A mast sheave is the wheel mounted inside the mast head (or partway down, for a mid-mast halyard exit) that the halyard runs over as it travels from the head fitting down to the deck. Harken's High Load Sheave in the 57mm size (HAR727) carries a 12mm max line, an 8mm max wire, and a 2,245 kg working load with a 4,491 kg breaking load — sized for halyards on a mid-size cruiser's main or genoa. MAURIPRO Sailing stocks Harken and Ronstan and Antal mast sheaves options, listed in Sailboat mast steering sheaves.
Mast sheaves at a glance
| Product | Brand | What it is for |
|---|---|---|
| Harken Micro Sheave - Big Bullet - Sheave Dia: 1 1/2 in (38.0 mm) | Harken | It's crafted from durable Delrin and features Delrin ball bearings, ensuring smooth operation and reliability. |
| Ronstan Sheave - Ball Bearing - Series 50 - Pin Dia: 2.0 in (50.0 mm) - Alloy | Ronstan | Ball-bearing sheave with a 50 mm pin diameter and an 8 mm max rope; lightweight corrosion-resistant alloy construction for higher-load halyard or control-line turns. |
| Antal Mast Block 120 mm Sheave | Antal | 120 mm sheave with an 18 mm max line and a verified 5,000 kg safe working load (SWL); hard black-anodized aluminum frame with insulating washers for corrosion protection. |
What do mast sheaves actually do?
You'd add or replace one when a line exits the mast at an angle that doesn't line up with your deck clutch bank, when an existing block is cracked or undersized for a line you've upgraded, or when you're adding a new control line that needs its own lead aft. The Harken Teardrop Mast Collar mounts around the base of the mast to redirect a halyard or control line toward its deck-level lead — a bolt-on job, not a mast-wall modification. An exit block is a through-wall fitting that mounts directly over a cut or pre-formed slot in the mast tube itself, creating a brand-new point where an internal line leaves the spar — at any height, not just at the base.
How do you match mast sheaves to your boat?
This is written for the ~30-45 ft cruiser or performance-cruiser owner who has found a worn, glazed, or noisy piece of hardware along the halyard's path and isn't sure which part to order — a masthead sheave, a mast base block, or an exit block.
What separates the mast sheaves options we stock?
Harken Micro Sheave - Big Bullet - Sheave Dia: 1 1/2 in (38.0 mm)
It's crafted from durable Delrin and features Delrin ball bearings, ensuring smooth operation and reliability. These sheaves are specifically designed to accommodate rope, making them an ideal choice for various sailing. Durable Delrin material for long-lasting use.
Antal Mast Block 120 mm Sheave
Mast block, sheave 120, SWL 5000 kg. Hard black anodized aluminium frame with insulating washers for corrosion protection.
What build details matter on mast sheaves?
Equipped with Delrin ball bearings for smooth performance. The use of Delrin in both the body and ball bearings ensures resistance to wear and tear, providing sailors with a reliable component for their gear.
What else decides the choice?
Harken's 57mm High Load Sheave is built for the primary halyard turn at the top of the mast — a 2,245 kg working load with a 12mm max line. A mast base block mounts at the bottom of the spar, at or just above deck level, and redirects a line that has already come down inside or alongside the mast toward a cleat, rope clutch, or winch on deck. Harken's Protexit exit block mounts over a slot cut in the mast wall itself, creating a new exit point for an internal line — rated 400 kg working load at an 8mm max line. A mast sheave turns your halyard at the top of the mast; a mast base block redirects it once it exits at deck level; an exit block creates a new through-wall exit partway up the spar.
Which should you choose?
- Harken Micro Sheave - Big Bullet - Sheave Dia: 1 1/2 in (38.0 mm) — It's crafted from durable Delrin and features Delrin ball bearings, ensuring smooth operation and reliability.
- Ronstan Sheave - Ball Bearing - Series 50 - Pin Dia: 2.0 in (50.0 mm) - Alloy — Ball-bearing sheave with a 50 mm pin diameter and an 8 mm max rope; lightweight corrosion-resistant alloy construction for higher-load halyard or control-line turns.
- Antal Mast Block 120 mm Sheave — 120 mm sheave with an 18 mm max line and a 5,000 kg safe working load (SWL); hard black-anodized aluminum frame with insulating washers for corrosion protection.
Frequently asked questions
What should you check first when buying mast sheaves?
Start by identifying which of the three parts actually failed — a masthead sheave, a mast base block, or an exit block — since they aren't interchangeable, then measure the line or wire that currently runs through it so the replacement's maximum line rating isn't undersized for what you're already running.
What size mast sheaves fits your boat?
A smaller 38mm Micro Sheave (HAR265) takes a 10mm line at a 136 kg working load, appropriate for lighter control lines or smaller boats, not a primary halyard on a 35-45 ft rig.
What are mast sheaves made of?
Resin sheaves (aluminium sheaves for 100 and 120 mm only) with composite fibre main bearing (not on size 40) and double side self-captive ball bearing.
How are mast sheaves mounted or set up?
A masthead sheave rotates on a pin inside the mast head casting itself; a mast base block bolts to a bracket at or just above deck level; an exit block through-fastens directly over a cut slot in the mast wall. Each has a different mounting pattern, so match the replacement to how your existing hardware is mounted, not just to line size.
What load can mast sheaves take?
Harken's Protexit 40mm single (HAR1202) is rated for an 8mm max line at a 400 kg working load and 800 kg breaking load, and mounts with through-fasteners directly to the mast wall.
Where can you buy mast sheaves?
Shop Sailboat mast steering sheaves →
Shop Harken sailboat mast steering sheaves →
Shop Ronstan sailboat mast steering sheaves →
Shop Antal sailboat mast steering sheaves →
Shop Sailboat blocks mast base blocks →
Last updated: September 22, 2026.