Propeller rotation under sail?

[Guest]

Is a shaft brake necessary? Does a rotating propellor offer greater drag or has it more to do with reducing wear on the stern gland etc? My main grumble is the sound created by the rotation—it's hardly loud, but it's noticable. My gearbox (engine off) doesn't prevent the shaft rotating either in forward or reverse (it's an old Hurth Syncromatic which is well over the hill and crunches when going into reverse). Maybe replacing the gearbox with another which brakes the shaftt in gear would remove the need for a shaft brake?

 

[Guest]

In theory, a prop with absolutely no friction in the stuffing box, bearings etc. should create less drag then a locked one. But in real world there´s a lot of friction in the driveline, resulting in more drag when allowed to spin. Furthermore, not all gearboxes can cope with a rotating shaft for a prolonged time, because of the lack of proper lubrication. Just for fun, I´ve tried both alternatives on my yacht (having a huge 24" three blade and a hydraulic disc brake on the shaft) and when locked speed increases between 0.1 - 0.2 knots. Furthermore, I got the impression that the steering feels more "sluggish" when the prop is allowed to rotate, it´s probably creating more turbulence along the rudder. Finally there is the irritating sound. No, I suggest, lock it!

 

[Guest]

Oh dear - here we go again!

This has been discussed time and time again with no obvious conclusion. I think it's a bit like religion - you need faith. I believe that a locked prop is the answer but can't think of a way of convincing anyone else. We have seen a lot of pseudo-science and the guys who really know what they are talking about go over everybody else'e heads. You choose.

 

[Guest]

Oh dear - here we go again!

This has been discussed time and time again with no obvious conclusion. I think it's a bit like religion - you need faith. I believe that a locked prop is the answer but can't think of a way of convincing anyone else. We have seen a lot of pseudo-science and the guys who really know what they are talking about go way over everybody else'e heads. You choose.

 

[Guest]

Re: Oh dear - here we go again!

I hope not Glynn, its simple enough, either you can stand the noise as your gearbox wears itself out or you cant! and you dont need a shaft brake. The differance in speed either way isnt noticable.

 

[Guest]

I have a 20hp Bukh in my Westerly, which, when sailing I just leave in neutral. By "locking", do you mean I should put it into gear or is this potentially damaging?

 

[Guest]

My engine/gearbox does the same when "watermilling". I just stick it in gear to shut it up! No problems in 15 years.

 

[Guest]

Science and the alleged practical boat owner

A long time ago PBO(i think)had an article about propeller drag. Wear and annoyance not considered.

The least drag depended on mainly two variables:

- friction to rotation

- speed thru water

+ pitch and dimmensions

The relationships were not somple. May I request that this article come op in PBO agasin soon.... please...

 

[Guest]

Question is, does locking in reverse damage g/box or engine?NM

x

 

[Guest]

I remember someone years ago giving details of how to use a Bicycle Caliper brake for locking the shaft - ingenious adaptation of a simple piece of kit .....

The wear on a gearbox is subject to drive medium - mine's a hydraulic box and doesn't care whether in or out of gear, BUT the stern seal / cutlass bearing does - but again the rate of rotation added to actual shaft speed under engine ..... does it make that much difference ? My shaft when 'auto-rotating' doesn't turn very fast, so needs a lot of miles to add up to anything. With wife / others moaning and the sound of beer ring-pulls, I don't hear the shaft whirring away !!!!

As someone said ..... again !!

 

[Guest]

The best explanation I have seen uses the unpowered Gyroscope/Gyrocopter analogy.A Gyroscope with a locked blade will plummet to earth:a Gyroscope with its blade rotating will glide to earth.It is clear that the rotation increases resistance. From all points of view,the answer must be to lock your prop in gear,or by other means.

 

[Guest]

Doesn't anyone use it for turning an alternator anymore.....or did I miss something???

 

[Guest]

According to practical sailor which did some extensive testing in the US there is less drag with a rotating prop.I have a Hurth gearbox and except for the noise its just fine.

 

[Guest]

and the noise means WEAR! bearings shafts etc god knows what its doing to a hydraulic box !

 

[Guest]

Now that WOULD cause drag !

 

[Guest]

shaft gen

My friends used the shaftgen om transatlantic cruise om PRM transmission.

When the velocity is so high that wavemaking drag is is significant, the generator will not slow the boat much.

 

[Guest]

Propeller rotation Hurth!!!

Don't do it!!!!!!

I have just had to buy a new Hurth gearbox! It was only a little more expensive than refurbishing the old one. 2500 Oz dollars! The plates wore out because the previous owner let the prop spin.

The new gearboz works wonderfully!

Fair Winds Captain Mudbank

 

[Guest]

Ah yes, mate....BUT

A gyrocopter with its blade rotating to cause LIFT and DRAG will slowly descend. However think about the blade spinning the other direction, driven by the medium through which it is travelling, as is a freewheeling boat prop. I suggest you go up in a helicopter and ask the pilot to try it. It is called "autorotation". Now kids, don't try this at home!! Captain Mudbank