Aquila Hydro-Glide Foil System with 35% fuel saving

It looks like a pretty simple foil that takes 40% of the weight and saves up to 35% of fuel. Presumably only at a specific dialed in speed. All the stability is provided by the still submerged hull.



I'd be curious what the thoughts on advantages and disadvantages are. It seems with a simple addition like this you could either save 35% fuel and also use a 35% less powerful and cheaper motor, or go 35% faster.

 

You might want to google up the 'Hysucat' story, mostly in South Africa; there are boats using the same working principle at all kinds of scales, and a couple research papers. It's not really a simple add-on: structure needs to account for the concentrated loads, and you better figure out which part should break away if you hit something. Probably more sensitive to weight and COG variations than a simple powercat - the power-trim can only handle so much. Yes it's a bit of a single-speed optimisation: the window between the foil not producing much more than drag and the boat rising high enough for foil (and propellers!) to ventilate is narrow. I had a long talk once with a sales rep; seems they sell on efficiency to pros, and on seakindness (less seasickness actually) to the RIB go-fast crowd. He said they struggle a bit to sell on engine downsizing because the bragging rights for big engines or high topspeed matter so much to the guys.. An unusual aspect of these boats is that they don't bank in turns.

 

Thanks so it's not really "the first time a production boat will be offered with hydrofoils". I have been foiled again by marketing!

And yeah "addon" is badly put. But if you are designing a new boat and do CFD analysis then adding this shouldn't be that hard and basically save money with a smaller engine. It seems like such a straightforward "gimmie" if you design a new boat.

 

Yep, not quite As for series leisure powerboats, these are from the early 60's: Volga-Hydrofoils: Gallery https://www.volga-hydrofoil.net/gallery/ . River shuttles like the Raketa (hydrofoil) - Wikipedia https://en.wikipedia.org/wiki/Raketa_(hydrofoil) took off in the 50's, and there already was a production (!) foiling sail multi 30 years ago: the WindRider Rave http://sailingmagazine.net/article-395-windrider-rave.html

 

So why are they not more popular? (EDIT: for power cats)

 

Fouling with floating detritus may be a factor.

 

If you could get rid of that center strut you could think of it as a guard for your propellers. Like those bumper guards for trucks

 

They certainly have merits. I made one for a tunnel hulled boat, seemed not to be hugely effective, but certainly lifted the boat, and had it banking in a way it did not previously, that is, more inward lean. The outboard was a few feet behind it, and no interference effects were evident. Come to think of it, it probably was affording some protection to the outboard leg, it would have hit first.

 

I read some papers about "Hysucat", because I was interested to take advantage of this principle regarding my motorcat build. But after reading and calculating I gave it up, because my boat will be a slow goer with 2 small electric outboards, 1.4 kW each. To gain less drag while partly foil borne you have to take more drag before the lifted state sets in. My intentionally small driving power will not be able to provide the necessary speed to develop the amount of lift that will fit.

It is really helpful if your boat mostly operates in a certain speed window as e. g. a ferry.

 

I was planning something similar Heimfried, for ~10 knots on very low solar power (10kw?). I still have to finish reading that hydrofoil book and then find out how much lift/drag you can actually get and calculate... it all seems a bit weird. I've found foil L/Ds that seem way too good to be true. What did you use to calculate?

But once I actually done calculations and IF does add up to an increase in speed or range it sounds like a great idea. But there certainly would be a break point where the speed is too low to compensate for the wetted surface drag.

But for normal power boats this should always be possible. Look at how small that foil is on the aquilla, if you assume half the speed you need twice the surface area of the foil for the same lift so you should still reduce the overall wetted surface and wave drag. (EDIT: This is wrong, you need four times the area at half the speed)

Basically since there are linear (or near linear) relationships between lift, displacement, foil surface area and power needed for lift it should be useful in a wide range. Except in very low power craft where the gain is negligible and added weight and complexity for foils outweights the gain.

I wish there was a simple to use CFD tool for marine applications based on openFoam out there. Something that is also free or cheap enough for us amateurs. Something like michlet.

I think it would be ok for any cruising power boat to have one specific optimal cruising speed. The added drag at lower speeds shouldn't be too big an issue.

And you could design it so that it still allows beaching / drying out in the ebb.

 

Oh wow awesome! Did you measure the fuel efficiency?

Maybe partial lifting hydrofoils don't help much with increasing speed and overcoming exponentially growing resistance. But it should improve fuel efficiency and reduce the power needed at the same speed. So maybe that's just less visible and why it didn't seem to make much of a difference?

 

Don't forget, that the lift formula is dominated by the term "(rho/2) * v²" so the square of the velocity has to be compared with the foil area. That means half the velocity requires a four times larger area. And because the lenght of the foil is limited by the beam of the boat you would have to increase the span. That means the aspect ratio is going adverse. The coefficients are disregarded in this coarse consideration but I don't think they will be of favourable influence.

Did you look at some NACA coefficient curves? A lot of this stuff is published in the web.

 

It certainly added lift, as I say, the boat banked in turns where previously it was rather flat, I suspect the extra drag from the foil and strut took away a fair bit of what speed gain there should have been from raising the boat, but there was also the matter of this boat having a centrally mounted outboard, that was working in the water downstream of the foil and strut. I probably should have been a little less liberal in the dimensioning of the strut. or left it out completely. But, it worked, I just did not notice much extra speed, may have gained 100-200 RPM, not much.

 

Sorry you are right of course, surface area increases by the square when halving speed. But the drag over that area surface area also decreases by the square, so power required is kind of linear with speed and weight. There is also the reynolds number to consider of course.

You're also right with the beam. Something like a 10m² foil probably isn't very practical haha. But I think you could add multiple high aspect ratio foils spaced enough behind each other over a catamaran hull. But essentially 35% energy efficiency is not worth it at too low speeds.

I have still a lot to read and understand and only read and searched so far yet as to see if this is a viable thing to consider.

I haven't found a very easy to read / use foil database but I haven't really been looking yet. I have the "Theory of Wing Sections" as a pdf but it seems kind of a nightmare to navigate. There are so many many foils. I'd love a database witch lets me filter experimental results.

 

How about the compare function on airfoiltools.com ?

For all kinds of eyeballing aids and rules of thumb, do you have https://www.amazon.com/Hydrofoils-Design-Build-Ray-Vellinga/dp/0982236115 ?

Maybe there is a missing piece in the reasonning developped so far in this thread: For a super-low-power boat, buoyancy can be very helpful. Fat semi-displacement monos aren't really efficient at any speed, but slender displacement or semi-displacement cats do great for the most common low-power application: sailing. If you make your cat raise, you'll loose some of this 'free' lift. You'll have to make up for it with the foil and live with the induced drag. Sure the foil can reduce wetted surface and wavemaking if you raise high enough, but do you have the power for that? And what is the break-even speed ? (Have you looked at these solar boat races between university teams https://www.yacht-club-monaco.mc/en/regattas-events/20-18/monaco-solar-energy-boat-challenge/ ? It seems to me that they operate at higher speeds & power, or at least a different relation between power and lightness (aka high-techiness and expense) than what you're after. I wouldn't be surprised if they need all their power at takeoff though, and in that state some of them would be comparable to what you describe).