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| Original 2-blade, fixed-blade propeller |
Cilantro came to us with a 50hp Yanmar
diesel engine and a fairly standard 2-blade, fixed-blade propeller.
Curtis had done a lot of reading on propellers during his months
(years?) of boat research, and he found that many cruisers chose to
upgrade from fixed-blade to folding or feathering props. One downside of a 2-blade, fixed-blade
prop is that, if it happens to come a stop with its blades sticking
out to the sides like Mickey Mouse ears, rather than straight up and down in line with the
keel, the blades create significant drag as the boat
sails through the water, as much as 1 knot. And on a sailboat whose
typical sailing speed is only 5-6 knots, a 1-knot drag cuts that
speed by as much as 20 percent. A folding
propeller has gearing that folds the blades away into a tapered shape
like a tulip when not motoring. A feathering propeller has gearing
that “feathers” or adjusts the blade as needed: blade face forward for motoring, blade edge forward for sailing.
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| Propeller shaft, sans propeller |
When you think of sailing, a propeller
is probably not the first thing that comes to mind. More likely, it's
the mast, or sails, or keel, or wheel or tiller, plus relatively slow
movement through the water. But ocean-going cruising sailboats
usually have an inboard diesel engine (referred to as “auxiliary
power,” or sometimes as the “iron topsail”) – plus
transmission, propeller shaft, and propeller – for those times when
there's not enough wind, or too much wind, or wind straight out of
the direction you are trying to go, either for convenience's sake or
to outrun a storm. And even though maximum motoring speed is not
much more than maximum speed under sail, auxiliary power turns
out to be an essential part of cruising. So...Curtis went looking for a
propeller solution that would (a) have sufficient power to move all eight tons of Cilantro in all kinds of weather and current and (b)
minimize drag when sailing. He did not go looking for a solution that would be easy on
the wallet.
Curtis was also concerned about “prop walk.” Prop walk, a common feature of powerboats, is a complex force that exerts a rotational force on a boat in addition to the forward or backward forces. It is created by the action of the propeller in combination with different vertical and turbulent movements of water over the rudder and stern. Prop walk is present when motoring in forward gear but much more noticeable in reverse, and it has a handedness: right-handed propellers (that spin in a clockwise direction as seen from astern) push the stern to port, or left, in reverse; less common left-handed propellers (spinning counterclockwise) push the stern to starboard, or right, in reverse. Once you know about and are used to prop walk, it can actually be helpful for certain docking maneuvers, but in general, it makes it hard to back up in a straight line. Different propellers seem to create more or less prop walk, but because hull shape and other boat-specific factors contribute to prop walk, too, it is impossible to predict how a specific propeller will perform.
Curtis was also concerned about “prop walk.” Prop walk, a common feature of powerboats, is a complex force that exerts a rotational force on a boat in addition to the forward or backward forces. It is created by the action of the propeller in combination with different vertical and turbulent movements of water over the rudder and stern. Prop walk is present when motoring in forward gear but much more noticeable in reverse, and it has a handedness: right-handed propellers (that spin in a clockwise direction as seen from astern) push the stern to port, or left, in reverse; less common left-handed propellers (spinning counterclockwise) push the stern to starboard, or right, in reverse. Once you know about and are used to prop walk, it can actually be helpful for certain docking maneuvers, but in general, it makes it hard to back up in a straight line. Different propellers seem to create more or less prop walk, but because hull shape and other boat-specific factors contribute to prop walk, too, it is impossible to predict how a specific propeller will perform.
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| VariProp feathering propeller, in the box |
There are many kinds of feathering and folding propellers, including Max-Prop, AutoProp, Martec, and Gori. We
bought a 4-blade Variprop, manufactured in Germany but with a dealer
in Maine. It's a big, heavy item (23 pounds) that should provide Cilantro with plenty
of power. The blades are individually geared to articulate edge-on to
the flow of water when we're not motoring, so drag should, in theory, be minimized. And unlike the scooped, asymmetrical profile of a traditional
fixed-blade prop, the blade shape on a Variprop is flatter and more symmetrical, features that are supposed to provide good power in reverse as
well.
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| Mikey installs the VariProp, in the aperture forward of the rudder post. It will never look this shiny again. |
We had Bittersweet Landing install the new VariProp for us, along with a new cutlass bearing (to stabilize the propeller shaft and lubricate it with seawater), Spurs line cutter (to keep lobster pot lines, or warps, from entangling and damaging the propeller), and dripless shaft seal (to lubricate the propeller shaft and keep seawater out of the engine compartment).
We also decided to add an extra-large
"zinc" on the hull near the propeller. A zinc is
a chunk of zinc metal installed on an underwater structure to serve
as a galvanic or sacrificial anode. Metals corrode constantly
underwater, due to electrochemical processes, so zincs, because
they corrode more readily than other nearby metals, are used as
a kind of scapegoat. Our new large zinc will be physically linked to the propeller shaft, and it
should, in theory, protect the prop from galvanic corrosion. Zincs are have to be replaced either annually or when they have been seriously eaten away, but they are much cheaper than propellers!
We'll report back on the performance of the Variprop after we've used it for a while. Not having experienced the action of the old prop, however, we won't be able to compare it in terms of power, drag while sailing, or prop walk. There is already a difference, however, in the wallet.
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| John of Bittersweet drills a hole in the hull (Not another hole in the hull!) to install a plate for the new zinc. |
We'll report back on the performance of the Variprop after we've used it for a while. Not having experienced the action of the old prop, however, we won't be able to compare it in terms of power, drag while sailing, or prop walk. There is already a difference, however, in the wallet.
