



Mike Dobbs
Tornado CAN 99 "Full Tilt"
Re: H18 SX diamond tension [Re: Tornado] #187230
08/06/09 03:11 PM
08/06/09 03:11 PM
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Memphis, TN
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Viper USA 132

1984 Hobie 18
Re: H18 SX diamond tension [Re: mikeborden] #187238
08/06/09 04:01 PM
08/06/09 04:01 PM
Joined: Apr 2003
Posts: 440
Graham, NC
WindyHillF20 Offline OP
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Thanks! A good read but really not applicable in my situation. My new sail will have curve in the luff and a squaretop so adjusting anything to work with the dacron pinhead will not help me. I have been sailing a Tiger ST main and was not able to get the mast bent to its liking, close but still not right. I am going to take the new sail and fit the mast to it as best as possible, play with downhaul and rotation until it feels right and see what she does. Its the only way I know to do it.

Re: H18 SX diamond tension [Re: WindyHillF20] #187260
08/06/09 06:57 PM
08/06/09 06:57 PM

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Scarecrow
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The attachment point does make a difference. Having the attachment on the front effectively gives you more spreader rake.

Last edited by Scarecrow; 08/06/09 06:57 PM.
Re: H18 SX diamond tension [Re: ] #187330
08/07/09 11:34 AM
08/07/09 11:34 AM
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Originally Posted by Scarecrow
The attachment point does make a difference. Having the attachment on the front effectively gives you more spreader rake.


Technically correct but you can still get to the same prebend with diamonds further aft on the mast...either by cranking more tension or sweeping the arma slightly more, or both. Main advantage of having the diamonds on te front is to make a single adjustment point, not two. The Tornado carbon mast no longer uses the single front mounts adjuster...the lower diamonds enter the SIDES of the mast near the base and connect to a single adjuster under the mast foot.
Here you can see the lower diamonds connectint to the metal strap entering the mast section:

[Linked Image]

Here is the adjuster bolt under the foot that the strap attaches to:

[Linked Image]

Note how far aft on the section this is...no where near where the previous alu mast diamonds attached. C-mast prebends just fine.





Mike Dobbs
Tornado CAN 99 "Full Tilt"

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