The height at which fenders should be positioned on a boat can vary depending on several factors, including the size and design of the boat, the docking conditions, and personal preferences. However, here are some general guidelines to consider:

1. Boat Size: Larger boats typically require fenders to be positioned higher on the hull compared to smaller boats. This is to ensure adequate protection and prevent potential damage during docking.

2. Docking Conditions: If you frequently dock in areas with strong currents or rough waters, you may need to position your fenders higher to account for potential fluctuations in water levels or increased impact forces.

3. Docking Configuration: The placement of fenders should be based on the specific docking configuration. Identify the areas of the boat that are most likely to come into contact with the dock or other objects during docking. Common positions for fenders include the bow (front), stern (rear), and along the sides of the boat.

4. Fender Length: Consider the length of your fenders. Longer fenders may require higher positioning to ensure adequate protection, especially if your boat has a higher freeboard (distance from the waterline to the deck).

5. Personal Preference: Ultimately, the placement of fenders is a matter of personal preference. Some boaters prefer to position them at the waterline or slightly above, while others prefer them to be placed higher for additional protection.

It is important to experiment and adjust the position of your fenders based on your specific boat and docking circumstances. Additionally, consulting the manufacturer's recommendations or seeking advice from experienced boaters or marina staff can provide valuable insights for your particular boat type and situation.