# Bilge Diagonal

Discussion in 'Boat Design' started by PAUL XAVIER, Apr 10, 2014.

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### PAUL XAVIERJunior Member

In every drawing, in the half breadth view there is "bilge diagonal". Why it is actually drawn, what is the purpose of that?
TY

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### gonzoSenior Member

It is to check for fairness. Diagonals are used in other sections too. For example, in the flare of the bow.

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### daiquiriEngineering and Design

Bilge diagonals are actually not present in every drawing. In the past, when boats were made of wooden planks, they were a pretty common feature. Today they are much less so.

Bilge diagonals have a double purpose:
1. They approximate the direction of planking, hence making it easier for the lofter to evaluate the required length and shape of planks.
2. They approximate the shape of streamlines of the water which flows in close proximity of the hull at low speeds, thus allowing a trained eye to make a quick qualitative evaluation of it's hydrodynamic worthiness.
Cheers

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### jehardimanSenior Member

As said it, a diagonal is for fairness, and is usually the primary line used to fair the run. Think of it this way...the waterlines and buttocks intersect the shell at fairly acute angles at the turn of the bilge, so a little error can greatly change the slope of the shell. Diagonals intersect the shell almost perpendicular, so they are much better at controlling the sweep of the run when lofting from a table of offsets.

However, as daiquiri points out, modern digitial lines which go straight to CNC cutting of molds and frames have reduced the need for a plotted diagonal. If lofting by hand on a mould floor then diagonals really help get a fair hull.

See the lines plan in this thread for an example of how diagonals better control the shape than the buttocks or waterlines.

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### navalexJunior Member

In the case of sailing vessels you can use it to see the angle of the run in different angles of heel, as well as to see how flat or rounded is the run for those angles of heel.
Obviously to achieve this effect you have to draw these lines perpendicular to the hull in the section view, as jehardiman says.

Last edited: Apr 10, 2014
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### daiquiriEngineering and Design

Sorry for my linguistic ignorance, but - what do the expressions "to fair the run" and "angle of the run" mean?

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### navalexJunior Member

The run is the aft part of the hull, more or less 1/3 of the length, so the angle of the run is the angle of the aft part of the hull between the hull and the waterplane.

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### daiquiriEngineering and Design

Ok, thanks. So is the expression "to fair the run" intended in the same sense as the point n.2 in my first post?

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### jehardimanSenior Member

Yes, If you look at the lines I posted in the link, the waterlines and buttocks have flats, hollows, and kinks while the diagonals are long sweeping curves. The diagonals are much more suited to showing the flow on vessels that sail "on their ear" with significant heel.

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