Adding Kort nozzle to an existing propeller

[TrustedShips]

Hi,



I am a new member here.

I would like to know the opinion of senior people here with an experience in propeller design about the following issue:

If there is an existing barge designed for river voyages, how much additional speed can she gain by adding a Kort nozzle to its propellers.

Number of propellers = 2
BHP = 2 x 325 (1500 rpm)
Propeller RPM = 400
Max speed attained is abt 8-9 knots
Target speed = 12 knots (without touching hull shape and engine)
Cb = 0.88

Regards,
Devu C

[Submarine Tom]

5 - 10 % at best.

-Tom

[gonzo]

Nozzles usually give less speed but more thrust. That is why the common uses are for trawlers, slow tugs, etc.

[Frosty]

Thrust equals speed does it not.

[baeckmo]

Are you shure the rpm's are propeller shaft speed? If so, your propellers are working very highly loaded, and would certainly benefit from nozzles.

Generally speaking, we use "Bp" (=rpm*HP^0.5/Va^2.5) as a measure for propeller loading (Va is speed of advance at propeller position, in your case ~0,8*boat speed). With Bp>=60 and upwards, nozzles should be considered, particularly if propeller diameter is less than optimum due to draft restrictions.

[Tackwise]

To explain gonzo text with a bit more words:
Generally speaking the following applies to nozzle propellers:
At lower speeds a nozzle generates additional thrust compared to an open propeller. At higher speeds the nozzle will no longer create thrust but drag.

This is nicely illustrated in the attached graph, which shows a rudimentary Propeller graph for a Ka propeller in a 19A nozzle. The graph shows the thrust coefficient of the Nozzle (KTN), and the total thrust coefficient = KTT (thrust of both nozzle and propeller combined). If you look at the graphs of KTN, you will see it drops below the zero line at J = 0,9! Above J =0,9 the thrust of the nozzle is therefore negative which equals to additional drag! (note: J does not equal the ships speed but is a coefficient related to the +/- ships speed divided by rpm and the propellers diameter)

Now as usually, the problem is far more complex than can be explained in a few words. At which speed a nozzle is no longer effective depends on the situation: (speed, rpm, hull shape, navigational profile, etc, etc etc!!) To answer the OP question: A lot more information will be needed to determine whether a nozzle will be effective! However based on the supplied information I can make the following comments:
1) I understand you want to use the original open propeller: This will generally be less effective as the propeller and nozzle are not designed for each other! You will for instance lose efficiency due to the gap between propeller and nozzle.
2) The stated speed of 8-9 knots is generally a speed for which a nozzle can be effective on inland water way vessels, however not always!
3) Depending on the water depth, the amount of additional speed achieved with a nozzle may turn out to be minimal!
4) Using a nozzle will generally also mean changing your propellers rpm. Again generally speaking a propeller in a nozzle needs a higher rpm compared to an open propeller. This may mean a new gearbox!

For Inland waterway vessels using nozzles has the added benefit that nozzles are very forgiving when the operational profile does not follow the design profile of the propeller! In the case of IWW, the operational profile can change very often (varying water depth/blockage coefficient of waterways, etc, etc)

[gonzo]

Nozzles create drag, so at higher speeds the extra torque is less than the extra drag. The cross point where that happens depends as Tackwise says on many aspects of the design.

[PAR]

Nozzles have the added benefit of directing the wash directly aft, which is good if operating in very shoal conditions or near shore. They're easier on the bottom, not ripping up sea life and easier on things along the shore, experiencing less prop wash related turbulence.

[TrustedShips]

Much impressed with all the replies. Thank you very much everyone. It was far beyond my expectations.

[TrustedShips]

Originally Posted by Submarine Tom

5 - 10 % at best.

-Tom

Thanks Tom. Could you please provide some links to what these figures are based on.

[TrustedShips]

Originally Posted by gonzo

Nozzles usually give less speed but more thrust. That is why the common uses are for trawlers, slow tugs, etc.

Thank you Sir. I went through a lot of online reference material, including that on this website, and the bottom line of all that was the same as you said. However, it was difficult to find some specific details.

[TrustedShips]

Originally Posted by Frosty

Thrust equals speed does it not.

Thanks a lot for that query. That was exactly what I wanted to know. I read somewhere that Kort Nozzles or Rice Nozzles are used on tugs to improve the bollard pull. I puzzled as to why that thrust can not be translated into speed.

[TrustedShips]

Originally Posted by baeckmo

Are you shure the rpm's are propeller shaft speed? If so, your propellers are working very highly loaded, and would certainly benefit from nozzles.

Generally speaking, we use "Bp" (=rpm*HP^0.5/Va^2.5) as a measure for propeller loading (Va is speed of advance at propeller position, in your case ~0,8*boat speed). With Bp>=60 and upwards, nozzles should be considered, particularly if propeller diameter is less than optimum due to draft restrictions.

Thanks you very much Sir. That is a nice piece of information.

The RPM that I mentioned was that corresponding to Engine BHP, at the Engine output end. There is a gear box, after which the propeller shaft rpm reached to about 400 per minute. (adding it above)

The Bp value in this case is coming below 60.

But I am interested in speed not more than 12 knots. So is there any modification that I can do.

[TrustedShips]

Originally Posted by Tackwise

Generally speaking the following applies to nozzle propellers:

Now that's what I call an expert advise. I am still studying this. Will keep you informed.

Thank you very much Sir.

[TrustedShips]

Originally Posted by PAR

Nozzles have the added benefit of directing the wash directly aft, which is good if operating in very shoal conditions or near shore. They're easier on the bottom, not ripping up sea life and easier on things along the shore, experiencing less prop wash related turbulence.

Yup, that's another thing which we in India are not providing much attention to.