How to Measure for a Boat Shaft Seal (Shaft and Stern Tube Sizing)
When ordering a dripless propeller shaft seal, the sizing has to be right. Most problems people have, leaks, overheating, premature wear, come back to one of two things: incorrect measurements, or mixing metric and imperial sizes.
The good news is that you only need a few measurements, and you can usually take them with the boat afloat (though any removal work should be planned carefully).
When ordering a dripless propeller shaft seal, the sizing has to be right. Most problems people have, leaks, overheating, premature wear, come back to one of two things: incorrect measurements, or mixing metric and imperial sizes.
The good news is that you only need a few measurements, and you can usually take them with the boat afloat (though any removal work should be planned carefully).
What you need to measure
You should measure these three items before choosing your seal:
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Propeller shaft diameter
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Stern tube outside diameter (OD)
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Space available between the stern tube and the coupling
Also strongly recommended:
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Check the shaft for wear, scoring, nicks, or corrosion where the lip seal will run
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Confirm the shaft is running central in the stern tube, because alignment affects seal life
Step 1: Measure propeller shaft diameter
Best tool: vernier caliper.
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Clean the shaft first (wipe off salt and grime).
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Measure on a smooth, unworn section close to the existing seal area.
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Take two readings, rotating the caliper around the shaft to confirm it is consistent.
Important: do not “convert and round” sizes. A shaft that is 25 mm is not exactly 1 inch, and ordering the wrong standard is a common mistake.
Step 2: Measure stern tube outside diameter (OD)
This is the outside diameter of the stern tube where the seal’s hose clamps on.
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Measure the stern tube OD with calipers if accessible.
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If access is tight, measure at multiple points to confirm it is round and clean.
If you cannot use calipers, you can wrap a paper strip around the tube, mark the circumference, measure that length, and calculate diameter (circumference ÷ 3.1416). This is less accurate, so only use it as a backup method.
Step 3: Confirm available space
You need enough room between the stern tube and the coupling to fit the seal assembly and clamps. This is specifically called out in dripless shaft seal sizing guidance.
Tip: take a photo with a tape measure visible. It helps later when you compare options or speak to support.
Metric or imperial: choose the standard, then stick to it
Many boats in the UK and EU are mixed standard (for example, an imperial shaft with a metric stern tube, or vice versa). Measure both items and then choose the correct seal range that matches your actual measurements, not the numbers you wish you had.
Planning hulls and water feed notes (important for later)
Lasdrop lip seals (DrySeal and EliteSeal) require water for cooling and lubrication, and faster planing hulls must inject water into the seal from a clean source after the raw water pick-up.
