Designing catamarans with fatter sterns?

Would it make sense to design catamarans and trimarans to be wider in the stern, to increase stern bouyancy? Are there any examples of this? What would that do to performance?

The faster a displacement hull moves through the water, the father back both the bow and stern waves will move, causing the vessel to climb the bow-wave, while simultaneously losing the support of the stern-wave, causing the stern to sink.

There are several mitigation techniques in use; but, is it possible to reimagine the hulls, to correct for this effect, without resorting to foils and stern-planes? Can hull bouyancy be manipulated this way, to reduce or eliminate this effect, by shifting it rearward?

Also, can permanent "planing" features be added to the hull sterns to further mitigate the tendancy of the stern to sink at higher speeds? What about hybrid or blended designs that benefit from two points in-front to divide the water in two, but with a stern more like a monohull?

With aircraft, stable supersonic flight was only achieved after the invention or discovery that the "coke-bottle" shape could be used to shift the supersonic shock-waves rearward and redistribute them to reduce drag and improve aircraft stability and maintain control-surface authority in the transonic region.

 

A sailing hull is designed for a range of conditions. Optimizing one mode of operation will cause a performance degradation in another. For a catamaran to plane, the hull needs to look more like a water ski. The loss of performance will occur during turns, which occur faster with more rocker in the keel. A planing hull sailing upwind will spend a lot of time in displacement mode, which will be slower than a hull with less wetted surface area.

The compromise is to design a semi-displacement hull so that flow separation occurs at the stern when at hull speed.

 

Thank you for the thoughtful and detailed answer.

Do you have links to any images of the hull types you describe that would be suitable for multihull use?

I'm also very interested in learning more about moving flow-separation to the stern. Can you recommend any relevant lit?

 

Here are lines drawings for two catamarans.

Tornado Experimenting with an old idea | Port Townsend Watercraft BLOG http://ptwatercraft.com/blog/?p=1711
America's Cup AC45 Chevalier Taglang: AMERICA'S CUP - AC45 PLANS - AC45 LINES AND SAIL PLAN - ACWS CATAMARAN http://chevaliertaglang.blogspot.com/2012/04/americas-cup-ac45-plans-ac45-lines-and.html

 

There are two different "area rules" for drag reduction which appear to have been confused, and there effect on aircraft misunderstood.

The Fenzl/Hayes/Whitcomb area rule applies at transonic speeds, typically Mach numbers from 0.75 to 1.2. The rule correlates shock wave strength and drag with the rate of change of cross-sectional area. A lower rate of area change results in reduced shock wave strength and drag. One implementation of this rule is "coke bottle" fuselage shape with reduced fuselage cross section in the vicinitiy of the wing of a transport type aircraft to reduce the cross-sectional area change. Another implementation is the long swept back transitions at the the wing roots of transport aircraft.

Another area rule is the RT Jones supersonic area rule which applies at supersonic speeds, typically Mach 1.2-1.3 and higher. That rule says drag will be reduced with increasing Mach number as the cross-sectional area is shiffted aft.

Neither rule has anything signficant effect on stability nor control surface authority at transonic speeds.

 

A wider hull aft can reduce "squatting" at higher speeds. To minimize drag at lower speeds the immersed transom area needs to be minimized. Many boats intended for operation at semi-displacement/semi-planing speeds have wide and shallow sterns.

 

That depends upon the purpose/SOR of the boat.

If it is a displacement hull, this does not occur. What you're describing occurs on a planing hull, not a displacement hull.

What you're describing is a feature of the design that is a function of the hull and its shape at certain speeds.
If you wish to avoid this, it is a "simple" fix....depending upon your SOR.

 

The keel slope drives the flow separation at the stern. In the picture, you can see the friction coefficient on the surface of the back portion of a catamaran hull.
The definition of the friction coefficient is the signed norm of :

where i and j are unitary vectors on the surface. The subscribe "p" means "parietal". Taup is positif when oriented in the direction of the upstream flow.

The region in blue is where the flow is separated from the surface. An increase of the keel slope will enlarger this region. A decrease of the keel slope will diminuish this region, up to a point where the flow separation will blow and suddenly become very wide, when the keel line is too flat. Pay attention that, the picture that I make here is strongly related to the speed ( Froude Number ). Playing with the slope to control the flow separation, as @ADHOC already said, may be just a fix among several, whose accordance with your SOR should be reviewed. One last and small comment : Trimaran or catamaran hull are not made to sail in planning mode. There are made to sail gently in displacement mode even at high speed, minimizing the water pushed by supporting (very ) light boats. If you wish to sail in a stabilized planning mode on an heavy boat, change to monohull.

 

On a Tornado, all of this is controlled in a very simple manner.
Going upwind, the crew weight is moved forward, until the bottom of the stern is at or very near the water surface.
There is no squatting, or need to widen the stern, or make a planing surface.
On a reach, where there is maximum power available, crew weight is moved aft. This raises the bow, reducing the tendency to nose dive into a crash. The broad near semicircular stern is depressed, with water cleanly separating from the hull. Any issue of increasing drag due to submerged stern, is immaterial due to the abundance of power. Of course, at varying wind strengths, it is beneficial to shift crew weight somewhat fwd if the bow is not driven too close to the water surface (lower wind) , and it is necessary to shift all weight as far aft as possible during a heavy wind, opting for survival rather than any drag reduction.

This is to point out that any discussion needs to include a lot more conditions than the simplistic question you posed.
The type of catamaran also changes the discussion. A heavy overloaded cruising boat won't care about weight shift.
To get a real answer, you are going to need boat shape, width, power, weight, and as much engineering data to describe how the boat sails. Then you need get new data for the changed boat shape.
Not a trivial discussion.

Good luck.

 

Sterns have been getting wider and flatter since the mid 1980s. Before then the idea was that the water should be brought back together at the stern, so pointed sterns were preferred.

Flat sterns are good at damping pitching. And the curvature aft of a wider stern produces more downforce to keep the bow out of trouble.

This is from small sizes to large. But have to remember the context that though the stern can be wide relative to the maximum waterline beam it might not be very wide in absolute terms.

 

The Tornado was designed in the early 60's. Not a pinched stern. The stern is as wide as the mid body.
The more modern daysailing cats are just catching up.
And the Tornado is still faster.

OP was talking about boats without foils, so your last two don't count. And nothing shows that those are any better when hull borne.
Fashion is a very strong force in how boats are designed.

 

as the boat passes over a wave upwind, will the wider stern not tend to put the boat into a more bow-down attitude? If the boat starts to fly a hull, will not the same occur, as there is much more reserve buoyancy at the stern than at the bow?

 

So the question is - why would you care?
Is there something going to happen that makes it bad, assuming you are correct?
What kind of cat are you talking about?

 

The remark of TANE is very rightfull. Too wide stern indeed put the boat into a more bow-down attitude. This pitching moment increase to the point where a violent nose-dive can happen. On some particular small sport catamaran, downwind, the elmsman could have to step and push hard on the transom to counter this effect. On big racing catamaran, nose-dives are very dangerous, because every one onboard could be sent flying in the air in a blink of an eye, before being crushed onto any obstacle. The mast or/and the rigging can also break, creating an additionnal danger for the boat or the crew. On recreationnal cruising cat, if the mast and the rig are correctly designed, only the clingelin sound of the fine porcelain dinner service of oma is to be heard, if the boat is not pushed too far...

 

talking about cruising cats
& imho bow-down is never good: CE moves forward, less reserve buoyance at bow anyway, so fatt stern. I don't know...