Stepped hulls

Right on Oneeye,

I would like to know where a resolution of the air/water/resistance thing can be found. Two posts give opposite opinions on the question. Which is correct? Are they both correct under different conditions? Who's on first? :?:

 

Now we're into mixed-medium fluid dynamics...... there is some REALLY nasty math involved if you want to go farther into this!
I can't resolve that one for you.... I doubt anyone without a master's in fluid dynamics could give a full, completely accurate answer. There's still a fair bit of disagreement over exactly what is happening down underneath a stepped hull, even among those who design and build them. (For that matter, we still don't have accurate methods of predicting things like wake, and prop size- we can guess with math, but the only way to find out for sure is to test.)
Any designers here with the laboratory and CFD experience to explain the air-hull-water dynamic interface? Now you've got me curious.....

 

My opinion is based on some empirical evidence from a towing tank. For my senior thesis in college I was investigating whether there were any performance benefits to be found in dimpled bottom bodyboards that were on the market at the time. We unable to find any advantage with the dimples, but one unexpected result that we had was that the resistance of both the dimpled and undimpled boards dropped when we towed them into waves. This seemed counterintuitve since head-on waves tend to increase resistance normally. We had a mirror and a camera set up to take photos of the bottoms of the boards as they went by. What we found was that the extremely flat planing surface of the boards, which also had a very low trim angle, would entrain a large amount of air when they encountered waves head on. This entrained air produced thick patterns of bubbles along the bottom. We were seeing the resistance drop by about 15 percent at the highest speeds. The trend was that the greater the speed, the greater the difference in resistance between the waves and still water.

We stumbled upon this discovery only by accident, since it was not part of the objective of our thesis. I certainly can not claim to have performed a thorough investigation of the phenomenon, but the small amount of evidence that I do have was quite clear in demonstrating that entrained bubbles do reduce resistance, at least in some circumstances.

 

Your oberservations agree with the father of seaplanes,Glen Martin. He encountered a brick wall on the first seaplanes. They could not take off of the surface with more than enough power. They flew around stuck to the surface with their V shaped boat hull. Quick as a wink he adds a step . Boats now fly. ALL sea planes have at least 1 step in the hull and floats. The step BREAKS THE WETTED SURFACE suction or wicking effect. The air bubbles or sheet of insulating air, drops the speed and horsepower to almost dry land takeoff requirements. The ELCO WWII PTB was 80'. 1/2 was out at 40 knots. 40 ' divided by 6 steps = 5' between steps. That may explain the 15 knot increase. Why the improved handling? Does the LACK of suction allow quicker turning in a less viscous water mixture? The hull is now on slipperier STUFF and can respond faster. Am I having another greyout?

 

5 deg. sould be the best trim angle for high speed...
I have invented the ASD (Arrow System Design).
This shape of hull reduces the trim angle at high speed (I've tried until 86 knots with a 12m craft length). I have more wetted surface...sure but what about propulsion? It is better for the propulsion to give thrust more parallel as possible to water surface or inclined by 5 deg with porpoising ?
In my web you can hear the noise of an engine of 205 bhp (the boat is named REFOLO ASD and there is the possibility to watch a video).
The noise is constant... this means that the propeller is working well and even if I have more wetted surface, the gain in thrust is larger.
The advantage of this shape is that the heeling angle even at speeds over 50 knots is less than 5 deg. try with stepped deep vee boat...

OTTO RANCHI

 

Ranchi, I have the actual 8 MM films from Elco PTB showing the 80' @ 40 tons boat doing a 40 knot 180 degree turn and staying on plane in the East River under the Brooklyn Bridge in less than 3 seconds. Fantastic cornering power and maybe a 10 knot drop in speed. They did 0 to 40 k in 8 seconds. No typo error at 4500 hp and geared down. The HIGGINS PTB with larger and 3 rudders was a much faster turning boat as reported by people who ran both. Ugly as a shoe box, with a point at 1 end.

 

The ELCO PTB, from which I have seen some pictures, had the following characteristics :

(1) L.c.g. located well fore 40/42 % l.w.l.
(2) 3 rudders and appendages that give vertical lift

Today most of the high speed boat have the l.c.g. well aft because the propulsion system.
How to reach 40/42% l.c.g. with a waterjet installation?

RANCHI Otto

 

Appearence and maximum usable interior are the most important features of pleasure boats today. Some are so designed that way, that they can not enter strong running tide inlets without capsizing. We have had many TUBBIES capsize in NJ inlets. Fact of life.

 

First design beds (as much as possible, even mini beds overall) then design the hull shape around and then ... take a drink ..... moored at pier.

RANCHI OTTO

 

You can design USA tubbies any day!

 

You could become #1 here, if you would as a option, have a 3 car attached garage on the boat. -----OK , safe off shore design, only 2 car attached.

 

The really scary part of joking like this, is some big ego is now going to want to be the first in his marina or waterfront dockage. Hee Hee. --------------------------------------Could you make it 7,000 to 9,000 sq. ft.? "Mr. Otto is the best designer in the world." A quote from your first client. Good luck Ranchi.

 

I agree with you but it is true that " Does the customer have always right ?"
NOT AT ALL !

 

Thanks for the lighted joking of a serious problem facing the boat industry. A party of people like you would be enjoyable. Thank you.

 

Don't joke with the sea

Every Naval Architect must have this proverb in his head !