Measure once. Specify with confidence.

A: Propeller-shaft diameter

Measure the bare metal shaft with suitable measuring equipment and record the result in the original units. Include any existing shaft specification or drawing if you have it.

Keep metric and imperial sizes separate. For example, a 1-inch shaft is 25.4 mm, while a 25 mm shaft is a different nominal size. Rounding one to the other can send you to the wrong product.

B: Stern-tube outside diameter

The bellows attaches around the outside of the stern tube. The required measurement is the tube outside diameter at the attachment area, not the shaft diameter and not the outside of an existing hose.

If the tube is concealed by the current seal or hose, say that the measurement is unavailable. A photograph and existing product reference are more useful than an estimated number presented as exact.

C: Available installation room

Record the space along the shaft between the stern-tube end and the coupling. Also show nearby floors, structure, pipework and other obstructions. The selected unit must fit in its working position and remain accessible.

Published drawings use specific measurement points. Uncompressed length, installed length and the amount of bellows compression describe different things. Send your measurement endpoints with the dimensions so they can be compared correctly.

Add photographs with context

Take one clear view of the whole assembly, then closer views of any product markings and the bellows attachment. A ruler in the same plane can help explain location, although it is not a substitute for accurate diameter measurement.

Keep your notes together. Labelling each image with the vessel name and the measurement it shows makes the enquiry easier to review and reduces repeated questions.

Complete the picture

Include vessel make and model, engine arrangement, approximate operating speed and the reason for replacement. Mention a known shaft-log bearing or any unusual installation detail. If the seal has a service record, add its date and model.

Do not loosen a below-waterline assembly just to obtain a quote measurement while the boat is afloat. Where access requires dismantling, arrange that measurement with the person carrying out the work.

• Shaft diameter, with units.
• Stern-tube outside diameter, with units.
• Available axial space and clear photographs.
• Vessel details and the existing seal reference.