Hydrofoils/SWATH question

In the calm water picture, she was displacing 8.2 metric tons. (13.1m LOA)

And whilst digging out that bit of information from old test reports, I double-checked the power number and during that 32.1 kt run she was actually using 408 HP (304 kw) of the 500HP available from the pair of 250s.

Design operating point (the craft was a 1/4-scale model) was 25 knots and at that speed the power was around the 300 HP level.

We never ran her much above that 32 knots, even though we still had another 100HP of "overhead" on the engines, because it required to much additional ballast to allow her to run that fast; the exhaust for the inboard auxiliary engine would be submerged when at rest and that was what limited total displacement.

 

Pretty respectable figures for an 8-tonner at 25 knots.

 

Yup. And the seakeeping ability of the craft was out of this world. Motions in 1.5m seas were essentially nothing..you could leave an open beverage container sitting on the consoles and it would not spill a drop..


The full-scale vessel was intended to be a passenger ferry, of course, but that market had just about dried up when the design and testing was completed over a decade ago. The Navy showed some interest in the concept as a highly stable unmanned surface vessel....but then went off a couple years later and paid someone else to design the same thing from scratch.

 

Yes of course.
But not important (?) at high speed I suppose. But at low speed, and at the mooring, with all the algees, barnacle and other nuisances, it is an enormous area compare to a simple hull. Just calculate the area of all the components, under and upper side, front side, lateral side both way and then you will understand what I mean.
I am a practical man, long term cost effective maintenance and long term efficiency is important in my view. After six month, I want the same efficiency than when launched, and not to have to go on the hard to clean the mess.
But perhaps they have good paint now, and yes at high speed the area can be cleaned by them self.
So it is a question for me to have an honest answer about the real performance all around of such a vessel, with such very complex appendage. If the goal is to have just one aspect right, leaving all the other aside, perhaps it is a success, but can be a very narrow utilization.
In my view I see a lot of trouble manning a ship like that for a long period.
How many fins and pods and horizontal fins will be necessary in the next one? I don't really see the point.
The Swath make sense du to her seaworthy capability.
http://www.youtube.com/watch?v=qNwGlp_1560

 

Like any hydrofoil, it is only designed to operate with the hulls clear, so the surface area is quite low; and of course that is one of the reasons the craft was remarkably efficient for its relatively high speeds of operation.

As for bottom maintenance, we found it to be no better nor worse to deal with than many other foil-assisted or foil-supported advanced high-speed marine craft we've dealt with over the last 30 years.

 

Exactly. Like nearly all advanced marine vehicle designs. This one was optimized for the fast passenger/car ferry role.

 

Make sense. Thanks

 

Ok, now it makes sense...the first picture made her look bigger than she was. Now that I see what you are doing, dskira is right, there are a lot of ways to clean that up. But, as you say, she is a one trick pony because she would be so sensitive to longitudinal weight shift. Did you have a ballast system to compensate or was it all done by lift? Otherwise was it difficult to get her up flying between full load and burned out?

It is interesting to note that even with the demi-hulls/lift fin her displacement length ratio was only 107...so without them would she have been faster (D/L of 70-80?)?

 

It also used active stabilization, or am I not remembering correctly?

 

Demi-hulls? You mean the amas/sidehulls? Those are hydrodynamically irrelevant, of course. They simply provide upright stability when the craft is not flying. Or were you referring to the bouyant lifting bodies with the wing between?

As a "one trick pony" that pony exceeded the propulsive efficiency and seakeeping abilities of a wide array of passenger ferry catamarans of similar capacity. Propulsion performance was roughly equivalent to that of a well-designed surface effect ship ferry whilst the motions were nearly as good as a well-designed and stabilized SWATH. Or put the two together and the craft nearly equaled the performance of the Boeing jetfoil types without the price tag.


The craft used s distributed fuel tankage system and carried some water ballast as well. We did keep it "trimmed" that way over the longer term, but having full control of lift via the flight control system operating the main and aft foils gave us a tremendous degree of real-time trim control.

 

It could not fly without it. And was a full flight control system..dynamic control of pitch, roll, heave and yaw.

 

Hi BMcF

Are you going to be able to share any info about your 'high-speed SWATH' project, or is it hush-hush?

 

Buoyant lifting bodies then. With them you didn't need to lift the total weight, looks like only about 70% of it. As said, there may be ways to reduce the interference drag and increase the volume without decreasing lift or increasing wetted surface <shrug> Which fits into the second comment.

"Similar" as in how? Length, "size", or capacity? Those are very different things. Same with "performance", what standard. FWIW Boeing jetfoils ferry performance was 1.125 to 1.40 passenger-nm/hp-hr (i.e. 250 psngr@45 knts to 350 psngr@40 @ 2x 5000 hp installed) . To match the lower performance standard at 32.1 knts and 500 hp installed you would need to carry 18 passengers (wt 1.4 LT)...on a 40 footer displacing 8.2 LT...something that is doable. But as you said, there is really no market. Even if your capacity was 25% lightship, 2 LT is not that much...25 passengers or one hardpoint with sensors.

 

The company that developed the craft is starting to push some information out, but I'm not at liberty to do that myself. Look for some more interesting results to become public in the July/August time frame. I know they are gearing up to attend and even present at some marine technology conferences and symposia this year.

 

I already stated the testcraft was a scale model. I assumed my audience knew what "similar' means in that context.

The foil-assisted tri was intended to compete with other passenger ferries in the 35-56m range and a range of design variants was developed accordingly. The testcraft was specifically designed as a 1/4-scale model of the 56m full-scale version but, of course, was a scale model for any other variant as well.

I headed the design team that developed the most efficient SES passenger ferry built; the 40m 400 pax SEMO SES. A 400 pax designvariant of the foil-assisted tri compared very favorably in terms of fuel economy. I ran the company that designed, built, installed and tested all the stabilization packages for the SWATHS built by SWATH Ocean Systems, San Diego ( and numerous other SWATH around the world) and the motions of the foil-assisted trimaran were nearly as good as the best of those, on roughly an equal payload basis.

And so on....


A clarification or disclaimer, by the way: My company did NOT invent the foil-assisted trimaran concept nor did we design that hybrid lifting body package that later replaced the original main lifting foil. We designed and built both to our client's specifications and then made it all fly. So when I make comments about the performance of the craft and how it compares with other types I've helped build and test over the years, its actually quite an unbiased opinion I offer. We've been involved in projects where the result was a total pig too.