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Old 02-03-2019, 09:53 PM   #1
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Definition Of Full Displacement

So...

What is the definition of full displacement.

Is it a specific hull design? Where no matter the engine HP it will always stay within the FD formula. Hull Speed = 1.34 x √LWL

Or

Is is a boat with a limited hp power plant that can never exceed FD speeds based on the LWL? Even if that hull design with bigger engines would result in a boat that would exceed the formula!

Can you even describe a FD bottom?
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Old 02-03-2019, 10:04 PM   #2
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FD Bottom:
1. Rolls like a log in water.
2. Put 10,000 HP engine on one and it will never plane but the end result and accident report will be spectacular.
3. They go slow.
Any more questions please advise.
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Old 02-03-2019, 10:06 PM   #3
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FD Bottom:
1. Rolls like a log in water.
2. Put 10,000 HP engine on one and it will never plane but the end result and accident report will be spectacular.
3. They go slow.
Any more questions please advise.
OK, we will put that down as definition #1.
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Old 02-03-2019, 10:43 PM   #4
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Not a thing you can define w words.
But has to do (almost entirely) w the shape of the stern underwater.
Has a lot to do w wave lengths, timing (speed) and the relationship between the hull length and the wave.
I know one when I see one but can't really define it.
Here is a stern shot of an example.

A definition that I used for years was .. "a boat with all of it's transom out of the water at rest". Not bullet proof but easy to apply and identify boats almost instantly so it's a fairly good rule of thumb. But as you can see by the pic below there's no transom on my boat and there are other slight problems w the def but give it a little lattitude and it's good enough for all practical purposes. And a very small submerged part of a transom dosn't take it out of FD category either. Some FD Nordhavn's have a bit of submerged transom.
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Old 02-03-2019, 10:47 PM   #5
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Typically a round bottom (no hard chimes or flat stern) that doesnít (canít) exceed hull speed. The intended is to minimize water displacement resulting in an efficient hull form that can also handle seas well. For trawler, think wide body sailboat hull without the deep keel.

Voyaging Under Power is a great resource for FD vs SD hull forms.
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Old 02-03-2019, 11:04 PM   #6
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Not a thing you can define w words....
I know one when I see one but can't really define it....
Per HH Judge Ramon Burke:" It`s like a rhinoceros, hard to describe but if you see one, you`ll know what it is".
I identify one operating in the water by the 2 waves the hull generates. I believe the effect of the 2 waves limits the achievable speed, thus the expression "hull speed".
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Old 02-04-2019, 12:08 AM   #7
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Actually full displacement hulls can exceed hull speed with enough hp. Think USN. Do the math on a destroyer or carrier.
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Old 02-04-2019, 12:14 AM   #8
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Actually - they can exceed hull speed in some cases (1.34 x √LWL) if they are skinny enough. The 1.34 coefficient is an average. It changes depending on efficiency of the hull shape.

Full displacement hulls are designed to operate in the water rather than on the water. I guess that would include submarines.
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Old 02-04-2019, 04:46 AM   #9
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Actually - they can exceed hull speed in some cases (1.34 x √LWL) if they are skinny enough. The 1.34 coefficient is an average. It changes depending on efficiency of the hull shape.

Full displacement hulls are designed to operate in the water rather than on the water. I guess that would include submarines.
Quite simply it's a hull that is designed to ride always in the water rather than partially or totally on top of the water thus limiting it's practical speed to one dictated by it's length. And no they don't all roll abominably and in general make a better seaboat.
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Old 02-04-2019, 06:16 AM   #10
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"And no they don't all roll abominably and in general make a better seaboat."

All boats will roll , for a Seakindly ride the question is not how far but how fast and how does the roll check and reverse.?

Happily to some extent this can be tested before purchasing.

With slack lines in a slip or anchored a boat can be rolled by simply moving from rail to rail. The roll period can be timed (usually about 4 seconds or so) and how the roll stops and reverses can be felt.

A quick roll with a hard check can create a vomitorium in a beam sea.


A full displacement boat is seldom bothered by the added weight of cruising gear , an added 2000 lbs of necessary stuff may add 3HP to the engine load or about 1/5 of a GPH.
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Old 02-04-2019, 06:35 AM   #11
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"And no they don't all roll abominably and in general make a better seaboat."

All boats will roll , for a Seakindly ride the question is not how far but how fast and how does the roll check and reverse.?

Happily to some extent this can be tested before purchasing.

With slack lines in a slip or anchored a boat can be rolled by simply moving from rail to rail. The roll period can be timed (usually about 4 seconds or so) and how the roll stops and reverses can be felt.

A quick roll with a hard check can create a vomitorium in a beam sea.


A full displacement boat is seldom bothered by the added weight of cruising gear , an added 2000 lbs of necessary stuff may add 3HP to the engine load or about 1/5 of a GPH.
Hence my word "abominably", all boats roll to some extent it's the nature of the roll that determines whether or not it's a comfortable roll. Here is a quote from Michael Kasten "Displacement vessels (sail or power) will usually have a less aggressive roll motion, a longer roll period, and a more gentle "return" at the end of the roll than semi-displacement or planing types. This is primarily due to displacement types having a proportionately less wide waterplane and greater displacement. We find that comfort and seakindliness are enhanced by keeping beam to the least amount necessary for initial stability and / or for sail carrying ability.
Conversely we observe that adding ballast will be counter-productive in terms of comfort.". I agree that you can get some idea of a vessels roll period at the dock as well as measure stability but at rest at the dock doesn't show the whole picture just a hint of what to expect at sea. I don't mind the motion of a full displacement boat with a round bilge but I have been on some downeast hulls that will jerk the coffee cup out of your hand in a beam sea.
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Old 02-04-2019, 06:59 AM   #12
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I don't know of any boats with hard chines that are labelled full displacement? (probably excluding rowboats...)

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Old 02-04-2019, 07:35 AM   #13
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I don't know of any boats with hard chines that are labelled full displacement? (probably excluding rowboats...)

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Old 02-04-2019, 08:56 AM   #14
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Full displacement?


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Old 02-04-2019, 09:01 AM   #15
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Full displacement?


Yes.
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Old 02-04-2019, 09:22 AM   #16
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Lotsof sailboats are hard chine and full disp. May have been even more common in the dayys of wood boats.
Hard chine actually has nothing to do with FD.
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Old 02-04-2019, 09:31 AM   #17
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Actually full displacement hulls can exceed hull speed with enough hp.
This ^^^^^


"Hull speed" is not an absolute limit. It is simply the point where it begins to take dramatically more power to push the boat any faster. But given enough power, no matter the hull shape, you CAN push "full displacement" boats past their theoretical hull speed.


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I guess that would include submarines.
Yeah, I think a submarine would be the ultimate full displacement boat!
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Old 02-04-2019, 09:49 AM   #18
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This ^^^^^


"Hull speed" is not an absolute limit. It is simply the point where it begins to take dramatically more power to push the boat any faster. But given enough power, no matter the hull shape, you CAN push "full displacement" boats past their theoretical hull speed.



Yeah, I think a submarine would be the ultimate full displacement boat!
As some sailboats have discovered while being towed by a ship when a displacement hull is driven past an SL ratio of 1.34 and it's hull is shaped in a way that will not provide enough lift to plane it sinks into it's own wave and ultimately sinks below the surface. So with some hull shapes there is indeed an absolute limit.
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Old 02-04-2019, 10:17 AM   #19
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Full displacement?


No hard chines there!
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Old 02-04-2019, 10:32 AM   #20
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Hard chines have nothing to do with it.
Even soft chines don’t.
Lots of lobster boats have a straight run aft, are planing hulls and have speeds over 25 knots. By “straight run aft” I’m tacking bout the hull bottom .. not the sides. Soft chine will pull some water up the sides causing increased drag but that dosn’t increase dramatically as speed increases so planing speeds are attainable. Just takes more power. Whith FD anything near planing speeds are not attailable
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