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To step or not to step –that is the question

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To step or not to step –that is the question

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Old 05-19-2011, 10:07 AM
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Default To step or not to step –that is the question

As promised on Steve’s Powerplay thread, I’ve finally had a minute to kick off this discussion on stepped hull design:

Air lubrication-that’s the most common reason sited for the increased efficiencies associated with stepped hulls but once you understand the physics involved, low pressure air lubrication of planing surfaces doesn’t make any sense at all. We design boats to plane when the energy required to push a boat through the water exceeds that required to lift the boat out of the water AND propel it forward at the same velocity. A chap named Froude pioneered the early science on this but for brevity we’ll just accept that a boat of say 30 feet of length and 8.5 feet of beam will require less power to travel at speeds over say 20 knots in the planing domain than that required to push it through the water in the displacement domain. So what is planing? It is lift -hydrodynamic lift (meaning the lift is derived through the dynamic force of the hull impacting the water at a positive angle of attack) as opposed to hydrostatic lift for which the weight of the boat is supported wholly via the static displacement of water, the mass of such displaced water being equal to the weight of the boat.
So given the above it’s also reasonable to conclude that the less drag we generate in creating all of this nice lift, the less power we need –which is the whole point. Since wetted surface area is a huge component of hydrodynamic drag it therefore stands to reason that we’d like the planing surfaces to generate as much lift as possible from the smallest area. So how do we achieve this? Well the first way is to increase the angle of attack of the planing surface (which is why we trim up to gain speed) but once you exceed 4-5 degrees the cost of that lift becomes increasingly higher in the form of increased induced drag. Therefore for maximum efficiency we’re stuck in the 3-5 degree angle of attack range. However here’s the problem. As we go faster, the stagnation line will move further and further aft causing the hull to seek an ever decreasing angle of attack (and under 3 degrees the hydrodynamic drag increases quite dramatically). Again this is why you find that you need more trim at higher speeds -unless of course you’re in a cat where aerodynamic forces are acting well in front of the lcg and are thus providing an aerodynamic positive trim device -we could start another thread on this topic. However in a conventional V hull (subject to lcg position-which we’ll go into later) you’re essentially using horsepower (in the form of a deflected thrust angle) to hold up the bow and maintain the ideal angle of attack…and it is here that you’ll find the first hint of why properly designed stepped hulls are more efficient than their non-stepped counterparts on many high performance hulls.

To be continued and apologies for the delay,

ND1
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Old 05-19-2011, 04:14 PM
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I’ve been boating for over 45 years, own a non-stepped high performance boat, but have driven many stepped hulls at the marina where I work part time. I’m also a Mechanical Engineer by degree, but didn’t spend much time with hydrodynamics. So most of what I’m going to say comes from seat of the pants experience.

Even in the “old” days with non-stepped hulls, everyone knew that reducing the wetted surface area of a hull made it go faster. One of the common techniques was to “trim the hell out of it” to get the bow up and thus reduce area and drag. Unfortunately, this also wasted some thrust from the motor / prop and also made for a real white knuckle ride. This technique generally only worked in fairly calm water and in a straight line. Just try to find that on a weekend!

A common design technique to reduce the wetted surface area was to add a “pad” to the keel of the hull. The boat then rode up on this narrow pad and went faster. That’s the way my boat’s hull is designed to work and as long as you know how to drive it (due to chine walk), life is pretty good. But, most folks don’t know how to drive through chine walk and messing up generally means getting wet – and maybe a lawsuit for the manufacturer.

Then came steps. They’ve actually been around for along time if you look, but for some reason no mainstream manufacturer grasped the value. Reggie Fountain – love him or hate him – was probably the first to really jump on steps. He found that the step (or steps) “lubricated” the wetted surface area and allowed the boat to go faster without a ton of positive trim. This had several benefits. First, one could go faster and not have to know how to drive through chine walk and other things. Second, the additional speed was literally free.

Unfortunately, early stepped hulls were literally experimenting with how much lubrication one could add and not get so slippery as to allow the boat to spin out. As most of us know, more than a few manufacturers found out they had more design & development work to do. The boats were fast in a straight line, but corners got really interesting. So the lubrication effect could work for and against you.

Finally (a few years ago) enough was known about how many steps, how deep they should be and where they needed to be placed that most of the nasty handling gremlins of stepped hulls had been exorcized. Almost anybody can drive the newest stepped hulls and go fast – without spinning out. All in all, it’s probably better for everyone out there. Personally though, I still think a non-stepped hull can corner much harder than a similar length and beam stepped hull.

I will offer a couple of reasons that a non-stepped hull might be a good experience though. First off, if you can drive a non-stepped hull at the limit, you can probably drive anything. Second, some hulls really don’t need any steps. When folks ask me why Kevin doesn’t add steps to his Progression hulls, I show them the picture below. Where the hell would you place a step to lubricate that little bit of the pad that’s actually in contact with the water?

For many folks though, I believe the benefits of stepped hulls far outweigh the “advantages” of non-stepped hulls. As further proof, try to find a competitive race boat without steps these days. They’re about as common as an honest politician.

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Old 05-19-2011, 09:02 PM
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Kevin's boats may be poppin in the US but those high trim angles needed to get them to run has the same effect as holding a sheet of plywood to the wind. Check this out from about 3:03 to 3:20. And this is any day of the week with the same weight boat. Modified 2.5 on the Progression and a stock 2.5 on the Phantom. In the rough the progression is nearly 5MPH off the pace.

http://www.youtube.com/watch?v=tjlCv...eature=related

The twin step bat boat kills them both rough and calm, same weight, same power.

I say 2 degree angle of attack and 2 degree on the stepped surfaces puts you right at that sweet 4 degrees.
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Old 05-19-2011, 09:44 PM
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Great take guys! I am curious to hear more!
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Old 05-20-2011, 05:30 AM
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Originally Posted by Lauderdaleboats View Post
Kevin's boats may be poppin in the US but those high trim angles needed to get them to run has the same effect as holding a sheet of plywood to the wind. Check this out from about 3:03 to 3:20. And this is any day of the week with the same weight boat. Modified 2.5 on the Progression and a stock 2.5 on the Phantom. In the rough the progression is nearly 5MPH off the pace.

http://www.youtube.com/watch?v=tjlCv...eature=related

The twin step bat boat kills them both rough and calm, same weight, same power.

I say 2 degree angle of attack and 2 degree on the stepped surfaces puts you right at that sweet 4 degrees.
I won't disagree with you there, but remember, the bat boat has something else going on. There's a bit of aerodynamic design to the bat boat hull. The "wings" at the stern give the stern aerodynamic lift at speed which allows it to run with less positive trim. Nice design and one fast hull indeed.

On a related note though, anybody remember which hull held the record in that race for many, many years?

A Progression 22.
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