Forced ventilation for stepped hull?

[erik818]

Using air is a way to reduce water friction for a planing hull. Unfortunately, a stepped hull doesn’t seem to work for small boats at moderate speeds (14 kn on a crude 5 m long experimental ½ scale hull). My guess is that the suction isn’t great enough at these speeds to provide a sufficient air flow.

I’ve seen data that for one boat indicated that the effect started to kick in at >30 kn.

Does anyone have any information on using forced ventilation (I’m thinking of using a 1 kW fan) for the step, to achieve an air flow also at moderate speeds?


Erik

[BMcF]

Forced ventilation (air lubrication) of hulls has been around for eons but there is little, that I am aware of anyway, in way of hard design data to support the selection of the pressure and flow requirements for your air mover(s). Not much in way of hard design parameters for the key geometry parameters of the steps or fences either, for that matter. The Russians (then-Soviets) invested heavily in air-lubrication schemes for large fast vessels and were peddling that technology for sale some years back but I never got my hands on much of the empirical data or design info.

I was approached by a fella in Mayport Florida once upon a time, right after we docked an SES nearby we were delivering up the coast to NY. He recognized our vesel as an SES and proudly pointed to his old Gulf-built 'traditional' crewboat and took me over to show me his air lubrication additions to it. He had simply welded a step on the bottom in a 'V' that was located where he guessed the forward boundary of the planing surface 'would be' after he made it go faster with the air lubrication added. Got to give him credit for both optomism and ingenuity there. He was correct in his assumptions and his execution....I watched him head out of harbor (he made daily runs in and out to ferry MSC vessel crews back and forth under a transport contract) and that silly old boat flew like the wind for the moderate power he had in the old girl. If I recall correctly, he picked up an astounding 10 knots of speed improvement with his air-lube modification...it was around a 23-knot vessel and was now a 33-knot vessel.

The fella used a row of used GM/Detroit roots blowers mounted/driven head-to-tail (they are built to be joined up and driven like that..) on top of the plenum box he installed over the step in the hull and drove them all from one end with a large electric motor. It worked superbly..but even he had no idea how much airflow and at what pressure he was supplying air to the step.

I guess the morale of my story is: It's well worth trying..but I can't tell you much about how to go about it.

[erik818]

It's at least promising that someone has tried to use air for lubrication with success. I'll give it a try.
Erik

[Frosty]

Its very interesting Eric I hope you keep us posted I for one would be very interested in your progress.

I would like to do something similar in the front bows of my displacement cat. being displacemrnt I must have a lot of wetted surface area drag. I was thinking of feeding air onto the bulbous bows.

I have already made some air feeds and I was hoping the suction alone would bring air down into the feeds and leave a plume of air at the fornt of the boat. Oh well one day .

Actually I am hauling out next week but wont be fitting the mod as I am altering the stern as well so if its a bummer I wont know which it was.

[FAST FRED]

The fella used a row of used GM/Detroit roots blowers mounted/driven head-to-tail (they are built to be joined up and driven like that..) on top of the plenum box he installed over the step in the hull and drove them all from one end with a large electric motor. It worked superbly..but even he had no idea how much airflow and at what pressure he was supplying air to the step.

EACH blower on a DD 6-71 will need about 75 to 100HP to drive it .

That must have been one really BIG electric motor!!

FF

[BMcF]

Originally Posted by FAST FRED

The fella used a row of used GM/Detroit roots blowers mounted/driven head-to-tail (they are built to be joined up and driven like that..) on top of the plenum box he installed over the step in the hull and drove them all from one end with a large electric motor. It worked superbly..but even he had no idea how much airflow and at what pressure he was supplying air to the step.

EACH blower on a DD 6-71 will need about 75 to 100HP to drive it .

That must have been one really BIG electric motor!!

FF

On an engine where they are supposed to be..of course. But the setup he had was running at 'whatever' the motor would turn it at I guess..which clearly means it wasn't producing a lot of pressure/flow to get the job done. I wish I had taken better 'notes' at the time. I do not even recall what units (4-71 probably?) but IIRC there were 4 of them.

[erik818]

Jack,
right now I'm also working to get my main boat in shape for putting it back into the water. Hopefully I'll have time to run the forced ventilation experiment in May or June.

My guess is that I won't need much pressure because the suction will help. Flow will be more important. My experimental hull has a very shallow draft which also helps reducing need for pressure.

I'll try to improvise a meaurement setup and will share whatever I find out with this forum.
Erik

[mydauphin]

My boat will use exhaust and compress air to lift boat into plane at a lower speed.

I would like to trade ideas with anyone out there. thanks

[BMcF]

Using exhaust has always proved problematic because engines do not like the backpressure. I'm more familiar with the larger turbo-diesels than other options and they have really strict backpressure limits. So strict that we could never run the exhaust directly in to an air cushion on an SES, for example..at a 'paltry' 20 inches water column.

[RANCHI OTTO]

At 14 knots for a 5 m long model, the waves resistance is 77% of total resistance...the friction resistance is only 23% abt. (I have taken some tank tests results for same length and Froude length number.)

A reduction of 5% of the friction resistance is less then 2% abt. of the total resistance.

The air bubbles don't have a scale effect and for this reason you don't have probably the expected resistance reduction if the boat will have 12 m length or more.

I have seen large boats (25m) with a system of compressed air under the hull. The good results obtained in the model basin were very unsatisfactory once the ship tested.

[BMcF]

Originally Posted by RANCHI OTTO

At 14 knots for a 5 m long model, the waves resistance is 77% of total resistance...the friction resistance is only 23% abt. (I have taken some tank tests results for same length and Froude length number.)

A reduction of 5% of the friction resistance is less then 2% abt. of the total resistance.

The air bubbles don't have a scale effect and for this reason you don't have probably the expected resistance reduction if the boat will have 12 m length or more.

I have seen large boats (25m) with a system of compressed air under the hull. The good results obtained in the model basin were very unsatisfactory once the ship tested.

Are you referencing results for fully-planing monohulls?..or displacement hulls of some kind? I too have seen some pretty dismal results with non-planing vessels where model tests predicted a benefit and it was not seen at full-scale. In fact, we worked on one 38m air-lubricated fast displacement catamaran project where the vessel was actually slower than it's sister vessels that were previously produced without an air-lubrication system. The yard removed the mess and sold it as a conventional cat.

From what I have seen, the benefits of air lubrication have only proven to be 'dramatic' and worthwhile when a lot of air is 'stuffed under', and fully encompassing the planing surface of a planing craft. For the case of a planing craft, the reduction in frictional resistance is a much bigger part of the total drag composition.

[RANCHI OTTO]

the reduction in frictional resistance is a much bigger part of the total drag composition.

Planing hull shape but with high displacement

this is true but only at high speeds, I have taken a planing hull testes in model basin.

If you are interested, the results were as follows:

L craft = 24m L model = 2.4m
D = 68 t

Vk CF*10^3 CR*10^3
---------------------------------
25 1.974 11.460
30 1.949 8.492
35 1.930 6.315
40 1.912 4.849
45 1.888 3.741

The frictional resistance is larger than waves resistance only at high Froude volume number.

[BMcF]

Understood. And I agree that the most benefit of forced air lubrication applies only to 'high planing speeds' for the most part, even though it often appears to offer benefit at other speeds in the model tank.

The Russians had a 'cute' demo set up in one of their model basins many years ago. A slender monohull was rigged with a constant-force towing arrangement and you could turn on or off the air supply with a flick of a switch while the model was being towed. It made for an impressive demo..you could easily and clearly see the towed model accelerate to a higher speed when you switched the air on. Alas..as we subsequently learned - and you have correctly noted - the results don't scale well.

Perhaps I digress, but silliness often begets more silliness in this business. Ranchi, have you picked up on any of the 'polymer drag reduction' stuff that is being researched for reducing drag at high speeds on naval craft? Here in our office, we call it 'snake oil injection'...

[RANCHI OTTO]

The air has not a scale effect and for this reason has more effect on a model than on a long ship.
Smaller the model= more effect...

An european country have tried to inject under the hull instead air an high lub. oil with low density.......
They have built the ship....EXPENSIVE!! and.....what was the result..?

NO EFFECT!

Another problem is the laminarity of the water on the model bottom at low speed...what is the interference of the air with the laminar flow?

[mydauphin]

I got idea from friends boat that he had the exhaust built into the hull. With the engine idling the whole boat lifted out of the water 6 inches.

I figures this reduces wetted area and consequently drag. I have a 20 meter planning aluminum light boat. If I direct exhaust about 1/3 the way from transom and inject compress air in several location about mid ship - the air will be trap between the curve areas formed in planning area until it exits in rear.

I have surface drives so they should not be affected by air or exhaust.

As far as back pressure, I am using none turbo 671 with bypass exhaust for idle. I am also building a vent to create suction as water flows under boat and spread exhaust.

I am looking to plan at a lower speed with less power by essentially lowering ship displacement on a light ship already 60000 lbs.

There was a French ship I heard that was able to increase cruise by 10 knots by turning on system like I describe. The only problem I see with this system is that performance will drop if sea state cause escape of air. Then the fast ship becomes slow. Not a good commercial system.