Gooseneck geometry

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Gooseneck geometry

There are a number of questions I've puzzled over about the gooseneck and kicking strap. I've written an article (first appeared in Marine Modelling International, August 1999) that is an attempt to answer some of them. What is the effect of the gooseneck pivot being offset from the line on which the mainsail pivots? What is the effect of a gooseneck that isn't exactly parallel with the mast? How much twist does one turn of the kicking strap adjuster introduce? There is a small spreadsheet (23kb) which models these questions.

Gooseneck pivot offset

The effect of the pivot offset is to increase the draft of the lower part of the mainsail as the boom is sheeted out. When the boom is square to the centreline on the run, it is as though the outhaul is moved towards the mast by the amount of the pivot offset. So just how much draft does the pivot offset introduce into the sail, and how much is the entry angle increased as a result, as the boom swings out from close hauled into the reach?

On my Ikon, the gooseneck pivot offset is about 5mm. The foot of the main on the No.1 rig is about 345mm (335mm between clew and tack eyelets), and I have the outhaul adjusted so that the clew is brought towards the mast by about 5mm. This gives a draft of about 25mm or about 7% in the centre of the sail. When the boom sheets out square, the clew is effectively brought towards the mast by about 10mm, and this changes the draft to about 35mm or about 10%. The table shows how the draft and entry angle changes according to the sheeting angle of the main boom, for three different pivot offsets.

My mainsail is attached with luff rings, rather than with a bolt rope within a luff groove. The luff ring arrangement in fact increases the "effective" pivot offset, since the sail pivots about the mast centreline, rather than around the mast training edge. With a 12.7mm diameter mast, the "effective" pivot offset is in fact about 11mm.

What is interesting is that, when moving onto the reach from close hauled, even a modest pivot offset of 5mm causes an entry angle change of between 3 and 5 degrees on the lower part of the main, effectively freeing the main by this amount. Readers may recall my earlier article on sheeting angles, where I suggested that the main and jib booms should run roughly parallel on the reach. This might be desirable when the jib and main have similar drafts, but if the main draft increases due to the gooseneck pivot offset, then the jib may need to remain sheeted out a few degrees more than the main to accommodate this effect.

Draft % & Entry angle in degrees

Sheeting angle

Pivot offset

 

5mm

10mm

15mm

5

7.3%

17

7.3%

17

7.3%

17

30

7.8%

18

8.2%

19

8.5%

20

45

8.3%

19

9.1%

21

9.8%

23

60

8.9%

21

10.1%

24

11.2%

27

75

9.5%

23

11.2%

27

12.5%

31

90

 10.1%

 

12.2% 

 

13.8% 

 

One practical application of the table is that, while my "effective" pivot offset is about 11mm for the No.1 rig, it is about 5mm for the No.2 and No.3 rigs, since both of these rigs use a bolt rope in a luff groove. Hence, the sheeting geometry of the No.2 and No.3 main booms is different from the No.1 to allow for the fact that more draft, and hence a larger entry angle, is introduced into the No.1 mainsail by the gooseneck pivot offset as the boom swings onto the reach.

Gooseneck axis tilt

The effect of the axis tilt is to change the tension of the mainsail leech as the boom is sheeted out. If the tilt angle is positive, then the clew is eased up on the run, just as though the kicking strap had been released. If the tilt angle is negative, then the clew is lowered on the run, as though the kicking strap had been tightened. So just how much twist does the tilt angle introduce or remove? And if the gooseneck body is to be tilted with respect to the mast, what thickness of shim needs to be inserted to achieve a desired amount of axis tilt?

On my Ikon, the kicking strap is adjusted with a M3 screw having a pitch of 0.7mm per turn. Turning the screw one revolution has the effect of raising or lowering the boom by about 1 degree, thus raising or lowering the clew by about 5mm (the boom is about 340mm long). The gooseneck body is about 50mm long, and placing a 0.7mm shim under the bottom part of the gooseneck body has almost the same effect. In this case, the body tilts by about -1 degree and the clew rises by about 5mm. Of course, in the close hauled position, the kicking strap would be eased by one turn to restore the desired twist if the gooseneck axis was tilted in this way. It should now be clear that, as the boom swings out onto the run, the effect would be as though the kicking strap had been tightened by one turn. This is perhaps the best way to express the effect that tilting the gooseneck pivot has -- in terms of the number of turns on the kicking strap adjuster screw to obtain a similar change in leech twist.

The table shows various effects depending upon the shim thickness, and at intermediate sheeting angles between close hauled and the run. The table can be used for shims placed either under the top or the bottom of the gooseneck body, since the number of "effective kicking strap adjuster turns" will either ease or tighten the leech respectively.

Interestingly, the table shows that most of the effect of tilting the gooseneck occurs between the reach and the run, with relatively little effect between close hauled and the reach.

"Effective" turns of kicking strap adjuster screw (assumed M3 with 0.7mm pitch)

Sheeting angle

Shim thickness

  0.4mm 0.7mm 1.0mm
5 0 0 0
30 0.1 0.1 0.2
45 0.2 0.3 0.4
60 0.3 0.5 0.7
75 0.4 0.7 1.0
90 0.5 0.9 1.3

One practical application of this table is that I have a 0.4mm shim under the TOP of my SailsEtc No.1 rig gooseneck, because I want an increase in twist for this suit as the main sheets out. On the other hand, I have a 0.7mm shim under the BOTTOM of my Rod Carr "HT" No.1 rig gooseneck, because this suit already has considerable twist cut into it, and I want this reduced for the run.

Axis tilt due to mast bend

Gordon Stout has helped me remember that the gooseneck axis will also tilt off the vertical when the mast is bent. The diagram gives a crude illustration of the effect. If the mast is significantly bent to match the mainsail luff curve, the gooseneck and the gooseneck pivot axis will tilt "negatively" such that, on the run, the leech will be tightened. This may be undesirable, and it may be worth putting a permanent shim under the top of the gooseneck body to restore the axis tilt so that it lines up with an imaginary line drawn from the mast foot to the mast head. The spreadsheet estimates this tilt due to mast bend. For example, with an IOM No.1 rig mast of about 1600mm, and a mast bend of 12mm -- about half an inch -- measured as the gap between the mast and the imaginary line from head to foot, axis tilt is about 0.86 degrees -- as though the kicker had been screwed a full turn. That is a significant effect....

Adjustable axis tilt

Adjustable gooseneck picture

The photo shows the simple arrangement I have to make my gooseneck tilt adjustable. I use one of the SailsEtc goosenecks that is suitable for any round mast, so the gooseneck body has a "Y" cross-section. It is screwed to the 11.1mm groovy mast using a single #4 20mm self-tapping screw through its centre. Inserted inside the mast is a short length of joiner section which takes the threads of the screw. A short 10mm length of 12.7mm alloy tube is slipped over the mast and underneath the bottom of the gooseneck body. When the screw is tightened, the tube is clamped firmly in place. This tube has a wall thickness of 0.7mm and hence forms a 0.7mm shim when placed right at the bottom of the gooseneck body. To increase the tilt, the tube shim is moved up as required. When a few mm short of the screw, it provides an effective shim of about 1.5mm.

PostscriptTwist due to the wind gradient while reaching or running is a significant factor in setting the sails.  Check this out before deciding how much to change the gooseneck axis tilt.

2005-12-18


2011 Lester Gilbert