My thundercat

[Matt]

Originally Posted by Roflhat

Just hard to experiment with the hull with it being fully inflatable, Jacques at Ceasar says they have been playing with the hulls in the last year or so, moving the nose cone down and transom up so you have less lift from the hull. Sounds like just what I need, that or he's just trying to sell me yet another boat!

I've played with the motor height, going higher gives more rpm but no more speed. It's at the optimum where it is I would say. Yeh LWP nose cone on it

Slip increasing. Which may mean you could need a bit more blade area or diameter to run higher. I assume cleaver style props - very flat?

[Roflhat]

Not got anything on the top currently although I've been wanting to try extending the top of the nose cone back for a while. Just again a compromise, you certainly wouldn't want to be stuck underneath it in a flip. Plus it means the copilot couldn't move forward if the bow did start to lift

Yeh I've tried many props, around 90 different ones by this point. Fastest are cleavers yes, already big diameter for this motor any more and the speed decreases. I think prop/engine wise the setup is close to optimised

[Matt]

I think you’ll be getting a fair bit of aerodynamic drag off the transom. If you can find a way to smooth the airflow over the top, this could also help you. Agree re needing to move around, but am thinking controlling the airflow rather than stopping completely.

Some of the old phantoms, once they start really going, get a lot of drag off the open back seat area

[Roflhat]

Was thinking of a partial cover, from the back of the nose cone for about 1m. Then adding sort of fairings to direct the air over me and over the engine cowling, use see through plastic for the cowling in front of me. You reckon that's worthwhile?

[Matt]

Might be worth sticking some yacht sail telltails at prudent points on the deck etc. I reckon you’ll be seeing a vortex in the crew area of the boat that creates drag. Which would show either telltails lifting, or even flowing forwards

Can you post some pics of the boat - running surfaces and deck?
When you go for record runs, can you and crew sit on the centreline?

[Roflhat]

I definitely would, have seen blades if sand etc. stuck behind the nose cone due to to the vacuum effect there.
Driver and copilot have to be on opooside sides really, unless you had a consolte instead of tiller

[Matt]

Good pics, thanks. So the actual running surface isn't rigid at all? I hadn't appreciated that, I had assumed that bit was solid.

That feels ever so squeeky bum to be doing 70+ on rubber sausage. 70mph = 70psi water pressure, so a bit of a moment and all the inflatable stuff just gets outta the way. It's the fundamental issue with high performance ribs - when it really comes to it, you stuff the thing at 70mph and the tubes will deform or disintegrate right out of the way. But people say - yeah, but look how tough they are when I inflate them - to 2 psi or so! 70psi is 31kg of force on a single square inch! Anyway, I digress....

Thinking just about your top speed quest.....(and I assume you're not worried about one-design class rules, you're just going for outright speed records - maybe I'm misunderstanding your goals?)

Have you considered longer, adjustable flaps at the back? Some people have put vertical damns (rubber flaps) on the backs of the tunnels of their cats. That always seems like a workaround for hulls that don't produce enough lift - but in your case, it feels like you could have a horizontal flap extending the tunnel - and even statically adjustable a few degrees down. This could increase aerodynamic lift and move the centre of lift back a bit?

I was previously wondering about wedges to create some simple steps on the running surface - but if it's not a rigid surface, I think lets stay away from there for now.

I also wonder if you are running out of water pressure before you run out of prop? Am purposely asking about remote water pickups, not low water pickups on the nosecone. The goal is to get just the lower half of the bullet in the water, because then it's like a little hull shape and you get lift off the bullet then - but if there's water going over the top half too, this counteracts it so you end up with drag, but not as much lift. (FWIW, I think Toffen with a phantom 25 that runs 90 suffers from this - I think he's getting lift off the bullet, but his hull design doesn't want that)

I assume that tank is not your coniston tank? It's nice having an open transom so airflow will be better than expected, but you've got the tank in the way. Can you have a bladder tank for coniston? I believe zapcats run bladder tanks, so might be able to get something.

Also see attached pic. I reckon if you put some sail telltails where I put the green stars, you'd see them lifting. So, some kind of cover from the bow down to the floor to improve airflow there. Very flexible material, held on with bungees, so your crew can put there butts there when they need to get the bow down in a hurry. Or even shaped, so there's a hole for you and the crew to sit up through - something not unlike a canoe deck?

Other ideas - can you get your crew and you more on the centreline. You can't move the engine, but would a modified tiller allow you to all be on the centreline? So at least the hole in the air that you and the crew have to make - is the same hole that the engine goes through.

Oh - and torque tab on the skeg? I reckon you're pulling a load of force on the tiller at v-max? With just the skeg in the water. Might balance the engine load out a bit better so you're not having to have it held turned slightly to go straight ahead - use all the thrust just to push you forward and not have to use any to offset the prop paddlewheeling.

Apologies, all quite vague - but you're, I reckon, way beyond all the simple big changes and into the small changes/marginal gains territory. Which is interesting, but also hard.

[Matt]

Just been going back through all your pics on this thread. Phenomenal job getting it to go this fast already.

[Roflhat]

Originally Posted by Matt

Just been going back through all your pics on this thread. Phenomenal job getting it to go this fast already.

Thanks, definitely an obsession at this point. But love coming up with ideas and concepts of how to improve performance!

[Roflhat]

The whole hull is inflatable, can be folded up onto a pallet. Saying that the hijackers are inflated to 12psi and main tubes to 6 so it’s fairly solid. Also getting a carbon floor made up to stiffen it some more. It has some thick rubber sections on the bottom of the hijackers too.

You mean extending the floor of the boat? I had thought about it, seen as the pivot point is currently around the transom so that should increase stability. Also considered a big rear wing mounted far back enough and wide enough to counteract the air under the hull trying to flip the boat. But difficult to achieve without adding a huge amount of weight and drag, both of which the boat is very sensitive to.

I’m confident the boat is running on the bottom half of the gearbox bullet, difficult to describe in words but I can feel when the stern lifts up onto it. Felt an immediate difference after adding the nose cone. And the speed went up around 1.5knots.

The Coniston fuel tank was tiny, 7L eberspacher tank so just as you say the open transom wasn’t blocked off which would cause a lot of drag.

The torque steer issue has also been addressed, the skeg itself I shaped so it counteracts the prop torque. A trick I learned from Rolf, very clever!

Absolutely loving the ideas, please keep them coming!

[Matt]

I should add - that by having a trailing edge type of cover on the bow, you're getting pretty close to having an aerofoil section with the hard bow and the cover too.....

Not saying it would work, quite possibly too many obstructions in the way, but that was part of my thinking that I didn't explain above. Even going so far as wondering if you could get your & the crews's butt in the middle, you could have an aerofoil section running from the bow right down to the transom holes on the stern....

Almost an argument to do a bit of a wind tunnel session with you guys onboard.......except they're ridiculously expensive.

[Matt]

Re. Torque Tab. Yeah, I was thinking about re-shaping the skeg even as I wrote it. I wrote what I did to send you down a "research path" if you'd not already done it - I'm not going to sit here and say drill a hole right here or there, you're way beyond that. But I am trying to highlight interesting paths to research further, if I can think of any.

Stick some sail tell tails on the engine cowl too. I reckon there's likely a bit of drag coming from the aft face of the engine cowl - and with a small deflector, you might be able to reduce drag from the trailing surface of the cowl too. Or even sharpen the edges of it so you get a clean break. Probably going into too much minutae now though.

You either want laminar flow everywhere (almost impossible here), or where you're getting turbulent flow, you want it to be controlled. This is where people often screw up when they start thinking about golf balls. The dimples control the turbulent flow better, but this is still worse than if you could get laminar flow round a golf ball.

[Roflhat]

Would really love to do some wind tunnel testing. I did model the boat in cad, did some cfd analysis although I don’t know enough about it. Need to get someone more knowledgeable onboard. Scale model testing could be an option?

The cowling actually smooths off nicely towards the rear, like a kamm tail. But the front I’m sure could be improved, rounded profile

[jwalker]

Matt's obviously thinking on the hoof as he's further considering the situation around the airflow. Personally I think this is now an essential and you need to do all you can to find out the aerodynamics of the hull at full chat. Safety is paramount and since you've reached flipping point it should now be priority. It'll be preferable to maiming or dying. Wind tunnel time is what you need.
Plainly this is major and possibly impossible. However, if this is the case you have to do what you can with what you have. But before suggesting a long polly tunnel facing west with a smokey bonfire at the front you need to consider the forces at play with the aim of improving longitudinal stabilty whilst also improving aerodynamic drag.

Viewing your boat from the side, when you're close to the limit it's in unstable equilibrium with the centre of pressure unknown but somewhere around the two patches of running surface. All the forward forces are in balance with the rear forces...until the forward forces increase, and increase rapidly.

At the rear, the moment is very short since the tail end of the boat is short, I can't possibly know but I suspect you're effectively running on 3 skids, the engine being the rear, central one. At present you cannot know the forces over and around the hull and since this whole section is out of the water they'll be aerodynamic. At least that simplifies things in that you don't need to consider the water except that it's there. There may be a water effect but we can't know that at present so let's work with what we do know or can make some educated effort to find out.

So, looking at your boat in profile the front can be considered to be flying with unknown distribution of pressure while the stern has the engine as a balancing force rearward. Plainly, if the engine has some upward force it will be balancing the rise in the bow and if it has downward force it will be lifting the bow. Either way, the distance the engine is rearward will lengthen it's moment and therefore, for the same force it will have greater influence.
Given that you've reached flipping point the forces forward became greater than the counterbalancing forces at the rear. Since the rear moment is short and since we don't know any of the distribution of forces at the front it would seem reasonable to consider mounting the engine further back to increase the rear moment and to ensure there is some lift at the engine to counterbalance the lift of the front section. You're moving the rear skid further back. This will reduce the sensitivity a bit as well as improving the stability of the triangle formed by the two running surfaces and the engine.

Now, essential to moving forward...pun intended!...is to try and gain some understanding of the air movement around the hull with limited resources. Matt has mentioned tell-tails and I think this is a good way to start but you need to cover the whole of the boat in tell-tails and try to video it while air is passing over it. I can think of various ways you might attempt this, especially as winter is coming and where you live! But, you also might be able to arrange a chase boat to video the cat in action at a reasonable speed on a calm day. It would at least give you a starting point and food for thought.

Two things to remember when deliberating...firstly, 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.

I think I'll leave now.....

[Roflhat]

Moving the engine further aft I have considered, even moving it the 14mm back with the jack plate helped. But any more and it starts to put a lot of strain on the transom.

Possibly worth trying an extended cavitation plate? Although at high speed that’s well out of the water so not sure if it would be of any benefit.

Will try the tell tale idea I like that, shouldn’t have to wait too long for a 70mph storm at this time of year.
I did try that fairing in front of the engine cowling, thought it would help but looking at it now it was just generating downforce which isn’t what I need. A more rounded front on the cowling would help.

If the front wing works I think that will be the way to go stability wise. After that would be nice to reduce overall drag

[Matt]

Is there any other way, unless it's your boat and you've got access to the little black book recording all the setup changes, weather conditions & performance.

Originally Posted by jwalker

Matt's obviously thinking on the hoof

[jwalker]

Originally Posted by Matt

Is there any other way, unless it's your boat and you've got access to the little black book recording all the setup changes, weather conditions & performance.

Oh, did I come over as being critical? Apologies for that, I just meant you'd had your thinking cap on!!

[Matt]

Moving engine aft
It's nothing compared to the loads they take when you are bouncing across waves and getting big air. Properly nothing.
My approach to increasing stability had been about putting steps/wedge on the running surface rather than moving engine back - because ideally we want aerodynamic CoL, hydrodynamic CoL and CoG all to be as close as possible.
I can see where shifting the engine slightly further increases the footprint of the (sort of) 3 point hydroplane you're running which means hydrodynamically you have a more stable platform, but I didn't like the fact that the consequence of doing that is that you'd shift the CoG back a little further, away from the aerodynamic CoL. So I get it, and maybe it'd be fast, it just wasn't a direction I was going in at this time.

Cav Plate
Won't help, I think, but who's to say. Might mean the prop slings off slightly less water and you can therefore run it 1/8" higher...? Might slow you down. Probably will do nothing.

Airflow Testing
Put it on the roofrack of a car? Certainly in the pics of you running, even the lifejacket collars are sticking up like a spoiler. So some work on where and how you guys sit is probably worth doing. As above, I think if you can find a way to have you, crew & engine present a single face forwards, this would help. Maybe as JW says. I was starting with a simple hypothesis we could test & validate and then expand depending on what you find. But certainly no bad thing to plaster the whole boat in em. What about removing the ropes & flaps on top of the tubes? I can see there are other handles to hold.

Other Things
ATF in the gearbox.....just for coniston.

FWIW, I reckon the front wings will reduce stability but add lift. The AoA you need to run at before it becomes unrecoverable will be lower. Although....if combined with moving the engine a little bit further aft - this gives is a bit of a moment to keep the bow down when the bullet is generating lift.....

Originally Posted by Roflhat

Moving the engine further aft I have considered, even moving it the 14mm back with the jack plate helped. But any more and it starts to put a lot of strain on the transom.

Possibly worth trying an extended cavitation plate? Although at high speed that’s well out of the water so not sure if it would be of any benefit.

Will try the tell tale idea I like that, shouldn’t have to wait too long for a 70mph storm at this time of year.
I did try that fairing in front of the engine cowling, thought it would help but looking at it now it was just generating downforce which isn’t what I need. A more rounded front on the cowling would help.

If the front wing works I think that will be the way to go stability wise. After that would be nice to reduce overall drag

[Matt]

Not at all.

I assume matey (sorry, I don't know your name) has a little black book with all his setup variations in. Probably an excel spreadsheet these days I guess.

Without us having this ourselves, I'm assuming he's tried most "normal" things already, so I'm purposely trying to look for things that are a bit more "out there" that may go against conventional wisdom. The hypothesis being that it's a non-linear system (with far too many degrees of freedom), but that while he's already at a "local maximum" for performance, it's possible (but certainly not definite) there's another one that is even higher...ie like this graph.

Conventional theory, starting where x=0 would optimise to the red circle, despite there being a higher one in there too.

Originally Posted by jwalker

Oh, did I come over as being critical? Apologies for that, I just meant you'd had your thinking cap on!!

[Roflhat]

I like the way everyone is thinking!
Already had a lot of the handles removed from the boat, usually has two grab handles in front of the nose cone, extra foot straps etc. Also had the copilot rope moved from the top of the tube to behind the nose cone for less drag.
Also run atf in the gearbox

I agree in normal running the front wings will add a bit of lift, but hopefully they’ll come into their own when required. As you say I think aero testing is where the gains are to be made from here on it, there’s very little boat in the water and most other avenues I’ve already been down.

Saw Ciaras lifejackets collar sticking up but only after Coniston sadly! It’s quite a well shaped air brake.