Foil assisted multihull design

As a rough rule of thumb, HYSUCAT foils are effective when the Volumetric Froude number, FnD > 2.20 For other foil arrangements it can be less but i propose you work with this number initially.

To calculate the exact performance improvement you will get - you will need to follow the process as described by BMcF - that is the only way.

 

That right there is what I observed to be the single most "destructive" factor that arose from the frenzy of fast ferry design and construction throughout the 80s and particularly the 90s. Too many "experts"..too few real successes..and quite a few total debacles. As a consequence of that, some very good marine technologies have been all but abandoned. At least for now.

 

sounds like we have been through the same thing. The latest big disaster is the foil design of the WindCat 101: http://www.windcatworkboats.com/#/fleet/wc_mk4/wc101/

Attached is an image which shows the foils on that boat. Notice how the foil is located well above the keel almost at the waterline. This foil must have been popping in and out of the water all the time in waves and apparently made the seakeeping atrocious. The owner of the vessel is furious as he paid a lot of money. He took the foils off and has been spreading the story of what a load of crap hydrofoils are for catamarans when in fact it was only a crap designer! Now try and get any Windfarm vessel operator to adopt foils... Not!

 

Ok I can understand the iterative process and I should be able to do the calculations.

next question, is determining a configuration... I noticed earlier sottorf suggested a tandem configuration for a low speed requirement. Im curious as to how the different foil configurations serve different requirements in terms of speed and load carrying applications. Any guidelines for initial iterations?

 

This is highly likely by RENERGY Marine, a French company that uses a different system/arrangement/methodology, but still with foils.

 

I found this neat little spread sheet that attempts to predict hydrofoil lift including near surface effects etc. The formulas were provided by Dr. Konstantin I. Matveev.

What do you guys think?

I also found and had a look at the YS-930 and YS-915 foil sections by Eppler-Shen and their respective Cl Cd alpha polars... i think the 930 section might be useful to me. Max Cl/Cd = 155.4 @ 2.75 degrees AoA @ Re# 1M. Does this look ok so far?

 

The surname Matveev is well-known in this field. The Excel spreadsheet in your post is based on formulas from this paper: http://www.foils.org/02_Papers dnloads/Matveev - Tool for Predicting Performance.pdf . The paper explains in plain terms the difficulties in estimation of a hydrofoil perfomance due to interaction with wave systems of other foils in proximity and with the hull. The Perotto's spreadsheet ignores these interactions and is hence valid only for lift-prediction of an isolated hydrofoil.

Another interesting paper from Matveev is this one: http://www.balancer.ru/forum/punbb/attachment.php?item=78203 . It shows some structural and maintenance problems encountered during the operation of Russian hydrofoil-assisted fast ferries.

Cheers

 

Groper's boat is a lightweight catamaran around 35' between 2.5 and 4 tons. With it's flat bottomed hulls and straightish underwater lines, there is no way it won't be a planing hull beyond 20 knots. ( He may disagree !) This boat is going to have some very lively motions underway pushed beyond 20 knots, given its width and lightness, with any sort of a lump. It might stay in one piece, but I wouldn't bet on it. This foil idea is beyond the scope of this vessel, imo, because those sort of speeds where foils are attractive, are faster than what this boat can handle. 30 knots, sure, on flat water a bathtub looks to go OK, but I don't fancy this boat is going to work in the rough at those speeds, at all. It may well be it goes like a bomb in flat water with foils, but his outboards will have to be propped out to high pitch wheels that will become a real handicap if it becomes necessary to throttle back to 15 knots when conditions dictate. I am of the view you have to prop engines according to what speed you will be cruising at 90% of the time, not 30% of the time, as dictated by sea state.

 

You might be right Mr E, but without any calculations or data to backup your opinion, its just that - an opinion. What is your opinion based upon? My intuition tells me the same as yours, but im not going to let it get the better of me unless i can prove it to myself with better data or calculation.

You see, several people have commented that they dont beleive the foils are feasable unless the speeds are high, sottorf suggested a volumetric-froude number of 2.2 for example. This is probably based on his experience with other vessels at a guess. I dont know where these feasability boundaries are, but it varies depending on many things, so whilst it might not be a good idea on 1 particular 35' vessel, it might work great on another if the weight or desired speeds are vastly different. My boat is one of the different ones in this regard. There are not many 35ft lwl * 17ft beam catamarans that have a dry weight just over 2 tonnes. The closest analogies are some of the modern sailing cats. Thus the standard boundaries may not apply here and i cannot approximate this because little to zero empirical data exists from other vessels as they there arnt any in service that im aware of with close enough parameters...

So the goal is not to travel at 30kts... because this burns heaps of fuel regardless of whether your foil born or planing or in a high speed displacement regime, going fast burns fuel any which way you look at the problem. So my problem, relates to a more moderate speed, ideally 15kts minimum to 25kts maximum for cruising speeds. The minimum speed is derived from experience operating many different vessels in these waters, 15kts is where most boats of this size are comfortable when conditions are rough. When conditions are favourable here, any speed is possible, only limited by excessive fuel consumption which is why i chose 25kts??? foil born - if no foils are present, then the nominal cruise speed is 20kts. Id cruise at whatever speed i would normally get at 20kts for the same fuel burn / distance travelled. At the lower end of the speed regime, if i can do 15kts with the same drag as no foils, thats fine with me as i get a seakeeping advatage from having the foils present. If the added drag is too great at these low speeds, then id deem the idea as unfeasable.

Now the dimensions... because i dont have alot of weight to lift, and i have the ability to accomodate a rather large span foil between the hulls, the spanwise foil loading can be quite small and the aspect ratio can be quite high. This means a very efficient foil can be used and therefore the efficient speed regime should be lower. The biggest challenge will be structural should it be feasable as the foil will be very thin.

More research is needed, and i may have to build a model to fully test things should a workable solution become apparent via empirical or numerical calculation...

 

But the modulus of the foil is proportional to the span^2. Thus the greater the span the greater the required modulus. This is where one of the "cross overs" if you like, comes into play. Structural. Since increasing the depth of the foil to gain more stiffness will add drag. Using higher modulus materials then comes into play to keep the depth (thickness) of the foil at a minimum, but that is versus increasing cost.

 

Indeed, i have the ability and experience to fabricate in carbon fibre if need be. However full span foils are not the only configuration possible, although it is appealing for essentially a 2D flow characteristic... like i said in my opening post, another option might be a pair of eliptical foils cantilevered from the inside edges of the hulls for example, easy for me to fabricate myself in composite, not so easy or cheap in metals with expensive CNC milling...

 

No struts to support these elliptical foils ?

 

You also have the issue of slamming loads on those flat-bottomed hulls, that would be far less with your original conception, without foils lifting them and exposing them.

 

You don't really have a good fix on how your boat will behave without foils, so you are only extrapolating from an unknown quantity, anyway. I think you need a good fix on what the actual performance figures and "feel" of the boat are, sans foils, because if you fit foils you won't really know what effect they are having, never having trialled it without them.

 

Whatever option we have, would be a bolt on job regardless mr e... if they fail miserably, I will unbolt them and throw them away... easy fix...