Stepped hulls- 50 knots plus. Why?

On Quad configuration and suspension, check out Nauticraft 4Play, if you haven't, and Joachim Grenestedt. (patent). I don't see why things need to be near that complicated--just use a swingarm with a mountain bike rear shock (or whatever's appropriate) to provide spring and damping. Attach to float with a U-joint for free pivot in pitch and roll.

I am a big skeptic of hydrofoils, although I envy their ability to be unaffected by waves. (SWATH boats are also nice in this respect.) But durability, susceptibility to fouling, and most importantly pitch control, are issues. And I think planing will beat 'em, on flat water. Hydrofoils take a LOT of power, all the time. Planing needs lots of power to crest the hump, but once you have, a whole lot less (as long as you're on flat water). Dunno how planing and suspension combine on bumpy stuff, although the links to videos above sure look promising to me.

 

We are getting far a field from step hydros but if someone wanted to start a thread regarding horizontal fin propulsion I'd be glad to participate.

 

There are two things that contribute to an increase in performance of stepped planing hulls.

Consider two constant weight, beam, deadrise and length hulls, A and B travelling at say 30 knots

A is not stepped.
The dynamic lift begins ahead of the stagnation line starting at atmospheric pressure and increases to the stagnation line where the lift per square inch is at the maximum amount. The highest lift per square inch. The water ahead of the stagnation point moves toward the bow (and outward) and the water behind the stagnation point heads toward the transom. Certainly, depending on the deadrise, there is a component of the water that is moving to the chines.

Ignoring for the moment the additional stagnation point that is created due to an introduced step. As the step takes away from the wetted surface and corresponding friction drag, but the hull would actually settle lower in the water creating a higher dynamic drag as the hull would have to move more water out of the way to support its weight as there is less dynamic lift area due to the lost area.

B the stepped hull
The step is made and air is allowed to enter behind the first step. The air pressure in this region would be close to or lower than atmospheric. There is less friction drag as this
area is removed for the step but the most important gain is that there is another stagnation line created on the second aft hull, ie max lift per square inch.
So in essence two stagnation max lift areas on the hull for the stepped hull.

 

You need to clarify this statement.

I think you need to read some basic books on hydrodynamics.

I refer you to my previous reply above.

Hmmm…again, I refer you to my previous reply.

Oh dear….

Oh, is it??...then don’t into the vile water.

Hmmm..i wonder why!!

 

Thanks for the analysis, @Barry. I had forgotten that the step introduces a second stagnation line at the transom and thus increases lift there. Very good reminder and explanation.

So this is a good picture of conventional steps, but what of variations? Can they do even better? I'm thinking of designs such as Eugene Clement's Dynaplane design and Jürgen Sass's (HJS) midship interceptor designs, discussed elsewhere on this forum. Dynaplane has a knuckle, or lip, in front of the step, while Sass's design uses an interceptor in front of or in place of the step. Both increase lift at the step relative to the transom, and in fact substitute a submerged, stabilizing hydrofoil for the transom. Advantages? Disadvanatages?

I'm all for reducing wetted area, and I like higher-than-typical AoA because it supports my own crazy view of planing, but I don't want the complexity, drag, and fragility of a stern hydrofoil. I'm also not yet sold on either knuckle or interceptor. One thought I've had is to make the aft hull (float, in my case) concave when viewed in profile. See below.



Here, moving from right to left, the forward (flat) bottom is angled at three degrees, plus 1 degree of overall trim. The step is just behind midships of LOA. Vent tunnels behind the step angle to an intake cowl over the deck. Behind the tunnels, the aft bottom angles down slightly at one degree, all the way to the transom. That's what I'm calling "concave." Blue block is attachment point at CG/CL.

A concave hull is known, I believe, but it's not common. My reasoning here is that I very much want to minimize contact along the aft bottom, but I do want some contact at the transom for stability. A "concave" aft bottom should extend the length the hull is ventilated, while also increasing AoA at the transom vs. simply continuing the bottom level to the deck (or angling up to the transom). I'm a fan of high AoA. Will this get done what I want done? Or will I be liable to going nose-down in chop? I did build in some ability to move CG fore and aft. Probably aft, just looking at my drawing.

On venting--I confess I don't understand why standard practice is not to vent to the deck. Venting at the waterline requires a minimum elevation of the hull to clear the openings and is subject to blockage by waves. Dynaplane and Sass both vent to the deck through tunnels. Benneton Air-Step and a few others do as well, but not many. Why doesn't everyone? You must vent efficiently, or you might as well not have a step. Worse than nothing, actually. Is it uncommon simply because of the complexity? Or the concern for sealing? My plan is to essentially have three transverse daggerboard cases chopping through the middle of my floats, as they're unoccupied. See my pictures at the top of the thread. Sure, more complex and subject to leaks, but these seem to be routinely overcome in many sailing dinghies. Assuming these challenges are addressed, is there any reason not to vent to the deck? Is there anything the conventional model provides that deck venting doesn't?

 

The second stagnation point occurs within 1/3 pf the leading edge of the rear "hull" behind the step. Similar as on the front section ahead of the step.
Not at the transom

 

Of course. I was being sloppy. Very nice graph! I suppose lift would be further toward or at the transom if one hooked it back there with an interceptor or trim tabs. Don't think I'm loving that. Propulsion is applied elsewhere, so it shouldn't have a direct effect on trim.

 

I meant that modifying a traditional hull, even with steps, would need more speed to reduce wetted surface. I answered the question, which did not include foils, starts withe kinetic energy reserve, etc.

 

Maybe, even pigs can fly, by riding a film of bubbles, hydrocopter per Francis Reynolds patent, or forced hydroplane when given enough power ?

Here's a link to a previous discussion along the same lines

Pedal Powered Boats https://www.boatdesign.net/threads/pedal-powered-boats.23345/page-151#post-972134
Post #2262

Building Hydrofoils - THE INTERNATIONAL HYDROFOIL SOCIETY - ESTABLISHED 1970 https://foils.org/academic-papers/building-hydrofoils/

Post #2

 

My second build was flat bottom , vertical freeboard slider. Transom was rectangular and also vertical , because of that I must took out the trim pin .
I choose this config consciously and because it was very simple design and build.
I analyzed a lot of our small motorboats, almost all of them had too much stuff on the bottom, vees, keels, strakes, reverse chine flats etc. All these things are common but they always take away your speed. Every change of flow direction have penalty of drag. You achive some better boat behaviours but not without sideffects .Huge improvmets of speed are props and their setups.
Passive venting thru the deck I'm sceptical . How big speed improvement ?
Almost flat bottoms or tunnell hulls i don't worry about speed .I worry about sudden take off.Especially after I found relatively cheap way to build light Frankenoutboards .
As for the concave bottom in the back of the hull, this is a good example of a polish made microskiff
(German Hille delphin )
Ogłoszenia - Sprzedam, kupię na OLX.pl https://www.olx.pl/d/oferta/motorowka-delfin-CID767-ID164QY3.html
the designer used a minimally concave bottom to stabilize the planing with weak CCCP 25hp engines. I have seen more than once that people put on laminate to make the bottom flat, because otherwise the braking effect occurs.This concave is minimal:
Hille Delfin 350 - Zmiana kształtu dna http://forum-motorowodne.pl/budowa-remont-modernizacja-%C5%82odzi/hille-delfin-350-zmiana-ksztaltu-dna

 

I previously posted this on an earlier thread: Steps for a slower planing hull https://www.boatdesign.net/threads/steps-for-a-slower-planing-hull.66882/ The link in the that post no longer works so I updated it below.

Rob Kaidy has designed numerous stepped hulls and has developed tools for analyzing the designs. He made an excellent presentation on stepped hulls at IBEX in 2013. It appears his slides from that presentation can be downloaded at Advanced Topics in Stepped Hull Design - Robert Kaidy - IBEX Session104 - 1 | PDF | Navier–Stokes Equations | Computational Fluid Dynamics https://www.scribd.com/document/307140643/Advanced-Topics-in-Stepped-Hull-Design-Robert-Kaidy-IBEX-Session104-1

His list of advantages:
Primary Advantages of Properly Designed Stepped Planing Hull over non-stepped:
•Reduced Resistance
•Increased Speed
•Improved Efficiency
•Improved Seakeeping
•Compelling Marketing / Hull Story

Kaidy's explanation of how steps provide benefits:
• Reduced Wetted Area
- Reduced Viscous Resistance
• Optimal Trim Angle
– Planing Surfaces Operating at Best Lift/Drag Ratio
• Higher Efficiency Planing Surfaces
– Multiple high Aspect Ratio Lifting Surfaces versus One Very Low Aspect Ratio Surface
• Favorable Effects from Planing Speed to Max Speed
• Trim Angle Remains Optimal at nearly all Speeds, and constant at max velocity
Potential disadvantages and problems listed by Kaidy:
•Higher off plane trim angles and resistance
•Dynamic instability / Porpoising
•High Speed Maneuvering Instability
•Potential for Hooking
•Surge in Seaway
•Structural Discontinuities
•Potential for improper or incomplete ventilation
•LCG Sensitivities
•Off Design or poorly designed craft with higher Resistance than Conv. Hull

Kaidy's list of myths about stepped hulls:
• Air Lubricated
• Air Bearings
• Air ... pretty much anything
• Ram Air Lift
• Big Steps = Fast Boat
• Little Steps = Fast Boat
• More Steps = Fast Boat

 

How so? More power, yes, but why more speed?

I think it's uncontroversial that increasing angle of attack increases lift, up to some point, for a given speed. OK, so hold vessel weight constant. For a vessel operating on the interface between two media of dissimilar densities, this means that a vessel will move toward the volume of lesser density--in this case, up--as you increase AoA. It rides higher, essentially by definition. You can do this through trim, or you can do it by keeping trim constant and changing the angle of your bottom when you design the thing. If we think of the hull as a wedge, which all planing hulls have to be in some fashion to attain any lift, then less of the wedge--less length of hull--is in contact with the water the higher you ride and the steeper your AoA.

To see this "in action," screen shot or open my float drawing above in whatever default tool you have. Better, draw yourself something where you replicate the drawing but have easily movable elements (lines). You can consider it a 2D flat plate, if you like (or two, separated by the step) and just replicate the bottom. One degree at the deck, three degrees on the forward bottom, one degree on the aft bottom. I assume many here use CADD. I use PowerPoint, for now. Any old drawing tool will do, as will pencil and ruler and protractor, but it helps to have something to look at and manipulate rather than just envisioning in the mind. So now, add a level waterline. For simplicity, just consider the forward bottom. Now move the waterline vertically down (hull lifts up). Keep doing this. The intersection of hull with water--the stagnation line--pretty rapidly moves aft, until it reaches the point where there is contact at only the very tip of the step (and transom, incidentally).

Now take the same drawing, and increase the angle of the forward bottom while keeping the same trim angle of the overall hull. Doesn't take much--a single degree will do. Move the waterline up and down. Note where waterline and hull bottom intersect, for a given change in vertical height.

When you're dealing with shallow angles, small changes in vertical elevation above the water make pretty striking changes in where the stagnation line intersects with the hull, thus how much wetted surface there is. It quite surprised me.

Anyway, if you grant that you'll get more lift from the steeper angle at the same hull speed, then you should see how the stagnation line moves back further the more you increase your angle and the more you increase your ride height. Make yourself duplicate floats, put one above the other and align, and compare. If you use shallower angles, like between one and two, it's quite dramatic and hard to miss. I spend a fair amount of time goofing around with this "phenomenon," musing about what bottom angle is ideal/appropriate, and mostly guessing about how much lift increases. One can calculate, using a flat plate for simplicity. I bookmarked a paper on it somewhere, complete with test tank data. I go by smell because I'm allergic to funky Greek symbols. No WAY am I set up to experiment and measure.

Big picture, though, even without calculation or experiment, is that as you increase bottom angle, you should get more lift and thus ride higher at the same speed, thereby reducing your wetted contact area. Not magic. Not pigs flying. Quite straightforward. Adding steps does the same thing, but adds a gap of no surface contact between the step and the point of reconnection with the water further aft. Try it out. See?

You'll have more induced drag from the increase in AoA, for sure, so will need more power to counter that. Unless your reduction in skin friction wins out. Which intervention will win this tug of war, and exactly where the "sweet spot" is, I have no idea. Savitsky says around 4 degrees, if memory serves. One can find his seminal paper on it with a bit of web surfing, or I can dig if anyone's interested. He basically lays it out this way.

I plan to go slower than Savitsky's probably contemplating, and I think more is going on than just lift and drag, so I want to juice my "impact" at the stagnation line by going with a pretty steep angle. 3 or 4 on the forward bottom looks and feels about right, with 1-2 degrees of trim, mediated by wherever the transom is contacting the water based on my fore and aft loading. In my world, maximizing impact--the suddenness of the pressure rise rather than its peak per se--is a key goal, because I want to smack the hell out of the water before it can react. This desire is also what informs the flat bottom, no taper in planform, slab sides, zero rocker, and other things that make my floats look like boxes, or the proverbial kitchen table. Or say, Bolger's "Skimmer." Skimmer is reported to be very quick to plane on low power, and very "slippery" when on plane so that power can be reduced or speed increased quite a lot with minimal increase in power. I'm pretty sure Tom Lathrop, Tom28571, RIP, would have agreed that these sorts of interventions do what "Skimmer" demonstrates.

You really can't predict what behavior will be if you don't look at dynamic and perhaps kinetic viscosity, but that's Navier-Stokes territory, and there be dragons, for people like me. Even Savitsky doesn't go into viscosity. It's also bloody hard if not impossible to set up a tank test to measure things like moving volumes of water. But you can read anecdotal accounts. And look at waves. Conjure up memories of what waves do and what water feels like on plane and at speed, when you're in a motorboat or on waterskis, if you have any such memories.

That last is really the only departure of my design from more conventional wisdom and well-supported observation. The effects on wetted surface area of increasing bottom angle and adding steps are quite well known and pretty basic, or at least it seems so from my puttering around and playing with PowerPoint. I think Barry's analysis supports that.

Sorry for the length.

 

The lift is constant from when the boat is floating in the water or planing with a reduced wetted surface. ie a boat weighing 3000 pounds at rest has a "lift" of 3000 pounds all due to bouyancy.
As the boat moves forward, and lets bypass the hump issue as you do not have to look at many fuel burn, mpg, curves in the hump speed range to see that with this very high angle of attack that it is does not offer an attractive fuel efficiency. (efficiency being fuel burn per mile per gallon)
So lets take a range to discuss beginning speed of 15mph, (well past hump speed for most boats) ( and also ignore a increased drag or lift due to aerodynamic effects just to keep it simple but
at higher speeds they extremely influence drag) and a top speed of 50mph. ( the thrust angle of the prop would also factor into the contributing factors is the AoA changes)

The boat hull is a constant deadrise 12 degree and 3000 pounds.
I have attached a graph that shows the optimum AoA for various deadrises and the optimum angle for a 12 degree hull is 5 degrees. Optimum angle being the angle that the combination of
bouyant forces and dynamic forces providing 3000 pounds of total lift. Note that the AoA changes for various deadrises.

As the speed goes from 15 to 50mph, the bouyant force effect diminishes as the hull is raising out of the water and dynamic forces, (forces developed on the hull due the hull accelerating the
water out of the way) dominate.

As the cross section of water moved diminishes as the boat rises higher in the water due to the fact that as you increase the speed of the boat, you also increase the acceleration of the water to get it out of the way, the lift force per square inch increases. Skin friction, viscous drag, increases as speed increases. I have been told by the national salesman salesman for Berkeley Jet, back in the very far past, that wetted surface drag is about 60 pounds per square foot at 60 miles per hour. (if anyone can provide a resource to confirm, I would be grateful)

Those who have ran fast boats will know that after you are in the 40 plus range for say a 21 foot boat, with an outboard, you can trim up the nose a couple of degrees, which reduces
wetted surface and increases speed without touching the throttle.

As above the lift force, the combination of bouyancy and dynamic forces is the same though the contributing factor of each changes. So I would not grant that the steeper angle provides more
overal lift. Certainly, the force per square inch would increase, Ie the stagnation pressure would increase, but if for this increase in the stagnation pressure increase, increased AoA, you
add a huge component of additional drag, what is the point. Or rather what is the goal. The vertical lift component of the force acting on the hull

I would assume if you leave out hull performance factors such as turn stability, pounding effects, etc, that you are trying to influence the specific boat hull shape to
either increase speed for a given horsepower or reduce fuel burn. I don't think that you have clearing defined this.

 

I'm not the guy who talks about bubbles. That's the OP. I believe he lifted many of my ideas from threads a few years ago and presented them as his own, since the combinations are just too similar and numerous as to be coincidence. I long ago deleted the threads, because I was pissed. But it's why I posted my model several years after THIS thread's original post date, to demonstrate who came first, and whose ideas about low-speed planing these are. And why I suspect he's too ashamed to participate in the thread he started, as he should be. It's not the first time. Really, I'm happy to share, but proper acknowledgment is not negotiable.

Anyway, please don't automatically equate the two of us. Not ALL of the ideas jakeeeef presented originate with me, just many of them. Chine fences are mine. Big air vents up to the deck, across the whole beam, are mine. "Vertical vanes" are mine. I used to think bubbles and air cushions were neat, but now I consider them to be just something to fill in a vacuum behind the step. There has to be something there, might as well be air.

Sorry I've sort of hijacked this thread, or at any rate seem to have started it up again. It's going kinda all over the place, but at least people seem to be engaged and participating. I think there are some good ideas folks are contributing, no? Let's just be mindful of ad hominem attacks, and this applies to me as well. Constructive criticism good, snideness and dismissiveness not helpful. If you think it's baloney, great, don't post, and read something else.

 

Very cool that you have some empirical data to go on. No substitute for experience.

On changing flow direction, I agree, and it's why I don't want any deadrise or taper in planform. Square-nosed, flat bottom, slab-sided scow, and I mean SQUARE. Kitchen table with sides. Cut-your finger corners. Ugly as sin. Phil Bolger was a big fan of square-nosed things, sharp chines, flat bottoms, etc. I think he was on to something, and not just ease of build. You turn water ONCE, along a single transverse stagnation line, where it provides direct downward lifting force, and that's it. Nothing else.

Of course, you have to consider conditions, and square and flat things tend to be quite unseaworthy in non-flatwater environments. Slamming like no one's business. Which brings up the consideration of suspended tri-or quad hulls to handle the bumps, which is where that tangent comes from, for those reading. They seem to work quite well, in the few examples I note somewhere above. I mean, REALLY well. Grenestedt is flabbergasted about how the entire world fails to see his video and not go to this geometry and suspension model for all planing boats, post haste. Can't say I disagree. MAN, his little yellow model goes like it's on rails, blasting right through waves and wakes. And if you look at his patent, sure enough--square-nosed, flat-bottomed, slab-sided scows for floats. Suspension gives you the luxury to do that.

Suspension is not required for my own project, which is the human-powered sprint speed record in a single run on flat water. But it's so NEAT, and Grenestedt's model is so compelling, I feel I must try it, should I ever get around to building. Doesn't even add more than a few pounds. Quite simple. Mountain bike rear shocks. Swingarms. Build U-joints out of PVC pipe.[/QUOTE]