Stepped hulls

[EPClement]

Chase 687 asked about a Dynaplane design‘s sensitivity to loading The extensive testing at DTMB of stepped models with trim-control devices showed favorable characteristics for this type with regard to loading. First, its low drag at planing speeds is achieved with a CG location close to the mid-length point of the hull. Accordingly the full length of the hull can be used for carrying useful items of payload. This is in contrast to the case for a conventional unstepped planing boat for which the CG is usually located an appreciable distance aft of mid-length in order to counteract to some extent its inherently inefficient performance. If the CG of this type is moved forward to a point near mid-length the resistance will be substantially increased. Also, the testing of the Dynaplane-type models at DTMB included tests to determine the effects on resistance of changes in CG location. The result found was that the moving the CG either forward or aft of the design position had a small effect on the resistance at low speeds and no effect at all on the resistance at planing speeds.

Another interesting test related to loading yielded the result that when the weight carried by a Dynaplane-type model was increased by 23% the efficiency of the hull at the heavier loading (as measured by the L/D value) increased by 8%. This was a surprising result. However the Dynaplane type has the advantages over the conventional hull-type of operating with higher aspect ratio, optimum trim angle, and efficient camber shape. I believe that there may be capabilities and welcome surprises in this combination of factors that still remain to be discovered.

In answer to another question, the afterbody of the Dynaplane design runs clear and dry above the water surface, over a range of speeds, when in the planing condition. The relation of the afterbody to the level of the water surface when in the planing condition is shown by two of the sketches of Figure 3 on the web site:
www.dynaplaneboat.com

The model testing at DTMB was done in smooth water, and unfortunately numerical information regarding performance in rough water is not available. However, the feature of trim-control does confer important benefits for operation in waves. The trim angle can be reduced to a low value to reduce impact accelerations when running in a head sea. Alternatively it can be increased to a high value to avoid burying the bow in the backs of waves when running in a following sea.

[chase687]

Thank you EPClement,

Since my last post I have read your booklet that was available on the www.foils.org website. It is very interesting reading. I plan to print it then reread it and also look up the reference sources to gain more information into the subject. My background is in aircraft so for me a lot of the concepts just made sense.

I am considering building a powered model similar to the design outlined in your booklet for a developable boat. What resources would help me in accomplishing this? Also if I may bother you for more of your time, what steps would you take in order to build such a model?

Sincerely,
Chase

[EPClement]

Doing a Google search on the topic, “stepped planing boat” is now less satisfactory than it was in the past. One reason is that someone at Google is now confusing the word planning with planing. This results in references to numerous inappropriate sites - “Planning a boat trip” - for example. Also, someone doing a search on “stepped planing boat” or on “dynaplane boat” would be helped by being given links to my web site, “www.dynaplaneboat.com” since the topic of that site is a stepped planing boat. Does anyone know how to “get the word” to Google?

[marshmat]

Google's engine uses inter-site linking as one of the factors in its calculation... hence why "miserable failure" turns up George Bush and Mike Moore- rivalling bloggers all put links to Bush's and Moore's sites with those titles, and Google's spider took the bait. Paying Google $$$ for priority placement is another option if you don't have 5,000 bloggers linking to your site. EPC, I'd also suggest fixing the link in post #78 while you're at it.

[Davor]

There is a very old paper on stepped hulls : Kafali V-forms.

Applicable for v shaped planning hulls and very high speeds. Effect of steps is reduction of wetted area and resistance of course, as :

R= 0.5 * Ro * V^2 * Wetted Area * C

[EPClement]

The equation for the resistance of a stepped planing hull given by Davor in his Message #80 only gives the frictional component of the resistance. There is an important pressure component of resistance also which is equal to the weight of the boat times the tangent of the running trim angle.

[Leo Lazauskas]

Originally Posted by EPClement

Doing a Google search on the topic, “stepped planing boat” is now less satisfactory than it was in the past. One reason is that someone at Google is now confusing the word planning with planing.

Back in the late 1950s, a set of conference proceedings were sent to be printed. Apparently the typesetters decided that there was no such word as "sea-keeping" and changed all instances to it to "bee-keeping".

Good Luck!
Leo.

[Davor]

Kafali method gives total resistance, not only frictonal.

[Jimboat]

Actually....

the format of the resistance (drag) formula is the same for ALL the different contributors to total hydrodynamic drag (resistance) ... R= 0.5 * Ro * V^2 * Wetted Area * C

...where C is the specific drag coefficient

the accurate approach is to measure/calculate the friction drag, induced drag, profile (form) drag, and even wave-making and/or bouancy resistance. The coefficients for each of these contributors to total drag are based on many, many different design and operating conditions. For any particular combination of design/configuration/operating influences, it is usually a bit of an oversimplification to summarize into one single coefficient, as the resulting approach (coefficient) would not be applicable to other situations.

[Davor]

I wrote this formula just to explain influenece of wetted area on resistance.

Kafali method for stepped hull is not that simple of course, and it includes a graph for "famous" coefficient C.

[mcollins07]

As Lorsail, my interest in the step is in sailing vessels. I have an idea for a stepped hull in an integrated-keel-hull. The goal is to get some ( ever so lightly) lift from a portion of the hull. And still maintian low drag. A hydrodynamic foil sees an increase in the pressure on its sides up to a point of maximum width then the pressure drops. I have designed a hull which I believe will have this effect when the hull is heeled. The point the preasure drops is a small step. My drawings are currently only on paper, so I'll have to describe the hull shape. The hull has a plumb bow and is quite narrow, a 12 degree wedge. This wedge has a flat bottom back to the position of the step. Aft of the step, the 12 degree wedge continues but is angled upward at an angle slightly greater than 12 degrees, say 15 degrees. When the boat heels to port, the starboard side of the step will move outboard and the port side of the step will move inboard. The starboard side is no longer at the bottom, but the port side is at the bottom. The heeling creates the hydrodynamic foil shape. There are some winglets to reduce slipage around the bottom of the keel. There is a shorter distance along the port side from bow to the low pressuer regions then on the starboard side. As the water passes the step, it is retruning to its non-displaced position, but in a vertical direction instead of a horizontal direction. This is interesting in that there is not hydrodynaic foil until the boat is heeled. The amount of lift should increase as the boat heels, up to a heeling angle of 45 degrees.

[EPClement]

This is in response to Message #86 by mcollins07. The most efficient way in which to utilize a step is to incorporate a cambered planing surface. A design booklet is available on the website of the International Hydrofoil Society (www.foils.org) that explains how to design cambered planing surfaces for ranges of values of speed, lift, size, deadrise angle, and angle of attack. The booklet is entitled “A Configuration for a Stepped Planing Boat Having Minimum Drag (Dynaplane Boat).” It is Item 7 in the section on the website entitled “Downloadable Hydrofoil Papers.”

[JorgenBeyer]

Originally Posted by cyclops

IT really comes down to getting a film of air under the entire bottom skin. Any and all methods will work. Steps or a load of planing strakes . Whatever you do should be a big help. ----------------A after thought. A flat bottom hull helps greatly at getting on plane quickly. Now you have to trade off some of the V ride and handling for the higher average speed. You can go back to a bow keel and rudder to retain steering and prevent the wind and current skidding. Your R & D. ----------Old bones like smooth V's. Wipper snappers like speed. And so it goes.

Are you saying that air lubrication is what makes up the drag reduction?

[JorgenBeyer]

Does anyone have any papers on the matter of stepped and high speed hulls?
I am particularly interested in IKEDA´s work: "A study of transverse stability loss on very high speed boats".

Thanks
Jorgen

[drhull]

jorgen, I also was interested in the air induction under the hull. I posted some ideas but I never followed through with any designs. My best thought was V hull with strakes/chimes to hold/guide any air induced under hull from 1/4 to 1/3 length from bow to 1/4 length from stern.Any air induced would be shut off if the boat was turned sharp not determined yet to prevent skipping across the water. Air would be either induced fans/blowers or ram air, just ideas.not much force not to create hovercraft a skin when on plane or higher speeds.Almost working like a drain plug.positioned at locations throughout to provide best air dispersion for each hull design. Best of luck. Mike