Fighercat Cheetah 6s2p

[properchopper]

I'm following this thread with interest- I've built one Cheetah for a customer and one for myself (both single motor ones). I've set both of them up the way I've set up all my Cats -

CG @ 30% forward from the rear running surface

strut rather high from the sponson bottom to minimize rear lift which causes nose down "wet" running

fairly positive strut angle to keep nose up under power but allows hull to drop down while off throttle for turns

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I've found (so far, testing still underway) that this airs out the hull under power - in fact partial-to-full-throttle wheelies

I'll agree that the Cheetah sponson architecture isn't anywhere near optimum BUT I'm not convinced that it's the rear step that keeps the nose down - increasing the aoa of the front sponson should cure the wet running/stuffing problems but I'm open to thoughts. It seems, to me at least, that the same lack of understanding of stepped-bottom design that created the psuedo-stepped oingo-boingo bottom of the Geico and it's variants went into the Cheetah sponson design. A correctly designed stepped bottom is NOT just a straight shot front-to-rear with some notches cut out to simulate steps.

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(for contrast/comparison the yellow hull is my Drifter S - the most perfectly aired out hull I've ever encountered)

DSC03921.JPG air8.jpg

[properchopper]

Then again, (after five days of being rained out and being bored out of my skull) by adding tailfins there might be some benefit, 'tho I have no idea what that might be

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[tiqueman]

With all do respect to you and your talent Tony, im gunna have to disagree somewhat. I understand that a truly designed step indeed is not flat, step to step HOWEVER, if it goes: flat, flat angled down, as the Cheetah does, theres no doubt in my mind that its whats holding the nose down. If all three surfaces were angled down a bit it would push the entire hull up, but, thats not the case. Only the last is angled downward acting as a 6" long trim tab. Its exactly what "hook" is in a mono, and for an even better example, take a mono hull and put the tabs down a bit. The last bit of negative in a hull will push the nose down if the rest of the hull is all running a tru ride flat angle. By raising the strut and puting positive into it, your just counteracting the trim tabs. Unfortunately in a twin, I dont have that raising ability... BUT, I do have filler.

EDIT And will be sure to post back after Ive modded mine.

[sampit]

I was wondering how much the lenght of the rudder afects the nose being pulled down,on the Cheetah, a lot of the Cheetah builds have quit long rudders.
would it improve by making the rudder even shorter and as close to the transom as possible?

[tiqueman]

I dont think that will make that much a difference. it needs the rudder to turn correctly and help to keep from spinning out. Iv run the same length rudder on other cats and been fine.

[sampit]

Ok back to the filler method, but where to begin, I will keep a close eye on your step filling and see what i can do with mine

[Chilli]

So has anyone seen the Cheetah run in race water? Just curious how she handles the turns in chop compared to the Sprint Cat. I would love to see the bottom profile of a sprint cat with a ruler to it. If memory serves, I don't think my Mean Machine had angled steps but she sure turned well. My River Cat has the true angled steps and makes a great SAW boat but turns like crap. Perhaps the Cheetah is a hybrid of the two??

IMHO - Regardless of angle of the last step, if it's bottom is no lower than the other steps or keel line, I don't see it acting as a trim tab. I'm wondering if that flatter second step is what helps holds the Cheetah in the turns? I always assumed the sole purpose of steps was to ventilate the bottom reducing drag.

This is good stuff!!!

[properchopper]

So has anyone seen the Cheetah run in race water? Just curious how she handles the turns in chop compared to the Sprint Cat. I would love to see the bottom profile of a sprint cat with a ruler to it. If memory serves, I don't think my Mean Machine had angled steps but she sure turned well. My River Cat has the true angled steps and makes a great SAW boat but turns like crap. Perhaps the Cheetah is a hybrid of the two??

IMHO - Regardless of angle of the last step, if it's bottom is no lower than the other steps or keel line, I don't see it acting as a trim tab. I'm wondering if that flatter second step is what helps holds the Cheetah in the turns? I always assumed the sole purpose of steps was to ventilate the bottom reducing drag.

This is good stuff!!!

This is getting weird. The straightedge on my MM shows the middle sponson higher by about 1/6" than the front and rear sponsons. I have no idea why or what that means

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[Chilli]

Cant tell if these variances are designed or hull warpage. I give up. LOL

[properchopper]

Interesting thread. And although I hesitate to share what I "think" as opposed to what I "know", since I just dropped my car off for a brake job which will consume my entire 2013 battery budget I'll hang in this thread instead of contemplating my financial "Devcon 1" disaster. {can I get a Boo-Hoo ?}

Tiq, I believe we're in synch with the general idea that the Cheetah bottom in stock form isn't optimal. But I agree with Chilli that if you consider the rear step as acting as a trim tab, you then might also view the effective wetted length of the hull bottom to terminate at the rear of the second "step". Using that as a theory, reducing the depth/penetration of the back of the rear step (as in raising the trim tab) you'd end up with a rounded bottom profile front to rear. Result : the hull would rock back and forth on the middle step much like the oingo-boingo/wah-wah Geico. Filling in the rear "notch" shouldn't have any effect since the part of the rear step that tends to keep the nose down is the rearmost portion and likely not the middle "fill area". Again, it's from the "I think" portion of my brain so it's not supported by any real world testing.

My first and only impression of the running attitude of my Cheetah was that at anything less that WOT it ran with the entire bottom in the water (or wet, to use that term). Turned great like that. With my CG fairly amidships (9.5" from the back of the last step), the prop up @5/32" and right around 3 deg positive, at WOT the boat got the nose up enough to run nice and loose (and low temps indicated that the electronics weren't struggling to plow the hull). That setup combined with the tunnel air-pack had the boat looking pretty much how I want it to.

My MC is set up the same way. The whole hull in the water for stable turns, but quite aired out at WOT (and it RIPS - took Second at the Nat's behind Brian's MC).

As an aside, the Cheetah is 34 1/4" and will not fit into NAMBA P Class length limits - no fudge factors allowed. And no, I ain't gonna do any sponson-tip surgery so I have other plans for that class come WW9 next Feb [watch out, Pags ! ]

[properchopper]

More pictorial data for the "What's Going On With These Sponsons Anyway" file :

The MC bottom with actual "mild" STEPS : (awesome handling boat, btw)

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[tiqueman]

We will just agree to disagree. that last step being negative and on different planes then the other logically will hold the nose down. I dont see how it wouldnt. If the last inch of a mono is kicked down (spooned or hooked) why is everyone so concererned w/ blueprinting????????????????????? Oh wait, because it makes the nose stay pinned to the water and the hull rides wet when hooked. its the same thing, step or no step. Now if the step before the last was at the same negative angle, it too would create lift and it would not run so wet because in theory, the water is going to leave the hull at whatever angle or ride surface it last leaves. And if thats a negative angle, like angling a tab down, its going to push the back of the hull up. In my mind its simple physics. We will see as Im doing each mod to each of my Cheetahs. My twin is getting the last ride pad surface filled to be parrallel and level and true with the pad before it. The single drive, which on on the build bench, is getting the 2nd to last step added to to match the neg angle of the last.

Chili, I think its safe to say it doesnt matter if the last step is the same height, if it were completely out of the water (higher), it would not matter at all, but even if an inch of it is in the water, its going to effect it. Do you have a mono hull? turn a trim tab screw down so its an eight of an inch in angle, then raise it so the back of the tab is the same level as the hull. Id bet anything that it will pull the bow down. Until its high enough that its not touching the water at all, it will have a bow down effect. now take that tab and make it 6" long. large pendulum, more effect w/ less angle?

I guess I just cant see how anyone could argue against that.. but, its what makes the world turn and Im done expressing my knowledge. 23yrs of boats and setting up and Im still learning.

And Tony, my future hobby expenditures are getting blown on very needed truck street tires for the F-150, so I feel ya and will boo hoo right along side you

[properchopper]

I'm going to stand down, listen and learn. Four reasons :

1: I don't feel right putting forth ideas that I haven't personally experienced coupled with real-world verification

2: Too frequently I put something forth, then hours later realize I was wrong (from a hangover song I recently wrote for a band called Choyce : "Woke up this morning with my head on fire, I tried to think, but my thinker's a liar")

3: Cheetah is just a fun boat; have seven raceboats to seriously prep for WW9 next Feb in AZ


4: Need to devote all my energy to feeling sorry for myself due to financial difficulties

[Chilli]

tiqueman, you said if the first two steps are flat, an angled last step acts as a trim tab to hold the bow down. What I'm trying to convey was that IMO an angled rear step whose bottom is level with the keel line would have no more effect on the bow of the boat than a flat step or step less tunnel of the same waterline length. At least not in a cat that runs flat like my Fightercat 32. In the grand scheme of things, I'm not a boat designer so what do I know. But it's still fun to talk theory and exchange ideas.

[tiqueman]

ill fill my sponsons and see what happens