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The Appliance of Science

A few weeks ago, I promised to give the technical slant on the story of Snark. It is a bit involved and lengthy, but I will try to get it done in one go.

In their heyday, in the early 20th c, spritsail barges were the epitome of the phrase, form follows function.

  • They were simple to make, mostly a wooden box with some heavy planking on each side of a box frame with minimal curvy bits at each end.

  • They were built to last long enough to make a profit but not to be indestructible.

  • They were a manageable size to enter small creeks but big enough to carry an economically viable cargo of between 60 – 100 tons,

  • The sprit sail rig was a fine balance compromise between efficiency and short-handed sail handling,

  • They were loaded with geared winches (typically five some with multiple spindles).

So why change anything? Well, there is a strong argument for that, not least that you know the old version works. Our contemporary new build ‘Blue Mermaid’ has taken exactly that approach even to the point of having no auxiliary engine.


However, other things have changed. It used to be that standards were set by insurance groups, typically Lloyds in the UK, who were concerned about the value of the vessel and cargo but had little concern over the people involved, life was cheap. One hundred years on we take a more serious view of crew and passenger safety and we are now bound by far more rigorous regulations (or Coding requirements) on construction, stability, crew training, maintenance obligations, etc. As a result, traditional Thames barges, including Blue Mermaid, are now covered by an exemption regulation that limits their areas of operation to more sheltered estuary waters and they can no longer operate commercially as a coastal vessel.


When we started looking for a barge to buy and operate all of these problem came to the fore but being architects by profession we applied our joke mantra ‘We don’t have problems we have opportunities’ and set about finding solutions. We engaged the expert marine engineers Longitude to look at key issues related to construction and stability and started doing our own in-depth research into the technical design of the rig, the leeboards, steering etc.


Load Line Length


The Code we were applying MGN280 is for smaller sailing vessels and is simpler to achieve and maintain than the regulations for larger sailing boats. However, to achieve this we had to cut 1.6m out of the centre of the hull to shorten the load line length to 23.95 m just under the 24m limit. Measuring LLL is in its own right a black art and if your boats steel consider doing it in the winter, a 24m m steel boat will expand up to 10mm in the summer.

We did not reduce the beam (width) making her proportionally wider than the original design. This helps with the stability as below but makes balancing the sail/leeboard/rudder loads more complex.

Construction

Snark is a steel barge based on the lines (hull shape) and scantlings (structural dimensions)

designed by J G Fay in Southampton in 1898. They complied with Lloyds regulations of the day and these are still surprisingly relevant.

When Longitude reran the calculations the framing and plate thicknesses met modern expectations closely enough to be accepted by the inspecting surveyors. The steel strength was lab tested and again proved just okay. The only fail was in the welding. Since the quality of a weld is very difficult to test in situ the Code requires that the welders doing the work are certified as competent to do the type of welding applied. David Speight did not have the necessary qualification or skill. The solution was to cut out all of the plate welding and redo it. The heat creates plate buckling which took a lot of remedial work to get within the regulations!


Stability


Boats need to stay up right even in big waves and strong winds. That may seem obvious but can be surprisingly difficult to achieve. Simply put either the centre of buoyancy has to be above the centre of gravity as with a modern keel ballasted yacht or the form of the boat needs to resist overturning, in the extreme like a catamaran. As barges are flat bottomed, they rely largely on form stability. Historically they would sail with a heavy cargoes most of the time which significantly improved the righting moment. One of the unique features of a spritsail barge was that they could be sailed un-ballasted, saving the cost of loading and off-loading dead weight when sailing empty to pick up a cargo.


The second factor in stability is called down flooding, the point when openings in the deck end up underwater. The removable hatch covers on a barge are not considered sea proof (though they often ended up underwater in the past) and this limits their stability to 65 or 70 degrees instead of the 90 degrees required.


We added 16 tonnes of solid steel plate at low level above the bottom frames and built a raised steel coach roof over the hatch opening which also allowed us to put waterproof windows into the comings (the upstand around the opening) This was then tested by putting heavy weights on alternating sides of the hull and extrapolating the heel angle out using a standard formula. We exceed the stability limit of 90% by over 10%. A great result.


The rigging and sails


We changed the rig from the more conventional river barge large main and very small mizzen to a rig with a larger mizzen based the layout on Thalatta, a muley rigged barge; a format more commonly used for coastal barges. This gives a wider range of sail combinations but along with the length reduction required moving the main mast forward and re-positioning the lee boards to ensure the boat was balanced. Lots of diagrams and calculations ensured that when we finally got to sail her she is remarkably well balanced.

The weight of the rig is critical to the stability, too heavy and it will tend to pull the vessel over when it is heeled, too light and its not strong enough to withstand the wind and shock loads caused by manoeuvres and waves. Barge rigs were not really designed but evolved to resist the loads when the vessel was fully ladened, and the righting moment was high. But we sail with an empty hold and our righting moment is only about 40% of a fully loaded barge. This means the loads from the wind are much lower and fittings spars and shrouds etc can be proportionally lighter. The Norsk Veritas, another insurance driven code, sets out a simple method o calculating mast sections, rigging loads and righting moments. These in turn satisfy the UK Coding authority who surprisingly do not have a standard for rigging.

We opted to return to the traditional approach with timber masts and spreet as they are lighter and absorbs shock loading as they flex, something the steel masts and spreets don’t do. We used the modern synthetic rope Dynema instead of steel wire for shrouds and fixed stays as it is much lighter and easier to work. It does stretch a bit but long turn buckles deal with this.


We replaced the hand wound drum and crab winches with electrically driven capstan winches and incorporated a system of ‘Constrictor’ rope locks to simplify the operation of the lines to allow guests to participate in the sailing. Lastly and critically we decided we must be able to operate everything from the deck, so no ratlines up the shrouds to tie in the top sail and no working out to the end of the bow sprit to dowse a flapping headsail.


This set a challenge to James Lawrence Sailmakers but they did a brilliant job of updating the traditional barge sails to suit our purpose. We deliberately didn’t use the traditional red/brown sails as we are not a traditional vessel in that sense. In any case the colour was just the protective oil and iron oxide dressing applied to the natural canvas often a few years after the sails were made. Whether Jimmy would approve, himself a barge master before becoming a sail maker, we don't know but we think she looks splendid with her natural coloured sails.


The leeboard and rudder


The sails only work when sailing to windward if there is an opposing pressure from the hull. As a barge is very shallow draft and has little ‘grip’ on the sea, this must be provided by leeboards which pivot down on the leeside of the hull and are raised and lowered on each tack. We started out with great ambitions of highly designed asymmetric foils and were given some advice on this by the Professor of Hydraulic Engineering at Southampton Uni. However the optimum profile is quite fat and the volume of this underwater is buoyant so needs to be ballasted to keep it down. The overall weight proved too difficult to handle though they did work surprisingly well. We currently have thinner but still carefully profiled oak boards. Steel fabricated designed profile leeboards may follow at some stage.

The rudder on a traditional barge doesn’t project below the bottom of the boat so in light mode it only goes 800 mm into the sea, not much area for steering or to contribute to lift. It was also very chunky as being transom mounted it was prone to damage in a busy harbour. So, we set about designing and fabricating a lifting rudder with a properly designed profile its taken a few modifications but now we can lower it as a 1.8 m deep rudder at sea, an 800 traditional paddle in shallow water and raise it right out of the water to avoid the flapping loads they suffered from wash and waves. It does work extremely well and makes helming much easier in a seaway.


Auxiliary power


And finally, we did decide to install engines. Two 115 hp Nani diesels running self folding Brunton Autoprops 590 mm in diameter. Careful positioning means these work pretty efficiently even though our draft is only 750. The folding reduces drag when we are sailing and also prevents damage if they touch a stone when we dry out. We also have a generator engine which provided hydraulic drive to a bow thruster and the anchor windless. Oh, and yes, we have abandoned the 130 kg fisherman’s hook for a state of the art Rocna anchor.

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