20241007

Why Hatseflats Has a Stiffy

Second revision 17-10-2024

When I was designing Hatseflats I examined existing designs to learn about mast dimensions for lug rigs. I based my calculations on Iain Oughtred's rule of thumb that unstayed masts for small boats should have a diameter of 1:50 of the height above deck and that yards and booms about should be 1:60 of their length.
Instead of building wooden spars I decided to use slightly lighter spars with comparable stiffness from anodised alloy tubes.
I used 50x2 mm tubes to build the yard and boom and a 80x3 mm tube for the mast.
The yard and boom bent just a little, but never caused problems. The mast was very stiff and would still have been perfect if I had not tried to drive it through an 11kV overhead electricity cable in the summer of 2019. This left a slight kink in the mast near the mast gate. I repaired the mast gate and fitted the mast upside down and continued to sail Hatseflats hard in the next years.

For the 2023 season I fitted hiking straps which Rene and I used to the full during the Semaine du Golfe. Although the mast held up well I was aware that hiking flat out put a lot of strain on the mast and the mast gate. For the 2024 season I reinforced the lower mast with an inner sleeve.
Since then I have sailed Hatseflats in even stronger winds without any problems.

Out of curiosity I measured the deflection of my 80mm alloy mast loaded with 0, 10, 20 and 30 kg as suggested by Michael Storer. It was not surprised that it was super-stiff: even when loaded with 30kg the mast bent only 38mm.

Why a Lug Rig?

Many sail and oars boats have lug rigs. For example the Vivier-designed Ilur and Seil, the double-enders designed by Iain Oughtred and Michael Storer's Goat Island Skiff. The majority of these boats were built from scratch by amateurs including the wooden spars.
The lug rig is extremely simple as it is set on an unstayed mast and only has three controls:

• the sail and yard are hoisted by the halyard;
• it is tightened by the downhaul on the boom, and
• the entire sail is controlled with a single sheet

The lug rig with its short spars is ideally suited for sail and oars boats in the Low Countries where the rig must be dropped and raised quickly to slide under low bridges. This is possible because the mast is unstayed and the sail is not tied to the mast but only to the yard and boom.
When approaching a low bridge, the skipper dumps the entire rig into the boat and lowers the mast while the boat is still moving. Once past the bridge the mast is put upright and the sail hoisted with the bow heading to the wind. The whole operation can be completed in a couple of minutes.

Spars For Lug Rigs

Much research has gone into bermudan rigs since they began to dominate the sailing scene in the 1920s. Hot competition in one-design classes and experimentation in development classes like the International Moth and Australian 18 foot skiffs led to many innovations and a widespread knowledge about masts, sails and rig tuning.
In comparison, hardly any research was done on the humble lug sail of the small fishing boats of the early 1900s. Their sailors merely required that the standing lug rig had to be easy to use and robust to be used in any conditions. Balanced lug rigs were seen as impractical as the boom was always in the wrong place and were only seen on pleasure boats.

Wood

Francois Vivier had made a study of the remaining lug-rigged fishing boats in Brittany when he designed the Aber, Ilur and other boats based on traditional working boats. Other designers like Iain Oughtred, John Welsford, Jim Michalak and Michael Storer used similar specifications for the spars of their lug-rigged sailboats.
The result is that many of today's lug rigged sail and oars boats are happily sailing with stiff wooden masts. Solid wooden masts surely look nice but are expensive, time-consuming to build and heavy in use as a typical 500cm mast may weigh up to 14kg and cannot be lifted through a hole in the foredeck.
Therefore some builders made their own hollow 'birdsmouth' masts to retain the looks of a wooden mast and reduce (a little) weight.

Aluminium

Alloy tubes are a highly practical, strong and inexpensive alternative to reduce weight. Untreated alloy tubes may stain your sail, so be sure to buy clear anodized tubes - they look nicer too.
Aluminium is easy to work with and you can simply bolt or rivet fittings to it.
Aesthetics may be the downside of aluminium. For the same stiffness they require a bigger diameter than solid wooden spars. For some, untapered alloy tubes look agricultural on traditional boats, which is why they are not very popular.

Carbon fiber

Several years ago French builder Emmanuel Conrath started using light weight carbon tubes for masts, yards, booms and gennaker poles and has stopped creating wooden spars. These light carbon spars are just as strong and stiff as the wooden ones and still look nice if a traditional appearance is required. During the last few years many French sail and oars boats made the change to carbon spars.

Carbon fibre tubes are expensive, which prompted some home builders to create spars from old windsurfer masts.
For example, Koos Winnips successfully created the spars for his Artemis sailing canoe from old windsurfer masts. Yard and boom were made from cutdown windsurfer masts and Koos used the lower part of another windsurfer mast as the main mast after adding unidirectional carbon fibre for extra strength and stiffness.
On the other hand, Cockie was struggling upwind with the windsurfer mast on her Poo Duck Skiff during the 2020 Dorestad Raid. It had been OK in light airs but now proved too bendy to flatten the sail in windy conditions. Cockie found it scary to continue and decided to row until the wind had gone down to a F2. After the raid, Joost remedied it by replacing it with a stiffer new carbon tube.
Jelle also got into trouble with the windsurfer mast for his new lug rig on his Ness Boat. At the 2024 Zonnewende weekend the big sail was hoisted for the first time. The sail clearly proved too big to handle in the SW4 (14kn) wind on the lake. The mast was bending dangerously and seemed to be on breaking point in the gusts. Jelle and Marlies wisely pulled out and safely made it back to the camp site.

How much stiffness do you need?

It is acceptable if the yard and boom bend a little under load, say 5cm over a length of 250cm. The yard and boom on Hatseflats bend about 4-5 cm under load which seems to be fine.
On the other hand, the yard should not be too flexible: Joost Engelen found that his bendy 32x19 mm carbon fibre yard caused an undesirable crease in the sail. Replacing it with a stiffer 40x2 mm carbon tube solved the problem.

Mast bend is much more critical. According to Michael Storer's write-up about lug rigs for the Goat Island Skiff, the mast must be 'relatively stiff'.
As an illustration I give my own observations:
When I first tried to sail upwind I found that Hatseflats could not point upwind. Joost Engelen fixed this by setting up the rig much like a Goat Island Skiff with lots of downhaul tension. With the luff now ultra-tight, Hatseflats was suddenly able to point as well as any other boat.
Another finding is that a diagonal crease appears every time I sail Hatseflats in gusty conditions. This crease runs from the throat at the yard to the clew at the aft end of the boom. It may be due to a little slack at the halyard cleat or flex in the yard but most probably it comes from the mast bending an extra couple of cm in the gusts. It is not a big crease and the sail is still controllable if the wind is not too much. If I pull the downhaul with my entire weight the crease disappears for a while. With the luff tightened up and the crease out of the way, the sail feels easier to control. I have to repeat this procedure 2-3 times before the crease finally goes away.

My theory is that the luff can be a little slack in light conditions but must be really tight for upwind work and for opening the leech to depower the sail in gusts when it blows. Very much like a Laser, Moth or any cat-rigged dinghy.
That's probably why Michael Storer wants to pull the downhaul 'crazy tight' when it blows and specifies stretch-free dyneema for the halyard and downhaul.
He writes that the diagonal crease (resulting from too little downhaul tension) may also cause problems when gybing. This is even worse because a bad gybe may cause the boat to capsize. Happened to me twice.

Therefore my preference is for ultra-stiff masts and my personal motto is: the stiffer, the better.

Spar materials overview

I created the table below with alloy and carbon fibre alternatives for solid wooden spars. It is by no means complete but may be useful if you are moving away from solid wooden spars.

Legend:

• EI kNm2: flexural rigidity indicating 'bending stiffness'
• Alloy AOM: available from www.aluminiumopmaat.nl
• Carbon Fibre PF: available from www.princefibre.com
• Carbon Fibre C-Tech SM: available from www.c-techcomposites.com (standard modulus)
• Carbon Fibre EC: used by Emmanuel Conrath
• Treat weight and stiffness as indicative values.

What about windsurfer masts?

Old windsurfer masts may be used for yards and booms but are too bendy for masts, except for very small rigs (<5m2).
When you are considering an old windsurfer mast, look at the IMCS value which indicates the stiffness. Around 30 may be useful for yard and boom. Note that the Hatseflats mast would have an IMCS of 150!
Joost Engelen's excellent 'Viola 14 Lug Rigging Guide' on www.storerboatplans.com contains much useful information.

In Summary

Lug rigs are very simple but powerful rigs which require a tight luff for pointing ability and to spill wind in gusts. Since the sail is only tied to the yard and boom, the mast must be stiff enough to deal with lots of downhaul tension. The yard and boom are less critical but should not be too bendy.
Wooden spars are stiff and look nice but can be too heavy if you must frequently lower and hoist the mast.
Alloy spars are extremely practical but not everybody likes their looks.
Off the shelf carbon tubes are light and stiff but come at a price. For most lug rigs, old windsurfer masts are not stiff enough to be used as a mast. However, the bottom of old windsurfer masts with an IMCS around 30 can be used for yards and booms.

And now, finally here are the mast measurements for Hatseflats:

Mast Bend Hatseflats alloy D=80mm t=3mm L=5000mm

Weight kg	Test Length mm	Deflection mm	Reduced Test Length mm	Deflection (mm)
0	4900	8	4500	6
10	4900	17	4500	13
20	4900	28	4500	20
30	4900	38	4500	29