# Drag forces on extremely small watercraft

Discussion in 'Hydrodynamics and Aerodynamics' started by seanGT, Jun 18, 2011.

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### seanGTNew Member

I am designing a hull for a small remote-controlled watercraft on the order of 100 lb. and 4 feet in length. I am trying to acquire a basic understanding of the relative contributions of the different drag forces involved.

From my preliminary reading, it looks like I should be concerned with form drag, skin drag, interference drag, and wave drag. As I understand it, changes in hull shape that would reduce one of these drag forces could act to increase one or more of the others. For example, a hull with a high fineness ratio would have less wave drag, but greater skin drag for the equivalent amount of buoyancy.

I would like to get input from people with practical experience who might be able to steer me in the right direction in terms of what sources of drag are likely to make the biggest contribution to the overall drag force.

Hull speed, in particular, is of interest to me since it seems to be primarily dependent on boat length. Is there much I can do to overcome this besides just making my hull longer? My quick calculations give me a hull speed of only 2.7 knots for a four foot hull.

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### jehardimanSenior Member

First, there is no such thing as hull speed...the vessel will always go as fast as power applied will drive it. That said, there are points where the power increase is very great for a small increase in speed, and conversely there are speeds were the vessel will see large increases in speed with no increase in power. Just the physics of the humps and hollows in the powering curve...

As you pointed out, the big items are form drag, skin drag, interference drag (there is no "wave" drag, it is just the manifestation of the form drag). And as you say, form drag and wetted surface drag play off against each other. For a given required speed and a given required displacement in a given required environment, there is an minimum power length, beam, draft, and hydrodynamic shape. This doesn't mater if your vessel is inches or furlongs (though the dimensions and shape will change with size, the math is the same). Now again, that said, there are generally other requirements that drive the hull off this optimum mostly conserned with propulsion, prime mover, and cargo requirements. Additionally, some designs and shapes will require significant appendages to make them controlable and/or stable so therefore these will have significant interference drag.

In the final analysis, the answer is that no single factor can be pointed out as "the biggest contributor". It will all depend on the overall mission requirements and design decisions that drive the final form.

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### Oldsalt31New Member

A can of entertaining worms

Hi Sean, You probably already realise the size of the can of worms now exposed to you. Hullform exposed me to this same can some time back. Jehardiman explains consisely the scope of your inquiry.Attached is documentation of two hulls I have drawn using Hullform
These hulls havethe same basic dimensions. The drag difference is obvious. Each hull is "built" using Terho Halne"s spreadsheet ( in other posts) so the stability and performance estimates can easily be compared. Reading Richard Woods contributions is informative reading also. I might add that there are probably 1000 hesigns between these two modest design attempts. Have fun, I have. Regards, John

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SeanGT

JEH gave you a very good summary. However, is your desire for a basic understanding, simply to learn about "this stuff" in greater detail, or simply to know how much power your 4 foot model requires, and/or hence, what speed she'll do?

Since former, is a review of hydrodynamics and there are endless books about to assist in this, but the later, really depends upon many factors, which JEH noted. An understanding or rather, a review of hydrodynamics does not automatically equate to knowing what speed your boat/model shall do. The SOR (statement of requirements), what the boat/model is designed, for, is required to know what power/speeds is required.

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### yipsterdesigner

As is said above it starts with a mission reqirement
a statement of requirements that determin the design emvellop
A displacement model will fall under the hullspeed rule,and a Froud number
Shure catamarans and very high lenght beam can do better speeds on the same power
Planing and flying boats use some other physics but it starts
With requirements bfore design and calcs

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### rwatsonSenior Member

What scale is a model of 50 kilos, and 4 ft long ? It sounds way out of proportion.

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### jehardimanSenior Member

I didn't read it as being a model as the OP stated that he is "designing" a hull. 4 feet by 100 lbs is a pretty common size of AV's/UV's now.

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### rwatsonSenior Member

Ooooooh Ok, Whats an AV,UV ?

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### yipsterdesigner

guess that wont be flying boats, but eh.. x2 whats an AV,UV ?

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### Olavnaval architect

I guess AV/UV stands for "Autonomous Vehicle" and "Unmanned Vehicle" respectively; afaik these and similar terms are more common abbreviations with underwater research stuff: AUV, UUV, and ROV.

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### jehardimanSenior Member

Yep, I thought about using AUV/UUV, but there is no reason to overspecify it for underwater. AV/UV is a general term, ASV/USV is used for surface, UAV and less often AAV, is used for air. ROV now more implies a tethered unit. FWIW, this stuff is years old.

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### ancient kayakeraka Terry Haines

4', 100 lb . . . ! If we are discussing a submersible here, maybe we should confirm that first. There's a significant diference in hydrodynamics.

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### pistnbrokeI try

perhaps we should also ask what he intends to power it with and how many HP or watts..that will give us a ball park figure for the speed .....

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