Air lubrication question

I was referring to the boundary layer drag exclusively. Perhaps I was unclear on my initial question at the beginning of the thread. I was looking for a ballpark ratio of skin friction drag to hydrodynamic drag of a flat plate 24" wide x 240" long, at 11-12 knots (semi planing). My bad for not being specific.

 

Tiki, it sounds like you have already purchased your new pontoons - or are you still thinking about it?
If the latter, then take note of the general consensus above (ie don't do it), and aim for building hydrodynamically efficient hulls in the first place.

What material are these pontoons made from?
An empty weight of 1 tonne sounds rather heavy to me.

Re your length of 20', and beam of 8'6", are these the maximum dimensions based on (eg) the berth size available?
Or maybe the limit of 8'6" is so that you can easily tow the boat on a trailer?
If it is at all possible to go longer and wider, then it would be well worthwhile doing so - ok, there will then be an extra cost in materials and labour to build, but you will get a major advantage of better fuel efficiency based on hull shape.
And even if your water taxis are only operating for 6 months of the year, over 5 years the savings in fuel costs will be considerable, and the extra initial cost will be paid back fairly quickly.
Hull shape is everything - the power cat in my avatar is a coastal water taxi, and she has been working here very happily for the last 20 years.
Initially she had some 'competition' (ha!) from another power cat, also locally designed and built - both of them could carry the same number of passengers up the coast at the same speed, but the other cat burned 4 times as much fuel as mine, simply because her hull shape was abysmally bad.

Have a look at Richard Woods' 18 ft. Skoota in the link below.
Sailing Catamarans - Skoota 18 trailable 8ft wide daycruising or fishing http://sailingcatamarans.com/index.php/designs-2/6-powercats/489-skoota-18

Richard notes that with just a single 20 hp O/B motor, "This gives an economic cruising speed around 9-10 knots and top speed in the low teens."

OK, the deadweight (passenger carrying capacity) of this Skoota is much less than your desired 2,300 lbs - but you could start with a concept like this proven hull shape, and make the hulls a bit longer and wider to allow for a payload of 2,300 lbs.

However if you cannot go larger than 20' x 8'6" for whatever reason, then that does throw a spanner in the works, and you are stuck with bath-tub pontoons in order to get the buoyancy required.

 

Alright.....I go with the pro consensus.... no air lube.. Keep it simple stupid. I have looked at Richard's designs, particularly the 28. My problem is I need to stick with aluminum construction simply for the ease of repair and durability (drivers crash boats). I had asked Richard if aluminum construction was possible with his designs, and his reply was that the weight of the material would make it impractical. 26'9" is my maximum length before we get into a different level of certification for Transport canada, thus higher costs.

 

Re the Skoota hull form, it would be relatively easy to make the hulls a bit wider to allow for the extra displacement required, both in terms of construction material, and payload.
Re your maximum length of 26'9", that is just over 8 metres - is there any restriction on overall beam?
You could easily have an overall beam of 4 metres (13') on a cat length of 26' - unless this creates logistics issues re transporting the boat from the builder's yard to the waterfront in Toronto?
Although 13' should be easily do-able - my avatar cat is 16' wide, and she was transported approx 4 miles by road from the Builder's yard to the waterfront (albeit at 4 am, when there was not much traffic!)

 

Oh, there I was refering to the case of a "bubbly layer", which in fact has a lower effective density than clean water; it is primarily a question of Archimede's principle. Take a look at a Plimsoll mark for a view on the impact of density. An additional aspect of a homogenous gas/liquid mixture is that its effective viscosity (both kinematic and dynamic) can be considerably higher than the value for each single component, and on top of that, it has non-Newtonian properties with a high start friction.

Next time you are washing up the plates after your dinner, and you have a nice foam layer on the sink surface, put a finger into the foam and move it around. You will notice how the bubbles are "locked" into a grid and the movement is first deforming the grid and then (with increased speed) bubbles are dislocating within the cluster (much like chrystal dislocations in a metal object under strain) to cope with the increased stress level. In addition to the density and viscosity changes, you also get a reduction of the critical wave velocity ("sound speed") in the mixture, so you will find yourself trying to deal with a supersonic flow with a non-Newtonian fluid of unspecified density; not many of us have the tools to cope with that (certainly not me...). Put altogether, unless the chemistry of the gas/liquid (including the relative surface tensions) is changed, there is no realistic possibility to reduce friction with a bubbly layer.

Then I referred to the case of a big, single gas volume and exemplifyed with the Russian type of rigid wall "air cavity" vessel, since it is closer to the hull types in question here. Of course this goes for the other types of pressurized air cavity variants as well. My point is that bubble mixtures don't work, but gas cavity types do, but only if the gas volume is clearly separating the hull skin from the water, and only if the friction reduction covers for the pumping power spent, not forgetting that an air cavity is a pressure disturbance and causes waves. Again, this requirement is not possible to maintain in "real life" in a thin layer moving along the hull, since the surface laminae are inherently instable due to the interaction between friction, surface tension, gravity and air velocity above the surface. Even a minor disturbance will set up ondulations on the surface, which eventually may touch the hull skin, and from there you have an uncontrolled slug flow. This is why I concluded that the only commercially realistic approach is the air cavity type.

These events are dictating flow conditions, heat transfer and economy in a variety of industrial applications and there are a lot of studies available (see "Effect of viscosity on gas-liquid flow calculations in a dynamic process calculator" by Tomi Heikkila, Aalto Uni Finland 2014 for a treatment and an excellent reference list). You just have to adjust your "topographical image" inside-out to find that what happens inside a pipe is very similar to the events outside a hull, even the waves are there.

 

The beam is limited, as we must tow to our yard for storage and maintenance. It's my understanding that there is an optimum ratio of overall beam to length ratio on cat hull spacing in order to minimize wave interference between the hulls. What would that be, and at what threshold would the hulls be considered too close together? Currently, I am exploring multiple ideas for new hulls, as I am considering replacing all the existing round logs I currently have. However, those boats are limited to 9'6" x 26'.
https://www.harbourfrontcentre.com/images/marine/watertaxi/Five Star Water Taxi.png

 

Your hulls definitely are too close to be called ideal from the interference angle, why does it need to be a catamaran ? A semi-displacement monohull might suit that speed range.

 

By using Pontoon type hulls, the stability calculations are a no brainer. Our load/unload conditions dictate bow-on docking (while powering in) and a rectangular deck facilitates carrying the cargo (bicycles, strollers) that come along with all the passengers. Richard Woods stats on his 24 foot boats seem to suit our speed/power/capacity requirements, except for the beam. What would be the optimum hull spacing for 24' hulls?

 

It is speed related of course, and someone will direct you to a resource that gives you a guide, but at ten knots, it would be somewhat more than your 8' 6, however does it really matter that much, is the question, how much difference would it make to overall fuel consumption on relatively small boats, with the duties involved. As I mentioned, you would probably be better to look at single engines, if that is allowable, to get the fuel down, you could probably knock 25% off it at a stroke, but a central mounted engine can be tricky, though easier on smooth and semi-smooth waters, it might even be worth looking at triple pontoons, and a single engine is easy there. But the loss of the redundancy that twin engines gives, may be unacceptable.

 

How often do you have to do this?
Do you store the boats ashore every evening?
Or just at the end of your operating season?
What is the maximum beam that you can have on a trailer on your roads?
If you only have to do it twice a year, would it be worthwhile getting a police escort (or whatever is required) in order to do this?
If you can tow a boat with a beam of 12' with special permission, then it would be well worthwhile building your new boats 26' x 12' - this will give you 50% more deck area, better spacing of the hulls, and the opportunity to re-design the hulls to be more efficient.

If you have to stick to 8'6" beam, you don't need to worry about hull spacing - you ideally want them further apart, but you can't.
I presume that you operate for 'only' 6 months of the year?
I also presume that you keep detailed accounts regarding petrol consumption (and cost!) for your boats?
I am thinking that a more efficient hull form than the pontoons shown in your link could most probably save you 25+% on your fuel costs.
It is your call.

 

I have read, and will take all into consideration. How does this sound 22 ft overall.... Skinny aluminum hulls (a-la-Woods style), variable beam (8.5 for trailering, 12.5-13 for operations), ... extra bow buoyancy for unloading and loading passengers (by using an adjustable dock float slung under deck). Twin 20's. Boat and motors under 2200 lbs, 5000 lbs fully loaded. Screw the bubbles.