Is your propeller a drag?

If you think anchor selection is a controversial subject you should try talking about propellers. I don’t know why these subjects should cause such angst, but they do. So I’ll just dive right in.

The first issue is drag: Under sail with the engine stopped does a fixed propeller create more drag when it’s locked or when it’s allowed to freewheel? You would think the answer would be unequivocal – and it shouldn’t need rocket scientists to work it out. But just to be sure, some rocket scientists, or their marine equivalents, did work it out and their answer is unequivocal: There is less drag when the propeller is allowed to rotate.
Scientists at MIT and at Strathclyde University agree on this. It is fact.

So, we know we get less drag with the propeller rotating but what are the other arguments for and against allowing the prop to turn?

Noise: The rumble from a rotating propeller can be quite intrusive, particularly if you’re off watch in a stern berth. Some people can’t stand the noise whilst others find it interesting; they like to judge the speed of the boat by the level of noise.

Energy recovery: If you want to run a generator off the shaft it has to turn – simple.
Wear: Where there’s motion there’s wear and tear, if not damage, to drive train bearings and seals.

Gearbox damage: Clearly you shouldn’t be risking damage to your gearbox or losing your warranty protection just to get a half a knot of boat speed under sail or to get a good night’s sleep in the quarter berth.

It seems that Yanmar became so concerned at the number of requests they received for clarification on the best practice for their engine/gearbox combinations that they issued a directive: The gearbox must be in neutral when sailing or your warranty will be invalidated. If you want to stop the shaft use a shaft brake, they say, not our gearbox.

I have to admit I sailed for many thousands of miles with my Yanmar 3GM30F in reverse gear to stop the shaft rotating and I never had a moment’s trouble. Just lucky?

If you have a Hurth/ZF gearbox you must not select forward gear when sailing forwards. Or reverse when sailing backwards, obviously. Apart from that, use the gearbox in reverse to lock the shaft or let it run free, it’s up to you.

With a Borg Warner Velvet Drive transmission you can do what you like, it will rotate anyway.

On some gearboxes damage can occur because the engine needs to be running to provide lubrication, with splash lubrication there isn’t usually a problem, so check the manual.

It boils down to this: If you are obsessed with squeezing out the last fraction of a knot under sail you need to let the prop freewheel. You’ll be happy to accept any wear and tear on your cutlass bearing and you’ll issue ear plugs to those that find the noise is keeping them awake.

If you’re worried about wear or can’t stand the noise you’ll want the shaft stopped and whether you do that by using the gearbox or a shaft brake will depend on your gearbox manufacturer’s advice, and whether or not you’re going to obey it.
Simple, really.

Tuning your rig

It’s really important to have a properly set rig to get the best performance from your boat without placing excessive loads on the rig or hull. The best way to achieve this is with a rig tension gauge made by rigging manufacturers Loos & Co, available from Saltyjohn.com here in the UK.

For the average cruising boat, you’ll be aiming to set up your rig like this:

Forestay tension – masthead rig: It is almost always advantageous to set the forestay tension as high as possible within the limits of structural strength. Generally, it is possible to use 15% of the breaking strain of the wire as the forestay tension. The backstay should be adjusted to maintain a straight mast with the desired forestay tension. The backstay tension will usually be less than the forestay tension because the backstay makes a greater angle to the mast than does the forestay – some catamarans are the exception to this rule.
Note that rollerfurling jib tension can only be set by adjusting backstay tension.

Forestay tension – fractional rig: Because the forestay tension cannot be directly balanced by the backstay tension some mast bend is accepted and the sails are cut to accommodate it. Forestay tension of at least 15% of the wire strength is desirable but, if this should result in excessive mast bend, it may be necessary to back off the tension.

Upper and lower shroud tension – masthead rig: The initial rig tension should be high enough that the leeward shrouds do not go slack when sailing close-hauled in a brisk breeze. The proper tension for your boat can be found by a few test-runs under sail and then the Tension Gauge can be used to record and maintain this value.
For many boats, a shroud tension of 10% to 12% of the wire strength is adequate. In some rigs it may be advantageous to carry a bit more tension in the uppers than the lowers.

Upper and lower shroud tension – fractional rig: In most cases the same comments apply as for masthead rigs. However, there is one exception. Where the upper and lower shrouds on a fractional rig lead to chainplates located aft of the mast – swept back spreaders – most of the forestay tension is balanced by the upper shrouds. A shroud tension as high as 20% of the wire strength may be required to achieve the desired forestay tension. Never exceed 25% of the wire breaking strength.

A well-tuned rig and correctly set up spreaders (ban the droop!) will ensure you get the best from your boat and avoid structural damage. The key is the Loos tension gauge.

Another job for the Metz VHF antenna

Lift boats are the service vessels of the offshore oil industry. Their most obvious feature is the three giant cylindrical legs towering 250 ft above the deck. When the lift boat gets alongside a rig these legs are lowered to the seabed and the whole boat is jacked up so that it can act as a work platform to service the rig a hundred feet above the surface.

They are hardly mere boats – these things weigh 2,500 tons, measure 200’ by 120’ and are propelled by a pair of 7’ diameter screws driven by two 1,500 HP diesels. They house 20 workers and carry all the equipment and spares necessary to service the rig. There’s a helicopter landing pad and a 10,000 square foot working deck.

Lift boats carry 2 or 3 cranes to assist in servicing, reconstruction or repair of the rig. Each crane can lift 200 tons and can reach out 150’.

And here’s why lift boats caught my attention recently – each of the cranes has a vhf radio to coordinate crane movements and the builders of the Dixie Patriot, the largest lift boat in the world when it was commissioned in 2003, specified Metz Manta vhf antennas.

So, we can add the Dixie Patriot to the list of Metz antenna users that includes the Clipper Fleet, the US Coastguard, several British Search and Rescue organisations and thousands of individuals who require the very best from their radios and AIS engines.

 

 

 

 

A summer sail in Scotland

Just back from a few days sailing in Scotland. The weather was disappointing for August but we got a couple of good days sandwiched between a major thunderstorm and torrential rain.

 

 

 

 

 

 

A boat with a pilot house is a definite advantage when the weather decides to be miserable.

 

 

 

 

 

 

The scenery is always spectacular in this part of the country and lovely anchorages abound. It was a most enjoyable time in excellent company  and in one of my favourite areas for sailing, no matter what the weather.