Propeller while sailing

I have a Volvo S120 saildrive from 1991 and have never had any lock-up problem when put into reverse with the engine off. As stated before I need to set it in reverse to make sure the propeller closes. The whole design of the propeller is made to open up as soon as it starts spinning, even if the spinning rate is fairly low (to enable manoeuvring at low speeds).
 
Logic says that the fixed prop will cause more drag.

Volvo say the question needs to be answered with reference to the engine/gearbox.

Some should be allowed to rotate, some need to be put in reverse and some gearboxes are so likely to be seriously damaged by starting them whilst spinning in reverse that the risk outweighs the benefits of doing so.

Mine is an md21 b and It should definitely be put in reverse.
 
As you say, Volvo state that for fixed or folding props.

Item 17 of the VP2003, states:-

[ QUOTE ]
"When under sail, the control lever should be in the neutral position if the propeller is a fixed propeller. If the propeller is a folding propeller, the control lever should be in the reverse position. Start the engine and run it for five minutes every ten hours when on long-distance cruises."

[/ QUOTE ]

I asked Volvo why, and the answer has been given in many of the previous posts why not to leave it in "reverse" if a fixed prop. Leaving it in ahead wears the clutch cones.

Basically, if you don't follow their guidelines, you have no come-back if something goes wrong.
 
Tank tests show the fixed prop MAY often show less drag, it depends on the area of the prop blades compared to the total disc area.

Logic says the bumble bee cannot fly /forums/images/graemlins/confused.gif

Personally I cannot live with the whine of a rotating prop, so went for a folder which also improved sailing speed significantly.
 
Lets answer a few posts in one hit.

Prop lock or free to rotate has been debated to death on here many times and I would agree it is counter intuitive but to quickly reiterate the main reasons. When rotating the energy to propel the boat is doing work to force the prop & shaft to rotate. The drag of doing this work is more than the drag of going pass the obstruction of a stationary prop.

When we bought our boat the Jeanneau manual stated engage reverse to stop prop when sailing but the Yanmar manual specifically warned not to and that there was a serious risk of damage. I went back to Yanmar and they said that many modern gearboxes are damaged if used to hold the prop stationary while sailing and they suggested that the Jeanneau manual was just stating what was best for sailing fastest and probable a standard statement that was OK for old gearboxes but no longer correct for some modern gearboxes.

Moral - Don't trust the boat manual go to the individual components manual in this case Yanmar.

I suspect the clue to what gearboxes are the type you should not engage reverse when sailing is that they lock up and you have to start in gear. This happened to us and I was certain that the selectyor lever locking up could not be good for the gearbox let alone what other damage may be being done internally.
 
Far be it from to be pedantic, but I believe that it matters what kind of folding prop you have. Bruntons, of Autoprop fame, say leave the engine in forward gear. Seems to work for us.
 
I think its theory, that reputedly says, "bees cannot fly".

As they do actually fly I think logic would tend to suggest that they can.

In simple terms the vessel, when motoring, is pushed through the water by the prop.

Just as a vehicle is pushed by its driven wheels acting on the road.

The road surface acts on the non driven wheels, and causes them to rotate.

The more resistance to rotation in these wheels the more drag.

When the vessel is under sail, the water acts on the prop and rotates it, preventing this rotation will tend to increase drag.

Will it not!!
 
[ QUOTE ]
I think its theory, that reputedly says, "bees cannot fly".

As they do actually fly I think logic would tend to suggest that they can.

In simple terms the vessel, when motoring, is pushed through the water by the prop.

Just as a vehicle is pushed by its driven wheels acting on the road.

The road surface acts on the non driven wheels, and causes them to rotate.

The more resistance to rotation in these wheels the more drag.

When the vessel is under sail, the water acts on the prop and rotates it, preventing this rotation will tend to increase drag.

Will it not!!

[/ QUOTE ]


No,


There was a very good article in one of the mags recently and they tested props in the fixed and rotating position at various hull speeds.

There was minimal difference at lower speeds but when getting towards max hull speed the drag of a rotating prop was nearly double that of a fixed prop.

A very interesting article and put an end to the ongoing argument for me.
 
All I know is that tank test data shows that fixed props have less drag.

The "theory" that states/proves that bees cant fly is based on an ancient dubious static data model.
 
I know it is counter intuitive, but as my historian daughter would say, 'Do you have documented evidence of that?' Logic is not enough.
 
[ QUOTE ]
Far be it from to be pedantic, but I believe that it matters what kind of folding prop you have. Bruntons, of Autoprop fame, say leave the engine in forward gear. Seems to work for us.


[/ QUOTE ] Seems odd - I thought the whole point of a feathering / folding prop was that it reduced drag when it was stationary. There should be no tendency for a feathered / folded prop to rotate, so why it should be left in gear is a mystery to me.

Our Varifold 2-blade folder will occasionally start to rotate if we kill the engine while sailing at over 5 knots. We flick the gearbox into reverse to fold it then put it back into neutral.

- W
 
Many years ago I had to perform a wind tunnel test with a completely flat rectangular plate on a spindle mounted on the axis of the tunnel. There was no obvious reason why the plate should do anything but stay still. As the wind speed rose the plate began to rotate and reached a high speed of rotation.

The reason was that the slightest asymmetry in the plate or the air flow would start the rotation and it would continue as autorotation. We had a 2 blade folding prop at one time and the propshaft was at a slight angle to clear the rudder. Sure enough the prop rotated when folded.
 
The reason that a fixed prop offers less drag than a rotating prop is simply due to the fact that a fixed prop is a stalled hydrofoil, whereas a rotating prop is acting hydrodynamically.

Since we are more used to the concepts with sails, this basically translates to a stalled sail generates far less power than a sail operating in the aerodynamic sense.

Arguably a surface operating at a very thin angle of attack may produce less power, but the friction of shaft gearbox and everything quickly takes the prop angle attack to above the stalled power.
 
its counterintuitive, but your explanation of the forces sounds reasonable, the question is what sort of power loss are we talking about, 1/2 knot or more? I must admit Iv'e never in all my years of sailing ever given it much thought, my current boat is fixed 3 blade and cruising I don't give it much thought. My racing machine was a folder so again not much thought. /forums/images/graemlins/smile.gif
 
I presume the speed difference really depends on the proportion of the overall drag that the prop represents. A low drag racer with clean hull, you'll notice the difference far more than a less sleek cruiser with a bit of weed on it.

First time my physics degree seems to have been of any practical use !!
 
[ QUOTE ]
There should be no tendency for a feathered / folded prop to rotate, so why it should be left in gear is a mystery to me.


[/ QUOTE ] The instuctions for our Max-prop are to put the gearbox in reverse to ensure that the propellor feathers properly. You are right that it shouldn't then tend to rotate, but locking the shaft ensures that it stays feathered.
 
It only locks up if you have the fixed blade which is trying to turn, I discovered this after loosing a prop blade on the folding volvo prop, so changed to the spare fixed prop. I'd been leaving it in reverse whilst sailing which helps shut the folder, only to find that when I went to put it in neutral to start the engine the gear lever won't budge.

First time I suspected the gear lever cable set up and stripped this down at sea (not fun). If it happens you have to start the engine in reverse and then it will free.
 
You must log in or register to reply here.

Other threads that may be of interest

R
Moving on.
2
Replies
29
Views
1K
Greg2
Greg2

Share this page

Top