Hull Speed

[comsnark]

FWIW, Island Packet is predominantly a sailboat manufacturer and from the website it seems this is their only powerboat. It is powered by a 100hp Yanmar diesel and it does have a 5000lb keel. I couldnt find any photos of the underside of the hull but the article describes it as being sailboat-like (yet it'd draft is only 3'8")?

I missed that this was an Island Packet in your first post.

The forward hull and cabin are VERY similar in appearance to their sailboats. The gelcoat color is also very "Island Packet". Island Packet sailboats are very, very nice.

Island Packet sailboats tend to have a "full keel" as opposed to a "fin keel". A fin keel will be like a knife sticking down, with a huge hunk of heavy on the bottom. The rudder is a smaller knife at the stern. For a full keel, the keel starts shallow, and extends down and aft all the way to the rudder at the back. There is no open space between the keel and rudder. This allows for similar resistance to being pushed sideways by the wind while (theoretically) requiring less draft. The flip side is that a full keel boat can be MUCH harder to turn in light air than a fin keel boat.

A sailboat like keel is simply not needed for a power boat, as you have much less cross sectional area up top (no sails).

Since this thing looks so much like a sailboat. . I wonder how much the design is adversely influenced by sailboat thinking.

 

[Henry Boyd]

Jim:

Your explanation is exactly as I understand the concept of hull speed, i.e., one wave at the bow and another at the stern. Thanks for taking the time to explain it in such an understandable fashion!

I would further think that, for the most efficient boat one would want the max. efficiency of the engine to occur approximately at or near hull speed. Right?

Cheers,

Bill

Bill,

You are kind of close. In the introduction section on Resistance in 'Volume II of Principles of Naval Architecture' SNAME 1988, van Manen & van Oossanen illustrate the point best;

" The task of the naval architect is to ensure that within limits of other design requirements , the hull form and propulsion arrangement will be the most efficient in the hydrodynamic sense. The ultimate test is that the ship shall perform at the required speed with the minimum of shaft power, and the problem is to attain the best combination of low resistance and high propulsive efficiency."

So it is more like a package deal kind of thing where outside factors such as what and how much stuff you need to carry, from and to where the stuff will be carried, and over what period of time. This creates the spec for the hull form on which the math can be done to determine the size of the propulsion system. Note the classic definition calls for the lowest shaft power needed to propel the hull at the desired speed. This may not be the most 'efficient' point for the power plant because the power plant may have to be sized larger to get the thing moving, e.g. getting a planing hull on plane.

Henry

 

[vernh59]

A semi-displacement hull (such as a lobster boat-type like the Mainship Pilot series: http://www.mainship.com/ ) has a sharp, deep vee bow with a deadrise transitioning to a nearly flat deadrise at the transom. They are not as efficient at or below hull speed as a full-displacement hull, but handle the transition between hull speed and planing speed fairly gracefully.

http://www.emma-maersk.info/

Hey! I resemble that. My hull transitions to a shallow V near the stern and a keel from midships to the stern where the keel is about 18" deep.

 

[rondds]

Since this thing looks so much like a sailboat. . I wonder how much the design is adversely influenced by sailboat thinking.

Jim
The article also has a footnote that refers to the introduction of nearly identical model but rigged like a sloop. So you can hang out the laundry as you add to your carbon footprint. Guess that baby will get better mileage than a Prius.

 

[Jim240]

Okay, I still think there are people more qualified to make heads hurt here... I've been a stress head all my life, but along the way took tons of fluid and aerodynamic courses and I hang out with way too many aerodynamacists (they're a fun crowd, really!). I'd be better at telling you how to make your boat go Mach 2!

Before going any further, if you're looking for the limits of efficiency, barely moving is often the most efficient. Notice how you can move your boat in almost any direction just by hand at the dock. Newton's law (F=ma) works at it's most basic form here. To get it moving, you're only fighting the inertial force (the property of it just having mass, sort of like the guys on the space station moving large structures that you see on TV). Once it starts getting going, not so easy, as drag is dependent on speed and you're fighting more forces.

Drag is made up of a lot of components; wave drag, friction drag and pressure drag to name a few. Your engine fights to overcome all of them. For the most part, the faster you go, drag will always increase.

Okay, there are exceptions. When fluids flow real nice and smooth such that there wouldn't be mixing between parallel layers, this would be called laminar flow. Laminar flow can be really good as it's counter is turbulent flow, where there is a lot of mixing. Mixing usually takes energy and things that take energy are usually bad for efficiency... plus the mixing usually means the distance of the affected region into the fluid (perpendicular to something like your boat, <far to port or starboard>) is much greater and you don't get that for free... it means you're usually expending more energy. Where the exceptions often occur is in the transition region... going from laminar flow to turbulent flow. We could also talk mach buffet (supersonic transition) but I doubt it's going to have much of an effect for anyone here.

BTW, if you're having trouble thinking of water flowing, since you might think the boat is moving and the water is stationary, just change the reference frame. If you as the skipper are the center of the universe, your boat is not moving, but the water below you is and it strikes your boat. It then flows all around your boat. This is just how wind tunnels work for aircraft... why move the model when you can just move the air!

If we look at efficiency curves for most Sea Ray's, we see great efficiency when the boat starts moving. It slowly drops as we move toward hull speed. Then it really drops, jumps back up when we get on plane, usually keeps going up for a short time and starts to fall as we go faster and faster.

To me the difference is pushing water (displacing) to getting to a speed where you're skipping on the top of the water (planning). Going really fast means the engine, propeller, etc. aren't going to be at their sweet spots of efficiency and friction drag is always going up, so after you reach your peak efficiency on plane, hope it's a gentle ride down.

And in my interpretation, you can adjust for shape factors in a generic sense. I saw the Froude Number mentioned... a dimensionless number that helps equate (scale) things. If you want to understand more, search on the Buckingham Pi Theorem. Many have probably heard of the Reynolds Number (Re)... as in it was a high Reynolds number airfoil we put on that race car? What a lot of these numbers do (at least in my world) is help you make little models and run them at odd flow speeds and they help you predict how things will work when they're full scale in the real world.

Researchers will spend huge amounts of time testing all sorts of different parameters a and making charts (at least in the old days). Today, we get cool computers to do the initial work.

Now HiFi asked me to elaborate... so here goes.

First off as I stated above, in my view, efficiency is usually best near zero speed and generally decreases as we go faster. Because of other drag factors and also the design point of the optimal efficiencies of the engine and drive train; overall efficiency for displacement operation is most often below the hull speed. Flow is a hair steadier below this point too... and unsteady flow is often detrimental to things like displacement operation.

So generating short, small waves that run down, multiple times, the hull is more efficient that one long, deep wave, right? Those short, shallow waves didn't take much energy to create. The longer, deeper wave takes more energy to get that depth in the middle. But what's nice about the long wave is that the wave height comes back up and I guess you'd say the aft half of the wave causes the boat to surf forward. That's what I mean about recapturing the wave.

As an engineer, we tend to just look at the difference in height between the bow wave and the stern wave and equate that to the height of water to push. But another way to look at it is that at displacement speed, approximately the front half of the boat is climbing up the wave and the aft half is surfing down it. Once you go faster and loose that aft wave, now it's all uphill baby!

Does that sound better?

 

[tmhudson2]

Jim, you have an ability to explain technical stuff in terms a non-technical person has a real chance of understanding without his head exploding. Have you considered writing "Naval Architecture for Dummies"?

 

[Jim240]

Thanks for the compliment... but I'm really not all that knowledgeable about boats, that's why I'm here! Now aircraft I understand, and there's a few things that the two have in common.

As for being able to explain things... it probably comes from years and years of having to deal with management! But since they make more and tend to have a lot less responsibility or accountability, I'm finding that my altruistic side is giving way to wanting a much larger boat. Where's the Kool-aid! I'm ready to drink!

 

[tmhudson2]

If you're ever down this way, give me a call. I'm not much on Kool-aid, but some other stuff good for sipping....

 

[Four Suns]

Well... being the "Head Pot Stirrer", I am going to have to disagree with your explanation Jim... It's not correct. A wave coming up and supporting the back of the boat (I assume via a buoyancy force) and "getting out of the trough?! Ummm.. "No"... that's not even close.

If you have had a bunch of classes in aerodynamics and fluid dynamics, you need to go back and look at the course work in small perturbation theory and wave drag. A boat hull approaching hull speed is the same wave drag theory of an airplane approaching the speed of sound. However, you don't have a compressible fluid so waves are created on the water surface. Approaching "hull speed" in a boat and approaching the "sonic speed" in an aircraft follow the same wave drag principles. I don't recall their being a buoyancy force holding up the back of an aircraft as an explanation why the wave drag was rapidly increasing... The difference in the boat is that water does not compress (much) and you are not going to break the "barrier" like in an aircraft but you'll climb over it and get on plane. No matter how loud the sound you make, the pressure waves in air travel at the speed of sound. Looking at how fast pressure waves travel in water, you'll figure out where the "1.34" came from in the 1.34*sqrt(LWL) formula (you can also review a "sears-haack body" on ideal shapes and their relationships to wave drag).

Jeff's comment about prismatic coefficient was probably the closest thing here that was on the right track. It's basically a measure of the "area rule" on a hull planform. "Hull Speed" and transonic aerodynamics in aircraft are very similar. No matter how loud the sound you make, the pressure waves travel at the speed of sound.

I'll try to post something up later that doesn't involve Fourier Series and decomposition of waves but involves a stone dropped in the water and waves propagating out but I don't have time right now. The explanation proposed here is not correct though.

I will say that the getting out of the trough on a planning hull is not about losing buoyancy force at the rear of the boat but it is climbing on top of the water because PHYSICS and wave theory dictate how fast you can push water out of the way via a pressure wave and if you are going faster than physics allow, you have to climb on top of the water and plane.

I can also assure you that the boat in question did not violate any laws of physics. Contrary to what the smart magazine writer said.

I have to go stuff a turkey.

 

[Four Suns]

a sailboat like keel is simply not needed for a power boat, as you have much less cross sectional area up top (no sails).

what?!

 

[Presentation]

Well... being the "Head Pot Stirrer", I am going to have to disagree with your explanation Jim... It's not correct. A wave coming up and supporting the back of the boat (I assume via a buoyancy force) and "getting out of the trough?! Ummm
.. "no".

If you have had a bunch of classes in aerodynamics and fluid dynamics, you need to go back and look at the course work in small perturbation theory and wave drag. A boat hull approaching hull speed is the same wave drag theory of a plane approaching the speed of sound. However, you don't have a compressible fluid so waves are created on the water surface.

Jeff's comment about prismatic coefficient was probably the closest thing here that was on the right track. It's basically a measure of the "area rule" on a hull planform. "Hull Speed" and transonic aerodynamics in aircraft are very similar.

I'll try to post something up later that doesn't involve Fourier Series and decomposition of waves but involves a stone dropped in the water and waves propagating out but I don't have time right now. The explanation proposed here is not correct though.

I will say that the getting out of the trough on a planning hull is not about losing buoyancy force at the rear of the boat but it is climbing on top of the water because PHYSICS and wave theory dictate how fast you can push water out of the way via a pressure wave and if you are going faster than physics allow, you have to climb on top of the water and plane.

I can also assure you that the boat in question did not violate any laws of physics. Contrary to what the smart magazine writer said.

I have to go stuff a turkey.

Gary, so you’re not just an unemployed rich dude that should not be allowed to use power tools? You’re an unemployed smart rich dude that should not be allowed to use power tools! Just giving you a hard time my friend. LOL

I have decided to add some ballast in the forward most part of my v-birth. I will get better gas mileage because I’ll be going down hill. How much ballast should I add?

A bumble bee flies due to "the power of its own ignorance."

Using this same approach I should be able to add ballast to the front of my boat and increase economy as long as I don’t think about it too much.

Have a great Thanksgiving dinner. I am looking forward to reading your follow up.

 

[Four Suns]

I'm not smart, rich, or unemployed anymore.... because of the following reasons:

1. I get more dumb the older I get (actually... I think one just realizes how dumb they are the older they get)

2. The market took care of that rich problem.

3. My wife made me go back to work.

So now I just stuff turkeys.

I have decided to add some ballast in the forward most part of my v-birth. I will get better gas mileage because I’ll be going down hill. How much ballast should I add?

That right there is funny.

 

[Jim240]

Gary,

I respect most everything you write here on this forum. I also appreciate being stirred. But in my opinion, split-density boundary flow is a very different animal than compressible supersonic flow (maybe I've had too many classes). I get where you're trying to go with the comparison and when I first started writing up what I did, I thought about going down that path too, but it ultimately leads to more headaches than anyone really wants to see, or you end up with too many obvious contradictions.

For instance, if we're going to use an analogy of hull speed and supersonic flow, the first thing that should jump out is that hull speed is dependent upon the length of the hull. Is the speed of sound dependent upon the length of the airplane? The fineness ratio may jump as a key parameter, but it doesn't change the speed of sound, only your efficiency. And then if we're going to compare shape factors and supersonic flow, then I'm just waiting for someone to tell me that I should have a slenderness profile of a trapezoid (I sort of figured as head pot-stirrer that would be you! But you went for sears-haack <naming it gives it more authority>, which is a bit old-school... heck, even trapezoidal area-rule is old-school, I don't even remember which one came first, it was long before my day).

So in my view... as supersonic flow is about pushing faster than a pressure wave will propagate in a continuous medium and it's ultimate discontinuities, it's going to have too many exceptions to trying to use it to explain in layman's terms a hull pushing at the boundary of two markably different density fluids.

And maybe I don't know enough about boats, but I know quite a bit about wave drag of aircraft and I don't see it as directly applicable (perhaps in it's most elementary form, but I'll be interested in how you make it easy to understand). Today, mach buffet in a turbulent medium is nothing like hull speed (which I hope if the designer knew what he was doing is dominantly laminar). But most of my knowledge is fairly current in supercritical airfoil design, maybe I'm missing something that looked similar in grandpa's day of NACA manuals. (so I like to stir a bit too!). And I think I pointed out that 1.34 is just a simplification for the density and viscosity of water. But heck, I didn't look up the equation so if I'm wrong, sorry, but I doubt it's going to matter much here.

So off the cuff, I'll try to make something of where you're going, and I'm sure it will be wrong in your eye. I throw a rock in a pond. Of course, as the rock breaks the surface tension force and then begins to displace the water below it as it falls to the bottom (I'm assuming the rock sinks... as an aside, this was an actual interview question... man sits in a boat in the middle of the lake, takes a rock that was in his boat, throws it into the lake and the rock sinks... did the level of the lake go up, stay the same, or fall?). This energy creates a displacement wave as all things in the universe try to find equilibrium. The water was disturbed and the lake absorbs the energy with the wave (and slight temperature increase) and dissipates it as the wave naturally expands. Be sure to note, the water is not going anywhere. The water in wave doesn't go just up and down, more of a circular motion, but don't think the water is moving along with say the crest of a wave.

Now what's important about this is that most everyone recognizes the crest is above the mean free surface of the lake, but the trough is also below. The difference is the amplitude. The amplitude subsides as the wave expands. The wave propagates at its natural frequency.

So Gary, were you now planning on taking this to doppler shift? In other words, if you throw stones in and move them forward, you'll see the waves in the direction get tighter. Yep, this is train whistle thing where the pitch is higher as it travels toward you and lower as it goes away from you.

Now my problem with this explanation is that what happens next??? As soon as the boat reaches the speed that the wave freely expands (which is only dependent on the water and has nothing to do with the length of the boat) am I to think that the boat instantly jumps out of the water to get over the wave?!

I need to go run up a mountain before dinner... I'll check back later to see where this goes. And I am truly interested as I said in the beginning, I'm not a boat expert and there's a lot more knowledge here than I'll ever have. And I do appreciate being stirred! Maybe I say that because I know you won't find a picture of me out there!

 

[Jim240]

OOps! I was just re-reading it quickly... waves don't propagate at their frequency, they oscillate at their natural frequency. Now if I remember it properly, the speed of sound is significantly faster in water than it is in air, which means the wave propagation speed at the surface has more to do with than just density but also... Gary, I'm not looking it up right now! But I'm guessing viscosity and surface tension.

 

[rondds]

did the level of the lake go up, stay the same, or fall?

It went up (Archimedes' Principle)????

Do I get the job?

Geez, what have I started with this question?

 

[tmhudson2]

Gary and Jim,

How is the wave created by a boat moving through the water at the surface different from the pressure wave created by a submerged submarine moving through water? I know there has to be a difference, but why? If both vessels are the same displacement and traveling the same speed, why does it take more energy to move the one on the surface than the one submerged?

 

[osd9]

It went up (Archimedes' Principle)????

Do I get the job?

Geez, what have I started with this question?

It went up when he took the rock with him in the boat. Tossing it overboard added nothing extra to the lake then was already there in the first place.

 

[rondds]

If a woman is screaming in the forest at a man that isn't there, is the man still wrong?

 

[sr360aft]

If a woman is screaming in the forest at a man that isn't there, is the man still wrong?

Now that depends on who you ask; him or her

 

[chuck1]

Now that depends on who you ask; him or her

So true......:thumbsup: