mast pre bend

g'day im jerry first time user sail A CLASS MK4 at kingston s.e south australia water is generally flat average wind varies between 10kts to 20kts .could anyone advise me on mast set up as im still playing around with prebend and chain plate settings . cheers jerry

Hi Jerry, I also have a Mk IV and other than the downhaul adjustment I have not changed anything on the diamond wire setup. On windy days the mast will start to slap from side to side after hoisting the sail easing the downhaul until launch keeps that to a minimum. Have you asked the guys over at www.ahpc.com.au yet? It might be a local call for you and Im sure they will know the answer. Keep intouch I'm learning more about cats on this site than anywhere else. Great group here!

Jerry,

I've got a decent light air setup on my boat and as soon as I get a chance to extract my notebook from the trailer, I'll list them here.

Jake

Sure ya will, I've heard that before.

thanks every one for replying to my to my questions .sorry about the delay at getting back to you.any imformation on mast rake ,pre bend ,or general set up's is appreciated.


thanks ready

Rake in general, less wind, set up stright, more wind, more rake. As far as prebend goes, I think you will need info from the manufacturer or other owners.

Jerry-
I'm sure you'll love the "A" cat! Very hard to give specifics on these boats as the masts can be different (the older Mk IV's even had aluminum masts though most have upgraded to CF- but could be Saarberg, Hall, Fiberfoam, etc. CF mast) and the sails can be very different materials (Dacron, Pentex, etc.) built by different sailmakers (Goodall, Ashby, Danger, Landenberger, Glaser, Ullman, etc.). Best thing to do in your particular case is to note type of mast (if possible), year of boat (if can't ID mast will at least give a clue), sailmaker and year of sail (if possible) and E-mail the sailmaker for suggestions as to prebend, rake, spreader length, etc.

Enjoy!

Kirt

Jerry,
You are in the right place. Call either Glenn at Ashby Sailcraft or Greg at Australian High performance. Both are very intimate with all of the Boyer and Flyer A cat designs. They will be able to give you basic mast set ups and give you a base rake setting.

thanks guys ill get in touch with boyer or glen ashby
and then start to set things up

Could you leave a post when you have received reply from Glenn? I mailed him about one week ago with a similar question but haven't received any reply yet. May be my message is stuck in his spam-filter ...

However, I would like to understand the subject in general. Right now, I have an average spreader rake but pretty high diamond tension. I guess, I can achieve the same mast bend with higher spreader rake and less tension on the diamonds. How will this affect the performance of the boat, is one option preferable over the other? If somebody could give me advice on this ...

Me too.

How much pre-bend is too much? This thread is about A-Cat stuff but I think the concept is universal.

I have a new main for my H20 and using Mike Krantz's technique of matching the mast bend to the sail luff curve, I was able to make a perfect match.

However, to do that I ended up with 4" of spreader rake and 800# wire tension.

I can find no reference in the various tuning manuals and posts of anyone using more than 2 1/2" of rake. Have I gone too far?

The sail looks great (we will find out this weekend).

Spreader rake will change how your mast reacts to pressures from wind and control lines. More rake will turn loads into mast bend faster than less rake will. Loads include side forces from wind, compression from trapezing weight (additional leverage), downhaul, and mainsheet tension.

I still treat spreader rake as a method to fine tune the mast reaction based on the weight of the sailors on board. Heavy sailors will want the mast to bend less quickly in response to additional pressure (higher wind) so they will want less spreader rake.

My impression of this is changing a little through conversation with some of the a-cat guys but I haven't completely digested what they're doing with tuning spreader rake.

Mast bend is fine tuned with the Cunningham right? I thought the prebend would only be re-set for a sail change. Watching the racing cats video the sail shape changed a lot as the mast came back the sail got fuller there was a mainsheet trim with every Cunningham adjustment. Have to watch that one again I believe it was a Tornado used in the video they covered rake of the spreaders as well. From my limited experiance on the MK IV every adjustment on the water results in a slowdown before speed increases. I was told by another Boyer owner to set the rake using the trap line as a ruler it should be about the same at the back crossbrace and at the forestays you can adjust for heavy or lite wind only on the beach.

Jack, you're talking of Mike Krantz's technique - is there a document or a link you can provide? And Jake, do I understand you right that less crew weight results in higher spreader rake and vice versa?
I have to re-adjust all parameters (spreader-rake, diamond tension, mast-rake, and side stay tension) since I put a pretty recent sail (Ashby 2004) on my A-Kat from 1997. There are so many parameters to gamble with - where to start? With the current set-up I have the feeling that my boat is too sensitive to gusts.

A few years back, Mitch Booth showed Dave and I how to tune our Tiger mast to match the luff curve of the main. He had us flip the boat on it's side, connect the main on the halyard hook, connect the downhaul, and mainsheet and sheet in hard as though you were going up wind. Note - the main is not in the luff track, it is outside the mast. Look at the luff curve of the sail, and compare it to the curve of the mast track. Adjust your diamonds and spreader rake accordingly to get them to match. That is your new "base setting". We found that we were carrying anywhere from 800 to 1,000 lbs of diamond tension to get some mainsails dialed in.

Mike Krantz quote

... do I understand this right: 800 - 1000lbs diamond tension? This converts to 360 - 450 kg - this is enormously much! Currently I have 270kg (= 600lbs) and feel already uncomfortably with it. But again, with more spreader rake and less tension I should be able to achieve the same results ...

Jack, you tuned your mast according to Mike Kratz's technique and want to make a try upcoming weekend? Please let me know how it has worked!

We found that we were carrying anywhere from 800 to 1,000 lbs of diamond tension to get some mainsails dialed in.


Seems to me if it takes that much to make it happen you might want to look at having the sail re-cut.

i think that you have mis-understood what the boss was saying. That curve is the MAX mast bend that you want with full down-haul, you just need enough prebend to allow the downhaul to get you to this point.

Question:
If you "overset" your pre-bend so when you max downhauled... you went past "fully flat" ... would that put more draft in your sail and power you up?

In my opinion more diamond tension takes out camber of the sail but not necessarily opens the leech (better for pointing) and that cunnigham open the leech and pull the depth forward (better for reaching). There must be a difference.

It would bend the mast more and give you a realy flappy leech on the back of a really flat front third of the sail - 'orrible

I agree with TeamVMG's remark. Wouldn't it make more sense to rig the boat correctly (= sail in the masttrack), lay her on one side and pull downhaul, outhaul, and mainsheet to a maximum as you would go upwind in extreme conditions and adjust the diamonds that they are just tight? Then you have enough mastband to get there if you need it but avoid over-tension.

emailed glenn ashby on setting mast up ; swing the trap wire forward tie extra sheet onto the trap wire place next to forstay in middle of hull mark the sheet.place a mark 100ml in front of rear beam . swing trap wire to the stern .mast is in correct place when the trap wire touches these two marks this is for light wind /heavy wind 20knts + mark 100ml behind rear beam.mast stay wire tension 35 on loos guage .

mark 100mm in front of the rear beam? That sounds way too far forward. I don't think I understand your measured positions - can you clarify?

... the value of 35 on the loose gauge remains the same on all wind conditions? I thought that mast rake remains fixed but the tension of the side stays gets adjusted according to the wind.
May I ask which adress you used to mail Glenn? I mailed him too, but received no reply - possibly I used a wrong address?
Kai

As some of you (and I) expected, we were way too flat at the extreme prebend settings. Even though the wind was 20+, we could not power up in the lulls (20-).

That was with rake at 4", tension at 800#.

The second day, with 18-20, we backed off to 3" rake and 600#...we were still underpowered except in the puffs.

Sooo....I guess the conventional wisdom is still about 2 to 2 1/2" rake and 400-700 tension.

That is some prebend with the ability to downhaul to as much as it takes. This is with an alloy stick of course.

There are a few different boats on this thread and different rigs and adjustment options with each. On my cat the side stays have only 5 holes to set the stays in before the forestays are tightened with some 1/8th braided line. Jerry I am going to try looking at the trap wire measurement next time I set up. Too bad we don't have any rake adjustment underway. As the wind comes up or dies that would be useful.

sorry for the confusion it should of read diamond wire tension is 35 on the guage. and the rear mark should be 100ml in front of the rear beam in the centre of the same hull thanks jerry

You actually made more sense after the Christmas party than before. That must have been special punch you were drinking.

So you're measuring like this? That's completely different than where I am on my Boyer MKIV. My experience is to have the trap line measurement fall even with the rear port for light air and halfway between the rear port and the stern for heavy. I'm MUCH more raked back than this measurement and could really feel that having it measured at the port for 15knots was way over powered. The second I raked it back more, the groove was steady and I had great speed:

Jake,
what did raking the rig back do for helm balance? I am subscribing more to the thought of finding a good setting and depowering with downhaul. What is your opinion?

Hi all, first post here.

I've dug around in my computer and found these comments from a tuning guide written by greg goodall, not sure when it was written, but probably around the beginning of mark 5s. Principles should still apply to the mark 4 and is consistent with Jakes comments. You also need a bit more rake for downwind in the Mark 4s and 5s. Exact tuning would depend on your mast, but its a good start. Jerry, PM me if you want a copy of the guide as it is no longer on the AHPC web site. There is also this quide on the Landenberger site, but I find it a bit hard to follow.

http://www.landenberger-onedesign.com/spip.php?article21

Hope this helps


Extract from Greg Goodall A class tuning quide. Date ??

Mast Rake
On our Auscat boats we set up the mast with about 5 deg. of aft rake for all conditions. We have found that it doesn't seem to make any significant difference if we change it for different wind conditions. Although we do increase the mast rake for rough water, i.e.. 1 meter+ waves.
On our boats we set the mast rake by using the trapeze wire. Measure the height of the trap ring off the deck at the front chainplate and then take the trap towards the stern. With normal mast rake the trap ring is the same height off the deck about 250mm behind the rear beam.

Rig Tension
Rig tension on an A-Class doesn’t need to be tight when compared to other classes. The tension is about 50 –60 Kg.

MAST SETUP
Spreader Rake, Diamond Tension and Mast Prebend.
Spreader rake.
Spreader rake is the method of tuning the fore aft stiffness of the mast below the hounds. The ideal amount of spreader rake is dependent on the fore-aft stiffness of the mast and to a lesser extent, the amount of luff curve cut into the sail. Stiff masts require more rake to make the mast bend sufficiently, soft masts require less rake.
The spreader rake is measured by placing a straight edge or string-line between the diamond wires at the spreaders and measure the distance to the back of the mast. On the Saarberg and Australian masts we are currently using 50 – 60mm of spreader rake.
To answer how much is sufficient rake can only be determined by sailing the boat and knowing what to look for.
If you have excellent height, but lack boat speed up wind and the boat does not want to accelerate in the wind gusts, then you need more rake. This helps the mast bend fore and aft which allows the sail to flatten and the leech to open in the wind gusts.
If you are lacking height and "grunt" in light weather, then you have too much spreader rake.
Diamond Tension
This primarily controls the side bends of your mast. Loose diamonds allow the middle of the mast to bend to leeward and the top of the mast to hook to windward. This tends to cause the boat to heel very easily in wind gusts. Very tight diamonds do the opposite.
Downwind, tight diamonds keep the mast bent reducing camber and power. On our boats the diamond tension is set at 30 - 34 on the loose gauge.

Funny you should ask - it actually balanced out my helm a little more when I raked back more. I suspect my rudders are raked a little forward of the center of pivot. With the rake measurement taken from the forestay to directly over the rear access port I had a lot of weather helm making it a real beast sailing downwind in any kind of air. I would really struggle to keep it under control while making an adjustment or two because it wanted to round up so quickly. Upwind in 12 to 18 knots, the groove was very difficult to find and the boat felt like it couldn't get out of it's own way in puffs. No matter what I would do, I couldn't get the bow down under pressure. In light air, however, the boat as setup was a rocket.

I then raked it back to the position between the port cover and the handling in big air became dramatically better. I was shocked, actually, at how much better it had become. The constant struggle to keep the bow down and the boat charging when puffs hit was gone and it was a matter of making the minor trim adjustment to handle the extra pressure. What also surprised me was that the weather helm also became less dramatic...leading me to believe that the rudders are raked a little too far forward. I've got a new Ashby sail coming so I'm not planning to do any more tuning until then.

Thanks Jake. That reflects my thoughts on the subject. Rake should be adjusted to the boat setup and the sailors, if the sailor is skilled enought to take advantage of it.
The "best practice" trimsheet did not suit us at all on the T with the sails we had. If we had sailed with stock sails, I guess "best practice" would have suited us well.

Jake,

do you expect recommendations for spreader rake / diamond tension / mast rake to come with your new Ashby-sail? I own a 2004 Ashby sail but I'm unable to get in contact with Ashby Sails to obtain the correct settings. If you could post the settings fpr your sail here or send me a PM, I could start with these on my boat - this would be very kind of you!
Kai

" I'm unable to get in contact with Ashby Sails to obtain the correct settings"

GONE SAILING - BACK SOON!

I think Glen will be out of the office for a few days defending his A Cat world championship.

Fred