Aluminium Frame

[DavidF]

Steve, my boat weighs in at 7.5 tons, and I can easily push it around with one hand and light pressure. Thats how I push it from one piling to the other when tying it up.

[sthone]

I'll take your word for it then, I'm not going to get into a debate about it. you can build your Rov as heavy as you want I guess.

-Steve

[turtle]

I'll take your word for it then, I'm not going to get into a debate about it. you can build your Rov as heavy as you want I guess.

-Steve

[DavidF]

As heavy as I want?? I cant afford that LOL and my back sure wont like it either!! The point I was trying to make was that the resistance to push a large flat faced rov would be high and would require more power to move it thru the water verses something with low frontal area. Kinda like trying to hold onto a sheet of plywood upright on a windy day. and no I would not recommend a trolling motor on the Titanic, but I dont think an aluminum frame is going to make any difference over a pvc frame in this case.
Now here is a good question for ya, If you have a 55 gallon drum full of water, how much does it weigh under water??

[bikerbones1968]

I will answer that for you!! A gallon of water weighs 8.34 pounds, 55 gallons of water then would weigh 458.7 pounds plus the weight of the barrel. For argument purposes lets say the barrel weights 20 pounds. So the total weight is 478.7 pounds regardless if it is on land or in water.

[DavidF]

I think im going to pull out a 5 gallon bucket, fish scale, and a video camera.... I could be wrong.....

[bikerbones1968]

By Archimedes' Principal it weighs less by an amount equal to the volume of water displaced by the volume of the object. Mass does not change, so mass of the barrel under the water is the same as above water. Now here comes the fun part.

If you tie a rope around the barrel, attach a scale to the end of the rope, and then slowly lower the barrel into water, you will see that the reading on the scale decreases by a large amount as the barrel sinks into the water. So your now thinking it weighs less?? This is the result of the water exerting an upward (buoyant) force on the barrel equal to the weight of the water which is being displaced by the barrel as it is being submerged.

However weight is another matter, weight = mass times gravity. Soooo technically the barrel weighs the same on land as it does in water. Mass is constant; weight depends on where you are in the universe. We use the terms weight and mass in most cases incorrectly. That apparent weight loss being buoyed up by the water will vary depending on the VOLUME of the barrel. As the barrel slips under water it displaces an amount of water equal to its volume. This water has weight. So the amount of water pushed out of the way is equal to the volume of water pushing back on the barrel providing it with a buoyant force. How much? The force is equal to the weight of water displaced. Are you confused yet?

The barrel underwater will seem to weigh less. And due to the bouyant forces exerted on it, it will measure less. Just rember mass is constant and weight is weight.

No wonder I hated Physics

[DavidF]

LOL Bones, I knew I liked you for some reason LOL... I was thinking you would only measure the weight of the barrell in a sense...

[thegadgetguy]

The other aspect of it is that acceleration = Force / Mass. The greater the mass, the smaller the acceleration at any given force value, making your ROV sluggish in the water - slow to stop and start. Slow to stop and start = extremely annoying

[DavidF]

The other aspect of it is that acceleration = Force / Mass. The greater the mass, the smaller the acceleration at any given force value, making your ROV sluggish in the water - slow to stop and start. Slow to stop and start = extremely annoying

I can see that as being an issue, picturing a 1000 lb rov heading for a brick wall under water and not being able to stop befor it impacts into it. "POP" goes the rov. But on a different note the mass could benefit in an area with swift and changing currents like in the surf. Im taking the easy way out with my rov, a tank treaded 1500 lb bottom crawler, there are down sides to this as well. Like falling into a hole or off a cliff and not being able to craw out