Stepped hulls- 50 knots plus. Why?

I've heard in several places, none particularly trusted, that the tradeoffs for stepped hulls only start to pay with very high performance boats. Normally 50 knots plus is the figure offered.

1) Where has this come from and how true is it?

2) What is the mechanism? Is this because high speeds are required to set up the pressure differential that is large enough to draw air into the centre of the he area behind the step, all the way from the sides of the boat? So at slower speeds air cannot be drawn enough into the middle areas of the boat, so not enough air is drawn in to offset the drag of water going over the step. Is this why modern stepped hulls use aft raked step channels? To make an easier path for the air to get the the keel area at lower speeds?

How does it differ with step design?. If my rough premise in 2 is correct to some extent, then the deadrise of the hull must be significant in this. In a higher deadrise hull you are having to suck air deeply underwater against its natural inclination. In a flat bottomed hull, once it's planing you are only having to move the air sideways ( roughly).

I'm wondering whether steps that are aerated directly from above will pay off at a lower speed than traditional single hull designs. I'm not talking about pumping air mechanically into steps from above, although I understand this can pay off from very low speeds. I'm taking about the inside, forward part of the step being a slot, entirely open to the atmosphere by mounting the steps on a double hull. Inner hull keeps the boat from sinking, outer hull allows directly aerated steps.

What my question is, is does the benefit of hull steps come at a lower speed ' than usual', if the step is open to atmosphere from above across its entire width (apart from some small vertical vanes to achieve structural integrity between the steps)?

With this system I'd also be inclined to incorporate small downturned planing rails in the shoe area of the outer, stepped hull to discourage the air/ water air mix, exiting the sides. Perhaps also a full width interceptor at the transom to keep an air chamber.

I've got a boat at the moment that can easily accommodate a slip on/ bolt on central stepped hull component that can be easily arranged to obtain all its step air through a wide, open transom, plus perhaps a bit of ram air from the front. But it's a 30 knot boat, so wondering if it's worth bothering with.

 

You mean like this? (Pardon the wavy paper--I'm trying to go very light on the model and also simulate a full-sized version made of Coroplast). This is one of four floats to be attached to a central fuselage. Full decks for stiffness, only the small well for attachment points.

 

You are describing something very similar to what Thornycroft designed and built back in 1910.

 

Basically the step is reducing the wetted surface area of the hull, in contact with the water....which leads to less frictional resistance.
In a nutshell.

But, the science of how big and where, of a step, is not so straight forward and many hulls, have them merely for show, without any logical behind them.
Unless you are going ultra fast, very high Fns, forget it, not worth it.

 

O.P. didn't anticipate that the boat would have to turn sometimes .Hydroplane stepped hull is well ventilated design . Increasing angle of attack or modifying bottom shape and surface dimension , maybe is possible to build lower speed hull.
At higher speeds you have to be more careful about aerodynamics, especially near the water surface. Backflip at speed is not very pleasant.

 

Won't work. To reduce wetted surface there has to be more dynamic pressure on the hull = more speed.

 

M class hydros in the 50s were three point designs. That means that they had steps. These were the smallest of the APBA racing classes. The M, or midget, speeds were usually in the sub class 30 MPH but they were true hydros that sat on three points, two of them being steps. Tunneled air was not a factor at those speeds unless running into a strong headwind, in which case the little boat would do a fancy little dance and worst case scenario, could experience a blowover.

 

Totally don't understand (or buy) this contention. For starters, increasing "angle of attack" (trim) at a given speed will put more dynamic pressure (lifting force) on the hull and thus reduce the wetted surface area needed to provide a requisite amount of lift, without increasing speed. Hydrodynamics 101.

As for montero’s speculation about modifying bottom shape, this too can be optimized to minimize wetted surface area needed to generate sufficient lift for planing at a given hull speed. Many interventions are suggested by the OP, including flat bottoms, vented midships steps, and interceptor(s). Chine fences (“downturned planing rails”) should act like “winglets” to reduce spanwise flow and thus increase lift and reduce wetted area.

If by “surface dimension” montero refers to the shape of the contact patch, then a beamier, shorter hull will provide a higher aspect ratio for a contact patch of a given area, thus more lift for a given speed. FWIW, my proposal for multiple short, rectangular, beamy “hulls” (floats) in combination with an elevated fuselage attempts to further increase effective aspect ratio by presenting four contact patches that are in aggregate both shorter and wider than that of a typical monohull--much like the three-point “thunderboat” hydroplane design that inspires them, as messabout observes. My proposed floats also incorporate all of the OP’s suggestions/speculations, including dead flat bottoms, transverse steps vented to the deck and reinforced with “small vertical vanes,” (possibly) interceptors, and chine fences. My goal is to attain and sustain a plane at extremely low speeds and power inputs. As in, about 8 knots at 250 watts. Human power. I would say the answer to the OP’s question of whether stepping a 30-knot hull is “worth it” would be a resounding “yes.” With a few caveats, like effort, expense, seakeeping, “slamming”….

Nothing is magic, and it may be that the increased induced drag from many of these modifications would negate any advantage from reducing skin drag by presenting a smaller contact patch. But one can certainly reduce wetted surface area while maintaining the same lift without increasing speed. To verify, spend ten seconds in a swimming pool with a cookie sheet.

 

I read a bit of Jakeeeef's posts. Interesting.
@Horton Gonzo meant that these games in general require some minimum speed.
Your expectations that air pressure phenomena will occur with such little available power are unrealistic.
If you want to go fast with such little power ,
"To verify, spend ten seconds in a swimming pool with a cookie sheet." is a good lead if you use a sheet of metal as a hydrofoil. This is the only solution with such low power.

 

@montero--I'm not sure what Gonzo meant, but what he said is that "more speed" is required to reduce "wetted surface." This is not so. There are many other ways to reduce wetted surface area without increasing boat speed.

Yes, some minimum speed is required. Hull speed on a 5ft long float is 3 knots. Same thing for four floats--3 knots. Granted, we want to exceed hull speed by "some minimum" amount to get on a decent plane--so, shall we say, 5 knots? 6? As a teen, I could handily maintain a plane on a pair of wide, flat, jumping waterskis at 8mph or less, and coast for a fair distance after letting go of the tow rope. Wakeboards and skimboards will plane and "coast" just fine at these speeds. There are lots of examples. Here's one: https://youtube.com/shorts/PK2Nh1qvOuA?si=cKSRlkMUTbawinVo. Coasts pretty good, huh? At about 8 mph. On "human power." In deepish water. Now use four of those, arranged in a square, like a water strider, to give some good rest buoyancy and facilitate getting over the hump when you don't have a running start from land. Think that'll plane when it's up on step? I kind of do. And sustain a plane on 250 watts? Well, maybe. At 400 watts and, say, 12 knots, as from a very fit cyclist or rower, I'd say probably. And an elite athlete can deliver more like 1500-2000 watts for about 5 seconds, to clear the hump. Pretty sure it won't take anywhere near that, and if it does, I'll just make the floats (even) bigger.

Whether this setup would yield more or less drag than a hydrofoil or conventional, say, racing shell, is another question entirely. But he DO coast pretty good, don't he? Look at him go! Makes one think.

A hydrofoil (like Decavitator's "speed foil") needs to have about an 18" span and 2.5" chord to clear the hull for a 160-lb rider and 47-lb boat at around 12 knots, if memory serves. OK, that's pretty small, and hard to match. I'm expecting something more like 4" "chord" and 80" "aggregate span" (beam) on my flat contact patches. Hydrofoils have cambered sections with topside suction, which a planing boat does not have. Submersion gives hydrofoils a lot of lift per unit area. But they also have skin drag on the top surface, and they have post drag, which is by no means nothing. More than that, though, they do not plane. They displace, downwards. They shove/pull water down to create lift. That gets their hull out of the water and reduces skin drag. Fine. But it's not planing.

A planing boat is doing something very different, and very odd. Its boat speed is exceeding the propagation speed of its own bow wave. (A hydrofoil is NOT doing this, and can't.) But what does this weird behavior--"clearing the hump"--MEAN? It means that on plane, the rate of the hull's attempted deformation of water exceeds the rate at which the water can accommodate that deformation, as dictated by its dynamic viscosity. It's not the same thing as lift, or not only that. Instead, the water "seems harder" in an accelerated frame of reference, because a flat, blunt, wide hull at some fairly significant AoA (4-5 degrees? 6?) is "slapping" (accelerating) the water faster than the water can "get out of the way." It's what you feel when you do a belly-flop, and what can kill you if you jump off a bridge and land flat. Concrete. That's what "true," "proper" planing is. Smacking the cookie sheet down on the flat surface of the water, if we're still doing cookie sheets. That's why my floats look the way they do.

That skimboarder looks a bit like he's skating over the water, because he is. He's encountering a lot less wave-making resistance than either a displacement boat or a hydrofoil, because the water "gives less." He's making smaller waves. He's in fact making shockwaves, with a nice Kelvin wake pattern. Skin drag, schmin-drag--the goal here is to reduce wave-making drag. Water is very dense and sludgy and vile. Let's get out of it altogether, stop shoving it around, "harden it up" so we stop sinking into it, and then just trip the light fantastic atop it. Skitter over it on the tips of your step "transoms," is the idea. Plus perhaps a little at the transom-transoms. Air in between. Look at Dynaplane. Look at our friend Jürgen Sass.

No one else much buys this explanation of planing, I have found, or favors such measures, including Savitsky. But my conviction of its potential is why I favor planing over either conventional displacement craft or hydrofoils for going fast on low power. Hydrofoils are not the "only" solution to get high speed from a low-power application, as our wakeboarding friend demonstrates.

 

I will answer you later . Miscalulations. The principle of conservation of energy is not water propulsion.
Foiler dimensions :
https://www.youtube.com/shorts/PsmBu5QTcBw

 

"8 knots at 250 watts" I wish you well but based on experience over the years it seems improbable. A six man water safari canoe can maintain that kind of speed for a while and back off a bit and they can maintain speed for many hours. Strictly displacement mode and highly optimized for those specific conditions.

The water skimmer video is interesting but the operator is entering the water with a considerable reserve of kinetic energy. I no longer have the math skills (or desire) to calculate the overall effect but the comparison of a 1/4 mike dragster and a sedan at a constant speed come to mind.

 

Quad configuration is interesting. How the front float will affect the rear .I need to think more about the Quadramaran with suspension.

I also like planing boats . Hydrofoils are naturally dangerous.For sea going boats I have one or two designs utilizing foils.

Interesting , interesting but if you use more of power.

 

@Skip--thanks--I agree it's a long shot. Although rowing shells (single sculls) are human-powered, and they top out at around 12 knots in a dead sprint.

Yeah, the water skimmer is starting out (and continuing) with a lot of kinetic energy, which is sort of cheating. "The rules" (IHPVA) don't explicitly say you have to have a water start, but I think it's only sporting. And yeah, human-powered planing may be a "dragster-like," sprint endeavor, on flat water. Say a 100m flying start, which is the situation where Decaviator continues to hold the world speed record. THEY optimized for specific conditions, so I figure I can, too.

 

@montero -
I've seen pumpafoil. I like the idea, especially its oscillating foil propulsion. Don't like hydrofoils for support, even though combining lift and propulsion is cool. Not new. Aquafoil, Trampofoil (VERY fast). Wasserlaufer was probably first. The oscillating foil is symmetrical, I would assume, and rigid, and doesn't have variable pitch. Much. Or so would be my guess. And the foil travel isn't very long, but mostly, you're putting a lot of energy into heaving your body weight up and down, aren't you? Or is the idea that you're building potential energy that you will then cash out on the down "thrust?"