# Wave piercers

Discussion in 'Hydrodynamics and Aerodynamics' started by ShaneK, Feb 21, 2018.

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### ShaneKJunior Member

Can anyone explain in simple terms, how modern hulls can easily exceed hull speed ? Is it that lack of dynamic lift forward, allowing the boat to move fast without climbing it's own bow wave ?

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### Gunnar SommerlundMarine Engineer

1) Wich boats?
2) Planning only downwind if sailbot.
3) Foiling
4) there is no "simple terms"

Look what happens to water in a pipe. the velocity increases in the narrow section and pressure drops.

I,m gonna leave this hanging here and let you come with some inputs on how you can reduce pressure drop in the middle and thus reduce the wave heght =)

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### Gunnar SommerlundMarine Engineer

If you wanter a really indepth mathematical explanation.

Google This: Michell’s thin ship theory

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### ShaneKJunior Member

Most recently seen in "Sailing La Vagabonde" Their 45 ft? cat, not planing hulls, doing 22 knots. A broad reach is the fastest point for sailing.

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### ShaneKJunior Member

Michell’s thin ship theory is VERY dated, and, I suspect, doesn't cover the recent design practices that have been proven to work very well indeed. Just look at the VOR boats, that whiz along on the merest breath of wind. They are almost always close hauled as the apparent wind is almost always from well ahead. They are very wet boats though, going through waves rather than over them.

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### TANSLSenior Member

Much less energy is wasted, and the wind is better used, going through than going over waves. Correct?. What do you think?

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### ShaneKJunior Member

nearly constant speed, unless they "crash"

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### BlueBellAhhhhh...

ShaneK,

Michell's thin ship theory is very dated, as you say, but don't dismiss it on that account or you'll be missing out.
Wave piercing hulls often create huge drag when piercing.
Check out some of the interviews with the crew on EARTHRACE the 70 ton wave piercer that eventually sank after being rammed in a whaling incident.
They reported jarring decelerations that were relentlessly crippling over time.
As bad or worse than pounding in a conventional hull.

Of course they're wet boats. Reduce total reserve buoyancy and enclose the cockpit.
Call it a semi-submersible if you like.

It is generally accepted now that "modern hulls", as you call them, are considerably less constrained by hull speed if they are narrow... very narrow.
Like 20:1 l/b ratio narrow.
So, you're either looking at a multi hull or a stabilized monohull (AKA trimaran).
The other thing that goes out the window, however, is planing.
So, one has to rethink the flat bottom, correctly angled planing hull...

Are you a student?

Last edited: Feb 27, 2018
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### ShaneKJunior Member

No, a 60y/o engineer with a rather large boat (106ft, and my home) that I would like to improve efficiency on, at close to hull speed. It is most likely to be a futile pursuit, but I try to take advice on board from several sources before I try it or give up. I will certainly be extending the hull aft to eliminate the sharp transom below the w/l, and to provide a platform for MOB recovery and general convenience. My dad was a naval architect, mostly for sailing yachts, but he isn't around now.

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### Gunnar SommerlundMarine Engineer

Hello. Sory i thought you ment a monohull. But as Bluebell mentioned your multihul is so slender that is creates extremely low wave rising ressistance witch is acountable for the biggest drag at near hull speed. With that sayed, making a normal sailboat a feet narrower wont give you ane impressive result. I,m pretty sure you can find a Naval architect here on forums and PM him/her for an advice. PS. Dolfiman https://www.boatdesign.net/members/dolfiman.62547/

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Firstly there is no such thing as ‘hull speed’.

It is a ‘simple’ concept, but not so easy to explain without a certain amount of basic engineering background. So a very simplified version to get the basics across may help.

If you look at a typical ship shape moving through the water shown below:

A variation of waves is produced at differing parts along the hull.

These waves are also changing the pressure distribution around the shape. In addition as the shipshape moves through the water is has what is called ‘boundary layer’. A certain amount of the fluid is ‘dragged’ along with the hull, so to speak. The amount increases as to go aft…and this changes the distribution of pressure along the hull length, as shown in the shaded region of this image below:

As you go faster there is more of a pressure differential between the front and the back of the hull, owing to the shape of the 3D hull and the speed of the fluid flowing over it.

For “normal” shape hulls, hulls that are designed to carry a lot of weight for their length, have shapes that are similar at the front and back of the boat. Except at the back there will be a slight change to allow for a prop, for example. This slight change creates an even greater pressure differential from the front of the boat to that at the back.

What occurs is that the difference in pressure becomes so create the back end is effectively pulled down, or sucked down, simply to remain in equilibrium. Otherwise there would be no water under the hull.

This massively increases the resistance as the back end is now dragging deeply in the water. This change in resistance is so great that, very roughly, for every knot you want to go faster doubles the amount of power you need. So the faster youw ant to go the massive increase in power required for even just a small amount is next to impossible.

Solution, change the aft shape of the hull so the pressure differential does not occur so the hull is not sucked down into the water. That is called a planning hull.

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### Surfer Naval ArchitectNaval Architect

Hello Tansl,
I am curious about your comment . Can you explain me more about what you mean and in which case you think is valid "your" theory?

Thanks a lot,

RF

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### TANSLSenior Member

What I mean is that crossing a wave produces a lot of friction but if the bow has a lot of buoyancy reserve, a lot of flare, the boat jumps and energy is lost moving the boat up, not in the direction of the advance, and a wave train that consumes much more energy than the friction of the first case.
A submerged submarine does not consume energy due to wave formation and, therefore, can go much faster. The same happens if the stem goes through a wave instead of jumping on it.
I do not think I have explained myself very well but I am not capable of more.
Thanks for your curiosity, my friend. I love talking to you because you defend your ideas with fullness.

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### Surfer Naval ArchitectNaval Architect

congratulations about your first part of the explanation, the second part I found a bit reductive to help our friend. For istance:

It is not so true with difference value of the Cb it would be nice to show him how change the shape of the bow (vortix included between the end of the fore part and the parallel body) and the aft part of the vessel.

Another thing that I think it is a bit reductive is your last explanation:
In which way do you think he should change the aft part of the vessel and what will bring changing the shape of the aft part? I think it is important to explain about trim optimazation for the desidered speed, alpha and beta index for this kind of boat and so on...

I can also give a contribute about your explanation when I will have time

RF

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