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Old 12-30-2014, 11:10 AM   #41
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Quote:
Originally Posted by eyschulman View Post
Scary if we try to apply what is the best form for long distance blue water particularly in the sub 50 foot size I would agree that it is a FD boat. But I would not put forth a motor boat at all. Sail would be the answer overwhelmingly so. What I am pointing out is that SD can do it. Same as motor boats can do it. But probably a much larger group of blue water cruisers are in sail boats. You think its a FD power boat they think its a sail boat and personally I don't ever want to do blue water in any boat, but that's part of what makes life interesting.
Eye,
And sailboats are FD boats all w stabilizers as in masts and sails.
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Old 12-30-2014, 11:48 AM   #42
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Originally Posted by skidgear View Post
I was cruising Yachtworld and noticed a Fu Hwa 38 double cabin with a listed maximum speed of 14 mph....semi-displacement territory. Did they make a variety of hull designs? Just curious...
That is a twin turbo engine boat and they are talking MPH and not knots. Our boat is a single and I suspect 9 or 10 knots is max. We normally cruise at 7 knots and 1650 RPMs on our naturally aspirated 135Hp Perkins 6-354. Fu Hwa did make a number of different models but I believe they all used the same FD hull for the 38'. The claim of 14MPH seems questionable to me too.
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Old 12-30-2014, 11:57 AM   #43
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Do you have a photo of your hull below the water line?
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Old 12-30-2014, 11:59 AM   #44
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Originally Posted by psneeld View Post
Didn't RickB post not only links to several good articles on on the subject but also rattled off a bunch of larger yachts that were semi displacement that traveled the world?
Yup, most are in fact.
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Old 12-30-2014, 12:00 PM   #45
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Do you have a photo of your hull below the water line?
Oh oh, the purist police are out
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Old 12-30-2014, 12:26 PM   #46
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Do you have a photo of your hull below the water line?
Thought I had one with the survey but it doesn't really show the hull below the waterline very well as it is the whole boat on the lift. From the picture the hull looks just like the one that claims 14MPH on YW. Our boat is like the one at Myrtle Beach.

Kind of gets me wondering at what point there is demarcation between FD and SD?
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Old 12-30-2014, 12:28 PM   #47
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IMO it doesn't matter what so and so drove to Antarctica if your cruising will be coastal.


People seem to get all caught up in the nuances of boat design that they will never use.
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Old 12-30-2014, 12:31 PM   #48
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Do you have a photo of your hull below the water line?
Sounds like the old Albin lineup.....which Albin describes as SD...but my wake becomes pretty obnoxious over about 7.5 knots and my boats belly is pretty rounded for an efficient SD.

I have no problem calling her an "inefficient FD" too....
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Old 12-30-2014, 12:36 PM   #49
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Is this hull "FD"?

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Old 12-30-2014, 12:43 PM   #50
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Lots of tred on the water. Naval architects can get sticky about something they call the buttock angle(I think). It has to do with the way the water flows by the submerged stern. You would have to submit good side and aft pictures of rear of boat and maybe some angle measurements for a verdict. Having a keel and deep forefoot may not exclude SD lobster boats have those things.
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Old 12-30-2014, 12:48 PM   #51
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Originally Posted by cardude01 View Post
Is this hull "FD"?
It is a sailboat hull so it is FD.
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Old 12-30-2014, 12:57 PM   #52
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Ah yes....The old buttock angle...

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Old 12-30-2014, 12:58 PM   #53
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Quote:
Originally Posted by Donsan View Post
It is a sailboat hull so it is FD.
Just because it is a sailboat hull doesn't make it FD...plenty of sailboat hulls plane.

Hard to see in those pics...I would dare to make that snap judgement.


A shot from directly astern would help, even then I detest making the call from just pictures......
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Old 12-30-2014, 01:07 PM   #54
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In defense of SD trawlers

It has an extended swim platform that flattens out the stern quite a bit. Kind of hard to see in my pictures.
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Old 12-30-2014, 01:18 PM   #55
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Found some better pics.

Also just realized, looking at these pics, that the set of used weaver davits I bought off Craigalist might not work with my swim platform. I didn't realize it had that reverse rake on it.
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Old 12-30-2014, 02:43 PM   #56
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Quote:
Originally Posted by eyschulman View Post
Naval architects can get sticky about something they call the buttock angle(I think). It has to do with the way the water flows by the submerged stern. You would have to submit good side and aft pictures of rear of boat and maybe some angle measurements for a verdict. Having a keel and deep forefoot may not exclude SD lobster boats have those things.
Just one feature(buttock angle) won't do it. Yes, as a very wide generalization buttock angle (rise) might be flatter on a semi-displacement hull than on a full displacement hull. But, I am a Naval Architect and I rarely look at buttock angle. I first look at prismatic coefficient, I look at displacement/length ratio, I look at intended operating speed(more precisely speed/length ratio), longitudinal weight distribution, HP to weight, and finally volume distribution (sort of back to CP).

I'm sure I've posted this before but for the record. Naval Architects define hull type; planing, displacement, or semi-displacement, by operating speed/length. We use something called a Froude number.

Fn = V/ sqrt Lg

V is the vessel speed (metres/second)
L is submerged length (metres)
g is acceleration due to gravity (9.8 m/s^2)

Displacement hulls operate at Fn less than 0.4
Planing hulls operate at Fn > 1.0-1.2
Semi-displacement vessels operate at Fn between about 0.4-0.5 and 1.0-1.2

To convert knots to m/s multiply by 1.9425

So a 30' waterline boat traveling at 7 knots is operating at Fn = 0.45

A 35' waterline at 14 knots is Fn = 27.195 / (sqrt 10.66 * 96.04)
= 27.195 / 31.99
= 0.84
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Old 12-30-2014, 02:58 PM   #57
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Tad:
What is meant by operates? Can some boats exceed those numbers? Certainly all can go slower.
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Old 12-30-2014, 02:58 PM   #58
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I'm the one that posted the buttock line ""angle" definition. Using the word definition rather loosely.

If one wants to think of a very easy cutoff point so you can glance at a boat and make the call (for all practical purposes) just observing how much transom of a typical boat is below the WL is enough. But it's not definite. Fronde numbers, quarter beam buttock line numbers ect ect will get you a little closer to black and white but it's quite safe to say all boats w the transom above the WL are FD.

But from experience I know 999% of TF skippers aren't interested in specifics so I offer the WL and transom method for impure simplicity.

From the pic that cardude01 posted I'd call that boat to be w a FD hull. There is a small bit of the transom below the WL but comparing the size of the hull to the submerged transom it's fly stuff. Not worth considering. But if that much transom was submerged on a 16' aluminum skiff it would classify it as SD and if the skiff was really really light a planing hull.
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Old 12-30-2014, 03:03 PM   #59
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Tad: What shape hull rolls least?
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Old 12-30-2014, 03:04 PM   #60
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Quote:
Originally Posted by Tad Roberts View Post
Just one feature(buttock angle) won't do it. Yes, as a very wide generalization buttock angle (rise) might be flatter on a semi-displacement hull than on a full displacement hull. But, I am a Naval Architect and I rarely look at buttock angle. I first look at prismatic coefficient, I look at displacement/length ratio, I look at intended operating speed(more precisely speed/length ratio), longitudinal weight distribution, HP to weight, and finally volume distribution (sort of back to CP).

I'm sure I've posted this before but for the record. Naval Architects define hull type; planing, displacement, or semi-displacement, by operating speed/length. We use something called a Froude number.

Fn = V/ sqrt Lg

V is the vessel speed (metres/second)
L is submerged length (metres)
g is acceleration due to gravity (9.8 m/s^2)

Displacement hulls operate at Fn less than 0.4
Planing hulls operate at Fn > 1.0-1.2
Semi-displacement vessels operate at Fn between about 0.4-0.5 and 1.0-1.2

To convert knots to m/s multiply by 1.9425

So a 30' waterline boat traveling at 7 knots is operating at Fn = 0.45

A 35' waterline at 14 knots is Fn = 27.195 / (sqrt 10.66 * 96.04)
= 27.195 / 31.99
= 0.84
Thank you for a reasonable explanation...

For years some of us have been trying to poo poo the one pic is worth a thousand NAs.... philosophy......it just doesn't seem to take root though...
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