propulsion of kort nozzle

[kschung]

now i`m analyzing and comparing the ducted propeller using B4-70 screw series chart in nozzle 19A and conventional propeller without nozzle using B4-70 screw series. i found that the thrust coefficience of ducted propeller is lower than the conventional propeller without nozzle. In analyzing i assume the RPM is constant. anyone answers my question?

how the increased advance velocity produces greater thrust?

[jehardiman]

Use of an accelerating nozzle does not increase absolute efficency or thrust, what it does is flatten the loading curve and allow higher loading at low ship speeds, U. It fools the propeller into thinking that it has a higher speed of advance, Va, than it really does. For the same RPM and ship speed I would expect the thrust coeficient,Kt, for the nozzle to be lower than an open water wheel for similiar sized propellers.

Edit

Remember a high Kt is not the same as high thrust, but rather is a ratio of how much energy can be added to the water by the torque applied to the propeller as a function of Diameter, RPM, and speed of advance.

[DaveB]

The nozzles section is foil shaped and generates lift (thrust) as the water flows over it. At low speeds the lift it creates is greater than drag so there is a net increase in thrust, but at higher speeds it causes more drag and so isn't beneficial...

Just my two cents...

[kschung]

thanks for answering. Mr. Jehardiman, you mean i can`t compare the both charts with same RPM, diameter, pitch ratio ? assume the RPM, diameter, pitch ratio are same for both propellers, but just attach nozzle to the one of them, what do you think about the open water test result for both? i still get the lower Kt for the unducted one? how you explain the objective of the ducted propeller is to improve thrust for heavy load vessel and low speed?

[DaveB]

The best way to illustrate this is to look at a kt plot for similar props, one ducted and one open... I realize that there are other subtle differences, open props have rounded tips while ducted ones are squared off, but I've seen some that are close enough... same p/d, BAR, etc... if you look at the kt vs J curve the one for the nozzled prop is higher for low J and then drops below the open one as J increases... That's the cut-off for design... if you plan on operating at speeds (J=Va/(nD)) above the intersection you're best without a nozzle...

Having said this, I'm referring to common accelerating nozzles like the marin 19A and others in that series, as this was the type referred to in the beginning of the thread. There are straight nozzles that are intended just to protect the prop and then there are also decellerating nozzles which are used to make the flow into the prop more uniform... These other nozzles don't tend to provide more thrust.

[jehardiman]

Originally Posted by kschung

you mean i can`t compare the both charts with same RPM, diameter, pitch ratio ?

As I understand your original post you are comparing a B4-70 in open water to a B4-70 in a nozzle. As DaveB points out, you should use something like a Ka4-70 in the nozzle, but I will address your question of identical propellers. You could use a open water chart for both, but you have to adjust the J for the one in the nozzle to account for the flow acceleration. If the inlet area is 10% larger than the section at the prop, then Va is apparently 10% greater and therefor the J(nozzle) is greater than J(open water). Looking at the curves for Kt vs J and you see that as J increases, Kt decreases. So we should expect that the Kt(nozzle) should be less than Kt(open water) for identical RPM, diameter, and pitch ratio.

As an aside, you could redraw the propeller curves for J as a function of ship speed (U) vice speed of advance (Va) or as a function of the speed of advance of the nozzle (Va(nozzle)).

Originally Posted by kschung

assume the RPM, diameter, pitch ratio are same for both propellers, but just attach nozzle to the one of them, what do you think about the open water test result for both? i still get the lower Kt for the unducted one?

No, you will get less thrust and a lower Kt for the ducted one, BUT the power absorbed by the ducted one will be less for the same RPM, diameter, and pitch ratio! . Notice from above that the J(nozzle) is greater than the J(open water). This means that Kt(nozzle) AND Kq(nozzle) are less than Kt(open water) and Kq(open water). For most loaded propellers at low J, increasing J increases efficiency (eta).

Originally Posted by kschung

how you explain the objective of the ducted propeller is to improve thrust for heavy load vessel and low speed?

Remember the idea behind powering is to get maximum thrust for minimum power at a given ship speed. You keep holding RPM constant in this problem. This is the wrong way to compare a propeller in a nozzle to a propeller in open water! For a fixed RPM, a nozzle reduces thrust and torque, which decreases power requirements and increases efficiency. Conversely, if you allow RPM to increase for the nozzle propeller, you get more thrust for the same power. This is why slow speed, heavy loaded hulls such as tugs, towboats and fishing craft use nozzles.

PS Maybe you should move this topic out of "Education" and into somewhere more appropriate, such as "Design". You may get more and better answers there.

[kschung]

thanks for both help. you both absolutely solved my problems.

[Joel]

so the nozzle is shaped like an air-foil which reduces drag...

thanks!

[jehardiman]

Originally Posted by Joel

so the nozzle is shaped like an air-foil which reduces drag...

NO.... all nozzles have more drag than an un-ducted propeller. What the nozzel does is manipulate the speed of the water flow into the wheel to change the apparent speed of advance, "J". Depending on how good you were at it, the thrust increase for a given horsepower input will be greater than the increased drag and you will see increased overall thrust. Miss the optimum point and overall net thrust will be less!

Hmmm... looks like i got troll bit....