Your hull type

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".

Nordic Tug

From the Nordic Tug website,

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

Click to expand...

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?



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.

Semi displacement

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

brian eiland said:

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

Click to expand...

TY Brian! Cool video - Real nice boat! However, I need FB.

brian eiland said:

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?


View 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.

Click to expand...

Please don't be another poster that tries to guess at hull performance from still picture...it says so much about an individual.....

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.

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.

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 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?

WEIGHT of vessel is key

FF said:

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.

Click to expand...

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,

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.

Click to expand...

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

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.)

Conrad said:

(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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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.

SeaHorse II said:

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.

Click to expand...

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.

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.

>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.

FF said:

>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.

Click to expand...

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.

Marty

manyboats said:

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?

Click to expand...

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.

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.

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.

manyboats said:

...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.

Click to expand...

Please define for me what makes and IG or a GB a semi-displacement boat??

manyboats said:

Brian wrote;....
"But remember most sailboats (at least monohull ones) all operated BELOW hull speed" .... .

Click to expand...

Don't know where you gat that from. On my sailboat, wind and direction determine my speed. Some if not most summer days I operate below hull speed. Fall, winter and spring, I operate above hull speed a fair amount of time by the order of a kt. to a kt and a half and sometimes 2 kts. A full displacement hull can operate above hull speed, not by much, but it can.
It can be noted that the efficiency will drop dramatically in that it will cost a heck of a lot more power to this. While wind is still free, the cost is not a factor. In a trawler with full displacement hull, going above hull speed can double or even triple the fuel requirements just to gain a very small amount of speed. Eventually, it will reach a point where it will not go any faster. If you had the power to make it faster, it would just porpoise and not gain any greater speed. It would dig in, rise out and bob.

manyboats said:

Brian wrote;.... 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.....

Click to expand...

I don't know how you are describing 'efficiency'. To me 'efficiency' is a factor of how much speed you are getting for a given amount of fuel. Technically, your above statement is true. From a practical point of view, most displacement hulls reach a great efficiency just a hair below theoretical hull speed. So if a designer wants to go a particular speed, he can design the hull just a tad longer so as to bring his theoretical hull speed up just a tad faster than his intended speed.

Theoretical hull speeds don't really work for SD or planning hulls. The hull length and angle of attack change with speed.

For the poll, I have a semi.

With that out of the way let me add this to the semi displacement vs Displacement debate.

When many people say the vessel is a semi displacement is it really? The answer is yes and no. The reason being, is that many people forget about the buttcock angle of the semi displacement hull. And here is what I mean by that.

I'll use this as a base line. 2 vessels. 1 Full displacement 1 semi displacement. Both are 72' 4" at the water line. both weight 160,000 lbs.

The SL ratio on the Full displacement is 1.62 which would give a max hull speed of 13.80 knots. Power needed at the prop too reach that speed would be 961.78 HP.

The SL Ratio on the Semi Diplacement with a 2 degree Buttcock angle is 2.5, which would give the vessel a max hull speed of 21.27 knots. Power needed at the prop would be 1,359.72 HP

Now here is where it gets fun.

Same semi displacement vessel but with a different buttcock angle of 4 degrees.

SL Ratio 2.1 which gives the vessel a max hull speed of 17.87 knots. Power needed at the prop is now 956.78 HP

Same semi displacement vessel with a buttcock angle of 6 degrees the

SL ratios is now 1.7 which gives the vessel a max hull speed of 14.46 Knots. Power needed at the prop is now 608.21 HP

Now just for the fun of it we change the buttcock angle to 7 degrees on our semi displacement vessel.

The SL ratio is now 1.5 which will gives the vessel a max hull speed of 12.76 knots. Power needed at the prop too reach the max speed. 459.61 HP

Now our Semi displacement trawler is now slower then the Full displacement vessel trawler and the SL ratios is lower. Now my question is? Is it a Semi displacement or a full displacement Vessel?

I would say both. That is the beauty of a semi displacement hull, it can play in 3 different worlds all rolled up in one hull.

Just something to think about.


Happy cruising to all

H. Foster

Here's a link to the Bill Dixon designed "Dual Mode Hull" on the Azimut Magellano series boats. Arguably the latest in semi-displacement design thinking. You'll have to click on the "how Dual Mode Keel works" link when the page pops up, as I couldn't get it to go directly to the interactive page. Click on the circles when the interactive page appears. There's also a boat test of the semi-planning/displacement 43 in the August issue of Power and Motoryacht (accessible on-line).

Azimut Magellano Yachts | Azimut Yachts Collection | Home

A real poll for me would be how many cruisers with semi-displacement hulls generally operate their boat below or at hull speed to save on fuel.

Bay Pelican is full displacement and we generally cruise at 80% of hull speed to save fuel. For us the savings goes a long way toward paying for dinner.


Marty

me too....but I'm NOT of the opinion most trawler hulls are semi-displacement...they might have some lift but unless specifically designed to cruise at 12 or above....then calling them semi-displacement to me is a real stretch...

So yes I go slow (6.3 kts seems to be good for me) to pay for dinner out when I want on my annual 2-3000 mile cruise.

Full Displacement

FF said:

>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 ...

Click to expand...

Flat-bottomed but not for speed:



WWII LCS(L)-102 "Mighty Midget" gunboat under restoration (more guns per ton than any other ship built for the USN)

brian eiland said:

Please define for me what makes and IG or a GB a semi-displacement boat??

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The low QBBL angle and submerged transom.
And the almost straight run from amidships aft.
The observation that one can achieve 1.5 X hull speed.

FD craft don't have these features or capabilities.

Marty I actually cruise a bit higher than that at 87% of hull speed. Had'nt looked at it as a percentage of hull speed. I get there w just a little bit less than 50% of engine load.

Skid I looked and could not find out much about Bill Dixon's design. No drawings or pics of the after body of the hull. So I don't know what it is but it sounds like it's too good to be true and .....................
I did notice that he's stretched the real WLL and it's full enough in the bow to have even a longer "effective" WLL. In the picture it looks like his hydrodynamic "wheelbase" is long indeed. I think I'd like the design if I could see it.

H Foster wrote;

"For the poll, I have a semi.

With that out of the way let me add this to the semi displacement vs Displacement debate."

Not a debate I hope. But there are things about it to debate.

In your text you're talk'in a religion I'm unfamiliar with. How you can compute how fast (exactly) a boat will go w "X" amount of power and a buttock angle of "Y" ignoring all the other many variables is beyond me.
But NO ... an underpowered SD boat that can't achieve hull speed is still a SD boat. As defined by her hull form. And putting a sizable keel on a planing hull does not change her type. just lowers her performance with increased wetted surface.
There was a discussion on this matter on BoatDesign.net and someone came up w a QBBL angle that divided the two types. Above that angle was FD and below was SD. I don't think one can do that. Too many variables. But I'll bet it comes close. Perhaps very close.
I post the pic of the NT26 again as an example. The QBBL is not flat nor is it 0 degrees. That would indicate that it could be a FD hull but it's clear that the line is almost straight and there's very little curve or rocker at all. So the QBBL is closer to a planing hull than any other hull form. So without further theories or numbers applied it must be a SD hull.

So to your question .. yes. Yes because your example changed into a FD hull w the high QBBL angle but no it can't be both. However some boats are so right in the middle between two types that anybody's call would be a guess. Some of the DeFevers (48-49'?) are in this uncommon grey area. I think of them as SD and perhaps underpowered but I think most owners think of them as FD. This is a grey area and my opinion is just a guess without more information but even w further information it could still be a guess and 12 naval architects could be evenly split on the call. But the NT 26 is (no doubt in my mind) a SD type.

Tony B.
I don't know much about sailboats. I'm very surprised to hear a normal sailboat can sail regularly above hull speed. News to me. Thanks.