Stepped Planing Hulls/small sailboats

Thanks very much, JS! Would the interceptor be installed in a straight line across the bottom of the boat?
-------------
Found this as a reference to a transom mounted interceptor: http://www.owenclarkedesign.com/da/11189

"Performance was enhanced further by a reduction in hull drag of between 10% and 16% by use of an interceptor system (the first time this has ever been done on a sailboat) to alter trim at different boat speeds. For more information on this unique development go to: The Daily Sail ."

 

The interceptor shall be installed in a straight line across the bottom of the boat at present, until you know how that works and what you want.

You can try to stretch a line around the hull and see what happens. It might work if properly balanced. The line diameter can be around ½ - 2 % of the forward whetted length. How long that might be?

Good luck!

JS

www.sassdesign.net

 

Interesting discussion folks.
For those that might take interest, I had some experience making a 17ft step hull sailing catamaran in Sydney in 1982 and registered the design with the patents and trade marks office about that time.
I photographed it on slide film for a presentation shown to my design year at industrial design dept University of Technology Sydney (then the S.C.A.) in 1984.
It was a follow on from my collaboration with Alex Cirrigottis and our original development of the tiny Itzacat planing cat which had 12ft hulls canted at 10 degrees to each other so the leeward hull would sail flat when the windward hull was just out of the water.
The 17 foot cat similarly had canted hulls but parallel vertical daggerboards to gain some slight hydrofoil lift when heeled.
It also had a second step aft of the mid step and rubber surrounding the crossbeams in an attempt to soften the ride of the approx 2ft wide flat planing hulls. At speed on the step they would pound quite hard in chop on Pitwater and it was quite a cough inducing sensation compared to a displacement cat at speed.
http://www.mckeewildthings.com/Wet and Wild Fluid Dynamics.htm
Having read numerous David Taylor model basin tests of step hulls and having made numerous RC seaplanes, hydrofoils and step hull sailboat models, my brother Bruce and I tested various step configurations finding a sharp wedge-like abrupt kick down about one centimetre deep just forwards of the step was effective. Those old papers mentioned a small kicked down step was as effective as a deeper flat step and our experiments seemed to confirm those results.
The kick down step edges were similar to the spoilers on the back of racing cars, (imagine the cars upside-down) and interestingly I had a habit of tow testing scale model cars underwater in swimming pools. A scratchbuilt model of MAD MAX's V8 interceptor with its roof spoiler forwards of, and just above the rear window proved to plane on water very effectively when inverted, the steep angle of the amidships roof spoiler acting like the interceptors of this thread!
We never thought to make vertical interceptors as we were concerned enough about abrasion of a beach cat on sand wearing down our wedged steps! We thought that a production version could have removable stainless metal step wedges that could be replaced or sharpened after wear and tear.
Although having made rc catamaran models with horizontal foil stabilizers on the rudders to prevent pitchpoling, on the 17 it seemed to take away the fun of sliding about as human ballast to control trim.
On our 17ft boat, we used 2 step ventilators in each hull based on comments from seaplane pioneer Glenn Curtis in a book I owned on early seaplane hull designs, and we used simple pvc conduit as as form to mould fibreglass tubes that connected the deck to the hull aft of the step. Underway the 17ft boat made slurping gurgling noises until air was able to enter the step from the sides of the hull. For a production version which we never could afford to tool up for we had intended to use a snazzy looking NACA inlet duct for the ventilator inlet.
On our step hull sailboards we just use GRP fishing rod as step ventilators.
I have some old faded photos of our 17ft step hulls being shaped in polyurethane green foam and Alex still has a photo of the mold I believe.
The estate of my engineering science lecturer David Mackie hopefully still has the colour slides of the finished boat.
As a uni student at the time we didn't have sufficient funds to continue the project but later continued in the aquatic vein making robotic sharks and whales etc.
Our 17 ft cat was regularly parked on the shore at Bayview near the Pitwater Aquatic Club and was eventually removed and I believe converted to land-fill by Council!
Bruce, Alex and I have talked for years about knocking up another one as a fun beach cat in the 17 ft range as our original was sensational to sail even with a pathetic non rotating mast 16ft skiff rig with no jib.
A Hang-glider maker in mona vale at the time was keen on making us a double surface wing rig but we never managed to scrape together the $1000 to make it. We settled for a 2nd hand Tornado rig but when we arrived to install it the council had already struck!
It might sound like a comedy of errors but was a great experience and gave us confidence that a hilariously fun beach cat could be made based on the planing/step principle that would provide a very different set of sensations to a conventional cat.

 

A boat is not an airplane.

A seaplane is not a boat with steps. The aft part has just the right static lift when the plane is not in speed. When the plane then accelerates the stern shall not have more lift than that the angle of attack on the wing may be right at take off.

A flying boat has also aerodynamic transverse stability with the wings. It needs less hydrodynamic stability than a corresponding boat. In addition, they wanted to have as little drag as possible when the aircraft was well up in the air.

When flying boats were designed it was also obliged to take into account that no water spray had come into the propellers.

So the experience of seaplanes has limited value when I design my interceptor boats

JS

www.sassdesign.net

 

================
Great stuff, Greg!
See this thread in "multihulls": http://www.boatdesign.net/forums/multihulls/planing-catamaran-30415.html