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Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Interesting. As far as I can tell, for the 12V motor they use, it works out a little more than 1lb of thrust per amp, so the small motor is a little under 1/2hp and the big one is a little over.
The whole craft also seems to weigh about 15-20kg so it is a little below the 1hp per 10kg rule of thumb.
These numbers seem quite sensible given my limited experience of boats, but they are an order of magnitude below Andy's estimate.
As an aside: I read an article that suggests that the speed of a (displacement hulled) boat is roughly proportional to the one-ninth power of the engine power, if you assume that the bigger engine is proportionately heavier and causes the boat to displace more water, increasing the drag.
I take from this that the engine power of a boat doesn't really matter that much, there is a wide range of powers over which the boat will move at a useful speed. If Andy built his proposed giant kontiki with 28 batteries and 500lbs of thrust, it would probably not perform that different to the 20kg Seahorse one, except that it wouldn't notice the drag of the line.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
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Posts: 4245
Steve Conner wrote ...
As an aside: I read an article that suggests that the speed of a (displacement hulled) boat is roughly proportional to the one-ninth power of the engine power, if you assume that the bigger engine is proportionately heavier and causes the boat to displace more water, increasing the drag.
I take from this that the engine power of a boat doesn't really matter that much, there is a wide range of powers over which the boat will move at a useful speed. If Andy built his proposed giant kontiki with 28 batteries and 500lbs of thrust, it would probably not perform that different to the 20kg Seahorse one, except that it wouldn't notice the drag of the line.
The simple formula I gave above comes from the days of sail. Increasing engine power in a displacement boat won't increase the speed much, once you reach the hull speed, as the boat is trying to climb it's own bow wave. Submarines experience a similar phenomena, however I don't know if the formula is the same. If the boat is designed to climb over it's bow wave it starts planing. Some boats operate in the intermediate region and are referred to as semi-planing, or, more usually, semi-displacement.
The design of the submarine hull is important. It needs to contain everything inside, yet needs low drag. The bow shape is critical to get anywhere near laminar flow, and so is the 'turtle back', in order to get laminar flow into the prop.
What about stabilisation? Contra-rotating props or sail? Apparently, an 'X' arrangement at the stern is better than '+'.
If you're referring to the equation F = 0.5 * rho * v^2 * C * A in the wiki article, F is 337.5N for a C of 0.5, or about 1kW of power needed. The C of 0.5 is for a sphere, so a more streamlined hull has likely a lower value. A sphere of 0.15m^2 cross section will displace about 44kg of water.
Steve Conner wrote:
I am good for about 1/2hp
Whoa, please post a photo of your arms
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.
Looks non obvious to me. The Reynolds number, being defined as the ratio of inertial to viscous forces is quite high in water at these speeds and lengths, e.g. 10^4 - 10^5. A floating hull won't cause water to move very much, a submerged one will. I guess the effective cross section of a long and flat boat bottom is much less than that of a sphere. I'm not an expert about this, just a transient thought.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
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Uspring wrote ...
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.
Looks non obvious to me. The Reynolds number, being defined as the ratio of inertial to viscous forces is quite high in water at these speeds and lengths, e.g. 10^4 - 10^5. A floating hull won't cause water to move very much, a submerged one will. I guess the effective cross section of a long and flat boat bottom is much less than that of a sphere. I'm not an expert about this, just a transient thought.
The main factor here is moving the water out of the way. Water, or anything else, can't change velocity instantaneously. It takes time to accelerate. The trick is to accelerate it with constant acceleration, for greatest efficiency and speed. A kayak displaces very little of it's volume, most of it is out of the water. A sub displaces all of it's volume when submerged.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
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Ash Small wrote ...
A kayak displaces very little of it's volume, most of it is out of the water. A sub displaces all of it's volume when submerged.
A 100kg submarine must displace the same volume as a 100kg kayak, 100 litres of water. If the submarine displaced more than 100 litres it would rise to the surface, and if it displaced less it would sink to the bottom. It follows (to me at least) that the drag must be similar.
wrote ... Whoa, please post a photo of your arms
Racing boats have a seat that slides on rails, so most of the driving power ends up coming from your legs. You still need a fair bit of upper body strength, if only to keep your balance and transmit the force without injuring yourself.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Steve Conner wrote ...
Ash Small wrote ...
A kayak displaces very little of it's volume, most of it is out of the water. A sub displaces all of it's volume when submerged.
A 100kg submarine must displace the same volume as a 100kg kayak, 100 litres of water. If the submarine displaced more than 100 litres it would rise to the surface, and if it displaced less it would sink to the bottom. It follows (to me at least) that the drag must be similar.
wrote ... Whoa, please post a photo of your arms
Racing boats have a seat that slides on rails, so most of the driving power ends up coming from your legs. You still need a fair bit of upper body strength, if only to keep your balance and transmit the force without injuring yourself.
The simple answer here, Steve, is that the kayak has a lower drag coefficient. It produces very little wake.
It takes half the waterline length to accelerate 100 litres of water, and then half the waterline length to decelerate it again, a very smooth transition.
The sub, on the other hand, only has the 'pointy bit' at the front to accelerate the same 100 litres of water to similar velocities. It is this higher rate of acceleration that requires more power.
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Joined: Fri Jun 25 2010, 04:25AM
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an interesting read
I dont think you can compare a kayak/boat and a sub directly, as one pierces a free surface and the other is fully submerged - two rather different conditions.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
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Posts: 4245
2Spoons wrote ...
an interesting read
I dont think you can compare a kayak/boat and a sub directly, as one pierces a free surface and the other is fully submerged - two rather different conditions.
You can make some comparisons, but they are limited. Interesting article. Seems to back up what I've said
It also emphasises the parabolic nature of the bow and turlte back.
It follows (to me at least) that the drag must be similar.
Tend to agree, if the volume of water displaced by the kayak has the same shape as a submerged vessel. There are some differences, though, a) the submerged vessel has a top on which friction can occur and b) a kayak can displace water to rise above sea level, a submerged vehicle can't.
Interesting is the part on page 10 and 11 of the doc, where it says, an optimal hull width to length ratio is 1:7. I believe that depends much on the speed, since friction effects rise linearly with speed, whereas pressure drag with its square.
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Boats/Submarines and the HP of the motor to speed of the craft
