Blocks redirect lines, create mechanical advantage and help sailors control substantial loads. Selecting the right block requires more than choosing a sheave that will accept the rope. The block must suit the line diameter, expected load, amount of movement, attachment method and direction of pull.
Our Block Selection Advice explains the principal Ronstan block ranges and the applications for which each has been developed. The following process provides a practical way to narrow the choice:
Start with the application
Begin by considering how the line and block will operate.
A sheet or control line may run frequently and needs low friction for fast trimming and easing. A halyard turning block may remain highly loaded for hours with relatively little movement. Other applications, such as running backstays or high-load sheets, combine substantial loads with repeated movement.
These different duties place different demands on the bearing system and block structure.
Frequently adjusted lines
Mainsheets, jib sheets, spinnaker sheets and many control lines benefit from low starting and running friction. Ball bearing blocks are generally well suited to these applications.
Ball Bearing Utility Blocks and Orbit Blocks™ use a two-stage bearing system. Ball bearings provide low friction under moderate loads, while a secondary bearing supports the sheave as the load increases.
High dynamic loads
Heavily loaded sheets, halyards, running backstays and powerful control systems need both structural strength and an efficient bearing system.
Core Blocks™ use a two-stage bearing system which provides excellent performance across the full working load range with an integrated thrust-bearing feature and the resilience to handle high dynamic or static loads. Soft Attachment Blocks use full-contact composite journal bearings and thrust washers for heavy dynamic and static loads.
For highly loaded lines that require frequent and smooth adjustment, Keelboat Orbit Blocks™ use load-bearing Torlon® needle rollers to provide a large bearing contact area, high strength and low friction under load.
Prolonged static loads
Halyard turning blocks, mast-base blocks, backstay systems and some vang systems may carry high loads for long periods. A full-contact or plain bearing spreads the load over a greater area and is often better suited to this duty than a conventional ball bearing system.
Ronstan Special Purpose Blocks and selected Utility Blocks are designed for applications involving high or prolonged static loading.
Where an application combines prolonged loading with the need for smooth adjustment under load, Keelboat Orbit Blocks™ provide a high-capacity option, with Torlon® needle rollers supporting the primary load and captive acetal or Torlon® ball bearings managing side thrust.
System Schematics
For common applications see our guides on:
Estimate the line tension
The load on a block begins with the tension in the line. This may be estimated from sail area, wind pressure, sail-plan geometry, purchase ratio or known loads elsewhere in the system. For complex or highly loaded systems, consult the boat designer, sailmaker, rigger or a Ronstan representative.
Consider the highest loads the system may experience, including:
- Maximum expected wind strength
- Reefed and unreefed sail configurations
- Shock loads during gybes, tacks or flogging
- Loads generated by winches, hydraulics or powered systems
- Repeated cycling and fatigue
- Offshore use and the consequences of failure
A purchase ratio gives the theoretical mechanical advantage at the working end of the line. It does not provide the load carried by every block, becket or attachment point. Friction and system geometry can also produce uneven tension between line segments.
Account for the angle of deflection
The load on a block depends on the line tension and the angle through which the line is turned. A small change in direction produces a lower block load than a 180-degree return.
At a 180-degree turn, the block carries approximately twice the line tension. At 90 degrees, it carries approximately 1.41 times the line tension.
The relationship is:
Block load = line tension × 2 × sin (deflection angle ÷ 2)
| Line deflection | Approximate block load |
| 30° | 52% of line tension |
| 60° | 100% of line tension |
| 90° | 141% of line tension |
| 180° | 200% of line tension |
A line carrying 500 kg of tension and turning through 90 degrees applies approximately 705 kg to the block. The same line turning through 180 degrees applies approximately 1,000 kg.
This calculated load should be compared with the block’s Maximum Working Load.
Select by Maximum Working Load
We define Maximum Working Load, or MWL, as the maximum static or dynamic load at which a product will continue to function without excessive friction, distortion, wear or permanent deformation.
Breaking Load, or BL, is the load at or around which major structural failure can be expected when the product is new. It is a destructive test value and should not be used as the normal operating load.
Choose a block with an MWL above the highest calculated service load, with suitable allowance for:
- Shock and impact loading
- Cyclic fatigue
- Uneven load sharing
- Poor alignment or side loading
- Wear, corrosion and ultraviolet exposure
- Uncertainty in the original load estimate
MWL must never be exceeded and that no product should be used above half its stated Breaking Load. Some applications require a greater safety factor, particularly where failure could injure crew or damage the boat.
Match the block to the line
Every block also has a published maximum rope diameter. This is the largest line that the sheave groove and cheek spacing are designed to accommodate.
Using the maximum possible diameter is not always the best choice. A line that fits too tightly can rub against the cheeks, bind as it enters the block or create additional friction when alignment changes.
Line selection should consider:
- Required strength and elongation
- Cover construction and abrasion resistance
- Handling and crew comfort
- Compatibility with cleats, clutches and winches
- Grip on ratchet sheaves
- Repeated bending around the sheave
Modern high-modulus ropes can carry substantial loads at relatively small diameters. The strongest acceptable line may still be too small for comfortable handling or reliable cleating. A larger line may be easier to handle, but it also requires enough room within the block and a suitable sheave diameter.
Larger sheaves generally reduce the severity of the rope bend, which can improve running efficiency and rope life. Check the rope manufacturer’s recommended minimum bend radius or sheave-to-rope diameter ratio when selecting equipment for high loads or high-modulus lines.
Individual products in our block range list the sheave diameter, maximum rope diameter, MWL and BL.
Choose the bearing system
Once the load and line size have been established, choose a bearing system suited to the duty.
Ball bearing blocks provide low starting and running friction. They suit sheets and control lines that move frequently and need to ease freely. A two-stage bearing system add secondary support as load increases and are used in several Ball Bearing Utility, Orbit and Core Block ranges.
Torlon® needle roller bearings
For highly loaded lines that require frequent and smooth adjustment, needle roller bearings provide greater contact area than ball bearings, spreading the load across a larger bearing surface.
Keelboat Orbit Blocks™ use long, load-bearing Torlon® needle rollers, with captive acetal or Torlon® ball bearings to manage side thrust. Combined with fully machined aluminium alloy cheeks and aluminium sheaves from 60 mm to 200 mm, this bearing system provides minimal friction at high loads and a high strength-to-weight ratio.
These are specialised blocks developed for demanding keelboat and racing applications. Their materials and construction make them a higher-cost option, so they are generally selected where the loads, performance requirements or weight savings justify the investment.
Full-contact and plain bearing blocks
A full-contact bearing distributes the load across a larger surface. It suits high static loads, prolonged loading and applications where the sheave moves less frequently. It can also provide efficient performance in demanding high-load dynamic applications when designed for that purpose.
Soft Attachment Blocks use full-contact self-lubricating composite journal bearings and thrust washers to handle high dynamic and static loads.
Ratchet blocks increase the holding power available to the sailor by gripping the loaded sheet as it passes over the sheave. They are commonly used for mainsheets and spinnaker sheets on dinghies, sports boats and smaller keelboats.
The ratchet reduces the effort required to hold the line. The block must still have sufficient working-load capacity for the full sheet load.
Choose the configuration and attachment
Blocks with the same sheave and bearing system may be available as singles, doubles, triples, fiddle blocks, cheek blocks, stand-up blocks, cleated blocks or versions with beckets.
A swivel block can follow a changing line lead. A fixed block provides controlled alignment where the lead direction is stable. Snatch blocks allow a block to be added to a standing line, while soft-attachment blocks provide lightweight and flexible installation options.
The block should be able to align with the loaded line unless it has been designed for a fixed installation. Side loading can force the sheave against the cheeks, increase friction and place unintended loads on the attachment.
The complete load path must also be checked. The working capacity of the system is limited by its weakest component, including the block, shackle, soft link, lashing, becket, padeye, fasteners and supporting structure.
For soft attachments, use a smooth, well-rounded mounting point without sharp edges or burrs.
A practical selection process
- Identify how the block and line will operate.
- Estimate the maximum line tension.
- Calculate the block load from the angle of deflection.
- Allow for shock, fatigue, environment and uncertainty.
- Select a block with sufficient Maximum Working Load.
- Confirm that the rope diameter and bend radius are suitable.
- Choose the appropriate bearing system.
- Select the required configuration and attachment.
- Check every component in the load path.
- Confirm that the installed block can align correctly.
The aim is a safe, efficient system that performs reliably in real sailing conditions. When the block size, bearing system and attachment are properly matched to the application, the hardware works quietly in the background. The crew can trim, ease and manoeuvre with confidence, and spend more time enjoying the sailing rather than thinking about the blocks.
