too much mast bend and rake?

[FourPoints]

trying to determine if I have too much bend and or rake in my mast. I suspect I may have too much rake because in order to flatten the sail the boom ends up at a 5-10 degree down angle, and I wonder about the bend because I always seem to have quite a bit of a pocket above the gooseneck by the luff.

in the pictures, note the main halyard is pulled tight, and gives a good reference to what is straight when looking at the mast bend. at the mid-point of the mast the halyard is a few inches aft of the mast

 

[PaulK]

Mast looks fine. If you changed your rake much, the spreaders might have to change their angle vis-a-vis the mast, and it looks like they're supposed to be pretty much at the angle they're at. It's hard to tell what kind of fitting (fixed or hinged) they have from the photos. What we need to see are pictures of the hoisted sail with the cunningham rigged. If you're sailing in Stamford, the sail may be cut large on purpose for the light air. You may also have stretched the leech from not reefing soon enough in a blow.

 

[ffaubert]

Ok, I'm by no means an authority on this and hence, probably wrong but, I was told," Forestay, -tightest of the tight, Backstay, -tight, Upper shroud, - no slack,either side, head into wind, Lower Shroud, - no sideways bend (of the mast), head into wind."

YMMV.

I went out on Lake Erie (that name is spelled wrong as this lake is full of "Holy Crap, WTF!?!") not using this, and my jib was a full 15° to leeward (and I do mean the forestay was pulled) like somebody's undershirt hanging from a clothesline.

Needless to say (but I'm going to do it anyway), the rig will be tuned a little (Ok,... a lot) better than before.

No, I didn't have my wife with me.

 

[Sumner]

Mine looked about the same and I didn't like it. I tightened the lower shroud yesterday and that made the mast straight from the deck to the spreaders and I'm still able to rake the top back with the backstay and the whole mast itself is raked back, but now it is straight up to the spreaders.

I don't know if what I did is correct or not or will hurt sailing or improve it, but it seems to now look more like what I envision it should look like which is a mast that is raked back about 3 degrees, but straight to the spreaders and bends from there up backwards depending on the tension of the backstay.

I'll also be interested in others thoughts on it. I have a loose gauge and I'm now closer to the figures they give for my wire size. Ours is also a fractional rig.

c ya,

Sum

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[BobM]

I am still a rookie at this, as I read up on it last year when I changed my back stay but that looks like a lot of bend to me, but I don't have swept back spreaders either. There are two things involved, rake and bend. If you have too much bend you may be losing some light air performance, as the main can't have as much belly (see text below for a way to diagnose). A ball park for rake is an inch for every five feet of mast, but I think the bend should be nearer an inch than three. I look forward to seeing what more experienced tuners post.

Luckily for us mortals there are a few more obvious clues. Mainsail luffs are cut with a curve that has to approximate the mast bend. When a mast bends about 30-40% more than the designed mainsail luff curve the sail will start to develop diagonal wrinkles. These "overbend wrinkles" are a very consistent and visible indicator of how much depth the mainsail has at that point. They serve as a reference point. Any further bend will only increase the unfairness of the mainsail and performance will start to drop. At the point where we start to see overbend wrinkles all the "external shape" has been pulled out of the sail. What we are left with is "internal shape". Internal shaping, or broad seaming, is the shape built into your sail by putting together curved edges of flat panels. Some sailmaking companies advertise that they are accomplishing internal shaping through large three dimensional molds, but this is misleading. Mylar film, the foundation of all laminate sails, is only available in flat sheets. So all laminate sails must have broad seaming seams, whether they are difficult to see or not.
The other end of the spectrum is a mast that is too straight for the luff curve. (or a main that is too curvy for a mast). In this case the indicator is more subtle. The sail will look too deep and too draft forward. When this happens the sail has too much external shape. There is only so much external shape you can jam into the front of a sail before you end up with a gutter just aft of the mast.
So the area in between these extremes is the working mast bend. Every mainsail will have a MAX power shape and a MAX depower shape. During most sailboat racing the mains will be in one of these two settings. When you find yourself in between these settings then angle of heel, and rudder pressure are the best indicators of how to trim the main. Constant tinkering with the mainsail controls can pay huge dividends in this condition especially if the wind pressure is erratic. (from the link below)




http://www.thunderbirdsailing.org/Articles/Tuning_guides/masttune.html

 

[FourPoints]

I'll try to get some pictures with the main up tomorrow, today is help the girlfriend move into a new place...

 

[RichH]

Bob M-
The on-going discussion going on here is for a SPECIFIC boat (H27) with a FRACTIONAL rig, a rig that is 'basic set' up with the mast in a 'bent/curved' position. Such a boat has its mainsail specifically made .... with a special shape to its luff (called luff hollow) cut into the mainsail. Such 'class' boats with such a basic-set up (mast bending) usually bend the FRACTIONAL mast to maximize the speed/performance under a narrow range of wind/wave conditions. Generally the mast is bent 3+ inches for normal condition, 6+ inches for speed sailing and for sailing in 'extreme light winds' in FLAT water conditions with the mainsail somewhat flattened by the mast curve; less mast curve when sailing in LARGE CHOP, (main 'powered-up'). Some fractional rig boats with 'tapered' masts will bend the mast sometimes up to several FEET of mast bend.

For a typical **masthead** rig a sailmaker 'expects' that such a mast will have (for normal conditions) only 3/4" mast pre-bend (forward bow) if the boat has single spreaders and 1 to 1-1/2" prebend for a mast with 2 sets of spreaders .... and will cut the mainsail for these expected 'dimensions & 'shapes'. Radically bending the mast on a 'cruising' masthead rig, although possible, will radically FLATTEN the mainsail and produce a very flat sail that 'may' be OK for speed sailing, but wont have much 'power' to punch through waves. BIG difference with mast bend between a masthead rig and a fractional rig.

 

[Deleted member 78819]

The mast is good. Show us a picture of the sail. If it is blown out/stretched you are going to have problems with sail shape.

 

[FourPoints]

The mast is good. Show us a picture of the sail. If it is blown out/stretched you are going to have problems with sail shape.

first couple pictures I cranked the halyard up as described on this thread:
http://forums.sbo.sailboatowners.com/showthread.php?t=120970

the rest of the photos the halyard and outhaul are tensioned roughly where I think they should be

The last two pictures are with the vang cranked down pretty hard.

 

[RichH]

A few pics from the cockpit 'looking up' at the sail when beating in 12-15kts. would be the most helpful. Nevertheless, in a well shaped and designed/cut/trimmed sail what you are typically looking for in 12-15kts. when hard on a beat, with proper halyard & outhaul strain, etc. is that the second from the top batten 'naturally' (and by the sail design parameters) has its aft end somewhat parallel to the boats center line. Too Flat because of excessive mast bend and that second from the top batten will 'usually' be too far to leeward; too drafted
because the mast is 'too straight' and the second from the top batten will usually be 'hooked up to weather'. The aim is that the airflow on both the leeward and windward side of the leech of the main is exactly the same (leech tell tales coming straight aft) and most sail designers and design programs tend to design the shape based on this parameter - and all this 'flow' is validated by watching the tell tales. Mast bend, therefore, causes the leech to be either 'tripped' or 'hooked' ... and under the specific conditions one, to attain the max. speed, has to (should) adjust the mast bend to meet those conditions ..... not merely setting a static amount of mast bend and then forgetting the need for 'variability' of the mast bend to 'meet' the days' conditions. A dynamic on-the-fly backstay adjuster is needed to do this.

The amount of mast bend will flatten or increase the amount of draft and in accordance with the current wind/sea states: Flatter for 'speed sailing' in relatively flat water (especially in 'light' wind conditions), and the mast less 'bowed' for a more powered-up shape for sailing into heavy chop, etc. There is a distinct correlation between the 'shape' of the sail and the SPEEDO for all wind/wave conditions for one to get to the 'optimum'. So therefore, dont depend on ONE trim or shape, or mast bend etc. factor if you want a FAST boat .... as only the SPEEDO (VMG, actually) is the prime indicator to validate if any trim/shape (and mast bending) alteration is correct .... and every day's wind/wave conditions are DIFFERENT; and, so should the amount of 'bend' in your mast be different for 'each' day's sailing requirements .... if you want a FAST and well performing boat.

At least keep records of all this so that you dont have to 'laboriously' go through and 'test' each trim/shape ... as the accumulation of data will enable you to 'quickly' choose the close-to-correct settings, etc. so you can quickly 'hone-in' on the final trim/shape.

 

[Deleted member 78819]

Looks to be the original main...? Also looks a lot like my own main, which I know is the original (1991 in my case) for my new to me this spring H28. I can't really flatten mine either and I know it is because mine is blown out and needs to be replaced. I can see a pocket in mine even when there is no wind and there isn't anything about it that is crisp any more. Experience with previous boats leads be to believe that a new main will make all the difference in the world for me and I expect it would for you as well.

 

[Joe]

The black band at the boom end shows max allowable length of mainsail foot. It appears to have another inch or so available. See if you can increase your outhaul a bit... one way would be to reset the clew connection to give the outhaul a bit more room to pull... that would mean disconnecting the outhaul, pulling out some slack and then re connecting... Maybe you've tried this already... just a suggetion.

I will tell you that cranking down the mainsheet and vang will do little to take the belly our of the lower part of the sail... that is the outhaul's job. Closing down the leech of the mainsail with the sheet has the same effect as moving the jib lead forward so you may be actually promoting a deeper draft by the action. Try easing the vang and sheet a little to give the outhaul a bit more operating room before you start trimming down hard. In fact, release the vang and the sheet when hoisting so you get full halyard extension.