Yachting myth busters

[ghostlymoron]

A lot of us are stuck with the props we've got!

I agree with Sailorman, a sailor isn't going to be using a fixed 3 blade prop unless there's a pressing reason - can't afford a change, has to use the boat as a tug ?!

Let's see the figures for a fixed 2 blade behind a skeg, or a feathering 3 blade prop ? The latter seem to make a living charging quite a lot, so people must think there's a good reason to have them.

No-one has countered the idea that a freewheeling prop wears the shaft, bearings etc yet; I'm saying it does !

 

[JumbleDuck]

Just thought of another one.
Stopping the prop produces less drag than letting it freewheel.

That entirely depends on how much friction/drag there is in the gearbox. A truly freewheeling propeller will create practically no drag, but as the torque required to turn the shaft increases, so does drag.

 

[sailorman]

That entirely depends on how much friction/drag there is in the gearbox. A truly freewheeling propeller will create practically no drag, but as the torque required to turn the shaft increases, so does drag.

I have a hydraulic box & it is not recommended that it free-wheels for extended periods but i do have a MaxProp

 

[Seajet]

OK Pete,

I am saying neither has had a proper ( if you pardon the pun ) calibrated impartial test.

Having said thay I'm sure there have been plenty, but the variables of which prop ? Which shaft & gearbox, Which hull shape etc bugger it up.

Personally I go for the locked prop, even if someone says it creates more drag - which I thoroughly doubt - it wouldn't be knackering my propshaft & bearings, gland, gearbox etc !!!

But any boat I own would get a folder anyway.

 

[lw395]

That entirely depends on how much friction/drag there is in the gearbox. A truly freewheeling propeller will create practically no drag, but as the torque required to turn the shaft increases, so does drag.

Depends also on the prop.
A narrow bladed prop stalled is quite different from an out board prop where the blades nearly overlap.
But on this occassion I agree with Seajet (shock!) all proper sailing boats should have folding or feathering props.

 

[PabloPicasso]

Some of my best friends are sailors:encouragement:

Some of my best friends are gay... ....this is a sailors forum isn't it?

 

[PabloPicasso]

Power gives way to sail. Especially in big commercial ports with dredged and buoyed channels for the big ships

 

[sailorman]

Power gives way to sail. Especially in big commercial ports with dredged and buoyed channels for the big ships

That Big Dredgers take no notice of with poor look out by the OOW

 

[jacktarjohn]

If you had your prop on a freewheeling shaft, common sense tells me it would spin when it was dragged through the water.Resistance is futile.

 

[KAM]

Depends also on the prop.
A narrow bladed prop stalled is quite different from an out board prop where the blades nearly overlap.
But on this occassion I agree with Seajet (shock!) all proper sailing boats should have folding or feathering props.

Many people overestimate the benefits. On my boat the 2 blade folder reduces drag by less than 6.3% at 5 kts reducing to less than 2.3% at 7 kts (rapidly becoming negligible) when compared with the same size fixed prop. A lot of people probably start motoring just at the point where they get maximum benefit from a folding or feathering prop.

 

[lpdsn]

Many people overestimate the benefits. On my boat the 2 blade folder reduces drag by less than 6.3% at 5 kts reducing to less than 2.3% at 7 kts (rapidly becoming negligible) when compared with the same size fixed prop.

That's interesting. Where do you get your figures from?

 

[VO5]

Well, I don't know. All I know is I have a Bruntons 3 blader and the boat goes faster.

 

[KAM]

That's interesting. Where do you get your figures from?

I could not find any reliable real data on the web so did some simple calculations for my own benefit. I can't make any guarantees and am quite happy to be proved wrong but here are some more numbers for my 10m light to moderate displacement fin keeler with a 2 blade prop. All the numbers are conservative so the real numbers will be even worse. Fixed prop drag 44N at 2.5kt, 180N at 5kt, 350N at 7kt ignoring any skeg effect. I assumed folded prop drag was zero again not true especially for a feathering prop. I then used the standard formulas for viscous drag and wave drag and estimated these at 2809N at 5kts and 14690N at 7kts again these numbers are very conservative with the real world case being much worse due to boat heeling and protuberances as well as fouling. As the boat approaches hull speed the percentage drag reduction becomes negligible. This is a very simple approach but gives a conservative answer. It would be interesting to compare with some measured data from a real yacht sailing but I could not find any on the web. I am only keeping my folder because I think its less prone to fouling debris when sailing.

 

[ghostlymoron]

How would I stop the prop on Cobblers with a volvo penta MD1 engine. I think preserving the prop shaft and gearbox is more important than an extra half knot.

 

[sailorman]

How would I stop the prop on Cobblers with a volvo penta MD1 engine. I think preserving the prop shaft and gearbox is more important than an extra half knot.

That isnt a Myth

 

[lw395]

I could not find any reliable real data on the web so did some simple calculations for my own benefit. I can't make any guarantees and am quite happy to be proved wrong but here are some more numbers for my 10m light to moderate displacement fin keeler with a 2 blade prop. All the numbers are conservative so the real numbers will be even worse. Fixed prop drag 44N at 2.5kt, 180N at 5kt, 350N at 7kt ignoring any skeg effect. I assumed folded prop drag was zero again not true especially for a feathering prop. I then used the standard formulas for viscous drag and wave drag and estimated these at 2809N at 5kts and 14690N at 7kts again these numbers are very conservative with the real world case being much worse due to boat heeling and protuberances as well as fouling. As the boat approaches hull speed the percentage drag reduction becomes negligible. This is a very simple approach but gives a conservative answer. It would be interesting to compare with some measured data from a real yacht sailing but I could not find any on the web. I am only keeping my folder because I think its less prone to fouling debris when sailing.

IRC offers a generous allowance for having a fixed prop, but nobody does, because the drag is worse than the rating allowance...

 

[vyv_cox]

Because he set it up to match his (presumably fairly typical) yacht? What should he have done instead? Made it not like a yacht?

What about the MIT or University of Strathclyde studies, what don't you like about those ones?

Pete

Here's the link http://strathprints.strath.ac.uk/5670/

 

[sailorman]

Myth fred rift

 

[KAM]

IRC offers a generous allowance for having a fixed prop, but nobody does, because the drag is worse than the rating allowance...

At what speed

 

[lpdsn]

IRC offers a generous allowance for having a fixed prop, but nobody does, because the drag is worse than the rating allowance...

That was why I asked for KAM's calculations. Thanks KAM. The answer is non-intuitive, but wouldn't be the first set of calculated figures that have shown intuition to be wrong.

I know one boat that did have a fixed prop - it changed the class that they were rated in. But it was a small two blade prop and we were always careful to line it up with the keel when the engine was stopped (somebody would get the job of going below and rotating the shaft into the correct position).

By the way, KAM, saw your second question about speed - IRC doesn't take speed into consideration. It would be a fiddle factor that they say are validated against real race results, but as lw395 says nobody thinks a fixed prop gives them an advantage (the case I mentioned above was to avoid being the smallest boat in a class where they were sometimes just buried at the start).