boat design - art or science?

Thanks, Bruce, you've jogged distant memories regarding some
anomolous results for finite depth I saw many years ago. (I
forgot all that at 6am when I posted.)

I've attached two graphs that show different results for a
L/B=2 rectangular pressure distribution.

The scanned plot is from:
Barratt, M.J., "The wave drag of a hovercraft",
J. Fluid Mech, 1965, vol. 22, part 1, pp. 39-47.
It was also reproduced in:
Doctors, L.J.,
"On the use of pressure distributions to model the
hydrodynamics of air-cushion vehicles and surface effect
ships", Naval Engineers Journal, March 1993.

The other is my calculation for the same pressure distribution
and plotted on the same log y-scale as used by Barratt. One
shows the result you want, (i.e. lower drag in shallow water),
the other doesn't. I'm not sure if the differences were ever
resolved. Maybe some good experiments could help, but they'd
have to be conducted in a very long, very wide basin.
Leo.

 

Yes, but as you must realise - its soo much fun to do so, especially when they dont realise you are doing so....

 

Err..it's Bill..not Bruce. Bill McFann..

The shallow-water effects are well-known to operators, but not well-characterized in the 'science'.. The USCG guys used to 'cheat' the shallow water in the Carribean with their BH110 SES patrol craft to gain speed; during builder's trials, we used to routinely run the 50 knot SEMO 40m across a known shallow bottom area to gain as much as 4 knots on the top end when 'showing off'; I used to bring my SH-2 5-pax hovercraft in close to shore and shallow water to gain a 'lift' and get the thing back over hump and accelerate faster..etc. My 'best guess' was that the phenomenon had to do with the bottom, in shallow water, restricting the formation of the cushion wave depression as deeply as it is in deeper water.

 

Sorry, Bill, I don't know where I got "Bruce" from. It's
worse with your offsider, Loheed. Every time I see his name
in my email box I whacky-parse it as Lockheed and imagine
offers in the millions.

The differences in the results for shallow water might just
be a failing of linear theory. Perhaps lubrication theory
is more appropriate in some cases, or perhaps non-linear terms
are required. But I take your point about shallowness tending
to suppress the formation of a large depression. Maybe that's
what I need to prevent the 36m hole under our "optimum"
pressure distribution. I'll see what Ernie Tuck has to say on
the matter when I see him next.

Modelling in shallow water (and towing tanks) is further
complicated by the formation of a bottom boundary layer,
but that would normally increase drag.

I have long suspected that builders use tricks during trials
to increase speeds by a few knots. In modelling the
performance of an Australian catamaran I found that the
claimed top speed of 52 knots seemed achievable, (and was
achieved in practice), but only in the empty condition, and
only because in that condition dynamic sinkage and trim let it.
But with a fairly small load in my model, the cat just didn't
seem able to get into the right attitude to reach 52 knots.
Maybe it is Ok to describe it as a 52kt cat. Or maybe it's like
60 Minute Drycleaners - it's just the name of the shop. It
doesn't really mean that your clothes are ready in an hour.

Leo.

 

Rick's last name used to be Lockheed..but he changed it so he would stop getting all the junk mail queries asking for large donations of cash. (kidding)

'Tricks' to inflate speed numbers?? I have no clue what you are talking about. Although I must admit found it a bit odd that we achieved speeds in whisper-calm fjords in Norway that were 'seldom seen' ever again after delivery. Probably something to do with the wind shear off the sides of the fjord cliffs..or maybe the water runs downhill a bit as you are headed toward the North Sea. One thing is certain..it wasn't shallow-wter effects in that case. But runnign speed trials in amongst the archipelago of islands off the southern tip of Korea..whole different story.

 

And then again it might have been. Depends on what lane you were driving in.

 

Off colour - rude and insulting.

To DanishBagger:

Glad to see you are now in agreement with me.
" .....people are worth more than a plane in the end."
As you say; people are less disposable than a plane is.

However; we all trying to grasp how you intend to tie:
kilts, fire brigades, American women, card board houses
and a small solar panel - into the original question ?
i.e.
".....art or science ? "

- To the Moderator:

When on these Forums does:
".....just kidding with you."
- Go over the line to; off - colour, rude and insulting ?
- To off - topic ?

With Regards.

 

Military planes/ships/subs are certainly not designed to be disposable. They are designed to the very highest standards and place an enormous emphasis on protection and survival. If commercial ships were designed, built, operated and maintained to the same standards as naval ones there would be far fewer accidents and lost ships (but the would be prohibitlvely expensive...).

Low Safety Factors do not mean taking greater risk with the deign. They imply a better understanding of the loads, material properties etc, so that the designer has more confidence in the precision of his sums. Aircraft loads are more predicitable and more research has been undertaken in aerospace than yacht design, so aerospace safety factors are lower (which they need to be).

 

Tri-star,

If you can't take a good-humoured jab, then I'm sorry.

The kilts, women and so forth tie in with the topic, just as much as your own remark that fighter jets and military personel are disposable. If you actually read what I wrote, then it might be a bit clearer, dear.

Again, I'm sorry that you feel the need to cry "moderator" because I, from the first remark simply disagreed with you that people and fighter jets were "disposable". But then again, I see you're out of reach, and that you will not accept being countered. Not to mention that you think you can speak on behalf on everyone (i.e. "we all trying to grasp").

Don't worry, you'll be ignored by me from now on.

Edit: I am certainly not in agreement with you. You're twisting my words, in an effort to make it look like you're right. It's a strawman. (and before you ask "how strawmen tie into the discussion", look it up.

 

SES yacht

A true SES' (not air-lubricated Burg or Harley stuff) have all suffered from an "almost usable deck 5' to 7' deep, of cross structure under the first deck, this results in a very tall/large frontal area compared to a conventional hull/superstructure. In the speed range everyone agrees a SES should shine >50knts, it is the massive aerodynamic drag that is the great majority killer of a common SES's efficiency. The speed range demands a sleek profile which is also absent from previous designs.

In my new design I applied a wave parting centerline keel to reduce that extra structural deck height and cut the total aerodynamic drag. True, the center hull will add some drag in a seaway but the preponderance of it remains out of the water. The side hull drag is also reduced 80% due to the short sponsons, which still lend the stability and directional assist that remains an advantage over full hovercraft.

Your insight and educated comments regarding this concept are most appreciated see the basic design shape at: http://www.industrialobject.com/50pb.html

 

That's a nice looking boat, but your central hull is just a
variation on a theme used for many years by Incat. Whether
you can get away with a lighter cross-structure really depends
on the mission. A large cross-structure might be unavoidable
if SES are required to carry very dense loads such as army
tanks or some presidents.

Your point regarding air drag is certainly valid.

Here are a few numbers to consider (when you get past the
dazzling artistry of my drawings).

My example hybrid vessel has similar proportions to a
260t Skjold SES. As you can see it has a fairly large
frontal area.

Drag components (non-dimensionalised by the weight) are
shown for an on-cushion condition with 80% of the weight
supported by the cushion. I had to estimate the skirt clearance
height hskirt and a few other air cushion parameters, but they
shouldn't be out by too much.

I've used an air drag coefficient of Cdair=0.4 in the drag
calculations which is probably a bit low. As you said, above
about 50 knots air drag is significant, more so if a larger
value of Cdair was appropriate.

I'm still not sure if your idea of tucking some above-water
volume under the wet deck will reduce the drag by very much.
I'm also not sure how strong and heavy the central hull must be
to withstand the wave loads it might be subject to. Will the
central portion weigh the same as the material you save by
reducing the height of the superstructure? Or will it be
heavier?

Also, what do you mean by "The side hull drag is also reduced
80% due to the short sponsons"?

Good luck!
Leo.

 

seapart SES

Yes the central hull is like the Ncat, but when on cushion only the aft end of this hull skims the surface with allocation for the propulsion units. Structural weight as a function of pressure loads calculated by DnV rules are not really driven by speed. Off cushion slam loads are the pressure driver, so the weight is similar as if flat, just increased in depth at center minimized on ends like a good girder span should be.

Side hull wetted surface is going to be cut significantly compared to a standard SES, these sponsons are shorter.

As a yacht, this vessel is not "demand" operated, they can choose to avoid the roughest water, where the centerline hull form would be most wet, however it would still be less drag/hull weight overall than a wave piercer.

I have been intimately involved with the LCAC, Bell Halter 110 and 70', Destriero design and a dozen other exotic hull forms real and imagined, so it is a compromise as they all are, but one geared for a earth ranging yacht. Efficient high density loads will always be in the domain of a sea plow.

Here are a few dimension for your measure : cushion hgt 12',

main deck, cushion to bridge dk 10.5', bridge 8.5' And a picture of the primary skirt, w/o cvk and transverse divider. Thanks

 

Thanks for the dimensions, but they aren't much use on their own

Will this vessel have conventional skirts fore and aft? I couldn't see their arrangement from the drawing on your www site.

Regards,
Leo