Why hull weight lies
Every manufacturer publishes a hull weight, and almost none of them publish a ready-to-use weight. The gap between the two is routinely 60–100 kg, which is more than enough to take a tender from comfortably within your davit rating to dangerously over it.
A buyer deciding between a 2.9 m aluminium RIB and a premium fibreglass RIB needs the total davit load, not the boat-only weight. Two boats that look 20 kg apart on paper can be 45 kg apart in the water.
The calculation
Add every item that lives in or on the boat when it hangs:
Ready-to-use weight =
Hull
+ Outboard
+ Fuel (full tank)
+ Battery
+ Anchor
+ Rode
+ Safety gear
+ Console (if fitted)
+ Launching wheels
+ Electronics
+ Lifting gear / bridle
Worked example
Take a nominal 3 m aluminium RIB with a published hull weight of 48 kg. Fit a 15 hp four-stroke at around 45 kg, a 12 litre tank at around 12 kg full, launching wheels at around 6 kg, an anchor and rode at around 8 kg, safety gear and a bridle at around 6 kg.
Hull weight: 48 kg. Ready-to-use weight: roughly 125 kg. If your davits are rated to 120 kg, the brochure said yes and the boat says no.
What to do with the number
- Get the davit or lifting-point rating in writing before you shortlist.
- Weigh the outboard you actually intend to buy, not the lightest one in the range.
- Decide early whether fuel lives in the tender or gets carried separately — it is often 10–15 kg of flexibility.
- If you are within 10% of the rating, choose the smaller boat. Davit failures happen at sea, in a swell, at the worst possible moment.