Caged propellers

Regarding a displacement boat, 120 feet long, dwt of 220T, 620Kw power, swinging a 1200mm prop, would a ring cage improve overall performance or not.

 

Depends on the type of cage. If it is simply a safety guard, then they will probably have a detrimental effect on prop efficiency.
If you're talking about a ducted prop (which means a properly shaped duct with tight clearance from blade tips), then the overall efficiency will increase at low speeds, with the peak gain at zero forward speed (infact they are used for boats requiring high bollard pull) and will steadily deteriorate at higher speeds. Infact they are commonly used for boats requiring high bollard pull (tugs, for example) and in aeronautics for UAVs (which need an efficient hovering capabilty).
You can find a discussion (plus some friendly and constructive flames between me and Rick W ) on that topic here:
http://www.boatdesign.net/forums/boat-design/shrouded-propeller-v-s-nonshrouded-opeller-24756.html

 

Taking a 1200mm 4-bladed prop running at 800rpm at 10kts will require 500kW on the shaft. The pitch is 700mm.

If you run a shrouded 4-blade prop of the same diameter and same speed the power drops to 320kW.

If the ship is more easily driven and capable of 14kts at 500kW unshrouded the power drops to 390kW with shrouding.

So a considerable fuel saving would be possible with a well designed shroud. The more easily driven the vessel, the less benefit from the shroud.

The prop and shroud need to be matched to the vessel to get the best result. The design would be based on the target cruising speed. If it is a work boat required to tow then the shroud will provide even greater benefit. For the purposes of the shroud design the velocity ratio is 1.6 in the later case and 2.1 in the 10kt case.

The above calculations are open water prop conditions operating without significant interference from the hull.

Rick W

 

Have a talk with Olds Engineering in Maryborough I think, they seem know about these things, regards from Jeff.

 

The attached link shows the section of a proper shroud. The area ratio is the same as the velocity ratio for best results:
http://commons.wikimedia.org/wiki/File:Kort_nozzle_2.png

Rick W

 

To shroud or not to shroud

I think this may help...or not!

 

With a 1.2m prop, I really think you mean a duct (i.e kort nozzle or shroud, a device to change the inflow/outflow characteristics to the prop), not a cage which is fitted to exclude debris up to and including people. If what you really mean is a cage, then it will always decrease performance and efficency.

As daiquiri pointed out, a ducted propeller will never match the maximum efficency of an unducted prop. In certian situations where prop diameter is limited, you can improve apparent efficency during the case of extreme blade loading, like daiquiri and Rick W mentioned, but if you are not a towboat or a torpedo, then a properly designed prop would be better.

See the following two threads which ask the same question you are about ducted and unducted props.

http://www.boatdesign.net/forums/education/propulsion-kort-nozzle-4525.html

http://www.boatdesign.net/forums/education/just-question-5547.html

 

Thanks fellas, yes I was thinking of along the Kort Nozzle type of shrouding.

Unfortunately, due to the owners desire to have a garage in the back of the boat we now have a large flat transom area, this has reduced the diameter i had allowed, so I now have a boat with a 1200mm prop.

Rick, I am surprised how much difference it can make, the hull speed is 15knots, it needs the 620 to do that, but certainly we would be running more economically down to about 10-12 knots max for most open water operations.

The vessel is designed as an ocean explorer, we have come a long way with the project, and it does look like I will be doing the shrouded prop thing.

The yard in China where i currently work is making sea tugs at the moment for Singapore, so I have seen first hand the shapes etc, but wondered if they would be suitable for our baot as it certainly in not designed por pulling, but if it helps so much in the push stages too, then we will be doing it.

Thanks again for the links, I will do some more homework on the subject as we progress now.

 

I would need to look at the drag curve to estimate the likely benefit. If it only requires say 250kW to do 10 knots and that is economic cruising then the benefit might not be great. It would certainly help in heavy going.

As far as I can determine the best nozzles are produced by Rice:
http://www.ricepropulsion.com/
But I have no direct experience. It is that their technical information agrees with what I have determined as the best solution.

To do a proper analysis you need to actually design the nozzle and see what extra drag it contributes. I only made a guesstimate in my prediction of the drag. I also did not make any allowance for wake factor so the lines at the aft end could be an important consideration. Obviously the faster the boat goes the more significant the drag from the nozzle and the less benefit from reducing the induced drag on the blades.

A good nozzle with a matching prop will effectively eliminate induced drag and this is a very significant loss when the prop is heavily loaded.

If you do move down the path with a reputable nozzle supplier I would be interested in any technical data you are able to offer for my own education.

Rick W

 

Landlubber

You're facing the age old 'compromise' issues of design, fun but not not always!

I'm not sure where others have interpreted the 620kW as being at the prop, this could be at the gearbox, or where 4 blades comes from as these are not in your post either. What is the RPM at the prop too? What tip clearances do you currently have, how many blades, what is the design speed of the prop?..are there any appendages which may affect the flow? Too many Qs really that need answers before any real quantitative answer can be sought, otherwise it'll be just qualitative.

Bottom line is, you need to establish, unless you already know, what the SOR of the boat is, what is important and what is not.

Such as, you say the lines aft have now changed. Do you have a tank test report of the 'before' hull? Have you told the client the effects of the change, since all these may have more of an effect and any gain in using a shroud may well be minor.

A shroud will only help if you can establish the effects and if the effects are at the design SOR. Since there is no point having a shroud if the gain is at a speed range which is not applicable to the requirements of the vessel.

 

The original design concept was to use an exiating small coastal freighter hull, and do a "nice" super onto it, thus negating a lot of base work in design, underwater at least.

we have now progressed a lot on this project, Tad was a great help along the way with my basic underwater shapes, and now the owner has done his bit upstairs. He has employed a local drawer to do renderings of his desires and we now have a complete boat. It will of course now have to go back to basics and have hydro testing done as well as a tank model test to see just what we have done.

It was in the early stages that the owner desired his garage for the toys, so the hull shape wss totally altered here to suit his accomodations (he is the owner after all), I lost my big prop and straight shaft, and ended up with a nice sized engine room, but now we have to go 10degrees to get the 1200mm prop into the hole allowed. I have 10% prop tip clearence at that.
We do not have an engine yet, that is another story as I desire a midspeed diesel of about 500hp, we actually need about 750hp now with the shape we have to drive the bloody thing, and the owner wants it to do hull speed (15 knots). We will certainly rarely do this as it uses nearly twice the fuel of 9 -10knots, so I am hoping to come up with a decent arrangement for him to do the cruise speed as economically as possible, and run her out if we ever have to do hull speed, maybe to clear weather or some unforseen circumstance.

The prop has not been designed yet at all, i was thinking of 4 or 5 blades for smooth and quiet operation.

Hope this enlightens more.

 

Landlubber

wow, you've got your hands full there!

Again, difficult to say with seeing the underwater shape and location of the prop. However:
10% tip clearance is not enough you need as a minimum 15~17%, 20% is better if you can.
10 degrees, wow, not a small angle, this wont help.
5 bladed will be better lower forces on each blade, but you'll need to check the BAR against cavitation too. 5 blades are also less likely hood of vibration problems in terms of resonant frequencies.

With such a wide variations in the design, as you noted, the prop efficiency will have an affect where you place the eff.Opt for your prop. Hence, which is more important, the 9~10knots or the 15knots, cant have eff.Opt for both.
Will you use Gawn Burrill charts?

But all this is rather wasted if the weight estimate is wrong...so lots to do

 

Landlubber, if you browse my gallery theres a rice nozzle(twin engine) that I did the line of shaft for on a mates motorised crane barge, one of his "issues" was to keep as shallow draft as possible & as such these reduced the compared to open prop tip clearance & therefore the draft & for his purpose the pushing & pulling capacity/control, as the vessel is still to be launched we havn't seen the goods at work yet but the theory all seems sound & the quality & service fron the suppliers(Olds Engineering) was good. Check out the "up to" performance claims made of the Rice speed nozzle, something like this might suit even if you halve or quarter the "up to" advantage. All the best with the project & regards from Jeff http://www.olds.com.au/

 

The new flat aft hull shape, plus your problems with prop inclination, tip clearance and the need for quiet operation makes me think that you might also take into consideration the Compact Azipod (or similar) drives:
http://www.maritime-suppliers.com/imidocs/90580P006 Compact Azipod.pdf
http://library.abb.com/global/scot/...$File/Product Introcuction_Compact Azipod.pdf

 

daiquiri,

Yes, indeed we have looked at these, cost is a major factor in this baby, unfortunately, conventional shaft drive is still the cheapest means to go forward. Ta .