It depends on who "they" are. I think the olympic racing rules are along the gunwales for some boats. When said in regards to efficiency, all that matters is what is in the water, so it would refer to a width at the wateline.

> When they say that the maximum beam should be located a certain distance
> from the bow, do they mean straight back down the keel line or along the
> gunwales?

> Thanks,

> Paul.