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Old 08-21-2013, 11:44 PM   #181
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So would my old sub would be a full displacement, or maybe a semi-displacement, or how about variable displacement, or negative displacement?

My wee Nordic Tug would hardly be a semi-planing boat. I think semi-displacement seems appropriate.

Edit for Marin (Hi Marin!):

Because it wallows like a pig! It does not even begin to see the glory of planing (I'm free, I'm free!) You lived in Hawaii as have I, have you not surfed, can you not feel the difference? That wobbly feeling when you put your knees up and the wave susses by you...

Semi-displacement vs. semi-planing is akin to glass half full to glass half empty. Of course, the engineer in me says "wrong size glass".
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Old 09-12-2013, 06:27 PM   #182
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Nordic Tug

From the Nordic Tug website,
Quote:
At Nordic Tugs, we've met this challenge with our unique semi-displacement hull, designed to meet the demands of serious inland and coastal cruising. At low speed, it conserves fuel and reduces roll. At high speed, it rises up to reduce drag. A versatility that neither a full displacement nor planing hull can match
These bottoms on all their models were designed by Lynn Senour as I understand it?

Does anyone have some good photos/drawings of these 'unique' hull shapes that allow such great performance under displacement and 'semi-displacement' speeds?


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This Nordic Tug 39 looks to be trying to get up to a planning form?....with 380-hp single engine.
2013 Nordic Tugs 39' Flybridge Power Boat For Sale - www.yachtworld.com
I would think that a 'semi-displacement hull design' (if there is such a thing) should be able to do better than this. To me it appears to be a displacement hull form being pushed onward to a planning situation that it can't quite get to in totality? ...or perhaps it can, and just has arrived their yet.
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Old 09-12-2013, 07:13 PM   #183
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Semi displacement
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Old 09-12-2013, 07:55 PM   #184
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Nordic Tug 34

This 34 foot model plans off rather nicely:
Nordic Tugs*Nordic Tug 34 (2013-)*2013* Reviews,performance,compare,price,warranty, specs,Reports,Specifications Layout, video | BoatTEST.com
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Old 09-12-2013, 08:24 PM   #185
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TY Brian! Cool video - Real nice boat! However, I need FB.
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Old 09-12-2013, 08:42 PM   #186
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Quote:
Originally Posted by brian eiland View Post
From the Nordic Tug website,


These bottoms on all their models were designed by Lynn Senour as I understand it?

Does anyone have some good photos/drawings of these 'unique' hull shapes that allow such great performance under displacement and 'semi-displacement' speeds?


Attachment 22851
This Nordic Tug 39 looks to be trying to get up to a planning form?....with 380-hp single engine.
2013 Nordic Tugs 39' Flybridge Power Boat For Sale - www.yachtworld.com
I would think that a 'semi-displacement hull design' (if there is such a thing) should be able to do better than this. To me it appears to be a displacement hull form being pushed onward to a planning situation that it can't quite get to in totality? ...or perhaps it can, and just has arrived their yet.
Please don't be another poster that tries to guess at hull performance from still picture...it says so much about an individual.....
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Old 09-12-2013, 08:54 PM   #187
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Very interesting video/test. Very nice boat with lots of goodies and benefits. I didn't know about the boat leaning toward the outside on hard turns, but I suppose that's what you get with planing speed and a full keel. Now I can understand why Eric (Manyboats) likes them.
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Old 09-12-2013, 11:37 PM   #188
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Yup .. I sure like them even if they are a light boat.

Semi-planing? That would be partly planing in my book and these photos of a NT26 in our yard show a bottom more capable of partly planing that many or even most on this forum. Owner says 10 knots w 55hp Yanmar.

Semi disp? Partly disp. Sounds like a hull closer to planing that has some tendency to perform fairly well at speeds that turn a planing hull into a dog.

I think the NT is not FD or planing and most use the term semi-disp. I'd like to see more rocker or/and a more convex aft buttock line ... the curve of the aft run. In other words semi disp but closer to FD.
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Old 09-13-2013, 05:20 AM   #189
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A displacement boat is mostly held up by its displacement , a plasining boat is held up by the water pressure on the plaining surface , usually a modest section of hull well aft.

As a plaining boat gets >over the hump< its fuel burn per mile will usually go down as the hull is now more efficient, as its not climbing over its bow wave.

Semi displacement is a hull that can not truly plane , just wallows the entire hull up onto less of the bow wave , but NEVER sees the increase in efficiency available to a genuine plaining boat.

It pays for this >performance< 100% of the time by its less efficient (at displacement speeds) hull shape and hugely over sized engine(s) .

Only advertising can overcome this normal handicap.

The FAST TRAWLER oxymoron.
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Old 09-13-2013, 09:33 AM   #190
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I suspect Northern Spy just dosn't like the word planing associated w his boat. I'm sure there are others here that feel the same way so I'll just use semi "displacement" rather than "planing".

FF wrote;
"Semi displacement is a hull that can not truly plane , just wallows the entire hull up onto less of the bow wave , but NEVER sees the increase in efficiency available to a genuine plaining boat.

You could also say;
Semi displacement is a hull that can not truly plane , just wallows the entire hull up onto less of the bow wave , but NEVER sees the increase in efficiency available to a genuine "full disp" boat.

Having a SD boat FF is that what you do is "wallow" around? Or is that just what other TF members do?
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Old 09-13-2013, 06:00 PM   #191
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WEIGHT of vessel is key

Quote:
Originally Posted by FF View Post
A displacement boat is mostly held up by its displacement , a plasining boat is held up by the water pressure on the plaining surface , usually a modest section of hull well aft.

As a plaining boat gets >over the hump< its fuel burn per mile will usually go down as the hull is now more efficient, as its not climbing over its bow wave.

Semi displacement is a hull that can not truly plane , just wallows the entire hull up onto less of the bow wave , but NEVER sees the increase in efficiency available to a genuine plaining boat.

It pays for this >performance< 100% of the time by its less efficient (at displacement speeds) hull shape and hugely over sized engine(s) .

Only advertising can overcome this normal handicap.

The FAST TRAWLER oxymoron.
I think most of your 'definitions' are spot on. Semi-displacement is a marketing term.

There may be another way to view it, per your quote,
Quote:
A displacement boat is mostly held up by its displacement , a planing boat is held up by the water pressure on the planing surface , usually a modest section of hull well aft.
I'm really coming to believe that the real difference one could experience between the performance of a pure displacement vessel and a semi-displacement vessel is NOT related so much to the subtleties in bottom shapes as much as its related to the WEIGHT of the vessel.

Lighter weight vessels are easier to get up on plan, and with less HP to sustain that planning mode. Additional surface area can also be beneficial to planning which helps explain that built-on 'swim platform extension' to the hull on the later Nordic Tugs
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Old 09-13-2013, 09:11 PM   #192
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Our NT42 is, according to most writeups on the NTs, is a semidisplacement hull. Perhaps not for the purists, I don't know.

But it gives a very decent ride at 8 knots and burns 8.7 litres/hr, both calculated (using FF's formula) and measured. When we need the extra speed for those rare and short duration moments, it will produce 12 knots at 8 gal/hr according to the flowmeter. No wallowing, just more speed when needed.

It fits our requirements perfectly, and at no time do I feel we are compromising anything.

(For NT aficionados our NT is hull #1 of the 42 model so has a 350 HP Lugger vs the more common 450+ HP Cummins engines.)
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Old 09-13-2013, 09:19 PM   #193
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(For NT aficionados our NT is hull #1 of the 42 model so has a 350 HP Lugger vs the more common 450+ HP Cummins engines.)
What exactly is a Lugger? I'm told it's a John Deer that has been blue printed and renamed.

Nordhavn swears by them and earlier Offshores (55 & 58) also had them.
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Old 09-13-2013, 09:30 PM   #194
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Originally Posted by SeaHorse II View Post
What exactly is a Lugger? I'm told it's a John Deer that has been blue printed and renamed.

Nordhavn swears by them and earlier Offshores (55 & 58) also had them.
Luggers are diesel engines marinized by Northern Lights. They use a variety of blocks from a variety of manufacturers, John Deere engines being among them. Ours is a marinized Komatsu, and from all reports is a very strong engine. I'm told that this particular engine, if looked after properly, is a 20,000 hr engine before rebuild.

Northern Lights is also recognized as a quality builder of gensets.
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Old 09-14-2013, 12:54 AM   #195
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Brian wrote;

"I'm really coming to believe that the real difference one could experience between the performance of a pure displacement vessel and a semi-displacement vessel is NOT related so much to the subtleties in bottom shapes as much as its related to the WEIGHT of the vessel."

Hard to believe you said that Brian. I totally disagree. Kayaks and canoes are all full displacement boats. I'll bet you'd not say that on BD.net. Re your wetted surface comment .. what have you been drinking?

Interestingly that statement is quite applicable to the definition of the word Trawler .... re the yacht type of course.
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Old 09-14-2013, 05:45 AM   #196
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>subtleties in bottom shapes as much as its related to the WEIGHT of the vessel.<

The bottom shape very much detirmines the ride.

An almost flat bottom will climb up at lower speeds and with less power .

A nice deep V will keep you and the vessel intact crashing into 6-8 ft waves at 30K.

For displacement efficiency a semi circle well immersed is least wetted area per ton.
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Old 09-14-2013, 09:17 AM   #197
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>subtleties in bottom shapes as much as its related to the WEIGHT of the vessel.<


For displacement efficiency a semi circle well immersed is least wetted area per ton.

Thanks, you just explained why the hull of my Krogen 42 is like a barrel.

And in a beam sea we roll (sans stabilizers) like a barrel of monkeys.

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Old 09-14-2013, 11:35 AM   #198
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Brian wrote;

"I'm really coming to believe that the real difference one could experience between the performance of a pure displacement vessel and a semi-displacement vessel is NOT related so much to the subtleties in bottom shapes as much as its related to the WEIGHT of the vessel."

Hard to believe you said that Brian. I totally disagree. Kayaks and canoes are all full displacement boats. I'll bet you'd not say that on BD.net. Re your wetted surface comment .. what have you been drinking?
Please realize that I was addressing the performance (the speed performance) of the vessel, NOT the handling characteristics. As FF pointed out a flat-bottomed shape is probably the easiest to power up onto plane and maintain it. It certainly wouldn't be a pleasant experience in many cases. But it would require the least HP. And the lighter the weight of this flat-bottom shape, the easier it would be to get it up on plane.

As far as canoes and kayaks are concerned I don't know that I follow your reasoning? They really are more of the 'slender ship' sort of hull form.

Yes, wetted surface is an important factor. Its particularly important in sailing craft where you wish to get the best performance fro the lowest amount of sail-area. So you shoot to cut down excessive wetted surface, which usually results in semi-circular hull forms,....most displacement per least wetted surface. But remember most sailboats (at least monohull ones) all operated BELOW hull speed,....full displacement vessels you might say. There are very few 'semi-displacement' sailboats, and almost no planning ones (other than surfing conditions).

So if I'm looking to make a trawler go a little faster than her 'full displacement speed', I believe the weight of the vessel is a more important factor than the extra wetted surface of a hard-chined design verses a semi-circular design. I get more flat area per boat length to push up against with the hard-chine, low-deadrise hull.
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Old 09-14-2013, 11:37 AM   #199
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My Willy fits the barrel talk well.
She looks more like a whale than a boat in this pic.
Fortunately there is, in this wine glass form a rather large "stem" (keel) in the wineglass or there'd be very little to stop or dampen the considerable roll. And also unfortunately there is a lot of wetted surface in the "stem" or keel and the rudder (in the propwash) so efficiency isn't what it could be but still not too shabby. A catamaran would obviously have the advantage here ... along w all the disadvantages cats have. The Krogen is better I think (in efficiency) and as I recall the Krogen's hull is more V shaped. Giving less wetted surface but perhaps a bit less roll dampening. I believe in the book Voyaging Under Power the Krogen is presented w a bit more efficiency. What I like most about the Krogen is her beautiful stern and above very slow speeds I think more efficient.

But re this hull form discussion I present the Willard 30 w her very pronounced whale like hull.
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Old 09-14-2013, 12:01 PM   #200
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Brian wrote;
"As far as canoes and kayaks are concerned I don't know that I follow your reasoning? They really are more of the 'slender ship' sort of hull form."
I pointed out (very appropriately I think) that even common very light boats are of the FD type. So much so no one could argue otherwise.

Wetted surface does not enter into the defining difference between FD and SD forms. In a cause and effect way FD boats are frequently heavier than SD but the weight difference is only an effect. No amount of weight (or lack of it) will be a factor in establishing hull type. Hull form (shape) is all. If you take a SD type like an IG or GB no amount of added weight will turn either into a FD boat.

"But remember most sailboats (at least monohull ones) all operated BELOW hull speed" .... And most all FD powerboats do as well. If a designer is contemplating a boat to operate usually at hull speed he would give it a SD form. Hull speed is just too inefficient for FD boats.

"So if I'm looking to make a trawler go a little faster than her 'full displacement speed', I believe the weight of the vessel is a more important factor" Indeed is is in my opinion as well Brian. Wave making resistance becomes the most important variable and hard chine soft chine has almost nothing to do with it at normal trawler speeds.

Just my opinions.
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