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Old 01-06-2005, 11:47 AM
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Default What is a displacement hull?

What is a displacement hull?
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Old 01-06-2005, 12:09 PM
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Default RE: What is a displacement hull?

Displacement in marine terms basically means the weight of the boat.
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Old 01-06-2005, 12:12 PM
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Default RE: What is a displacement hull?

a displacement hull is a hull that does not plane. Trawlers, sail boats, big ships, etc. all have displacement hulls.

Displacement hulls are much slower moving than a planing hull. The benefit is that they require much less power to move though the water instead of over it.

The larger the hull the more speed will be able to be developed. There is a formula that equates the length and shape of a ships hull to the amount of speed it can generate. I don't know what it is off the top of my head.

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Old 01-06-2005, 12:13 PM
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Default RE: What is a displacement hull?

A true displacement hull is one that is NOT designed to plane and will be limited to it's theoretical 'hull' speed.

Doug --- www.onewolf.net ---

Added: The formula for the 'hull' speed is square root of the water line length in feet * a constant value of approximately 1.2. So a 28 foot sailboat with a 25 foot water line length will have a hull speed around 5*1.2=6. The constant value varies slightly depending upon the actual hull design.
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Old 01-06-2005, 12:16 PM
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Default RE: What is a displacement hull?

Speed equal to the square root of its waterline length

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Old 01-06-2005, 12:16 PM
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Default RE: What is a displacement hull?

There, I learned something new today.
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Old 01-06-2005, 12:17 PM
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Default RE: What is a displacement hull?

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Nickys Toy - 1/6/2005 12:47 PM What is a displacement hull?
A displacement hull displaces or plows through the water to move ahead

A planing hull tries to float on top of the water . . . higher speeds can be obtained with planing hulls

For similar sized and weight boats, it generally takes much less power for a displacement hull to go from a to b, when compared to a planing hull . . . but it takes longer

That is*the simple answer . . . after this the explanations get more complicated . . . hope this helps
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Old 01-06-2005, 12:18 PM
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Default RE: What is a displacement hull?

Another definition might be:

a hull which is designed to acheive its maximum speed while supported solely by its bouyancy, rather than lift generated by movement throught the water.
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Old 01-06-2005, 12:30 PM
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Default RE: What is a displacement hull?

Displacement hulls will plane if they have enough power, its just that they are more power efficient if they do not try to go beyond their imperical hull speed.
Displacement hulls usually are rounded on the bottom as comapred to the flat planing surfaces found on a planning hull.
But given enough horsepower, any shape will plane or skip like a rock, even if its a bowling ball.
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Old 01-06-2005, 01:04 PM
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Default RE: What is a displacement hull?

Does a displacement hull basically have a flat bottom, I ask this because Raymarine says that an in-hull transducer is typically used on displacement hulls because they need to shoot straight downwards. Does this mean I can't use one in my boat with a 19 degree deadrise?
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Old 01-06-2005, 01:17 PM
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Default RE: What is a displacement hull?

I just ordered a Raymarine FF with an in hull transducer for my boat with an 18 degree deadrise. The Airmar in-hull transducer they use says in the specs 'easily adjusts to any deadrise angle up to 22 degrees'. The important thing is that the bottom of the boat is solid fiberglass, it won't read through coring. I had an in-hull transducer on my last boat which was also a planing, deep-v boat and it worked great.
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Old 01-06-2005, 01:19 PM
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Default RE: What is a displacement hull?

The inhull will work fine 90% if there are now air voids between the laminates. This can be tested on a ziplock bag prior to installation to determine the best location while underway.
Displacemt=round shapes
Planing = flat shapes.
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Old 01-06-2005, 01:24 PM
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Default RE: What is a displacement hull?

Come on guy's*and*gals! Enough with the mathematical mumbo jumbo.

A displacement hull does just what it says, it DISPLACES the water or moves thru it, even at speed.
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Old 01-06-2005, 02:10 PM
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Default RE: What is a displacement hull?

Below from Powercruising.com seems to be a good explanation of the difference between a displacement hull and semi-displacement hull, the latter of which is somewhere between displacement and planing:

"The two main types of hulls for trawlers are the full displacement and the semi-displacement. An example of the semi-displacement hull is the Grand Banks line of trawlers and the Kadey Krogen 42 is an example of the full displacement hull. The difference between the two hulls can be simplified by stating that the semi-displacement hull is designed, that with enough horsepower it is able to go faster than hull speed, while the full displacement hull can't exceed hull speed....
An advantage of the full displacement hull is that the weight of all the equipment, food, beverages, fuel and other material carried on board on a long cruise won't greatly affect the performance of the boat since the design of the boat is to stay at or under hull speed and thus the extra weight won't be a great factor in performance. In a planing hull or even a semi-displacement hull, the opposite is true, as it takes a lot more horsepower and fuel to achieve planing speed if you increase the weight.
Basically, if you want longer range and more blue water performance go with the displacement hull boats, if you do more coastal cruising and need more speed then look at the semi-displacement boats."

Thought it was a good explanation of the three types, hope it wasn't too wordy
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Old 01-06-2005, 02:20 PM
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Default RE: What is a displacement hull?

So basically, anything with a displacement hull is LARGE.
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Old 01-06-2005, 02:38 PM
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Default RE: What is a displacement hull?

I discussed this with some designers at Northrop Grumman / NNS one time. If I can recall correctly...

There is a practical limit to the size of a planing hull, 150 foot, or something like that. All the large ships are displacement hulls, hull speed being determined largely by length. The most efficient hull is long and narrow, like a canoe. As the hull speed increases, the bow wave "wavelength" (???) starts to get longer on the hull. As MAX speed is approached, the bow wave reaches toward the stern. MAX speed is when the bow wave length on the hull is equal to the hull length. Anyway, that's what I was told. ???. To go larger and still "lift", they submerge part of the hull as a lifting structure, a teardrop wing of some type or use a simple hydrofoil. That is another matter entirely.

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Old 01-06-2005, 03:10 PM
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Default RE: What is a displacement hull?

Quote:
Nickys Toy - 1/6/2005 2:20 PM

So basically, anything with a displacement hull is LARGE.
A canoe is a displacement hull, so is the USS Ronald Reagan

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Old 01-06-2005, 03:55 PM
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Default RE: What is a displacement hull?

Quote:
jelly - 1/6/2005 1:16 PM

Speed equal to the square root of its waterline length
Incorrecto... square root of the waterline length multiplied by 1.2 or 1.4, depending on the hull (primarily a function of beam for the multiplier). This value ("hull speed") is the point at which the boat is trying to climb out of the trough created by its bow and stern waves. Planing hulls can climb over the bow wave and leave it behind, displacement hulls cannot.
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Old 01-06-2005, 03:56 PM
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Default RE: What is a displacement hull?

Quote:
Nickys Toy - 1/6/2005 3:20 PM

So basically, anything with a displacement hull is LARGE.
No, my rowboat is a displacement hull.
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Old 01-06-2005, 08:30 PM
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Default RE: What is a displacement hull?

All hulls are displacement hulls when they are not moving though

-Jacobi
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