Rim driven propellers and riblett skin

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1 September 2012 at 10:33 #3896
Paul T
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@pault84577
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1 September 2012 at 10:33 #36230
Paul T
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@pault84577
In my never ending quest for speed I am now looking at rim driven propellers and riblett hull coverings for model boats.

The rim drive reduces cavitation and increases the thrust efficiency, applying a riblett covering (shark skin) to the hull reduces drag and so increases speed.

Given that a few warships are using this technology I am surprised that the new Type 26 hasn't adopted them.

Paul

Edited By Paul. on 01/09/2012 10:39:43
1 September 2012 at 16:40 #36237
ashley needham
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@ashleyneedham69188
There must be some down-sides to this, possibly cost or noise generation? The actual blades do not look very hi-tech??

Ashley
1 September 2012 at 18:33 #36241
Paul T
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@pault84577
Ashley

I don't know about the cost comparison between Rimdrive and the normal prop shaft / prop arrangement but I do know that the Rimdrive is quieter and produces significantly less cavitation than traditional units.

Paul
1 September 2012 at 19:03 #36242
Colin Bishop
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@colinbishop34627
Paul,

An interesting concept. I see that Southampton University state that they have made rim drives down to 50mm diameter which is bordering on model size. Obviously building the propeller into the motor saves weight in running gear (couplings, shafts and props) but what sort of efficiency does this type of drive offer?

I have heard reports of the 'sharkskin' coating reducing friction but I believe that the RN are currently using a special paint which significantly achieves a similar result offering a greater top speed and more economical cruising. There are also bubble systems being applied to merchant ships which effectively reduce friction by pumping out air through holes along the bottom of the vessel introducing an air layer between the hull and the surrounding water.

Slightly off topic, there have been experiments with concrete road surfaces to try and reduce the very high tyre noise levels they generate. These involve grinding out shallow grooves along the length of the road surface. There is an experimental stretch just west of Ipswich and the effect is quite magical, apparently it doesn't compromise road holding either.

Colin
2 September 2012 at 02:41 #36248
big bill
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@bigbill
rim drive might be ok, but its hard to get away from the no complications of normal prop drive , that everybody understands .

also most full size improvements in drives , dont do much more in model form ,ie. feathering paddle wheels

good luck w your drive
2 September 2012 at 07:00 #36249
Paul T
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@pault84577
Here is an example of Riblets being used on an Airbus.

Its odd that a ribbed surface produces less drag than a smooth one.

These are cross sections of typical riblet types
2 September 2012 at 18:45 #36276
ashley needham
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@ashleyneedham69188
Hydro and Aero dynamics is a funny thing.

Golf balls with hexagonal dimples fly further than those with plain dimples!! (apparently).

I should imagine that for naval use there might be also be a question of battle damage to consider. The repair of such a unit would be nigh impossible I would have thought without proper facilities and they might be more susceptable to underwater shock. Just as a for instance.

The underwater coatings used nowadays I am led to believe are self abraiding, so organisms get washed off the surface by and by. The sharks skin coating may be another expense over benefit thing similarly? An underwater sharks skin would be difficult to keep clean and barnicle free.

Ashley ps do you think covering one of my boats with sand paper would be effective??
2 September 2012 at 18:59 #36278
Colin Bishop
Moderator
@colinbishop34627
Its odd that a ribbed surface produces less drag than a smooth one.

Not really if you think about it. Just imagine putting a sheet of sticky backed fabric such as Fablon on a smooth surface and then trying to pull it off. Then try doing the same on a ribbed surface. It will come off much more easily in the latter situation as there is less friction/stickiness to overcome.

This is also why modern ships. especially warships have transom sterns. The abrupt break in the water stream stops the surface layer of water 'sticking' to the hull and creating drag.

Ultimately it is all about reducing the friction between two moving mediums.

Colin
3 September 2012 at 09:57 #36284
Paul T
Participant
@pault84577
On a related subject the QM2 had a design change to the bow bulb, it was lengthened to alter the seawater wave form as it travelled down the length of the ship. This change increased efficiency and enabled the design team to achieve the speed required by the owner without increasing engine output.
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