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Oct 31, 2007
Vessel Name
Vessel Make
Willard Nomad 30'
Thats where the planing surface is supposed to be.What part of your boat is still in the water at 25knts? A full planing hull has a flat or at least a straight run aft with sharp hard chines. It also has fairly low weight and high power. A semi displacement and a semi planing hull ( SD @ SP ) is harder to classify. The word " partly " is used to define " semi ". So an SD hull is like a planing hull but partly not. An SP hull is like a disp. hull but partly not. So Carey's lobster hull is a SD hull. All it would need to become a planing hull is to have hard chines aft. An SP hull is one that partly planes. It is slower than an SD hull and faster than an FD ( full displ hull ). A FD hull is usually defined as a hull that operates at or below 1.34 x sq. root WLL.If this is a speed / length ratio of one then FD hulls operate below an SL of 1.4T. Here are three ways to identify a FD hull. It will not be possible to drive it to a SLR of 2 ( about 15 knts for a 30' boat ). Also at about 75% power it's bow will not have begun to rise. This happens at about a SLR of about 1. With a good working load 100% of the transom will be out of the water.. as in not submerged.The geryest area is between SD and SP hulls. Some boats that I think are in this zone are: Eagle 32 @ 40, Nordhaven 35 @ 47, some Devlins and the Selene Archer 36. The GBs and NTs are in the narrow grey area between FP andSD hulls.
Eric Henning
Thorne Bay AK

Probably largely semantics as you say, but from what I have read from boat designers on the topic, semi-displacement and semi-planing mean exactly the same thing in terms of what the hull does, only semi-planing is the more hydrodynamically correct term.

Lobsterboats need more than just hard chines aft to be able to plane completely. Actually, they have very tightly radiused chines aft so they are "almost sharp" as it is. To fully plane, they need to lose the keel, obviously, but the big requirement would be to extend the lifting surfaces much farther forward than they are now. They would need a hull more like a scaled down WWII PT boat in order to be able to achieve a full plane. The rounded front half of the typical lobsterboat hull, plus the weight, plus the normal power on these boats, prevents them from achieving a full plane (assuming the keel was gone already).

But then you'd have a very "poundy" boat, so the forward part of the hull is as it should be for this particular application.

But in terms of how his hull acts, Carey's boat is semi-planing, as is our GB. He just has way more power and considerably less weight so he can take more advantage of his hull shape than we can.

This is about as simple as I can get and and still be definitive.
Looks like you should know a great deal about design. What identifiable feature of a planing hull distinguishes it as a planing hull?
What part of my post didn't make sense to you?

Eric Henning
Thorne Bay AK

Are you following me around? Good. I'll always have somone to talk to. Your'e very dependable.
In calm weather who cares what the bow of Carey's boat looks like's completly out of the water! Most lobster boats are flat on the bottom like a canoe with the same rounded chines. A planing hull needs hard chines to break the out board flowing water free so it dos'nt suck the hull, down. Water tends to follow a curved surface. Your GB has the wide flat aft section and the hard chines. The keel is fine as long as one has the power to drag it along. The GB is a planing hull with way too much weight and way too little power. You just won't admit it. Your'e right about stability though. But if you're high on the face of a wave broadside, hard chine will get you capsized where the rounded chines will suck the high chine down, the other chine will ride high allowing your boat to slide down the face of the wave instead of tripping on on the sharp edge of t5he hard chine.
Good night Marin

Eric Henning
Thorne Bay AK
Actually, the front of a lobsterboat is not completely out of the water, calm or not. Take a look at this website which clearly shows the difference between a planing hull and a lobsterboat hull. They use the term "semi-displacement," where I prefer the term "semi-planing" but in terms of function they're the same thing. The lobsterboat hull does not have enough lifting surface to truly plane the boat even if you replaced the soft chines with hard chines unless you put a tremendous amount of horsepower in it. With enough power, you can plane anything, even a rock
And in fact, I do agree with you: a GB hull could be planed with enough power and enough weight reduction. But since it has neither, it is "semi-planing."

And while I have nowhere near the experience with hull shapes as Mike does. I've been around a lot of planing hulls for a couple of decades in the form of floatplane floats. I have also done a lot of reading and research about PT boats in preparation for the book I am now writing, and we own a small planing boat in the form of a 17' Arima. From what I have observed regarding what defines a true planing hull, they have lifting surfaces that extend almost to the front of the hull. As power is added, enough hydrodynamic pressure must be generated to lift the boat out of the water, thus reducing drag, which in turn enables the hull to be driven faster, which creates more hydrodynamic lift, which lifts the hull farther, which reduces more drag, and the cycle continues until the hull is riding on the minimum possible wetted surface.

The forward lifting surfaces are what's missing on a hull like a lobsterboat, Grand Banks, etc. So they cannot efficiently generate enough hydrodynamic lift to get the forebody up and start reducing drag enough to get onto a plane. The only way they can overcome this is to use a huge amout of power to generate sufficient hydrodynamic lift on the aft lifting surface to get the whole hull up. But most boats of this type are not fitted with engines this powerful, so you start to get up onto a plane but then can't go any farther because of the lack of sufficient lifting surface and power. Hence they are "semi-planing."

So my opinion is the most identifiable characteristic of a true planing hull is the lifting surfaces that run from the front of the boat, or very near the front, clear back to the stern. A PT hull is an excellent example of this because it's so big (80') and the configuration of the lifting surfaces is very obvious. Another very obvious example of this is a floatplane float.

-- Edited by Marin at 02:15, 2007-12-05
Airplane designers say you could fly a brick to paris and back if it had enough thrust. I imagine my displacement hull would plane if I had three thousand horsepower. It wouldnt plane well, but it would eventually lift itself out of the water (unless of course the hull collapsed under the stress).

My flat bottomed mastercraft ski boat is a good example of what a real planing hull looks like.

-- Edited by holohonu at 23:25, 2007-12-05
I totally agree with Marin on this subject although power or weight has absolutely no bearing on hullform or what we call the only has a bearing on its performance. What needs to be remembered is that there are advantages to a SD hull. One of the big ones is a smoother ride in the range of planing speeds. Whereas a planing hull has flatter hullform forward(as Marin stated) which can cause pounding in the chop, a SD hull neither pounds nor does it even just goes right through the water cutting thru the chop. A smoother ride for sure but sometimes a wetter ride for the same reason. Chine shape has really nothing to do with it either although I think Willy might be refering to chines as a totally rounded bilge aft. A chine really is just the edge of the hull. You can have a totally flat no deadrise hull(max lift) and still have soft chines. You can also have a full disp hull with hard chines.

The reason why SD hulls are used in trawlers unable to achieve planing speeds(due to be underpowered) is for form stability or static stability as we call it in airplanes. Due to keeping the flotation all the way out to the edges(chines), the boat has the initial tendency to resist rolling. This flotation is also a disadvantage as once the boat is upset, it has a tendency to snap back quickly whereas a full displacement boat does not have this flotation at the chines so it does not snap back(although its initial stability is not as good so initially, it rolls easier).

Willy, my boat and boats like it are planing hulls from about midship back and FD hulls forward. Obviously there is a gradual transformation along the length of the hull. I have some good pics of my boat outta water that I will put up....
Look at this pic.* Notice how from the bottom of the boat up to that spray chine.* If you drew a line from the bottom to the chine, there would be "air" in between the hull and that line.* On a planing boat the hull would at least meet that line and most likely "bulge out beyond it.* It would also not be so deep of a "V" providing even more dynamic lift.


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John makes a good point about semi-planing boats tending to have wet rides. If you want to experience a wet ride, try a GB quartering into wind waves with a good 15 knot breeze blowing everything the hull kicks up back at you. A lot of trawler-type boats have significant bow flare to help direct water out and even down a bit. But not a GB. Bellingham Bay is rough more often than it's not, and our windshield wipers are often the most imporant components on the boat.
I still disagree with you guys about fwd lift. See if this makes sense. When a boat reaches a speed length ratio of 1 the hull is riding right in the middle of the trough. The bow is on the backside of the bow wave and the stern is on the face of the stern wave. Both ends of the hull are equally supported and the boat moves forward in a level attitude ( neither bow up or down ). Now we try to go faster. The boat moves forward and in doing so assumes a position on the backside of the bow wave because the center of the boat is supported by the bow wave and the stern drops into the trough of the two waves. The stern droping and the center of the boat rising is what causes the bow to rise. The bow is just along for the ride.

Eric Henning
Thorne Bay AK

I think you're wrong about this. I've had the whole hydrodynamic principle of floatplane floats--- which are no different in form and function that a planing boat hull--- explained to me in painstaking detail when I was writing my floatplane instructional book back in the '80s by the guy who ran Edo floats for decades. Without that lifting force in the forebody of the float as well as farther back, there is not enough hydrodynamic force to lift the float onto a full plane (the "step" as it's called in seaplane flying.

The stern down-bow up squat you are talking about is not the kind of force needed to lift a hull. The stern drops in part because the prop is "digging" water out from under it. I (and Carey) have experienced this in canal boats where the boat can go over very shallow spots at idle, but if you put in power, the prop pulls the water out from under the rear portion of the hull, the hull squats, and it hits the bottom. Since you're standing on the stern with these boats, you can feel this happening. And canal boats are about as full-displacement as you can get with a boat

Merely supporting the bow with the bow wave will not provide enough upward force to lift the hull and reduce the wetted surface enough to reduce drag and allow the boat to accellerate and enter the "more lift = less wetted surface = less drag = more speed = more lift" cycle I mentioned before.

Planing is not about the angle of the hull, it's about the reduction of drag. The only way to do that with a boat is to reduce the wetted surface, and the only way to do THAT is to generate enough hydrodynamic force--- lifting force-- to get as much of the hull out of the water as possible. To do that efficiently the hull must be shaped in such a way as to generate as much lifting force as possible with the the minimum amount of power.* Ideally you would want the ENTIRE hull out of the water for true efficiency, which is what a hydrofoil is all about.

The most efficient planing surface is a sheet of plywood, but obviously that introduces all sorts of control and stability problems. The ubiquitous "John boats" used by duck hunters, fishermen in the bayous, etc. have absolutely flat bottoms and they can be put on a full plane with surprisingly small motors. ANd of course the flat bottom is what you want when operating in shallow water like most of them do.

Flat bottoms are not what you want to be riding around in when there are waves, so you start getting compromises with sharper entries, keels, and so on, all of which make it harder to generate the lift you need to get up onto the plane.

But the bottom line (literally) is that if you want to plane, you have to get that hull up out of the water and minimize the wetted surface. Doing that with any degree of efficiency requires getting as much of the hull as possible to contribute to the generation of lift, and that means the front as well as the back. Once you're on the plane, the front is no longer a factor as it's pretty much out of the water. But that forward lifing surface is essential to get the boat up onto the plane. Without it, you either need gobs and gobs of power, which often isn't practical, or an amazingly light boat, which in a trawler isn't practical either.

Since a boat like a trawler, lobsterboat, etc. doesn't have the lifting surface at the front like a true planing boat--- ski boat, fast sportfisherman, etc.--- you end up with a boat that starts getting up onto a plane but never makes it because it's too heavy, too low-powered, and doesn't have a hull design that generates a lot of lift as speed is increased. In other words, you have a "semi-planing" boat. It tries, but it just doesn't have the right combination of weight, power, and hull shape to get away with it.

-- Edited by Marin at 15:45, 2007-12-06

Marin, again, I agree with you. But weight and power have absolutely nothing to do with a semiplaning/displacement hull.....only it's performance. I have a boat with a semi-planing hull and it does plane. It is still a semi-planing hull even though it has more power and less weight which allows it to operate in the planing range. If you put more lift forward on my boat I would get more speed/efficiency with the same weight/power combination but it would also not be as sea-kindly.....that is what the trade off is.

Perhaps the difference stems in how we each define a boat. If you have a boat that planes--- achieves a full plane, not plowing along partway up--- then to me, it's a planing hull regardless of what the hull shape happens to be. I have always categorized boats by what they can do--- plane, partial plane, not plane at all, and so on.

So by my definition--- and I'm not saying it's the one everyone should follow--- if I have a rock and I can strap a great big engine to it and get that rock up onto a plane, it is a planing rock as far as I'm concerned. By my definiton, a planing hull must have the right combination of weight, power, and hull shape to get onto the plane. They all contribute to the boat's ability to plane. To me, a semi-planing boat is one that doesn't have the right combination of weight, power, and hull shape to get onto a plane.

However....*if you are using the term semi-displacement to define a particular hull configuration, then I have no argument with your statement.* *If your boat has a sharper entry, deeper and more rounded forebody to cut through waves rather than slam into them, and then a flatter cross section aft to allow it to achieve faster-than-displacement speeds (and better stability in some situations), and that is how we define a semi-planing hull, then yes, I would say your boat has a semi-planing (or semi-displacement if you prefer) hull even though you can get it up onto a full plane.

So if we accept that I define a boat type strictly by what it can achieve in terms of performance*and you define a boat type by its physical configuration and how it behaves, then we're both right.

-- Edited by Marin at 17:21, 2007-12-06
Your'e dealin with apples and zucchinis! Ever heard of a boat whith the thrust line 22 ' above the keel. I'll bet you'd need some fwd lift or you'd go right down in the water. Planing IS all about angle of attack. No angle of attack .... no lift...none zip nada. Whats fore and aft on an airplane float may be confusing to you also as the effective transom at 10 knts becomes the step.

You are correct, Marin. It is how we define the hull. Anything will plane given enough power....that does not mean it has a planing hull. The rock might be planing, but it does not have a planing hull nor does it have a hull at all. We are in agreement. I was talking about the hull configuration....not what that particular hull was doing at any particular time.

You're correct but in a planing hull the angle of attack is built into the angle of the forward lifting surfaces. Just saying "the bow comes up" is not a very accurate explanation of what's creating the lift. In a truly planing hull, the forward part of the hull begins developing lift before the bow begins to rise because of the upward angle of the lifting surfaces. As soon as the lift starts to build the bow starts to rise, but the bow does not have to rise first in order to develop lift in a planing hull. It does in a semi-planing hull, which is why a semi-planing hull either can't achieve a full plane at all or if it can, it does so relatively inefficiently in comparison with a true planing hull.

As to floatplanes, the parallel is apples and apples. The center of thrust is higher than the floats, but only about five feet in the case of the Beaver I fly. And the height of the center of thrust is more than offset by the use of up-elevator at the begining of the takeoff run, which is why you use up-elevator.

Once a floatplane begins to accelerate, the afterbody of the float--- the portion aft of the step--- is irrelevant. The only reason it's there is to keep the plane level and on top of the water in the displacement mode Once the speed starts to build, the rear half of the float is just along for the ride. It contributes nothing to the lift needed to get the plane onto the step. The only part of a floatplane float that is important when getting onto and staying on the plane or step is the forward portion, from the bow to the step. In this regard, the float acts exactly like the hull of a planing boat, which in fact, it is.

-- Edited by Marin at 19:24, 2007-12-06
I'm bored.* Let's get out of our underwear and go boating!
Let's go boating in our underwear!!!
Wait....! Is there a way we can use underwear to increase the ability of a semi-planing/semi-displacement hull to get on the plane? And what are the best methods for attaching underwear to the hull?
I suppose we should have female underwear for a boat. Good I like that. Somebody mentioned an article in PMM about hull form and I looked for the message but couldn't find. I went through my magazines and found the article. I have a catagorized list of about 75% of PM magazines as to what articles are in what to subject matter. Ask me and I'll do a spot of research for you.
The article about hull form is in the winter 97 issue. It is written by Charles Nevile who is a Naval architect who designs mostly heavy steel trawlers. I like his boats and I like what he says. I may need to apologise to the group. I think I may have made things a bit more complicated than nessesary. Mr Neville lumped all hulls not FP or FD into the semi displacement category. No mention was made of the expression semi planing. I wrote my introduction to this thread off the top of my head and perhaps I should have put a bit more time into it. Does anybody know if there is more than one official classification for this middle ground? I think that I assumed, some time ago, that the two expressions stood for one end of the range. Could be, over time, I was just making things up. Mr Neville puts the upper end of FD at SLR of 1.42 and the beginning of FP at SLR of 2.5. That would about 17 knts for a 30' boat and 19 or 20 knts for a 35'. He stresses the value of ratios and numerical comparative data. The alaska state ferry Malispina has disp 3600 tons 8000 hp 18 knt cruise 352' WLLand 2.2 hp per ton. The Nordhavn 46 has 4 hp per ton and my willard has 5. The Nordhavn is my favorite boat and it gives me pleasure to see that it is very much like my Willard, only much larger.

Eric Henning
Thorne Bay AK
Willy, no need for an apology. We're just shootin the breeze here. The article I was referring to happened in the last coupla years. It was a good one. The thing to take from all of this is there is a lot of grey area.
Hey, it could be way worse. This could be an aviation site and we could be having a "discussion" about what creates aerodynamic lift. Been there, done that, and it ain't pretty. (It's got nothing to do with Bernoulli by the way). Boating--- at least with the trawler folks--- is a way friendlier deal.
Uhhh, it does have something to do with bernoulli....
John and a myriad of others.....Marin is exactly right! it doesn't have anything to do with Bernoulli's principal. When in your car......stick your hand out the window with closed fingers and "fly" it up and down, like when we were kids. I'll stop right here as the explanation takes too long to present in a believable fashon.
I am sure it does. But I will leave it at that. I know what y'all are getting at ad we have had this discussion before. But the Bernoulli principle is not completely and totally irrelevant as y'all make it sound. It is very relevant. It is extremely relevant.
Nahhhh. But if ya really wanna hash it out, prolly the off topic area would be the best place to do it.....doesn't really relate to the original intent of this thread.
Mr Baker,

So it was you. Please tell me the year and month of the article. I would be grateful.
As to the Bernoulli stuff I don't want no hash out. My opnion is not contemporary but if such a thread were to come to pass I would make a statement ... nothing more...One statement. You've heard the " old pilots and bold pilots " .... Well the reason there are no old pilots is that they all become us.

Eric Henning
Thorne Bay AK
Willy, I honestly don't know and I do not kep back issues(I'm not that organized and I used to live on my boat so we never had the space). Maybe the PM site has a search function?
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