What does this measurement rule mean?

You need a measurement form, or a set of plans to decipher this. Or just phone up the class measurer and ask him?

Originally posted by Peaky I’m really struggling to understand what this means . Halfway between the bow and the stern, 229mm above the keel band the beam should be no less than 1168mm? What is the 40-150mm thing, and how do you measure a point 229mm above the bottom?

Its usually best to take these rules in stages, and you can then remove the bits that are to do with loopholes and weird shaped boats and understand the meat.

4.3.3 At midlength, measured 229mm above the lowest points on the outer surface of the skin

OK, so its a rise of floor rule. Most box rule classes have them. The idea is to prevent the boat being too skinny at a point that vaguely approximates to waterline beam.

(or on a straight line, bridging any hollows, parallel with the vertical plane through the fore and aft centreline) between 40mm and 150mm from the fore and aft centreline of the hull at midlength,

This is 'weird shaped hull' stuff, given a normal boat you can ignore it. If the boat has a normal convex rockerline and no concavities n the outer surface it won't come into play.

shall not be less than 1168mm.

So this will be the actual beam restriction. They have either worded it IMHO clumsily or else appear to have stuffed the proof reading and left a word or two out! The usual way of checking these measurements is to make a jig consisting of a piece of wood 1168mm wide, with a two other pieces sticking down 229mm at each end - a very shallow U shape. If both prongs don't touch the hull then the boat is too thin.

This measurement shall be taken to the straight line bridging adjacent lands if any.

And this is more weird shaped hull stuff - in this case allowing for clinker built hulls.
Does that help?

Edited by JimC - 04 Jan 19 at 7:08am

Ah, thank you!

Page 21 of the measurement manual will help you.

https://www.national12.org/downloads/MeasuresManual.PDF

For info, 1168mm equates to 46 inches; 229mm is 9 inches.

Otherwise, the rise of floor rule is a basic type forming rule for a box class rule such as the N12. Boats built tend to be bang on max length, max width and have minimum rise of floor as this will give minimum wetted surface area.

Rise of floor can be difficult for a builder and it is not uncommmon to see small measurement bumps -

You're ignoring rocker and waterplane, which make quite a difference to how the wetted area works out.

But yes, rise of floor rules can be rather type forming, doubly so if the measurement is at midlength, which is not where you want the widest part of the boat. The Australian (and pre 1984 UK) Cherub rules are notably influential in that way,leading to some unexpected shapes.

When we were drafting the new IC rules a lot of attention was spent in attempting to get a rule that would have the desired effect in mandating the targeted width of boat without forcing the designer down a particular shape.