"Displacement" Hulls

They're one of a variety of convex bottom designs, effective when used in combination with other elements of design. Here's a portion of something I wrote a few years ago that talks about the rounded convex bottoms you find on displacement hulls:

Bellied, or rolled, bottoms are generally rounded convex features. They can be subtly rolled, or highly domed. What sets bellied bottoms apart from other convex bottom shapes are their ability, under certain conditions, to generate a combination of lift and reduced wetted surface, and as a result, higher speeds. Due to the fact that a small region of the dome – it’s peak along the stringer – is nearly flat and parallel to the deck, the lifting force it creates is straight upward, in direct opposition to the force of the rider’s feet pushing down. As the entry rocker of the bottom lifts the board up onto plane, it begins to plane higher and higher, on a narrower and narrower planing surface, eventually planing only on an elongated, narrow portion of the bottom of the board when compared to flat or concave bottom designs… but only if the board is able to reach a high enough speed to do so. This is because a significant amount of lifting force, generated only by entry rocker, is required to hoist the board and rider up and out of the water to plane on the peak of the dome, and this degree of force can only be reached at high speeds. However, once up and planing on the peak of the dome, the reduced wetted surface allows the board to reach even higher speeds, and is the trademark design element of the “hull” shapes perfected by Greg Liddle. His boards are said by devotees to have a “fifth gear” that is only reached in long, fast point surf, where these designs excel and can be exploited.

This type of bellied dome should not be confused with another specialty design, invented and perfected by Geoff McCoy, called a “loaded dome.” McCoy’s loaded dome design is a softly rounded, pyramid shaped dome, with flattened panels on the sides, front and back. Although it is technically a displacement feature, the front panel actually creates a considerable amount of lift, and a high pressure “pivot point” at the top of the dome for turning. How this loaded dome is blended with rocker, rail, tail width, and other design elements is highly detailed, and a quite sophisticated feature of design.

 

Surfers Journal just had an interesting article about some displacement hull boards that george greenough shaped fairly recently, worth a read if you get a chance.

 

Greenough's flex spoons were shaped as a hull design, but with enough flex to "feel" the wave face and respond. The guy was/is a brilliant, intuitive designer and a master craftsman.

 

I heard they are hard to ride. I don't want hard to ride.

 

While I enjoy learning about different designs, this one, as neat as it is, seems completely inappropriate for the waves I typically surf, and the style in which I surf them.

@LBCrew Your write up is a cool easy to understand description though.

 

The experience of hull riding is in the connection with the wave flow for the rider rather than the onlooker. First of all planing. Planing is when the craft skims over the surface of the water. Displacement is when the craft ploughs it's way through the water, very much like the hull of a boat.

Modern displacement hulls are usually very rounded and rails turned up in the nose. Very rolled midsection, rolled or flat panel Vee in the tail with rails turned down with little or no edge. Flattened rockers too.

Definitely sit lower in the water while riding. Suited more for point break waves.

Boards tend to be ridden from center and not on the tail so much.

Few I built this year. Full blown Hulls, and Semi-hulls.

 

Barry Snyder or LBCrew: I'm trying to square watching video of guys like Greg Liddle or Steve Krajewski just blazing down the line at Malibu on hulls, seemingly all about glide and speed, with this description from a well known shaper of such craft:

"A displacement hull has a theoretical hull speed, above which the water actually sucks the hull deeper into the water (I’m simplifying here). Take a sailboat or any other displacement hull and tow it. At anything above the theoretical hull speed, the boat begins to submarine, actually being pulled deeper into the water. Old, soft edged boards are the same, as are any true displacement hulls being produced today. As soon as you put an edge at the tail, you release the water and the board begins to plane. The modern surfboard, most “hulls” included, balance these principles to achieve the desired effect or feel.

Displacement hulls, by their very nature, are not as fast as planing hulls. They may “feel” faster in a section, but without the release, they are constantly dragging more than a sharp edged board would. Now this is not a bad thing. The feeling of a well balanced hull is one of the great pleasures of surfing that most people fail to credit. Surfing one well takes a different approach, especially if you are stepping off a thruster. Single fin riders tend to have an easier time"

 

I had a 7'2" Doyle with a hull bottom. kinda like a rounded inverted vee. I liked it a lot. turned on a dime. Fast as crap. I filed the fins down(been stabbed twice) and they would hum on the bottom turn. Sounded like blowing over a bottle. Perfect of Hermosa close out perfection.

 

Here in Ca, we don't have many point breaks. Mostly beach breaks. Not very well suited for the waves we have. I do have couple of friends who surf them at Rincon and Malibu.
Definitely a feeling of being closer to the wave and feeling its natural energy.

 

I was always under the impression that surfboards are essentially hybrid displacement/planing hulls. A 7' to 8' malibu style egg shape is still pretty much a planing hull IMO. Really round and thick 10' logs are the closest surfboard equiv. to a displacement hull. And boy do they have a speed limit, they never seem to pop up on top and go fast. Displacement= float, planing=skim. Surfboards do a little of both don't you think? You almost must decide how much you want of either. Concave bottom performance shapes- all plane power & no (little) float. Soft railed tanker logs- all float & no (low) skim. More planing and quicker up on plane is much more fun. But to much plane may make the board bounce or misbehave (hard to control). The combinations are mind bending & infinite, keeps me coming back for another.

 

Hulls will have increased drag at lower speeds, where they feel smooth and flowing, and give that "one with the wave" feeling. But at higher speeds I have never felt this mysterious "suction" force that pulls them down, and can't explain the physics of how this could happen. In my experience, the faster they go... the faster they go. A longboard with belly, and a true hull design, are two completely different animals, intended for two completely different rides. I have ridden a number of true hulls, and an even greater number of displacement bottom boards that are not hulls. They are worlds apart.

 

Boats have displacements hulls. Who the heck wants to surf on a boat??

 

That's the one. While thats a boat-load of money, I can't say that I'm all that surprised.

 

They are fun. Very different feeling with the belly under the board rather than flat or concave. once up to speed, it feels almost like you are floating but still connected to the wave. Very cool once everything clicks but not for everyone. Some really don't like the designs because they are meant for lined up points that most don't surf everyday.

 

Personally, I don't think so. I think they're a little tricky to ride well in general, but no difference between front- and backside for me. People say the same thing about fish... that they're difficult to ride backside. I don't find that to be true either. However... I'll qualify that statement by saying that I learned to surf backside, and rode nothing but lefts growing up.

 

"Displacement Hulls"

None of boards mentioned above are displacement hulls, they are planning hulls. Let me explain.
A true displacement hull approximates the shape of a U, with vertical lines on the sides. It takes the lowest amount of energy to move these through the water at lower speeds, and that is why giant freighters are displacement hulls.
The only stand-up surfing crab that even comes close to this are the ancient Polynesian Olo and another variety ancient Polynesian surfboard which I forgot the name of as of right now. I do not at all include in this boards made by somebody in New Zealand with diproportionately high price tags.
What are known as displacement hulls are really planning hulls. They might have convex features. That is why Greg Liddle calls them MTDH: modified transitional displacement hulls. These have a flat area along the stringer. The low rocker rock bottom and entry makes these boards very fast as well as the long tail and the fin position and flex. Without flex fins they don't work.
The the pinched rails that everybody likes especially those extreme knifey tails of the death machine slow this board down at potentially high speed waves. The wide point forward and a long narrow tail also contribute to this. The board is not hard to ride if you are a front footed Surfer and you dedicate time to this type of board and this type of board only for about 2 years. It works well in the 7-foot range any smaller than that and it still works good but it doesn't harness wave energy as well and it doesn't do the banking over that it's so well known to do with the nose buried in the water. Baseboards and trim are much faster than flat or concave bottom boards especially and steep waves up to a point. What I mean is the board is not meant for super Hollow waves.
A much Superior board to all is the loaded dome by McCoy.
It combines the skatiness of a fish the stability of a gun and in a fast Hollo wave is way faster than anything out there if it's in a single fin with a wide tail. With a low rocker model of this such as a low rocker Potbelly that is not too small for the writer it flies and Maneuvers better than any fish can and holds. On a high rocker model it also goes fast and Maneuvers better and is amazing in the barrel. Again this is with the Single Fin with the wide tail. The foil of these boards are thick from nose to tail making the board fast. And this keeps the momentum going. The roll of the bottom changes frominch to inch, from the nose to the tail and there are no flat spots. McCoy has made changes to the boards from year to year even from month to month recently and also from times past to fine-tune the board. The bottom is so complex yet so simple once you know the secret behind it.
The real start out as a completely full 5050 rail in the nose then it becomes a 60/40 rail with a soft underage that tux under away from the rail almost like a bevel that is Blended smooth and round and then toward the tail the rail the tucked under soft Edge or Blended bevel comes out and disappears but the rail stays soft if it's a Single Fin although it becomes a little sharper it's still soft to hold the board onto the water without sliding out as a single. It only becomes hard the way the rails are on a 3 Fin at the back end of The Single Fin box. For release.
This design is also a planing hull. It does not suck down into the water because the rails are full although they are not Boxxy. They do have some slant on the deck side so the rails are full enough two plane or act as a planning surface when on rail but just slanted enough to go into the water and go through the water without losing speed.
The speed and maneuverability is amazing. It is a totally different Beast in terms of technique from most any other board and complete opposite of a displacement Hull because it is a back for the board meaning you steer it by rotating the back ankle in putting pressure on the back foot instead of rotating the front ankle.

 

Continued from above: The most powerful turns on the widest tail models are like stalling the board and trying to plow through the water but the board will not slow down because of the bottom Contours and the width and thickness of the tail. It throws the thickest tallest sheet of spray I have ever seen and it throws it forward. In fact just doing a simple bottom turn or fading back we'll throw so much spray off that maneuver it's radical. But you have to learn how to surf it off the back foot again you have to put in the time and commit to that board only and study videos of Cheyne Horan and on these boards or Occy from the eighties especially in the movie filthy habits.
There is a whole lot of Mystique behind the Greg Luddle Displacement hulls and the loaded dome McCoy boards. I've had 3 Greg Liddle displacement hulls and I still have 2, and I've had a total of 5 McCoy's. It wasn't until I got the single Fin McCoy very wide Astron Zot and wrote it in very fast very Hollow powerful waves that I felt something I have never felt in my life surfing it felt like I was literally hovering above the water like on a hoverboard but with perfect control. All the stuff spoken about the displacement hulls I felt in a much much greater degree on the McCoy. Then I had a smaller one made but still very wide and thick and short and it has an amazing feeling of lift. On the small one sometimes I just stand on it straight up as if I was writing a classic Pig longboard with very low rocker I actually have one like this I just stand on the small McCoy with the wide round thick tail and the soft rails he uses and a fuller nose then he used to use. I just stand on it like a longboard sometimes and it just cruises sometimes I want to hot dog and it is more maneuverable than any so-called modern board out there.
The bottom from rail to rail fits into the wave face so it doesn't want to go to the bottom like a concave or flat bottom board. It wants to stay mid face or wherever the best placement of the board on that wave is because it actually slots and positions its own self. I can go into why it does this but it would take me too long. So the board on a hollow wave even on the small very Holloway where most people would be at the bottom losing speed and getting hit by the lip this thing stays in the right place without too much effort. If you know how to rip the Single Fin rippes better than anything and it just goes so fast. The rolled bottom is rolled enough to let water flow around certain areas and it changes to cause lift in other areas and then it changes to cause a self slotting effect. Taking off under the lip on a very steep sucking double up in shallow water is child's play on this thing. Sometimes you don't even have to paddle. On the day I had it in double overhead fast Hollow grinding burning barrels it lifted itself out of the water and beat sections other boards cannot with one fin and I was able to get into waves that other boards could never get into because my board was heavy beating the offshore wind and due to the thickness and the bottom Contours which made the water flow around it so much better I was able to get into waves earlier than any paddle boarder or longboard I could ever hope to get into just buy paddling on my belly. So whoever applies the properties of a displacement Hull to these aforementioned boards does not understand what a true displacement Hull is and that these boards are planning hulls with rolled bottoms.