20241007

Why Hatseflats Has a Stiffy

Revised 05-11-2024

When designing the spars for Hatseflats I used Iain Oughtred's rule of thumb that an unstayed wooden mast should have a diameter of 1:50 of its height above deck and that the diameter of yard and boom should be about 1:60 of their length.
To save time I made my spars from anodised alloy tubes which were just as stiff and quite a bit lighter than solid wooden spars.
The yard and boom were made from 50x2 mm tube and bent a couple of cm under load. The stiff 80x3 mm alloy mast would still have been immaculate if I had not tried to drive it through an 11kV overhead electricity cable in the summer of 2019. After repairing the mast gate and fitting the slightly kinked mast upside down I continued to sail Hatseflats hard.
Yet during the 2023 Semaine du Golfe I found that hiking out with 180kg on the rail put a lot of strain on the mast. Before the next season I inserted an inner sleeve in the lower mast. This made the mast immensely strong, so it was only out of curiosity when I measured the deflection of the mast loaded with 0, 10, 20 and 30 kg and found that the mast only bent 38mm when loaded with 30kg.

Why a Lug Rig?

The lug rig is deceptively simple: it is set on an unstayed mast and only has three controls:

• a halyard to hoist the sail, yard and boom
• a downhaul to pull the boom down and tighten the luff
• a main sheet to pull the sail in.

Spars For Lug Rigs

Francois Vivier designed the Aber, Ilur and other traditional looking boats with solid wooden spars after the original working boats and their crews had already disappeared, so I suppose that the sailing rigs of his designs were based on common sense as well as documentary evidence.
Note that the lug rigged boats designed by Iain Oughtred, John Welsford, Jim Michalak and Michael Storer have solid wooden spars with similar dimensions.
So in the absence of hard and fast rules, designers seem to agree that the spars must be stiff and strong.

Stiffness

The yard and boom on Hatseflats bend about 4-5 cm with full power on the downhaul. Too much bend is not desirable: Joost Engelen found that his 32x1.5 mm carbon fibre yard caused a big crease in the sail of his new Silmaril and had to replace it with a stiffer 40x2 mm carbon tube to solve the problem.

Mast bend seems much more critical. According to Michael Storer's write-up about lug rigs for the Goat Island Skiff, the mast must be 'relatively stiff'.
Based on my own experience I think that a 500cm mast should not bend more than a couple of cm under full load:
During my first outings I was disappointed that Hatseflats would not sail upwind on a starboard tack. Joost Engelen later hinted that the gap between the boom and mast was too big so I added a 'bleater' to tie the boom closer to the mast. At the start of the Raid NL, Joost helped to remove the bleater and mast ring and set up the Tirrik rig just like a Goat Island Skiff and added lots of downhaul tension. Thanks to the stiff mast, the ultra tight luff now kept the boom close to the mast and Hatseflats suddenly pointed upwind as well as any other boat.
Another indicator is the diagonal crease that appears every time I sail Hatseflats upwind in gusty conditions. It runs from the throat at the yard to the clew at the aft end of the boom. Maybe it's due to a little slack in the halyard or 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 I can still sail, but it feels as if I am losing height. Pulling the downhaul extra tight makes the crease disappear and the sail becomes easier to control. With the 4:1 downhaul I guess there is about 150kg tension on the luff. I have to repeat this procedure 2-3 times to make the crease finally disappear.

Summing up: even if the luff can be a little slack in light conditions it must be tight for upwind work and brutally tight to flatten the sail in a breeze and to open the leech in strong gusts.
The need for a tight luff also applies to sailboards and racing dinghies like the Finn, Laser and Topper. Yet these have bendy masts and sails which can be tuned to suit all points of sail or weather conditions. Tightening the luff on these boats makes the mast bend to match the luff round of the sail and flatten the sail.
The lug rig works very differently, because the sail is suspended from a yard. The unstayed mast is only there to hold the yard up. Here the sail is flattened by pulling the boom away from the yard with the downhaul. This puts a lot of stress on the halyard, downhaul and the luff of the sail. Flattening the sail only works if the mast is stiff enough to withstand these forces.
Sailmakers Frank van Zoest and Marten-Jan Giesing said that you cannot flatten a lug sail enough by pulling the downhaul on a bendy mast. The sail will remain too full and will be very hard to control. Marten-Jan added that trying to sail with a reefed lug sail a bendy mast is particularly bad because both the yard and boom will fly away from the mast until the mast breaks or you capsize.
Frank later added that the luff rig needs extra reinforcement to cope with the forces on the downhaul.
Note that Michael Storer wants to pull the downhaul 'crazy tight' when it blows and specifies stretch-free dyneema for the halyard and downhaul. He also writes that the diagonal crease from too little downhaul tension may cause the boat to capsize during a gybe. This happened to me twice and I'd rather not go there again!
That is why my motto is 'the stiffer, the better'.

Spar materials

Wood

Most of today's lug rigged sail and oars boats are sailing with sturdy wooden spars. Solid wooden masts look nice but are expensive and time-consuming to build. A typical 500cm mast weighing up to 14kg becomes very heavy if it is lowered and raised all the time, so some builders make their own hollow 'birdsmouth' masts which looks like a fun thing to do and may save several kg.

Aluminium

Alloy tubes can be a practical and relatively inexpensive alternative for hollow wooden masts. I found them easy to work with and completed the spars for Hatseflats within a single day. Be sure to buy clear anodized tubes: they look nicer and don't stain your sail. Note that alloy spars require a bigger diameter than solid wooden spars for the same stiffness. Alloy spars are virtually maintenance free but their slightly agricultural look does not make them a popular choice for lug rigs.

Carbon fiber

Several years ago French builder Emmanuel Conrath started to use light weight carbon tubes for masts, yards, booms and gennaker poles. The diameter of carbon spars can be smaller than solid wooden spars which improves both handling and looks. Soon enough many French sail and oars boats converted to carbon spars.
Note that you must laminate fittings to the carbon tubes, so this requires a lot of extra care. See Joost Engelen's excellent 'Viola 14 Lug Rigging Guide' on www.storerboatplans.com.

Some home builders used old windsurfer masts with varying degrees of success.
For example, Koos Winnips successfully created the spars for his Artemis sailing canoe from old windsurfer masts. He made the yard and boom cut down windsurfer masts and used mast from the lower part of another windsurfer mast after adding unidirectional carbon fibre for extra strength and stiffness.
On the other hand, Cockie found the windsurfer mast on her Poo Duck Skiff too much to handle during the 2020 Dorestad Raid. It had been OK in light airs but now proved very bendy in windy conditions. Reefing did not help to get the sail under control, so she decided to row until the wind had moderated to a light breeze. Afterwards the windsurfer mast was replaced by a stiffer new carbon tube.
Similarly, Jelle found the new lug sail on his Ness boat too big for his windsurfer mast. On his first outing at the 2024 Zonnewende weekend, the mast was bending dangerously and seemed to be on breaking point in the gusts. He wisely pulled out and safely made it back to shore.

Old windsurfer masts may be suitable for yards and booms but are too bendy to be used as masts (except for rigs <5m2).
When buying an old windsurfer mast, be sure to check the IMCS stiffness indicator. Around 30 may be useful for yard and boom but will be too bendy for a mast. Note that the Hatseflats mast would have an IMCS of 150!

Comparison Table

I created the table below to compare (untapered) spars of solid wood with alloy and carbon tubes, with the properties of alloy and wood based on the Wikipedia page about Young's modulus.
The table is by no means complete and all numbers are indicative. Please get the exact specifications from the supplier before you buy!

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

Summary

Lug rigs are deceptively simple rigs which are ideally suited for dinghy cruising. However they require a tight luff to point well upwind, to flatten the sail and to spill wind in gusts. Since the sail is only tied to the yard and boom, the mast must be stiff to deal with lots of downhaul tension. A stiff mast is critically important when sailing upwind with a reefed sail. 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 may be a practical alternative 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.

If you are still awake, 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