Propeller THRUST

Hi,
Comments Welcome,
Case A: Vessel with a inboard engine at 85HP, with some popeller delivering some mount of THRUST at X rpm.

Case B: new inboard engine at 100HP, others same than CASE A, same propeller, same X rpm.

Question: We have more or same THRUST in case B?

 

Could be designed for more or less. Identical 5.0L mercruiser engine. 2 barrel carb gives 220 hp @ 4800rpm. 4 barrel carb gives 230 hp @ 4800 rpm which will generate more thrust. Always the 2 barrel, as it has a higher manifold vacume which gives a higher maximum torque at a given speed. Raise one value, the other is reduced. Thrust, is the direct result of available torque. Dutch canal barges have 1 cylinder 700 ci. engines that turn 1 revolution every 2-3 seconds. Almost no horsepower, but they move the 5 to 10 ton barges with ease. Torque, is what makes everything on this planet move. Horsepower makes it move faster.

 

If you have the same propeller and the same RPM, you have the same THRUST, since :

THRUST = Kt * ro * (n)^2 * (D)^4

where :

ro = water specific weight (t/m³);
n = velocity (rotations per seconds);
D = propeller diameter (m);
Kt = is the trust coeficient, that is a function of the propeller's diameter and velocity.

By increasing the amount of power given by the motor, you may achieve a higher RPM, than you will have a higher Thrust.

 

Or.....
If you have more Hp at a fixed rpm (i.e. more torque at x rpm) you could up the pitch and get more thrust.

 

the best thing

Dear Williams,

If you want or need more thrust, you can change two features in your problem:the diameter or the rotation. As Mr Genjuro posted, the Thrust dependes of four values: the water density, the rotation (RPS) and the propeller's diameter.
In my opinion, the best way to have more thrust is modifying your propeller's diameter. Because the Diameter is (D)^4. That way you will have a increase in your thrust with the same rotation of your engine. And a problem with four variables reduce in one, just the Diameter.

Thrust = Kt x ro x n² x (D)^4

 

If we have a same THRUST (T), but not same engines and following with the sample of 100HP and 85HP, so:
Case A: engine working at 85HP (Input) given a thrust=Ta
Case B: engine working at 100HP (Input) given a thrust = Tb

Same propeller, same rpm, so Ta=Tb (Output), and thinking a energy level where goes the 15HP of Input energy if Output is the same?

 

Well, if you have the same RPM in both cases, I would say that your engine B is not running in it's maximum continuous rate, or it is much less efficient then engine A.

In the first case, the engine is not using it's full capability to develop torque. In the second, the engine is wasting energy in ways of heating.

 

the right propeller sizing and selection is based in the 100%MCR of the engine?

 

Yes, all the hull-propeller-engine integration must be considered, and doing so, you should select the most effective propeller for the maximum power you can deliver to it.

Actually, saying 100%MCR is quite confusing, because MCR already stands for the maximum power the engine can develop and sustain, although some times you may pass this point.

Not always your engine will be running 100% power, this is just a design point. For example, the engine B, may not be running in it's MCR, but a little lower, this would explain why it is delivering the same RPM of engine A. In this case you would have more power to develop, increasing the RPM and of course, the Thrust delivered by the propeller to the water.

In selecting the right propeller, you should fix some variables in order to solve the problem. Usually you fix your propeller max. diameter, since there is a limit given by the boat's draft. Then, you may observe the possibility of propeller racing occurrence, by making a seakeeping study (of course there are more easy ways to do this, mostly by pratical experience).

Once you've determined the propeller diameter, you may define others properties like the Ae/A, P/D and the number of blades. Done that, you can now define the propeller's velocity (you will need the propeller's diagram of thrust, torque and efficiency), this will be given by the point where the prop. achieve the required Thrust (assuming that you already have made the boat's resistance calculations, and have estimated the wake and the thrust reduction factor).

Usually, there is a power margin added to the final required power, that will be your MCR. For instance, if you add a 10% margin, you may have a Normal Continuous Rate (NCR) that is 90% of the MCR. This may be the point the boat will operate in most time.

 

For workboats (24/7/365) the most common setup on gensets is 80% of rated load ar 90% of rated rpm.

This works fine as an engine guide , but most engines are oversized for sea state requirements.

A vessel may need 40% MORE hp to keep the same speed in heavy going , but few will have that excess avilable.
20% is more reasonable , and you just slow down a bit in the REALLY rough stuff.

So the propulsion engine selected would be run at 70% of rated Hp at 75 or 80% of rated RPM, for longest service.

FAST FRED