Boat measurements are crucial for maritime safety, efficiency, and compatibility. Navigating these measurements is vital for manufacturers and boating enthusiasts. Proper understanding ensures vessel selection, marina compliance, and safe navigation.
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11 Main Boat Measurement Terms
- Length Overall (LOA): The total length from the bow to the stern. It determines mooring space, marina fees, and navigation in confined waters. For example, a boat with a 30-foot LOA needs a slightly longer berth.
- Length on Deck (LOD): Measures the hull’s horizontal length, excluding extensions like bow sprits. It provides a more accurate measurement of usable space. A boat might have a 35-foot LOA but a 30-foot LOD due to a 5-foot bow sprit.
- Beam: The width at the widest point, affecting stability and maneuverability. For instance, a boat with a 10-foot beam is more stable but less maneuverable than one with an 8-foot beam.
- Draft: The minimum depth required for the boat to float. A sailboat with a 6-foot draft cannot navigate waters less than 6 feet deep.
- Displacement: The weight of water displaced by the hull, reflecting the boat’s weight when loaded. A boat displacing 5 tons handles differently than one displacing 2 tons.
- Freeboard: The distance from the waterline to the upper deck, indicates how much of the boat is above water. A boat with 4 feet of freeboard handles choppy water better than one with 2 feet.
- Deadrise: The hull’s angle relative to a flat surface. A higher deadrise (sharp angle) cuts through waves better. A boat with a 20-degree deadrise performs better in rough water than one with a 10-degree deadrise.
- Air Draft: The height from the waterline to the highest fixed point. Important for bridge clearances. A sailboat with a 50-foot mast must consider bridge heights.
- Gross Tonnage: Volume measurement for large vessels, based on total internal space. For example, a cargo ship with 50,000 gross tonnage has a large capacity.
- Ballast: Weight added low in the boat for stability. A sailboat with 2,000 pounds of ballast is more stable than one with 1,000 pounds.
- Waterline Length (LWL): Length at the waterline, affects speed and stability. A boat with a 25-foot LWL is generally faster than one with a 20-foot LWL.
How to Measure a Boat Hull
- Length Overall (LOA): Measure from the furthest point forward to the furthest point aft.
- Length on Deck (LOD): Measure the deck length, excluding overhangs.
- Waterline Length (LWL): Measure at the waterline.
- Beam: Measure the widest part of the hull.
- Draft: Measure from the waterline to the lowest part of the hull.
- Freeboard: Measure from the waterline to the upper edge of the hull.
- Deadrise: Measure the angle between the hull bottom and a horizontal plane.
- Hull Depth: Measure from the deepest point of the hull to the top of the deck.
Coast Guard Boat Length Measurement
The U.S. Coast Guard measures boat length using the Length Overall (LOA) method, from the bow’s tip to the stern’s end, including structural parts but excluding removable attachments. This measurement is crucial for regulatory and safety reasons, influencing boating laws and safety requirements.
I worked as an officer in the deck department on various types of vessels, including oil and chemical tankers, LPG carriers, and even reefer and TSHD in the early years. Currently employed as Marine Surveyor carrying cargo, draft, bunker, and warranty survey.
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