Propellors: how much drag?

[john_morris_uk]

Sorry if this has been asked before and kicked to death, but are there any independent authorititive figures for how much drag there is on a fixed prop? The Max-Prop site suggests 15% improvement in sailing performance - but they would wouldn't they and whole knot at seven knots is an attractive thought. We have never had a choice before (Sigma 33s always have two bladed folding props) so with the next boat, I am beginning to draw up the wish list. Of course the price of a Max prop at £2000 odd quid might persuade me as well!

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[Jacket]

Could you make a wood cut out the same size as a fixed propellor, fit a bridle so that when towed it remains perpendicular to the water flow, then tow it behind the boat to see how much speed you loose?

A bit rough and ready, but would give you a ball park figure to work with.

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[john_morris_uk]

Well I could - but the results would only be approximate as the water flow wouldn't be the same as the water flowing past the propeller when its in situ near the hull etc. Also I was hoping someone would have already done the research and have the answers... Its obvious that a folding or feathering prop has less drag - but how much less - or more importantly how much faster do you really go?

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[Redmond]

In racing terms the penalty (which is quite high) was massivly outwayed by the speed increase. I have had a Max prop for 10 years and they are well worth having. When ours died after 19 years of use we didnt dream of having anything else.

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[brianhumber]

All I can add is that replacing the small folding prop with a three bladed twin pitch GORI has been worth every penny. Extra 1.5 kts in flat waters, better stopping in astern and quieter on the ears. These pluses enabled a pottering trip non stop to South Brittany on a windless weekend from Torquay without getting deaf or stressed out.

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[brians]

Volvo have done the study. Results are shown in their parts guide which any dealer will have.

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[gunnarsilins]

I cannot give you..

...any scientific figures, but when I got a 3-blade Maxprop instead of a fixed 3-blade there was a significiant improvement in speed (> 0,5 knots) under sail in especially weaker winds.
Also the bite in reverse was dramatically improved.
Despite the high price (the prop is 24") I´m sure this was a better value for the price investment then the new sails I ordered.

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[maxi]

Seem to remember an article suggesting that there was approx 65lbs of drag on a 13" prop at 5knts sailing speed. Can't for the life of me remember where the article was to be found.

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[AndCur]

On my Achilles when i lift the outboard out and insert the sailing plug which makes the bottom of the totally flat there is a speed increase of between 1/2 to 3/4 of a knot.

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[charles_reed]

It\'s marginal

the reduction in drag between folding and self-feathering props.

The more significant advantages come in improved drive especially in reverse.

If you are basing your decision purely on drag-reduction you'll find your most cost-effective answer lies with the folding prop.

I've had folding, fixed and self-feathering on my boat and whilst I think the last is best, I'm still not sure if it's really been worthwhile in simple economic terms.

But of course owning a boat is economically nonsensical.

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[spark]

The drag of a fixed blade propeller is (more or less) constant regardless of the speed that the boat is sailing.

The drag of the hull, on the other hand, increases exponentially with speed.

Therefore, the drag of the propeller is a significant proportion of the total drag (hull + propeller) at low speeds and becomes less significant the faster you go. When you get up to "hull speed" you'll probably not notice the extra drag.

So, if you are interested in improving your light wind sailing performance then a feathering/folding prop is a very good idea but if you tend to motor in light winds much of the advantage is lost, unless you go for an Autoprop/Maxprop/etc. which has other motoring advantages as described some of the other posters.



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[beneteau_305_553]

A standard fixed prop with a free rotating shaft when sailing cannot have any significant drag?
Just make sure the gearbox is in neutral.

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[john_morris_uk]

I don't really want to start this argument - but whilst I understand your logic you will find lots of people who would argue against allowing props to spin when sailing on drag and mechanical grounds. For me the conclusive argument is the one against wear in the cutlass, seal etc, and perhaps most importantly, some gearboxes aren't designed to be spun continually by the shaft. There have been numerous posts and arguments on whether to leave the boat in gear or not. I believe the balance of opinion falls down on not.

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[charles_reed]

I think your opinion

is that of a minority.

Could you produce figures to support the assertion?

Certainly, without exception, all the physical tests I have seen have proved the contrary. A free-wheeling prop causing, under most circumstances, more drag than a stationary fixed prop.

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[Dipper]

Some years ago, I fitted a 2-bladed folding propeller to my 26ft boat and it made a very significant difference to sailing performance. Average speed probably increased by about ½ to ¾ knot. This doesn’t sound much but over a 5 hour journey at around 5 knots boat speed, it means you are 30 – 45 minutes further ahead than you would have been with the fixed blades. Also, with the fixed bladed propeller I could feel the boat was being held back by the drag. Even with the sails set well, it just didn’t quite feel right. I also point significantly higher now, especially in light winds. The downside is a slight loss of grip especially in rough water eg when I get the tides wrong through a race although feathering props would perform better than folding props.

When I first started sailing, I used to let the propeller spin to reduce drag until someone told me it should be in gear. It doesn’t seem logical but it works! I suppose one way to explain it is to think why sycamore seeds are design to spin as they fall to the ground. This can only be to increase drag and to keep them in the air for as long as possible. The same logic applies to the freely rotating blades of a gyrocopter – stop the blades spinning = less drag = plummet to the ground!


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[charles_reed]

The theory about induced drag

is that the propellor drags with it the "ball" of water affected by its rotation, this is about x2 the diameter of the prop so you're effectively dragging a disc x2 the prop diameter through the water.
In practice, from the work done at Wolfson, there's about half as much drag from a fixed than a rotating prop. The math is quite interesting because the drag effect forms a parabolic curve, the highest drag being at about max displacement speed.

There's a little section in one of the Marchaj books if you're interested.

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[rex]

Re: The theory about induced drag

I still can't see how this works. Can someone point me to test tank results where the same fixed blade prop was towed at the same speed and the drag measured when fixed and when rotating, giving actual figures. Likewise the magic variable pitch prop giving increased speed when motorsailing. Surely this just alters the prop power curve and so uses more fuel at lower revs?

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[oldsalt]

Re: The theory about induced drag

As I understand it, a freewheeling propellor needs energy to rotate. It's also rotating the shaft and parts of the gearbox, there will be tranmission losses to overcome as well as the energy needed to rotate these components. This energy comes from the boats forward movement, thus slowing it down. The sycamore seed is the best analogy, it spins and has high drag thus falls slowly to the ground.

This thread is useful:


http://groups.google.co.uk/groups?h...UTF-8&oe=UTF-8&q=freewheeling+propellor+drag+



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[charles_reed]

Try this

http://www.wumtia.soton.ac.uk/

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[rex]

Re: The theory about induced drag

I had a look at the link you gave but there is a lot of theory and lift to drag ratios for totally different blade shape/area in air. I don't think there is a similarity between helicopter blades in air and a boat propellor in water. Has no one seen actual results for pulling a boat propellor through water locked and free-wheeling. I see that turning the gearbox produces noise and heat but when locked a lot of energy is used causing turbulence round the blades you don't see. There must be some real figures somewhere.

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