Props and rudders, a techie problem!

[Mike2822425]

Last week you all helped me out with a fuel tank problem (diesle tank being used next for petrol) BUT now I have come across another prob!!!

In my Folkboat under total refurbe.

Previous engine was BMW diesel with large prop (poss. 2/1 reduced gearbox) now I am installing direct drive Dolphin engine with small ( about half the size) high speed prop.

Question is "do I have to reduce the cut out in the rudder for the Dolphin prop to increase its effiencey and by how much? I obviousley will put this to Dolphin but wondered if you have any comments!

It realy begs the question " does a lage prop moving (2/1) slowly, move more or less water than a small prop moving at twice the speed and what mods. if any do I need to do to my existing rudder!


Mike the garden navigator

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[ccscott49]

The cut out in your rudder, shouldn't need reducing, but you will see some difference in response to your prop/rudder, as the new prop wash, probably wont touch your rudder, and the prop walk will possibly be less, due to the reduced diameter of your new prop, but the new response just needs learning.

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[jbate]

Just a few thoughts:
Reducing the amount of prop wash passing over the rudder will have the effect of reducing the turning ability of the boat, this could significantly affect the handling of the boat.
You generally get a reduction in overall efficiency when reducing the prop diameter, although this will depend on how well matched the engine/prop are.
Check that you will not get a cavitating prop, i.e. that the prop can carry the thrust from the engine especially at low/stationary speed - this could really affect the handling and even safety of the boat if you get thrust collapse.
JB



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[oldharry]

does a lage prop moving (2/1) slowly, move more or less water than a small prop moving at twice the speed and what mods. if any do I need to do to my existing rudder!

In theory yes, in practice no. Visualise a tube with the diameter of your propellor. For each turn of the prop a certain quantity of water is sucked past it. The larger the prop diameter, the slower that volume of water moves. In other words a small prop will move the water quickly, the large one will move the same amount slowly.

In actual practice propellors are very inefficient, and the faster they are turning the lower the efficiency - there are tables available (somewhere - eg Lancing Marine) which give the level of prop efficiency for given siz/RPM/pitch.

The bottom line is that the faster the prop is turning, the less efficient it is at converting power into forward thrust - which is why reduction boxes are a good idea on heavy boats.

You will find your smaller faster Dolphin prop less efficient at moving the boat in difficult conditions than your old 2:1 large blade prop. It simply does not grip the water so well.

A different set of rules comes in with planing hulls, where the hull speed can as it were keep up with the prop speed, but prop speed on a displacement hull should not exceed around 1800 - 2000 RPM, otherwise the prop will cavitate and lose power.

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[neil_s]

Look on the bright side, your folkboat will sail faster with less prop drag!

You could fill in some of the propellor aperture, making sure you leave sufficient clearence for the prop blade tips and also putting the helm over. I think 10% of diameter is about right. This will improve the flow over the rudder and reduce drag even more.

Neil

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