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Old 09-14-2013, 12:18 PM   #201
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...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.
Please define for me what makes and IG or a GB a semi-displacement boat??
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Old 09-14-2013, 01:24 PM   #202
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Brian wrote;....
"But remember most sailboats (at least monohull ones) all operated BELOW hull speed" .... .
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.

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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.....
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.
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Old 09-14-2013, 01:53 PM   #203
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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.


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Old 09-14-2013, 04:42 PM   #204
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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).

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Old 09-14-2013, 05:35 PM   #205
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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.


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Old 09-14-2013, 05:57 PM   #206
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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.
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Old 09-14-2013, 06:30 PM   #207
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Old 09-14-2013, 07:59 PM   #208
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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 ...
Flat-bottomed but not for speed:



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Old 09-14-2013, 08:57 PM   #209
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Please define for me what makes and IG or a GB a semi-displacement boat??
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.
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Old 09-14-2013, 11:14 PM   #210
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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.
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Old 09-15-2013, 09:39 AM   #211
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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.
I got the impression that QBBL is not a key factor in Dixon's approach to semi-displacement...
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Old 09-15-2013, 10:00 AM   #212
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Skidgear,
Without seeing more I assume Dixon is marketing his hull like the others calling it "go fast go slow" or "fast trawler". In this age of engineering excellence and the everyday scientific breakthrough claiming whatever can snag a lot of followers. Dixon's boat looks really good but I don't see anything new.
As to QBBL .. I think it's a very important part of every design that's not a planing type. Whether he measured the numerical number in degrees and compared it to some standard or drew many many lines until he was happy with one I don't know .. of course. Whether he got extremely numerical about it or applied his extensive experience and good eye could be his secret but I like what he's done above the WL.
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Old 09-15-2013, 11:49 AM   #213
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Skidgear,
Without seeing more I assume Dixon is marketing his hull like the others calling it "go fast go slow" or "fast trawler". In this age of engineering excellence and the everyday scientific breakthrough claiming whatever can snag a lot of followers. Dixon's boat looks really good but I don't see anything new.
As to QBBL .. I think it's a very important part of every design that's not a planing type. Whether he measured the numerical number in degrees and compared it to some standard or drew many many lines until he was happy with one I don't know .. of course. Whether he got extremely numerical about it or applied his extensive experience and good eye could be his secret but I like what he's done above the WL.

Dixon isn't marketing anything...Azimut asked him to design a hull for their Magellano trans-hump/improved efficiency line of boats. Agreed that there's nothing new in the individual design elements, but the combination and integration of those design elements is an interesting approach to the semi-planing compromise. At least it's not a jaunty "trawler" deckhouse on a "fast trawler" planing hull as per the new Grand Banks or the various "tugs".
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Old 09-15-2013, 03:20 PM   #214
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Dixon isn't marketing anything...Azimut asked him to design a hull for their Magellano trans-hump/improved efficiency line of boats. Agreed that there's nothing new in the individual design elements, but the combination and integration of those design elements is an interesting approach to the semi-planing compromise. At least it's not a jaunty "trawler" deckhouse on a "fast trawler" planing hull as per the new Grand Banks or the various "tugs".
The combination of round sections forward with hard chine vee sections aft is nothing new at all in the semi-displacement world. Builders really like having unique key-words to define the product, thus the "Dual-mode" moniker.

At least this builder has acknowledged the reality of owner's operating their boats at less than 20 knots. 30 years ago when Bob Harris was designing the Jefferson line (Taiwan Bertram copy) we knew no one would actually use them at 25 knots. But they had to have deep-vee hulls and huge engines to sell against the competition. People actually use them at less than 15 knots, because it's more comfortable, quieter, and uses tons less fuel.
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Old 09-15-2013, 04:23 PM   #215
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On the link Skidgear provided I could only find some hyped marketing talk describing the hull design.

I'm keen on 12 knot boats so anything promising to be new to that end I'm right on it. And since there's not much of anything new I look at older designs a lot. Most of the newer stuff that's rather efficient is light and mostly flattish on the bottom. I'm still look'in at Atkin's Tang. I think it's a better design than Handy Billy .. that I also like.
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Old 09-15-2013, 06:30 PM   #216
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On the link Skidgear provided I could only find some hyped marketing talk describing the hull design.
After the first paragraph of gobbledygook there's a line "How Does Dual Mode Hull Work, Click Here" That opens a new window with a picture and some sections of the hull. It's tiny and hard to see but gives the basic idea. Also click on the resistance graph and note this boat crosses their "standard vee-bottom"(whatever that is!) curve at 21 knots......
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Old 09-15-2013, 07:54 PM   #217
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...........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.
.

They normally use the term "Theoretical Hull Speed" rather than just "hull Speed".
Anyway. it is not a magic number that stops at the edge of the world. It can be exceeded to a certain point and is exceeded quite regularly. Not by much, but it is done. What it really tells us more than anything else is that as you approach it, it will require a disproportionate amount of additional power to increase speed only a very small amount. Like maybe 300% more fuel to gain 20% more speed. Those figures are probably not accurate, but you get the idea.
At some point past the theoretical hull speed, the bow actually dips down, then pops up as the boat starts to 'porpoise'. With enough power, it will go faster and at some point at which the boat is not designed to go, it will probably end up committing suicide.

For a trawler with a displacement hull to approach or slightly exceed Theoretical Hull Speed it would be costly. For a sailboat, the wind is free.


The above info is my understanding of the whole thing. That don't necessarily make it right.
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Old 09-15-2013, 07:57 PM   #218
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Thank you TAD but it seems I have a problem w my "flash player".

Your light description of the hull brings up images of many that I've seen in the past. Not much advantage. A bit more initial stability and probably a little flatter turns. And a bit more wetted surface. All of it being basically fly stuff.

Thank You
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Old 09-15-2013, 07:59 PM   #219
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When many people say the vessel is a semi displacement is it really? ..... many people forget about the buttcock angle of the semi displacement hull. And here is what I mean by that..
Semi Displacement. (Don`t ask, I`m not going to justify it.)
hfoster, You gave comparisons but never really defined "buttcock angle". Googling it may swamp my anti virus system, can you explain?
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Old 09-15-2013, 08:12 PM   #220
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The real choice between a SD hull and a full displacement hull is really a choice between speed vs. economy. Generally speaking, you can't have it both ways.

The major advantage of a FD hull is fuel economy and that is a real biggie.
The disadvantage of FD hulls is not just reduced speed but greatly reduced interior usable space.

Most of the old wooden shrimp boats in this area have what appears to be FD hulls. Not so much in the newer steel hulls. Since I know nothing about shrimping, I will assume the newer hulls are sacrificing fuel economy for larger cargo holds and Ice-making capability.
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