SailNet Community - View Single Post - Long versus short overhangs
View Single Post
  #10  
Old 03-13-2008
Plumper's Avatar
Plumper Plumper is offline
Sailor
 
Join Date: Nov 2007
Location: Vancouver Island
Posts: 845
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 7
Plumper is on a distinguished road
Quote:
Originally Posted by Jeff_H View Post
Obviously you have not been reading my comments. If you go back to yacht design texts, working water craft histories and cruising books that predate the CCA era, there was a strong concensus that the CCA driven narrow beam, and long overhang designs have no place offshore. If you read CCA era design critiques, there was a real outcry against CCA type boats as being 'unwholesome' for offshore work. If you read some of the post Fastnet research and some of the pre- CE directive research on suitable offshore vessels, the short waterline lengths and long overhangs come into the crossfire for their negative impact on motion comfort, lack of stability, and poor carrying capacity.

At least amoung the current crop of offshore vessel designers there seems to be a near unanimous sense that long waterline/ short over hangs are the way to go from all perspectives; ease of handling, sea keeping, motion comfort, carrying capacity, not to mention overall performance.

Which also brings up a related issue. When you look at idealized values for such surrogate formulas as L/D, Motion Comfort Index, and Capsize Screen Formula, the numbers that we all are used to were based on CCA era short-waterline, long overhang designs.

If you look at an equal length on deck boat from the CCA era vs one from today, you'd be surprised that the overall weights of these boats are not all that different, but the waterline lengths of the newer boat is typically as much as a third longer than those of the CCA era boat. The newer boats also often have greater depth and higher ballast ratios as well, meaning lower VCG's relative to the Vert center of Buoyancy)

What this 1/3 longer waterline does is make the equal weight modern boat seem overly light (in other words, what we would consider a moderate displacement boat of today with an L/D of 170 would be the same weight and length on deck as a CCA era boat with an L/D of roughly 350 which would have been considered quite heavy)

Historically a LD of 170 would be considerd too light for offshore work, unable top carry adequate supplies, and the other formulas would suggesting less stable/ seaworthy, and prone to a less comfortable motion, when in fact the longer waterline/equal weight boat should actually be less prone to capsize, have a more comfortable motion and have greater carrying capacity.

But beyond that these CCA era almost by necessity are sailed at very high heel angles, and compared to more modern designs tend to scoop up a whole lot of water over the bow and be pooped over the stern making them miserable to sail in heavy going.

It is for that very reason that I cringe whenever I see someone suggest that boats like the Alberg's, Ariels, Bristol 32 and to a lesser extent 40, Triton, Vanguards and the like make any sense of offshore work.

Respectfully,
Jeff
So how do you explain the fact the ships aren't built with plumb bows? At what point does the traditional overhanging bow become correct again? 30', 50', 100'?

I suggest that if everything you say here is true then modern ships (especially warships) would have plumb bows and sterns. We know the age of the Dreadnought is long over.

Specifically, the negative trend toward CCA boats was because they were designed to fit a racing rule instead of seaworthiness, so many aspects of the design contributed to their poor seaworthiness not just overhangs. Contrary to your argument, most of todays offshore cruisers do have overhangs. They are not as big as more traditional designs but they are there. The overhang provides a drier ride not a wetter one. It is plumb bowed boats that have a tendency to drive through the waves rather than over them.

Regarding your claims of L/D ratios, your assumption that if the waterline length approaches the LOD then the L/D drops making it a boat as safe as an older boat with a higher L/D is flawed. The point is that as the boat with the shorter water line length (and consequently higher L/D) gets over run by a wave, the extra boat above the waterline comes into play and lifts the bow up. There is a reason why submarines don't have long overhangs.

The real reason some modern boats are built with plumb bows is for boat speed and interior room. A plumb bow boat better meets the demands for "Floating Condos" in a shorter hull than does the boat with graceful overhangs. They are also faster with longer waterlines. Proper modern bluewater cruisers still have overhanging bows if for no other reason than to prevent the anchor from flailing the hull and to sweep aside that log that you don't see....
__________________
There is a tide in the affairs of men,
Which, taken at the flood, leads on to fortune;
Omitted, all the voyage of their life
Is bound in shallows and in miseries.
Shakespeare, Julius Caesar IV, iii, 217

Last edited by Plumper; 03-13-2008 at 05:08 PM.
Reply With Quote Share with Facebook