The sizing walkthrough: finding every number the Builder asks for

The Builder asks five questions, and every answer is written somewhere on your boat. This is the guided tour — take your phone, photograph everything, and let the pictures answer half our follow-up questions before we ask them.

The engine plate (power and max RPM)

A metal plate riveted to the block — usually on the rocker cover, heat exchanger or engine foot. You want rated power (hp or kW) and rated/max RPM. Don't trust memory or the broker's listing: engines get de-rated, replaced and mis-remembered. Photograph the plate.

The gearbox plate, by brand

• ZF / Hurth (HBW, ZF-M series): silver plate on top or the aft face of the box — ratio appears as 'i = 2.45' or 'Ratio A/B'.
• PRM Newage: black or silver plate on the top cover; ratio stamped under 'Reduction'.
• Kanzaki (behind most Yanmars): plate on the housing side; KM-series boxes stamp two ratios — ahead and astern — read the ahead figure.
• Technodrive (TMC): plate on the top face; ratio as 'R=2.60'.
• Twin Disc / Velvet Drive: plate on the side of the case; ratio within the model code.
• Volvo integrated (MS/HS): plate on the bell housing; some ratios must come from the model + serial — photograph both and we decode.

Shaft diameter and taper

Measure the shaft with callipers (or a wrench across it against a ruler in a pinch) somewhere clean between the gland and the coupling. Common sizes: 25, 30, 35, 40 mm — or 1", 1¼", 1½" on imperial-era boats. The taper at the propeller end follows either the SAE or metric standard; if you're replacing a propeller, the old hub tells us, so photograph the stamp on the current prop too — decode it here.

Tip clearance in five minutes

With the boat hauled: lay a batten across the hull above the propeller, measure the gap from blade tip (rotated to its closest point) to hull. You want roughly 10–15% of propeller diameter. Less than 10% explains a drumming aft cabin at cruise — and constrains any upsize, which is why the Builder asks.

Rotation, drive and the rest

• Rotation: from astern in ahead gear — clockwise is right-handed. Twins should be opposite-handed; note which side is which.
• Saildrive owners: the model (110S, 120S, SD20…) is on the leg's plate — that plus the anode style is all we need.
• Not sure about anything? Leave the field blank and attach the photo. 'Blank' beats 'maybe' every time — a wrong ratio sizes a wrong propeller.