How to calculate the area of your boat's bottom
Do you need to find out how much primer or antifouling you need for your boat?
Whether you are painting for the first time or switching to a more environmentally friendly solution like Hempel Silic One, it is important to know how much paint you need. A correct calculation ensures that you have enough paint for the job and avoids waste.
We have created 3 methods for how you can calculate the area of the bottom of your boat.
- The very simple one (estimate)
- The slightly more advanced
- The exact one
Why is it important to know the area?
- Quantity of paint: By knowing the area, you can accurately calculate how much paint you need to buy.
- Economy: Avoid buying too much or too little paint.
- Planning: This gives you a better overview of the project and how long it will take.
How to calculate the area
There are several methods for calculating the area of your boat's bottom. Here are some of the most common:
1. The very simple one (estimate)
- The underwater area of a sailboat with fin keel: LWL x (B + D) x 0.50 = area *
- The underwater area of a long-keeled sailboat: LWL x (B + D) x 0.75 = area *
- The underwater area of a motorboat: LWL x (B + D) x 0.85 = area *
The calculations are only indicative, but give a good idea of the amount you need.
* LWL = length of the waterline
B = Width (The widest point)
D = Depth (from waterline to lower edge of keel)
2. The slightly more advanced method:
- Rectangles and squares: If your boat's bottom is primarily made up of rectangles and squares, you can measure the length and width of each section and then calculate the area of each section. Finally, add all area calculations together.
- Circular parts: If your boat has round or oval parts, you can approximate them as circles or ellipses and use the formula for the area of a circle (A = πr²) or ellipse (A = πab).
3. The more precise method:
- Division into smaller sections: Divide the bottom of your boat into smaller, more regular shapes (e.g. triangles, trapezoids) and calculate the area of each one.
- Using a computer program: There are specialized software programs and apps that can help you calculate the area of complex shapes by entering measurements or by uploading drawings of your boat.
Tips to make the calculation easier:
- Draw a sketch: Make a simple sketch of your boat's bottom and measure all relevant dimensions.
- Measure carefully: Use a tape measure and make sure to measure all dimensions accurately.
- Round up: Once you have calculated the area for each part, you can round up by 10-20% to make sure you have enough paint.
What do you need?
- Measuring tape: For measuring length, width and height.
- Pencil and paper: To make a sketch and note the measurements.
- Calculator: To calculate the area.
- Computer: If you want to use a computer program to calculate the area.
Remember:
- Irregular shapes: If your boat's bottom has many irregular shapes, it may be difficult to get a completely accurate calculation. In such cases, you can contact a professional for help.
- Surface roughness: Remember to take surface roughness into account as it can affect the amount of paint needed.
Once you have calculated the area of the bottom of the boat, you can then use this area to estimate how much antifouling paint you will need. Paint manufacturers always state how much paint to use per m2 in their technical data sheets (TDS).
Example (the simple calculation):
You have scraped your 30 foot fin keel sailboat to prime and sanded it smooth. Now you want to give it 2 coats of Hempel Light Primer, 1 coat of Hempel Silic one Tiecoat and 2 coats of Hempel Silic One.
- LWL (length at waterline) = 6.9 meters
- Width (widest point) = 2.96 meters
- Draft (from waterline to lower edge of keel) = 1.55 meters
- Area (LWL x (W + D) x 0.50 = the area *) = (6.9 x (2.96 +1.55) x 0.5 = 15.56 m2
Now you will find the data sheet for Hempel Light Primer, which states "Theoretical coverage 8.2 m²/L [334 sq ft/US gal]". Since you will be applying 2 coats of Light Primer, you will need 2 pcs. 2.25 liters (We recommend 1 white and 1 grey so you can distinguish the layers). For Hempel Silic One Tiecoat the theoretical coverage is "Theoretical coverage 10 m²/L [401sq ft/US gal]" , which means that 1.56 liters are required. This means that if you round up, you will need 3 x 0.75 litres of Hempel Silic One Tiecoa t. For Hempel Silic One the theoretical coverage is also 10 m²/L [407sq ft/US gal. Which means that 3 x 0.75 litres per coat are required and since 2 coats are to be applied, 6 x 0.75 liter Hempel Silic one .
Do you want help?
Are you still unsure about how to calculate the area of your boat's bottom? Contact us at Alpina Marine Equipment and we will be happy to help you. We can also help you choose the right antifouling for your boat and give you advice on application.