Virtual Design - part 6

Another try at the hull lines

Let's go quickly through this one.
While reducing the displacement, we are going to sketch a hull with more emphasis on speed and less on long distance cruising.
It may not be exactly what we want but we may use some elements and combine the two to get our "ideal" hull.
Here is how it looks.

In black, you can see the "loft" lines from which we did loft the hull and cut transom etc. Those 3D lines are very fair.
We added a deck with cabin profile sketch plus a sail plan.
The sail plan is more performance oriented: it goes with the type of boat. This one will require swept back spreaders and running backstays, more about that later. Not shown are the appendages.
Note that we strongly reduced the rocker.

This bow view shows one major difference with the previous hull shape: the fine entry created with the low chine. It is a factor of speed but there is a price to pay.

The stern is also wider: nice for planing and it provides ample volume for an aft cabin and cockpit.

A view from under the hull.

The hydrostatic calculations show a displacement of 2,200 liters: much better.
Almost same beam max. and at WL but 50 cm more length at DWL, more stable in X and Y.
We did not need calculations to know that this hull can carry more sail.

The curve if areas shows a noticeable shift of volumes towards the stern. While this boat will be faster thanks to its planing potential and its finer entry, she will also be less forgiving to handle.

Summary: more potential for speed, ability to carry more sail, nice volume aft but there are cons.
The low chine will create a less smooth roll in the waves. We are not talking about a long roll but a jerky movement as soon as that chine gets in the water with stability increasing suddenly. Not a problem for short sails or while racing but it will tire the crew during long passages.
The hull will also heave differently: we did not mention it but the PPI increase very quickly because of that lower chine.

Parenthesis: heave is the up and down movement. A boat hull under way in waves goes through 6 different type of motions and all of them affect the comfort of the crew and safety. A tired, seasick crew doesn't function well.

  • rolling: every knows that one
  • pitching: same movement but fore and aft
  • yawing: going straight or not, zigzags
  • heaving: up and down
  • surging: acceleration positive and negative
  • swaying: drift or leeway

All of them are affected by the hull shape, by the distribution of volumes in the hull.
End of parenthesis.

The movements of the previous hull will be much softer, nicer, all of them.
For this hull, the slight unbalance of the volumes fore and aft will require much more attention at the tiller, again, not ideal for cruising.
Under autopilot, it will drain the batteries in no time.
While the increase in volume aft is nice, we loose volume forward to the point where we may not be able to fit a double berth in there anymore.

Let's compare the lines of the two hulls and see if we can reach a compromise.

The lines of the 1st hull are in red, the new wide one in black. The profile view shows the big difference at the chine forward and the reduced rocker of the new hull.

The plan view shows how much the new hull (black) is pinched at the bow but the difference is not that marked at the stern except for the sheer line.

How about keeping the lines of the 1st sketch but with the reduced rocker of the 2nd hull and also, wider sheer line at the stern? That will give us the displacement we need and a little bit more planing ability but we'll keep a very balanced, well behaved hull. The wider stern will gives us more cockpit room, if we need it.
The sheer line of the 2nd black model is also nicer looking, it blends in very well with the pilothouse style cabin.

OK, now we can design the final (or almost final) lines and proceed towards the next step.

STOP: this page was written in October 2002. The project was put on hold at that time, quasi abandonned. When we re-activated it in 2005, several potential builders had insisted on real cruising qualities and redesigned the hull. We will discuss it in the next articles and explain the choices.

Before we proceed with the design, we will, in the next pages, study the tools used to design our boat.

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