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Old 02-06-2014, 07:18 PM   #1
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Full Displacement or Semi Displacement

I offer this new thread for those that want to help define the difference between the two. There seems to be strong opinions about what is and is not a full displacement hull.

It looks to me like there's a mentality here that says that the most serious trawlers are FD. Or real men have FD trawlers and wimps have SD. What is it that makes so many here want to be seen and known as owners and operators of FD boats?

So given the need for the label what do you think the definition of a full displacement hull should be. What's the difference?

Should the best trawler be FD or SD?
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Old 02-06-2014, 07:52 PM   #2
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Actually i don't get that sense at all.

There are a lot of people here passionate about what they do and write about, that's why the spend the time here, to both help others and to learn more.

The way I have interpreted this thread during the last two days is that many times it's more about marketing than reality. Most boats are not true full displacements because the rolling would drive most crazy.

But one thing I have gleaned from this thread is there is no real dividing line, other than at the extremes. And also no matter how they are defined, almost all are happy and proud of their boats, as they should be.
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Old 02-06-2014, 08:22 PM   #3
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All boats at rest are full displacement.
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Old 02-06-2014, 08:59 PM   #4
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All boats at rest are full displacement.
Pure wisdom
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Old 02-06-2014, 10:55 PM   #5
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Without over complicating things, full displacement is a boat that only operates at displacement speeds and semi-displacement will go faster. Now we know that we put some leeway in there because many "full displacement" boats would have a displacement speed of 8 knots and operate at 10 to 12, so wouldn't fully meet that definition. But the point remains that in a full displacement boat, adding horsepower will have minimum effect.

Why an issue, because once you accept a hull as being full displacement, you feel comfortable with lower horsepower and often a single engine. This then means lower fuel and maintenance costs. While true that a semi-displacement hull will operate at displacement speeds, it will generally have more horsepower so will not do it as inexpensively.

It's really a matter of taste. Full displacement is the most economical and if speed is no object has many advantages. Personally, we just can't accept the speeds so fully informed as to the cost differential are only interested in semi-displacement.

There is no right or wrong, just what is right or wrong for you.

In larger boats one other factor comes into play and that is hull material. Steel boats are only going to be full displacement so if thats the choice it's fixed.
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Old 02-06-2014, 11:07 PM   #6
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A full displacement boat cannot be driven up on plane no matter what size engines you put in it.

Any boat that can be driven up on plane is a planing hull.

There is no such thing as a semi displacement boat.
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Old 02-06-2014, 11:21 PM   #7
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There is no such thing as a semi displacement boat.
At the risk of disagreeing with naval architects I completely agree.

Your either pregnant or not, but never semi-pregnant.
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Old 02-06-2014, 11:31 PM   #8
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I'm already liking this.

Start from the basics. The thought of FD as the most economical is brilliant. That hasn't come to me from any source but the larger a vessel gets the more that becomes apparent. So if you were to have the biggest boat capable of carrying the greatest loads it would be FD.

ksanders that's interesting although somewhat simplistic. You're not dodging the question are you? So what is a GB or an IG?. Looks like they can plane to some degree but not capable of going over the hump and steeping out w lots of spray. What is the difference between an older wood CC planing boat and an IG? The Chris is narrower and the IG has a big keel. I suspect if you lightened up an IG and took off the keel 25 knots would be a reasonable expectation. But even w the extra weight and big keel it can go much faster than my Willard. I think we need a middle ground (SD) as there are boat that are part planing and not that far from FD but basically aren't either. But it simplifies the issue in that there's only one grey area. But it's bigger and fuzzier.

Sorry Craig my Willard is not preg ...... What made you thing of that anyway?
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Old 02-06-2014, 11:33 PM   #9
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I prefer the term semiplaning.
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Old 02-06-2014, 11:56 PM   #10
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I suspect we are entering the world of semantics. I suspect there is pure SD, pure planing and everything in between, which people can and will label however they like.
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Old 02-07-2014, 12:35 AM   #11
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Eric

My description came right out of VOYAGING UNDER POWER almost word for word.

I am traveling and do not have my copy here but the concept was simple. (you already know this Eric )

A full displacement hull has a shape, such that no matter how much power is applied the boat will not climb over its bow wave.

Now, the concept of semi displacement, and I have used it myself is pure BS. Either a boat can climb over its bow wave or it cannot. Generally this occurs at 2.0 x sqft of the water line length.

Engine size does not define FD or SD status. Hull geometry does.

You can put small engines in a boat that COULD plane but that does not make it a FD hull.

You could put gigantic engines in a FD hull and that won't turn it into a anything more than a overpowered boat.

So, the question people are starting to ask is why would you ever build a planing hull boat and put little engines in it.

The answer to that is stability, and fuel economy

A planing hull boat will exhibit higher initial stability than a FD boat. This is easily demonstrated by comparing a FD boat and a planing hull boat at anchor. The FD boat is much more tippy. People don't want tippy. They want stable.

The smaller engines all came about due to the sudden increase in fuel prices in the early 1970's. This opened up a new concept, that being a large recreational boat that was economical to operate.

Now lets Fast forward from the 70's to the 90's. The slow boats of the 70's and 80's were just too slow. People had to make a living to buy the boats and they needed to have a faster boat.

So, what did the marketeers do, well they took the same basic non tippy (planing) hull designs that were so comfortable and put bigger engines in them. These boats typically operate at just under 2.0 X sqrt WLL, and burn a hell of allot of fuel doing it, but they met a market need for speed.

Then designers actually started optimizing hull forms to be more efficient at these slower than text book planing speeds and the boats we call "semi displacement" were born.

My boat is 100% typical of that. It is called semi displacement, but in reality it is a planing hull boat, that has been optimized to operate without its bow sticking straight up in the air at slightly less than traditional planing speeds.

What is better??? Well, they are all better at certain things.

A stabilized FD boat is far and away better at holding the fuel necessary, and providing safety to cross oceans.

A boat like mine is far better at providing people with choices. Go slow and be comfortable and efficient. Go fast and avoid the really rough weather, making it to a safe port in 1/2 the time.
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Old 02-07-2014, 12:52 AM   #12
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I prefer the term semiplaning.
That term makes far more sense to me than semi-displacement because it describes what it is. An under powered planing hull.

Eric, if ya put enough horsepower in a Grand Banks 36 would it plane? It may be ugly and awkward doing it but still...
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Old 02-07-2014, 01:03 AM   #13
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Most people here have boats with two engines and two helm positions, doubling some costs and maintenance. They have two engines so they can exceed hull speed, thus should have semi-displacement or even full planing hulls. For me, simplicity and cruising at 6.3 knots (a knot below hull speed) is fine, and having a heavy boat evens out movement when encountering waves. Also, a low-maximum-revolution engine reduces irritating (to me) engine whine.

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Old 02-07-2014, 01:29 AM   #14
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I vote with Markpiece and ksanders!! Amen boys, amen. Al
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Old 02-07-2014, 02:42 AM   #15
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I am a bit surprised no one actually looked up any definitions of the hull shapes. A full-displacement hull is supported in the water entirely, or virtually entirely by buoyancy. As Northern Spy noted, at rest, all boats are full displacement. As the hull moves through the water, a full displacement hull does creates no, or virtually no, hydrodynamic lift. Wetted surface remains the same no matter the speed. As we all know, speed is limited by waterline length.

A planning hull creates hydrodynamic lift as speed increases, this "lifts" the boat, reducing wetted surface and decreasing draft. Thus, higher speed as less wetted surface and less draft means less water is moved as the boat moves. At top speed, very little of the boat weight is supported by buoyancy, most is supported by the lift.

The continuum between the two is simply the amount of hydrodynamic lift created by the hull form as it moves through the water. So, a semi-displacement hull can run the entire range from creating just a very little hydrodynamic lift to just short of a full planning hull form. So, ful-displacement is sharply defined and full planing is sharply defined, but there is a huge area for a hull to be called semi-displacement.
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Old 02-07-2014, 02:58 AM   #16
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Semi-Displacement is one of those terms that is literally inaccurate and thus will always be subject to challenge.

My understanding of a semi-displacement boat (and I could be wrong) is that it is not a smaller engine but a design of a planning hull that is geared to some extent more toward fuel efficiency at lower speeds than a hull designed to reach higher speeds. Thus like everything else on a boat it is a compromise sacrificing higher speed for more fuel efficiency.

It is unfortunate that the marketers have called this arrangement semi-displacement - but then the term planning hull sacrificing a lot of speed for more fuel efficiency probably wouldn't sell.
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Old 02-07-2014, 05:56 AM   #17
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A full displacement boat cannot be driven up on plane no matter what size engines you put in it.

However this only limits speed with low powered fat boats.

A cat with a 12-1 LB ratio never plains but its >hull speed< is way beyond the fat boat.

>So, the question people are starting to ask is why would you ever build a planing hull boat and put little engines in it.

The answer to that is stability, and fuel economy

A planing hull boat will exhibit higher initial stability than a FD boat. This is easily demonstrated by comparing a FD boat and a planing hull boat at anchor. The FD boat is much more tippy. People don't want tippy. They want stable.<

And stable less tippy is great at DOCKSIDE , climbing on and off the cottage.

Underway the way the boat checks , stops the roll and starts to roll the other way is key to a good ride in rough conditions.

The planing boat attempts to climb up on top of the waves and simply bash through or leap from wave to wave to smooth the ride.

Fine if 1/2 MPG is worth it for a quickie get home.

The chines on the planing boat make most far to stiff , away from the dock at displacement speeds to give a good long term ride.

> Go fast and avoid the really rough weather, making it to a safe port in 1/2 the time.<

IF your not so far from shore that you run out of fuel.
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Old 02-07-2014, 07:16 AM   #18
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FD will always displace their weight with water and will always have the same draft (more or less). They will always displace their weight in water.


PH will use lift to rise up using the lift provided by the reaction of water on their hull form to increase speed by reduced water drag. at full plane the wake will reduce as the boat rides on top of the water and at this point will require less power to maintain this condition. At full plane they will displace most of their weight with lift.



Fast non-planing boats (SD?) will have so much power and some lift built in to there hull that they will achieve a partial plane with slight reduction in surface drag and reduction in draft but with a much higher wake production and with no reduction in overall drag (never over the hump) regardless of use of full power. They will displace a small amount of their weight with lift but most of their weight will be displaced at speed by water.



Its like an airplane, the FD has not enough wing (lift) to fly so its really a bus. The SD is like a hover craft lots of power and can fly up to about 3 ft. The planing hull can fly.




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Old 02-07-2014, 07:16 AM   #19
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Sorry Craig my Willard is not preg ...... What made you thing of that anyway?

Maybe not but if it were this would be the offspring.

I suppose she is a 30' too but in this configuration looks much smaller.
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Old 02-07-2014, 07:40 AM   #20
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Simply as many want to say...there is no clear break between designs....change a little here and there and a full displacement boat can become semi...and a semi can become a planning.....there's no magic number or shape that jumps a boat from one cat to the other. That's hull design.

Then there's "mode"...all boats at rest are in the "displacement mode"...a planning boat goes throught the 2 slower modes until it's hull it totally lifted by pressure (oversimplified).

So between the hull design type that is not absolute for many hulls as they are somewhere in the design between "hull types averages" and how a person runs the boat (like a ridiculously underpowered planning or ridiculously overpowered semi boat)....there's so much overlap labeling a lot of boats is just an exercise of armchair boating.

Sure, take the calculations off of a design, work out the numbers of the fully finished boat from actual weighing and waterline measurement...and you can go back in and find out where in the curve your boat may fall....

But the discussion like Rick often points out is splitting hairs in many ways...

A displacement hull pushed to it's hull speed that has a lousy prismatic coefficient (read cinder block) in all likelihood will be less efficient than a low powered semi with a really slippery prismatic coefficient run below her hull speed (read really slippery Lobsteryacht).

So simply...it isn't ....not the way some here try to simplify it.....
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