HYSWAS anyone?

In a model, the square-cube relationship will strongly affect this tradeoff compared to the full-scale boat. At the same time material density is going to be lower and the material strength and stiffness comparatively larger, so the weight won't necessarily scale as size cubed, either.

You have some fundamental decisions to make, such as the proportion of lift that will come from buoyancy vs lift from the foils. Your choice of takeoff speed also plays a big role, here. Induced drag from lift on the foils decreases with the square of velocity, while parasite drag from the wetted area increases with the square of the speed, and the combination is minimized when they are approximately equal.

If you are looking for performance, then the tradeoff is getting rid of most of the wave drag and wetted area of the surface hull vs the lift-induced drag of the underwater foils. The wetted surface of the pod is also more than for a hull of the same volume because you are talking about full circle sections instead of half-circle sections. You have to gain back the drag of that wetted area, too. So you start with a really big penalty in added wetted area that you have to recover from the wave drag.

A program like Michlet will be essential for making these tradeoffs. You'll need to estimate the drag of the lifting foils and add that to the Michlet estimates.

Human powered vehicle research may help you come up with a laminar flow shape for the pod. Transition occurs earlier in water, but at model speeds you may be able to achieve a useful amount of laminar flow to reduce the skin friction.

With regard to stability and control, I assume this will be a radio controlled model and you'll be manually flying it to stabilize it. If it's to be free running, you'll either have to develop an electronic stabilization system for it, or you might be able to adopt Bradfield-style wands to sense the surface and mechanically feed that back to the foils to regulate flying height and roll. I recommend using the forward foil to regulate height and the rear foil to stabilize in pitch in order to have stable pitch-heave coupling. Shutt-strut foils may also be a way to go, especially for the forward foil. The rear foil could then just follow passively.

By and large, I don't see a HYSWAS having better performance than a slender catamaran unless the competition rules are based on waterline length. Then you can go with a very slender HYSWAS, like a pencil with foils.

Stevenson Projects did an electrically powered personal watercraft like this. The batteries were in the pod.



They did a 1/3 scale model, which might be similar to what you're wanting to build.

 

Mr, Speer,

Very informative.

Very helpful.

Very much appreciated.

Let me "chew" on what you've said.

-Tom

 

Tspeer,

Did you really mean "Induced drag from lift on the foils decreases with the square of velocity,"?
I suspect so, wow that's pretty cool.

Some of your questions addressed:
Yes, radio control but only two channels allowed (supplied) so speed and rudder.
No cats allowed. Mono hull but no mention of submersible pods or hydrofoils...
My intension was Bradford style wands at the three "corners". The forward two controlling ride height and roll, the after one just height.
What is this pitch-heave coupling you reference, porpoising?

The rules are 3 kg above the waterline (static), 3 kg in the hull (bottom of the pod seems the place for that).
90 cm max length, 20 cm max draft. Foam construction. Motor, rudder, prop, batteries, radio are all standard issue, no changes, no mods.
Speed, static stability, design and maneuverability are the judging criteria in desending value.
It is a 90 meter course with a standing start, 45 m turn around buoy with the width of a swimming lane to do it in and a flying finish.
Stability is measured at 5 cm from centre, amidships in unknown weighted increments.

My thoughts following your post are a 40 cm wide, 90 cm long shallow, flat (stable) surface hull.
Two forward struts to the main (80%) hydrofoil on a 3 litre pod and a single aft strut with rudder behind he prop and a 20% lift foil.

My thinking is, the airborne surface hull could provide ground effects.

Additional note: the best thus far is 2.4 knots from a traditional displacment hull and 7.8 kg total.

I'll draw up a picture of what I have in mind.

-Tom

 

Side and bottom view.
Front view.

Only managed 1 litre in the pod in order to keep the diameter to surface ratio below 3.

Top foil adjustable but fixed while underway.

Pod ~4 kg, surface hull~3.5 kg

 

Yes when compared on the basis of equal lift, which is generally the case (lift=weight). That's the whole reason planing boats and hydrofoils are used for high-speed craft. Parasite drag, however, increases as the square of velocity. So the whole point of hydrofoils is to trade dynamic lift and its associated induced drag for buoyancy and its associated wetted surface & skin friction drag. But the tradeoff isn't favorable at low speeds where the induced drag is high.

You might just need the two forward wands. The stern can be allowed to trail freely, with a fully submerged stern foil used to stabilize the boat in pitch.

The three conditions you need for static stability are, starting from a condition of equlibrium,
- if the craft is disturbed in heave, there must be a reduction in lift as it rises and an increase in lift as it descends,
- if the craft is disturbed in pitch, there must be a bow-down moment generated if it pitches up and a bow-up moment generated if it pitches down,
- if the craft is disturbed in heave, there must be a bow-down moment generated if it rises and a bow-up moment generated if it descends.

The first two conditions result in the craft wanting to come back to its trimmed flying height and attitude. The last condition is for stable coupling between the pitch and heave motions. To see why, consider having a fixed forward foil and a split stern foil so your two wands control the heave and roll at the stern. As the boat accelerates, the foils will produce more lift and it will rise. The wands at the stern will reduce the lift there, so the stern will rise very little. But the forward foil will lift the bow, and the increase in pitch attitude will increase the angle of attack on both the bow and stern foils, increasing the lift further. The wands will compensate for this at the stern, but the bow will rise even further. The result will be the boat will zoom up, out of control, until the forward foil stalls or breaches the surface.

Now consider the opposite situation, with the wands controlling the forward foil. As the boat accelerates, the lift is increased on both the forward and stern foils, and the boat starts to rise. The feedback from the forward wand will reduce the lift on the forward foil, slowing its rise compared to the stern foil. As the stern rises, the boat pitches bow down, which reduces the lift on all the foils, helping to return to the equilbrium flying height. In effect the wands keep the forward foil pinned at the same height while the stern weathervanes about the forward foil. This is a stable situation.

Batteries and motor in a deep pod would contribute a lot to the static stability. You could also go with a low fineness ratio to the hull for more stability because it would only be in the water at low speed. The depth of the pod would contribute stability and also allow the boat to heel steeply into the turn for maneuverability.

You need to get the definition of "catamaran" clarified. Sometimes this is enforced by requiring that there be no hollows in the girth, often measured by rocking a straightedge around the hull and requiring no daylight if the straightedge touches at more than one point. If the girth is only around the canoe body of the upper hull, then a scow hull would be appropriate for the requirements. If the girth includes the pod, then you have a problem.

A single strut forward might have less drag.

Forget ground effect. Even if you could get significant aerodynamic lift, the span would be so small that the induced drag would be high.

 

Very good.

I understand.

I don't want to narrow the hull too much as I'm going to get great initial stability from it (points) and shallow draft which means more clearance for a larger pod diameter (increased volume/lift). Would you agree three diameters below the surface is a good target depth to avoid surface wave drag from the pod?

Are you in agreement with my upper, fixed (but adjustable) foil to aid getting the hull out of the water at low speed. Laddering?

I suppose the next step will be Michlet and lift drag figuring.

Thanks again for your interest and assistance Tom.

-Tom

 

I have missed most of the middle of this thread, but I do have one comment based on the image of your design. A long thin centerbody is probably not the ideal shape for minimizing drag. The surface area to displacement ratio is higher than it needs to be and it will not be as effective for promoting laminar flow. A body with equal displacement and a L/D ratio of about 6-7 is probably better. If it can be run smoothly, laminar flow can probably be achieved for alot of the body if properly shaped (like a laminar flow airfoil as a body of revolution).

 

I don't really know - I've never run any cases with submerged bodies. But it sounds reasonable. Michlet would give you a better handle on the wave drag.

There's going to be a tradeoff between the wetted area of the struts and the running depth of the pod. But you control that with the hydrofoils, so it's something you can tune by experiment.

I think the upper foil is mostly adding wetted area. It is close to the surface, so the induced drag will be high, too. If you're going to use a ladder foil, I suggest you mount it lower and add dihedral. That way there is a gradual reduction in area as it comes out of the water. Sudden changes make for difficult control.

Yes. You need to start doing performance predictions so you can see how the performance changes with differently sized elements. There's no way to know if something is good or bad just by looking at it.

 

Before you do, have a look at this thread for some guidance.
http://www.boatdesign.net/forums/hy...sistance-predictions-using-michlet-42258.html

 

Hi John,

I believe this was addressed in post #27.

Thanks.

 

I wonder how they got the power from the engine(s) down to the shaft. In the video it appears to be an ICE (maybe diesel), likely located in the main hull. Did they drive it with two ninety degree shafts....or some sort of belt between parallel shafts??

 

Maybe diesel electric with the motor in the pod...

Who knows...?

 

The MAPC HYSWAS was simply a Cummins diesel down in the hull..direct drive to the prop via a transmission.

Access to the engine was only possible when it was on the hard, via removeable panels.

 

Now THAT is a big hydrofoil.

 

Somehow, that doesn't look right. The construction is just off for a SWATH and the blocking looks too insubstantial to be supporting that much hull.