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Conventional V-bottom hydronamics question..........

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Old 12-20-2007, 11:48 PM
  #31
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Originally Posted by mccaffertee View Post
What happens to the stability of the boat if most of it is out of the water?
Get a cat.
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Old 12-21-2007, 12:16 AM
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Get a cat.
lol
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Old 12-21-2007, 02:31 AM
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The cats will naturally be more stable because they have a wider footprint.

There are two ways you can make a boat go fast:
1. Figure out how to make it run on top of the water
or
2. Figure out how to make it run more efficiently through the water

If you try to make a vee run on top of the water, it will have a smaller footprint and become more unstable, and will be thrown off balance much easier by wakes, because you are trying to balance the boat on a very small surface. It's kinda like standing on a basketball. This is the idea that the velocity, allison, hydrostream, and many others use, and although they are fast, you have to be right on top of the wheel. Obviously a cat can run right on top of the water and still be stable because it has a wide footprint.

If you use steps and a smaller pad like a fountain (or no pad at all- OL, Cig, Hustler), the boat runs a more wetted surface area, giving it a bigger footprint, but the boat is going through the water more efficiently because the steps aerate the water underneath the hull.

Hope this helps some
-Jason
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Old 12-21-2007, 04:55 AM
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Originally Posted by Semper Fi View Post
Griff also mentioned the pad bottom. I just find it interesting that the non-stepped boat can be as fast or faster. The reason I say this is because when you take a conventional hull-like a 38 Top Gun with 500EFI's the boat will probably run 72pmh at best. Compare it to a TS Top Gun with the same 500EFI's and WOW!!! This boats runs almost 10mph faster!!! There is a huge speed improvement.

Weight difference is at least 10% between a straight bottom and TS Gun. That counts for some of the speed gain. The step and a completely new hull design add the rest.

Cig tried just adding a step to the straight bottom. Speed went up, but handling suffered. It had to be totally redesigned.
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Old 12-21-2007, 10:29 AM
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OK.. I'll put my .02 into this, having driven a step bottom in the late 60's.
First there are many different steps for different reasons, so let's not get into that. "V" bottoms can "ventilate" the hull if the ribs are "hyperbolic", In general, think of it this way. Steps are generally faster because they don't stick as much to the water as "V"s, The step reduces the "suction cup" effect, thus reduces drag, faster speed. Pads raise the boat, more speed. "V" generally cut through the waves. Deadrise is a major component in wave crushing and also speed.
Velosity's rakes and ribs are not true parallel to the sides or the bottom / keel, they divert some of the water to the outside so the boat stay more on top. (I don't know what the deadrise is) Less friction = speed. Less friction, harder to handle, squirmy. More weight=less speed, better in washing machine. Anyone figures out how to get "all" we want from one hull. Let me know.


statement. " buy a fountain"
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Old 12-21-2007, 10:30 AM
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Baja-- I think you may have a point. I was just looking at a few different boats as comparisons.....

38 Fountain - 9,800lbs
38 Cigarette - 9,900lbs
39 Nor-Tech - 9,200 lbs
38 Hustler - 8,500 lbs
39 Velocity - 8,000 lbs

In the smaller boats (under 30') the weight difference is alot smaller, if any

reinforcement for said statement as the fountain is the fastest
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Old 12-21-2007, 10:35 AM
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Not too jump on the thread but Brian brings up something I don't agree with, I know he is on the A.T. payroll and has a nice 37 AVH, but how many builders only have a single step ? I know A.T. has it and Cigarette had a few POS single step boats for about a year but as much as I can remember in the last ten years the fastest step bottom boats have 2-3 or 4 steps.
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Old 12-21-2007, 10:41 AM
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Here's my take on what steps do or how they work on a V hull.

I'm not an engineer but I had the Gancia dei Gancia and Ebel at my disposal and we ran them everyday we could both in the US and Italy up on the lake. So, we gained allot of experience with step-bottomed V hulls before anyone here in the US even played with them.

First, have you ever done the spoon under the faucet trick? Where you hold the backside of spoon against the downward flow of the water and the spoon is sucked into the water further not pushed away? That represents the rocker bottom on a V hull and how it will stick.

Now, to drive a V hull fast in the water (for open ocean racing, not on the bay or the lake for top speed) you need to have your drives tucked under or negative trim. The hulls rocker will naturally let the bow come up and your drives at negative should lift the rear and raise the whole boat, thus it goes fast.

Our big diesel race boats aside from being heavy all had fixed drives (now called Trimax) with a negative angle, much more so than you would run with an out-drive. That angle constantly pushed the bow down just like trying to get on plane with the drives tucked under.

What the steps did was create and very exaggerated rocker to set the stern back down and with all the weight they would stick like glue. The steps helped to break that tension that is demonstrated buy the spoon trick. So, we had a V hull with extreme rocker, extreme negative trim and steps to break tension. No one could ever catch us (when they ran good) if you remember.

Before someone asks about the wing Ė here what it did mostly. Our boats were narrow and always trimmed bow down because of the drives, they chine walked like crazy. The wing provided lift to help stop the chine walk. On Ebel the boat I ran 3 times a week, ever week, if you take off the wing you could not run more than 85-90 mph without Sh*tting your pants, you needed to throttle back constantly to get it under control. Wing on and it flew straight.

Again, donít beat me up this was from Buzzi directly, I was an engine guy not a naval architect.

Why is a non-stepped hull just as fast some times - I think weight, balance and a straight bottom all add to it. Again - the SPOON trick - bottom is not straight, has hook, bad rocker it will actually suck down into the water. That's why when you blueprint the bottom, the area between the front of the engine bay and the transom is the most important.
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Old 12-21-2007, 11:13 AM
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Originally Posted by LapseofReason View Post
Not too jump on the thread but Brian brings up something I don't agree with, I know he is on the A.T. payroll and has a nice 37 AVH, but how many builders only have a single step ? I know A.T. has it and Cigarette had a few POS single step boats for about a year but as much as I can remember in the last ten years the fastest step bottom boats have 2-3 or 4 steps.
Greg,

I'm not on the AT payroll.
I'm not up to anything. Its OK to disagree; but no insinuating is necessary.
Anyone can call up Harry Schoell and ask him about bottom design. He's a nice guy. http://www.schoellmarine.com/

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Old 12-21-2007, 11:23 AM
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Originally Posted by monstaaa View Post
reinforcement for said statement as the fountain is the fastest
I'll agree.

It also has less hull in the water a.k.a running surface; which provides for reduced drag.
A 26' boat and a 28' boat with like bottom designs that weigh EXACTLY the same; down to the ounce, with IDENTICAL power... Which one will be faster?
The one with the least drag.

Last edited by Sydwayz; 12-21-2007 at 11:25 AM.
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