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step hull

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Old 08-31-2006, 03:12 PM
  #51
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Default Re: step hull

Oh: I forgot. It has NOTHING AT ALL to do with getting air under the boat. Lots of people thought so originally, and many people think so today, but experiments proved conclusively long ago that air pipes leading to the bottom of the boat, or to steps, have absolutely ZERO effect. They do add weight and complexity, and more holes reduces strength and stiffness, so they actually slow the boat down.

The reason for lifting strakes that run fore-and-aft on the bottom of the boat is to cause the water layer (bow wave, or bow spray) to break away from the hull, reducing surface area drag.

But the whole air-under-the-hull story is totally bogus, and has been proven to be so over and over and over.

One of the most recent experiments I read was introducing bubbles under a planing surface via pressurized air. The INCREASE in drag was quite a lot.

Why does getting air under a hull, even a planing hull, INCREASE drag? Because each bubble introduces the force of surface tension, and the surface tension of water is quite a lot actually. You are trying, a zillion times over the bottom surface, to rip the bottom of the boat free from the surface tension of those zillions of bubbles. No single bubble is a lot of force, but a zillion is a measureable amount of force.

No, you want smooth flow under the boat, not aerated flow. No different from any other fluid flow problem: laminar flow (smooth flow) is your friend, turbulent flow (like bubbles in water) is a HUGE disadvantage.
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Old 08-31-2006, 08:55 PM
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Default Re: step hull

Quote:
Originally Posted by LostinBoston
Soon......

soon...?
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Old 08-31-2006, 09:12 PM
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Default Re: step hull

Now thats a lot of info!! Thank you guys for the help and your time. You must of been tracking boat hulls a long time crash to no all that history. It is somthing how a boat hull can be such a sience.
So what is a aeration tub Mark and what does it look like. When I looked a your boat I could only see what I call lift straps.
Another thing I did on my last hull was I cut my lift straps back about three feet from the back. I no that you had told me that I might try going all the way out the back this time Mark. Why did I do it!! Well I look at every hull I see and I seen that Scarab did that on there boat I seen. Why would they do that because after you think about it you are losing lift. I just thought that they would no best on that. But like you said crash all diferent boats run different.
Thanks a bunch
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Old 08-31-2006, 11:08 PM
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Default Re: step hull

Quote:
Originally Posted by carcrash
Oh: I forgot. It has NOTHING AT ALL to do with getting air under the boat. Lots of people thought so originally, and many people think so today, but experiments proved conclusively long ago that air pipes leading to the bottom of the boat, or to steps, have absolutely ZERO effect. They do add weight and complexity, and more holes reduces strength and stiffness, so they actually slow the boat down.

The reason for lifting strakes that run fore-and-aft on the bottom of the boat is to cause the water layer (bow wave, or bow spray) to break away from the hull, reducing surface area drag.

But the whole air-under-the-hull story is totally bogus, and has been proven to be so over and over and over.

One of the most recent experiments I read was introducing bubbles under a planing surface via pressurized air. The INCREASE in drag was quite a lot.

Why does getting air under a hull, even a planing hull, INCREASE drag? Because each bubble introduces the force of surface tension, and the surface tension of water is quite a lot actually. You are trying, a zillion times over the bottom surface, to rip the bottom of the boat free from the surface tension of those zillions of bubbles. No single bubble is a lot of force, but a zillion is a measureable amount of force.

No, you want smooth flow under the boat, not aerated flow. No different from any other fluid flow problem: laminar flow (smooth flow) is your friend, turbulent flow (like bubbles in water) is a HUGE disadvantage.

I'm gonna need to see this proven on mythbusters. The bonus will be seeing Kari Byron in a swimsuit again.
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Old 09-02-2006, 05:46 PM
  #55
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Default Re: step hull

So why do Outerlimits and Nor-Techs have multiple steps (ex 6 or 7)
If its not for speed, it must be for production cost. These small steps must be easy to build, compared to 2 big ones. The big steps are often shaped as a pear or triangle. This effekts the strukture and stifness much more.

MTI who seems to be fast now, have 2 big steps on their hulls.

But Skaters are as fast, have multiple smaler steps. etc Baccardi Silver skater.

Anyone that know why they coose different.
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Old 09-05-2006, 07:13 AM
  #56
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Default Re: step hull

Anyone ?
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Old 09-05-2006, 05:30 PM
  #57
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Default Re: step hull

OK,
I can tell you this, I was bench racing with the owner of one of the BIG guys mentioned above and he told me when I asked about the effectiveness of multiple steps...

" I don't know if it does anything, but I'm not going to loose a sale because another builder has them ( multiple steps)"

Which means to me that we the buyer asked for them.

By the way, the aerating thing, don't confuse introducing air on to a flat surface with introducing air into a negative pressure area like behind a step or a notch.
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Old 09-05-2006, 06:14 PM
  #58
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Default Re: step hull

Quote:
Originally Posted by carcrash
Oh: I forgot. It has NOTHING AT ALL to do with getting air under the boat. Lots of people thought so originally, and many people think so today, but experiments proved conclusively long ago that air pipes leading to the bottom of the boat, or to steps, have absolutely ZERO effect. They do add weight and complexity, and more holes reduces strength and stiffness, so they actually slow the boat down.

The reason for lifting strakes that run fore-and-aft on the bottom of the boat is to cause the water layer (bow wave, or bow spray) to break away from the hull, reducing surface area drag.

But the whole air-under-the-hull story is totally bogus, and has been proven to be so over and over and over.

One of the most recent experiments I read was introducing bubbles under a planing surface via pressurized air. The INCREASE in drag was quite a lot.

Why does getting air under a hull, even a planing hull, INCREASE drag? Because each bubble introduces the force of surface tension, and the surface tension of water is quite a lot actually. You are trying, a zillion times over the bottom surface, to rip the bottom of the boat free from the surface tension of those zillions of bubbles. No single bubble is a lot of force, but a zillion is a measureable amount of force.

No, you want smooth flow under the boat, not aerated flow. No different from any other fluid flow problem: laminar flow (smooth flow) is your friend, turbulent flow (like bubbles in water) is a HUGE disadvantage.
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Old 09-05-2006, 07:01 PM
  #59
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Default Re: step hull

Now I'm just checking here, T2x,

When you built the Challenger 21 with the step in it... remember... do you recall that it went better with or without aeration tubes in it?
I remember the difference.
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Old 09-05-2006, 07:12 PM
  #60
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Default Re: step hull

Interesting. T2x, you agree with Crash?
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