2 bladed prop alignment

[splashman237]

Any bright ideas about how to determin the alignment of a two bladed prob while in the water ? Dont fancy a dip in the briny at mo as its freezing ..

(Wanting to align it with keel while sailing, as can get another 0.5kn apparently)

Yes I know I should have done it while she was out of water but forgot :0((

Would turning shaft 1/8 turn and then checking average speed give me an indication or would other fluctuations in windspeed etc negate any differences ?

 

[Twister_Ken]

Cheap underwater camera on a pole?

 

[fisherman]

Simple water glass? Length of plastic drain pipe with cling film stretched over the end, get in the tender and peer up your backside through it.

 

[Tranona]

Not convinced you will get that kind of speed improvement, although intuitively hiding the prop behind a wide keel should reduce drag. Doubt you will be able to measure any change accurately under sail. You also need to find a way of stopping the shaft in the right place reliably - usually with a shaft brake.

Assume the boat is a long keel - probably better to dry out alongside a wall to mark the position rather than trying to find it in the water.

 

[splashman237]

Simple water glass? Length of plastic drain pipe with cling film stretched over the end, get in the tender and peer up your backside through it.

Hmmm sounds like this waterglass might work, might need a waterproof torch strapped to side as well .

Im guessing sticking it in gear once prop is aligned will stop it turning , or does it then just turn the gearbox over ?

Is it more efficient to let the prop freewheel when sailing or stop it with brake or gearbox ?

 

[splashman237]

Yep that would work , how 'cheap' is a cheap underwater camera , you mean the disposable cameras Im guessing ? how would you activate the shutter remote once its under the water ?

 

[Salty John]

Is it more efficient to let the prop freewheel when sailing or stop it with brake or gearbox ?

There's less drag if you let the prop freewheel, but there are other factors that will determine whether or not you let it do so. Some gearboxes will not be lubricated properly if turned by the freewheeling prop, then there is the noise issue.
Lining up a two bladed prop behind the keel is probably less drag than a freewheeling prop, but you need a convenient way of doing it.
Whether you lock the prop with the gearbox or a shaft brake will depend on the type of gearbox and the manufacturers recommendation.

 

[Twister_Ken]

Is it more efficient to let the prop freewheel when sailing or stop it with brake or gearbox ?

Don't even go there! It's even more contentious than 'which anchor is best'.

 

[Salty John]

It's no longer in doubt, but I'm sure it's still contentious.

Both MIT and University of Strathclyde conducted tests which confirm freewheeling provides significantly less drag. If that's not enough, forumite MaineSail on the cruising forum did his own tests and confirmed these results.

At one time I thought a non-rotating prop would be less drag but I was converted after seeing these test results.

This is the subject of a recent SJ blog, by the way.

 

[AliM]

You want to align the prop because it would give you another .5kn sailing? I.e. this is for a practical, not a theoretical, reason.

Ok, well, try it out while sailing with the shaft clamped at 30 deg intervals, and include the free-wheeling test in that. If you can find no difference, then it clearly makes no difference, and it's not going to be worth investigating further!

It sounds like something to do to pass the time next time you are sailing in low, fairly constant winds.

 

[fisherman]

Im guessing sticking it in gear once prop is aligned will stop it turning , or does it then just turn the gearbox over ?

QUOTE]

If the box is hydraulic, then the shaft will spin, if mechanical, it will lock. Once the position is established you could have a simple paint dot on the shaft and a bit of string to a coupling bolt.

 

[Seajet]

Tippex !

Used by good mechanics I know to mark cam belts, with suitable de-greasing by white spirit works a treat on prop shafts - if they are visible ?

If the prop' is left turning, think of the drag as a disc, let alone the mechanical wear...

 

[awol]

My 2-bladed prop is in a skeg aperture and I firmly believe ~0.5kt improvement over free spinning from locking it vertical. Proving it is more difficult as speed change is not instant, the log has hysteresis and the wind is not constant, but I am convinced it is worth the effort, and anyway the rumble of the shaft turning is annoying.
You should be able to see the prop on a calm day from pontoon or dinghy and certainly with drainpipe/clingfilm. Mine is marked on the shaft and stern gland with retroflective tape. Locking in position is done by selecting reverse/neutral/reverse until the tapes align. With my Yanmar KM2 gearbox, reverse locks the cone clutch while forward allows rotation - yours may differ.

 

[splashman237]

My 2-bladed prop is in a skeg aperture and I firmly believe ~0.5kt improvement over free spinning from locking it vertical. Proving it is more difficult as speed change is not instant, the log has hysteresis and the wind is not constant, but I am convinced it is worth the effort, and anyway the rumble of the shaft turning is annoying.
You should be able to see the prop on a calm day from pontoon or dinghy and certainly with drainpipe/clingfilm. Mine is marked on the shaft and stern gland with retroflective tape. Locking in position is done by selecting reverse/neutral/reverse until the tapes align. With my Yanmar KM2 gearbox, reverse locks the cone clutch while forward allows rotation - yours may differ.

Ill have to check that reverse /neutral reverse locks shaft, can hear the thrum of it turning anyway but easy enough to pop down and view it turning or not.

 

[splashman237]

presumably the laymans explanation of the freewheeling prop is that is 'screws' its way through the water rather than acting like a pair of flat plates dragged through the water . Got a link for the Strathclyde research ?

Presumably if you turned it even faster then it would drive the boat along like a propellor - arghhhh someones already done that ...

 

[Tranona]

You don't say what boat/engine/box you have, but most small gearboxes stop the shaft turning in reverse. Many people automatically put in reverse when sailing to stop the noise of a turning shaft.

 

[splashman237]

Yep purely practical , extra .25->.5 kn would make quite a difference after a days sailing ...

Have to think about calibrating the speedo and the GPS STW/SOG accurately and do it at slack tide or across tide. might need to adjust sensitivity of the log too. Is there a way of recording NME data to track whats going on ie wind Speed and STW and SOG alongside each other?

 

[Salty John]

I've always put the engine in reverse to lock the shaft but Yanmar strongly advise against it:

Advisory Number: MSA08-003:

DATE February 8, 2008 Dealers and OEMs
TO: All Marine Distributors
SUBJECT: Gear in Neutral While Sailing All MODELS:

All Sailboat Engines

We continue to get questions regarding the correct gear position while sailing with the engine OFF. This advisory is issued as a reminder; Yanmar requires that if sailing with the engine OFF (not running) the transmission shifter must be in the neutral position or internal damage to the gear or sail-drive will result. This damage will not be covered by Yanmar’s Limited Warranty. Please instruct customers and dealers who deliver the sailboat to the customer, of the correct (Neutral) position for the marine gear while sailing.

If the customer desires that the propeller shaft not spin while sailing, either a folding propeller, shaft break, or other suitable device may be used. However, Yanmar accepts no responsibility for the selection, installation, or operation of such devices. Please also refer to Marine service advisory “MSA07-001_Yanmar Sail Drive Propeller Selection” for additional information.

If you have any questions regarding this advisory please contact a Customer Support representative."

I never had a problem with my Yanmar, but some users have reported that, after a long sail with the stopped engine in reverse, the gear shift sticks and requires considerable force to free it.

Hurth/ZF allow using reverse to stop the shaft. VelvetDrive and other hydraulic boxes don't lock the shaft when in gear so you need a shaft lock if you want to stop it turning.

 

[rickym]

Volvo 2002/3 engines recommend leaving the box in neutral while sailing, could lock it behind the deadwood with light string to a bolt as someone previously mentioned

 

[Tranona]

Volvo 2002/3 engines recommend leaving the box in neutral while sailing, could lock it behind the deadwood with light string to a bolt as someone previously mentioned

Volvo have been inconsistent about leaving in gear or not when sailing. Never seen any evidence that it does any harm. Gearboxes are generally bullet proof. The only difficulty is that you sometimes cannot go back into neutral without starting the engine.