main backwinding - masthead rig

[ShaunG]

We have an old halftonner, masthead rig with 140% no2 and 160% no1 genoa's

these are not normally used when cruising, we usually use the 120%, however this year we have moved up a bit in our abilities and have started to use them.

However, whenever we are going to windward and the genoa is sheeted in hard (but on trim, all telltales parrellel with leeward side lifting occassionally) the main will backwind even though it is on the centre line, we have tried all sorts such as flattening the main etc but backwinding remains, even tried using the traveller to hoist to windward (i know it's wrong but was worth a try)

The backwinding is about the first 20% of the sail and forms a near perfect 'bubble' from the luff. We just cannot remove it, all the main telltales still flow well with the top telltale just folding behind the leach and the 'bubble' does stays constant and does not flap.

I am guessing the slot is wrong, however if i open the slot to remove the backwinding then the genoa is not on trim which slows the boat down drastically, as she is mainly headsail driven.

Any advice on how to get rid of this or is it just a sympton of larger headsails and smaller main? I have never had this before and just can't find a way to deal with it

thanks in advance

 

[robertj]

no you're sailing it just right!!
the first 20% should back wind as long as its the first and not all along the main.
Is she going well and pointing high?

bob

 

[sarabande]

Presumabl y used in lower wind speeds, say up f3 ?

Adjust the outhaul and/or vang and/or halyard and/or traveller on the main to flatten it.

For the genoa, is the leech line too tight ? If you have tried opening the slot, there remains the track, which might be too far forward causing the flow to hook up to windward into the lee side of the main.

Finally, and expensively, are the sails cut properly or worn out a bit ?

 

[sarabande]

Robertj

"the first 20% should back wind "

Ummm, a backwinded main indicates the flow is not linear, and is therefore not generating power.

If the backwinding cannot be adjusted out by the existing control systems, then the sail is badly cut.

 

[ShaunG]

robert - going like a train but not pointing that well, can normally get to around 35 but just touching 40 with this problem, i am aware that i am likley to lose a bit. of pointing in the higher wind ranges for the sail

sarabande

The no1 is used in upto top end of a F3 the no2 upto top end of a F5. (in keeping with the boat data and the polars i inherited)

The problem is more apparent (bubble very steady) at the higher end of the genoa wind ranges. For instance 20knts on RTI this year and main had constant bubble on when going to windward. I have very long tracks for the genoa cars, i have tired all the way forward to all the way back (almost the stern quarter to level with the mast, 30ft boat and tracks are 14ft long)

I have the normal dacron main and a brand new laminate main from kemp (came with the boat) i thought the same about the sail being stretched, however it does the same with both mains, unless they were both cut badly, i guess it's possible.

i have not played with the genoa leech line much, or the main. they are both set to just eliminate flutter of the leech and no more.

The cunningham and main halyard and both been full on as as the outhaul.

The kicker/vang however is a bit c**p, it hydraulic and i am reluctant to use it too much as the return on it useless, it is next on the list for replacement.

Maybe i am not putting enough on the kicker to fully flatten, any thoughts on this?

I have looked through a number of old photos of half tonners and a few seem to have a 'bubble' and i assume this is a general problem with mastheads with big genoas, any help on removing it is much appreciated

 

[fantasia7628]

When this occurs, look straight up the leach of the genoa and see if the slot is parallel to the main and also if the leech of the genoa is open. If it is hooked, that could be causing turbulence in the slot, move the car positions and if you have a leach line ease it. You might also try opening the slot by moving the car slightly outboard. Good luck.

 

[Twister_Ken]

You say you have the cunningham on full, but is there a corresponding flattener in the leach - like a mini-reef? If so try pulling that down as well - it may help.

Otherwise, sounds like you've tried most things and it may be something you have to live with. Is there a sister ship? If so, track them down and ask what they experience?

Trying to remember what we did when I raced the things back in the late 70's. I was never main trimmer, but I do remember there was quite a lot of work moving the mainsheet car up and down the track, which was combined with sheet trimming. Also, backstay on hard upwind in any air, though that was mostly to help the genoa luff stand up.

I wouldn't sweat too much over it. Trim the genny to perfection (don't compromise the genny for the sake of main trim) then trim the main as best you can. Also, I can't recall ever pointing very high with big genoas. Pointing at 35º is probably a degree too far with that rig. Go for speed over pointing.

As an aside, the boat I used to moor against until last year was an ex-racer from that era (a Contention 33) which nowadays is single-handed most of the time, and sails under genny only. The owner reckons it's quite fast enough like that!

 

[lw395]

If the genoa leach is too tight or hooked, that will make this problem worse.
Try moving the jib car back.
Release the leach line, then pull on just enough to stop excessive vibration of the leach, you ca nlive with a little.
Main needs to be flatter, and boom very near centreline, so try moving the main traveller up to windward more?
Too much main halyard tension (or cunningham) may pull the fullness too far forward, try flattening with outhaul and mast bend not luff tension.

More genoa halyard tension might move the genoa fullness forwards.
If you want highest pointing, then backstay tension will be needed to tighten forestay.
If the sea is not flat, you may be better cracking off a touch, maybe barber haul the genoa sheet outwards a couple of inches to open the slot/compensate for letting the main a little off centre.
If the problem is mostly higher up the main, you may want more kicker or sheet tension to reduce twist.

Have a look at North and Kemp sails for tuning guides on the web.

 

[dt4134]

I'd start out with working out just how badly or not you're performing at the moment. Hard to do with instruments. Is it possible to get another half-tonner to sail against to work out how low & slow you are?

Just sometimes, back-winding of the main seems to work. I raced on a boat that needed the rig re-tuned each time we swapped between the mainsails. One was much fuller and needed more pre-bend. For some reason in one regatta we just couldn't get rid of the back-winding, but the boat was fast.

Having said that, the genny is the obvious place to look. lw395's list is pretty comprehensive. I'd look at it in approx this order:

1) is the forestay tight enough (I've seen that on it's own cause back-winding)
2) check the leech line
3) Where is the max draught in the headsail? More luff (halyard) tension to move it forward
4) try easing the cars back just slightly.

You really need another boat to compare against as you fiddle with the trim if you're to get the best understanding of the effect.

 

[westernman]

1) As others have said try opening up the leech of the genoa - e.g. by moving the car back a bit

2) Slacken off the cunningham on the main to allow the fullness in the main to move back a bit. However, tighten up on the halyard and the clew to stop he sail getting too baggy. Having the cunningham tightened too much will pull the fullness forward and might just be putting it where you don't want it.

However, really sounds like the main might just be too old and/or baggy for top racing peformance to windward.

You might get something out of recutting it to take some of the fullness out of it (for example straightening out the edges of the panels a little way back from the luff).

 

[Twister_Ken]

Gents,

I think many of you are missing that the OP is talking about 160% and 140% gennies. That's a heck of an overlap, and opening up the slot is not going to do much except ruin the pointing. This IOR style rig uses the smallish main almost as an extension to the genoa, to effectively have a single aerofoil. I reckon this bubble is a design feature, not a fault.

I spent several seasons in the 70's racing boats of this ilk, and the genoa does nearly all the work. This image of Smokey Bear (poor though it is) gives some idea of the balance between the two sails...

 

[Blueboatman]

This is such a late night post that you might regard it, quite rightly, as 'silly'......However when you eat those Wheatatrim brecky munchies and perhaps peruse this, you may care to reflect that a) Posh Swedish Ken -AKA Twisded Swister Ken-(Now there was a fast boat,but I digress )- really knows his sail trim, and b) Fleming in another v. recent post neatly summarised modern sail development as the reduction of foresail area in favour of foresail shape, net result being you go flatter and faster..
IMO, play with , learn with and enjoy, break and ignore those giant overlapping headsails, and then when Earnie next delivers on the premium bonds, consider maybe going next for something in exotics that can be flown in up to 20kn ( so a beautiful blade) and enjoy the '' oh my God its a goddam wing and it works'' experience.


Have fun sailing fast.

 

[Bajansailor]

Just a story about bubbles..... I used to sail on a lovely old S & S yawl, who went like a train on rails, always perfectly balanced, and invariably with everything happening in cheerful defiance of 'modern' sailing trim theory.
She had a bubble..... in fact it was more than a bubble, the whole of the main was backwinded by the genoa, and she was sailing on the leach only. And with the boom eased waaay out.
Crank the main sheet in hard, as the gorillas did with enthusiasm in her first regatta in the Med under new ownership, and she would just stop dead.
And she did. Much consternation and swearing.
Fortunately the gorillas had Olin Stephens sailing with them as Afterguard on that occasion (at the grand old age of 90 something), and it was only after Olin dropped some polite hints about what to do with the mainsail that they cottoned on..... and took off again. Fast.

 

[flaming]

I've not done too much sailing on the type of boat described, I'm a bit too young to have caught them in their heyday.
But, the yarns from those who have describe this as the "speed bubble". It seems to be a feature. Certainly when we used to have 140% genoas on a fractional rig we would tolerate a small amount of this, as you just couldn't sheet the genoa in hard enough if you didn't. I'd concentrate on Genoa trim, and using the main to balance the helm.

This, btw, all changes as soon as you are slightly off the wind, when a speed bubble just indicates drag.

 

[robertj]

Saraband is right, your gen is probable old and is it hooked? If so can it be sorted by easing genny leech line if not it's definately baggy.
A new genny will cure this to a certain degree but there will always be some sort of backwinding on the main.
If the main backwinds a little like you indicated it's going fine.
This I take it when pointing high yes?
If backwinding still occures off the wind then it's the genny too baggy/ blown out a little.

Options are :

1 new sail
2 recut old one
3 live with it

suggest you head out and get close to a fleet of racers on their windward beat, look at the mains, the new sails hardly back wind but you will see that they will when hard on the wind will have their mains backwinding a little.

All the race boats I have raced on were not 'no expence ' yachts and the main trimmer had that slight backwinding when hard to windward.

Bob

if it's going well don't worry what it looks like!
In various races the best trimmers sometimes have the sails looking not quite right yet ease past the fleet.

Bob

 

[flaming]

In various races the best trimmers sometimes have the sails looking not quite right yet ease past the fleet.

Bob

Ain't that the truth! I used to do quite a bit of match racing in the RYA's old fleet of first class 8s. To get them to windward fast you had the boom about 2 feet over the centreline. Completely contrary to what you would think, but the top guys proved again and again that was what you had to do!

 

[ShaunG]

Thank you everyone for your response, i have tried most of your suggestions already but there seems to be focus on a couple of issues,

Move the genoa car backwards - increase twist in the genny. I have done this previously, however i will move it as far aft as i can. Although i suspect the increase in twist will slow us somewhat from previous experiments

Leech Lines - i will play with these much more and make sure they are loose, then tighten to remove about 70% of any flutter


Move the main traveller to windward - i have moved the car about 6" to windward previously, i will now take a big armful and move it further to windward so the boom when fully sheeted in is right on the centreline

New Sails - i bought the boat 4 years ago and she came with several sets of brand new sails, the invoice inidicated the previous owner had spent £26000 on new kemp sails 3 months before i bought the boat. we have not used some of these sails at all until recently when this problem has shown, we have the following mainsail, no1,2,3,4,5 genoa we have these these in both dacron and laminate giving 12 sails in total to play with.

It is just like it is - i had wondered this and the pictures from the time do show a bubble in the odd pic.


off the wind the sail trim is great, the only issue if going to windward, i though long and hard before posting this as from a physics point of view i really struggled to see how the main could not backwind bearing in mind the relationship between the main and the genny. Using Ken's reasoning that the main acts an extension of the genny makes some sense to me, the bubble seems to form on the main from the genny leach to the luff of the mainsail, ie if the genny overlaps the main by 20% then the bubble will be 20%, if the genny overlaps the main by 30% then the bubble will be 30%


Please keep any suggestions coming

Thanks

shaun

 

[Deleted member 36384]

This is what happens and why the "speed bubble" works, up to a point. This is from another post I made and the information is verifiable from a sail design book I have, which I assume is correct. The choking bit is what is causing your bubble but its also helping the Genoa to work better.

Its to do with the flow of air over a single air foil, the head sail in this case. There is an area of "stagnation" to the lee near the leech on a head sail only which limits the efficiency of the foil shape.

Add in a mainsail and it alters the air flow across the wind ward side of the head sail. The stagnant point of the head sail moves back beyond the leech of the mainsail. The combined head sail and mainsail are now behaving more as a single air foil which as we know is far more efficient way of powering a sloop under sail.

What happens in the two sail scenario is the slot causes the air flow across the windward side of the head sail to be some what choked (compared to no mainsail being there) and this resistance is what diverts more of the effective air around the head sail to leeward than would other wise happen in the single head sail case. For the correct angle of attack to the wind (say close hauled) this air has to move faster so causes the pressure on the leeward side of the head sail to drop more than it would by itself only.

 

[r_h]

Ain't that the truth! I used to do quite a bit of match racing in the RYA's old fleet of first class 8s. To get them to windward fast you had the boom about 2 feet over the centreline. Completely contrary to what you would think, but the top guys proved again and again that was what you had to do!

Two feet sounds like a lot in a small boat, but I don't doubt that it worked. In light airs on Farr 45s we sheet the main with the bottom batten on the boat's centreline, with the boom therefore slightly to windward.

 

[Neil_Y]

I would say a limied amount of backwinded main (in the lower sections) is normal for a well set up boat trimmed for sailing upwind. A bit of searching to find a suitable picture came up with the wb-sails ltd who have an excellent site with graphic and picture explanations of sail trim.