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Pro's & Con's between full tunnels and tunnels with center pods

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Old 11-18-2003, 11:28 PM
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Wow This turned into good reading. Tripps,thanks I'll stop over this weekend. I have heard that a center pod prevents the drive from getting beat up before. But most all cobra tunnel sterndrives are single engines without a pod and I have only heard that they work very well with very good #'s on MPH. I think Marty has had very good results with his.It sounds like some think that a center pod tunnel wouldn't do well in rough water which would be a concern cause I do use my boat on long island south bays with an occasional run into the ocean. My cobra handles this with no problem so far. that is why I was thinking 27 to 28 foot.What I get out of reading about center pod tunnels [Hot boat, Powerboat ect.] is if you build enough power into the boat to lift the pod out of the water than they come alive and a non pad tunnel might use less power to come alive. Any body running a tunnel with a pod in rough water? How do they handle? As for now I can't wait for spring to play with my cobra some more. I didn't have a boating season this year but plan to next year
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Old 11-19-2003, 04:02 AM
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In terms of cars, I will say that I am not that familiar with IRL cars but that F1 cars certainly do create a lot of suction force through venturi effect. However, the principle is based on channeling air into a narrower surface area, like under a car, in order to accelerate its flow and create a suction effect. Thus, it depends a lot on how effective the reduction in area is under the car. For maximum effect, cars must have very deep side "skirts" which almost totally separate the area under the car from the rest of the track. F1 cars do have those "skirts" but they are very small and their dimensions are strictly regulated. If this is even more so with IRL cars, and I am not sure what the rules are, the venturi effect is greatly reduced. So Spectre30 may be right to say that the aero downforce has become more important due to rules but there is most definitely some venturi effect left. Even though I do not know the IRL regulations, one thing goes in favor of saying that the venturi effect on IRL cars is not so great as perhaps thought. A very important part of the effect is how the air exits the area under the car. This is why on F1 cars you see very elaborate "diffusers" which work as fixed mechanical air extractors. On IRL cars, this rear end is not that elaborate, you just see the gearbox which makes me think that maybe the venturi effect is not that important to them probably due to regulations which severely limits it anyway. Correct me if I'm wrong.

Catmando, that's a good point. I always believed that Skater, MTI... did not use the center pod because this makes the boat faster, a reasoning I explained in a previous post. They are winning because they are faster not because they can handle big water better. Racing does not happen in really big water nowadays anyway. A small center pod will improve rough water handling in my book. I'd rather see waves splash a V-hull shaped surface than a flat surface tunnel. It makes a softer ride. However, when we're talking about very large center pods, sometimes larger than side sponsons, I have the feeling that boats behave more like conventional V bottoms thereby producing an even better ride in the rough stuff. However, the volume of the tunnel is greatly reduced there and the cushion effect is smaller so the boats are quite a bit slower.

BTW SS930, which driver are you thinking of? Ayrton Senna or someone else?
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Old 11-19-2003, 06:40 AM
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The pod will make more surface area in the tunnel and more drag when hitting the waves. The surface needs to be relatively flat because the boat is to behave like a wing and be 'flying'- some due to compressing the incoming air and also due to the shape.
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Old 11-19-2003, 08:23 AM
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ST,
Yes Senna. It's very possible that his car bottom out (if only for an instant) causing him to lose control. The steering wheel theory has been discredited to the best of my knowledge.
As far as the tunnel design goes, I still suspect that a pod-less design will not only be faster, but also better in rougher waters. I do agree with you that the pod will create a 'cushion' if a tunnel is going to bottom out on the water. It makes the transition from flight to total air evacuation under the tunnel much smoother, but it is also adding drag. I think Marty is right about a flatter tunnel will also create more lift. The more lift created, the faster and smoother the ride will be assuming all things are equal. Many pod-less designs have a slight "v" in the center of the tunnel as can be seen in post #51. My guess is that they're there to create a cushion for when the tunnel bottoms out on the water, while at the same time trying to maximize the flat wing effect which will maximize lift and air flow. These same boats do not have a design limitation that 'requires' the need of the pod. IMHO, I suspect most manufactures have pods only because they are required by the nature of their propulsion system and or cabin design.

Just my 2 cents.
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Old 11-19-2003, 08:32 AM
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Not surprised to see the CF registration # on that weird boat, T2x.
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Old 11-19-2003, 09:05 AM
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Concerning the tunnel designs, we seem to agree that a podless tunnel will be faster as demonstrated by Skater, MTI, Tencara... as there will be less drag and the "quality" of the air cushion will be greater. However, I still think that a center pod design will be at least more progressive, more comfortable in the rough stuff. Surely it must serve a purpose. Or maybe it's just a requirement of the propulsion system and accomodation? This is a question which needs to be answered. Anyone?

Concerning Senna, I don't know if the steering column theory was discredited. In Italy, they have a 10 year backlog of criminal investigations so nobody can really keep track. The investigation was officially closed then reopened again and so on... As far as bottoming out is concerned, the Tamburello curve where it occured was pretty flat so I do not know whether this is possible. However, some investigations led to the hydraulic system which could have lost pressure instantly and thus "dropped" the car on the track because of the active suspension system. The effect would have been the same as "bottoming out", resulting in an instantaneous loss of control. Very disturbing in a curve taken at speeds approaching 170 mph.
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Old 11-19-2003, 09:11 AM
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Quote:
Originally posted by cobra marty
ST, no problem. One day let's share a pitcher of beer.
Interesting the firehawk center pod is deeper than the sponsons. Sort-of like a V bottom with slots cut in. I wonder what the back end looks like at speed- Does it ride on the center pod and chine walk back and forth or does it lean in in the turns like a V?
The Eliminator at speed probably the center pod is up out of the water. A video of the rear of these boats at speed would be great.


I think it rides on the center pod and uses the outer sponsons for balance. There probably isn't enough area to the center pod for it to lift the entire boat out of the water. In all honesty, it's a compromise. The pod was brought down to help the boat lean to the inside. Mind you it still turns flat, but I get no sensation that the boat is leaning out. The boat is incredibly stable. Does not try to do anything funny or chine walk at all. It also has absolutely no tendency to porpoise. The boat will slide though. That probably has as much to do with my high x-dim as the the center pod. At the right speed, the boat will slide around a good 90 degrees before settling into the water. It's actually kinda fun.

I used to have a pic of the transom, but it's been lost. The boat is now in storage and I won't have access to it until mid December.
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Old 11-19-2003, 09:38 AM
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I contacted the mfr. Hopefully they can get me a pic. Two other catamaran types also need mention here. One is the Mod-vp bottom. I believe that these are popular on the smaller outboard boats, but there are a couple of manufacturers that offer them on larger boats. Stoker and Obsession are two that come to mind. A mod-vp is where they take a v-bottom boat and then cut two tunnels into the hull. If V-bottoms were on the right and true tunnel hulls were on the left, the mod-vp would be on the right, between a deep center sponson cat and a true v-bottom.

The other design is a pretty popular boat, it's the HTM. This boat has an incredibly wide, flat center sponson. The tunnels are also very shallow. Probably no more than 6" deep.


For the record, as long as you have the balls to hang it out, my boat will provide an incredible ride in the rough. It's actually pretty darn nice at LOTO-until you hit a big one. Then you are flying. The boat doesn't cut through the water, it goes over. And when you land, you land HARD. Those 3 sponsons produce a lot of lift really fast. At LOTO I'll land several times and it will hurt.
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Old 11-19-2003, 09:41 AM
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Senna rocked. We pretty much universally agree that his crash was related to an equipment failure rather than driver error. It's still a part of racing and always will be.

"Cats" and "Tunnels" have so many variations that I find it hard to follow conventional logic when trying to discuss them in two "camps".

While a "center pod" tunnel is an obvious distinction, you still have the whole realm of whether the center pod is a vee pod or a flat pad surface. Whether the tunnel cavity on each side of the center pod is a square air channel or an inverted vee channel. Whether the center pod's leading entry directs airflow down each side or whether it traps it and produces another concentrated high pressure air pad (evidenced by the late model STV center pod).

Then you have the "no center pod" hulls. On the surface, logic tells you that they would ride hard if they bottom the pad in heavy seas. On the other side, at high speeds, the air cushion works as a progressive spring and gets harder and harder to force to bottom because the air pad gets compressed just like one of those hemmerhoid cushions (and we know that it is almost impossible to bottom out a hemmerhoid cushion). Add to that the LARGE secondary lifting strakes on the inside of the tunnel and you have additional hydrodynamic lift working to prevent bottoming of the tunnel. You also got the outside strakes of the primary side hulls with several sets of strakes..

Here's my take: All-out straight line big water cats seem to excel with no center pod. Situations that involve a lot more emphasis on frequent cornering may benefit from some sort of center pod. Recreational hulls that will spend a lot of time at cruise speed will/may exhibit less weirdness if they have some sort of center pod (weirdness: loping, tramlining, wallowing).

You got (or at least USED TO HAVE) some racing series based on "vee hulls turned tunnel". What I refer to here is "modified vees". Initially, boats such as STV (Summerford Tunnel Vee) started out as vee hulls with little "training wheels" added to the sides, as the rules allowed them. Over time, the side hulls were allowed to expand into true hulls, slowly extending further and further forwards until pickleforks were finally allowed. Nowadays, the latest iteration of the STV has full picklefork side hulls with aeration steps and the center pod has evolved FROM a full vee hull into a center strip with an inverted air notch. The center pod is REQUIRED in the racing series, else it becomes a full tunnel and is required to compete with F1 style hulls, which are superior in every way (from a racing standpoint).

I have driven BOTH of the above outboard hulls. The F1 "Seebold style" hull was the superior performer, but wallowed, pitched, tramlined, pounded, bottomed the tunnel, and all other rude behavior AT RECREATRIONAL SPEEDS. The STV was a really NICE behaving hull at recreational speeds (we've skiied and tubed behind them - with goggles so the prop roost didn't tear out our eyeballs). At racing speeds the F1 hull is a whole different animal - wonderful.

Maybe some of the pros and cons of the two camps of cat/tunnel/centerpod discussion can be related to extending the zone of comfortable behavior of a given hull by using a center pod.
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Old 11-19-2003, 10:51 AM
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"Then you have the "no center pod" hulls. On the surface, logic tells you that they would ride hard if they bottom the pad in heavy seas. On the other side, at high speeds, the air cushion works as a progressive spring and gets harder and harder to force to bottom because the air pad gets compressed just like one of those hemmerhoid cushions (and we know that it is almost impossible to bottom out a hemmerhoid cushion)."

I completely agree with this but it's important to remember the air is not trapped on all sides in a tunnel hull like it is in a hemmerhoid cushion, but rather it is very heavily vented in the front and rear. So for this reason it looses much of the qualities gained under a hemmerhoid condition... and a few of these hard hits will give you hemorrhoids!

Good points mcollinstn.
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