Definition Of Full Displacement

[menzies]

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?

 

[Gulf Comanche]

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.

 

[menzies]

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.

 

[Nomad Willy]

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.

 

[Airstream345]

Typically a round bottom (no hard chimes or flat stern) that doesn’t (can’t) 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.

 

[BruceK]

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".

 

[Lepke]

Actually full displacement hulls can exceed hull speed with enough hp. Think USN. Do the math on a destroyer or carrier.

 

[AusCan]

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.

 

[Fish53]

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.

 

[FF]

"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.

 

[Fish53]

"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.

 

[ranger58sb]

I don't know of any boats with hard chines that are labelled full displacement? (probably excluding rowboats...)

-Chris

 

[Fish53]

I don't know of any boats with hard chines that are labelled full displacement? (probably excluding rowboats...)

-Chris

There's thousands of commercial vessels from supply boats to ships that have a relatively hard chine and a flat bottom.

 

[RT Firefly]

Full displacement?

 

[Fish53]

Full displacement?

Yes.

 

[Nomad Willy]

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.

 

[denverd0n]

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.

I guess that would include submarines.

Yeah, I think a submarine would be the ultimate full displacement boat!

 

[Fish53]

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.

 

[menzies]

Full displacement?

No hard chines there!

 

[Nomad Willy]

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

 

[markpierce]

If the boat makes wakes like these at cruising speed, it has a displacement hull:

 

[Fish53]

If the boat makes wakes like these at cruising speed, it has a displacement hull:

More correctly any hull being operated at displacement speed.This boat is being operated at the transition from full displacement to planning or semi-displacement. Note the characteristic depression almost to the transom and the building wave in front and behind. In full displacement mode the depression is midway as in your photo, at planning speed the hull has climbed over the wave forward and is riding n the surface.

 

[Delfin]

Actually full displacement hulls can exceed hull speed with enough hp. Think USN. Do the math on a destroyer or carrier.

Those Navy vessels are semi displacement because of flat after sections, so can exceed the physical limits of speed for a full displacement hull, which will not go faster than its hull speed, no matter how much horsepower you stick in it. Besides, a 500' destroyer would have a hull speed of 30 knots anyway if it was full displacement, which it is not.

 

[Delfin]

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.

Nope. Add more HP and you will simply suck the stern down, slightly increasing LWL and slightly increasing speed until the physical limit is reached. After that, the stern submerges. Just physics.

 

[Fish53]

Those Navy vessels are semi displacement because of flat after sections, so can exceed the physical limits of speed for a full displacement hull, which will not go faster than its hull speed, no matter how much horsepower you stick in it. Besides, a 500' destroyer would have a hull speed of 30 knots anyway if it was full displacement, which it is not.

Being emphatic doesn't necessarily make things correct, destroyers have nowhere near enough breadth to length to create a planning or semi-displacement hull, nor would you want one. The hull speed of a 500 foot waterline at an SL of 1.34 is 29.334 kts, destroyers may be able with sufficient horsepower to exceed this to 1.35-6 but at a horrific use of fuel.

 

[Delfin]

Being emphatic doesn't necessarily make things correct, destroyers have nowhere near enough breadth to length to create a planning or semi-displacement hull, nor would you want one. The hull speed of a 500 foot waterline at an SL of 1.34 is 29.334 kts, destroyers may be able with sufficient horsepower to exceed this to 1.35-6 but at a horrific use of fuel.

The ratio of beam to length has zero to do with whether a hull is full, or semi displacement. The Zumwalt class destroyer had been clocked at 50 knots, or around 80% more than hull speed. Why? Because it and every other modem destroyer has a semi displacement under body.

 

[Fish53]

The ratio of beam to length has zero to do with whether a hull is full, or semi displacement. The Zumwalt class destroyer had been clocked at 50 knots, or around 80% more than hull speed. Why? Because it and every other modem destroyer has a semi displacement under body.

I see now we qualify it by specifying a Zumwalt destroyer, which are being built right up the road from me, and ignoring the over one hundred years of destroyer design? Which you made no reference to in the previous comment. Thank you for your contribution.

 

[Delfin]

Pretty good explanation of hull forms from a naval architect: https://www.google.com/url?sa=i&source=web&cd=&ved=2ahUKEwjS16W426LgAhVhJzQIHZx5DKAQzPwBegQIARAC&url=https%3A%2F%2Fwww.marineinsight.com%2Fnaval-architecture%2Fchoosing-a-hull-form-for-ships-a-naval-architects-perspective%2F&psig=AOvVaw2XZhk37c6xfKh1hoFUJlY7&ust=1549391548722858

Pull quote on displacement hull speed limitations: "To put simply-the length of a (displacement hull) vessel puts a ceiling on its speed.".

 

[menzies]

Interesting article.

https://oceantrawleryachts.com/comparing-trawler-displacement-hulls/

Also, here is a photo of the bottom on a KK 52.

 

[Delfin]

I see now we qualify it by specifying a Zumwalt destroyer, which are being built right up the road from me, and ignoring the over one hundred years of destroyer design? Which you made no reference to in the previous comment. Thank you for your contribution.

Caught mid edit, but since you are having a hard time with the concept, as noted, every naval ship capable of exceeding hull speed has a semi displacement hull. Being snarky in your response doesn't change physics.

Since a picture is worth a thousand words, here is the semi displacement underbody of the Shoup, an Arleigh Burke class destroyer.

Does that stern section look like a full displacement configuration?