weight vs. cost

[rapscallion]

I found a shuttleworth article that shows how to calculate relative boatspeeds based on weight, sail area ect... I thought it would be a good way to quantify the performance differences of the same design of boat but built with different construction methods...

So the question is, how much weight difference is there between say a stitched ply construction vs a hand lay up foam core construction vs vacuum bag construction.

I'm just looking for ball park figures for a comparative estimate....

thanks in advance
raps

[Eric Sponberg]

You can't really say in any general sort of way. The weights of different construction methods depends on the details and general arrangement of the internal framing structure. And in this regard, the more framing, the less weight. I know this sounds counter-intuitive, but it is true. About two thirds of a hull weight comes from the skin, and one third comes from the framing. But as you add more framing, strength and stiffness go up exponentially, so the skin thickness can be reduced accordingly. This makes for a fast reduction in weight.

Also, in a cruising sailboat, hull and deck structural weight can be a relatively small portion of overall boat weight. The weight of the interior, mechanical systems, the rig, ballast if it is a monohull, all have their own contributions.

On an open class boat or a really stripped out racer, of course, the weight of the hull and deck will be a significant proportion of the whole boat weight, and maybe you could see some small differences. But would they translate into discernible differences in performance. I wonder. Again, it depends on the details of construction.

The only way to really compare is to take one given design, build it in two different methods and take advantage of the reduced weight of one design over another. Let's say then that one boat is going to be lighter than the other. The lighter boat can then maybe have a slightly smaller sailplan and perhaps a smaller ballast keel if a monohull. Then you would have to sail one boat against the other to denote performance differences.

I suppose an easier way to look at this is to study the long term trends in open class design. For example, look at the open class designs from the beginning of the BOC Around the World Race in 1983-84 to the present. Some of the early boats were aluminum, some fiberglass, and I don't think back then there were any carbon fiber boats. Trace that trend to today, and virtually all competitive open class boats are built of carbon fiber, and their weights have gone down and their speeds and costs to build have gone up. So there is a trend, certainly, that lighter weight leads to higher speed, but there is a commensurate price increase to achieve that performance.

To examine this in detail in a given design requires that you spec out the structures of that design in the methods of construction under consideration, and take advantage of the weight trade-offs in equipment that will be afforded to you. It is very difficult to say which method of construction will be better in simple general terms.

Eric

[rapscallion]

Thanks for the insight Eric. There is a lot of good info in your post. I was actually trying to compare say a F22 built out of stitched ply or out of e glass with a foam core. I am trying to gather info on building a trimaran, and your post raises additional questions.