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Old 02-08-2019, 12:46 PM   #181
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Well it wasn't as confusing as POTUS, or as excruciating.
Now I understand.
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Old 02-08-2019, 12:49 PM   #182
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Now I understand.
Finally.
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Old 02-08-2019, 01:08 PM   #183
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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.
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Old 02-08-2019, 01:19 PM   #184
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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.
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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Old 02-08-2019, 01:37 PM   #185
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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.
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Old 02-08-2019, 01:54 PM   #186
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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.
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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Old 02-08-2019, 01:57 PM   #187
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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.
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.
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Old 02-08-2019, 02:11 PM   #188
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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.
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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Old 02-08-2019, 03:49 PM   #189
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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!
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Old 02-08-2019, 04:10 PM   #190
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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.
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.
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Old 02-08-2019, 04:43 PM   #191
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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.)
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Old 02-08-2019, 05:06 PM   #192
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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.
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Old 02-08-2019, 05:27 PM   #193
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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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Old 02-08-2019, 05:32 PM   #194
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Yup .. didnít read all of Fish53ís 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.
That's about it, all determined on a drawing board before anything even gets wet. Thanks you've restored my faith in mankind.
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Old 02-08-2019, 05:48 PM   #195
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How 'bout these buttocks?


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Old 02-08-2019, 05:50 PM   #196
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How 'bout these buttocks?


I'm afraid for that we'll need illustration...Got any pictures?
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Old 02-08-2019, 06:24 PM   #197
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Definition

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Originally Posted by menzies View Post
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?

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
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Old 02-08-2019, 06:27 PM   #198
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Yup .. didnít read all of Fish53ís 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 canít 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 Iíd 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 CptnPeteís west Cove and would be easily attained by hp alone.
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. Name:  Iroquois2.jpg
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Old 02-08-2019, 07:38 PM   #199
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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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Old 02-08-2019, 08:08 PM   #200
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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.
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.Click image for larger version

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