Fitting out

Written by: Ashley Shepherd & Martin Egan

If you have a kicking strap system which uses an offset clip as show above (common on Winder Mk 3s), you would be well advised to check the offset clip for cracks on a regular basis.

Read more …

Written by: Professor Lars Larsson

Smooth Bottoms

Professor Lars Larsson asks how smooth should the hull be in order to reduce resistance? Is there a ‘roughness height’ below which there is no resistance increase?

The flow of water near the hull
When a dinghy or yacht moves through the water it drags the water along in a rather thin layer outside the hull called the boundary layer which is a few centimeters thick for most yachts.

Water sticks to the actual surface of the hull and so at this point there is no relative motion.

Thus, within the boundary layer the speed increases from zero, relative to the hull, to approximately the yacht speed at the boundary layer edge.

In the boundary layer the flow of the water is very irregular and turbulent, but there is a very thin region close to the hull, the viscous sub-layer, where the flow is mostly smooth, or laminar.

Permissible roughness of the hull
If any roughness of the hull is embedded in the viscous sub-layer it will not influence resistance; the surface is hydraulically smooth. This is good news, as it means there is a limit beyond which there is no point in further smoothing the surface.

This limit is dependent upon three factors: the hull shape, position on the hull and speed of the boat. The speed is the most important factor and a reasonable estimate of the permissible roughness height can be based on speed alone, as shown in Figure 1 below.

Here the roughness is in microns (1/1000 mm) and the speed in metres per second (knots divided by two, approximately).

As an example, a 36ft yacht will seldom go faster than 8 knots, i.e. 4 m/s – unless planing. This yields a permissible roughness of 25 microns, which is the grit size of sandpaper P600.

So we can say that there should be no roughness elements on the surface higher than the grits on this sandpaper! This is a tough call which would not normally be accomplished on a brush or roll-painted surface without sanding afterwards.

Brush and roll-painted surfaces
The unevenness left from a brush or a roller varies depending on the paint viscosity, which in turn depends on temperature and humidity. Other factors are the brush/roller quality, the skill of the painter and how fast the paint dries. In addition, the original surface may contain roughness elements caused, for instance, by old paint. In Figure 2 the increase in frictional resistance of the yacht for varying roughness size is shown. The results were obtained for a 25 foot traditional yacht, but are applicable to other yachts as well, as speed is the dominating factor.

At 7 knots, increases in frictional resistance as large as 20% may occur, even for a modest roughness of 100 microns which is common for a brush-painted surface.

For 500 microns (0.5 mm) the roughness resistance is 80% of the smooth frictional resistance. Upwind in a good breeze the frictional resistance is about 1/3 of the total. For a yacht that makes 7 knots under these conditions, the 100 micron roughness would yield around 7% increase in total resistance, which corresponds to a speed loss of 1.5%, roughly 3 boat lengths per nautical mile.

Other factors which will increase frictional resistance are barnacles, the keel and rudder.

Learn more…

Written by: Martin Egan

The building instructions (this version kindly provided by Mirror Sailing Development in Canada) cover the basic fitting out of a gaff on page 41 and page 42. If you are not going to race these may be adequate for you. If you want better performance, then the following may help.

Gaff band or slot & pin

There are two choices.

1. Most people in the UK use a gaff band to attach the main halyard to the gaff. You need to use the 'low profile' stainless steel Allen Brothers A4032, or something better. The original brass gaff band supplied in the 60's and 70's is utterly useless if you want to get the gaff up tight against the back of the mast. 
2. A few people have a slot in the front of the gaff which an eye in the end of the halyard goes into where it is secured with a pin. More popular in Australia than in the UK, although I'm pretty sure UK MCA Technical Officer Alan Bennett used this system on  69929 - Zarya. I've got my doubts about the wisdom of cutting a slot here since the front of the gaff where the halyard attaches is under considerable strain when the rig is fully powered up. I suppose if you are using a wire halyard, its only a 2-3mm slot,.....

You need to ensure the bottom edge of the band, or the pin, is as near the maximum permitted distance from the top of the gaff as is allowed in the class rules (currently 1733mm). This will ensure the gaff is hoisted as high as possible.

Before fitting a band, you need to ensure it fits the gaff snugly. If  your gaff has a square cross section at this point, it might just need the corners rounded off a bit. If you have a more circular cross section, then you will need to bend the band to match, using pliers etc. You are trying to get the main halyard attachment point as close to the front face of the gaff as possible. The point on the gaff where the band is fitted is where the gaff is under most stress (when a gaff breaks, it's nearly always at this point). Don't try and cut a rebate for the band, that will just weaken the gaff, exactly where you need most strength.

With a square cross section gaff, the screws holding the band on are in sheer. If you have a different section, you should aim to have the screws in sheer as well, that might mean re-drilling the fixing holes in the band. Because they are in sheer, there is no need to have oversize screws (which will just weaken the gaff). I find 12mm No. 6 pan heads are perfectly adequate on a new gaff. Needless to say, you need to ensure the screws do not protrode into the slot in the gaff, otherwise you won't be able to get the mainsail in or out of the slot.

When fitting a band, I use a couple of small G clamps to hold the band firmly in place with the halyard attachment point as close to the gaff as possible. I also make sure the pilot holes for the screws are bearing on aft side of the holes in the band - so the band is held tightly against the front of the gaff. Always a good idea to put some (wet) varnish, or (unset) epoxy in the screw holes before you put the screws in.

If you need to re-fit a gaff band, and there are old screw holes near to where you want to fit the band, one option is to cut a bit (50mm, 75mm) off the top end of the gaff, re-mark the "black" band and re-fit the band lower down (i.e. screws going into fresh wood). Needless to say, you need to check positions for the bands comply with the class rules. There is no minimum length for the gaff, and cutting a bit off won't make much, if any, difference.

Distinctive Coloured Measurement Band

Using masking tape when painting the band - Click to expand

Normally referred to as the 'black' band (but it does not need to be black), the head of the mainsail must not be pulled past the bottom edge of this band, so limiting how high the mainsail can be set. Details in the class rules, currently (2018) painted, not less than 16mm wide, lower edge not less than 76mm from the top of the gaff.

Gaff Jaws

The standard jaws can be greatly improved to reduce windage and stop other rigging, like the spinnaker halyard, catching on them. 

There is, in my view, no need for them to extend forward of the middle of the mast. This is, after all, where they bear on the mast.

Burgee Holder

I think a burgee or wind indicator is essential on a Mirror. The Allen Brothers A4066 burgee holder is now pretty well the standard fitting on gaffs or Bermuda masts. It works well with a Falker burgee which is my preferred make. Needless to say, it needs to be fitted as high up as possible, so the burgee is as high as possible above the mainsail.

If you want less windage, there is a way, using a system based on the ZephyrRace burgees popular in the 70s & 80s.

Written by: Martin Egan

The building instructions (this version kindly provided by Mirror Sailing Development in Canada) cover the basic fitting out of a gunter mast on page 42 and page 43. If you are not going to have a spinnaker, then these may be adequate for you.

A couple of comments from me on these instructions

• The type of the halyard cleat (wrap around jamb horn cleat) show in the Building Instructions is far from ideal in my experience. A clamcleat is a much better bet (it does not bend like the typical plastic wrap around ones and is kinder on the rope). Also, because you can tension the halyard while it is in the cleat, you can also get the halyard really tight. I favour the use of "open" clamcleats such as these.

Clamcleats with open sides like this allow you to pull the sail up with the halyard out of the cleat, and just hook it into the cleat when the sail is hoisted. Lowering the sail is the reverse, uncleat the halyard and then take it clear of the cleat to lower. This is much easier than having the halyard running through the cleat all the time, as happens with the "closed" type of clamcleat. Yet another advantage is that you can easily add a 2:1 purchase system to the main halyard.

• The location shown in the Building Instructions, one on each side of the mast near the foot, means that with the "wrap around" ones, the jibsheets get caught in the top "horn" every time you tack. It's also very difficult to pull the halyard up really hard when your hand is so close to the deck. I prefer to have the cleats about half way between the gooseneck fitting and the foot of the mast, located on the back of the mast and slightly angled, like this.

As you can seem, this mast is fitted with a handed pair with a silver finish - the part numbers for these are CL217MK2 & CL218MK2. They are located 285mm from the bottom of the heel to the bottom of the fitting. If I was fitting out this mast again, I would use one CL217MK2AN Mk2 Side Entry (Starboard) cleat and one CL218MK2AN Mk2 Side Entry (Port) cleat instead. The AN on the part number indicates a hard anodised (black) finish which is the best, espically for use in salt water. Whatever you decide, they have to be below the gooseneck to comply with the class rules.

The lacing eye/deck clip in the photo is for a 2:1 purchase system on the main halyard.

• If you can get hold of a pop riveter, I would do so and use pop rivets rather than self-tapping screws to hold fittings onto the alloy tube part of the mast. Also make sure you use rivets made of monel - a special alloy. These are more expensive and harder to get hold of than plain aluminium rivets, but they won't corrode and break, and so your fittings will stay put. You might need to use a spacer, some M3 or M4 nuts for example,  on the mandrel of the rivet to get it seated in the right place if the nozzle of the pop riveter is too large to get into the recessed fixing hole on the cleats. Putting some zinc chromate primer or an etch primer or similar in the hole and on the rivet before you attach the fitting won't do any harm. No need to wait for it to dry.

The spinnaker pole eye & spinnaker pole uphaul pulley location is covered in the flyaway pole article.

A Spinnaker halyard crane is a popular option. A lot of boats are equipped with the Holt-Allen HA4184 or Allen Brothers A4184.

These have a lot of friction and the plastic roller is prone to stick in one position, then the halyard wears a groove in it. If you have one of these the best bet is to shackle a small ball bearing block, such as the Harken HK404 to the front hole and take the spinnaker halyard down directly from the block, missing out the roller altogether.

I use this RWO crane on my masts with a small ball bearing block fixed to the eye.

I've fitted mine upside down, and slightly offset to port, so it does not foul the main halyard, as you can see here.

In order to get the gaff to go vertical with no gap between the gaff and the mast, and to ensure it rotates correctly, you need to cut away around the sheve on the aft side at top of the mast. Something like this:

I suggest holding the mast in a vice and rigging the gaff and gaff halyard and then pulling the halyard tight to simulate the situation when sailing. You should be able to see then what is stopping the gaff going up vertical and do something about it.