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Boats/Submarines and the HP of the motor to speed of the craft

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Andy

Sat Dec 06 2014, 04:38AM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
Location:
Posts: 874

I'm trying to build a kontik, its pretty much a submarine that takes a longline fishing line out to a certain distance from shore then stop. I started a thread about control(direction finding, hopefully cheaper than GPS), but the data on the internet is hard to find out, what the HP of the electrical motor needs to be to move the sub through the water.
On one website they said 1hp for 10kg of displacement as a rough figure.
How did they work that out, is it just drag, inertia etc?

Cheers

Dr. Slack

Sat Dec 06 2014, 10:22AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

In the limit of zero speed, you need zero power.

How fast do you want to go?

You probably need the capability to go at least twice as fast as the strongest current you expect to encounter, so that you always have some control authority.

Then with your minimum target speed settled, you can start to google for drag coefficient, and use that through frontal area and total wetted area to some estimates of power needed.

Ash Small

Sat Dec 06 2014, 03:58PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

For displacement vessels, as opposed to planing vessels, there is a formula for finding the most efficient speed (for surface vessels). The formula is the square root of the waterline length (in feet) is equal to the most efficient speed in Knots. For example, a 50 foot vessel will have an optimum speed of ~7 knots.

Once you try to push the vessel faster, the power required rises dramatically. I don't know if this also applies to subs, but I assume either this principle, or a similar one holds.

There is also an optimum formula for designing the front of the sub, which I forget, but which explains the strange shape of the front end of nuclear subs, and also of some supertankers.

Andy

Sat Dec 06 2014, 08:43PM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
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Posts: 874

Do these numbers look right

,with a rought coffeinect of 0.5

Area = 0.15m2
Speed = 3m/s

Fd = 371.25kg
P(watts) = 3341.25

Using 28 motorcycle batterys with two induction motors and a VFD about 20amp/hours at 340volt 0.5 DOD 3.4kwh, so about 1 hour run time.

Carbon_Rod

Mon Dec 08 2014, 12:04AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
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Posts: 1155

GPS does not give direction like a magnetic compass, but rather measures distance over time to get the direction of travel.

There are several dozen GPS flight path control systems for RC planes that should work with minimal hardware changes.

Note that depending on the region many rules regarding fishing gear sometimes carry a large fine.
...besides the ethical ramifications it is unwise to copy another innovator's products.

Andy

Tue Dec 09 2014, 02:12AM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
Location:
Posts: 874

Thanks Carbon Rod

Note that depending on the region many rules regarding fishing gear sometimes carry a large fine.
...besides the ethical ramifications it is unwise to copy another innovator's products.

Longline fishing has a max of 25 hooks per length, but I think it might have dropped down to 20. What do you mean by copy another innovators product?, konitiks have been around for 200-300 years, I'm sure the owner or grandchildren would have passed away.

Sorry I'm not making a product, just gaining knowledge in subs, and catch some fish.

Carbon_Rod

Tue Dec 09 2014, 06:48AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155

A small trolling motor would likely cost less than a Diver Propulsion Vehicle prop. Assuming a roughly perpendicular to shoreline path, one may prefer a tethered inverse-gear rudder steering system controlled by simple surface line tension and a tiller eye linkage. One could use the same line to trip a thruster cutoff (similar to a lamp switch) when it is taut enough at a specified line length.
Additionally, using two feeder spools on a ceramic bullseye could allow you to cycle the lines several times without returning the float to shore. For example, letting both ends of the line out would increase travel distance, braking both would stop the vessel prop, and reeling in one (baited) while letting out the other (clean) would cycle the line back to shore without tangling. Remember to use a natural cork plug in the hull so if your gear is lost it will sink eventually.

A few low cost units may prove more effective than some relatively costly AUV.

Steve Conner

Tue Dec 09 2014, 10:33AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

1hp for 10kg of displacement would be nice, but you can get away with a lot less. The sculling boat I use is about 27ft long and displaces something like 110kg with me in it. I am good for about 1/2hp and my speed in calm conditions is about 4m/s or a little under 8 knots. Oddly this is well over Ash's rule of thumb for optimum hull speed of a 27ft boat.

A long, thin torpedo shaped submarine would probably have much the same drag characteristics as a long narrow boat. At these low speeds the drag is mainly due to friction between water and hull, so I don't think it matters if it is floating on top of the water or submerged in it.

For the fishing application you suggested, the drag of the line would have to be considered, as well as the force needed to keep station against winds and currents. That may well end up more important than the hull drag.

If the fishing application doesn't work out, you can always sell it to some Mexican smugglers.

Ash Small

Tue Dec 09 2014, 01:34PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Steve Conner wrote ...

1hp for 10kg of displacement would be nice, but you can get away with a lot less. The sculling boat I use is about 27ft long and displaces something like 110kg with me in it. I am good for about 1/2hp and my speed in calm conditions is about 4m/s or a little under 8 knots. Oddly this is well over Ash's rule of thumb for optimum hull speed of a 27ft boat.

A long, thin torpedo shaped submarine would probably have much the same drag characteristics as a long narrow boat. At these low speeds the drag is mainly due to friction between water and hull, so I don't think it matters if it is floating on top of the water or submerged in it.

For the fishing application you suggested, the drag of the line would have to be considered, as well as the force needed to keep station against winds and currents. That may well end up more important than the hull drag.

If the fishing application doesn't work out, you can always sell it to some Mexican smugglers.

The calculation I gave is 'very' approximate, but is pretty good for conventionally shaped vessels. There are plenty of other factors, and other methods. Kayaks are always used as an example of non-conformity. A vessel that has very little in the way of wake will exceed the speed calculated using this very basic formula, but the longer the waterline length, the faster the vessel will go. This is why ocean going liners got bigger and bigger, so they could cross the Atlantic faster.

2Spoons

Tue Dec 09 2014, 09:28PM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615

something like this?

These guys have been around a while. The specs on their system should tell you if your calcs are in the ballpark.

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