Ronstan RF25109 Single Lashing Block 20mm – Ball Bearing Pulley

The Ronstan RF25109 Single Lashing Block 20mm is a compact, high-performance ball bearing pulley designed for modern sailing control systems where weight, efficiency, and reliability are critical.

Part of Ronstan’s renowned Series 20 Orbit range, this block features a precision 20mm acetal sheave running on a ball bearing system, delivering exceptionally low friction and smooth operation—ideal for fast-adjusting control lines.

Weighing just around 9 grams, the RF25109 offers one of the best strength-to-weight ratios available, making it perfect for dinghies, skiffs, and high-performance sailing setups.

The lashing attachment system with integrated stainless steel hub allows for flexible rigging using Dyneema or Spectra, eliminating the need for shackles and reducing overall weight and noise.

Constructed from glass fibre-reinforced nylon cheeks and a UV-stabilised acetal sheave, this block is built to withstand harsh marine environments while maintaining long-term performance.

With a maximum working load of 250kg and breaking load of 550kg, this small block delivers impressive strength for its size, making it a go-to choice for performance sailors.

Key Features

• 20mm single sheave – compact and efficient design
• Ball bearing system – ultra-low friction performance
• Ultra-lightweight (~9g) – ideal for performance rigs
• High load capacity (250kg MWL / 550kg BL)
• Lashing attachment system – flexible, lightweight rigging
• Stainless steel hub – secure and durable connection
• Composite construction – corrosion-resistant and marine-ready

Specifications

• Brand: Ronstan
• Model: RF25109
• Product Type: Single lashing block / pulley
• Sheave Diameter: 20mm
• Max Rope Diameter: 6mm
• Maximum Working Load: 250kg
• Breaking Load: 550kg
• Weight: ~9g
• Materials:
  • Acetal sheave
  • Glass fibre reinforced nylon cheeks
  • Stainless steel hub & bearings
• Attachment: Lashing (Dyneema/Spectra recommended)

Ideal Applications

• Dinghy and skiff control lines
• Cunningham and vang systems
• Traveller and leech line setups
• Lightweight cascading systems

This block is ideal where minimal weight and maximum efficiency are required.

Why Buy from Barton Marine NZ?

• Trusted NZ sailing hardware specialist
• Fast nationwide delivery
• Genuine Ronstan performance gear
• Ideal for NZ racing and cruising conditions

Pro Tip 

• Use Dyneema lashings with multiple passes to achieve full load rating
• Keep bearings rinsed with fresh water for long-term smooth operation
• Ideal upgrade where traditional blocks add unnecessary weight