Propellors: how much drag?

We had similar experience replacing fixed 2 blade prop with Gori folding 2 blade prop. The boat now feels much more lively and generally quicker, probably > 1 knot faster.

Regards, Jeff.

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Re: The theory about induced drag

I have always been intrigued by this question of fixed or rotating props. I remember when racing Westerly Storms we always hid the prop behind the P bracket. I could never make my mind up about the merits in both arguments until a couple of years ago. At that time we had a little Proctor Pirate with an outboard on a lowering bracket. On the day in question we were sailing in a good breeze with a boat speed of round 4 knots and the outboard kept tilting up on its pivot. It was then I noticed we had left it in forward gear! Putting it in neutral stopped this happening. Putting it back into forward started it tilting up again. This settled the argument for me.


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Absolutely not - minimum drag is with prop locked, so in gear please. Added to this gearbox will not be lubricated when the engine isnt running, so you will wear out the gearbox as well as having higer drag.

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Re: The theory about induced drag

But O/B props are very different beasts. They have a very high blade area ratio, and are usually of a very fine pitch compared with a slow turning displacement craft prop.

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Re: also

i believe by the same theory, in order to gain higher speed, freestyle swimmers are now coached to move their arms from side to side as they pull through the water. As a one-time swimming racer myself i know this isn't an involuntary action - but the fastest swimmers do it to "grab new water" as they say, or at any rate , the world record holder does it, so it must allow higher speeds, because there's higher drag, and more to pull against. maybe.



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Re: The theory about induced drag

<<<The sycamore seed is the best analogy, it spins and has high drag thus falls slowly to the ground.>>>

Hadn't ever thought of that good example.

Leads to the one of - Perhaps those who stick to the belief that freewheeling props have less drag should consider which they would prefer to be in - an helicopter with engine failure with a freewheeling rotor (high drag slow fall) or an helicopter with engine failure with a locked rotor (low drag very fast fall).

I have been told that helicopters are actually designed to fail to a freewheeling rotor status rather than a locked one, but I stand to be corrected on that.

John



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Low drag fixed blade propellors are designed for yachts and I suspect from the point of view of a cruising yacht the penalty is not a worthwhile driver for going away from one of those to a folding or other type of variable propellor unless one is super interested in speed or it is a high performance cruiser (you do not seem to have many of those in Europe /forums/images/icons/smile.gif as has been covered in other threads). Fixed props do, of course, induce a fatal penalty on a race boat.

All the anecdotal comparisons given are pointless because it depends very much on what fixed bladed propellor is used for comparison. From many of the propellors I see on yachts, I suspect putting anything else on would lower the drag when under sail. For someone to say that they had a fixed prop, which they do not describe, and then put some other folding, whatever, decribed prop on and that gave better performance gives no useful information whatsoever and may only mean that they had a load of rubbish for a prop in the first instance.

John

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I read an interesting piece by design wizard and propeller guru Dave Gerr.

If we had reduction ratios of say 4:1 in the gearbox we could swing perhaps an 18 inch two bladed folder instead of the usual 12 inch 3 blader. The two bladed prop would have adequate and realistic blade loading ( to avoid cavitation etc) , and would absorb the right amount of power to generate optimum cruising speed.

The two bladed folder is known to generate almost negligible drag when sailing.

The best of both worlds ?

He feels that yachts and motorsailors should have this feature at design stage.

Regards



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Interesting. I have to say that am not sure what I would go for fixed or other if we ever built another cruising yacht for ourselves (unlikely) but would probably stay with fixed if the comments of the propellor manufacturer (one of high reputation) we used are correct.

Their 3 bladed yacht propellors are claimed to have a Blade Area Ratio of 0.38 and they say that if one tows a can with that equivalent area then that gives you the effect of the drag from a locked propellor. That is, in our case where we have an 18 " dia prop that if we towed a can with the dia giving it the same area as 0.38 x the area of an 18 inch circle we would see the effect of the drag.

So, for a 3 bladed 18 " prop of that BAR the drag, they state, is the same as towing a 5.5 inch diameter can. For a 12 inch prop that is an around 3.5 inch dia can. Not a penalty for a cruising yacht, unless a performance craft, in my opinion. Many of the fixed props I see on yachts, the drag would be much higher, however.

The other offset which applies in our case, is that we only normally have to lift our boat for antifouling 2 yearly, the rest of the time it is afloat - I think that with a non fixed prop I would like to check it annually for bearing wear, etc and that would add significantly to the cost of maintenance.

John

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A caution to all of this

Don't lock your gearbox unless the maker says it's ok. The TMP hydraulic box common in Moodys can be locked, jammed in fact, by sailing with reverse selected: DON'T! It must be allowed to freewheel or be fitted with an external shaft brake.

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