Buttocks - not that kind!

Buttocks

Buttocks are also fairing lines and show the flow of water around the hull. Usually two buttock lines are used and are shown only on the profile as they will be straight lines in plan. They are drawn first on the sections, the blue lines,

and then the intersections transferred to the profile and then joined by curves.

You can see that we have the same hump at station 1 as we had in the diagonal. The flow aft appears good, perhaps we should have a third buttock line to better define that flow.

That third buttock shows a definite problem aft of station 5. This where the fiddly bits come in, adjusting all the lines so that they are fair to one another

Somewhat askew

Diagonals

Diagonals are fairing lines that help develop the shape of your boat so that it presents less of an obstacle to smooth movement through the water. Most designers will tell you that diagonals are shown on the plan view on the opposite side from the water lines. I prefer to simply use a different colour, in this case magenta. They will also tell you that diagonals are not shown on the profile. I prefer to show the diagonal as it helps me “see” the hull.

Diagonals usually go from some defined point on the centreline to the turn of the bilge, there may be more than one diagonal in a larger vessel in which case they are not numbered but identified by letters. In our case only one diagonal is needed.

The intersections are then transferred to the plan and profile using the method we have already discussed here.

The transferred intersections are then joined to form curves.

First in plan,

and then in profile.

These look very smooth and fair but if you look closely at the intersection with station1 in profile you can see a definite hump in profile and hollow in plan.

We'll put the buttocks in first before attempting any corrective action.

Sections Redux

Sections yet again

In the last post we had drawn in the waterlines in plan now we can go back and construct the sections in the same way as we did here. The thing is here there are more intersections and instead of joining them with straight lines we join them with curves.

In the first iteration the sections look like this,

So we'll go with that as it looks good. You'll notice that I said the first iteration but now comes the tricky bit, buttocks and diagonals.