New Hovercraft design change ?

[Jeremy Harris]

I remember seeing trials of a sidewall hovercraft back when I was young. The idea was being investigated to build a fast landing craft. I think the experimental craft was built on the Isle of Wight, UK, probably by BHC. It used very slim catamaran side hulls with flexible skirts fore and aft and centrifugal lift fans. I have a feeling that the propulsion was by water jet, but it may have used propellers. This was a long time ago, around the mid-seventies I'd guess.

The sidewalls do reduce spray and cushion leakage, and make the use of conventional fast craft propulsion systems possible (water props or jets fitted in the side sponsons). The disadvantage is that they significantly increase drag when compared to a hovercraft with an all around hover gap.

Ram air for lift is a non-starter unless the required cushion pressure is very low or the craft speed is very high. As has already been mentioned, the dynamic pressure from forward motion is only 1/2 Rho V². You can add wings to get lift, but then you're really talking about a WIG, rather than a cushion craft.

[DocScience]

If I can get fast enough speeds, I would like to use the ram air, but at this time, I need to design without it for now.
After seeing the speed necessary for Ram air, it is not for this stage.

I did some redesign.
Both front and rear skirts are rigid and I am hoping that they act like skis but with the one end of each on a hinge.


kach22i
I watched several of your videos, and I thought they were very interesting.
You illustrate the problems and solutions very well.

" The stern skirt design I tried once had a semi rigid flap like you propose,
it too was sucked down into the water and forced the tail down. "

I noticed 2 different possible effects that could happen.
The end of the rigid rear skirt will most likely need a small spring to keep it down, or the rear end will drop.

Can you give me an outline of this flap arrangement that you tried ?
.

[FAST FRED]

The use of flexible panels has great advantages.

They can be in contact with the surface , reducing the size of the lift motor , or its fuel consumption.

In addition they can be easily changed out if damaged , or when worn out.

Why bother with solid?

[kach22i]

Originally Posted by DocScience

" The stern skirt design I tried once had a semi rigid flap like you propose,
it too was sucked down into the water and forced the tail down. "
................................

Can you give me an outline of this flap arrangement that you tried ?
.

1. When the drag at the skirt contact line occurs at the bow, this can lead to "Plow In". This condition is very well documented and described in Alan Bliant's book. My point mentioning this is that when talking about the stern skirt, the same laws of physics and skirt design applies.

Link:
http://books.google.com/books?id=aJT...rcraft&f=false

2. The rigid flap at the stern I tried was very short lived. It happened back when I was using the clear skirt material. It was just a piece of 1/2" styrene rigid insulation duct taped to the underside of the hull, for a static test on the trailer. Remember this was on a hovercraft with adjacent flexible skirts on the sides, no rigid side walls. When I started the craft the flap flipped up and let all the air out. Not only did it dawn on me that I need a spring in compression above it or tie back ropes in tension under it to hold the flap in place, but the flap could also cut into the side skirts on a banking turn.

I felt pretty silly, but it amounted to about 1/2 hour of wasted time. I spent far more time planning it, and thinking about it, but that proved to be worthless. I encourage you to "do test", because there is no better teacher, and no quicker way to get definitive results.

The power of the "aha moment" is strong.

Originally Posted by DocScience

I am hoping that they act like skis
.

Try curling up the panel at the water contact line with a broad arc. I don't think it will work, but give it a try. If you make the hinged bow panel out of clear acrylic or polycarbonate (curl with blow dryer heat and something to bend it over) you may be able to video tape it and slow down the playback to see what is happening. A larger model (24"x48") made of rigid insulation and duct tape using a leaf blower would make a nice weekend project. A second weekend pushing or towing it around in a kiddie pool or pond and you will know something new.

[DocScience]

My theoretical results, my actual results may vary !!!

In the pics, the boat at rest, has the front end up, as long as the center of weight, is to the rear of the center.

" FAST FRED - The use of flexible panels has great advantages. "

What type of flexible panels do people recommend ?
.

[FAST FRED]

What type of flexible panels do people recommend ?

The simplest is to find a commercial craft about the size you are looking at and observe their panels. They are usually boxes that only get into shape , and support each other when pressurized.

There should be sources for either the materials or the made panels , usually in 3 or 4 shapes , depending on their location.

FF

[kach22i]

For your model use what every you have lying around, can score locally at a local recycle shop or can buy retail at the hardware and or fabric store. An old clear shower curtain would make a fine bow skirt on a quickie model.

The more flexible, and lighter the better, don't worry about wear or abrasion resistance on a toss-away model. Do worry how you will glue, or sew it together. Also figure out how you are going to secure it to the craft. A marine grade boat cover vinyl coated fabric will be fine for a model (use the light cheap stuff). You can use vinyl glue or high strength contact cement (the smelly stuff) to glue things like tabs on. If you use plastic zip ties, those too can be found at your local hardware store. You have to get your hands messy to learn, just my opinion.

You may want to purchase some of the booklets available at the HCA.

Link-1:
http://hoverclubofamerica.org/General/Merchandise.html

You may also want to consider joining the HCA and get the bi-monthly newsletter.

Link-2:
http://hoverclubofamerica.org/Membership/index.html

There are many sources of parts and materials, and a few are in the HCA link below. I like to use Slipstream when possible, but the others are cool too.

Link-3:
http://hoverclubofamerica.org/Links/index.html

[BMcF]

Originally Posted by Jeremy Harris

I remember seeing trials of a sidewall hovercraft back when I was young. The idea was being investigated to build a fast landing craft. I think the experimental craft was built on the Isle of Wight, UK, probably by BHC. It used very slim catamaran side hulls with flexible skirts fore and aft and centrifugal lift fans. I have a feeling that the propulsion was by water jet, but it may have used propellers. This was a long time ago, around the mid-seventies I'd guess..

You are remembering the Vosper Hovermarine craft...of which many of the HM218 model were produced and exported; Hong Kong once had the largest concentration of them but they ended up all over the world.


We - and others - experimented with numerous incarnations of seal types over the decades, including 'planing' rear seals. None of the planing designs were able to survive the real-world operating environment for very long. I was on a delviery run in a 35m LOA SES from Norway to Greece when the latest (and last, as it turned out) attempt at producing a planing non-flexible stern seal was ripped from the hull and left floating in Englich Channel somewhre off the coast of France.

The current 'batch' of SES in the world are the Norwegain navy fleet and a few remaining Norwegian- and Korean-built passenger ferries. All still use the tried and true rubber bow fingers and rubber multi-lope bag at the stern.

[Jeremy Harris]

Thanks for that, glad to know that my recall is only partially faulty!

I've just done a web search and found details of the HM218 and you're right, it is the craft I recall seeing in the Solent all those years ago.



Jeremy

[BMcF]

Originally Posted by Jeremy Harris

Thanks for that, glad to know that my recall is only partially faulty!

I've just done a web search and found details of the HM218 and you're right, it is the craft I recall seeing in the Solent all those years ago.



Jeremy

I have worked with all who were involved in all that activity back then. BHC..Westland..Hovermarine...Saunders Roe... Some of us still get together and 'reminisce'...

Sadly..its all history now. There is the [struggling] hovercraft museum there in ..in Southampton proper, I believe but don't take that as gospel. They have accumulated a good number of the various old craft but are somewhat lacking in facilites and, particularly, the money required to conserve and restore them.

I mispoke..its not ALL history. My good friend John Gifford and his fine crew at Griffon Hovercraft are still producing a lot of excellent hovercraft and, recently, completed the SES landing craft demonstrator for the UK MOD. Intersting, as it relates directly to this thread, the SES landing craft demonstrator has a fixed but adjustable-height plank-ramp at the stern. 'tis quite structurally robust and even then the design is not intended to operate in high sea condiutions; it is a landing craft after all.


To come full circle with this walk down memory lane..the inventor of the rather unique SES hull form that is the basis for the UK MOD landing craft demonstrator was the chief designer for Hovermarine back in the 60s and 70s..the man largely responsible for the successful HM218 design.

[Jeremy Harris]

Full circle indeed, as my first contact with sidewall craft was a proposed landing craft application for the UK MOD back around 1976. It came to nothing at the time, but I went on to help (as a young baby scientist working for the UK MOD) do torpedo launch trials from one of the Saunders Roe craft a year or so later.

[DocScience]

BMcF

I did some searches and so far I have not been able to find any of those on the internet.
I would find it useful to know what has been found not to work.

Would you be able to give me a brief outline arrangement of that 'planing' rear seals and others that have been tried ?
.

[BMcF]

Originally Posted by Jeremy Harris

Full circle indeed, as my first contact with sidewall craft was a proposed landing craft application for the UK MOD back around 1976. It came to nothing at the time, but I went on to help (as a young baby scientist working for the UK MOD) do torpedo launch trials from one of the Saunders Roe craft a year or so later.

LOL...funny you mentioned that. I was involved myself or knew folks directly that were involved in some interesting (too often, VERY interesting*) tests with SES and ACV that I would lump all together as being driven by "Neat boat..what kind stuff that shoots things can we fit up on it?"

*Did you know:

- that a Hellfire missils you just fired from the deck of your rolling and pitching SES can enter the water and then re-emerge from the water headed right back in the direction from whence it just came?? Who knew?

- what a 'hung trigger' on a GAU-20 gatling gun sounds like and the entire magazine goes off?


ah yes..fun with things that go 'bang'...

[BMcF]

Originally Posted by DocScience

BMcF

I did some searches and so far I have not been able to find any of those on the internet.
I would find it useful to know what has been found not to work.

Would you be able to give me a brief outline arrangement of that stearn seal and others that have been tried ?
.

You won't find much...a limited amount of the work done by USN in the 70s is available on CD-ROM from the International Hydrofoil Society, IHS (www.foils.org) That may seem an odd source for SES information...until you uinderstand that the old US Hovercraft Society was absorbed in to the IHS a good 15 or more years ago. None of the commercial yards that we supported, like BrAa in Norway published anything at all about the technoligy they were developing...either it worked and remained proprietary or it didn't work and went in to the round file. The mechanical 'planing' stern seal work fell in the latter category.

The fundamental physical laws that prevented mechanical stern seals from working are quite simple really; to function well as a seal in real world seas required that the seal structure be ultra-light, so it can track (or more correctly, be 'forced' to track) the water surface at high frequencies of wave encounter..and yet it must be structurally robust enough to survive indefinitely against hundreds of wave 'impacts' per hour. "Ultra-light" and "structurally robust" conspire very often to be mutually exclusive objectives.

[DocScience]

BMcF

It appears then, that there is a good chance that this information would most likely not be available anywhere.

Would you be able to give a brief sketch of any of the designs that you remember seeing ?
.