Boat suspension

Why does one need big twisting stiffness? My thought is, make it as soggy as possible. Offer little or no resistance. Roll away. What's it hurt? No one says you have to keep the float flat. It can roll up 15, 30, 45 degrees. If you picked the float up, you could twist it whatever--45 degrees up, 45 degrees back, 45 degrees one side, then the other, all combinations. Just no yaw. Otherwise, like an ankle joint. There are three other points, spread out a loooong ways, to keep the vessel stable overall. What one joint is doing doesn't really matter, as long as it isn't digging in or sinking. It can bob up and down and rotate around as dictated by the buffeting of whatever bumpy water it's encountering locally.

Why double swingarms? Maybe I don't understand what you mean. If we're just talking separation in, um, planform, the proposal is 6061 aluminum channel, 2" wide base (web) separating 1" legs (flanges) on either side. Do the legs count as "double swingarms?" And the 2" web the "frame?" It's not much, but I contend it should be plenty to resist sag and twist given the loads it should be subjected to. I could use separate, deeper arms, maybe each a channel, and bolt on cross members to separate, or use rectangular tube. But do I need to? The thing is on the order of a bicycle, after all. Swingarms are like chainstays, and those weigh a few ounces each. Human-powered, pretty much straight line. No thrust to speak of, total sprung weight for the vessel under 260 lbs, including 220-lb pilot. Unsprung weight around 15 lbs (all four floats and their attachments). Light weight and simplicity is paramount. An ATV might weigh 700 lbs, this thing should weigh no more than 45. No steel. Mostly Coroplast monocoque for floats and fuselage, carbon fiber and/or fiberglass tubes for support booms. The overall boat is 11.5 ft wide and 13.5 ft LOA. Draw a box between the anchor points on the four floats, it's about ten feet long and nine feet wide. Fuselage 10 ft long, 2 ft wide. For a single-person boat, pretty long, outrageously wide, ultra-low power and slow (15-20 knots peak) and ultralight. If that helps conceptualize. More strength and separation would be appropriate for more weight and power, but I contend not needed (or wanted) here. I could use 1x3 channel, but it's twice the weight (and cost).

Again, I am not very adept at building, so anything complicated is not gonna happen, but there seems to be an advantage to simplicity in terms of promoting adoption, at least for a simple human-powered incarnation. And in all cases, the fewer joints you have exposed to the water, the better. And keeping the transverse support arm high and rigid gets it out of the water and lets the floats pivot and roll underneath it. A wishbone or parallelogram would get in the way of that. This keeps the support beam (rigid stub arms) high but keeps the fuselage slung relatively low.

Does one need to let the support arm pivot, like Nauti-craft? Why? Swingarm lets one float rise up and down with respect to the opposite float, and free roll relieves any torsion forces on float itself and on the vessel overall. I might have 6" travel at the ends of the arms. Water itself , and buoyancy, provides some "give" (and is all the "suspension" most boats have). The thing is super-floaty--4 floats add up to 1.5 times the footprint and rest buoyancy of a big SUP, around 1.5" draft. At rest, that is-- more like 1/2" draft on plane, assuming it planes.

What I'm most worried about is fore and aft loads on the shocks, with that three-point triangle setup. I'm assuming they have quite a lot, and that a clever linkage would align the loads more vertically, so they aren't transmitted to the sides of the shocks' pistons. I considered sliding pillar suspension at one point, but side loads there would be terrible, I'd think, plus they'd require long travel shocks if you hung them way outboard as I plan. Hence the swingarms.

Can you give me a quick sketch of what you're proposing with "double swingarms?" Or a better way to reduce fore-and aft loads, assuming they're there, on the shocks?

 

Sorry--by fore and aft loads, I mean up and down loads, I think. Or perhaps a bit of both?

Shocks (struts, really) are designed to be loaded on the ends, not to take loads transverse across the cylinder. I don't like that shallow angle on the shock. Could I just extend the"fin" where the top of the strut attaches, to get the strut more upright? Would that help? Is it needed? One could make that fin of something stiffer, like channel or rectangular tube, if needed, to resist twist. Right now it's just angle aluminum on either side, maybe 1"x1.5" or something. Could go, like, 1x3. Can also get shocks maybe 50% longer. I'll try to get on a real computer and make some sketches.

 

All this suspension seems complicated, why not use some form of SWATH, at least for large craft and for deep water- would be my humble question?

 

I didn't approach it as Human Powered Boat. I was thinking about a "normal" boat.
It changes a lot especially since you won't do dynamics turns.
Swingarm doesn't have to be so stiff. But bicycle swingarm has an extremely short axis of rotation, this is due to the limited space between the cranks.
My advice even when it comes to HPV is not a double but triangle swingarm. The swingarm axis bearings widely spaced. It is as much as a free boom protrudes.
Wider spaced - less friction and swingarm could be light. Forget bicycle lego. Make swingarm with carbon tubes .So we have wishbone pulled swingarm and wisbone pushed swingarm .
Suspension spring and damping : you can use bicycle lego , one remark , for example full air dampers are light but they have kind of lag caused by high pressure seals .
On the other hand, it is possible to regulate force of air spring. It can be priceless unless you want to spend $ 1000 on various springs. What kind of damper depend of kinematics and load. And I'm sure that you need different springs fore and aft,

Allright , swingarms part will be easy . What next ? What kind of joint should we use ?

My yesterday toughts are kind of huge rubber joint but you need e.g. 45deg pitch and roll no yaw. No-yaw was my biggest concern on motorboat design . One point of attachment make big yaw twisting stress.

So you need free cardan joint with some limits let say 45 deg.

 

I've Started this thread because I found first film in the thread as impossible. SWATH I remember this concept, it was in a small book about experimental boats. Are there a lot of boat of this type in the world?

 

Yes.
But they are more of a niche, owing to their raison d'etre.

 

Konnichiwa !
I have to read about these boats. They were not in my area of interest. From what I remember from a book from 30 years ago, resistance to waves, I think there was also a special port with pier between the floats .
Regards

 

Just to go back up to the original sketches--the overall vessel is a car. The four floats are its wheels. The attachment points are its hubs. Like wheels, the floats "roll" (rotate up and down), in this case mostly pivoting on top of a midships, or even slightly forward of midships, step. Unlike a car, I guess, the "tires" are very wide--each about 2 ft beam. I couldn't care less about "camber"--it's a boat. Boats don't need to make sharp turns, as a rule. Wide, gradual, sweeping turns are almost always fine. Refinements are readily doable, just not necessary on a first order.

Here, end-on, the "tires" might stay flat as the fuselage leans, or the fuselage may stay flat as the floats kick in towards it or out away from it due to encountering waves. Or in on one side, out on the other, the fuselage leaning a bit in or out, or for that matter somewhat nose-down or up. Who cares? Whatever it is, it's a lot better than what it was without suspension. And I'm not steering with the floats, so I have no yaw to deal with. They're "four rear wheels," steered by a rudder that a car has not got. Hulls generally go in the same direction forward, and can do some sidewise skittering if needed. It is, after all, water. Side loads in the absence of turning are minimal, seems to me. Waves one encounters do not move sideways, they propagate their up-and down oscillation outwards. The boat is actually quite wide and low, like a Lamborghini, for what that's worth.

This is all self-evident to me, but it's never safe to assume everyone sees the same video you see in your head. It is not a "problem." It is not a question of "could it work. " Of course it will work. If you think you can't cantilever against gentle side loads over that length (about 17"), look at, oh, say the sliding pillar suspension on a P-51, or any nose wheel on a light airplane. It will certainly work for a model and a light, slowish craft, the question is really just the loads on the struts, for me. And even if you go big--P-51s weighed about four tons. Passive suspension. Coils or gas. You're making this too hard.

The point is, water at speed is not water. It's asphalt. It doesn't give, much, due to its viscosity combined with your speed. This really isn't up for debate--anyone who has driven a fast boat knows it in his or her bones, with or without slide rule. The slovenly convention of using water as your "suspension"--displacing it using deadrise, cushioning with buouyancy--isn't gonna cut it, at speed. You need SUSPENSION. Suspension-suspension. Damped spring suspension. REAL suspension, not *******, draggy water suspension. Without it, you're rattling over a very bumpy road with no cushioning at all. You rattle your brains out, and if you hit a big enough bump and catch enough air, you backflip. Well.... Duh. If you don't cushion the impact with spring in your knees when you cross the wake on waterskis, you'll chip your teeth. Has no one waterskied?

Which is why I say--can we get ON with it, please? Is there some reason we insist on living in the dark ages? Suspension is not "fringe" or outre, it's utterly obvious, and it should have been done about 100 years ago. And could have, quite straightforwardly. And should be, starting today. It's just a car, guys. Think dune buggy.

 

The natural periods of motion, of a Swath, are, in general, far removed from their encounter period with waves.
So they remain largely unaffected by waves. In a nut shell.

 

It's been done, over 100 years ago.
The Sea Sled rode on a cushion of sea foam at speed.
Think of it as air suspension with drag reducing benefits.
No hardware, no camber or caster to set, no alignment concerns.
No moving parts, no added weight.
K.I.S.S.

OR

Put the suspension in the seats you sit on like ShockWave does.
I've ridden on them in an 870 Zodiac in rough seas.
They sure take the high G pounding out.

OR

Just slow down a bit until the ride is more comfortable.
No cost.

 

Actually it's quite simple. The reason for it is to try to get the advantages of (stepped) planing in terms of greatly reduced skin drag and wave-making resistance, while mitigating the huge downside of planing, namely slamming (pounding) in rough water. Or even ripply water, when you're going 200 mph.

I grew up skiing behind an old 15-ft Caravelle with an 80-hp merc outboard. Top speed 30 mph. Even at 25, even at 20, in any kind of chop or crossing wakes, its stupid triple cathedral hull design made it pound so hard it was terrifying, to a 10-year old kid. I thought the old hull would break apart. Deep Vs were much better, but took so much power it was out of our price range. Thinks I, "this is silly. There must be a better way."

SWATHs are great for just avoiding waves entirely, but their skin drag is horrendous. And it would seem one would need to fiddle with ballast for positive and negative buoyancy to adjust to load. And I'd expect they'd hit a wall at about 50 knots, without fancy things like air bubble injection. Torpedoes do. Subs do. Hydrofoils do. Sailrocket 2 did, until they got smart. I can explain why, but on this thread I'd probably get my head bitten off, since I apparently can't read books, or think without them.

Anyway, planing is just all kinds of nice and low-drag, with the potential for ridiculous speed, were it not for those pesky waves. You know, if it weren't for those pesky waves, things like ekranoplans (Airfish 8) and other ground effect aircraft and aerodynamically alleviated marine vehicles could possibly be viable, since they wouldn't jar fillings loose in takeoff, and takeoff and landing would be far less fraught than using hydrofoils.

Suspension is all about dealing with the pesky waves. It's about making planing, and thus efficient high speed water transport, and possibly air transport over water, closer to reality.

(My goofy boat doesn't need it at all, probably, it's just an approachable and cheap test platform. Suspension is NEAT. I WANT it!)

 

Sold! I LIKE it. Definitely feels less rickety. I own quite a few carbon tubes, and they are so light and strong they are not of this earth, but they're a pain to work with. What about staying with flat things? Or channel. I really like channel for this application, because it's strong and light(ish) and you can bend it. How about 1"x1", (1/8" thick is all you can get), cut away the legs (flanges) and bend the remaining web at whatever angle you want?

6065 aluminum, like for windows and such, is garbage, but 6061 has a nice radius bend and is quite strong and very affordable. Fiberglass lighter but not bendable and the legs are too short (9/16").

WAIT! If we go 1x1, onlinemetals has 1/16" (.062") thickness. Think that's enough? Could go with rectangular tube, but we're cutting and bending anyway, plus we are using smarts and geometry and not brute materials. I think 1/16" should work. You?

 

"Priceless" is right. I bought two really nice SunTour adjustable rear MTB gas shocks, on ridiculous sale, still $140 apiece or something, and actually not that light. But I have them. They're long, like 200mm or 220mm, I'd have to look. So there's your adjustable spring force for the rear. You're probably right there, but, you know, you CAN scoot your load (pilot) forward. Even start with the fuselage a bit nose down, i.e., more weight on the front floats. Propulsion is from oscillating vertical foils (tandem, opposed), which will be quite deep (say 2 feet). So the moment on the whole vessel is pretty strongly stern down.

Crappy spring shocks for the front are more like $40 apiece. Not adjustable, but actually much lighter, especially if you go with short ones (120mm?). Like, 12 ounces. If you use them to set the base spring force, then you only need the two on the rear to adjust proportionally.

 

I just don't see yaw as a big issue. Look at the drawings. The longitudinal main pin is in a bracket on the bottom. The transverse pin goes through the main pin. It you don't like that, do it the other way (I actually forgot which way I drew it.) Or use bigger "pins" (pipe/tube). Or use a (probably wood) block, and have pins/tubes of uniform diameter protruding out four ways. You can make the yoke and brackets as long and big and large diameter as you like, for strength and stability. It's not really a "point" connection, more like four bearing surfaces. I'm thinking about 4"x3" or something, but make them 6"x6", if your wells are big enough (mine aren't). Quite simple, really. Two brackets, one wider than the other, big pipe, little pipe, or block with pipe ends sticking out of it. Pipe can probably be just PVC, but you told me not to use PVC. ABS? Aluminum pipe with nylon bushings? Whatever, doable. My limit on complexity is, can I make it with a hacksaw and some glue and a hammer? And some screws/bolts? My table drill press is nice, and I have a vise, but that's about it.

No yaw. Can't yaw. Way the Cardan joint (U-joint) is designed. Play with it. Doesn't yaw. Now that we have triangle swingarms, yaw is a thing of the past. The floats should track straight, under all conditions. If yawing's going on, it's the whole boat that's yawing, which I think and hope is less of a problem. I mean, it can TRY to yaw, but we shall prevent it. Make your joint and brackets as wide as you like, within reason.

I wanted the brackets to be narrow enough that I had enough width in the wells to pull a pin out, for disassembly. Suppose I could pull the longitudinal pin out. Or just spring the jaws of one of the brackets. So, wide(and long) brackets, to make montero happy. .

 

To eleiminate yaw twisting friction is possible to move float attachment point forward to make something like positive caster angle .Caster wheel.