# Tool that calculates total drag for a submarine

Discussion in 'Hydrodynamics and Aerodynamics' started by Bjorn Moren, Apr 11, 2021.

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### Bjorn MorenJunior Member

Can you please suggest a tool (software or online tool) that can analyze a submarine hull shape and give the total drag at a specific velocity.

A simplified tool would work too, where I select a general geometry, then input the dimensions and speed, and get the drag force.

I'm reading Submarine Hydrodynamics by Martin Renilson, where he writes "For a typical submarine the total submerged pressure resistance does not exceed 10–20% of the skin friction resistance. Thus, the skin friction dominates the resistance of a deeply submerged submarine."

I'm assuming that this is the case for slow speed only, and that pressure resistance is the dominating resistance at high speed, or submarine hulls wouldn't be so streamlined. Or am I wrong?

My application is for low speed, and I need a non conventional hull shape that is flat (box shaped), and I'm trying to understand how much penalty I will have to pay for this.

The standard drag equation is too simplified in my case, because it approximates both skin drag and pressure drag into one variable (the drag coefficient).

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### jehardimanSenior Member

That violates the Wassenaar Arrangement.

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It depends upon the slenderness ratio of the hull. (Length/diameter)
Since as the slenderness ratio increases, there is an increase in WSA, however the pressure gradient aft decreases, as does the viscous pressure resistance (Cpv).
But an increase in the slenderness ratio increases the wake and Cpv.

However, Im sure Martin goes into this in detail.... I can't confirm as I don't have his book.

Thus, like most things in design, everything is a compromise...

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### BlueBell"Whatever..."

John,
You're such a wet blanket.
BB

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### DCockeySenior Member

For the pressure drag to be small fraction of the skin friction of a submerged body the flow needs to remain attached for the full length of the body or until a small base area is reached. That requires a "streamlined" shape.
Incorrect. As long as a fully submerged body is moving fast enough for the flow to be turbulent over almost the full length of the body (not a very fast speed for a body such as a submarine) and the speed is not close to (or exceeds) the speed of sound the pressure drag changes only slighly with speed.

Another interesting characteristic of fully submerged slender bodies with more or less parallel mid-sections at speeds slow enough that compressibility effects are not significant: As long as the flow is full attached over the front of the body the pressure drag is insensitive to the shape of the front of the body.

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### jehardimanSenior Member

Yeah, it comes up from time to time. The following thread pretty well covered it, in all for its ordure.

For the record...

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### BlueBell"Whatever..."

Bjorn Moren,

Welcome to the Forum.
Lets try and answer some of your questions before we get too distracted.

No software I know of for calculating sub drag.
6:1 is going to be your optimum length to beam ratio for "low speed".
What speed do you consider low?

By "box shape" do you mean in cross-section?
Little to no penalty. Look at the box fish.

If you are building this then just measure the drag.

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### DCockeySenior Member

Reference??? Is this for on surface or submerged? What is mean by low speed? Presumably the optimum length to beam ratio depends on shape.

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I suspect he may be referring to:
Naval Architecture Aspects of Submarine Design, by Arentzen ES & Mandel P.Transaction of SNAME, 1960, No.68

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### jehardimanSenior Member

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### DejaySenior Newbie

Who hasn't dreamed of exploring the ocean in a submarine yacht?

I think michlet can predict resistance of submarines at various depths.

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### Bjorn MorenJunior Member

Thanks for the corrections DCockey. Very interesting about front body shape. So a cone shape with a rounded transition over to the mid section would perform about the same as an elliptical shape?

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### Bjorn MorenJunior Member

Thanks BlueBell. This is a hobby project for a UAV, around 1.5 m long, so I think that building a series of different mock-up hulls and running them in a water tunnel is the only sure way to know. I'd have to build me a water tunnel first though, so if you have any suggestions on how to do it please let me know. I'm thinking of just making a 10 m long water channel out of wood, lined with plastic, and pull the vessel using a wire running over pulleys with a weight attached.

Correct, I can pick between a circular cross section and a box-shaped cross section. Box is better for project specific reasons, but I assumed it came with a high drag penalty. From the answers I've got so far this doesn't seem like the case, so wet area is the 90% dominating part of the drag (skin drag dominates).

I expect cruise speed to be around 5 km/h (2.7 knots).

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### Bjorn MorenJunior Member

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### BlueBell"Whatever..."

You are most welcome.
Powered or is it a glider? Are you familiar with how "gliders" work?

A plastic lined wooden rectangle would work but don't you have access to a pool?
I don't think you're going to find any measurable differences in drag between shapes.

What would your diameter be round?
And square?
Hard to imagine your payload can't be incorporated into a round cross section.
However, the smaller the hole you have to make moving through the water, the less drag.

Fun project. Pictures?

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