Pre-Bend

General Sailing Discussion
Post Reply
fjviola
Professional
Posts: 90
Joined: April 21st, 2011, 4:06 am
Boat Make/Model: ARC 21 and Supercat 17
Location: Seabrook, TX
Contact:

Pre-Bend

Post by fjviola » April 11th, 2017, 5:52 pm

So how did I celebrate year no. 58 last week?

By putting "Pre-Bend" back into my Stick!
_FVA1537.jpg
100 lbs (5 gallons deck paint + two sheets of steel) hung off the diamond spreaders = 1.75-inch Pre-Bend
_FVA1539.jpg
We could really tell the performance difference last weekend when tacking in 20 knot winds with 3-4 foot chop in the bay. Not one tack missed! :D
Franklin Viola
ARC 2101

Disclaimer: Stick Pre-Bending did not last more than four (4) hours, therefore No seeking of immediate medical attention was required 8)

Eric2101
Professional
Posts: 205
Joined: August 16th, 2004, 2:42 am
Boat Make/Model: Supercat 15
Location: Davie, FL.

Re: Pre-Bend

Post by Eric2101 » April 18th, 2017, 5:52 pm

Great looking boat Frank......
Eric Arbogast
Supercat 15

Eric2101
Professional
Posts: 205
Joined: August 16th, 2004, 2:42 am
Boat Make/Model: Supercat 15
Location: Davie, FL.

Re: Pre-Bend

Post by Eric2101 » April 18th, 2017, 5:54 pm

I think the 4 hour rule applies tp pre straightening... Just sayin...
Eric Arbogast
Supercat 15

Bill Roberts
Expert
Posts: 474
Joined: November 17th, 2003, 7:13 pm
Location: Stuart, Florida

Re: Pre-Bend

Post by Bill Roberts » June 18th, 2017, 7:21 am

Hello fjviols,
I see much confusion here. PREBEND is a structural thing. It is not a performance thing. Prebend, ~3", tends to stop masts from inverting when carrying a spinnaker. Remember when the I20 first came out and owners were breaking masts left and right when flying spinnakers. THEY HAD NO MAST PREBEND ! Prebend is a forward bend in the central area of the mast , mid height, achieved by tightening the diamond wires with a" swept aft spreader bracket". With this arrangement, rig, as the spinnaker pulls forward on the mast above the hounds, the central area of the mast tends to try to bend aft, invert. This causes the diamond wire tension to increase and this increases the forward push of the spreader bracket on the central area of the mast thus avoiding mast inversion and breakage.
The PREBEND should be taken into account when the sailmaker is selecting the luff curve for a new main sail.
It really leaves me speechless when A Cat sailors come up to me and ask how much prebend they should run??????

Bill Roberts
Expert
Posts: 474
Joined: November 17th, 2003, 7:13 pm
Location: Stuart, Florida

Re: Pre-Bend

Post by Bill Roberts » June 22nd, 2017, 7:24 pm

Question: Does everybody understand mast PREBEND now? As long as you guys understand it. that's good. You all are the most important.

Bill Roberts
Expert
Posts: 474
Joined: November 17th, 2003, 7:13 pm
Location: Stuart, Florida

Re: Pre-Bend

Post by Bill Roberts » June 26th, 2017, 3:30 am

So, what's up? Does everybody understand what "PREBEND" is all about?

Kevin Keller
Professional
Posts: 332
Joined: September 23rd, 2006, 11:56 pm
Boat Make/Model: SC-20, ARC22
Location: Honolulu

Re: Pre-Bend

Post by Kevin Keller » August 3rd, 2017, 4:32 pm

Hey Bill,

Thanks for explaining prebend. I and many others didn't understand that. I was thought it was for flattening the sail to take the power out of a sail.

I wish (and maybe it does exist) this website had the ability to tell people when there are new posts. I have been quite busy lately and haven't been able to check this. I eventually get around to it.

Thanks for continuing to educate us.

Kevin

Bill Roberts
Expert
Posts: 474
Joined: November 17th, 2003, 7:13 pm
Location: Stuart, Florida

Re: Pre-Bend

Post by Bill Roberts » August 4th, 2017, 3:58 pm

Frank and Kevin
To put the prebend back into a mast after it has been removed, wires removed, for some strange reason.
Support the mast at the ends and sail track up. Hanging weights at the center of the mast is not needed or required. Pull a tight string from one end of the mast to the other. Measure from the string to the mast at the spreader bracket. Remember this number. This is not prebend.
Check to be sure that you have a prebend spreader bracket. Place a straight edge across the mast from spreader tip to spreader tip. This measurement should be about 3 ins. Check with Tom H for correct measurement. This puts the spreader tips about 6 ins aft of the center of the mast at the spreader bracket.
Now tighten the right side diamond wire until the mast has an obvious bend to the left, 2", 3". Now tighten the left diamond wire until the mast is straight. Check the prebend. Now tighten the right side diamond wire again until you can see a definate bend to the left. Tighten the left wire until the mast is straight. Check prebend. Repeat this process until the desired prebend is obtained. Check Tom H for correct prebend for your boat. The prebend in the mast is caused by the "tight diamond wires pushing forward at the swept back spreader bracket" which bends the mast forward at the spreader bracket. For the mast to invert under spin, this increases diamond wire tension even more which pushes forward on the mast even more at the spreader therefore preventing mast inversion. If your mast does not have a prebend spreader bracket, no prebend can be induced. Forget prebend.

Prebend will flatten your sail to the max when the prebend curve is in line with the plain of the sail, no mast rotation. Distance from bolt rope to sail leech is at its max length. AS the mast is rotated relative to the sail, the prebend effect is reduced, bolt rope to leech becomes less, so the sail becomes fuller. Also as the mast ratates more, the nose of the mast moves out to windward and this is an increase in airfoil camber or sail max depth to chord ratio also.
So in conclusion: With prebend, As mast rotation is reduced form 45 degrees for example, the mast prebend will flatten the sailcloth because the bolt rope moves away from the leech and the contribution of the nose of the mast will reduce overall sail camber also. As mast rotarion is increased above 45 degrees for examplle, the effect of prebend on the sailcloth is that the sailcloth becomes fuller, bolt rope moves closer to the leech and the nose of the mast moving out to windward increases over airfoil camber.
So, with prebend, which is a structural fix for the mast with spin, it turns out that the overall sail camber can be varied more than with a STRAIGHT MAST AND THAT IS A GOOD THING. The sail can be flattened more than with a straight mast with a prebent mast and sail.

Now, tell me something guys, Where have you ever seen the mast prebend thing explained before both from the structural point of view and the aerodynamic point of view? Was it in a Tornado news letter or a NACRA news letter or a Hobie news letter or an A Cat news letter or a G Cat news letter or a Prindle news letter or where???? Where does all of this stuff come from anyway???
Last edited by Bill Roberts on August 5th, 2017, 7:28 am, edited 1 time in total.

Kevin Keller
Professional
Posts: 332
Joined: September 23rd, 2006, 11:56 pm
Boat Make/Model: SC-20, ARC22
Location: Honolulu

Re: Pre-Bend

Post by Kevin Keller » August 4th, 2017, 4:58 pm

Bill,

This is the first time I have ever read an explanation like that. The Supercat was the first catamaran I bought, so I wasn't exposed to any other information from other boats. The SC has taught me a lot bout sailing. I look back at some of the racing I did in college and I see I had no clue to what I was doing.

I think Tom told me that that the SC-20SR didn't have any bend.

Are you doing much sailing down there in Florida this summer? I'd still like to come out and try the 30.

Thanks again.

Bill Roberts
Expert
Posts: 474
Joined: November 17th, 2003, 7:13 pm
Location: Stuart, Florida

Re: Pre-Bend

Post by Bill Roberts » August 5th, 2017, 5:20 am

Kevin,
Thanks for your comments and interest. I'm not doing much sailing this summer. I have a bum right shoulder. I'm doing much physical therapy every day hoping to avoid a shoulder replacement.
To me the most important thing you said in your last post was about your college sailing. In the US in most sailing schools the boats of choice are lasers and 420s. These boats will not teach you to be a good sailor. They are too slow and too sluggish. When these boats race you hear loud demands like, "come up", "luffing rights", starboard", "buoy room", "overlap", "give me room" etc. These slow sluggish boats develop "sea lawyers".
They do not develop excellent sailors.
In the last Am Cup, the NZ Skipper and main foil trimmer were 49er World Champs. The sail, wing, trimmer was A Cat World Champ. These are high performance, high demand boats that teach you to sail fast, get the most speed out of the boat. You know you have done this because the boat is so sensitive and responsive to sail trim and hull trim. These "boats are excellent teachers". We don't like boats with characteristics like this in the US because they are difficult to sail well and they turn over easily.

Post Reply