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m_kiel

Moth decksweeper and lowered mast stumps

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Hello fellow moth sailors,

while waiting for warmer weather I am considering some winter tuning projects for my Mach2.2. In context of the trend to low mast stumps and recently also decksweeper style sails the following questions have come up:

  1.  Do the decksweeper style sails use a significantly shorter mast or how do they get around the maximum sail dimension as measured from the throat point in the rules?
  2.  Do you think the small advantage of an endplate effect really outweighs the drawbacks regarding aspect ratio of the sail and  heavily disturbed flow on a significant part of the sail area?
  3.  In addition to cutting down the mast stump on my boat with KA sail and Mach2 mast there seems to be some space to cut down the mast, too - even with full cunningham there is some distance between the sail and the boom. Is that reasonable or not a very good idea?

I am looking forward to hear some opinions!

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Probably out of my depth here. Endplating only occurs when the sail is sealed to the deck, or some sort of other sealed transverse feature which serves to deter the creation of tip vorticies and seal the sail into the airflow across the deck. If you aren't sealing to the deck the "endplate" appendage is only very marginally moving the CoP down the sail. One example of actual endplating is the combined set of Mach 2 upgrades that extends the foot down and butts it against a semi-rigid foredeck extension. However, even this isn't endplating if it isn't in contact. Adding a feature that gives some elasticity to the seal is necessary to give an endplate tuning range.

As a comparison point for what's fashionable right now, take a look at the results from Wangi and bear in mind that Luca Damik's Scalpel is now ~7 years old and has, as far as I can tell from this distance, not been upgraded anywhere, barring fresh foils and sails. 

DRC

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My2c

DISCLAIMER: I have to agree with Dave. Do not think that any progress you will change performance much.  Low rigs are a bit overrated, at least with current boats geometry and with the impossibility to properly seal the foot.

CUTTING MAST STUMP: if you are already on a low mach2 mast stump you can cut a little bit but not much; going to stump heights like those of the Maguire (close to 20 cm from kicker attachment to end of stump) increases loads exponentially. I'm not really sure a mach2 can handle those loads without significant reinforcements to the kingpost area. 25 cm, which is close to the standard mach 2 low stump, looks more reasonable.

CUTTING MASTS: with a m2 mast with a KA sail you have room to cut a couple of cm of mast if you really want to. beware that you will have of course to run a little less rake from that moment onwards.

DECKSWEEPER LEECH LENGHT. Ka decksweepers go precisely to 5,185 lenght from throat point (I assisted to measurement in Garda). All other sails measured before july 17 were 8-10 cm inside max lenght. Latest Lennons (goodison sail at the Garda worlds) and I suspect new 2018 norths go to max lenght as well. Ka decksweepers of course had to cut a good chunk of the mast, to go down to foredeck; all others decksweepers will have to do the same.

DECKSWEEPER EFFICIENCY: I have no proper technical Info. What I have been told by more informed people is exactly what Dave says. I suspect that decksweepers will properly start to make a difference only if they are put on a boat designed for them and if they are able to combine endplating with good sail shape and with good enough ease of handling. There is a lot of developement on them right now, we'll see in Bermuda if someone really finds a proper breakthrough. Until that happens, performance differences with standard rigs will be very small IMHO.

 

 

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Time for a few home truths.

Unless you are battling to win the worlds with a name like Outteridge, Slingsby, Goodison or Burling. the modifications you are considering will make zero difference to your performance and your finishing position. Ask yourself whether you can regularly hit the same speeds as the top guys before the mods. If not, don't waste your money and note that Goodie won without a decksweeper

As for the "decksweeper", the name is misleading. Amac claims that in straight line speed testing by world class sailors, the KA "decksweeper" has proven to be faster, even without the special deck moulding and without the deck moulding, it clearly cannot be due to sealing the bottom of the sail. The question is whether it is due to the feature at the bottom or whether it is to do with the resultant change in overall profile, height etc.

7 hours ago, m_kiel said:

 Do you think the small advantage of an endplate effect really outweighs the drawbacks regarding aspect ratio of the sail and  heavily disturbed flow on a significant part of the sail area?

As said, here is no end plate effect but your comment about there being a drawback to sail area in heavily disturbed flow might be where you re going wrong. For most of the time, the Moth has more sail area than it needs and you are blading out the top of the sail to reduce power. What i think might be happening is that the new low down sail area is actually reducing turbulence in that area and as such, this is where the gain is coming from. Drag reduction seems to me to be one of the big areas that people are really working on and this could well be part of that strategy.

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1. The KA decksweeper rigs used at Malcesine and Wangi have the mast shortenned by about 250mm, 100 off the top and 150 off the bottom from memory. In Malcesine they had weird boxes fitted to the deck which sealed the foot. These did not fit in for the return trip, so they were not used in Wangi. There are only 4 of these sails/rig built so far. They are not yet commerially available. 

2. Last year we changed the way the luff limit is measured. Previously it was  band on the mast at 5.185m now we measure the sail with a 5.185 max. So previously stretched, now in relaxed state, so new sails are being made longer by about 150mm. Same area limits, but now slightly higher aspect ration. The KA sails with the boom between a double skin measure to this luff length. The newest Norths and Lennons are also built to the new longer limit.

3. AMAC says the endplate effect of the deck box was significant, but he did not bother making new boxes and the boats went pretty well at Wangi anyway.

4. The 4 boats with these rigs had previously had the stumps lowered. (AMAC's and Harry's are teh same boats at both venues, but Peter's and Scott's at Malcesine were different boats to Nathan's and Scott's at Wangi.) Not sure to what extent. Lots of M2 have had these modifications. You can apparently cut up to about 40m off the bottom of the stump and just bend down the comp struts, sounds dodgy to me. many cut off more and then cut and shorten the comp struts and rebond them. These look like the later version of the fork/wing bars sold as upgrades. A few others, like Scott,  have lowered the stup to the foredeck level and moved the comp struts to the hull. A smaller number have strengthenned the main tubes and deleted the comp bars.

5. A few people are making big efforts to close the gap between boom and sail. Not just the double skin but also some soft panels with shockcord fitting neatly to the boom. 

6. Lowering the rig seems to be more advantage to small people. Bigger people appear to suffer more in lighter air.

7. Lowering the rig shortens the leach of the sail, considering we still need space to get under the boom quickly. Some aerodymacists consier leach length more important than luff length.  The deck sweeper component of these sails only extends back less than half the foot length, again the aerodynamic theory suggests the end plate is more beneficial at the back of the sail/wing.

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GBR posted while I was writing. I totally agree with everything he has said. If you are not already doing 18 upwind and 29 downwind as well as nailing every tack and gybe plus all the shifts, just do not bother. Go sail your moth, keep learning and most of all have fun.

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Just curious... What % of Mothists are hitting 18/29 and nailing tacks, gybes and shifts?

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Hi guys,

thanks a lot for the swift and extensive replies. Just to clarify as it is not clear from my original post: The winter here is long and when I can not go sailing I do enjoy the tinkering a lot, too. Thus when thinking about changing parts of the boat the main motivation for me is simply that I enjoy the process of trying to understand something in theory and then implementing it on the boat. The possibility of increased performance is more of a side note - I am fully aware that if I had any of the top guys boats and they were in a Bladerider I would probably not stand a chance around a race course.

Now concerning the topic of a lowered rig in our German moth group Chris Rashley has been quoted to claim an increase of 0.5 knots over a standard rig. That would be more than one boat length every 100m, which I, based on my limited experiences when lining up with others that have the low rig, was finding hard to believe. I am happy that your general opinion seems to be that it is most likely not nearly as significant, especially as I am on the upper end of weight and height compared to the ideal moth sailor:

1 hour ago, Phil S said:

6. Lowering the rig seems to be more advantage to small people. Bigger people appear to suffer more in lighter air.

Currently I still use the high mast stump with a straight boom and have not had too many difficulties to get stuck underneath. Lowering the rig would also mean I have to kink the boom, which inevitably will add some weight, which is also true for modifying the compression bars and thus not helping low wind performance.

3 hours ago, 17mika said:

CUTTING MAST STUMP: if you are already on a low mach2 mast stump you can cut a little bit but not much; going to stump heights like those of the Maguire (close to 20 cm from kicker attachment to end of stump) increases loads exponentially. I'm not really sure a mach2 can handle those loads without significant reinforcement.

This is also a very valid point that had me concerned and will, together with the other points you mentioned, have me stay with my current setup: I do enjoy how resilient the Mach2 has been so far and I really don't want to change that.

Thanks also to Dave Clark and Team_GBR for clarifying some things concerning the endplate effect.

On the Lennon Sails website they have some short text about the A3.5m developed by Dylan, which is described by 'countering the direction of travel on [the other sails]' from which I read that the current trend is definitely not the holy grail yet.

 

 

 

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4 hours ago, Phil S said:

5. A few people are making big efforts to close the gap between boom and sail. Not just the double skin but also some soft panels with shockcord fitting neatly to the boom. 

 

YeeHaa, a move back to shelf foots and boltropes in the boom, circular development continues! :lol:

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16 hours ago, Dave Clark said:

As a comparison point for what's fashionable right now, take a look at the results from Wangi and bear in mind that Luca Damik's Scalpel is now ~7 years old and has, as far as I can tell from this distance, not been upgraded anywhere, barring fresh foils and sails. 

DRC

Luka’s boat has had significant upgrades. Significant lowering of rig and shortening of mast included.

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3 hours ago, teknologika said:

Luka’s boat has had significant upgrades. Significant lowering of rig and shortening of mast included.

Turns out I am out of my depth.
 

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15 hours ago, m_kiel said:

Hi guys,

On the Lennon Sails website they have some short text about the A3.5m developed by Dylan, which is described by 'countering the direction of travel on [the other sails]' from which I read that the current trend is definitely not the holy grail yet.

 

 

 

I think Dylan's sail was made to the old rules though, so this is also one of the reason the luff is shorter. Anyway I think he reads SA, so he may elaborate on that if he wants. We'll see what rig he and others will bring at the worlds.

Mic

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