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

* PLEASE don't let this thread turn into a stepped hull VS. non-stepped hull bashing issue!

There's no dispute that some people like conventional (non-stepped hulls) and some people like stepped hulls. There's also no argument that they both have their good/bad points, I have an intrigging question.

99% of the time, a stepped hull is faster than a non-stepped hull-- so why are some non-stepped hulls, like Velocity Powerboats as fast or faster than a stepped hull in it's own class?

I know it's not "rocket science", I do understand why a stepped hull is usually faster, because of less drag from the steps. So why is a Velocity (non-stepped bottom) so fast? What makes it so fast compared to other stepped hulls in its class?

The Velocity has large pad at the back. The boat runs with the pad on top of the water instead of a standard V that cuts through it.

Also I would think weight still plays a big roll in the speed.

I completely agree, however the weights of these boats are similiar to their competitors.

I wonder why other conventional V bottom builders aren't doing the same?

Because many people (like me) are looking for a boat that rides well in the rough. I true Vee will cut through the big waves. I pad will slam on top of them.

Everything about boating is a trade off. Speed vs. ride, etc.

Smooth ride, fast boat, low cost. Pick two of the three! :D

Like Clay says there are compromises. I spent a lot of time time running different 30 Velocity's and enjoyed them very much. The 30 is a drivers boat it is fast and nimble, carries weight well, and importantly demands your attention. They are completely capable of running with any rough water boat in the rough if the driver is experienced. It does ride harder. You could think of it ride wise like comparing a vette and a caddy. Both are nice with different strong points.

WHOAH Gerrard!! You opened up a can here!

There are a LOT of things that matter.

1. Weight is a bigger factor than anything.
2. The step itself is not what makes a boat faster. Its the natural pitch or trim that it places the thrust, as well as depth in the water in relation to the weight being pushed or displaced.
3. There are so many bottom designs out there, that you CAN'T generally title one as better (fast/ride/efficiency) than the other based on categorical design. You have to take into account strakes, hook, rocker, tabs, etc. etc.
4. Multiple steps are actually slower a single step; and a straight bottom should be faster. (Given ALL things equal, but how could you accomplish that?) Think of it in plumbing terms. If point A and point B are exactly 8' apart, and you have three equal diameter tubes to pour 2 gallons of water through; which one do you think is going to empty First? Second? Third?

When did "low cost" become an option??

I guess I missed that thread / didn't get the memo.

As far as different manufactures doing pads many do. There are variations in deadrise and size of the pads. Fountains, and numerous others have some variation of a pad. Some cats even have pads.

I guess I should have said high cost vs. uber-high cost! :p

Smooth + Fast = Uber-High Cost! :eek:

Sydwayz, I'm not sure your plumbing analogy applies. I agree there are many factors. Did you read the first line?

Yeah, I'm not getting the plumbing analogy either.
I understood it as introducing air under the hull = less drag/aerate the props.
I'm no engi-ma-neer so I could be completely wrong.

I just know they both are expensive - and as Clay pointed out, some are Uber-expensive

well what you are saying is that a non step is the fastest :drink:because the stright pipe will let the water move through it faster then the others .

steps allowed slow boats to run all most as fast as pad bottoms:evilb::food-smiley-007:

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.
:D:D

Pad bottom? Simply put; nothing supports weight like a horizontal surface now the trick (bad word) is how much of it?

Pad bottom? Simply put; nothing supports weight like a horizontal surface now the trick (bad word) is how much of it?[/QUOTE]

Yup.. In theory, a piece of plywood with a motor on it is the fastest design. :evilb:

Good responses.......I was wondering how the non-stepped Velocity can be as fast as the stepped hulls. I think Paradox summed it up best about how the steps work (I had a pretty good idea of that already)

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.

Velocity, are much lighter than similar boats with the same power and size, they are usually 15-20% lighter. Thus faster.

On a personal note WOT stepps don't even matter, how much of a boat is in the water at WOT, about 20" in the rear, way past the step, however a pad can make a huge diff.

Just my 2 cents:ernaehrung004:

WILL

You guys focused on the picture too much! Too many pretty colors!

Items 1-4 on my post were in order of importance.

Thanks for the analogy. I wasn't hoping to open up a can of worms.....just thought that it's an interesting subject! Hey, it's wintertime :)

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

i would be will to bet that none of those weight's are close to right on those boats. not saying you are wrong and i know you got them from the manf. but i bet they are not close

Mike,

I couldn't say for sure, as I was getting the weights off each own manufacturers websites

I enjoyed the thread and the various explanations on hull design and performance. I have a pad velocity and wondered how the pad would perform against multi step hulls at WOT. Thanks for the explanations guys. Merry Christmas!!!!!!!!!!

i know you did but what i have found is they all tend to lie a little about weight. some thank that heavy is stronger so they say theres is heavy and some say that light is faster so they do the same.

I don't follow how diverting water to the outside is going to lift the boat. If anything, it would think it would cause it to drop in the hole left by the diverted water, and plow even more water.

Michael

Hard to draw with a mouse, sorry, but I think you got the idea.
"A" standar design, "B" hyperbolic and faster. more lift, more air, less friction.
Attachment 315752

Have you ever seen a really fast V-hull boat? The thing is running on the back 2 or so feet of the boat (LESS on smaller boats like a hydrostream). My old hydrostream actually rode on the bullet of the lower unit more so then the bottom of the boat. At that spead and angle of attack the steps would do nothing in the way of reduced drag. I would think that they would hurt by adding wind turbulence.

Get a cat.

lol :D

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 :D
-Jason

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.

statement. " buy a fountain"

reinforcement for said statement as the fountain is the fastest

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.

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 – 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, don’t 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.

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/