Foil assisted multihull design

With the current bearing materials and titanium foil construction, only the increased cost of fouling maintenance remains as a cost factor any different from more conventional hull configurations. Modern hydrofoils seldom resemble the complex PHM/Boeing variety. The large titanium foils under the USN X-craft catamaran have been serviced only once in 7 years to renew the wear bushings..for example. Beyond that...nothing but cleaning and painting.

Rodriguez has done reasonably well in the commercial ferry sector; there are still a lot of their foils operating today. Certainly a lot of the various Russian/Ukrainian hydrofoils still out there as well. The Fjellstrand Foilcats are still in service..

Meanwhile...I'm not aware of any currently operating military hydrofoils.

 

I've literally hundreds...but need to dig them out and sort through my haphazard filing system. But around 90% of them are old paper copies; copied when I was a student and before the days of computers/pdfs. The one I listed on the previous page, is some 30-40pages long, and old photocopy print which has faded a tad over the years.

If you're interested in more up to date computer/CFD analsyis types, then:

"The effect of finite depth on 2D and 3D cavitating hydrofoils", S.Bai, or "Multi-fidelity optimization of a high-speed foil assisted semi-planing catamaran for low wake", M. Kandasamy et al. Or a bit more practical based, "CFD validation studies for a high-speed foil assisted semi-planing catamaran" M. Kandasamy et al.

 

Im a little confused sottorf, you said the above in a previous post and yet use sub cavitating profiles... can you clarify what you meant here?

 

I know those papers - limited value in terms of seakeeping. THere are hundreds of papers on hydrofoil0supported catamarans but very few provide anything useful about SEAKEEPING of FIXED FOILS.

I have posted a copy of the the Hadler 1974 paper on this forum electronically before for those who are interested.

 

Don't confuse cavitation with ventilation. Ventilation occurs when the foil gets too close to the surface of the water and the low pressure area on top of the foil is ventilated by air. When this happens the foil looses lift and drops deeper into the water. Ventilation stops and the foil starts lifting again, immersion reduces, foil ventilates and the cycle repeats...

Changing the profile shape will not help against ventilation. One has to design the vessel so that if the foil ventilates it does not create a porpoising instability. foil lift fraction and position relative to LCG are important parameters.

 

Out of all the references listed in this thread, i could only find this one for free on the web, Adhocs last reference above, CFD validation studies for a high-speed foil assisted semi-planing catamaran" M. Kandasamy et al http://www.google.com.au/url?sa=t&r...AW4gLOlHA&sig2=AceInQqu3mvbR8lsoIZaSg&cad=rja

I did find one other reference from Adhocs direction, relating to the USN and their ocean catamarans of the 70`s, HAYES and ORTOLAN. Its about the addition of the forward cross hull, keel mounted foil added to improve the seakeeping of the vessels - they had a problem of wetdeck slamming and the foil addition was deemed a big success in terms of seakeeping, reducing motions of pitching, rolling and corkscrewing. Notably, the bow pitching movements were reduced by upto 30%. It didnt contain any information relating to resistance however. http://www.google.com.au/url?sa=t&r...=Bvzj0-ENJNtzDl9PEMlCEw&bvm=bv.53371865,d.aGc

Ok i need help in selecting a foil section for my needs. From there, i think i can calculate the required foil dimensions, the induced drag and re calculate the new hull resistance by adding the foil drag and modified hull drag to arrive at the new total resistance. At least in a ball park kinda way anyway....

 

Ok, thanks for the clarification.

How does the Hysucat system cope with variation in LCG position? This could be an issue with a pleasure boat whereby people are moving around all the time, movements of people from the stern to the bow could be quite common and would change the LCG quite considerably...

 

This paper, i just found and seems very pertinent to my problem; "A numerical method for performance prediction of hydrofoil assisted catamarans", ive attached a copy of the PDF.

Its late and i havnt had a chance to run the calculations via the formulas shown in this paper yet, but ill get onto it when next i get a chance.


View attachment foil assisted cat resitance prediction.pdf

 

HYSUCAT system has worked well down to 6.5m RIB boats with passengers moving around. There was a 5.5m HYSUCAT RIB produced for a while but that one did not work as well. It was more difficult to balance out with passengers moving around.

For larger vessels it is generally not a problem. But all the above only applies if the rear foils are present and correctly located sized and on the hull to suite the LCG shifts.

During the design of the foil system, one needs to consider all the extremes of LCG variation and then choose a foil setup that works. It may be that it is only possible to carry 40% load fraction on the foils to avoid problems due to LCG shifts. Each case has to be looked at individually.

 

I propose you choose a profile based on the Eppler-Shen profiles or something similar. These work well for shallowly submerged foils.

Shen, Y.T. and Eppler R. 1981 “Wing Sections for Hydrofoils-Part 2: Nonsymmetrical Profiles,” Journal of Ship Research, 25, 191-200.

You will have to balance the thickness of the profile with structural strength. Make sure you use a hefty safety margin in your calculations to account for cyclic loads otherwise your foil will crack.

If you really want to study hydrofoils, you will find all the info you need on the International Hydrofoil Society Website. www.foils.org They have a couple of CDs for sale with a collection of the most useful papers on hydrofoils from Western (mainly american) developments.

 

Groper,
You might also consider the H105 section from Tom Speers.

 

The H105 section is designed for low Reynolds numbers, typically for sailing hydrofoils that have problems with laminar separation bubbles. These are not ideal for HYSUCATS because the boats are faster, the foils have larger chord (both resulting in higher Reynolds number) and they operate very close to the free surface which changes pressure gradients on the upper surface of the foil and in turn the boundary layer. Laminar separation bubbles are not a concern and only need to be considered if you are testing a scaled model in a towing tank.

Eppler-Shen profiles are a better option. Another good alternative is the Goettingen K-Series profiles.

 

Ride control foils - which are not lifting foils but rather motion damping appendages - are the only application where actively controlled foils are used to some extent. In Hong Kong the largest ferry operator in the World - Turbojet - took all the ride control foils off their catamarans because they were just a source of trouble and maintenance.

Both Rodriquez and Russian hydrofoils use fixed foils - as they are simple and cost effective. I heard Rodriquez has a development for a new hydrofoil craft with actively controlled foils but the program is stalled due to lack of funds in the present economic climate...

One FoilCat is still running in Hong Kong by Turbojet but they don't have much good to say about it because it is high maintenance and has a stupid waterjet intake system which vibrates and cracks. The FoilCat also had many teething problems with its ride control system which caused the vessel to bow dive in certain sea conditions. Needless to say, the FoilCat is out of production after only a handful of vessels being built and millions of Dollars of research funding spent.

Too many projects have shown that actively controlled foils are a lot of trouble and their use in my opinion will remain limited to ride control appendages.

I agree titanium is a great material for foils but far too expensive right now and so difficult to weld that it will also remain in realms of a few specialized military projects. The X-Craft is an Office of Naval Research funded project which is of course military.

Over 600 HYSUCATs with simple fixed hydrofoils have been built worldwide - in size ranges from 4.5m to 40m and with speeds up to 70 knots - over the last 30 years since its invention and interest keeps on growing. In other words the case of fixed hydrofoils is really strong based on past experience...

 

Having personally directed or at least been involved in the production and testing of many actively stabilized hydrofoil and foil assisted vessels, I have to disagree. As the head of the original team that designed and delivered the first active foils used only for ride control and not lift...I might know something about those too. They were very much an adaptation of our previous hydrofoil knowledge.

In fact, many years ago when Rodriquez first started delivering active systems with their foils, we were concerned that they might try to compete with us; outside of just their own production.
The Russian Katran class, and similar, boats have active elements integrated in to their foils for ride, balance and height control; I was involved in a project to upgrade the flight control packages (replace is a better word..the original control equipment was gorgeous..just not supportable).

And so on..30 years worth of "so on". It's been quite a ride.

 

I used to travel frequently on one of those hydrofoils. In rough weather they were real pigs. Yelps and screams from those on board less used to its odd motions when rough.