Propellers

[BrundallNavy]

Our boat has quite a large propeller which makes going in reverse “Interesting”. After watching a family on YouTube repairing a Lepard 50, they fitted some Max props. These props reverse the pitch when you go in reverse so my question is why are these not more common on the broads as they seem to completely eliminate prop walk. 

[marshman]

Cost?

[Smoggy]

When you say "reverse pitch" do you mean there's no gearbox in the system (other than neutral/reduction)?

 

[BrundallNavy]

1 hour ago, Smoggy said:

When you say "reverse pitch" do you mean there's no gearbox in the system (other than neutral/reduction)?

 

No the. Prop reverses so the leading edge leads which ever way you drive. 
 

[grendel]

probably cost, if it breaks down it may cost half the price of the boat to fix

[Popeye]

The problem with that type of prop is you cannot flip the weed off by going into reverse as you can with a conventional setup as the prop still rotates the same way whether in forwards or reverse. That is on top of the problems set out above.

Usually with that set up, neutral is achieved by feathering the prop which compounds the above problem.

[Popeye]

There is an interesting article in Wikipedia regarding this subject.

[annv]

Variable pitch props require a hollow prop shafts for control rod or oil gallery's to activate/control them, cost would be one thing and a prop-shaft of 1" to 2" dia would be impractical. John

[Jonathan]

Propeller blades have a "twist" in their pitch, from the hub to the tip to accommodate the differing speeds at which the root and tip travel through the water; also, the leading edge of each blade is aligned with the "disc" of the propeller to some extent, so that it cuts into the water and the blades then curve (slightly spoon shaped) so that the trailing edge is pushing the water backwards.

Both of these features mean that if you try to reverse the pitch, then the efficiency and effectiveness of the propeller would be reduced significantly.  Yes it will work, but will require a lot more power to achieve the same effect and I would guess that the "walking" effect would be far worse.

There are military propeller-driven aircraft that use reverse-pitch to stop when attempting short landings.  The noise made by the blades flailing air in all directions in reverse pitch is much higher than (for instance) a high-power take-off.

[Davydine]

I assume the Leopard 50 is a sailing catamaran? If so, the Max Prop makes sense as it feathers when sailing with the engines off to reduce drag. 
 

Not sure that the added cost and complexity would add any value to a motor boat

[FlyingFortress]

I was not sure which system you were asking about but now it has become clear it's Controllable Pitch Propeller CPP then I think most is covered above.

There are in my experience certain know hazards in CPP with regards to handling.

The ability to feather the pitch that is align the blades with the water flow is a big advantage on a sailing boat and I can imagine helpful on a multi engine power boat that sufferers a single engine failure. Never experienced this but can relate to experience on multi engine propeller aircraft. When simulating single engine failure on my multi course the instructor would pull the throttle back to idle and then when I had gone through the correct procedure would then advance that throttle about one quarter to simulate the loss of drag due to feathering.

Putting aside specialized vessels there are generally 3 types of propulsion systems in ships.

Gearbox very similar to what we have on our boats, tends to be smaller vessels

CPP tends to be on medium sized ships that do a lot of manoeuvring.

The big daddy is the big diesel directly coupled to a shaft. To stop the prop you stop the engine.To go astern you change the firing order of the pistons and the engine runs backwards.

My later seagoing career was on Offshore Supply Vessels such as are seen in GTY and these are specialized vessels which are very overpowered for their size but have to be very highly manouverable .An example the lowest powered OSV I was on had 4000 shp and was 65m long. An 80m coaster may have 400shp. I became a big fan of CPP. Not so much when I became a pilot and found the pitch very slow acting. In fact I am on record saying I would prefer the stop start engines.

So CPP 

As others have said above cost and complexity would be a major factor why these are not used on small boats.

CPP does not eliminate prop walk. It can be quite violent on some ships. In others it can be unpredictable sometimes acting one way and sometimes the other.

There is also major danger with CPP which needs to be known about.

When the pitch is set to zero the blades do not feather but form a spinning disc in front of the rudder. This can cause the rudder to stall due to having no water flow over it this is particularly prominent if you reduce pitch quickly. Think about coming into a mooring and you put your engine into neutral you will have rudder authority for quite some time after the prop has stopped turning. With CPP you can slow right down to almost nothing and still maintain steerage PROVIDED you slow down gradually. If you don't you will stall the rudder , take a large sheer and have a heck of a job regaining control the only option may be a crash stop, easy done in a small boat but not so easy with a ship.

Think about that yacht that makes an unexpected tack into your path. Very often a reduction in speed is all that is needed, not so good if you stall the rudder and sheer towards him.

Hope this has been helpful.

[BrundallNavy]

The Max prop in question has no external control it just reverses when the shaft spins the opposite way. They claim It reduces propwalk and gives over 90% efficiency in reverse. But as people have said the cost for my boat would be around 4.5k.  Back to the drawing board.  

[Smoggy]

What is the efficiency for a normal prop in reverse? And what is the efficiency of the max prop in forward compared to a normal prop? Do they have any cupping to the blades? (answers on the back of a £20 note to....)

I know which way I spend most of my time going, not the same for raggies as no-one knows which way they are going when they are going forward anyway.

Edit to add: When I had an outboard powered boat with 9.9 mariner 4 stroke I fitted a 4 blade prop and drilled a couple of holes each side just above the anti-cavitation plate to reduce the exhaust through the prop and reversing/stopping was transformed.

[annv]

Hi A Axium propeller gives a higher efficiency along with less weed pick up i have one on my present boat very pleased with it. It does cost a little  more. John

[FlyingFortress]

2 hours ago, BrundallNavy said:

The Max prop in question has no external control it just reverses when the shaft spins the opposite way. They claim It reduces propwalk and gives over 90% efficiency in reverse. But as people have said the cost for my boat would be around 4.5k.  Back to the drawing board.  

Oops Sorry.

Looks like a I was barking up the wrong tree with my post earlier.

A CPP shaft does not reverse.

[FlyingFortress]

I was also going to say at 4.5k I think I would have a bow thruster fitted but then thought . Can you do that on a wooden boat? Can't say I have ever seen one. Is there any reason why not? 

Or maybe just learn to live with the prop walk at that price. 

[grendel]

4 hours ago, BrundallNavy said:

The Max prop in question has no external control it just reverses when the shaft spins the opposite way. They claim It reduces propwalk and gives over 90% efficiency in reverse. But as people have said the cost for my boat would be around 4.5k.  Back to the drawing board.  

plus of course the more complex it is, the more chances for it to go wrong.

[Smoggy]

Once understood prop walk can be you friend when reversing into a spot, on mine using one engine it's a complete nightmare due to complete lack of keel, on both the ot's always the outside engine in reverse to take speed off and the inside engine forward to steer in to bank, this kicks the stern in nicely. (as long as no-one is looking otherwise it all goes wrong)

[stumpy]

A  alternative would be to fit a Kitchen rudder - no need for a reverse gear then.

[FlyingFortress]

2 hours ago, Smoggy said:

Once understood prop walk can be you friend when reversing into a spot, on mine using one engine it's a complete nightmare due to complete lack of keel, on both the ot's always the outside engine in reverse to take speed off and the inside engine forward to steer in to bank, this kicks the stern in nicely. (as long as no-one is looking otherwise it all goes wrong)

Outward or Inward turner's Smoggy?

[Vaughan]

10 hours ago, stumpy said:

A  alternative would be to fit a Kitchen rudder - no need for a reverse gear then.

Hearts Cruisers tow boat used to have one of those, as she was an ex Admiralty harbour launch.  Very effective, but a whole new method of boat handling!

[ChrisB]

I had a folding prop on a sailing boat and never really noticed the claimed advantages or to be fair the disadvantages (some said there was power lag)

Both my Centaur and Berwick were delivered from the factory with upgraded engines and larger 4 blade props. With a young family on board often I was short handed and deliberately set out to get "Motor Sailer" qualities.

The ability to motor into the teeth of a Channel Gale, make light work of the The St. Albans Race and the Hurst Narrows more than made up for any loss of sailing performance.

[BrundallNavy]

15 hours ago, FlyingFortress said:

I was also going to say at 4.5k I think I would have a bow thruster fitted but then thought . Can you do that on a wooden boat? Can't say I have ever seen one. Is there any reason why not? 

Or maybe just learn to live with the prop walk at that price. 

Yes you can   A company called Jet Trusters do one but again over 6k for a bow thruster and 9k for both ends. 

[FlyingFortress]

15 hours ago, stumpy said:

A  alternative would be to fit a Kitchen rudder - no need for a reverse gear then.

Had to look that one up.

Similar to a twin Schilling (spelling?) Rudder set up or the reverse thrust deflectors on a jet aircraft. I believe the use of an individual schilling rudder in a twin set up can be used to maintain directional control whilst performing a braking function on a vessel. A very desirable function indeed.

A former colleague of mine was a pilot at GTY when one of the ferries had a twin schilling set up. He did say it could be a tad challenging. 

I think the vessel was called The Ballard and was quite infamous for getting wedged across the river with its bow on the Gorleston side and the stern on the Yarmouth side after blacking out on a flood tide.. very similar to the situation the Ever Given Found herself in Suez but thankfully not aground so came free when the tide changed.

[FlyingFortress]

2 hours ago, BrundallNavy said:

Yes you can   A company called Jet Trusters do one but again over 6k for a bow thruster and 9k for both ends. 

That's a tad pricey