Air lubrication on displacement catamaran

Our company build 78" power catamaran. It was Malcom Tennants 72" Adriatic project http://www.tennantdesign.co.nz/index.php?page=adriatic---72

In 2011 we have have added vertical bow to reduce water spray on high speed and stretched aft +2,2m to accommodate two PWCs.

Boat name is Ulysses, she has two 600hp Cats, top speed 25 knots with fuel consumption 115l/h per side.

Ulysses has displacement hulls with vertical bow and round bottom. Length of the hulls is 23m, width 1,4m, draft 1,6m. She also has underwater exhaust system.

Recently I have heard about MALS - air lubrication system that allows to reduce water drag.

I was thinking what if to use water exhaust as an air lubrication system? All I need to do is to relocate engine exhaust from the rear part of the hull to the front, and divide exhaust to several points to achieve more even spread of air cushion.

What do you think about this idea? Is it possible to achieve higher speed or have some fuel economy?

 

A lot of research projects are currently looking into air lubrication. There are also a lot of theories and methods being investigated. Without going into the merits of these methods, one general statement can be made for air lubrication and that is that it needs to be kept along side of the hull as long as possible for it to have any significant result!

Most air lubrication systems are therefore applied on the flat underside of a ship, often with a slight bilge keel (bulge) to keep the air captive on the underside of the ship. Air lubrication on a vertical hull will generally not be as effective as the air tends to escape upwards and away from the underwater hull!

Your catamaran has, as far as I can see, only vertical sides and is therefore not very suitable for air lubrication. There are also a lot of other (very large) drawbacks on applying air lubrication, which can only be solved with significant design changes. I think it is therefore (relatively*) safe to say that applying air lubrication on your catamaran will have no benefits whatsoever.

* There is a good chance other forum members will want to prove me otherwise!

 

Then there is the possibility of aerating the water the prop is operating in, causing loss of 'grip'.

 

How do I planed to do it

Thank you Tackwise!

I understand that the main idea is to keep bubbles close to the hull.

Assuming that the speed of the boat is 12m/s and the underwater length of the hull is about 20m, so hull will slide along the bubble in less than 2 seconds, question is how far the bubble will go up during this 2 seconds?

Other question is that the air bubble will move sideway away from the hull, and there is really nothing to do about it.

Total square of the sides of 2 hulls that could be air lubricated is about 150 sq.m. I guess they generate substantial drag. Is it possible to estimate, how much power goes to wave creation and how much goes to wetted surface drag?

I have two ideas how to lubricate side of the underwater hull with air. One idea is simple, another is very simple

In first idea we use exhaust gas to ventilate the vertical tube along the bow. This tube have small exits (holes) to the outside. Exhaust gas come out of holes and lubricates the sides and bottom of the hulls. Pluses of this idea, is that it will not increase drag in any way.

Second idea is to install some kind of V shape hat before the bow. So that the hat and the bow looks like that " >> ". This hat will go up the waterline and suck the air inside and underwater, where it will create air cavity that will spread along the hull. The faster boat will go the bigger will be the cavity. This is very easy and cheap to do, the possible drawback is that drag from the V-hat will be more then the drag loss due to lubrication. In fact this V-hat could be made out of stainless steel and bolted to the bow, so that it will be possible to install and deinstall it in water. In such case it will be easy to tray different V-hat with different size and angle.

 

Faster Cat

LIG,

An alternative could be to put a skirt at each end and a couple of fans inside and run as a surface effect ship. Should give you +20/30% on speed; or for existing speed the equivalent saving in fuel.

Your present idea may be a difficult one to carry out in practice. Running hot exhaust right forward and then out will give very high back pressures for the engines to fight against. Best to check with the engine maker before making any serious structural modifications. You could end up with less power delivery.

Good luck with whatever way you go.

Best Regards,
Graham

PS I do have plenty of experience with SES design and operation if it helps

 

Wow, 1,200 horsepower, 60 gallons per hour fuel consumption? Your clients must be very rich.

 

LIG,

What Gwboats says is true, and is probably the biggest problem to overcome if you want to use the exhaust gasses.

Check out this site for some good reading on the subject of air lubrication.
http://www.smooth-ships.itu.edu.tr/conference_programme.htm

I am afraid that air lubrication is not a very easy subject. I am currently a bit to busy to give you any hard arguments as to why it will not be very effective on your catamaran. But in my honest opinion it is currently not worth the trouble!

 

From what I've read before steep hulls and deep vee planning hulls have only shown benefits from air lubrication when using micro bubbles. The micro bubbles need to be applied over a large surface area using higher pressure..not easy.
I would have thought such a cat could gain efficiency at that speed using a spanning foil? off topic though.

 

I recommend foils- I was looking at a Prout Panther 64 a few years ago,top speed was 31 knots and with foils it went up to 43 knots.

 

I'd say we think alike in our thinking outside the box, and you would probably soo love my design of a rather radical concept high speed, high sea state vessel that i had come up with several years ago. (i think i discussed it in some of my older posts).

But here is a youtube video titled " Aces air lubrication ", that may give you some insight to your concept idea. ...

http://www.youtube.com/watch?v=7jSBd-8sSQk

P.S. .... you may also want to check out a vessel called the " M-ship or Stiletto " for some design insight or ideas.... http://www.mshipco.com/

 

I would also recommend that you consider a lifting foil instead of pursuing any kind of air lubrication schme; few of those air-lubrication schemes have proven to have much benefit and the only ones I personally know of that did work to some extent:

a. used fairly high pressure 'compressor systems' to force the air through the vents and fences on the hull bottom;

b. were added to planing or at least semi-planing hulls with the air-lubricated area of the hull bottom nearly flat.

 

Ok after taking a closer look at your hull pic, ( great looking boat BTW ), even with a plenum at the bow, the air cavities under your vessel are way to large to obtain any benefit from any type of air pressure injection systems, no matter how many high volume blowers you install. Air is compressable, but water is not !!!. the obvious solution to that would be a purposly built vessel that has 6-8 shallow, divided rectangular depressions built into the under side of the boat, (these shallow depressions would sit just below the waterline when not underway), with the leading edges of those depressions having a tapered leading edge, and with the back edges of those "transverse lifting strakes" ending with an abrupt step (to reduce the suction and drag, as well as to create the shallow air cavities ). now when the vessel is underway it would essential be riding on a froth of micro-bubbles and no blowers would be necessary.
I remember that there was one small one of these types of SES vessels buit in 1969 as a prototype that showed great promise. I believe it was called the X-1, but after Vietnam the project was stopped, and now that vessel is nothing more than a small low speed harbor work barge. What a shame !!!.
In the 1970's the Russian navy was doing some research on these types of SES vessels as well, but i have never seen much about them or their progress. ( i think they switched more to the development of the hydrofoils (such as... the Meteor and the Comet).
Now back to your boat, there is a system that could be retrofitted for use on your boat, (it uses ride control lifting fins ) youtube won't open for me at the moment but it is amoungst the videos about hydrofoils, (i think it was called the Hyswea or something like that).
I hope the info helps.

 

HYSWAS hydrofoil ....
http://www.youtube.com/watch?v=ORuDmzI48B4&feature=related

 

The USN, the Russians, Norwegians and others have built many successful SES over the years..but the only proven technology for reducing the drag and/or improving the L/D ratio for the catamaran shown in the original post is through the use of a transverse lifting foil mounted between the two hulls with smaller balance foils mounted aft.. Incat Crowther Designs, for one example, has done more than one such and I have personally been involved in several others. The largest of them was a yacht catamran around 150' loa if I recall correctly; its been some 15 or more years since we worked on the yacht in question.

The simplest and most commonly applied lifting foil solutions are being delivered to designs by Dr. Hoppe in SA. I'll try to find a link.

Ah .here it is:http://www.hydrospeed.co.za/

 

*chuckle*

That is a 'half a SWATH'...with lifting foils appended to the submerged hull. Literally not one bit of technology on that demonstrator that has applicability to anything other than..another HYSWAS.