My thundercat

[Matt]

I think these 2 points need some deliberation.

I really feel like extending the tunnel aftwards with some flaps, probably only slightly angled down, is worth investigating to improve compression in the tunnel.

There's an interesting pic of an american cat I saw a few years back, lemme see if I can find it again. On the face of it - normal american music and big engines. But the interesting thing is you can see, once it was really motoring, that it was actually blowing air sideways out of the tunnel UNDER the sponsons.

Originally Posted by jwalker

where air is squeezed and accelerated the pressure within it drops and secondly, as a turbulent area builds it becomes more solid and can actually begin to shape the airflow around it. This can be used to your advantage.

[Matt]

Here you go. There's lots of things I really dislike about this video, but fortunately 3 things I do like.
b. 1m40s you can see it's running well enough - but still very wet.
a. 3m8s you can see now it's really motoring, it's "puffing" air out of the side of the tunnel.
c. The Brunette

God only knows why they think it's OK to run a 100mph boat without any kind of lifejackets on. Although I suppose if I was that safety conscious, I'd stay at home and not run the Arrow at all.

https://1drv.ms/v/s!AhLMmfVtRmsDkapd...X-_Mw?e=MZuEuN

[Matt]

Have you got a side on shot of the boat on the trailer, engine in the normal running position, please?

[Matt]

Oh, just working my way back reading the whole thread. Can see you've tried ATF in the gearbox. What effect did it have?

[Roflhat]

ATF didn't seem to make a huge difference, I still run it but minimise gearchanges when the engine is running

Another thought, extending the bottom of the hijackers further back. Would have a similar effect to extending the floor but possibly more so, would counteract the bow coming up

[Matt]

So this is a thing. Obviously massively exaggerated to make the point, but....

But having a step part way up the running surface it materially makes the footprint of the boat longer when you're running flat out. It also brings the centre of hydrodynamic lift way further forwards. The goal is to achieve a few things:

Moving you into the regime of the 3-point hydroplanes where you're intentionally using the bullet/prop lift to run the boat on. So you're taking that drag that it creates and using it to produce even more useful lift.

A longer footprint to improve stability - in all directions

And by stability, I don't just mean to prevent a blowover

Also means you can run a further forward CG and get the same AoA

Starts to give you options about step height and ramp angle independent of aero angle - meaning you can potentially further reduce the wetted surface area.

There are equations to calculate lift off a ski for varies sizes, angles & speeds - but it's too long ago since I studied it all for me to remember them properly I'm afraid. Although It then also opens the door to, for example, having a slightly concave step with sharp edges so that the usual ineffiency you get off the spray being deflected outwards - is not deflected inwards to increase lift.
(Look at the transom a Mannerfelt B23 so see what I mean - caveated by the B23 needing a slightly protruding single point in the middle of the V to meet class rules as a monohull!
And also - look at a monoski. They exhibit the same - albeit usually less sharp chines than I suggest here)

You can then also modify the step so instead of 10 degrees deadrise, you're running 5, or 2, or 0..

It's a big "step" going in this direction - and no guarantees. In fact, I'd be very keen for people to pick holes in this idea.

Oh - and clearly step depth, length & location are just to demonstrate the point! But somewhere near CG would feel about right to start with. Probably a fair bit further forward than I mocked up in the pic.

The goal - to see how close you can get the boat to being aerodynamically stable enough so that it's only the gearcase in the water.

[Matt]

What are the hijackers? Is this what I call the running surface?

re. ATF. I didn't think it was the dog clutches that take the hit, it's the gear teeth face that rely on the oil film strength to stop them making contact?

Certainly on my boat, I know I'm about maxxed out with the torque I can put through them using the proper merc high performance stuff. But everyone knows even the XR the uppers on a bravo drive are "lifed" once you put 600hp ish through them.

[Roflhat]

That’s good thinking, I was thinking cutting steps in but adding some on is a better way at least of testing.

The hijackers are the “speed tubes” basically the bits that make it a cat. The atf seems to come out clean enough each time, although for the difference it makes I may just go back to Mercury stuff.

[Matt]

Ah, OK the sponsons?

I was thinking about how you could attach steps. The guy I used to race with just sikaflexed mahogony wedges on to the running surface to adjust the step/pad configuration. Sadly the boat was just way too heavy to really wake it up - it was always a challenge just to get it to break free at all, he designed it for 3B, but ended up running it in 3C.

I was wondering if you could bond a layer of carbon, front to rear, onto the running surface and then use that as the base to experiment on. I dunno - can't think of anything particularly better at the moment.

The idea of steps is that everything aft of the step is then running clear of the water. Conventional 3-step design is more robust than that - it's not designed to be running like a 3-point hydroplane at all - but in an offshore hull, conditions are all over the shop - so nice neutral trim & balance, plus the efficiency of the step, is all good.

Where I'm thinking with you though is proper 3-point hydroplane. It will need some modelling - but I think you're already conversant with CAD & CFD, way more so than me (When I did my degree in it all a 486 DX2 computer running Windows 3.1 was high-tech).

As I think about it more - I'm thinking of another couple of ideas.
1. Steps could be mounted to the tunnel and the mount follows the countour of the toobs rather than fixed to the toobs themselves.
2. Which then got me thinking about mini foils. Classically, foils are pretty useless over about 40 knots since they start cavitating uncontrollably. Even some of the foiling trimaran sailing boats struggle with this, they are that quick. They are epic machines :

https://www.youtube.com/watch?v=QFnWEnbdZfI

However, I am sure I read that there has been progress on developing super-cavitating foils. It's always been the holy grail, for example, on torpedos - claimed >200mph top speed underwater : https://en.wikipedia.org/wiki/VA-111_Shkval

Anyway - probably time for me to climb back in to my box.

[Roflhat]

All sounds a bit complicated, didn't understand a word of the video!

A while ago myself and Andy were discussing adding air intakes in the nose cone which would feed under the hijackers to they'd be running on air. But probably more time and effort than it's worth.

Will look into adding steps to the hull though I've wanted to do that for a while, adding ones rather than cutting is a good idea

[Matt]

Yeah, I mostly posted that vid just because I think those boats are epic on every level. And to demonstrate they are not above having the same challenges everyone else does.

In short, the issue is that because water has quite a lot mass, you get to a point where to get enough lift to lift off at all, you need a particular aerofoil - but then it's has a speed limit where the water won't follow the curve of the foil and breaks away. Bit like cavitation on a prop. So you start having to do a bunch of stuff to manage that. For example, finite thickness trailing edges, like you can see on the vertical stabiliser of the North American X-15

I don't think foils, especially super-cavitating ones, are an interesting development path for you at the moment, based on my current knowledge of them.

I do have some ideas about how to stick steps to your running surfaces without destroying the tubes though.

[Roflhat]

Haven't done much with the thundercat lately, spent 3 months in Arctic Norway again.




Now working with Ineos Britannia towards the Americas Cup as chase boat captain


Going to sell my current thundercat, planning on buying another in white as I'll be based in Palma then Barcelona over the next couple years. Trying again to do some CFD testing, this time of different floor angles in an attempt to improve stability at high speed. All the manufacturers are currently lifting the level of the floor at the nose, which makes the boat far twitchier. Probably ok with 50hp and in the rough but not good for high speed


Also got a new model of propeller out, the Forge V3. 12x22, which is more suited to high speed. Planning on tuning a few for my own boat, they're close to the size of my fastest props

[GuyC]

Interesting jobs - both the one in Norway and the one with Ineos.

There must be someone in Ineos that can run some CFD for you.

[jeffstevens763@g]

Lucky lad looking forward to the new boat

[Matt]

Originally Posted by Roflhat

Trying again to do some CFD testing, this time of different floor angles in an attempt to improve stability at high speed. All the manufacturers are currently lifting the level of the floor at the nose, which makes the boat far twitchier. Probably ok with 50hp and in the rough but not good for high speed

Have you considered other floor shapes - an undercambered floor for example, might give you some opportunities to fine tune the balance?

[Roflhat]

I've read a bit about it, if anything I would flip that camber upside down to provide downforce. But I don't think there's any way of achieving aerodynamic stability once the bow starts to lift, no matter what shape the floor is once it starts lifting it will continue to lift. So the next best thing is low drag, flatter so the bow will rise slower and give you more time to react.
Other than my front wing concept the next best thing would be a sort of wheelie bar setup behind the transom with some waterskis behind the hijackers. Just above the waterline so when the bow starts to rise they make contact with the water and push it back down again

[GuyC]

I guess the difficulty is reacting to the change of trim quickly enough to do anything about it.

Looking at three point hydroplanes, they have two points forward in way of the sponsons and one aft in way of the engine. They also juggle the size of the 'air trap' and use a wing to optimise trim.

More guessing but I suspect that what gives the three point hydroplanes their stability is the longitudinal separation of the planing points. With the Thundercat, your three points are the aft end of the hulls and the outboard which have very little longitudinal separation and (more guessing) this will make controlling the trim much more difficult.

With the wheelie bar, could you connect it to the throttle so it cut the throttle when your trim became too much?

[Matt]

Yeah, the undercamber idea wasn’t to increase lift, I think you’ve said further up the thread that you have plenty. I was just thinking about control and stability.

[Roflhat]

I've thought about more setback on the outboard, as you say to increase the longitudinal distance from the running surfaces to the engine. But puts a lot of stress on the transom and also in my mind makes it worse because as the bow starts to lift you effectively have more trim? Prop pushes the bow further upwards still

The idea with the wheelie bar is to have the same 3 surfaces most of the time but then 5 if the bow starts to lift, well aft of the outboard

[Matt]

Are you sure? I might not be following what you meant, but as the bow lifts, it's effectively trimming in the stern - so you'll get more stern lift? But by that point, it's "gob" is so wide that a bloweover is inevitable anyway. I am pretty sure that on the big cats, if the bow comes up, punching the throttle to increase stern lift is one of the things they do. If the bow is coming up, you have to fight it, not bail out.

The other thing I thought about before was a kind of "ballistic chute" (not literally, but just to describe the principle) so if the bow reaches critical angle, you add a whole bunch of drag on the stern. More than your wheely bar - a stern brake. But concluded that was an interesting thought exercise but likely to be an absolute nightmare in practice.

Maybe an interesting thing about your wheely bar idea is you could kind of end up running a reverse 3-point configuration? But your more or less running with none of the hull in the water today anyway aren't you?

IMVHO on cats, I've always viewed, once you have enough lift to go quick, as stability and balance beeing the key. The closer to the edge you can run it without crossing the line, the quicker it'll go. Hence the undercambered idea is just about moving the centre of lift a bit further aft towards your CG.

On my old hydrolift I had even considered a venturi to create a low pressure zone to pack more air into the tunnel. So not just extending the tunnel, but having a leading undercambered slat that would pack more air in. Never got round to trying it though - life moved on.