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Old 08-10-2016, 05:35 PM   #21
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Jay wrote "leans out in a turn"

Wow, If a boat did that on sea trial I'd be say'in lets go back.

Baker,
Re #13 you're kinda stuck in aviation mode. I'm sitting here at Starbucks wondering if I want to talk about lift. I think not as this P v/s SD conversation has so little to do w it. Any boat has lift .. even my FD Willard. The bow rises at 7 knots and would do so w a planing, SD or FD hull. The stern pulling the aft end down w something like suction and pressure fwd caused by the mass of the hull being partly pushed up on the water that is reluctant to get out of the way. We can call it lift if you like.

The dynamics above pretain mostly to planing hulls. And rocker on the bottom (curved and convex) on FD and SD hulls suck or pull the bottom down so full planing speeds can't be achieved. Any boat w the transom out of the water at rest is in this catergory. And if the boat is emersed in the water the same aft (at the transon) as it is amidships it's a planing hull. It will have a straight run aft that prevents squating and hence allows planing.

Baker I can only guess at the hull form of the pilot. Never seen one out of the water nor have I seen good pics. But I have seen the older 34 and most of what I've said above pretains to that boat. There was a pretty red Pilot on my float about a year ago but all I remember is that it was pretty and the owner used the bow thruster a lot.

I agree w your post #16 and your coments mesh well w what I guessed about the hull. What part of the shape or what apendages promote you to say it's SD? I think it has roundish chines and some sort of a keel larger than most planing hulls have. Lobster boats have soft chines and significant keels. Most call them SD but I call them planing hulls. Again a straight run aft w zero degrees QBBL. Planing. Would plane better w hard chines and most or all of the keel removed. Lobster boats can go very fast .. over 30 knots I believe and on the right coast they race them .. 700hp and more.

But the old M34 is more of a planing hull than a lobster boat as it lacks the soft chine. Only the keel and big heavy diesel engine detract from it's being 100% planing. That brings me to another element of definition here that is most often overlooked. Grey area. There is no black and white. My Willard is definitely a FD boat. But there are features about it that make it more or less a FD hull compared to other boats. There are many boats that are more of a FD boat than my Willard. Some are trawlers. Many are sailboats.

But it remains that the shape of the stern mostly decides the dynamic hull type. No other feature tells more about it than the shape of the stern. A NT (a 32 at least) has enough rocker to put it firmly inbetween planing and displacement. A SD hull. So is an older GB.
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Old 08-10-2016, 08:39 PM   #22
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From Soundings Magazine

A semiplaning hull such as this is supported by both buoyancy and dynamic lift.

Semiplaning
That brings us to this month’s topic, although the preamble was necessary to fully appreciate what the semiplaning hull can do. Any design, whether displacement or planing or in between, has capabilities and limitations. What’s helpful is to understand what they are so you can make an informed choice about which combination of compromises produces the boat that best meets your needs.
A semiplaning hull is simply one that operates in the gray region between displacement and full planing, a speed range at which the hull is supported partially by buoyancy and partially by dynamic lift. It is further made gray by the observation that a 40-foot hull at 20 knots is running at an S/L of more than 3, so it is, in fact, fully planing. But 20 knots is close to the top of its speed range, not the bottom, and that makes a very big difference in the boat’s performance, as we will see.
In the simplest terms, the semiplaning hull bears little resemblance to the displacement hull, but quite a lot of resemblance to the planing hull. As such, let’s compare the semiplaning hull with its full-planing counterpart.

Bilges and chines
For a semiplaning boat that routinely cruises between 12 and 16 knots, either a round bilge or hard chine hull will do the job. At 16 knots and higher, however, the hard chine is the way to go. That’s because hard chines start producing significant flow separation — deflecting water flow and spray away from the hull surface — at these higher speeds.

http://www.soundingsonline.com/boat-...eet-many-needs
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Old 08-10-2016, 09:04 PM   #23
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From Soundings Magazine,
"In the simplest terms, the semiplaning hull bears little resemblance to the displacement hull, but quite a lot of resemblance to the planing hull. As such, let’s compare the semiplaning hull with its full-planing counterpart."

I like that.
Most SD boats do resemble planing hulls more than FD. But I think it's just because they are more popular. Probably because most people would rather go quite a bit faster than FD .. not just a little bit. But the difference is more profound in reality than as expressed above. I prefer the just a little bit faster boats and they are almost non-existant. SD boats at the upper end are everywhere. Look right here on TF.

Of course now that boaters are so much more frugal w fuel dollars we see a lot more SD boats running just a little above displacement speeds or even at displacement speed.
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Old 08-11-2016, 12:34 AM   #24
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If you don't want to talk about lift as it relates to this discussion, then you are basically pleading ignorance....and I mean that in the most respectful way. You are literally ignoring the most important factor....on purpose. Onward....no changing your mind.
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Old 08-11-2016, 06:31 AM   #25
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What is missed time after time, there is no "line" to cross from one hull design to another.

Emphasizing only "parts" of boat design fails to comprehend the whole.

Like this article supported yet again, sometimes only very subtle differences are in the design, for reasons other than powering the vessel.
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Old 08-11-2016, 07:40 AM   #26
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Fine entry forward, flat bottom at transom w shallow tunnel, hard chines

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Old 08-11-2016, 11:15 AM   #27
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Quote:
Jay wrote "leans out in a turn"

Wow, If a boat did that on sea trial I'd be say'in lets go back.
It's the keel that makes it lean out. It's not like it's going to tip over.
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Old 08-11-2016, 01:20 PM   #28
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Quote:
Originally Posted by jleonard View Post
It's the keel that makes it lean out. It's not like it's going to tip over.
Yep dang all those boars that lean out on turns...unseaworthy POSs....

Funny, come to think of it, I can't believe how many boats I have run or been on that do the same.........well st least some....
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Old 08-11-2016, 01:33 PM   #29
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Jay,
I designed a 28' boat, built it and used it for awhile w/o a cabin. Kinda like sailboaters do when building a sailboat. They use it as a powerboat before they can afford the sails and rigging. And they are amazed at the different roll characteristics. My boat turned absolutely flat .. no bank w/o the cabin. It was kind of a tri. but yet a monohull. Or a cathedral hull. Anyway when I put the cabin on she banked moderately .. more like a normal boat.

I can only surmise that it was a CG function. Changed nothing, power or bottom wise .. just added the cabin. And she banked the same amount with or without wind. Only thing I can think of is CG.

I know boats w soft chines bank more than hard chines as the water exiting the hull out from under the inbd side sucked the inbd side down. But I made no change to the hull at all.


Keysdisease,
Thanks for the pics. She looks like a typical V type hull. Looks long and narrow but I'm sure it's the photo length of the camera lens. Not flat at the stern like the 34 but nearly so. I'm guessing the trim tabs are down in the lower photo.
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