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Dangers of Tunnel Tab on 36 Spectre Cat?

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Old 03-28-2006, 08:37 AM
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Default Re: Dangers of Tunnel Tab on 36 Spectre Cat?

I totally agree with what everyone has been saying, but I would like to introduce another spin on tunnel tabs.

It's a matter of Bernouli's Principle. When you lower the tab, you decrease the cross-sectional area through which air is escaping your tunnel (which is essentially a nozzle thanks to the influence of ground(water) effect). This in turn produces a reduction in velocity of air passing through the nozzle, and thus increases the pressure(Bernouli). This increase in pressure further lifts the boat out of the water, and there is a subsequent decrease in drag - but as pointed out, it also decreases the effectiveness of your control devices.

As a steady state dynamic system, this works out great - The problem is when you encounter wave action. The moment that the boat even slightly leaves the water, and the ground(water) no longer has an influence acting as a 4th wall of the nozzle, the pressure decreases so rapidly under the boat, that it begins to fall very fast without the benefit of all that artificial lift - How it falls is usually an unknown, but more often than not, I would bet the stern will beat the bow back to the water, then you trip and stuff.

Another scenario, which is not too often discussed, is the tab itself - let's ignore for a second the nozzling effect, and look at the tab itself. It's a rather large flat plate held at a fairly substantial angle of attack ralative to the direction of travel. When air is passing under it, all is well, but when water strikes this tab with its drastically greater density and lower viscocity it doesn't deflect as easily as air and there is created a large, forceful impulse acting in the upward direction - action/reaction, water down, stern up and potentially bow piercing the next wave.

Like Kilo said, it's not a good idea at high speed.
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Old 03-28-2006, 08:37 AM
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Default Re: Dangers of Tunnel Tab on 36 Spectre Cat?

I think Kilo's comments hit it right on the nose, thats coming from the best. I would suggest taking his comments very seriously as we are talking about human life here and tunnel tabs should only be used in race applications with race experienced drivers, Tunnel tabs should not be on pleasure boats, and I believe most manufacturers will not put them on without a liability waiver, just not worth it.
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Old 03-28-2006, 08:38 AM
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Default Re: Dangers of Tunnel Tab on 36 Spectre Cat?

I agree with KiloKat.
The only time I would use a tab in your set up (and why it is there) is at lower speeds to reduce the lope that a full tunnel boat like yours will have. When your boat is trying to achieve lift at lower speed from its design (the tunnel), but is not traveling fast enough, it will start to lift and then drop. Kind of the same sensation as a V bottom porpoise. In this case you can lower the tab to create more pressure in the tunnel (less room for air to escape from the back). This will eliminate the lope. Now if you stab the throttles and forget to bring the tab back up, refer to KiloKats thread on how it can all go wrong.

If you didnt have the tab or dont use it, the way to eliminate the lope is to drive through it, go faster until the pressure in the hull is great enough to support the boat and run flat.

I do not have a tab on my boat and dont plan on putting one on.

Last edited by Tantrum; 03-28-2006 at 08:43 AM.
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Old 03-28-2006, 08:45 AM
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Default Re: Dangers of Tunnel Tab on 36 Spectre Cat?

Very nice CigDaze!
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Old 03-28-2006, 08:51 AM
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Default Re: Dangers of Tunnel Tab on 36 Spectre Cat?

Quote:
Originally Posted by KiloKat
Very nice CigDaze!
Showoff....using his big college theories....

Well said CD
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Old 03-28-2006, 08:51 AM
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Default Re: Dangers of Tunnel Tab on 36 Spectre Cat?

Bonecrusher,

You have a Great boat!. I too have owned a 36 Spectre cat w/tab. PM me your # Id be happy to discuss details of how that particular hull reacts to Tab adjustments at speed.

As many above have mentioned, inappropriate use can be tragic on ANY cat.

Marc
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Old 03-28-2006, 09:03 AM
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Default Re: Dangers of Tunnel Tab on 36 Spectre Cat?

Quote:
Originally Posted by KiloKat
Very nice CigDaze!
Mr. Scott, thank you kindly.

Likewise to Tantrum.
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Old 03-28-2006, 09:19 AM
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Default Re: Dangers of Tunnel Tab on 36 Spectre Cat?

Quote:
Originally Posted by CigDaze
Mr. Scott, thank you kindly.

Likewise to Tantrum.

showoff!!!!
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Old 03-28-2006, 09:33 AM
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Default Re: Dangers of Tunnel Tab on 36 Spectre Cat?

The only way I would put one on, would be to set it up where when it is fully extended, its only at about 10 degrees.

What happens is you forget its down and thats when you get in trouble.

Either drive through the lope or stay on the other side, on my boat is 53 mph.
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Old 03-28-2006, 11:34 AM
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Default Re: Dangers of Tunnel Tab on 36 Spectre Cat?

Sounds like alot of people here who know what they are talking about! I would be safe and follow their advise!
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