A purchase system multiplies your pulling force by running a line through a set of blocks, so a small effort controls a large load. Simple purchase systems use a single block set for ratios like 2:1 or 4:1, while compound systems stack one purchase on top of another to reach much higher mechanical advantage for demanding controls like the mainsheet.
Purchase systems at a glance
| System type | Typical ratio | How it works | Best for |
|---|---|---|---|
| Simple (1:1) | 1:1 | One block, one line; force equals load | Sail tension, raising a centerboard, dinghy sheets |
| Simple purchase | 2:1 to 6:1 | Additional sheaves multiply force, but you pull more line | Vangs, cunninghams, small keelboat controls |
| Compound | 8:1 and up | One purchase pulls the line of another, multiplying ratios | Mainsheets on racing yachts, high-load controls |
| High-ratio racing | up to 54:1 | Multiple stacked purchases for extreme loads | International Moth and other high-performance dinghies |
What is a purchase system on a sailboat?
A purchase system is a combination of blocks and line arranged to multiply the effort you apply. By threading a single line back and forth through sheaves, the load is shared across multiple line parts, so each part carries a fraction of the total force. This is the mechanism behind nearly every adjustable control on a boat, from the mainsheet and vang to the traveler and cunningham. Without blocks and cleats, none of these controls would function.
How does mechanical advantage work?
Mechanical advantage (MA) is the ratio between the load and the effort needed to move it. Adding blocks raises the MA to ratios such as 2:1, 4:1, or 6:1, letting you handle heavier loads with the same pull. The trade-off is line travel: a 2:1 system halves the effort but doubles the length of line you must pull. Higher ratios also reduce the tactile feedback sailors rely on, so the goal is to balance power against the "feel" you need to trim accurately.
What is the difference between simple and compound purchase systems?
A simple purchase uses one set of blocks and a single line for a fixed ratio, ideal for straightforward jobs like adjusting sail tension or raising a centerboard. A compound system combines two or more simple purchases, feeding the tail of one purchase into another to reach much higher ratios. Compound setups shine on heavy-duty controls like racing mainsheets, where a coarse 4:1 quickly brings the sail into position and a fine-tune stage of 12:1 or more allows precise adjustment.
How do I calculate and design a purchase system?
Designing a purchase system follows a repeatable sequence:
- Identify the purpose: Define the task, such as hoisting sails or controlling rigging.
- Assess load requirements: Calculate the maximum load the system will handle and add a safety margin.
- Calculate the MA: Divide the load by the effort you are willing to apply to find the necessary ratio.
- Choose components: Select blocks and lines rated for the calculated MA and your space constraints.
- Test and maintain: Install the system, test it under load, and set a maintenance routine for long-term reliability.
Which purchase ratio should I choose?
Match the ratio to the load and the control feel you want. Dinghy sheets and light controls often work well at 1:1 or 2:1, keelboat vangs and cunninghams typically run 4:1 to 8:1, and racing mainsheets use compound systems of 8:1 and higher for coarse and fine control. Always size your blocks and line to the calculated load, and remember that every increase in ratio means pulling more line for the same movement.
Frequently asked questions
What is a purchase system in sailing?
It is an arrangement of blocks and line that multiplies your pulling force, letting a small effort control a large load such as a mainsheet or vang.
What does a 4:1 purchase mean?
A 4:1 purchase reduces the effort to one quarter of the load, but you must pull four times as much line to move the load the same distance.
When should I use a compound purchase system?
Use a compound system when a single simple purchase cannot deliver enough mechanical advantage, such as high-load racing mainsheets that need both coarse and fine control.
Does more mechanical advantage always help?
No. Higher ratios increase line travel and reduce the tactile feedback needed for accurate trimming, so balance power against feel for the control in question.
What blocks do I need to build a purchase system?
Choose blocks rated for your calculated load with sheaves sized to your line diameter; V-Block style blocks are commonly used for high-load mainsheet and vang purchases.