Forced ventilation for stepped hull?

Interesting..and I guess if any diesel could stand the added backpressure, it would be the old GM 2-strokes. If his boat is actually 'lifting' 6 inches, that implies more than just air lubrication is going on..there must also be an air entrapment scheme involved, yes? Air lubrication alone does not change the 'lift' on the hull, it only modifies the friction in the hull boundary layer.

 

HI!! i am very interested in combining a forced ventilation on a stepped hull for my boat project!!! I found a interesting article (in swedish) with numbers and all that actually compared two similar hull´s, one with steps and one without. the test is unique!! i have never seen any other tests like this one, please let me know if you guys seen others...

but anyway, the conclusion of the test tis a crushing victory in both economics and speed for the air stepped hull..

link: http://www.nimbusmaxicenter.com/docpdf/17.pdf

// micke

 

Is this true only for the proposed boat modification?

Don't SES's and Hovercraft used air to get them over the bow wave hump to start planing as low as 5 knots?

 

Apples and oranges, Kach. Air-lubrication and air-lift are two different animals; the former only working when 'spread out' in the boundary layer over the water contact surface of the hull at speeds where the reduction of friction drag may be significant. But air-lubrication does not create 'static lift' as is present for ACVs and SES'; where it is also the case that the air lift separates hull from water.

That said..there have been some attempts at 'hybrids' that were supposed to provide both static lift and air lubrication. Harley and Burg are just two examples..but the results have been pretty dismal except at smal scale with very light waterplane loadings.

 

Thank you.

 

There seems to be diffent opinions on what "full scale" means. For some, a 5 m boat is already full scale... My target boat would be something like 10 m so the scaling problems when going up to 23 m or 38 m are not of immideate interest.

As for my 5 m hull, it is light, narrow (1 m) and has a flat bottom. It doesn´t make any waves of consequence at any speed, just small ripples. I doubt that wave resistance is a major contributor to drag. My calculations rather indicate that more than half of the drag at 14 knots come from friction, the rest is "induced" from planing and attack angle, so I hope to measure some difference in drag when using air lubrication. I could be wrong, but there is no point to argue until I have results.

The link Doppson (micke) provided shows a reduction in fuel consumption that would indicate a reduction of 50% in drag for a boat with a stepped hull compared to a similar boat without a step. Both boats are relatively heavy and short (almost 3 tons, 7.5 m) and have a significant V at the transom (20 and 24 deg), where the stepped hull actually has the larger deadrise. I'm amazed that air lubrication makes a significant difference for such a hull also at speeds down to 15 knots, but I would rather believe in measurements than in rules of thumb and equations that might not be applicable to the situation. Anyone has a theory explaining those test results?

Erik

 

This boat must have had a large "pocket" to hold the air?

 

I am researching a 40 ft sailing ctamaran with flat bottom hulls and a hull length to beam ratio of 17 to 1 I will have very small curtains about half an inch on the side to keep the air in....
Terry

 

Exhaust Back Pressure

A couple of things on back pressure that I have look at. I am using non-turbo engines with as straight a pipe as possible.

Also building a exhaust vent shaped a little like a hull step to generate a suction affect so exhaust will follow into it.

I am still looking for more info on web.