Prop Wash

[bendyone]

I have a simple question. - What is the best design of prop to reduce to a mimimum propwash?

I have a Hunter 26 with a Yanmar GM1 which kicks to port in reverse. At the moment it is fitted with a 2 bladed prop (size unknown)

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[Talbot]

It is my understanding that the fitting of a gori or brunton autoprop will not only make sailing a more enjoyable experience /forums/images/icons/smile.gif, but will also considerably reduce the wonderful kick.

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[MIKE_MCKIE]

All things considered, propwash can be very beneficial in many circumstances. Turning short round is one, berthing is another, but I find that if you plan ahead, you can make it work for you, as opposed to giving you a headache. Almost all boats have right handed props which give a left handed kick to the stern, when going astern. That is why many, many boats tie up port side to. You aim the bow slightly left of the intended point & approach from 20-40 degrees off the berth. On going astern the back kicks in a lot, & hopefully the bow kicks out a little, thus (all things being equal, all going well, weather & god permitting!) putting you parallel to the berth & stopped. Having done it over the last 40+ years, on everything from 20' boats to 50,000 ton motor ships, I can tell you it works (most of the time!!!) Doing the same thing without propwash can be a right pain.
There will always be the odd occasion when it doesn't work, but that goes for anything. I have a long keel sloop which has a lot of longitudinal stabilility, ie it don't turn on a sixpence, & the thought of getting it into & out of the berth in Brighton WITHOUT propwash would certainly add to the gaiety of the situation & no mistake!
Brgds
Mike

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[snowleopard]

<Almost all boats have right handed props >

this is true of boats with conventional prop shafts. saildrives tend to need left handed props.

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[Benbow]

You are describing prop-walk not prop wash. Prop wash is an even more useful and controllable effect where you put the helm hard over when stationary and give a squirt of power. The water flowing over the rudder acts like a thruster and pushes the stern in the corresponding direction.

I agree that prop walk can be a mixed blessing, I have the same boat/engine that you describe and reversing in a straight line can be challenging, the secret seems to be short bursts of power until you have enough way on for the rudder to work.

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[DMGibson]

The effect you describe is caused by the physics of the propellor and the water - the water at the bottom of the propellor is denser than that at the top, so it causes a difference in the thrust from the top and bottom edges of the propellor. The effect is small, and only noticable when the boat is hardly moving - once at speed a slight adjustment to the helm is all it takes to cancel it out.

The effect is more noticable on bigger diameter props, so the only way to reduce it is to go for a smaller diameter prop which will also mean changing the reduction ratio on the gearbox. A smaller prop needs more shaft revs for the same engine power so you will need a lower reduction ratio.

Unfortunately, the propellor efficiency also drops with a smaller diameter, so you won't get so much thrust and hence go along more slowly.

Most propellor designers go for the biggest prop they can fit in the space provided, to get the greatest efficiency, and you then just live with "propwalk" and make use of the effect when coming alongside as suggested in the other posts.

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[Benbow]

That is correct, so just to clarify;

Propwalk is a big effect when boat is stopped and engine is in *reverse*, little or no effect when engine is in forward. Caused by paddlewheel effect - the bottom of the prop 'walking' the boat sideways.

Propwash is a big effect when boat is stopped (or nearly stopped) and engine is in *ahead*. Caused by water pushed from the prop being redirected to one side by the rudder. No effect in when engine is in reverse because the water is being squirted forward away from the rudder.

Both effects vanish as the boat gathers way and the rudder works on water flowing past the whole boat. As has been said both effects can be used to spectacularly increase your control in tight situations. On big long-keel boats I have sailed, prop wash tends to be a very powerful effect which is used as a basic manoeuvring tool.


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[jimi]

But if you've got a longkeeled boat like a Twister or Tradewind where the propeller is in a "hole" in the rudder, I understand the propwash effect to be very small. It is maximised with a skeg hung balanced rudder just behind the propeller

<hr width=100% size=1>.. whit way roon should it be again ..

 

[Benbow]

Yes I am sure that you are right that it depends a lot on the prop/rudder location and type. I can see that a balanced rudder would increase the effect by catching more of the prop's stream. I can tell you that the effect is spectacular on this boat, which as almost exactly as you describe:

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[BrendanS]

You left a space between [image] and http


<hr width=100% size=1>There is no such thing as "fun for the whole family."