Why are hulls curved when looking from the side?

Beginner question here. I've tried finding the answer on my own, but without luck.

I get why hulls are curved when looking from the top/bottom, but why are they also curved when looking from the side? In the picture below I've colored the original hull shape in green. What would happen if the hulls were shaped like the red lines instead? The submerged part of the hull would still look like a hydrodynamic cigarr. Autonomous boats are also curved, so I guess it's not about ride comfort or creating enough space for humans.

 

I can think of a couple of reasons, but there may be more.

Firstly you need the boat to handle waves or chop where if the two ends are providing more of the buoyancy and the middle less, you're putting extra stress on the structure - you want most of the lifting to be done by the middle, while the front and back ends are working more to keep the boat level relative to the slope of the water surface.

The second reason is that you're trying to minimise drag from excessive surface area where you want a gentle transition as you go from no boat displacing water at the bow through maximum displacement of water in the middle and back to zero displacement at the stern, and varying the maximum depth of the boat as you move from front to back is just as important for that as varying how much it bulges out sideways - keep one constant and the other has to vary more, resulting either in a steeper angle of surface being driven against the water or needing a longer hull to maintain the same angle, which means extra surface area and more drag.

 

Hi @Boatyboatboat

Viewed historically I guess the main reason for a not-rectangle shape is it's not as hard to turn as a (in side view) rectangle shape.

 

Good question.

Number 1's already been mentioned, The hull with a straight bottom will have more wetted surface, hence more drag. Also the passage of a hull thru the water is a parting of the water in the front half of the hull and recombining in the stern. For a displacement type hull this needs to be a smooth gradual process. The hull with more volume in the bow will make a larger wave which means more drag. There are a lot of additional little nuances that factor in but drag is the big one.

Number 2 In a boat that's curved in cross section will be harder to build I think.. A 3 panel skiff out of plywood would be easy enough to build with a straight flat bottom but handling would suffer compared to a hull with rocker.

 

Phil Bolger drew hulls with square cross sections, made bottom and side curvature the same, and was surprised by how well they sailed. The explanation he came up with was that if the curvature of bottom and sides was the same, there would be no eddy-generating, energy-wasting flow across the hard chine. (I am not aware of him discussing leeway in connection with this idea.)

However, if you round the chine enough, you can have a totally flat bottom. Many canoes do, many cargo ships, the Dovekie, and the more recent of Rob Denney's Harryproas. Denney says the advantage is less pitching and a higher prismatic coefficient, which describes how large a proportion the hull fills of an imaginary rectangular block with the same cross section as the hull's largest section and the same length as the waterline length. A large prismatic coefficient means filling a large proportion, and that there is much buoyancy in the ends. Such hulls have lower wave resistance at speeds that are high for the waterline length. Hulls that fill a small proportion, having fine ends, have less wave resistance at speeds that low for the waterline length. Those proas are intended to sail fast, and a proa reverses direction, and so does not need to be designed to turn well while tacking.

So hulls with a flat bottom when seen from the side do exist, but you have to think about flow in three dimensions, and how easily the water can move from the curved sides to the flat bottom, because it will not just move sideways at constant depth. And you have to think about how easily the boat needs to turn.

 

I actually did a sailing canoe close to the profile red lines above, double ended in planform and square in body view. It was spooky- it turned ok, had moments when it was faster than you’d expect, but it just felt numb. Also was impossible to get the bow up, even when standing close to the stern. The bow had this unnerving tendency to dive going downwind, even in very flat water. I’ll see if I can find my YouTube of the first sail. Very light air, maybe 4-5 knots in the gusts. In the middle of the vid, you’ll see the thing really scoot in little air. After that I started standing near the stern to get the bow up, which you will notice doesn’t do the trick. At the end you’ll see very tentative sailing downwind as the bow wants to dive! Dive! Very little freeboard forward did not help, but even with a foot of bow, that would be weird. Moving the heavy aluminum mast aft might have helped?

 

Thanks for the report. I had forgotten the Bionic Broomstick, it was a total rectangle in side view, 14' long (Texas registration rules) 8" wide and 20" deep I think with a rounded bottom in cross section to cut down on wetted surface. There was plenty of freeboard but dang that thing ran bow down. It's good to know that weight shifting doesn't help much. If I ever do another it will be longer with a flat bottom and some rocker.

 

The reason for the curve provides a place for pressure recovery to occur and help move the boat along.
If the boat has a rectangular cross section as you illustrated, then the top view should have a curve at the back. Like the one shown in Post #7

It the boat curves up from the side view, this curved area provide this pressure recovery which reduces drag

A Boeing 747 does it a 500 knots plus and fast swiming fish tapers rearward as well.

"The increased pressure from compression pushes against the stern of the hull. By pushing from the aft side, the pressure reduces the total hull resistance. It pushes the hull forward. We call this pressure recovery, and it is a vital component of efficient hull design." Note that they use the word compression but this is not valid in a liquid.
from DMS Marine Consultants

A quick search will provide much more technical explanations on the effect

 

I did this experiment to see with my own eyes the drag caused by an immersed transom.
https://www.youtube.com/shorts/RVPvyYSDQ-Y

It wasn't untill I passed hull speed that the bottle was no longer sucked into the transom.

 

I don't have the detailed math and engineering knowledge to give you a certain answer. But, in addition to the points already made:

In the sea kayak community, which after all originated in the arctic, one of the reasons often given that front curvature is how easy it is to paddle and slide it onto a sheet of ice. A somewhat similar theory probably applies to paddling/dragging it onto land. sharp rectangular corner at the bottom would get stuck on things.

E.g., you will notice that the Viking raiding ships, which often were sailed/rowed/dragged onto shore outside standard docks, for covert reasons, had a very curved bow.

Of course, not all boats are paddled or dragged onto ice or land. And when they are, they are frequently loaded onto a trailer. So that argument doesn't apply to many other types of boats.

I could also argue that if you have to take any boat through dense seaweed or similar vegetation, an upward curved bow might be less likely to get caught and stuck in it. And that if it has to back up into it, the same applies to the stern. As more and more docks, rivers and bays get clogged with seaweed and other vegetation due to fertilizer runoff, this could increasingly become an issue.

My arguments only apply to the part of the boat at or near the waterline. But with different cargo weights, the waterline level changes.

One more thing: David Cooper mentions structural strength issues - pointing out that you get more stress if more lifting occurs at the ends. That makes sense. (Though I could point out that if the ends are completely out of the water, you have stress in a different direction.) In addition, though, convex curved shapes tend to be stronger against outside forces and impacts than straight ones, for a given weight. In somewhat the same way that an arch can be often stronger for a given weight than a straight beam. (Though that's complicated, and relates also to factors like the relative strength of the material in different directions, such as tension and compression.)

But I intend my next sea kayak to have a much straighter keel, so it can be shorter and therefore lighter, and still have a long waterline length.

 

One more minor thing: I suppose that expertly piloted ships rarely run aground. But recreational boats occasionally run aground, or onto a rock, which may or may not be visible above the water. A curved bottom at the bow means the boat deflects upwards and stops more gradually. That also reduces the stress on the hull. Perhaps no one else has mentioned this because they are too expert to ever run aground?

By the way, you can come up with disadvantages to a curved bottom profile too. E.g., since I am lighter than the people most kayaks are designed for, a lot of the length is out of the water. So the waterline is shorter, making it slower. And the boat is harder to control - wind and waves can push those ends that aren't in the water around more easily than if they are sort of "anchored" in the water. Again, I think you could argue that a craft of a given weight can hold more cargo weight if it looks more like a barge, as long as the waves aren't very high. And shallow drafts (corresponding to more or less flat bottoms) allow boats to go in shallow water.

So the optimal shape will vary with the intended uses and conditions.

 

think about the shape of fish

 

The o.p. asked why. Thinking about a reasonably efficient creature doesn't give you a reason - at best it implies that the shape is efficient - but why? Is it optimally streamlined? Easy to maneuver? Etc.?

Besides, there are flat commonly shaped fish, and there are eels (and water snakes - not fish, but they must be reasonably efficient in some sense too), and even "flying" fish, all of which move.

 

I'm always surprised that Bolger was surprised how well his sharpies sailed, since race results and rating schemes show that they were very slow even when compared to other cheap sharpie and hard chine types. The Dovekie is no doubt an excellent cruiser for many but it's rated only 3% faster than the 11'plywood gunter-rigged Mirror dinghy pram, for example. The Light Schooner is rated slower than the 7' shorter Norwalk Island 18 sharpie, and much slower than many 1960s cruisers of similar length.

Other people have designed far faster craft using cross sections very close to square, and they haven't made bottom and side curvatures the same. The fastest pre-foil International Moths had very square sections and very little rocker, but they had "pintail" sterns that allowed the stern to sink so that the hull could take on a positive angle of attack and the bow could rise instead of kicking the whole boat into a nosedive. Even the plywood pre-foiling Moths



Some British Cberub "skiffs" and Australian Skate class dinghies had similar "coffin" hull shapes, and some of the former were designed by top-class designers as were some of the Moths. So world-class creators of some of the fastest boats for their length and sail area didn't follow what Bolger wrote, and their boxy boats are dramatically faster than anything Bolger dreamed of.

High performance dinghies and skiffs show why boats have rockered hulls; for one, without rocker the bow will not rise and therefore the boat will nosedive. Secondly, the planing lift is developed at the bow and therefore there must be a positive angle of attack there or the boat won't plane. Thirdly, a dead straight rocker won't allow the boat to turn quickly. Fourthly, if the boat doesn't have enough rocker the bow cannot lift out of the water when planing and therefore the wetted surface will not reduce at high speed.

The "coffin boats" are also often slow in very light winds because their wetted surface area is much greater than a hull with more rocker. In addition, they tend to be problematic in terms of handling because they can "catch a chine" at speed, and if they are heeled they present a Vee shape to the water and go slow. The shape therefore works well, in performance and handling terms, on craft with big rigs and lots of crew-induced righting moment so that they can be sailed fast and upright. It doesn't work well on boats with conventional-size or small rigs and without traps, wings, planks etc.