Foil assisted multihull design

Thanks bmcf, I've already started playing with xfoil but again I'm pretty sure it ignores wave drag, and I'd appreciate a bit more detail about the biplane model and simulating free surface effects? Can you elaborate on that?

Peter, thanks for joining in...

The goals are 3 fold.

1. Improve fuel efficiency litres/mile at higher speeds ie Fnv = 2.5 and above. Cruise speed = Fnv 2.5-2.9.
2. Reduce vessel motions at all speeds including at anchor.
3. Preserve a shallow running condition and ability to "beach" the boat.

Just wanted to state, that outright maximum speed is not a goal, I'm not trying to do 40+ kts and I'm not looking to lift the boat clear of the water.

Cat dimensions are;
Max loaded disp = 4000kg
Lwl = 10.6m
Demihull centreline separation = 4.1m
Demihull length/beam ratio = 16.5:1.
Draft at max displacement = .43m
Draft in 50% lifted condition = .25m

There is a reduction in hull drag of 2kN @ 10m/s in the lifted condition. So provided we can generate 20kN of lift @ 10 m/s with less than 2kN of foil drag, there is a net gain in efficiency... Just need to derive the configuration to achieve it in terms of foil loading, and pushing the structural boundaries are far as possible to achieve the minimum thickness foils with maximum aspect ratio...

 

So if you do it with a foil spanning the hulls what size would it be? Less then 4.1mx? Are you having one central or fwd main lifting foil and a trimming foil? A canard and a rear foil? What configuration do you envisage? Peter

 

Dont know at this point! which ever configuration can be made with best efficiency... sottorf suggested a tanfem configuration for the lower end speed regime... this should give better pitch damping seakeeping motions if we can get the foils closer towards the ends...

 

Groper Foil

Hi Groper - So I've assumed the following. 1) We are having a canard and a rear foil. This is to maximise the spacing apart to help pitching and general stability 2) Are you intending them to be active or passive? ( i suspect active?) 3) Most foils have a drag bucket of 5degs so we want to operate inside +/- 5degs so the Cl~<0.75 I've picked 0.5 4) so at 50% displacement we need a foil 4mx0.2m 5) if its a 10% foil it needs to be 20mm thick 6) for structural reasons I'd camber the foil down so when it pushed up it pushed into the hulls. If made flat it will pull outward on hulls when it deflects 6) If passive we need to tune it tightly 6) if active we need to be able to trim say +/- 5dges to establish a trimmed position and a lift position to take advantage of the higher lift available at greater AoA. If you know a std NACA section or can give me a section you are thinking about I can do a quick check to see if it will support the load. If cambered down we maybe able to use FG as it becomes a compression problem not a stiffness problem. So would need to build a model with varying camber to establish how much is needed to get the right mechanics ie put the foil in compression rather then bending. First thoughts are it may need to be a solid laminate or not much hollow... Do these things have a central strut as well? or do they span right across? cheers peter s

 

Ha, its funny... i initially did all the same sums and arrived at the same numbers using the same assumed chord! Bit odd considering a just plucked the chord length out of thin air!

The hysucats ive seen pictures of, do use a slightly V`d foil spanning from hull to hull with a center strut supporting the middle - which puts it in compression just like you said. The span this foil then becomes 3.6m from hull to hull.

Sottorf also recommended one of the Eppler-shen hydrofoil sections... i looked at the YS930 and YS915. Theyre lift drag polars are here -> http://airfoiltools.com/airfoil/details?airfoil=ys930-il

For angle of attack variation and trim control, i was hoping to mount the rear foils on the cavitation plates of the outboard engines which have hydraulic trim control - makes it easy in terms of installation and also the ability to change the trim by the push of a button however this puts the foils in a planing condition which is much lower efficiency. We might have to mount them on the inner hulls back near the transom. I was hoping for a fully passive foil system to keep the simplicity and low maintenance.

The stability is achieved via the foils proximity to the free surface as i understand it. As they gets closer to the free surface, they dump more lift and help restore the normal running condition. But as Sottorf mensioned, it gets complicated to prevent porpoising with respect to the foil configuration and LCG etc... Need to remember that we are running these foils only 1 chord length below the free surface, and in waves the foils often/always ventilate...

Provided we can acheive a L/D ratio in excess of 10:1 - which should be doable, then we are in front on this...

 

Cats tend to pitch from the stern not midships. An aft foil will provide very little, if any, assistance in pitch damping as its too close to the centre of pitch. In roll, a single foil provides just as much resistance if the area is the same. It would only assistance in heave, but again, a larger fwd foil would do the same.

That may be well for a single in-plane direct compression load as you’ve described, although without running numbers to confirm, I suspect it shall fail in buckling.

However, what is the foil doing..lifting. Thus what is the load on the foil when lifting. Using your numbers and with your area, the modulus if the 200x20mm foil is a solid block, not even foil shaped, the max modulus you can get is 13.3cm3. When lifting 20kN, that is a bending stress of 501MPa, assuming built-in supports (best case) and a simple UDL, and thus not even taking fatigue nor deflections into account, is seriously under strength if a solid laminate. Thus as I noted before, the span tends to kill these simplistic ideas when faced with the reality of “doing the job”.

 

Groper Foils

Hi Ad Hoc - I agree on a couple of things. 1) I calced the inertia of a NACA0010 and get the flexure stress at 2m span at 498MPa same as you. Typical glass and carbon laminates I build have a flexural stress of ~1100Mpa. So typical static Sf=3 gives stress allowable at 366Mpa which means it would have a shortened life (not indefinite life, hard to estimate actual life) But it would work. So Groper perhaps the section has to be thicker then 20mm. 25mm would halve the stress. I'm involved in a fatigue testing program at the moment and in a month or so can comment on this further. If you went to two Struts then it would work easy, but more spray drag. If we design the span so its a parabola and put the foil into compression then this may not be a problem. Have to run a non linear FEA to calculate the correct camber required to do this. If you have selected the section then this is possible. Re fatigue using a 8% elongation resin and my types of laminates typically puts the fatigue stress of the laminate over the static design stress of most codes ie predicts infinite life of laminate. At about SF=2 static theory is about even. We are going to check this out over the next three months. It has to do with the cross ply strain level that cracks the resin. Very high elongation resins do not fail in this way. The aerospace guys use 25% strain resins on leading edges of aircraft for impact strength and I'd expect these to have infinate life at typical service stress levels. Aerospace are going to thermoplastics to get this type of performance. cheers Peter s

 

That’s significantly higher than values I use. That is also “static” stress , one shouldn’t be designing for static in this case, it must be fatigue driven.

You also need to be careful when testing. The compression cycle is just as important, since you can experience buckling of the fibres under compression and shearing between the fibre and resin.

When taking fatigue into account it gets terribly complex and is exacerbated by the QA. Thus you need a very large ‘sample’ to draw any kind of realistic (to others) conclusions. General rule for CF is an expected loss of circa 40-50% of its static strength, as a rough guide.

However, QA plays a major major role and must not be under estimated; one which is very hard for any “back yard” builder to duplicate compared to a well oiled dedicated composite builder. Thus, results from laboratory testing needs careful review to its applicability to others in non-controlled environments and has any sensitivity analysis been done for exploration purposes.

 

The foil span is only 3.6m - not 4m.

Also need to divide the total lift amonst all the foils ie front and rear foil which might be split into 75% lift on the front foil and 25% on the rear foils. So the main foil only has to lift 15kN...

Some dimentional considerations;
I have a very strong main bulkhead located @ 6m from the transom - potential front foil location. Its also the most forward part of teh hull that carries full draft before the forefoot starts to ramp up and draft is reduced forward of this point which then starts to limit the available submersion of a keel mounted horizontal foil.

LCG is predicted at 4.2m from transom. And this is confirmed by a running spreadsheet of every peice weighed and then added to the boat as its being built, so far there is very good agreement with the "on paper" design.

So the lifting moment occuring at the front foil in this location might be 1.8m*15kN = 27kN/m.

The rear foil thus should balance this moment, providing 5kN of lift and must be more lightly loaded foil for stability reasons? - not sure by how much tho - anyone have a rule of thumb for this? Therfore the rear foil should be located around 27kN/m / 5kN = 5.4m behind the LCG - this is aft of the transom! Therefore this doesnt work and a lower lift fraction on the main foil would need to be adopted in this location.

so perhaps a different lift balance is adopted... if we have main foil 1.8m forward of the LCG and the rear foil 3.3m aft, we end up closer to a 65% main foil lift and 35% rear foil lift.

An alternative might be with a canard arrangement, main foil located behind the LCG - i have another strong bulkhead located at 3.2m from transom = 1m behind the LCG. then put the canard closer to the 6m forward location. This would allow a greater lift fraction on the main foil. So many options...

Then there is the foil interactions between each other... the Hysucats seem to put the stabilizing rear foils higher up on the hull compared to the main foil at keel level. With optimal longitudunal spacing, the rear foils can surf the wave behind the front foil - anyone know how to calculate the wave hollow length?

 

Hi Groper,
To account for inertial effects I have placed the entire load on one foil. Yes if we evenly divided them the 20mm would be OK as the 500MPa max stress would drop to 250Mpa. Ad Hoc, I have been having laminates tested on and off for 20 years so know pretty closly what will happen. Especially with infused laminates they have been very close with different builders constructing the same laminate. Groper don't get tied into putting the foils at existing BH's. You need to optimise the spacing then make structural adjustments. If they fall on a BH then well and good. laminates do not drop "strength" in the 0deg direction. Its not valid to say that the laminate losses 50% say strength due to fatigue. laminates loss strength in the cross ply direction and this happens at about 0.004% or 0.005% (1e8 cycles) transverse strain range using conventional approaches. Its a big subject to discuss here. Theres a lot of fatigue data out there from wind turbines but unfortunately they use polyester resin. Recently they have moved to epoxy to increase their lifetime.
Heavy Duty boats are designed at about vertical 4g, normal duty about 2g so a SF=2 is Ok for starters that would include the inertial effects. So we are pointing at a foil say 20-24mm thick, is this what you expected? I think we can design the span shape to minimise bending so it then becomes a buckling problem. We know from moths and AC72's that cavitation is the big problem at above 30kn so this aspect should be covered. Hopefully we are being helpful. Cheers Peter s

 

Groper - I've just done some reading on the hysucat site. I think its worthwhile connecting the foils to the bridge deck or chamfer panel. If the foils are connected below the static waterline,if you hit something then the resulting holes could sink the boat. Have you thought about this? In this case the foils could extend through the deck and you could control their AoA??? Like the AC72s?? Cheers Peter S

 

yep pretty much... that also limits our material selection to carbon fibre doesnt it... i was thinking about steel but it get very weighty, and then titanium just gets too expensive... so CF is probably the best option for me, as i can do it myself. Same method as weve discussed previously, hotwire or CNC mill a 2 peice female mold forma from a block of foam for our chosen foil section, with an offset for hard surfacing thickness, then ill either infuse a solid laminate or simply vacbag it to alleiviate the thickness problems of infusing, and the 2 halves will bond together...

I hear you on the positioning, was thinking the same myself... this boat has an aft LCG location from LWL center, so a canard arrangement probably fits better with its natural weight distribution. The tandem configuration is also doable. However the aircraft or conventional configuration, will place the main foil close to the middle length of the hull and i doubt i will get as much pitch damping effect for the desired seakeeping improvement. A canard or tandem arrangement seems better in this regard. Either way, a significant portion of the load would be carried on the secondary foils - which may not span the entire tunnel width - they may be an eliptical planform foil cantilevered from the inside edge of the hull for example.

 

I have a false floor in the bilge of the hulls, which is watertight sealed. With the foils mounted near the keel, if the foils are torn out and hole the bilge, there is a secondary compartment to breach before flooding a hull. Should be enough?

 

Don't know until you hit something at 20kn+!! Peter S

 

I have nearly hit dugong several times, with your dugong rake, er, hydrofoils, you increase your chances considerably. There would want to be a big performance improvement in prospect to make it a worthwhile exercise.