Foil assisted multihull design

Not sure what you mean by adjusting the engines, but the cav plates are set so they are just below the transom bottom edge. So the props are always in the right place regardless of how far the boat is lifted unless the transom is lifted clear of the water that is... the transoms will take anysize engine upto 150hp per side, but the original plan was for 90hp per side.

theres no weed and very little general debris around these waters here so im not concerned with those issues...

sottorf, im not really interested in outright speed, but rather fuel economy. How will hysucat foils perform in terms of fuel economy at 15 kts through 30kts?

 

You face an uphill battle to power and prop your boat for a cruise speed of 30 knots with foils, but having to back off to 15 knots due to sea state, that will involve any kind of "fuel economy"

 

Well without any calculations to support that view, im not going to buy into it just yet.

Im also interested in the damping characteristics installing them will provide in terms of improved seakeeping manners and reduced accelerations.

So i guess instead a bit of research is in order, and hopefully we can get a decent information flow happening... i hope we will hear from people like BMcF and Tom speer etc...

I Found this in a thesis by Bryce Peirce relating to supercavitating foils for ride control in high speed craft; I thought its rather relevant for shallow running foils as they are most likely fully ventilated most of the time at speed.

The foil profile has a significant effect on the performance in either subcavitating
(i.e. fully-wetted or without cavitation present) or cavitating conditions.
The various profile types described in the following are also indicated
in Figure 1.1 as appropriate. Optimum sub-cavitating foil sections have an
upper speed limit of about 45 knots (Conolly, 1975). For stable operation
at higher speeds, where cavitation cannot be avoided, special supercavitating
foil sections were developed which operate with the suction side of the foil
completely un-wetted (Johnson, 1957, 1958; Wu, 1956). This is achieved with
the super-cavity forming off sharp leading and trailing edges, as shown in the
lower portion of Figure 1.1. Interest in the use of these supercavitating hydrofoil
sections was directed to high-speed hydrofoil borne craft, super-cavitating
propellers and seaplane hydro-skis (Conolly, 1975; Johnson, 1958; Tulin, 1964).
Sub- or supercavitating foils designed for high-speed operation, and at shallow
submergence, are required to have thin sections to obtain suitable efficiency
in performance5. Such thin sections, in addition to low camber, are necessary
on sub-cavitating foils to eliminate the occurrence of cavitation and a thin
leading edge is required on supercavitating sections to ensure cavity detachment
is maintained at this point. These thin profiles, together with the high
loadings, present significant structural trade offs in the achievement of suitable
practical high-speed foil sections of either type (Huang, 1965)

I found this interesting.

I guess in a high speed hysucat type configuation, they would opt for a super-cavitating foil section running very close to the surface A compromise might have to be made if slower speeds and more foil immersion is a design constraint, in which case a semi-cavitating foil might be a better option.

Improvements to tip loses (where present ie aft foils) might be possible from forward swept plan form. Ive also noticed the negative dihedral in the aft foils of hysucat and teknicraft - presumably for the same reason?

Ill add more as i find things and have time, hope some others can contribute...

 

Have you bought the 90 Suzukis ?

 

Richard,
The US Navy hydrofoil develop was done in Washington state and they they developed design criteria for foils considering impact of foils with deadheads. Impact with floating objects is one of the most important considerations and the foil system strength must consider this aspect.

A Boeing Jetfoil prototype actually struck a deadhead at 40 knots and the bow foil went straight through it. That is generally the design requirement you need to work with if you want a practical foil system.

Weed can be a problem if you are running is thick weed or kelp beds as this will catch on the foil. Normally by reversing the boat you can free the foil once you are out of the weeds. I only ever heard of one case of a 6.5m HYSUCAT boat running in thick kelp beds where the amount of kelp on the foil was so great that added weight of it ripped the foil off. I should add that that boat had a very weak foil-hull connection that was corroded (not my design).

Foils do have their disadvantages but these are generally worth the copromise as it is not that big a price to pay for 30-40% speed increase, up to 30% fuel savings, 30-50% reduction in vertical accelerations and 20-30% reduction in wake wash...

 

At 15 knots there will be no benefit in fuel economy for the foils. At 30 knots it should be roughly 20-25% as a guesstimate.

 

Generally, with foils fitted one can maintain a higher speed in waves and the ride is actually more comfortable at higher speeds because the foils ability to damp out vertical motions depends on having a certain forward speed for the boat.

I cant give specific data on this application without detailed analysis.

 

Groper,
that information is a little dated and is applicable to deeply submerged foils. Once foils operate close to the water surface, cavitation inception is well above 60 knots. Actually based on what I have seen cavitation will never occur on a HYSUCAT even at much higher speeds.

We therefore stick to sub-cavitating profiles even for 60 knot applications and we have never had any form of cavitation damage or vibration. Sub-cavitating profiles have a much better lift to drag ratio than super-cavitating profiles.

Regarding damping characteristics of fixed hydrofoils in waves, this paper gives a good indication of what is possible:

Welnicki, W, 1998a, The Enhancement of Seakeeping Qualities of Fast Catamarans by Means of Stabilising Foils, Polish Maritime Research, Vol. 5, No. 3, pp. 10-13.

I am traveling now and don't have that paper with me. I will try and remember to post it here when i get home.

 

There are many good papers out, one of the better ones is a paper published in 1974 written by: Hadler, Lee, Birmingham & Jones "Ocean Catamaran Seakeeping Design, Based on the Experiences of USNS Hayes". Good theoretical coupled with sea trail data. By today's standards the data is limited, but the theory is sound.

 

Good reference Ad Hoc..

if you have many good papers on the subject of the seakeeping characteristics of catamarans with fixed foils please do share the references. According to my knowledge there is very little available that deals with fixed foils. Most papers deal with active control which is of limited value here.

 

That is exactly the design lift distribution for the foils under our foil-assisted trimaran, which was designed to operate with approximately 80% of the displacement on the foils at speeds over about 18-20 knots, with a maximum speed of around 30 knots.

That vessel was 13.5m LOA and 9 tons displacement, btw.

All of our/my experience with foil-assisted and full foil supported cats, tris and monohulls is only for actively controlled foil lift though..so I don't have much to offer the discussion of fixed foil configurations.

 

carrying 80% of the lift on foils would only work in a actively controlled situation. BMcF why not carry 100% on the foils if you have active control? The only good reason I can see for not doing that is to keep a small part of the hull in the water so you can use conventional propulsion systems.

 

That. Same as with surface effect ships, which also operate at roughly an 80% lift fractoion..allows the use of readily available conventional propulsion.

 

And the resulting ride quality and stability is nothing short of phenomenal.

But with actively controlled foils and flight control systems comes expense, of course, of both the initial cost and long term cost varieties.

 

not to mention the cost of maintaining moving parts under water... only for governments with deep pockets!