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Old 02-17-2019, 04:38 PM   #321
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Originally Posted by sunchaser View Post
If I saw an M30 sitting at the dock, looking at its stern would suggest an FD vessel. 43 knots out of this beauty.
http://www.mooresyachts.com
Chaser,
It probably has a stern very much like the Bartender.
Both planing hulls.
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Old 02-17-2019, 04:58 PM   #322
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Almost every dragger I ever worked on had an immersed transom, including my own but it's important to remember that these vessels are designed to take on a considerable amount of load, far more than a pleasure boat of similar size. In practice minus ice and fish that may or may not be immersed. Oh boy this thread isn't dead after all.
Not dead HaHa ....
you mean this FD hard chine vessel?
Nothing to talk about. FD
But perhaps the partially submersed transom. I can’t think of a boat w it’s transom fully above the water as being anything but FD. But there are some, quite a few actually, that have partially submerged transoms. The design element that is decisive on these boats is the angle (fore and aft) that comprises most of the bottom well aft of amidships. These boats have a quite steep angle aft that identifies them 100% as being FD. If the bottom was straight and about horizontal they could be even planing hulls.

This angle that is called Quarter Beam Buttock Line by naval architects has in the past been assigned numbers such that a shallower # than “X” indicates a SD hull and steeper than “X” = FD.
But whenever one assigns a specific number to something that number fails to hold up under all circumstances. But the QBBL is probably the best decisive road to separting FD from SD.

One other way to separate FD/SD is to evaluate the area and it’s percentage of boat submerged to the deepest point and how much area is submerged of the transom. But I know of no hard numbers to attach to that.
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Old 02-17-2019, 05:12 PM   #323
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simple no flat sections aft. P and SD have flat sections aft.
I guess the FD prawn trawler guys never got your memo

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Old 02-17-2019, 05:27 PM   #324
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I thought I tried to be a bit ambiguous with that reply by making the loading observation.
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Old 02-17-2019, 08:11 PM   #325
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I guess the FD prawn trawler guys never got your memo

A SD hull that lacks the power to get on top of its bow wave of still a SD hull. With enough power, this boat could plane, because the flat sections aft will provide the lift to keep the stern from going under. It's a lot cheaper to build this shape hull, even if you never expect or want to go faster than displacement speeds.
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Old 02-17-2019, 08:17 PM   #326
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Why is it cheaper Delfin?
To build this shape of hull?
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Old 02-17-2019, 08:17 PM   #327
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A SD hull that lacks the power to get on top of its bow wave of still a SD hull. With enough power, this boat could plane, because the flat sections aft will provide the lift to keep the stern from going under. It's a lot cheaper to build this shape hull, even if you never expect or want to go faster than displacement speeds.
yeah
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Old 02-17-2019, 08:41 PM   #328
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Why is it cheaper Delfin?
To build this shape of hull?
Flat steel not requiring shaping, less steel.
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Old 02-17-2019, 10:03 PM   #329
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Delfin,
Don’t think bending is a problem on boats like this. I could be wrong as I’m no expert on steel building but I’ve seen a few come together w not very sophisticated equipment that had bends about the same as the bottom on these boats. It’s a bigger boat and takes more material but that shouldn’t be that much either. Bigger that a flatter boat bottom w/o such a big “underbelly”.
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Old 02-17-2019, 10:38 PM   #330
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Delfin,
Don’t think bending is a problem on boats like this. I could be wrong as I’m no expert on steel building but I’ve seen a few come together w not very sophisticated equipment that had bends about the same as the bottom on these boats. It’s a bigger boat and takes more material but that shouldn’t be that much either. Bigger that a flatter boat bottom w/o such a big “underbelly”.
All you need is an English wheel and a couple of guys that know how to use it. The vessel in question may have been intended for a significant load in shallow water such as carrying a load of pots, but curved steel would be stronger for the weight if the design would allow it. A large "underbelly" gives an offshore boat better carrying and seakeeping properties.
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Old 02-18-2019, 12:00 AM   #331
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Delfin,
Don’t think bending is a problem on boats like this. I could be wrong as I’m no expert on steel building but I’ve seen a few come together w not very sophisticated equipment that had bends about the same as the bottom on these boats. It’s a bigger boat and takes more material but that shouldn’t be that much either. Bigger that a flatter boat bottom w/o such a big “underbelly”.
Which stern do you think would be easier and cheaper to build - the above or the below?
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Old 02-18-2019, 01:28 AM   #332
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The Coot has a similar crescent-shaped bottom.
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Old 02-18-2019, 03:57 AM   #333
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Delfin,
My opinion would be the above by far. Lots of compound cirves on the “below” hull. The “above” hull is much more like a simple plywood rowing skiff but lots and lots of rocker. The panels or plates curve in only one direction like flat panels tend/want to do. The “below” boat has much more developed and complicated curved in many planes and in more directions. Rocker being the inverted curve of the bottom running fore and aft. Like the “rocker” on a rocking chair. Kinda like Marks boat on steroids. Marks boat has the same kind of rocker but to a far less degree. The “above” fishing boats are huge and very heavy. Marks QBBL would be much more shallow or put another way a lower angle than the big fishing boats. But the same type of hull basically. Most trawlers have fairly shallow QBB Lines. The IG’s for example have such a shallow angle it’s more like a planing hull. But none the less a SD hull due to her fairly heavy weight, big keel, and prop and gears suited more to heavy craft. Other trawlers like the GB are inbetween. More like a FD hull but still a SD.
All of this that I say is an opinion. And if you heard it from a NA it would likely be far more complicated.
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Old 02-18-2019, 04:05 AM   #334
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Mark 99.9 % of this question pretains to the stern, not the bow.
And your Coot has the simple plywood shapes of the “above” boats.
But your Coot has the “bone in her teeth” bow wave of a heavy boat having a fairly wide angle of entry. Unlike a “slicer” as you say.
But your hull is very much related to the “above” very heavy fishing boat hulls.
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Old 02-18-2019, 07:31 AM   #335
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Old 02-18-2019, 10:04 AM   #336
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George

Hull extensions or running gear forward can prove interesting.

Often with the rudder, keel and prop well forward from the stern, a vessel wanders a lot. The after market vertical wavelets on the stern sides are possibly an attempt to force the vessel to stay in a straight line.
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Old 02-18-2019, 10:12 AM   #337
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caltex I assume you’re looking for comment.

Looks like someone extended a FD hull.
This is a “head swim” example. Has a shallow buttock line angle suggesting a SD hull. She has more than most FD hulls have in submerged transom. Knee jerk reaction is “wow she looks really fast”. But that would assume a FD. Another thing I can say is that she looks like a boat designed to break the rules.

OK,
I’ll pigeon hole this boat as SD.
Why?
Buttock line is very long and almost flat. Transom submerged is significant for a FD and not much for a SD ... not a decisive element w this boat.
This boat is not at all like a FD ... well it is in several ways. She’s oviously built for speed and FD boats are not. She’s SD

But something I should bring into the discussion .... she’s VERY round and would be almost (if not dangerously so) scary in roll and perhaps lacks directional stability. See those little finns at the corners of the stern? I’m think’in they are add on’s because she may not come back out of a turn ... or at least be ruluctant to. Could even be a “bad design” but I may be over thinking it.

Can honestly say I’ve never seen a boat even closly resembling this one. I wanted badly to call her on the fence and so close to uncallable but the QBBL dosn’t lie. She’s SD

sunhaser I’m glad you agree to even some extent.
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Old 02-18-2019, 12:42 PM   #338
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The last pictured boat is a Nordhavn 62.
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Old 02-18-2019, 01:16 PM   #339
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Old 02-18-2019, 01:41 PM   #340
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The little aft outboard fins are aftermarket, but otherwise it is a stock hull.

https://photos.app.goo.gl/zBX7pigkggSEHBr78

And to Sunchaser's comment yes, typically boats with the prop and rudders well forward have issues in following seas. You see that on some old Hatteras motoryachts where an aftermarket cockpit was put on and the props and rudders left in their original position.
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