IOM specs

I would like to find out from some one who is in the know.
With a given length (1000mm) and a rocker depth of 50mm (being measured from the waterline) or even to the max which I think is 60mm and a a maximum weight of what is it for the IOM, 4Kg.
What would be the difference in wetted area be between all the different designs.
Should not the wetted area of all designs to the exact same weight say 4.0Kg be the same (displacement) even if it is a “skinny” or a broad design.
Could somebody please enlighten me.


Geometry makes all the difference!

The wetted surfaces is different between boats because of the difference in cross-sectional shape (amongst other things, but I think this will illustrate the idea pretty well). Say you look at two hulls with the same profile (looking at it from the side). Now, say one of the boats is made from a box-like construction so the cross-sections are rectangles. Now say the other boat is made using semi-circular cross-sections. And since they are displacing the same amount, the area of each cross-section has to be the same.

Say our hull depth is 50mm with semicircular sections. The area associated with that is (.5pi50^2) 3927mm^2. The perimeter of the semicircular cross-section (which corresponds to wetted surface) is (.5pi100) 157mm.
now, for the hull with rectangular cross-sections, the depth is 50mm, so to get the width, you divide the area by 50 (in this case 3972/50) which results in a beam of 79.44 mm. Now, figure out the perimeter (again corresponding to wetted surface): two sides 50mm deep with a 79.44mm width, resulting in a wetted surface of 179.44mm.

So the difference between these two hypothetical cross-sections is a little over 20mm, which when compared to the overall wetted surface, is quite a bit.

That was a purely arbitrary example, but it illustrates that the shape of the hull matters.

interesting to note that the “boxy” design is actually significantly narrower than the semi-circular hull.

Hope that helps


Mathematically, a circle gives you the minimum length (circumference) that encloses the maximum area. Any cross-section that isn’t “circular arc” will therefore have more wetted area than otherwise. It isn’t quite that simple, because it is a sphere which actually gives you minimum wetted area for a given volume, but you don’t see too many hulls in the shape of a sphere.

Lester Gilbert