Locking the Prop

[FWB]

Sorry if this has been discussed before but what are the pros and cons of locking the prop when sailing? Is it bad for the gearbox, or better perhaps. I should know this but uni was decades ago, is the drag greater locked or spinning?


<hr width=100% size=1>

 

[john_morris_uk]

Its has been discussed loads before. If you do a search you will find long and tortuous arguments, however, generally accepted that its less drag and less wear (on cutlass bearing, gearbox etc etc) to lock the prop and not allow it to spin.

<hr width=100% size=1>

 

[Robin]

You are right that it has been discussed before so a search will help.

There are still two schools of thought but IMHO:-

Drag is higher with the prop rotating, this is also causing gearbox and sterngland wear as well as making an irritating rumbling noise. I have always locked props on every boat I have owned both 2 bladed and 3 bladed and our current 3 bladed Brunton selfpitching/feathering must be locked in order for the blades to feather.

<hr width=100% size=1>

 

[Twister_Ken]

As you guess it has been discussed many times before. As I remember, the conclusiion was either that it's better spinning, or that it's better locked, depending on which football team you support.

There's a search function on the forum which will let you read the arguments in all their tedious detail.

For what (very little) it's worth, I generally lock my prop, unless I forget.

<hr width=100% size=1><A target="_blank" HREF=http://www.writeforweb.com/twister1>Let's Twist Again</A>

 

[Cotillion]

I always lock mine as it reminds me of the mother in law when left free. Continuous whining.

<hr width=100% size=1>

 

[snowleopard]

'common sense' says less drag if freely rotating.

science says less drag if locked.

tests prove science is right.

lock it!

<hr width=100% size=1>

 

[MainlySteam]

An easy way to see that the drag is greater when a propellor is not locked is to consider helicopters when they autorotate. If a helicopter has a power failure or a tail rotor failure the main rotor is allowed to freewheel and the helicopter can, with the help of the pilot autorotate to a safe landing (hopefully) with the "drag" of the free spinning rotor slowing the decent.

However, if the main rotor were to be locked, the helicopter would plummet straight down because with a locked rotor the drag is much less. With a freewheeling rotor the drag is high.

The supporters of drag being lowest with a freewheeling prop are invited to demonstrate the truth of their belief by trying a managed decent in a helicopter with a locked rotor - unfortunately they will find that their belief will die with them.

John

<hr width=100% size=1>

 

[PeteMcK]

Gearbox

Some hydraulic boxes can be jammed by locking, eg, the TMP, often fitted to Thornycrofts, Moodys. See:
http://www.thameswaymarineproducts.co.uk/index.htm



<hr width=100% size=1>

 

[StugeronSteve]

Volvo recommend free spinning for shaft drives and locked for sail drives.

<hr width=100% size=1>I only support two football teams. Notts County and whoever's playing Forest.

 

[Evadne]

I'm afraid I believe you're wrong on two counts. If there were one situation where a helicopter had more drag (blades locked or unlocked) then that would be the one to use as it would slow the helicopter down more. But that isn't why they use auto-rotation in case of engine failure. The helicopter lands safely because it extracts energy from the air rushing over the blades as it descends. To do this you want the situation that transfers the maximum energy from the airflow to the propellor. It uses this energy to build up momentum i.e. to spin the blades, then converts this to lift at the last moment to land at a slower speed.

On the boat, both methods produce drag, the spinning prop may or may not have less static drag but it gets the energy to make it spin from the boat's forward momentum as well. Me, I lock the prop because the biggest factor is wear on the bearing and shaft, not the 0.1 knot speed difference!

<hr width=100% size=1>

 

[charles_reed]

Goodness me

Like the bumblebee the autogyro can't fly.

Perhaps Dave you'd like to make history by telling them both.

<hr width=100% size=1>

 

[charles_reed]

There are 2 schools of thought - both are partially right - about whether a locked or a rotating prop produces more drag.

Tank testing, mainly at the Wolfson Institute, suggests that most sailboats have less drag at normal speeds with a locked prop.

The crossover appears to occur when wave-making drag exceeds skin friction below which point, due to shaft & bearing friction, most props are effectively fixed.
The exception could be with saildrives.
All, of course, depends on hull form.

<hr width=100% size=1>

 

[Evadne]

I think you missed the point

Never mind, it's not that important, unless you're a helicopter pilot with a broken engine.

<hr width=100% size=1>

 

[dickh]

If you have a Hurth Gearbox - the recommendation is to lock it in REVERSE gaer not forward - Why I don't know - I can't see the difference as it is filled with oil anyway - but I do it as I had to repair my gearbox last year and it was expensive! - and still don't know exactly what the cause was.

<hr width=100% size=1>dickh
I'd rather be sailing... /forums/images/icons/smile.gif

 

[gunnarsilins]

Before changing to a feathering prop I did extensive trials on my boat at different speeds with the shaft locked and free wheeling as well.

The boat was slightly faster with the shaft locked at all different speeds up to near hull speed, biggest difference at weaker winds/lower boat speeds.
The shaft and BW hydraulic gear is pretty hard to turn, so a boat with lower friction in the stern gear might show other results.

<hr width=100% size=1>

 

[jeffro]

another thought if you was sailing through a minefield of lobster pot bobbers would you prefer to lock or not ????

<hr width=100% size=1>

 

[gtmoore]

I have a Hurth gearbox and the prop will continue to spin when in forward gear - only in reverse will it stop.

It's probably the other way round if sailing backwards!


<hr width=100% size=1>Gavin

 

[Twister_Ken]

And Bow Thrusters?

Spin or stop?

<hr width=100% size=1><A target="_blank" HREF=http://www.writeforweb.com/twister1>Let's Twist Again</A>

 

[FWB]

Thank you all for your comments. I should know if the drag is increased or decreased with a locked prop but I can no longer remember the maths.
As to the effect on the gearbox---it goes into astern with a bit of a clunk and is hard to put back into neutral with any way on the boat. The gears are holding it still so there is presumably a strain there. Also when there's a gaffers wind the prop sings above 7kts--I can hardly hear the rumble of rotation but the resonance is very loud. Have to admit that 7kts is rare. Does anyone actually KNOW or are we all guessing? Anyway thanks again for input.

<hr width=100% size=1>

 

[gunnarsilins]

In my case I KNOW, the trials were made with a log showing variations down to +- 0,01 knots.

<hr width=100% size=1>