On the block

Why the fixation with Max Working Load (MWL), anyway? We examine what a sailor should look at when selecting new blocks.

Written by Jack O'Rourke
Photography by Harken

26 May 2020


It’s not breaking news that sailors are a competitive lot. And so it might follow that the companies who build and sell equipment for their boats, many of whom are lead by sailors, would be equally competitive.

Recently, there has been some dock talk about the term Max Working Load (MWL) as it applies to blocks. Harken-Australia’s Managing Director Grant Pellew says, “Different companies have different attitudes to MWL – two companies can differ about its importance as a measure of performance and reliability, and both design, build and market excellent high-performance blocks.

“The important thing is to understand that, not only might the companies have different priorities about what they like to market, but they probably also have different methodologies for how to measure and test MWL. Even grand-prix sailors probably don’t know how the testing is done.”

It would seem so simple – the higher the MWL, the stronger the block, the more value it should provide. Unfortunately, it’s not that simple.


There’s another concept that enters the picture and complicates matters: efficiency. Essentially, a measure of the percentage of an input to the block, trimming or easing, which acts on the load. The difference between input and output is energy (efficiency) lost to friction.

“When we perform efficiency testing at Harken, we cycle line with a 180-degree wrap around the block and a load cell on both ends,” said Matt Schmidt, Design Engineer and Small Boat Product Manager at Harken.

“The line is pulled against a hydraulic cylinder fixed at a pre-determined load. By comparing the two load cell measurements, we measure the friction/efficiency/loss on the block. If load cell measurements are nearly identical, then friction is low. The bigger the difference, the more friction/loss/lower efficiency.”

It would appear that key considerations for the sailor standing in the chandlery aisle should be what load is required for the rigging application and the efficiency the alternative blocks will provide at that required load. So relying on MWL alone is likely not enough to make the optimal performance decision.

Schmidt continued, “Typically, we start with low loads to establish a rough efficiency target. Load is progressively added to find a sweet spot – where efficiency still is good, but load capacity is also good. If efficiency starts to fall off, it’s an indicator that the bearing system is being pushed too hard. 

“Fact is, we won’t publish a Max Working Load figure past the range where the efficiency has begun to falter. We could. MWL doesn’t include an operational definition for ‘working’ that includes a common efficiency number – and it’s not even clear all companies even test for efficiency. But we won’t publish a higher number just to publish a higher number.

“Some blocks we’ve seen are designed for the sheave to revolve the centre rivet at loads higher than their roller bearings can handle. The sheave still turns, the block still ‘works’. They can publish a higher MWL but there’s a lot of friction – we just don’t go there.”

Grant Pellew added, “Harken believes tensile/failure tests should be performed separately from cycle and efficiency testing. We certainly break stuff.

“But then we compare tensile tests with efficiency tests so we can establish break load as well as max working load.  Typically, this is a 2:1 minimum safety factor for standard items in our catalogue. MWL is verified by cycle testing and efficiency testing to make sure the product performs long term.”

Long term. There’s yet another performance variable to take into account, so how long a block can deliver efficiently at the published MWL is also tested.

“Load, speed of line and dwell (waiting between cycles) are all adjustable and drive how realistic or aggressive the test may be,” said Harken’s Schmidt. 

“These factors all make heat in the bearing system. A block cycled at low load but high speed and no waiting can build heat and destroy a bearing system. High load/low speed/high dwell can do the same.

“Keeping speed and dwell realistic to the end application is just as important as load.”

In the tests of its 29-millimetre and 40-millimetre FLY blocks, Harken looked for less than 5 percent efficiency loss, with no physical deformation or concerns when doing its MWL tests.

From Schmidt’s notes:

  • 29-millimetre FLY

Efficiency stays at less than 5 percent loss till 400 kg. Under 4 percent loss til 300 kg. That’s why we set MWL at 350 kg.

The ball bearings start to dent the titanium sheave at 600 kg, and freely spin till 1200kg, but then the inner race deforms and starts to lock.

No catastrophic break where the line or load is dropped as is the experience with most other headpost type blocks.

  • 40-millimetre FLY

Same idea as 29 millimetre – efficiency stays good under 650–700 kg, then the efficiency drops off as the inner race starts to deform.

The block will spin freely at 2300 kg (4 x MWL), but the efficiency is unacceptable. Heat and metal dust start to develop under the high loads.

In the end, is MWL an important piece of information to use in selecting a racing block? Certainly, it is one thing to consider. But, as we have seen here, it is probably not the only thing on which to base a purchase.


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