Torpedo hull

The video I linked to advises the vessel was factory complete in July 1995.

Rick W

 

I too am a computer engineering student and have been thinking of trying to construct a small computer controlled swath boat with automatic buoyancy compensators in the hulls. I’ve read a lot of other threads and was thinking of pm-ing BMcF to see if he would give me advice (thanks for your other post about swaths, very interesting), but I saw this thread and decided maybe it would be more useful to post it here. I’m sure I could handle the controls part but had some questions regarding the hull / design itself.

My dream goal would be to build a very small long range ocean capable boat, I’m thinking slightly smaller than 10 meters. I want to make it as tough as possible for the size, basically a nutshell. How would such a small swath behave? From what I’ve read I believe it would provide a stable ride up to moderate seas, but how would it handle a storm or other undesirable situation? I’ve read other post stating they can’t be made self-righting. It seems as if maybe swaths perform very stable until they are overtaken by waves to the point of capsizing? Any advice/links appreciated, there is very little info I can find about this topic!

 

The smallest manned SWATH I ever 'played' with was built by SOS and had one of Doc Higdon's last analog computer stabilization packages installed. It was 34' LOA if I recall correctly, and served as a camera platofrom for ESPN during the America's Cup back in '87 (?..is memory serves).

The exceedingly tender nature of the SWATH is really evident when dealing with such a small version...getting people on and off at the dock required some coordination and she also ballasted down considerably to increase the stability at rest.

Underway..rock stable and very sea-capable little craft.

Fully stabilized SWATH vessels share a problematic trait with stabilized hydrofoil craft; their remarkable seakeeping performance can (and have) lead an operator in to trouble because there is nothing to indicate a worsening of sea conditions in a progressive or gradual way. You can operate yourself right in to serious and immediate trouble. The 'limit sea state' appears suddenly as a brick wall.

Including the sensors and related control algorithms to support a 'wave contouring' mode of operation is done for some. However, there is a 'grey area' between sea conditions where platforming control is both desirable and possible and the high sea conditions where 'contouring', or following the local wave slope, is the only means of survival.

 

Thanks. So do you personally think there is a better, more seaworthy hull for the size? Is my idea practical at all? I plan on using dc motors, one in each hull, perhaps the batteries and fuel/water as well. Rough estimate of 8 meters long with .5 meter diameter subs, fiberglass. I'm thinking of just building 1 submarine and see how that goes. Rather than the double cat like design mine would be more 2 cylinders with some buoyant arm holding them to the main hull.

 

The only merit would be ride quality. It will have a lot of wetted surface so a lot of drag.

I practical terms it will need a couple of mtres of water to operate so you will not be able to get close to shore although nothing different to a deep keel yacht. How do you get to a motor or wiring if something fails while at sea?

Rick W

 

BMcF

Yes well noted. That is often the problem with SWATHs, knowing when rough is rough! I was on sea trails of a SWATH many years ago, we were out in a hurricane, over 100knots of wind, the Ariel eventually blew off it was that bad! Only problem we had was turning owing to the high windage. From memory we heeled about 22 degrees during turns in those winds, other than that though, she was smooth as a babies bottom!

In answering the original question, the speed of a pure SWATH is related to the Monk Effect, excessive pitching, hence why SWATHs, pure SWATHs are stabilised for pitch.

My colleague designed currently the worlds fastest SWATH, some called it a MWATH, owing to its slightly greater WPA. We have designed/developed a 40knot waterjet SWATH several years ago, just waiting for a client to say "I want one", original client back out last minute, typical!..trouble is powered by GTs so very very expensive and limited application!

 

It seems to be rather difficult to get a small SWATH form to work well. Between the waves-over-the-deck issue and the weight shifts of crew moving around, I think there's a good reason why we don't see many SWATH ships under 15-20 metres or so.

The Canadian Coast Guard has some for hydrographic survey: http://www.ccg-gcc.gc.ca/eng/Fleet/Vessels?id=1095 Reported to be very, very stable and excellent platforms to work from. (I haven't seen one in action yet, but I've come across a few videos- they are almost completely unaffected by waves.) But note that they're only intended for coastal use in ice-free conditions. They run for shore if things get nasty.

I share Ricks concerns about draught and access to the hulls in an 8-metre-ish SWATH. By the time you get enough waterplane to be able to even get one arm down there, you're already almost a conventional catamaran.

 

Marshmat

The smallest SWATH my colleague and I have designed has been 21m LWL, but there is the small is too small aspect when considering a small SWATH. The main being as you pointed out movement of personnel would mean a very active ballast system being required which adds weight, so the tubes become larger, then deeper draft and so on and so forth...so there is law of diminishing returns when considering a small swath.

Not sure what you mean about a 'conventional catamaran' with respect to waterplane area, when "getting one arm down"...

 

Thinking in terms of, in an 8-metre boat, for the struts to be narrow enough that it behaves like a SWATH instead of like a normal surface-floating catamaran, you wouldn't be able to get to anything in the pods. If you make the struts wide enough to get more than one hand in there, it wouldn't be a SWATH anymore.

 

If it is just 8m, then why would it matter if you can't get easy access via the struts?...since 'just an arm' is hardly sufficient to perform proper maintenance on any equipment below in the tubes!

This is what design and being a client is all about. Accepting compromises. A SWATH a true SWATH is the ideal choice for seakeeping, but for speed, or cost etc, it is not. Next time you buy a car, is it for its speed, its weight or its functions or its price?!

Everything in design comes at "a price"..whether one is willing to accept the compromise or not, allows the design to be realised. Horses for courses. Focusing on one aspect of a complete design to say it is flawed is not 'designing'!

We can of course scrutinise and criticize an 8m design SWATH, but if that is what the client wants and is prepared to accept the compromises, then who are we to argue?

 

sorry forgot to add

what is your definition of a SWATH then, if you feel allowing a hand to pass between the struts renders the design "something else"..?

 

Thanks for all the input.

As far as maintenance, if something fails inside one of the subs then I would not try to repair it while at sea. While not ideal, the boat should be able to get by with just one dc motor. All the computer controls, wiring, etc will convene in the main hull. The only thing that could fail in the sub is the motor itself, or structural failure. My idea right now is the sub will be completely encased except for an opening for wiring/tubing in the middle and an end cap where some type of tray can slide out which holds all the equipment.

As far as the depth, that shouldn’t be an issue. The buoyancy could just be increased where it basically becomes a pontoon boat. This situation wouldn't be ideal either, but I don’t see it being a problem temporarily if it needs to pass shallow water.

My main concern or thought is still: if it is possible to build this design, how well could it handle a storm. Again, my main design goal is to make the toughest small boat possible. Everyone says this is the best design for seaworthiness, which makes sense in semi-rough conditions, but if the boat is overtaken by one bad wave I don’t see how it can recover. This would be true too for larger vessels also though so perhaps that isn’t as big of a problem as I foresee it to be? It just seems the natural tendency of the swath would be to flip over and stay there.

 

Small
Long Range
Ocean Capable
SWATH

Choose any 3, but you are never going to get all 4. As has been well pointed out in some of the above posts, SWATHS are geometry and weight critical in respect to thier hydrostatic and hydrodynamic properities. Get too far off the necessary relationships of length, seperation, buoyancy, waterplane, variable load, and fineness and you will have a poor performing vessel. FWIW, SWATHs basicaly fall into 3 LOA sizes, a poor performing 15-17m range (some that are called SWATHs in this size are really cats), a 25-30m size, and a 60-70m size. These sizes are not arbratary, they are based upon construction, strength, maintaince, and envrionmental constraints. Additionaly larger SWATHs beneifit from the Law of Cubes, which normally hamper fast vessels. One additional note, because a SWATH depends on it's geometry for it's performance, fuel consumption, tankage, and ballasting is a primary design driven.

Actually, capsize is the least of your worries. SWATHs are very inefficient structurally and a balanced SWATH design will most likely fail structurally or take a plunging wave before it can be hydrodynamicly loaded sufficiently to cause an actual capsize or the more likely pitchpole. You must understand that the interaction between static and dynamic response in a seaway of a proper SWATH is very different than a normal vessel.

In short (~1.5 LOA or less) waves a SWATH "platforms" the waves, i.e. it rides over them without heave using it's large mass to WP to prevent motion. In this mode, there is no need for active control and the control fins are zeroed to provide damping. This is very different compared to monohulls or catamarans which can only platform waves much smaller than LOA.

In long (2 LOA or more) waves on the other hand, a SWATH has be forced to "profile" the waves using active control. This is because the vessel is slow to rise to the wave due to the high mass inertia in pitch compared to the small WP. Without active pitch control to force response, it would be impossible to build sufficient crossdeck clearence

Because of the realities of sea spectra, open ocean SWATHs need crossdeck clearences of 2-4+m. Smaller SWATHs cannot acheive these clearences within weight and strength considerations and conesquently have much poorer performance.