Mast Tuning Inversion

[GCushing]

I am having trouble getting pre-bend in the mast. I have gone from inverted to about straight. With the inners relaxed I was able to get some pre-bend with the outers tensioned but lost most of it when the inners were tensioned. I started the process of tightening the outers witlh the backstay half-on, which I released before the inners were tightened.

From previous posts it has been noted that the spreader tips should be up slightly. Mine look horizontal. How do I raise the tips with the boat rigged and in the water? Possible?

I am a little suspect of the headstay length. I had a rigger take some of the rake out of the mast, that was set back a good ways by a previous owner, by shortening the headstay (shortening via toggle?). I race PHRF and asked for 31.10 as noted on a post on this site as a good setting. Will double check this as best I can with the mast up, tomorrow. Could be a bit short.

Any suggestions?

Regards,

George

[LarryE-old]

I was able to bump my spreaders up, slightly, by gently tapping them, at the shrouds, with the spinaker pole.
The ideal forstay length is 31ft 8 in.

[dave]

George, the 7.9 is funny that way sometimes. In all of the years that the boat has been on the market I have STILL never seen a tuning guide from any sailmaker that I agree with about the boat. They all [to one extent or another] seem to cut the sails and then try to force the mast to fit the main by tuning in some, to a lot of prebend. In my experience, getting any prebend at all in the 7.9 stick winds up with the uppers being drum tight and the lowers MUCH looser than normal. When you add this to the fact that the boat has a very heavy rig that is literally hanging by/from the headstay because of the severe rake, it's nye on impossible to get ANY headstay sag in light air or enough headstay sag to REALLY make the boat power up in light air. Therefore, they all cut the sails fuller than would be optimum to make up for the drive at the low end of the wind range, and then you wind up with a hard to sail beast when the wind builds because you can't get the sails REALLY flat!!!!

I'm sure that many on this board will think me insane for questioning the "bigshot" sailmakers, but as I've said before, I'm a sailmaker too and my niche seems to be taking what others have done [or NOT done] over the years on small to small/moderately sized PHRF type keelboats and doing things differently [A.K.A. sensibly] and making the boats go faster.

[dave]

Man................................. how boring is this? I can't believe that nobody has tried to call me out on this yet!!!

[Tim Bosma]

Man................................. how boring is this? I can't believe that nobody has tried to call me out on this yet!!!

I think we are taking a 'wait and see how it works for Dave' perspective. When will the boat go in?

[dave]

Tim, if you mean my boat, the point is already proven. When last I sailed her and built my "Frankenstein" main from the remnants of 4 different bolts and types of cloth combined with 500 yards of Kevlar tape, I cut the sail to the way the mast sets up with what I would call proper tuning. This results in a mast with no prebend, and as I was alluding to with George, in the lighter end of the wind spectrum the mast actually does want to invert slightly WITH NO SAILS UP. As soon as the sails are hoisted the weight of the jib just hanging on the headstay and even enough mainsheet tension to set the leech in 3-4 mph of wind makes the mast go straight to ever so slightly prebent, but it's so slight that I would have to call it straight. I even took the sail back to the loft and removed maybe 1/2" of luff curve [mostly down low] to make it better fit the very straight mast.

After I built that sail we were almost untouchable by the rest of our handicap fleet, including two other 7.9's [albeit not always well sailed] and a gaggle of J24's, one skippered by a two time national champ in another class and crewed very well. We are rated the same. In the first 3 month series without "Frankie" we won by 1 point after maybe 11-12 races, with the second place boat being a J27 sailed by the aforementioned guy and his crew. In the second "after Frankie" series, said skipper and crew jumped on a J24 thinking that they would have a better chance of beating us. We won by 11 points in a series with a higher number of races.

In all but the last few races of the last series, we also NEVER raised the board!!!!

The facts man, just the facts........................................

[GCushing]

I am releasing all of the backstay tension and releasing the mainsheet and boomvang while setting shroud tension. Is this correct? Also, the backstay is totally slack in the full release.

George

[dave]

Not to be Clintonesque, but that depends on what the meaning of "correct" i s !!!!

IMHO, a tuning guide for a non keel stepped mast should be written for the way that the mast WANTS to tune up, without going to extremes to make the mast do what YOU want it to do, and the sails should be cut accordingly. The KISS principal at it's finest!!

No matter who's sails you have and the tuning guide that comes with it, the WORST thing that you can do for your program is to blindly follow it, just like a cow to the slaughter house. Little things add up on a sailboat, even one that is heavy for it's size. I've never blindly followed a tuning guide in my life and have learned a heck of a lot because of it. Things like mast rake, spreader rake, etc, are usually a great place to start, but sailing is sailing. Some boats like to be sailed in more of a footing mode [Flying Scot, Melges 24] even though one is a heavy pig and the other is a light weight rocket, but the REASONS that these two totally different types of boats both like this sailing style are totally different. Likewise on the opposite end, some boats like to sail high IF you let them go through the gears to gain enough speed to let the blades work. The 7.9 is such a boat, and fiddling with the mast tuning and sail settings can make huge differences in speed and angle. It just takes time and some patience to see the benefits.

I purposely do things that go against many class tuning guides to see what more I can get [if anything] out of any boat that I sail. You never know until you try and trying hasn't cost you anything as long as you do it during practice. I use a GPS for not only speed tuning but as importantly for angle. The track page will graphically show you you tacking angles without having to decipher anything and is VERY helpful to the program. With a GPS you DON'T need another boat to get really dialed in as to what makes any boat go faster and/or higher or faster and/or lower.

[dave]

One further addition that just adds more to what I'm saying. Because of the aforementioned severe rake and the weight of the mast combined with the 7.9 being a fractional rig, the headstay attachment point winds up acting as a fulcrum. If you just step the mast and snug things up slightly but DON'T connect the backstay, the unsupported weight between the hounds and the deck will cause the the mast to sag aft just from gravity and of course the section above the hounds go forward because of that. This DOES cause the mast to want to invert. Because the backstay attaches at the top, the force applied there will act against the fulcrum [hounds] and pull the top back which moves the lower section forward some, bringing the whole thing back into column WHEN TIGHTENED. The problems arise when you want to sail with a loose backstay [such as in light air] and ESPECIALLY if you have water conditions that are relatively choppy/wavy for the wind conditions. The mast sagging aft in the middle is not in and of itself a bad thing IF the main is cut to take this into account AND one adjusts his/her thinking to what they are looking at as far as how a mast is "supposed" to look. Personally I don't like that look but it's the nature of the beast. The only way to keep the mast from sagging aft in the middle [let alone get ANY prebend] is to have the upper shrouds ridiculously tight for the conditions at hand and/or always have some tension on the backstay, neither one always being desirable for obvious reasons!

All boats perform at there best through a wide range of wind and water conditions if the tuning of the mast and the cut of the sails will allow the boat to have all of the gears that it needs, just as vehicles with 5-6 speed transmissions can derive more USABLE power than a 3 speed for a given engine. If a boat is always sailed with a relatively tight rig to counteract the above phenomenon, there's no way that the sails can be or have been optimized to get EVERYTHING POSSIBLE from the boat in the lower wind ranges without overpowering too quickly as the wind builds, for the very elementary reason that all of the gears aren't there to be had!!!

[dave]

George, I apologize for getting so windy but possibly not giving you much of an answer: "is this correct". I would use the guide that you have BUT go from there and do your own experimenting. The whole point is that without some backstay tension the mast on the 7.9 WILL want to invert for the reasons given unless the upper shrouds are REALLY tight, and IMHO this is NOT the proper way to tune and/or sail a boat. Just remember that prebend is neither good or bad, the mast just needs to fit the mainsail luff curve and at the same time the rig tension which goes to the headstay needs to be at a point where the two sails will change shapes and shift gears together. Speed and angle are the only factors the matter e in the end. How you get there matters not a whit as long as you get there.

Just a note of curiosity: This is the only class of boats that I have ever dealt with or read about where most all of the participants use the terms "outer and inner" instead of upper and lower for the shrouds. Where does this come from? Was there a Canadian in the wood pile back when?!?!

[GCushing]

Dave,

You are a great participant and have a wealth of information to share. This has certainly been helpful to me in learning how to repair and sail my boat.

I am new to racing so I am progressing through a learning curve where even the answers take awhile to assimilate. The more I sail and maintain the boat the nore the nuances of the advice begin to reveal themselves. I have off the shelf sails from North and have no idea how to determine how they were cut. I am sure that will come later. I am just trying to master the mechanics of tuning the rig, i.e. do I slack all of the tension on the mast while tuning (main sheet, vang, backstay). Too, when I had the forestay shortend close to class spec it became apparent that the previous owner had shortened the backstay to make it work in the previous configuration. The rigger lenghtened it for the new mast position. It is quite slack in the off position, I assume this is correct.

I have been through that tensioning of the of the uppers and lowers several times, am able to keep the mast straight and avoid a gross inversion, so progress is being made.

Still learning the terminology, inners/outers, uppers/lowers. Uppers and lowers make sense.

Regards,

George

[dave]

George, aren't you on the gulf? Where do you sail usually? I haven't seen any "new" North 7.9 sails but my experiences with them in the past were that the mains were too full, too draft forward and too long in the leech, and this also means that you are loosing lots of sail area up high in the roach. The only time that I could ever get one of those to look "right" was with almost max backstay tension, because they had way to much luff curve cut in to them. As I said, I haven't seen a newer model but design ideas are kind of like leopards: they don't usually change their spots!

If you sail in lots of chop that draft forward/full shape is sometimes very helpful IF not carried to extremes, but I would still rather have all of the gears. In a nutshell, the 7.9 mast WANTS to set up with no prebend even with a snug backstay. Trying to make the mast do something different and/or doing the same thing with the sails is making a simple job difficult, AND adding insult to injury when it comes to an already proven weak point/problem spot in the boat: the mast step area!!!

If you could take some on board pics looking up the middle of both sails with and without much backstay tension and post them here I would be glad to give you my thoughts as to what I'm seeing and whether I think you need to do anything differently.

[dave]

Oh yeah, I wasn't deriding you about the terminology thingy. Just curious, because everyone in this class uses that terminology!

[Gregg]

Hey Dave,

for comparison's sake - what numbers do you use on your loos gauge for the uppers and lowers?

Thanks,

Gregg

[dave]

I don't! I've been doing this long enough that I set the rig up by feel for the conditions at hand, then go out and let someone else drive upwind before the race so that I can sight up the sail groove of the mast on both tacks, and then tweek from there. The most important things for me are that the mast is in the middle of the boat [already should be done] and that it stands the same way on both tack, and that's not always board straight depending on the conditions. Anytime you have the mast standing straight from side to side and under a good load and then apply the backstay, the upper shrouds will become progressively looser as the backstay is tightening, on a mast with swept aft spreaders. This causes the the whole mast to bow out to weather in the middle and fall to leeward at the top which is VERY FAST, IF you don't over trim the freeking jib AND remember to trim the main hard everytime the backstay is pulled more!!!!

I would much rather error towards being too loose with the rig tension than too tight because you can always get the headstay tighter with the backstay, but if the whole rig setup is too tight you CAN'T get the headstay loose enough to power the headsail up when/if conditions change. I know that there are some very good sailors who don't agree with using the backstay much, instead they say to rely totally on adjusting your rig tension to the conditions. They also say that as the backstay is tightened the headstay becomes looser because of a bunch of theoretical geometry that's going on with the foretriangle. I don't and won't argue with them. I just say to put your tension gauge on the headstay and take a reading, no matter where your uppers and lowers are set. Now apply the backstay and go check the gauge. I GUARANTEE YOU that the tension has gone up and will KEEP rising as more backstay tension is applied, period.

I of course adjust the uppers and lowers for different wind/water conditions but once I've found my sweet spot for a specific boat I just go up or down from that spot, remembering that as the wind builds the lowers need to be progressivley tighter in relation to the uppers, for the reason mentioned in the first paragraph. The simplest way to do this is by remembering that the lowers are approximately half the length of the uppers and that they also have half the stretch [yes, they DO stretch under load]!!!. If I start with the lowers set to keep the mast straight from side to side in about in about 7-8 knots of breeze with the backtay just snugged up, that makes the lowers shrouds about half the tension of the uppers with the mast basically straight side to side. As the wind builds up to around 20 or so I have tightened the upper and lower shroud turnbuckles the SAME amount until I have reached a point where the lowers are now as tight or tighter than the uppers. No having to remember that I put 6 turns on the uppers and 4 on the lowers, etc. All I have to remember is one number that was used on everything!!!! If that max range winds up being 6, 8 or 12 turns tighter [whatever] it matters not, as long as I know and remember [write it down] that number of turns up from my base setting. It works the other way too. In lighter air I back off the same number of turns on both [but it won't take as many] until the lowers are starting to get to the "just floppy" point. This would be about 2-3 knots of wind. Sight up the mast again while sailing upwind and viola: The mast is standing in column or SLIGHTLY sagging to leeward at the speaders., which is perfect tune for those conditions.

Still, the sails have to match the rig, but try this with another brand and you may just be amazed at how well it works!