Air lubrication question

Hi everyone. I'm a long time lurker, first time poster. I currently own a fleet of water taxi pontoon boats in Toronto. Up until now, I have been purchasing the pontoons premade from one of the local kit builders, then building the boats up from there. However, in a move to become more environmentally friendly, I have embarked on a new build program.
Our operation environment is Toronto Harbour where we have a 10 knot speed limit. Any new boat I'm building will not have to exceed 15 knots without a load, and 11-12 knots fully loaded.

My new pontoons are rectangular bilge/flat bottom, 26" wide. The desired length is 20 ft, 8.5 foot beam, 2200 pounds empty, 4500 pounds with passengers/driver/gear. From the online calculators I have been able to find, it seems that I'm within the desired parameters if I run twin 20hp fourstrokes.

I have designed a method of getting air under the pontoons (through using blowers) which seems to be effective at creating a layer of air under the hull. Due to our low cruising speeds and relatively calm waters (most of the time) the amount of air required works out to approximately 100 cfm per pontoon. The pontoon construction creates a shallow channel of approximately 3/8 of an inch depth along the whole pontoon to prevent the air from escaping. The air injection would have 2-3 locations along the length of the pontoon which are normally submerged at planing speed. A valve would be used to change the injection location depending on conditions.
Now, the question I have for the engineers out there.... at 10-12 knots, without the lubrication what percentage of the drag on the hulls would be skin drag vs other forms of drag. This is my conundrum....In order to keep it uncomplicated, we could assume that the bottom is a flat plate..... This will dictate whether of not the system is even worth attempting to prototype.

 

I'm a bit confused about the speed limit, you say ten knots, but then talk about 15 knots ?

 

You are aiming at a speed range, where the dominant part of hull resistance comes from wave-making. The numbers you mention also tells me you want to balance on the "high side of the limit". That, and the resistance issue says that you should spend your money on a hull shape that has a good hydrodynamic shape, suitable for the actual speeds. This is not a suitable application for a "stretched brick" with some miracle machinery.

Air lubrication in boating is a dead end except for a very few, very special cases. The main point is that you substitute a high-density fluid (giving buoyancy to the hull) with a low-density fluid, with the result that the hull must sink deeper to float, i.e. it is "showing a bigger transverse shadow to the flow". There have been experiments going on for almost 200 years without results worth mentioning in an investment budget. The only variant that has come to practical/commercial use, is the air cavity, Russian style, and that is only suitable for higher speeds.

Compensating for the buoyancy loss by increasing bubble pressure costs power and results in a dramatic change in the energy gradient within the boundary layers, making them instable. And....and... and... So, unless you just inherited an awful load of money, stay away from bubbly things around the boat.

 

Aaahh.. Baeckmo beat me to it!!

Indeed, with a length-displacement ratio of 4.8, that is low.
This means your wave making will dominate...and you will most likely experience a higher degree of trim/squat than you may like too.

As B says, spend money wisely.... and ignore those bubbles.

 

What, John did you have the wallside bed this morning........? Always slows you down to climb over a warm body......

 

Well,... humm...indeed...
Anyway....Both make a good point here as, unlike the other relevant topic right now, Active Drag Reduction? https://www.boatdesign.net/threads/active-drag-reduction.65265/ , air lubrication really doesn't change the surface drag at less than planing speeds. Air lubrication works by reducing the surface in contact with the water; but by having the water with more air and less density, puts more surface in contact with it. It really doesn't gain you anything.
The introduction of air under the planing surface has some benefit, but again, it needs to be carefully look at based upon form drag (i.e. suction drag.)

 

Nah.. it was a beautiful early spring morning here...frosty start...sunny...so went off hiking early start into the mountains with friends.
Didn't get back until late-ish!

 

What, involving this air injection idea, or not ? Your fuel bill savings would be minimal even if it did work, you might save fuel, if that is the aim of this exercise, by switching to a single engine. Unless you need to walk the boat around with twin engines when docking.

 

I very much appreciate everyone's input, and value your expertise. Hence, I will stay to making bubbles exclusively in the bathtub, at no cost other than funny stares from the dog.
Mr Efficiency, the speed limit is 10, but who the hell drives the speed limit?

 

Hull air lubrication: future and challenges | SSPA https://www.sspa.se/ship-design-and-hydrodynamics/hull-air-lubrication-future-and-challenges

A most interesting article on the subject

Making the assumption that at 10 knots and 44oo pounds, the pontoons will not plane AND that there is one point on the hull that the pressure due to hydrodynamic and buoyant forces are (and only for arguments sake)3 pounds per square inch. (only because I want a number to refer to) perhaps at the stagnation point.

The air being injected will have to over come this pressure or it cannot be injected into the hull due to the combined water pressure. So the air say exactly at the nozzle is 10 psi, but only at the nozzle. The air pressure in an extremely short distance will equalize to the existing pressure along the hull to be the same as the surrounding water pressure to hull skin interface pressure.

I am curious about your comment the boat will sink as the pressure on the bottom of the hull will be the same with an air boundary layer or water boundary layer. Is this not correct?

Experimental investigation of frictional resistance reduction with air layer on the hull bottom of a ship - ScienceDirect https://www.sciencedirect.com/science/article/pii/S2092678216303077
A scientific paper on the subject

In the 80's we sold both Scat and Hovertechnics hovercraft. With no fan speed, the hull weight was carried only by buoyant forces, when power was applied the hull rose. The subsequent air pressure acting on the water
would be enough to carry the weight of the craft and crew and excess air pressure would escape under the skirt.

To Tiki, you might be better extending the length of the hulls and shape the front and rear of the pontoons to reduce hydrodynamic drag. Pointed front, pointed back with a proper transition.

What is the effect of aerating a planning boats on it's performance https://www.boatdesign.net/threads/what-is-the-effect-of-aerating-a-planning-boats-on-its-performance.50693/page-2

There a few threads on this topic in this forum, albeit old, that discuss this topic.

 

Probably get more performance improvement from well chosen bottom paint...think slippery..

 

Barry,
Experimental investigation of frictional resistance reduction with air layer on the hull bottom of a ship - ScienceDirect https://www.sciencedirect.com/science/article/pii/S2092678216303077
This article describes exactly what I had envisioned. In fact, the described plate is very close dimensionally to the immersed section of my pontoon design. I had also envisioned short side walls to keep the air from leaking out, not even considering that the vortices would be eliminated. Really, it seems that the balance is getting the correct amount of air injected to maintain the proper "air sheet thickness".

 

Do not take my initial post as supporting your concept. Just that there is scientific information available. There is a cost to supplying this air under your pontoons and cannot be ignored as your stated goal is better
fuel economy.
You may find that 3 narrower pontoons, with POSSIBLE less dynamic drag will save you the same fuel. Proper entry and exit design may reduce drag. You say that your calculations show that 2 20 hp engines will be enough to power your boat. I would assume that you have determined that you need 40 hp overall to supply the speed that you require but 2 20's working at max horsepower may not be the most fuel efficient way to get
40 hp into the water. Ie perhaps 2 30 hp motors operating below the max available will save you fuel. Or consider the High Thrust engines available, lower gear, larger props etc.

If fuel savings is the goal I would go down this path:
1) Hire a NA to provide a drag efficient shape and length for the pontoons and have him consider a tritoon option
2) Consider upping the engine package to not have to run the 20's at max rated output. Often, pinning the throttle is not the best fuel efficient setting
3) Consider the High Thrust engines in the market place
4) Be prepared to buy several prop sets to get the one that will be correct for your loading and speed.

These are static costs. Ie you will not have to pay for fuel to run a compressor, the subsequent capital cost, or the on going maintenance. I believe one of the papers that I referenced said that with larger boats
a 5% fuel savings is POSSIBLE. If you are burning 3 gallons per hour to produce 40 horsepower, a guess, at $5 per gallon, that translates to 75 cents per hour savings in fuel LESS what ever the cost will be to provide
fuel to for the compressor/fan motor.

It does not make sense to me to spend dollars per hour to save cents per hour

 

There were several factors for choosing the 20's. The availability of new motors which are lightweight (100 pounds each) and fuel injected, and the elimination of the transom between the pontoons. No transom allowed for lighter construction between the pontoons, and no transom drag. The 40HP would drive the boat faster than required, so this was already considered. As stated earlier, we have a 10 knot speed limit, which can be reasonably pushed to 11-12 knots without any adverse attention from the authorities. This speed limit is a funny zone to operate in, as it's too fast for displacement hulls (26 foot limit), yet too slow for full-on planing hulls. Ideally, we would likely want to explore electric propulsion, but that's a whole other ball game. We would have to produce the desired attributed using gas propulsion first.
The intriguing thing about the flat plate experiment is the high percentage of drag reduction. It's my understanding that part of the cost calculation is the energy required to compress and inject the air under a full-sized tanker, with a deep draft. That draft requires a lot more pressure than my pontoons which draft 8-10 inches. At 11-12 knots, the boat should be running in semi-planing mode, and the vast majority of the wetted surface is the bottom. Let's say 80%. With 80% skin drag reduction in experiments, I could safely assume to attain 50% reduction in the real world. 50% of drag on that surface would equate to an overall 40% reduction in skin friction (using the 80% value at semi-planing speed). I think the flat plate comparison is closer to my situation than a large tanker. Am I making sense in my reasoning?

 

Where do you get the 40% drag reduction? As one learned contributor mentioned above, hydrodynamic drag is higher than boundary layer drag.

A quick look at a horsepower convertor, 100 CFM at 5 psi takes about 6 horsepower plus say 20% for mechanical loss, so 7 horsepower plus duct loss, 40%, two plenums with lots of 9o degree turns, so maybe 10 plus horsepower. I recognize that I may be out quite a bit but just illustrating probably a low value.