# Marine propulsion

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1. Dec 22, 2015

### weerakkodi

I have a some problem about marine propulsion. Assume the 4 engine-fast gunboat is moved by forward two engines. The boat get 10 knots. Then if other Aft two engines operate gradually. You know the boat doesn't get 20 knots speed. What are the reason for that? Explain theoretically? Thanks for all replies!!

2. Dec 22, 2015

### Staff: Mentor

Welcome to PF!

Power is not a linear function of speed, it is at least a square function of speed if not a cube function. Ie, if you double the speed and that doubles the drag, 2x2=4x the power is required to achieve it.

3. Dec 22, 2015

### weerakkodi

Thanks russ, if you please explain further why the ship doesn't achieved double speed than its earlier speed? what are the factors to reduce that speed?

4. Dec 23, 2015

### rcgldr

Complicating matters is if the boat goes into a planing mode, rising out of the water, it ends up going faster than expected.

5. Dec 23, 2015

### weerakkodi

Thanks rcgldr, if you please explain why ship cant achieve its double speed?

6. Dec 23, 2015

### weerakkodi

Thanks russ, if you please explain further why the ship doesn't achieved double speed than its earlier speed? what are the factors to reduce that speed?

7. Dec 23, 2015

### SteamKing

Staff Emeritus
The resistance of a vessel is not proportional to its speed thru the water, just like the resistance of an airplane or other flying object is not proportional to the speed of the plane.

For air drag, FD, the formula FD = (1/2) CDρ A v2 is often used, where
CD - non-dimensional drag coefficient, depends on the shape of the object
ρ - mass density of the air or fluid
A - frontal area
v - speed of the object relative to the fluid

Notice that in the formula for the drag force, the velocity v is squared. If an object has a known drag force at velocity = v, then the drag force doesn't just double when the velocity doubles, the drag force quadruples. Since the power needed to maintain a constant velocity, P = FD × v, then the power also must increase exponentially:

At v = v0,
FD(v0) = (1/2)CD ⋅ ρ ⋅ A ⋅ v02
P(v0) = F(v0) × v0 = (1/2)CD ⋅ ρ ⋅ A ⋅ v03

Now, if v = 2 v0

FD(2v0) = (1/2)CD ⋅ ρ ⋅ A ⋅ (2v0)2 = 4 ⋅ FD(v0)
P(2v0) = F(2v0) × 2v0 = (1/2)CD ⋅ ρ ⋅ A ⋅ (2v0)3 = 8 ⋅ P(v0)

If you want to double the speed v, you must provide 23 = 8 times the original power.

Vessel resistance cannot be computed using simple drag formulas like those shown above, but the relationship between resistance and speed uses a similar scaling law.

At very low speeds, a typical vessel only needs to develop enough power to overcome the friction between the hull and the water. At higher speeds, the vessel's movement thru the water starts to generate waves which become greater in amplitude the faster the vessel moves, and this wave generation causes a sharp increase in hull resistance and the power required to maintain speed.

Typically for a large vessel with a design maximum speed of say 30 knots, one-half of the installed power can drive the vessel at a speed of about 24-25 knots. It requires the other half of the installed power to increase the vessel's speed that additional 5-6 knots to reach full speed.

8. Dec 23, 2015

### weerakkodi

It is very excellent answer..Thank you very much sir..

9. Dec 25, 2015

### cjl

I think you misunderstood what rcgldr said - if at 10mph, the boat is almost (but not quite) on plane, then when you double the power, you are likely to get more than double the speed (so the boat will go faster than 20mph with all 4 engines). There isn't just a simple relation - it depends on hull design, powerplant design, propulsion mechanism, and several other factors.

10. Dec 26, 2015

### Ronie Bayron

In practical sense, adding another engine or propulsion does not increase its speed but definitely increase its load capacity (thrust).
Take for instance, you add up the same size of engine but rpm is low compared to the other, you wont have any increase of speed but most likely adds up drag.
When all identical engines are synchronized, you would have the same rated speed achieved, yet certainly it does not increase speed even if you add up infinetum.

Last edited: Dec 26, 2015
11. Dec 26, 2015

### NTW

The drag is, as you say, proportional to the square of the speed (all the other variables left equal...) and as the power is drag x speed, the power required is proportional to the cube of speed...

12. Dec 26, 2015

### SteamKing

Staff Emeritus
This statement is only half right.

It is the total thrust which a boat can produce that gives it the ability of to overcome the resistance of the water when it is traveling at speed. This is basic physics, which, believe it or not, applies even to boats. When the thrust output by a vessel matches the resistance of the hull at a given speed, the vessel will travel at that speed until 1) the thrust changes, 2) the resistance changes, or 3) some combination of 1) and 2) occurs.

I'm not sure what this last bit even means. Multiple power plants in a vessel do not need to be 'synchronized' in order to produce additional thrust to drive the vessel forward. Typically, vessels which have multiple power plants tend to be fitted with plants of identical size and type, not with some collection of random power plants which are thrown into one vessel.

The amount of thrust output can be proportional to engine RPM, but this is not always the case. It depends on the manner in which the torque of the engine is converted into thrust. There are several different ways this can happen.

An engine connected to a propeller by a shaft is a common way to convert engine torque to thrust, and the thrust generated by the propeller can be proportional to the RPM of the engine if the propeller has fixed blades, but there are some vessels which are propelled by controllable pitch propellers, where the angle of the blades can be set such that the propeller is spinning, but no ahead thrust is being produced, or the thrust produced is such that the vessel goes in reverse. This is often the case with large vessels powered by gas turbines.

A prime mover, like a gas turbine or diesel engine, can be used to turn a generator or alternator to make electricity. This electricity can then be used to turn an electric motor which turns a propeller. The prime mover usually turns at a constant RPM in order to produce electricity most efficiently, while the motor is set up to be able to turn at different speeds, and even go in reverse when called on to do so.

Some vessels don't even use propellers to generate thrust. Many high-speed craft use the engines to drive pumps, which suck water from under the boat and then shoot it out the back, making thrust by a simple reaction mechanism.

13. Dec 26, 2015

### Ronie Bayron

What do you mean half right?
Is there such logic as half right? It could either be wrong or right, simple as that.
I speak in critical criteria of beyond what you said.

Yes there is resistance to boat's thrust and that is directly proportional to speed of the boat. The boat can not exceed the ideal thrust generated the propulsion whatever it is, either water jet, azimuth, podded, fixed blade etc.- it does not matter.

There's no way you would increase boat's speed by adding up identical propulsion system. I would rather say, size up the engine,increase rpm and increase propeller's pitch angle, that's the way to increase speed.

I agree with controllable pitch propulsion engine you cited, but your logic wandered around the bush. I certainly mean fixed blade propulsion and adding up identical engine as what OP's query is directed.

14. Dec 26, 2015

### SteamKing

Staff Emeritus
I was being generous, I thought, in giving you credit for the part of your statement that was correct.

Adding another engine can add to the thrust the boat can produce. That's not saying that adding another engine, by itself, increases thrust. Obviously, other parts of the propulsion system may have to change as well to realize the thrust which the additional engine power can create.

Saying that adding another engine cannot make the boat go faster is in direct contradiction to saying that more thrust = more speed.

There is a one-to-one correspondence between the speed of a boat and the resistance of the water produced on the boat traveling at that speed.

Adding thrust cannot not make the boat go faster. How much faster the additional thrust can make the boat go, that depends on a several other factors.
So the glass can't be half full or half empty?
I'm not sure what 'critical criteria' means here.

It's not the thrust of the boat which creates the resistance to motion, but the motion of the hull through the water produced by the thrust which creates the resistance.
While the thrust of the boat is greater than the resistance of the hull at a given speed, the boat will accelerate, in accordance with Newton's laws of motion. When the thrust of the boat is exactly equal to the boat's resistance, then the boat will travel at a constant speed.
It's not clear what the 'ideal thrust' means here.

As has been shown above, the resistance of a boat is, in part, directly proportional to the square of the speed of the boat.
Thrust is thrust. The boat doesn't care how it's created, only that it's there to be used.

Boats with multiple power plants don't always have to sail with all of the power plants running at the same time. If you have half the plant going, your boat goes a certain speed. If you bring the full plant on line, your boat goes faster, just not twice as fast as before.

Practical experience shows that adding engines can increase the speed of a boat. You find all sorts of recreational craft with one, two, or three identical engines hanging off the stern as outboards. If one engine breaks down, the boat doesn't stop (unless it's single engine); it just goes a little slower.

We don't know that the OP's gunboat is powered by fixed-pitch propellers; he never said what the method of propulsion was, only that he was disappointed the boat didn't go as fast as he thought it should by bringing additional engines on line.

15. Dec 26, 2015

### Staff: Mentor

That's not really what I said. I realize in some situations (like air resistance) drag is a square function of speed, but I'm not sure that is true for ships because ships sit on top of the water, cutting a line, unlike air resistance where the whole cross sectional area is exposed to the air. Then displacement hulls also have to deal with their wake, which causes the drag it increase much faster past a certain speed.

Here's a graph showing the different possible shapes of the curve:

Drag in ships is a lot more complicated than air resistance.

16. Dec 27, 2015

### NTW

Yes, but to a first approximation, it can, I believe, be considered as proportional to the square of speed, specially if the hull keeps its displacement more or less constant.

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