The Thin Green Line

The Thin Green Line

In my last post I said that I would explain the use of the multiple waterlines. Firstly, why are they green? Because I like them green, as soon as I see a green line I know it's a waterline.

Why are there so many? Because we will use them to develop the 3D shape of our boat along with other reference lines.

So far we have our midsection, bow transom and stern transom. The thing that joins these entities is the water line, By transferring the location of the LWL on these three sections we can start to develop the shape.

First we draw in several waterlines. I will explain further later.

Next we locate the point of contact in plan of the LWL and the three sections thusly, note the red circles,

and join the points with a fair curve, very easy on the computer.

Then we do the very same thing for each of the other waterlines like this,

Until we have this,

You can deduce from what we have just done the reason for the many waterlines.

Now we can go back and construct the other sections using the waterlines we have just created.

Onward

Onward

The question in the last post was where do we go from here. Well we go back to here and using the grid we constructed in the last post showing the profile and plan we add our new midsection. We can use the pram bow from RMS and we can take RMS's transom and round it to give us this.

You'll notice that I'm using the technique we discussed here to transfer the basic dimensions.

So now what?

Well we can transfer the sheer to the sections by using the same technique giving us this,

If we connect up all the intersections and the end of the midsection and bow transom correctly we have the sheer from bow to stern on the sections drawing like this.

You'll notice a couple of oddities, for one we don't have a section to the right of the centreline and there are extra green lines.

Lets deal with the green lines first, they are water lines, the LWL is the lowest one. I will explain their use in the forthcoming posts.

The missing section is section 6 and it's not there because we have no idea of the shape but we'll use what we do have to determine that shape as we develop the new round bottom boat.

Round and round we go

Redesign

As we saw RMS was designed as a flat bottom boat, but would it be better as a round bottom boat?

We'll have a go at a redesign based on RMS. Here's RMS midsection as originally designed,

That midsection has a submerged area of .87 sqft. With a prismatic coefficient of .56 and a length on the waterline of 12.24 ft. that's a projected displacement of 373 lbs.

A more rounded shape with the same over all dimensions looks like this,

If we overlay the new midsection onto the old midsection it is obvious that we would lose a significant amount of displacement.

The new midsection area is .76 sqft. Using the same prismatic coefficient and the same WL we get a projected displacement of 325lbs a loss of approximately 50 lbs. If we can live with that then lets proceed.

But how to we proceed from here?