The Ultimate Guide to Purchase Systems and Mechanical Advantage.
In this guide, we’re breaking down the nitty-gritty of purchase systems, from the straightforward simplicity of the “simple” system to the power-packed intricacy of the “compound” system. We will also take a look at how to calculate a purchase system and the steps you should take to design your own.
A purchase system is like your boat’s backstage crew, working behind the scenes to make sailing a breeze. It involves blocks (the unsung heroes) and ropes, creating a setup that multiplies your muscle power, making sail control and rigging adjustment a piece of cake.
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Increased Mechanical Advantage Tips
Mechanical Advantage Line Loading
Simple Purchase Systems
Let’s start with the basics – the “direct” approach. In a simple “direct” purchase system, there’s just one block, one line (rope), and one straightforward mission: to control the load. It’s a 1:1 ratio, meaning the force you exert is the force felt by the load. Perfect for tasks like raising the centreboard on a dinghy or adjusting basic sail tension. Simple, effective, and great for when you need no-nonsense control.
When using a 1:1 purchase system the length of line that is pulled will result in a direct amount of movement on the load. For example, if you pull the line 10cm, the load will move 10cm.
The following diagram is a good example of a direct spinnaker or jib sheet system found on many dinghies and small keelboats. The weight represents the load from the jib or spinnaker.
Mechanical Advantage
Imagine mechanical advantage as your boat’s superpower. With a simple, low-loaded system you can opt for a no-fuss direct purchase system of 1:1. But with a highly loaded system which would require superhuman strength to manoeuvre then you can amp it up using mechanical advantage. With mechanical advantage, you can create a purchase system of 2:1, 4:1, 6:1 and beyond!
The International Moth is known to have a purchase system of 54:1! And that’s only increasing as people find new ways to improve the system layout.
In simple terms – A larger purchase system ratio can lift a heavier load with the same effort.
The diagram below is a great example of a typical keelboat jib system. With the weight acting as the load from the jib. By adding an additional block to the clew of the jib, the mechanical advantage has increased by 1 extra line within the system, this in turn half’s the load of the line.
This is also a typical setup for a 2:1 halyard system, where you want to reduce the load required to hoist the sail. Just imagine the weight as the mainsail and the top block as the sheave at the top of the mast.
Increased Mechanical Advantage Tips
Now that you understand the very basics of why mechanical advantage (MA) is used. It’s worth noting a couple of knock-on effects of increasing the MA.
When you increase the number of pulleys (ratio) in a system, you’re effectively increasing the length of the line that needs to be pulled to move the object. This happens because as you add more pulleys, the line wraps around them multiple times before reaching the point where force is applied. This in turn gives you greater control of the load and makes it easier to make fine adjustments.
In a 1:1 system when you pull 10cm of line, the load will move 10cm. But if you add another pulley, the line wraps back on itself, so for the same amount of lifting, you need to pull twice the length of the line. So, in a 2:1 system to move the load 10cm, you need to pull 20cm of line.
With increased mechanical advantage, tasks like hoisting heavy sails become safer as the line is easier to handle.
However, some sailors argue that excessively high mechanical advantage systems can diminish the “feel” of sailing. The tactile feedback of trimming sails and rigging allows you to understand what is happening on board and if the boat “feels” fast or slow. The higher the MA the less “feel” you have.
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Mechanical Advantage Line Loading
When understanding purchase systems and MA It’s important to understand how the load is shared between the blocks and line.
The load will be evenly shared across each of the lines in the system. This means the attachment point of the block will be taking the full load of the lines plus any additional load required to move the load.
The force required to move the load will be equal to or more than the load divided between the lines in the system.
So, for example, a 3:1 purchase system would require 33.3+kg of force to move a 100Kg load.
Compound Purchase Systems
Now, let’s crank it up a notch. Enter the compound purchase system, the maestro of mechanical advantage. With multiple blocks and lines, it’s like having a team of block-and-tackle ninjas working together. The load is spread out among them, and suddenly, you’re controlling heavier loads with less sweat. Think of it as sailing’s secret weapon for adjusting sail shape, controlling rigging, or managing those vicious mainsheet systems.
A compound purchase system is essentially a mechanical advantage system which combines 2 or more “simple” purchase systems. The following diagram is an example of a mainsheet system you might find on a keelboat (imagine the 100kg load is the boom).
Now that might look a little confusing, so let’s break it down.
If you separate the two systems into their “simple” layouts you have the “coarse” side with a 4:1 and the “fine” side of the system with a 3:1.
However, when you pull the “fine” side of the system the “coarse” side also moves, which in turn multiples the mechanical advantage. This results in the “fine” side becoming a 12:1.
How do you calculate this? You mulitply the ratio of the two together 3:1 x 4:1 = 12:1.
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Compound System Advantages
It might seem odd to have two ways to adjust one system, but a compound system has some great benefits. Firstly, having a coarse and fine adjustable system gives a great deal more control. An example would be in a mainsheet system on a racing yacht.
When you need to pull the mainsheet in quickly you can use the coarse system (4:1 will move the load quicker than the 12:1) until the load becomes too high. Once the load becomes too high you can start to pull the fine tune purchase system to finish sheeting into the desired tension.
Having the fine tune system will also give you extra control and make it easier for you to manage a larger load.
How to calculate a MA system
Now you hopefully understand the basics of how the simple and compound systems work, so let’s move onto the numbers and how to work out what MA you might need in a system.
When configuring a purchase system, you need to understand how much load you are trying to control and what the maximum effort is that you can apply to move the load.
So, for example, if you want to move a load of 100 kg, and you know the effort you’re willing or able to apply is 20 kg, then you can plug those values into the MA equation to solve the mechanical advantage needed.
Load = 100Kg
Effort = 20kg
100 divided by 20 = 5
This means you need a mechanical advantage of 5:1 to move 100kg with 20kg of effort.
Setting up a new system
Setting up a new purchase system on a yacht or dinghy involves several steps to ensure it’s safe, effective, and tailored to your specific needs. Here’s a guide to help you through the process:
Identify the Purpose: Determine what tasks you need the system for. Are you looking to hoist sails, control rigging, or handle other loads on the boat?
Assess Load Requirements: Calculate or estimate the maximum loads you expect the system to handle. Consider the weight of sails, rigging components, and any other loads involved in the system. Always add a little extra as a safety margin.
Calculate Mechanical Advantage: Calculate the mechanical advantage needed to control the loads effectively.
Evaluate Space and Mounting Options: Assess the available space on your boat where you plan to install the system. Consider factors such as clearance, accessibility, and mounting points for blocks and tackle.
Choose the Right Components: Select the appropriate blocks, lines (ropes), and other hardware based on your load requirements, available space, and budget. Using ball-bearing blocks will remove friction and make it easier and quicker to adjust a system. For more information on different types of bearings check out the following articles.
Design the System: Determine the configuration of the system based on the desired mechanical advantage, available space, and intended use. Decide whether to use a simple or compound system.
Install Hardware: Install the blocks and other hardware securely on the yacht according to best practices. Ensure that all components are properly aligned, attached, and fastened to withstand the expected loads.
Rig Lines (Ropes): Rig the lines through the blocks according to the designed configuration. Tie secure knots or use appropriate splices to attach the lines to hardware and create purchase systems.
Test the System: Test the mechanical advantage system under controlled conditions to ensure it operates smoothly and can handle the intended loads. Make any necessary adjustments or reinforcements to optimise performance and safety.
Regular Maintenance: Establish a regular maintenance schedule to inspect, clean and replace components of the mechanical advantage system as needed. Monitor for wear, corrosion, and other signs of damage to ensure continued reliability and safety.
Examples of purchase systems
In the following slides, there is a large variety of different pictures of purchase systems. From mainsheets and travellers to kickers and Cunninghams. Purchase systems are vital to the operation of your boat, and without the blocks and cleats they wouldn’t function at all.
Using everything above you should now be able to work out the mechanical advantage needed for a new system to make sailing easier, more efficient and most importantly more fun!
If you have any questions or would like to use any of the above marketing material please contact sales@allenbrothers.co.uk
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