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Wednesday, March 21, 2012

A Class Catamarans – A Look at the State of the Art Part 7

We saw in the last post that current foil assisted A cats are inherently unstable in pitch.
As sail force and hence bow down trim increase, the angle of attack (AoA) of the foils decreases resulting in less bow up trimming moment.
Conversely, if drive force decreases and the bow comes up, the AoA increases and a runaway feedback loop arises: More bow up trim results in a greater AoA that gives more bow up trim… 
Until either the boat jumps out of the water or the foils stall. 
Both possible outcomes happen quickly and are slow in terms of time around the course!

Bow up feedback loop being allowed to continue.
Image credit unknown
The problem is compounded by the fact that effective foil dihedral INCREASES with ride height. 
So as the boat comes out of the water, the vertical component of the foil force INCREASES, causing the boat to want to rise further.


Instability in pitch is combined with instability in ride height
due to the foils effectively becoming more horizontal with increasing ride height
In the real world the twofold feedback loop resulting in pitch instability is mitigated by the following factors:

-          When trapezing downwind, the boats are sailed with ‘reserve’ bow up attitude so there is some margin before the angle of attack goes negative, flicking the bow down suddenly. Even so, the angle of attack does decrease instantly as the sail force increases. Incidentally this explains the sudden and spectacular ‘tripping over pitchpoles’ that are often seen when foil assisted A cats are sailed at speed, even in flat water, and with the bows seemingly clear of the water. If the foils suddenly switch from producing a significant bow up moment (positive AoA) to pulling down (negative AoA), the limits are reached suddenly and spectacularly. The stern plays a role here but more on that later.

-          When sail force increases, whilst the bow up ‘margin’ is being used, the boat also accelerates. The increased speed means the foils can generate more lift for a given angle of attack. If there is enough reserve pitch angle to keep the foil angle of attack positive, then the bow up moment can remain sufficient provided enough additional speed is obtained in time. 
In other words acceleration replaces pitch angle as the determining factor in the amount of lift produced.

-          As sail force increases, the skipper steers to pull the apparent wind around and sheets on. This redirects the sail vector more across the boat and reduces the apparent wind speed, moderating the sail force.

Sailing downwind with an AoA 'reserve' allows the boat to accelerate.
Higher speed and course/sheet adjustments can compensate for the reduction in AoA caused by increased drive force.
Sailing technique has evolved to deal with the limitations of existing curved foils, and foil assisted sailing is winning A cat races convincingly. 
In moderate conditions this requires very fast reflexes and agile shifts in crew weight. 
It can be thought of as analogous to riding a unicycle: It can be difficult to master and laborious but it is possible within limits. 

Time and again one reads in forums and hears sailors comment that in certain conditions the foils need to be partially retracted to limit the amount of vertical lift and bring the boat back under control. 
This is a serious limitation: retracting the foils simply reduces dihedral angle, giving up the full benefits of vertical lift just at the speeds where the greatest advantages occur (remember that as speed rises foils get more effective and hull drag becomes more expensive).

Retracting a curved foil (constant radius) reduces effective dihedral
Yet this limitation is accepted as inherent in the current winning configuration.
One manufacturer has even increased the radius of curvature of their updated foils.
They have made the newer foils straighter partly because they were getting 'too much lift'.
Surely a better solution would be to improve dynamic stability and avoid giving up the advantages of foil assistance, just as speed rises sufficiently for the advantages to become significant.

One way to push the limiting conditions further up the speed range is to increase volume and flat area in the stern. 
That is the current trend and will be the subject of the next post.

2 comments:

  1. DNA have increased the radius, but not to reduce lift. The foils are straighter but they are also longer, in order to produce the same lift but gain more resistance to leeway. In addition, the deeper board gives more "bite" when trying to pop a hull in marginal wild thing conditions.

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    Replies
    1. I was not referring to DNA but it is interesting that they have also straightened their boards. From their official blog:

      "In strong winds you can easily pull up the boards 10 cms as you easily have too much lift."

      The observation that too much vertical lift is produced at speed is common in forums and on the beach. The fact that lift must be reduced for the boat to remain manageable is an indicator of instability.

      Remember that vertical lift is related to sideforce through dihedral angle. Sideforce is determined by sail force which in turn is limited by righting moment. So sideforce cannot be increased for a given leeway angle.

      Since sideforce is a given, they moved the Centre of Effort (CE) of the boards down. This increases the vertical separation between sail force and foil force, increasing heeling moment. More heeling moment means earlier hull flying (same effect as making the mast taller).

      What sailors feel as "more bite to be able to trapeze sooner downwind" is simply more heeling moment that arises because the boards produce the same side force but lower down.

      Interestingly they suggest raising the boards by the same amount as the increase in span. This actually gives a net reduction in vertical lift at the old span if you do the math for the larger radius.
      They have reduced dihedral to get less vertical lift for the same sideforce with a lower CE when the foil is fully down and even less vertical lift when the foil is at the original span.

      The DNA is arguably the fastest boat out there right now. It is a great product that has served the class very well and their solutions clearly work.
      The purpose of these posts is to look at the physics as we study the limitations of current solutions in the search for future improvements.

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