What would be the the difference? I’m talking boats here.
Thanks
Don
Same question for rocker. Forward/aft and depth
Don
Very broadly prismatic coefficient and block coefficient.
Thee are measures of how imersed volume is distributed around the hull. Basically the more volume (in proportion) the hull has in the ends (i.e. the less the rocker, the fuller the ends) the more efficient it is at gigh displacement speeds - and vice versa.If you wanbt to know more see http://www.navweaps.com/index_tech/tech-004.htm . It’s based on merchant ships, but it works for anything - and will save me a great deal of typing.
There are, of course other things about wide sterns (rocker too, in some environments). These mostly invlve, fashion, witchcraft and rating systems. Most of what follows applies to full-size yachts. It might be taken as a warning to think about what is happening before you rush in and copy some full-size design feature.
First, you must distinguish between sterns that are wide and essentially triangular (frequetly well disguised, but read on and you’ll see what I mean) and one that are basically rectangular. The first type have is actually quite narrow as far as this water stuff is concerned but has huge out of water flare (at any intended angle of heel). Such sterns provide a location for various types of moveable ballast - water, deck-apes …
In the second, rectangular, version there are two reasons for using them. Unfortunately the requirements and results often conflict.
We can arrange the corners of the wide stern and a hard turn of the bilge amidships so that when heeled we end up with a clean line through them and maintain good hull balance when heeled with minimum curvature in a wide hull. The now rather elderly but still classic Farr 40s from Gaucho on are classic examples of this. The technique requires very careful design with great attention to changes in area and curvature, but if done right it certainly works - see Gaucho, arguably Farr’s materpiece.
On the other hand, very wide sterns are used by some planing boats to help lift them onto the plane - in aeroplane terms the ‘wing loading’ is reduced. This is a double edged sword. If you look at, say. Sydney Harbour 18 footers closely, they often have quite narrow sterns so far as the water is concerned - with huge flare + trapezes, hiking racks and any other means available for making flesh pay for itself. Note that the concomittant is that that there is excessive lift aft: it is necessary to counter the fact that the centre of pressure/lifr is way aft of the centre of gravity by moving (typically) crew weight aft. A narrower stern at water level will reduce this problem.
There are other configurations. True skiff hulls have a narrow beam configuration wihout any pronounced centreline. As the boats heels, its shape does not change very much in terms of the immersed watrlines - but their angle to the static datum eaterline does.
This is very far from a complete explanation.If you are genuinely intersted in yacht design (which you appear to be), I suggest that you get a copy of the book by Eiliasson and Larssin (correctly spelt - they are Swedish). Much of it is irrelevant to model ychts - but overall it is the best book on yacht design published in the last fifty years,
Hope this hekps a little bit
Found it on Amazon.com
“Principles of Yacht Design”
by
Lars Larsson & Rolf Eliasson
List Price $47.95
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