How do load cells work?

In very basic terms, a load cell is a unit that is able to measure force and a strain gauge is a type of load cell that measures the in-line force across a single component such as a toggle. The load cells we’ll be covering in this article are strain gauges.

Strain gauges measure the change in length of a metal component and then convert that to a measure of force by applying Hooke’s law which states that the change in length of an elastic component is proportional to the force applied.

Sensors can be used for accurate trim settings. Photo: Spinlock.

When a relatively large solid item (such as a rigging screw) is put under load the change in its length is microscopic and impractical to measure. So, load cells measure by attaching a very fine length of wire to the main component in the direction that force will be applied. The wire runs up and down a fixed grid, several times and is stretched and relaxed at the same rate as the main component moves.

As this fine wire changes length, so will its electrical resistance (due to the resultant change in wire diameter) and it is this measure of resistance that can be converted to a measure of force. Each strain gauge will also contain a temperature control wire to ensure only changes due to an increase in resistance are being measured.

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Types of load cells

Different types of strain gauges and interfaces can be used for various applications around a yacht’s deck. They include:

• Load pins: these are designed to replace the pins that attach standing rigging to the deck or hull and are ideal for sensing load in forestays and shrouds. I also use them for backstay loads on my IMOCA.

The strain gauge is housed inside the pin and measures the upwards force placed on the centre of the pin when under load. For accuracy it is essential the pins are orientated correctly and fixed in position.

• Bottle screw gauges: these are a retrofit alternative to load pins which will allow monitoring of forestay and shroud loads. The gauge replaces a conventional bottle screw.

Bottle screw gauge measuring forestay tension. Photo: Cyclops Marine

• Toggles: these can be used between the top of a furler and a cable end for non-structural cabled sails such as Code 0s. They sit in line with the furler and will relay the cable tension. If retrofitting to an older sail this may require a reduction in cable length.

• Running rigging gauges: these small gauges are simple links that can be used for various in-line applications to measure load in running rigging. Think of them as metallic links with rigging attached on each side, they can be used to measure mainsheet or vang loads, or even placed in-line with a sheet.

Be aware that you’re adding a hard component to soft rigging so avoid set ups that will flog on hard objects by keeping the strop one side of the gauge short. The benefit of these types of gauges is that if attached with soft shackles they can be easily removed and used in different locations, or used only for setup and tuning sessions but removed for casual sailing.

Smartlink running rigging gauge fitted to a J/111. Photo: Cyclops Marine

• Sail integration: we are now starting to see strain gauges being integrated with sails, mostly in the tack to measure luff tension in cableless Code 0s, however they can be used in clews to measure sheet tension. These are normally custom solutions focussed on the racing market.

How to use load cell data

It is all very well having this data, but you need to be able to use it in a meaningful way before the load cells are able to add value. Here are some of the ways load cell data can be used:

• Safety

Load cells will allow you to set up and view the loads in your standing rigging both on the dock and when under load in changing conditions.

Work with your rigger to understand what the safe and maximum working loads should be and have those numbers on display in the cockpit or by the helm. With rigs that are more adjustable this will allow on-the-water adjustment of backstays and tacklines to ensure the rig is stable, not pumping upwind and at no risk of inversion downwind.

Load cells in the shrouds can also be a good indication of when a boat is overloaded, triggering a reduction in the sail plan. This is particularly useful in cruising catamarans which, when loaded up with extra weight in the hulls, can feel stable and mask the signs it is time to reef leading to damage of sails, mast or rudders. For safety applications conditional formatting as mentioned above is helpful.

• Furling

Load cells will help successful furling of headsails both on and off the wind by enabling an exact tension to be replicated each time. Quite often a furl will not be successful because the torsion cable was too slack for the ‘twist’ to transfer to the head of the sail, or damage can be done by over-tensioning a cable and pulling the head of the furler into the top of the mast.

• Trim

Load cell data gives a numerical value to sail trim which can be replicated in a precise way. By measuring the force going through running and standing rigging we are not reliant on marks on ropes, or the deck or memory of sail shapes to sail well. This makes it easier to record and return to fast modes.

For race boats there are obvious advantages to this but it can also make life simpler for sailors who spend much time under cover. Fixed biminis and doghouses often leave a sailor with a poor view of the sails and working on sail trim from the cockpit can be challenging when you cannot see the sails and are on a single person watch. The development of load cells built into sails has the potential to make precision trimming easy for sailors across all types of vessels.

Interface options

Each strain gauge reading needs to be amplified and then transmitted as usable data to a display around the boat where the information can be used. This is done either through a wired, or wireless, connection with more versatility currently offered through wireless options.

Strain gauge info on a mast display unit. Photo: Cyclops Marine

Data can then either be sent directly to an app on a smartphone or tablet, or converted to NMEA 2000 and displayed on an instrument cluster or PC-based software.

Each of these solutions has pros and cons, but the most important thing to consider is what you are using the load cell data for, who is going to be accessing it and when it will be used.

For example, if only using a bottle screw type strain gauge for initial set up prior to going afloat then an app on a smartphone is perfect. If the data is helping with performance and will be monitored while racing, displaying the data on existing multigraphic screens which have been sited in view of the helmsman and trimmers may be a better option.

Forestay loads relayed to a phone app. Photo: Cyclops Marine

The other consideration when choosing displays is how the force value will be interpreted by your crew. An experienced racing crew may be used to working with numbers and so be happy to read forestay tension as a value in tonnes and make suitable adjustments.

A novice crew cannot be expected to interpret load cell values in the same way, so using a data interface with conditional formatting, that might for example show a dial, which is green then changes red when normal working loads are exceeded is more appropriate.

When racing my IMOCA I am often staring at banks of data where it is hard to pick out one number from another so I find dials and any visual display with colour a useful way to gain my attention if something is wrong.

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