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Old 01-29-2013, 07:07 AM   #1
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Displacement

Hi All,
Anyone help me how 2 boats by the same manufacturer that weigh the same:
1 is called full displacement
2 is called semi displacement?
From my old schooldays anything that you put into fluid caused displacement so how can something be semi displaced?????
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Peter & Ronie B
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Old 01-29-2013, 07:25 AM   #2
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Both boats displace the same amount of water i.e. a quantity equal to their weight. But that's not what the manufacturers are talking about.

The terms Full Displacement, Semi Displacement, and Full Planing have to do with how the boat rides in/on the water while underway. A full displacement hull always rides in the water, pushing water out of the way as it moves. Hulls like this are limited in practice to a speed of about 1.3 x square root of the LWL (I don't have the exact figure committed to memory, but that's close). This is also knows as "hull speed". For a 40' boat that's about 8 kts.

A semi displacement hull can be pushed faster than hull speed and behaves better when doing so, partly because it begins to lift out of the water and run partly on the water rather than completely in the water. Such a boat might be capable of 10-12 kts. Be aware that for any of these hull types, the HP and fuel burn required to go faster rises exponentially, so going faster comes at a price.

A full planing hull can be run even faster, and eventually reaches a full plan where it is riding on the water rather than pushing through it. Almost all small power boats are of this type, and as engine power has increased over the years, it has become possible to build much larger boats capable of planing. These boats can operate at 20 kts or faster, but hold onto your wallet. It can take 1000HP and 50 gal per hour to run at 20 kts.
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Old 01-29-2013, 07:48 AM   #3
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In addition to what the previous poster said which is correct, the way the water passes the hull at higher speeds is due to the shape of the hull. In a full displacement hull the bottom is usually round with the underbody tapering up to the water line at the stern with only a narrow triangle of hull in the water at the stern. This lets the water pass smoothly and efficiently but doesn't give any lift at higher speeds so the stern squats.

A semi displacment hull has flatter and wider planing sections particularly aft so that the water forces can support the hull and get over the bow wave as speed builds. A planing hull even more so, sometimes with a step to let the water break loose from the hull.

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Old 01-29-2013, 12:50 PM   #4
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While the term "semi-displacement" has become widely used it is actually an inaccurate term. "Displacement" is like "dead." You either are or you aren't.

By the same token I think "full displacement" is a meaningless term, too. Or if not meaningless, then certainly redundant.

A much more accurate term as supported by naval architects like Tom Fexas is "semi-planing." Once a boat builds up enough speed to start generating a lifting force under the hull it is entering the planing realm.

A lot of boats like Grand Banks have hulls that will let them make use of this hydrodynamic force up to a point to go faster without a completely ridiculous amount of power. Our boat, for example, can cruise at 10 or 11 knots with engines that are not that much more powerful than the ones we have now. That is because the fairly flat aftersection of the bottom is designed to "ride up" on the water pressure being generated by the boat's increasing speed. And the more of the hull you can get out of the water the less drag there will be and the faster the boat can go.

So our boat with bigger engines could get partially up toward a full plane, hence "semi-planing." Which I think is a much more accurate term for what the hull is actually doing.
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Old 01-29-2013, 01:17 PM   #5
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In case the OP hasn't guessed from the previous posts I'll clarify - the whole displacement/semi-displacement thing is marketing BS. A given hull will be labelled according to what the guy selling it thinks will make it more appealing. The words themselves have little relation to the hull they are applied to.

My Malibu Response is a full displacement vessel at dead idle which is about 4 kts. When I ski behind it at 32 kts the fins and the prop are about all that is in the water which makes it pretty well the definition of a planing hull. I can't imagine how much HP you would have to throw at Gray Hawk to make her plane but there are double ended hulls that plane so I suppose its not impossible - maybe some kind of a turbofan.

To the OP - you are right, Archimedes figured out that bodies floating (or sinking for that matter) in water displace water equal to their weight.
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Old 01-29-2013, 01:19 PM   #6
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there are double ended hulls that plane so I suppose its not impossible

The Herrishoff versions required lifting fins to keep the canoe hull from going under at the stern.
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Old 01-29-2013, 01:31 PM   #7
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Semi-planing

Yes I agree w that one Marin.

""Displacement" is like "dead." You either are or you aren't." Dead?

There actually isn't a sharp break-of point where FD ends and SD (or SP) takes over. The still slightly fuzzy point is most accurately defined by the QBBL (quarter beam buttock line) angle. The all important line is in-between midships and the transom, in-between the chine and the keel ... and runs fore and aft. The angle of this line relative to the WL at rest tells the story. I'll try to find the angle as a number.

An easier and less technical way to "usually" determine SP from FD is to observe the transom at rest. Basically if ant significant amount of transom is submerged the boat is SP. The important thing is that how fast or slow the boat goes has nothing to do what type it is. It's the shape of the bottom aft. With a steep QBBL some transom can be submerged on a FD hull and shallow angle QBBL hulls are all planing or semi-planing.

I know you all want to think your boats are full displacement because they are trawlers but it just isn't so. What marketing people call boats is in a word ... marketing.
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Old 01-29-2013, 01:47 PM   #8
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Semi-planing

Yes I agree w that one Marin.

""Displacement" is like "dead." You either are or you aren't." Dead?
No. I think a hull is (by design, not Archimedes law) either displacement or it's not. That's why I think the term "full displacement" is just as inaccurate as the term "semi-displacement."

I think a hull is designed in terms of cruise performance to be one of three things: displacement, semi-planing, or planing. I suppose there's a fourth if you include "submersible."

As Bob pointed out above, semi-planing and planing hulls operate in the displacement mode at slow speeds. But in terms of what the hull is designed to do in cruise (or "at speed" if you prefer), I think there are just three basic configurations.
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Old 01-29-2013, 01:56 PM   #9
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The Herrishoff versions required lifting fins to keep the canoe hull from going under at the stern.
George Calkins drew the Bartender that way as well. I'm thinking that if Gray Hawk ever got up on plane she'd be pretty tender. Not mention thirsty.
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Old 01-29-2013, 02:13 PM   #10
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Sorry Marin it's not black and white.

I found this on BoatDesign.net

"Please refer to http://www.westlawn.edu/news/Westlaw...d06_June08.pdf
"Practical Speed and Power calculations June 2008 page 12" Table:
Buttock Angle vs Speed
Semi-Displacement Boats:
Speed- Lenght Ratio 1.5 to 2.5
Buttock Angle 3 to 6 deg.
Angles to the at-rest waterline"



Speed length ratio at hull speed is 1.34.
So for a 35' boat:
Speed at SLR 1.5 = 8.5 knots.
Speed at SLR 2.5 = 14 knots.

So w a 6 degree buttock angle the 35' boat is limited to about 9 knots.
With a 3 degree buttock line mid teens are probable. All these speeds are rough except the numbers in the quote.

Marin, Bob,
There is no such thing as "mode". The water flowing around a 36' sp/sd hull at 6 knots is very different than on a same size FD hull. If you go slow enough ... say about a SLR of .6 or .7 you will be "displacing" 100% of your hull weight but the water aft will not be flowing gracefully around the stern like a FD hull. Think of a sailboat. Again the speed (fast or slow) does not determine it's classification. It's the hull shape aft.

Here is a pic of an unquestionably FD hull and the buttock line is clearly very steep. Note also that there is no transom at all to drag through the water. I over exposed the pic so the hull shape (painted black) is more clear. The opposite is of course a planing hull. And Marin there are soooo many variations of hull design that many have shape features that have characteristics matching more than one type.
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Old 01-29-2013, 02:37 PM   #11
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gain the speed (fast or slow) does not determine it's classification. It's the hull shape aft.
Depends on who your talking to, I think. To the naval architect you're probably right. To the manufacturer, particularly with regards to their marketing/branding/image folks I think speed is the way the boats are classified. And I think most buyers look at it this way, too.

"Displacement" means slow, efficient, seaworthy (however you define that).
"Semi-planing" means faster but still kind of efficient and seaworthy.
"Planing" means goes like a bat out of hell.

And that's how boats are perceived by the market, I think, and so is how these terms are used by most people. That's why you get Nordic Tugs marketed under the term "fast trawler" even though they're not all that fast and they're not trawlers.

The typical boat buyer's not going to understand or care one bit about buttock lines or water flow or any of the rest of it. I suspect if you asked, 99 percent of boat owners, and probably 100 percent of boat salesmen, won't even know what a buttock line is.

So two separate worlds. The naval architect/marine engineer world and the boat-buying public world. I've been using the terms as I think they are perceived by the boat-buying market and the manufacturers in describing their boats to that market.

However I am under the influence of some pretty strong cough medicine as I'm taking a sick day so perhaps I'm making even less sense than usual.
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Old 01-29-2013, 03:16 PM   #12
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" Nordic Tugs marketed under the term "fast trawler" even though they're not all that fast and they're not trawlers."

Not mak'in much sense? YUP
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Old 01-29-2013, 03:35 PM   #13
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Why'd you change your avatar photo, Eric? Your last one was much nicer in my opinion.
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Old 01-29-2013, 09:21 PM   #14
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displacement

Hi All ,
Thanks for all the replies now i have lots more to think about We are looking at 44-49' DeFever's so now i undertstand more and and that helps keep the old brain active
Cheers
Peter & Ronie B
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Old 01-29-2013, 09:45 PM   #15
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Why'd you change your avatar photo, Eric? Your last one was much nicer in my opinion.
I don't know about that Marin it's a toss up for me haha

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Old 01-30-2013, 06:41 AM   #16
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"Displacement" means slow, efficient, seaworthy (however you define that).
"Semi-planing" means faster but still kind of efficient and seaworthy.
"Planing" means goes like a bat out of hell.

An easy way to define a boat from paper specks is to look at the power installed.

At 3-5 hp per ton its a displacement boat , that should be cheap to keep.

After that its up to you to decide if the range at usually 1 nm per gal is useful for you.

Beware , many owners and boat assembelers will use statute miles , not nautical miles in touting their product, about a 20% difference.
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Old 01-30-2013, 07:10 AM   #17
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My background suggests that there are 3 "typical" marketed hull shapes (but with a bazillion variations to accommodate sales...and of course there are others like cats, proas, etc but not as popular as the big 3).

There is displacement, semi-whichever and planing.

Then there are the "modes" or phases that a semii-whatever and a planing hull go through. They all start in the displacement mode, then depending on design characteristics, go into the semi-whatever mode, then the planing hulls jump up into planing at some later speed.

A planing hull or semi-whatever hull is destined to always be in the displacement mode if never given a large enough engine to create the speed to get it to lift into the next 2 modes.

Speed is not an absolute determiner of planing...some can do it at much lower speeds than others....flat bottoms are often planing at a little over 6 knots...

A flat iron skiff and many barges present the same hull shape to the water. The skiff can plane because a few hp will lift it all the way up to make planing practical yet I have yet to see a tug company try and get 600 foot barge to plane, Arguably I have heard that nuclear aircraft carriers are semi-planing as there is quite a bit of lift when the pour on the coals...although I have never been on one, nor have seen it.

While many rec boats described as displacement hulls are rounded bottom...many commercial boats are clearly intended for displacement work have very angular bottoms.
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Old 01-31-2013, 06:33 AM   #18
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For those with a tech bent , this discussion may answer some questions on Plaining.

Definition of Planing - Boat Design Forums
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