# Aerodynamics - thundercat racing inflatable

Discussion in 'Hydrodynamics and Aerodynamics' started by Roflhat, Oct 13, 2015.

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### markdrelaSenior Member

As I said earlier, for a configuration of a given planform and CG position, adding rear amber or adding positive tail incidence has no effect on the aero pitch stability margin. What it will do is add nose-down pitch trim. To regain pitch trim it is necessary to move the CG back, and that will reduce pitch stability.

I think that trying to be clever with the hydro forces is counterproductive here. When designing a stable boat which in normal operation might briefly lose contact with the water, one obviously cannot rely on hydro forces when working out the airborne stability, because then they don't exist. To me, the natural first-iteration design sequence is pretty clear:

1) Design a configuration planform and CG position which has aero pitch stability with some chosen margin. The planform may or may not include a horizonal tail. For an "aero-clean" boat like a hydroplane, Vortex Lattice is the natural design tool at this stage. CFD or wind tunnel cut and try might be needed for less clean boats.

2) Decide on the amount of aero lift (L_aero) during normal on-water operation, at the on-water pitch attitude and at the design speed. The rest will be hydro lift. Clearly the aero lift has to be less than the weight, otherwise the boat will fly.

3) Decide on the aero pitching moment (M_aero) about the CG in normal operation. You clearly don't want the nose to rise after losing contact with the water. So the aero pitching moment should be zero or slightly nose-down (center of aero lift is at or slightly behind the CG).

4) Determine the pitching moment M_prop which is imparted by the thrusting prop or water jet in normal operation.

5) We now know the required hydro lift and hydro moment:

L_hydro = Weight - L_aero
M_hydro = -(M_prop + M_aero)

These tell you where the hydro lift needs to be applied:

x_hydro = x_CG - M_hydro / L_hydro

That's where you put your planing-surface patch. Done.

Obviously we now need to see what it does at off-design speeds and tweak things as needed, but the above should be a good start.

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### RoflhatThundercat - 70.10mph

Ok thanks. The pilot sits to the side (starboard) and the copilot sits on the port side. So the engine is in relatively undisturbed air. I was looking at designs on lorries, and I was thinking it may help, if only a wee bit, to add a sloping box at the rear of the cowl. like this:

It can run at redline as it is, but hopefully should be able to rev higher with the top hat design cylinder head.

I currently have about 11 different propellers, mostly surface-piercing (choppers, cleavers). Some are for use in surf, some for top speed etc. I've tried most of them but need to do more testing once the engine is set up.

The engine has an extended tiller, so I can sit further forward. But at low speeds it's often best to sit further back, to keep less boat in the water.

Yes the boats do go faster with the weight as far back as possible. The current record is set at 74mph, the total weight of the pilot and copilot was around 120kg.

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### AluminationJunior Member

Good information.

I'm surprised Jim Russel's name hasn't been brought into this discussion.

By locating CG ahead of CLA, any moment like a wind gust or rogue wave which creates a greater angle of attack can help produce more lift in the stern than the bow and prevent a blow over event.

The problem with additional wings or foils, which will not operate in ground effect, is that they won't produce the same amount of lift as the tunnel which is in ground effect. That is unless they are huge which will negate any performance gain due to drag and weight.

Since the pilot/co-pilot of this boat influences CG so greatly, it should be easy to experiment and control the balance.

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### AluminationJunior Member

Hydrodynamic drag has a much greater effect than aerodynamic so bigger gains will be had by addressing them first.

How high do you have the engine mounted?
Where is the centerline of the propeller relative to the bottom of the hull/sponsons?
About 2" above the bottom is usually the most efficient if the engine is set up for it. Low water pickup either on the engine or on the hull.

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### RoflhatThundercat - 70.10mph

It varies depending on the propeller. For the higher pitch surface-piercing props I raise the engine up. For use in surf and waves the engine needs to be lower for better bite. Also the higher the engine is mounted the harder it is to steer, I have lots of steering dampers and bungees set up but there is a lot of torque. I was thinking about putting a LWP on the engine, I may still do so, I'm going to do some testing and see how it goes.
At these sort of speeds it's not so much the lack of ability of the boat or engine as the stability that limits how fast I dare go.

Although I want to increase the stability and top speed, I don't want to compromise the driveability or versatility of the boat - I still want to use it in rough seas and in the surf. So if there are ideas which improve every aspect of the performance those would be my top priority.

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### AluminationJunior Member

If you are running without a LWP then your engine isn't nearly as high as it could be.

At what speed is the boat up and out of the water, running as high as it ever does?
I'm wondering if you have too much hydrodynamic lift, if you have too much hull/sponson bottom in contact with the water.
What are the sponsons made of? UHMW or the like? Delrin, Urethane like material? You might could fasten a block or strip of similar material to the bottom which would be narrower so more of it in the water at slower speeds and less touching at higher speeds when WIG is providing lots of aerodynamic lift.
Do folks already do this in the inflatable racing world?

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### RoflhatThundercat - 70.10mph

Yeh I could go a lot higher with the engine. I'll probably get a LWP, my biggest prop is 23p, which will probably need to be very high to get revs from it.

I would say at about ~47 knots the boat is as high as it gets, from there on it only lifts slightly. There's not much boat in contact with the water at 50+ knots, just the back 5-10cm of the hijackers.
The whole boat is made from PVC rubber. I was looking into putting a teflon coating or similar over the rear section of the hijackers but didn't do much research into it. Changing the shape could be an option, although it wouldn't be too easy to do I don't think.

I believe the newer models of the boats are available in different configurations, some for flat water, some for offshore etc. I think the flat water ones actually have wider pads on the bottom of the hijackers?

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If the cog is in front of the aero lift
What happens when the hull leaves the water..
Nose dive.

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### Mr EfficiencySenior Member

Yeah, some of the ideas proposed in this thread should not be tried "at home" ! There are probably hospital wards full of experimenters !

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The average punter doesnt realise the gearbox and skeg shape, the prop, the hull running surface,
The aero of the hull both on deck and running surface and the cog all are working togther when its in its normal operating range
Now add wind and wave variables
and where's the text book for that?

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### JimboatSenior Member

I agree with markdrela and tspeer.
A tunnel hull configuration is 'inherenetly unstable". When a tunnel hull, including a Thundercat/RIB design, experiences a “disturbance” that causes a “nose-up” rotation about its CofG, the increase in angle of attack causes an increase in aerodynamic Lift, which causes the boat to rotate “nose-up” about its CofG. A small increase in angle of attack tends to cause a bigger increase in angle of attack, rather than tending to “restore” things to the original state (as an airplane could). In this condition of dynamic instability even small changes tend to cause bigger changes, and is then in the region of easy “BLOWOVER”. [More in article on 'Blowover']

The performance tunnel boat gets much of its performance from its use of aerodynamic Lift. By necessity, it maintains its thrust (propeller) and control (steering, trim, etc.) by contact with the water surface. The performance boat must make most efficient use of aerodynamic Lift while never losing contact with the water surface. This is the dilemma that forces inherent instability - but also what makes tunnel boats so much fun!

There are hull design practices that can make a hull, including Thundercats/RIBs behave better more predictably than others, but the performance tunnel boat can always have the potential to react to disturbances due to it's inherent instability.

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and that why we get sponsors as the crashes are spectacular which is fine in a 6 point harness and safety cell

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### JimboatSenior Member

Yes, sponsors and spectators love the racing crashes, but believe me, even with a safety cell, blow overs are not fun!

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### AluminationJunior Member

The lesser of two weevils.

What area of his boat would you address first, how can he make his boat faster?

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### tspeerSenior Member

Not necessarily. It will pitch down, yes. But to nose dive, it needs to pitch down more than the hull can stand. This requires both a substantial altitude above the water and a comparatively rapid pitch rate.

The stable aerodynamic/hydrodynamic coupling will keep it from getting so far off the water in the first place. And the amount of aerodynamic stability can be tailored to keep the pitch rate in the tolerable range.

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