Boat design question


(Bob LLewellyn) #1

Some time back I made a joke about fighting pirates by blowing bubbles underneath them, there by sinking their boats. But that got me thinking about designing a load carrying craft like a barge with some of the basics of a hydrofoil.

Barges push the water under the craft instead of toward the sides. Therefore the largest resistance would seem to be that large flat bottom. I was thinking, what if we recessed that bottom in by an inch or so leaving a thin strip of metal down the side and across the front but open in the rear. We compress air and blow it into the crevice. This should reduce the resistance, as the barge would be floating on a thin layer of air. You would still have the resistance from the bow and sides but I would think that we should see a 25% or so savings? Maybe?

Anyone have any thoughts?


#2

So far, the gains haven’t been worth the complexity. Posted some of the latest on it, not long ago, but damned if I know when. They’re working to combine hydrophobic coatings and air, to reduce drag.


(Larry G) #3

There are at least three factors to parasitic drag in boat hulls. The first one is friction against the water, and this is what you’re addressing above.

The amount of wetted surface, and thereby surface frictional resistance, has a large effect on performance at slow speeds, before wave making resistance begins to become dominant. The shape containing the most volume within the least surface area is the sphere. Thus, rounded sectional shapes tend to produce less wetted surface than angular shapes with the same displacement. The trend toward fin-appendages, rather than true keels, is driven in large part by reduction in drag.

Skin friction is largely a matter of skin roughness. Fouling, even a small amount, produces far more skin friction or drag than the mere area of wetted surface. The importance of the amount of roughness to skin friction and therefore the light air sailing performance should always be kept in mind.

The second and by far the most significant is inertia of displacement, and it is mostly determined by how much water you’re displacing, how far you’re displacing it, and how fast you move it, including how fast it can “fill in” again after you.

If you’re moving fast enough to leave a cavity behind you, then in practical terms of energy expended you are moving the weight of the water currently displaced by your hull PLUS the weight of the water displaced by the air cavity behind you. Even if this is just a dip of a few inches depth across an area of a few square feet, this can be thousands of pounds of drag. Likewise, moving fast enough to create a bow wave means moving the volume currently displaced by your hull PLUS the volume of water contained in the bow wave above mean water level.

This is why speedy hulls have low angles of attack against the water ahead (bow is both raked and narrowed) and smooth, rounded sterns to allow water to flow in behind with as little cavitation as possible. This is one reason why speed and carrying capacity have always been measured as trade-offs against each other. The more you want to carry, the more volume of displacement you need to keep buoyant. The more displacement, the more energy it takes to move those TONS of water further and further away from “at rest”. The faster you want to move, it’s exponentially more energy to move those TONS of water and then pull against the drag of water filling in behind you.

"The idea is to part the water with as little fuss as possible, move through it easily, then leave it behind as gracefully as possible having created the smallest possible wave pattern. Lindsay Lord expressed this very well by saying, “According to Newton’s Second Law of Motion, the resistance encountered is equal to the time rate at which the body changes the momentum of the fluid. That is, the rate at which the driving force does work is equal to the rate at which kinetic energy is imparted to the fluid.” In other words, changing the momentum of the water abruptly with an un-graceful shape is very much a disadvantage in terms of resistance, and much more so at speed."

This is why motor boats try to get “on plane” as soon as possible. It’s partly a matter of less hull surface in contact providing less friction, but it’s mostly about not displacing any more water than absolutely necessary.

Third, the effect of “lift” created by curved surfaces (Bernouli’s theorem) is yet another aspect of drag that is not addressed in the discussion above, due to a bit more complexity. Suffice it to to say, for every action, there is an equal and opposite reaction. Lift occurs at angles to forward motion and the energy used in lift has to come from somewhere, so it appears to add weight/resistance/drag in opposition to thrust.

But then, I’m apparently an Oompa Loompa toodler, and I’m definitely, admittedly, not a marine engineer or naval architect, so what do I know?


(Larry G) #4

@JL_Frusha posted this some time back:

http://www.boatdesign.net/Directory/Software/Free_or_Low_Cost/

Here’s a discussion on bow bulbs:

http://www.brayyachtdesign.bc.ca/article_bbows.html

and some formulas:

https://web.archive.org/web/20090313074901/http://johnsboatstuff.com/Articles/design.htm

Stability of beam vs ballast:

http://www.kastenmarine.com/beam_vs_ballast.htm


(system) closed #5

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