Prop design

Flossdog

Flossdog

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To me, getting a rope around the prop is a big nightmare, especially where I sail, big tides, big rocks and often big winds! Why has the weedless prop design not caught on? Or what about a flat ring around the prop? Both would, I believe, help stop ropes (pot ropes) getting caught around the prop. I haved a disc rope cutter which doesn't work and will this winter, fit a stripper or spurs but why not prevent the problem in the first place?

Has anyone got any interesting comments?
 
For a propeller to work it has to suck water in. Preventing solid objects from entering the flow while still allowing a free flow of water is not particularly easy.
Evadne has a long keel which is less prone to getting ropes around the prop, although not immune, because it has a gert big keel in front of it. This has an impact in performance under power, both forwards but especially in reverse. Many modern yachts do not suffer this because their prop is in free water, but it is a price I am happy to pay.
 
Its all down to efficiency but also prop life, the low power weedless props would not be able to impart the ammount of thrust needed on a large vessel when you want to accelerate to or stop from 6 7 knts or more. The blade root needs to be quite thick as it is running under high loads.

I also think a weed cutting edge wouldn't be enough to cope with much in the way of rope or line.

The rings and protection don't help much as Evadne says as the prop sucks in water and rubbish. The latest fishing boat catamarans suffer worse as the prop is close to the edge and water is sucked down from the side of the hull, so even floating debris can find its way down at the prop.
 
Flossdog said:
Why has the weedless prop design not caught on?
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You raise a good question. British Seagull offered 2 weedless props, differing only in the hub attachment for reversable gearboxes. As to why this type of design never caught on - I can only suggest that this is probably not the most efficient design from a fuel and performance perspective.
Or what about a flat ring around the prop?
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Now that is easier to answer. The periphery of a propellor moves the greatest distance through the water, and at the highest speed - therefore an increase in wetted area at the prop's periphery will create a significant reduction in performance through added drag.
 
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I read somewhere that propellers on modern ships are only about 5% more efficient than the ones Brunel fitted to one of his ships (was it the SS Gt Britain?)

Not sure if that's true, but I always though it quite astonishing that we haven't made more progress than that with modern computer modelling. Also a tribute to Brunel's abilities.

Apologies for thread drift - but it is about prop design at least:)
 
electrosys said:
You raise a good question. British Seagull offered 2 weedless props, differing only in the hub attachment for reversable gearboxes. As to why this type of design never caught on - I can only suggest that this is probably not the most efficient design from a fuel and performance perspective.

Now that is easier to answer. The periphery of a propellor moves the greatest distance through the water, and at the highest speed - therefore an increase in wetted area at the prop's periphery will create a significant reduction in performance through added drag.
Click to expand...

Having spent many hours chugging up and down Poole harbour when we were developing those props (in the late 70's) all I can say is that they were better than the fans we used before. Our toughest test was to get up the Frome to have lunch on Wareham quay when the weed cutters had been at work and the new props did stop the weed balling up and stalling the engine.

However you need to remember than the power they were transmitting was puny and not sure the design could be scaled up. They would probably pick up anything tougher such as ropes and nets which is the problem with most props. The only answer is an effective cutter which will stop the rope etc from balling up in the centre and particularly getting round the exposed shaft.
 
sarabande said:
but what about the end-plate effect of having a cylindrical ring at the end of the prop ? ("ducted propeller")
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Ah yes - forgot about that ... Like end plates on rudders and keels to stop the water from slipping over to the other side. Trade-off between increased drag and blade efficiency, then ? Sounds like a good premise for an experiment.
 
It's evident that open propellers allow water to slip outwards - as you can see by the swirlings on the water. A ducted prop acts as a jet exhaust does, and squirts the water faster backwards.

(Sorry for the technical language :) )


I can remember the RNLI "Beryl Tollemache" stationed at Eastbourne in the late 50s had two long tunnels running from midway to the stern. In these tunnels were the props, with a clearance of about two inches. The disadvantage was that you could not get as much lateral thrust (prop walk).

:) I have just found that she is still in commission.

http://www.trawlerphotos.co.uk/gall...title=beryl-tollemache-ex-eastbourne-lifeboat

They knew how to build them in those days!
 
I am surprised that the Kitchen Rudder was not more widely adopted. It appears to have found more use over the years on model boats than on full size ones, though the principle involved is used on jet engine reverse thrusters.
 
Perhaps because the simple fixed prop in front of the rudder is pretty idiot proof and generally effective. You can trawl the literature and find all sorts of clever devices supposed to overcome the "failings" of the simple set ups, but none have ever really caught on in leisure boats.
 
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