s.w.a.t.h

hello all, like to hear the forums opinion on "swath" boatdesigns.

construction, draft, space, parking etc. may be a story apart and maybe they only excell in specific tasks but i find them intriging and expect to see more of these designs in the near future.

as i understand it "swath" can be any hullnumber, even wonder if it includes surface submarines.

Small-Waterplane-Area Tri-Hull or "SWATH" at
http://www.swath.com and at [URL]http://www.*****************/BugattiYachts/Advanced_Hulls.htm[/URL] [URL]http://www.*****************/images/SWATH_w_Outriggers_sm.gif[/URL] [URL]http://www.*****************/images/Copy_of_sw1.jpg[/URL] [URL]http://www.*****************/images/Copy_of_sw3.jpg[/URL]

 

I remember seeing this one quite a few years ago. Probably not a bad trip.

http://www.nektoncruises.com/Pilot/pilot_closeup_1.htm

 

yipster

SWATH is Small Waterplane Area Twin Hull.

They are quite common these days and are usually ocean research or ferries. Because of the small waterplane they do not carry much deadweght (cargo, oil, water. people etc) but they are a stable ride and have good propulsion characteristics but they do not plane.

Michael

 

a 12 meter recreational swath at http://www.swacat.com/swacat42.htm looking good.

ben

 

The purpose of the small waterplane area is to reduce the heave stiffness of the craft. With a small waterplane area, there is little change in buoyancy with a change in immersion, and thus little excitation of the vertical motion as the craft goes through waves. SWATH ships make it possible to have a response in a seaway that is like a far larger ship. The natural frequency of the response is much lower and the response to encounter frequencies above the resonant frequency is much smaller that a conventional craft of similar size. In this respect, they are similar to hydrofoils. Since the wave encounter frequency increases with speed, SWATH hullforms are good for high speed because they become less sensitive to chop.

The small waterplane area also means that the craft will experience a large change in immersion with a comparatively modest change in displacement. This makes them sensitive to changes in loading, including pitch and roll trim. It also means the craft is not well suited for resisting large steady heeling or pitching moments, which is why you don't see SWATH hulls used for sailing catamarans. Although they are insensitive to wave encounter frequencies above their resonant frequency, they can have poor damping near resonance. So that's why you often see hydrofoil stabilizers added to them. A feedback control system can sense the vertical motion and use the hydrofoils to damp it out.

SWATH vessels can be subject to unusual loads and thus demand careful engineering of the structure. The Monterey Bay Aquarium Institute operates a SWATH research vessel, the Western Flyer (http://www.glosten.com/projects_research.htm). When she entered service in 1997, she started to develop structural cracks (http://www.glosten.com/projects_research.htm). These were due to the waves spreading the hulls apart and squeezing them together as the ship straddled wave crests and troughs, combined with a notch in the hull contour near the junction of the upper hull and strut. The notch was intended to deflect spray, but served as a stress-riser leading to early fatique cracking in the aluminum structure. This necessitated an extensive refit, based on sophisticated analysis of the SWATH's dynamics and loads performed by Glosten Associates, a Seattle naval architecture firm.

So the SWATH configuration is a good design option for craft whose mission requires fast, stable performance under rough conditions; whose payload is relatively constant; and a large deck area compared to the displacement.

Cheers,

 

maybe i should have left this tread with the S.W.A.T.H. reply writing(s)... they explain all and more, and thanks for that.

i do very well understand the S.W.A.T.H. design principle to get out of froude's wave law, did read pitch and roll, just wanted to comprehend that buoyancy myself. reason for playing with glass piping now is probably because of reading to many PM magazines while only having a modest portion on science education and still want to know how things work. in this simple experiment a mono hull S.W.A.T.H. is easy; just need weight and acts like having a keel.

with twin hulls it only worked with hollow legs and remained a bit of a mindbender, bigger more buoyant hollow struts to the submerged twin hulls was the trick. i've been thinking of oil rigs and life raft bags, they all seem to use parts of the same idea.

i'm learning, a true S.W.A.T.H. design however remains atractive and intriguing, and by using foils to damp out its buoyancy it may well be -in its own way- as sophisticated as a hydrofoil. guess i'm now saying the same things in other words

thanks again, greetings, yipster

 

S.W.A...

think finally all my lights got on, S.W.A... is a jack of many trades and various other configurations can be imagined and work just as well. great design, love it.

yip

 

think i should have said jack of many trade(off)s, but i like..
so its back to WSA+ to learn more. (and see how green i am/was)
like to figer out the tekst "By way of background, the total resistance of a twin-hull SWATH ship at moderately high speeds is roughly 50% frictional and 50% wavemaking + form drag"
on http://www.swath.com so its into spreadsheet formulas but cant withhold just one late quik rough sketch here ok? spray?

yipster

 

It's not 100% clear to me just what you're trying to accomplish with this design. What are the mission requirements?

I'd ditch the bulbs on the amas (outriggers) and give the ama a narrow V cross section. The reason is that in flat water with the boat balanced, you don't want to have much ama in the water so as to minimize the wetted area and wave drag. But as the boat heels you want to rapidly pick up volume to prevent capsize. The ama's you've sketched do exactly the opposite - lots of wetted area at zero heel and almost no change in volume as the vessel rolls until the high-side bulb starts to come out of the water.

Were you aiming at a very nonlinear type of roll response that was based on the boat being stabilized by the ama bulb being exposed?

If you're going to stabilize the boat in roll by making it a trimaran, then you might want to rethink the hullform for the main hull. It looks to me like you still have a lot of waterplane area in the middle of the main hull, and this is the major source of drag in a seaway. A fineness ratio of 8:1 is fairly chunky by multihull standards, and 10:1 is the minimum most designers shoot for. And you can go with very round section shapes for minimum wetted area.

This might lead to a blending of the HYSWAS and displacement hullforms, with the rounded bottom half of the bulb being carried full length. At the midship section the hull might rise straight up and flare out above the waterline without any dumbbell-like shape to it. At the bow, the waterplane would come in as you've shown, forming a bulb. At the stern, the shape could be something like the bow, forming a bustle.

 

Tom,

Thanks for the advice, yes your rite, as you keenly noticed, in this drawing the main hull is still a little in the water. and isn't non-linear anymore as intended because I tried to incorporate a efficient single power plant, kept thinking of floorspace in a centre hull not only by being used so (in a tri the floorspace is much smaller anyway) it also keeps the deckhouse low.

Maybe have to reset these thinkings.

Mission is learning something on (experimental) boatdesign and hopefully building a 14m. liveaboard capable of cruising wider coastal mediterian waters economical at moderate (about 25km?) speed without doubling production cost, a lot to ask. Got impressed with the swath design, looked up the really sophisticated hyswath's you mentiont (here a tank pic but also saw a hyswash waterbike) and am drawing new sketches with the bulb carried almost full length, hull on keel(s), ama's don't know yet how... non-linear can also be with that narrow V cross section more imerged..

I better first study and check WSA, power, drag.. than make a new drawing, dynamic model etc. boating is / becomes a science! I still like it.

HAPPY NEW YEAR

yipster

 

I like the looks of the upper concept. It has the narrow amas I was talking about. If you carry significant amounts of buoyancy in the ama's, you'll pay a wetted area penalty for it due to the cube-square law (the larger hull can carry the displacement with less wetted area simply because it is larger). If you want the amas to carry part of the load, they should be as long as the main hull, turning it into a true trimaran, because multiple slender hulls have less wave drag than a single fat hull of the same length. Carrying the load with multiple short, fat hulls is the worst possible way to do it for a displacement craft.

You might judge your designs through a figure of merit like taking the square root of the quantity length times displacement and dividing the result by the total wetted area. That would give you a rapid, rough ranking of your designs. Michlet (http://www.maths.adelaide.edu.au/Applied/llazausk/software/michlet/michlet.htm) would do a good job of computing the hydrodynamic drag of these hullforms, including the interference between ama and main hull.

The middle concept looks to fat to my eye. I think you ought to aim for a minimum length/beam ratio of 10. The other thing that strikes me about this concept is how draggy the house is. A little rounding of the corners and smoothing of the contours will dramatically reduce the windage without sacrificing much internal volume or useable deck area. According to Hoerner's "Fluid Dynamic Drag", a sharp rectangular shape can have its drag cut by 80% with a corner radius that is only 20% of the height of the body. That still leaves 60% of the flat side untouched.

Take a look at http://steamradio.com/JSYD/Dogstar50-article.html for the state-of-the-art in sailing catamaran design. Many of the same principles would apply to your SWATH or HYSWAS. For example, if you look at his drag table and subtract the drag of the sail rig and keel, the aerodynamic drag of the hull is still greater than the hydrodynamic drag.

Take a look at his elevation profile: http://steamradio.com/JSYD/Images/Tektron50-photo.jpg. Notice how he's paid just as much attention to how the topsides close up behind as he does to the forward contours. This maintains smooth, attached flow to a much smaller base area than designs that are simply chopped off behind. The base drag is a huge contributor to windage. Most powerboat designers don't seem to pay much attention to this - their topsides look like wedges.

Imagine an exaggerated version of Shuttleworth's midship section (http://steamradio.com/JSYD/Images/Fig2-T50-and-D50-hull.jpg), adapted to the HYSWAS configuration. The in-water portion would be extended downward by adding a vertical segment to the sidewalls. Toward the bow, the bottom would blend into the bulb with a fine cut-water above. At the stern, the section would sweep up, forming a smooth run aft, with or without a bustle. The flare in the topsides would be higher to allow for the wave crests and extend outward to form the bridge deck connecting to the amas.

With regard to the bottom picture, notice the electrical cables running to the model. I suspect the model was actively stabilized with sensors measuring the hull motion and actuators moving the hydrofoils. The cables would supply power to the actuators and carry the feedback signals to a control computer located off the model. I wouldn't assume that the model was inherently stable without the computer control.

 

Tom

Most interesting info! Meanwhile i found Tony Grainger’s HYSWATH with intricate design details that also made me again realise the importance of even fractionally reducing WSA. Input and software required to elevate this hybrid are all still imaginable but leave specifically the high tech pure hydrofoil at the bow vulnerable.

For a stabilised real world SWATH your earlier remarks make more sense to me. They were refreshing and inspiring, still there is so much to learn on the various constellations. So be prepared, my fantasy is ignited, like to come back later with a pencil drawing for computer control...

At http://boatdesign.net/forums/showthread.php?s=&threadid=617&highlight=Grainger were I found this HYSWATH design (thanks to Jeff) I was surprised to read about PM magazines also, and Jules Verne, since partly the reason for starting this tread was based on reading about submarines. Most subs are much faster –even on battery’s- when submerged. A now hard to find back nautilus site, mentiont it was Jules Verne who thought up a water density unit that is still used today. I am not joking or trying to ridicule the necessary math’s, they are hard enough (with some equations even way over my head)

I learned some things here, many thanks.
yipster

 

resistance in SWATH

Dear mike
I am Msc student of naval architect in Iran
My master project is about" calculation resistance in SWATH"
can you help me about this proposal with sending pdf files or any data.
Thank a lot
best regards

 

SWATH resistance

Dear mike
I am Msc student of naval architect in Iran
My master project is about" calculation resistance in SWATH"
can you help me about this proposal with sending pdf files or any data.
Thank a lot
best regards

 

A SWATH on flat water?

A SWATH design has a large number of permutations and how it is configured would depend on the use it is put to. The bottom line it seems to me is efficiency. A small waterline means less energy lost to turbulence and wave-making at the water-air-hull interface. The hull(s) are really submarines with an above water passenger pod piggy-backing on them. As coupled submarines they should suggest a lot of interesting ideas. Like floodable ballast tanks for load compensation and fine tuning draft. Deep ocean SWATH vessels must have deep draft because otherwise wave action may expose the otherwise submerged hulls. However SWATH for relatively flat waters would make sense because of the efficiency, which translates to fuel use and speed. I am considering a SWATH design for my Solar Electric Protected Waters Cruiser which I have started another thread on. What if you want a stabile, efficient, platform with a 110 square meters of living space and are content to make 5 knots for 50 km a day? This is what I hope to achieve to cruise the inland waterways of Europe.

Aloha,

Jonathan