Definition Of Full Displacement

Fish53 said:

Well it wasn't as confusing as POTUS, or as excruciating.

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Now I understand.

sunchaser said:

Now I understand.

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Finally.

Regarding the original question in this 10 page post which has become a classic example of theory and reality. Many theorys have been submitted but the reality is....
Bricks, lumps of steel and conccrete are full displacement. They weigh more than the weight if the water they displace. All boats are semi displacement untill a hole in the hull renders them full displacement.

Gulf Comanche said:

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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Actually, if you could mount a 10k hp motor on a log, it would be quite fast, assuming the log is straight and missing the branches. Due to the waterline x beam ratio, a log (or a telephone pole) would be quite fast through the water.

My opinion of a displacement boat is one that sits in the water and when you move it, you apply pressure in the direction you want the boat to go and let the water flow out of the way and around the hull. Since water is thicker than air, you have higher resistance and therefore are limited to the speed you can move the hull. If you mounted the 10k hp engine on the hull, you would eventually push it out of the water and make it a planing hull, skipping across the water.

FD hulls don’t “skip across the water”
But they do tend to lack control if driven really hard.
I have driven my 18’ freight canoe (w a small flat stern) 13 knots. But w/o a person in the bow I need to be very careful at the tiller. Only the stern is in the water (midships aft) and I can do fairly tight figure eight turns but anybody witnessing that would be either clapping or calling me a horses ass.
Sure miss that red “OR”hat.

sunchaser said:

Just to keep this rolling, there are many SD vessels that are great and proven blue water vessels. It would be a tough call to say a FD Nordhavn 64 is a better blue water vessel than a SD Fleming 65.

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Really...
I would think that would be an obvious no brainer.
I know the Fleming is a highly respected boat but I’m having trouble imagining it in 12’ following seas and matching the Nordhavn for comfort and safety.

stubones99 said:

Actually, if you could mount a 10k hp motor on a log, it would be quite fast, assuming the log is straight and missing the branches. Due to the waterline x beam ratio, a log (or a telephone pole) would be quite fast through the water.

My opinion of a displacement boat is one that sits in the water and when you move it, you apply pressure in the direction you want the boat to go and let the water flow out of the way and around the hull. Since water is thicker than air, you have higher resistance and therefore are limited to the speed you can move the hull. If you mounted the 10k hp engine on the hull, you would eventually push it out of the water and make it a planing hull, skipping across the water.

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I spoke with a NA friend who has designed boats for 50 years about this whole question of displacement, hull design and speed. He said the same thing others have said, that is, that calculated hull speed isn't a speed wall, but what he called a soft barrier. I asked if that soft barrier had a limit and he thought so, but only for a true full displacement vessel. I also asked what happened if you tried to exceed that upper limit and he told me about a boat he designed that because of the power the owner insisted on had what he called an "auto-swamp" feature, meaning the transom would sink and take on water at full power. His solution was adding trim tabs, a.k.a. flat sections that allowed the boat to stay afloat and get to a semi-displacement status.

The reason FD vessels can submerge themselves if you push them hard enough is because the bow and stern wave generated are moving almost as fast as the boat. When a wave speeds up, physics dictates that it get higher. Go fast enough, and the stern wave is large enough that the stern sinks into a fairly deep hole, eventually going under.

I gather this will happen before the vessel gets up and starts "skipping along the water", but again, that is a function of whether the hull form lacks sufficient flat sections to provide enough lift to keep that from happening.

I still like my definition of full displacement as a hull form that generates more drag than lift as it speeds up.

sunchaser said:

Just to keep this rolling, there are many SD vessels that are great and proven blue water vessels. It would be a tough call to say a FD Nordhavn 64 is a better blue water vessel than a SD Fleming 65.

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I think Beebe had a pretty good definition of trawler (if I recall correctly) - a bluewater vessel capable of crossing oceans under power. No mention of hull form, although all of his boats for that purpose were FD.

I think Madison Ave. had/has the most profitable definition... use the old-world moniker "Trawler" for adding to peoples' romantic feelings toward a simple Pleasure Boat... so the dealer can make more sales!

Delfin said:

I think Beebe had a pretty good definition of trawler (if I recall correctly) - a bluewater vessel capable of crossing oceans under power. No mention of hull form, although all of his boats for that purpose were FD.

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No need to mention hull types IMO.
But sunchaser was talking bout “blue water boats” and to be honest I should have looked that up before posting.
I just thought it meant well ofshore .. not crossing oceans. But crossing oceans would seem to require a FD hull but people have done it in very small boats ... lucky ones I assume. And I’m sure at least onedid it w/o a FD hull. Oh I can even think of one ..Kon-Tiki .. a sailing raft basically.

But the fairly to mostly flat bottomed Flemming dosn’t seem to fit .. “blue water” to me. With or w/o the FD comparison.

An interesting thread with lots of tangents. But I cannot say I am any smarter in defining a FD hull. Nor do I really grasp to what end.


I normally run my boat below 8 knts like she were a FD hull. I know sister ships can utilize the built down lobster hull as SD and go faster with more power, I haven't tried yet. So I am gonna say it is both FD and SD depending on how she is used...


(Sorry dont know why/how hull appears upside down.)

FD and SD hulls have nothing to do with a specific vessels performance it's the operating speed range the designer created the hull to perform at, that's all. As was stated back a few pages it's simply a hull designed to operate at above or below an SL of around 1.34 for FD and SD and above an SL of about 3.0 is where planing boats live. Logs, bricks, dead bodies and other floating or non-floating objects are not designed for any speed hence no designation as FD or SD. This information is presented in numerous texts on the subject and I assure you I didn't make it up. So to simplify, if you have a boat that is designed as a semi-displacement boat and you have a 2hp engine and never go over 2kts it's still a semidisplacement boat. If you put 5000hp in it and it goes 100kts it's still a semidisplacement boat.

Yup .. didn’t read all of Fish53’s post.
FD SD have nothing to do w performance or speed.
It has only to do w hull form .. the shape of the hull aft.
You can’t change that w/o lots of cutting ect w tools to change the shape aft. Your boat is SD whatever speed she goes.
By the way I really like your boat.


But 5000hp and 100knots Fish I think it would be safe to call it a planing hull. I know of no SD hull that I’d bet would be controlable at 100 knots. But most trawlers (SD) would go about 20 knots w 3-400hp. So a SD boat could go at medium slow planing speeds. Medium planing of 18 knots for CptnPete’s west Cove and would be easily attained by hp alone.

Nomad Willy said:

Yup .. didn’t read all of Fish53’s post.
FD SD have nothing to do w performance or speed.
It has only to do w hull form .. the shape of the hull aft.

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That's about it, all determined on a drawing board before anything even gets wet. Thanks you've restored my faith in mankind.

How 'bout these buttocks?

RT Firefly said:

How 'bout these buttocks?

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I'm afraid for that we'll need illustration...Got any pictures?

Definition

menzies said:

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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Yes, your first guess was correct. "Full Displacement" refers to a hull design only, and has nothing to do with limitations on the hp of the power-plant.



All hulls have a 'hull speed' that is based on the square-root of the length of the waterline. Likewise, all hulls have a "displacement speed", where the hull is operating in a 'displacement mode' (essentially, this is when the running angle = 0 degrees). The unique thing about a "Full Displacement" hull is that it cannot ever climb over it's bow wave, no matter how much power you apply. This means you can never get up on plane and your speed will be strictly limited by the length of your hull at the waterline. Generally, a full-displacement hull will be the most fuel efficient hull compared to semi-displacement or planing hulls. This is why sailboats are built on displacement hulls.


More on the topic here:



https://www.nordhavn.com/35/overview_hull.htm

Nomad Willy said:

Yup .. didn’t read all of Fish53’s post.
FD SD have nothing to do w performance or speed.
It has only to do w hull form .. the shape of the hull aft.
You can’t change that w/o lots of cutting ect w tools to change the shape aft. Your boat is SD whatever speed she goes.
By the way I really like your boat.


But 5000hp and 100knots Fish I think it would be safe to call it a planing hull. I know of no SD hull that I’d bet would be controlable at 100 knots. But most trawlers (SD) would go about 20 knots w 3-400hp. So a SD boat could go at medium slow planing speeds. Medium planing of 18 knots for CptnPete’s west Cove and would be easily attained by hp alone.

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This is one of the fishing trawlers I used to own, she had 260hp but was designed originally for 100hp. Mine would do 9kts WOT, one with 100hp did 9kts WOT. Of course the hp was to tow a bigger net and mine weighed 50,000 lbs.

9 knots. Pretty good for 100hp.

Nice looking hull from what I can see w the black hull paint.

Ahhh towing the net.
Do you tow into the tide?
Don’t see how it could be done any other way.
But I also have a hard time imagining towing those huge nets w much tide current.

Nomad Willy said:

9 knots. Pretty good for 100hp.

Nice looking hull from what I can see w the black hull paint.

Ahhh towing the net.
Do you tow into the tide?
Don’t see how it could be done any other way.
But I also have a hard time imagining towing those huge nets w much tide current.

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Fair tide, head tide and abeam doesn't matter much, a little less or a little more throttle. Wind has more affect but the real driver is staying on a contour or working around obstructions called hangs. Fish like to keep near lumps, ridges and holes so you need to follow the bottom as best you can. That one had the aforementioned 260hp and a 4 to 1 gear turning a 38x33 four blade wheel so she pulled pretty well. This one that I used to run had 2500hp and also did 9kts. That's Iliamna volcano in the background.

I usta work at False Pass in the Nick Bezz days.

Nomad Willy said:

I usta work at False Pass in the Nick Bezz days.

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I fished salmon in southeast back around 1980 seining, I like Alaska and spent years there but I guess New England has too much pull for me to stay in AK. I used to go up to the Grand Banks when US boats still worked there, weather in that region can get brutal. I had the most fun in Ireland but made no money, Gloucester and New Bedford boats fishing on Georges was the best though. Being forced into retirement sucks.

With high tech laser measuring equipment:


It should be fairly easy that correct design conditions could fabricate add-on portions for any boat's bottom to alter its shape as desired... i.e. into a different form.


Therefore... D designed hull could become SD designed hull or even a P hull design. The reverse is also possible.

OMG - A whole new boat-product market just opened up. :lol:

Nomad Willy said:

Really...
I would think that would be an obvious no brainer.
I know the Fleming is a highly respected boat but I’m having trouble imagining it in 12’ following seas and matching the Nordhavn for comfort and safety.

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Tony has traveled with the 65' Fleming Venture I and II to some interesting far off places. Lots of YouTube videos of his travels to look at. Our own Steve D did the North Atlantic crossing with him.

But, my point was that there are some very well traveled SDs out there. A FD vessel is not the only blue water game in town. A trip offshore in snotty weather in a big sport fishing vessel is quite an eye opener too.

Thanks Sunchaser,
Even a planing boat at times can be excellent in big water. So yes FD isn’t required for big water.

But crossing oceans is different. You’re not going out on nice days ... you’re going out on all the days. Basically whatever happens out there you’re going to get. And every boat is different.

Some boats are good at head seas but terrible in following seas. My boat is great at everything but beam seas. Another boats bow interferes w directional stability and another has a stern that causes wandering or worse.
But there are those boats that do excellent (comparitively) at all things. And there’s no perfect boat.

Of the well known SD trawlers (under 50’) what boats are in the near perfect catergory for offshore running, do you think?

There's more to seaworthiness of a boat than just hull shape....


More than of a few of all hull shapes and boats less yhan 100 feet have crossed the Atlantic. Some a stunts, and promotions with safety boats...but sso have regular joes in things like production sportfish, etc.


And yes...more possible these days because of better weather forecasting.

Fish53 said:

This is one of the fishing trawlers I used to own, she had 260hp but was designed originally for 100hp. Mine would do 9kts WOT, one with 100hp did 9kts WOT. Of course the hp was to tow a bigger net and mine weighed 50,000 lbs. View attachment 85078

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That's what has me scratching my head a bit. Your CD38 (which i really like) has a very similar hull shape to my WC40 from what i can tell in the pic you have shared and the marketing brochure i have on it. Same Royal Lowell designer. Same beam. Different displacement (mine is half). And the CD has an extra foot of draft. But I wonder if your Iroquois had no trawl gear, nothing in the hold etc. and had a 200-300HP engine if she too would easily get up above 10knts like a SD. I dont know if the CD had ballast. Nor if it had an extraordinarily thick hull so i wonder where the weight came from in standard out of the box FD configuration.

CptnPete said:

That's what has me scratching my head a bit. Your CD38 (which i really like) has a very similar hull shape to my WC40 from what i can tell in the pic you have shared and the marketing brochure i have on it. Same Royal Lowell designer. Same beam. Different displacement (mine is half). And the CD has an extra foot of draft. But I wonder if your Iroquois had no trawl gear, nothing in the hold etc. and had a 200-300HP engine if she too would easily get up above 10knts like a SD. I dont know if the CD had ballast. Nor if it had an extraordinarily thick hull so i wonder where the weight came from in standard out of the box FD configuration.

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She had concrete ballast, about 3000 lbs. and it had all the gear, winches, net reel, mast, gallows, doors and net. The builders weight without any gear and a 100hp engine was 42,000 lbs. and a designed speed of 8kts., I believe to get 10kts. out of one you'd need to go over Niagara Falls. I should add the disclaimer that I only saw 9kts. once and it was probably fair tide. I usually steamed at 7kts. where it used about 2 gallons an hour in good weather.

Nomad Willy said:

Of the well known SD trawlers (under 50’) what boats are in the near perfect catergory for offshore running, do you think?

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Eric that is a tough question to answer in that the list is very long. PSN mentioned weather awareness. Couple that with a well tended vessel as represented by many on trawler forum and it is well established that go anywhere is the norm. Range of course becomes a limiting factor.

Recently I was in St Lucia. The types of MV that cruise there from Florida is large. Couple that with what we see in Mexico, the PNW, Alaska proper and Europe and it is fair to say most vessels with a SD hull work quite well in smart hands.

Last summer when in Petersburg we spent time with people who routinely cruise from SE Alaska to Seward on their GB 42. Some years ago I chuckled when reading Egret's (N46) posted blog. Several times they met up with a DeFever 44 when in the Southern Chilean waters around the Cape.

Heck, I know of people who bought a DF 49 in the Seattle area and delivered it to England on its own bottom. It really comes down to time, knowledge, vessel soundness and desire. SD vessels have few limitations as compared to FD. And some are faster than their comparable FD cousins. Think bad weather avoidance. An extra few knots is relevant on the open ocean.

Again, read and view Tony Fleming's F65 articles and YouTube presentations. Impressive as is the man. Keep up your good posts and unflinching interest in this subject.

Delfin said:

When a wave speeds up, physics dictates that it get higher. Go fast enough, and the stern wave is large enough that the stern sinks into a fairly deep hole, eventually going under.

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When a wave speeds up it gets longer, not necessarily higher. The height has more to do with the object creating a wave, it's displacement and shape. A planing boat creates a bow wave, the trough is well behind the transom. As a hull without a lot of lift speeds up, the wave it creates is the same length as the planing boat at the same speed but the energy required is much higher, and that energy is spent in building a higher wave.

Riverguy said:

The unique thing about a "Full Displacement" hull is that it cannot ever climb over it's bow wave, no matter how much power you apply. This means you can never get up on plane and your speed will be strictly limited by the length of your hull at the waterline.

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Never say "never". You can apply a tremendous amount of power to a boat, most normal hull shapes will eventually plane, though some will sink due to lack of freeboard first. Certainly something like a KK would plane. As a practical matter, you would not attempt to do it as the power required would be very large.

psneeld said:

There's more to seaworthiness of a boat than just hull shape....


More than of a few of all hull shapes and boats less yhan 100 feet have crossed the Atlantic. Some a stunts, and promotions with safety boats...but sso have regular joes in things like production sportfish, etc.

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The Atlantic has been crossed by kayaks, rowboats, wind surfers, beach cats (numerous times for all of those), inflatable boats, and on at least one occasion each, a floating pile of garbage and a person clinging to a mooring buoy. The smallest boat to do it was just over 5' long. The only challenge for a powerboat is carrying enough fuel.