### Author Topic: Underwater Thrusters  (Read 1900 times)

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#### Lacrosse24

• Beginner
• Posts: 6
##### Underwater Thrusters
« on: August 28, 2015, 11:54:02 AM »
If I have a flat floating object roughly 6 feet long about a foot and a half wide weighing around 9 pounds, and it had 2 underwater thrusters fixed on the bottom side of the back end of this object with each thruster having around 9 pounds of thrust, what would its top speed be or can you even figure it out with this information.

#### cyberjeff

• Full Member
• Posts: 114
##### Re: Underwater Thrusters
« Reply #1 on: August 28, 2015, 07:10:04 PM »
If I have a flat floating object roughly 6 feet long about a foot and a half wide weighing around 9 pounds, and it had 2 underwater thrusters fixed on the bottom side of the back end of this object with each thruster having around 9 pounds of thrust, what would its top speed be or can you even figure it out with this information.

Apples and oranges.

Thrust does not have a speed component, it is force pure  and simple. 9 pounds of thrust will move 9 pounds, probably very very slowly.

Let's look at an automotive example with thrust equaling torque.  Torque gets you started, but horsepower determines how fast you can go and in your case you don't have a power figure, just the thrust (torque)..

Even if you could hazard a guess as to power, you don't know what the losses are.

#### Lacrosse24

• Beginner
• Posts: 6
##### Re: Underwater Thrusters
« Reply #2 on: August 28, 2015, 09:32:57 PM »
Ok, I'm starting to understand this.  I'm still trying to figure out how much power these underwater thrusters put out and the speed they could get this floating object up to.  If you know the rpm of the prop, the diameter  or size of the prop blade and the amount of weight at which its pushing could you find the top speed?

#### Lacrosse24

• Beginner
• Posts: 6
##### Re: Underwater Thrusters
« Reply #3 on: August 28, 2015, 09:39:06 PM »
Also with what you said about 9 pounds of thrust will move a 9 pound object very slowly, isn't that vertically, moving in the opposite direction of gravity.  I'm merely talking about moving an object in the x-direction of gravity across the surface of water which doesn't put as much pressure on the object as when moving completely against gravity's weight.  I might just be talking nonsense but it's worth a shot.

#### cyberjeff

• Full Member
• Posts: 114
##### Re: Underwater Thrusters
« Reply #4 on: August 29, 2015, 06:39:44 AM »
Ok, I'm starting to understand this.  I'm still trying to figure out how much power these underwater thrusters put out and the speed they could get this floating object up to.  If you know the rpm of the prop, the diameter  or size of the prop blade and the amount of weight at which its pushing could you find the top speed?

I have no experience with water craft, there are all sorts of rules of thumb for boats and trolling motors, none of which talk about speed.

https://www.trollingmotors.net/trolling-motor-thrust-guide

http://www.psychosnail.com/boatspeedcalculator.aspx

With that information (above) and taking a guess at drag, you could estimate speed. At some point you have to give up on calculations and just build it. There are too many variables. And too many choices, tugs use large propellers geared down, speed boats are designed differently. Drag is different at planing.

Getting something to work is the first step. Getting it to work well comes later.