Equipment | Propeller
We had a standard three-bladed fixed propeller when we bought Jocara. It worked fine, but it looked like it would add quite a lot of drag under sail. I've always wanted a folding or feathering propeller to reduce drag, vibration, noise and wear (no need for a shaft brake) but I knew they were expensive.
After much searching about, and discussion of the various merits of folding versus feathering propellers, we came across a German company, SPW GmbH (Germany), that had just developed a design for a four-bladed folding propeller. I think they may have only made one before this, for a much larger yacht, around 134'. I believe the design was developed in partnership with Bruntons in the UK.
Two- and three-bladed folding propellers are common, but four blades are not. The extra blade allows more surface area to provide drive, reducing the risk of cavitation (which robs power and damages the blades) without going to a larger diameter (for which we don't have clearance).
SPW GmbH (Germany) in Bremerhaven offered a discounted price in partial sponsorship and we decided to go for it. In the interests of efficiency, conserving power, aesthetics, etc. And, let's be honest, just for the hell of having high-tech toys!
The new propeller has proved excellent, providing good, smooth drive in both forward and reverse directions, without excessive propeller walk. It also allowed us to design a diameter and pitch specifically for cruising at low revs, providing good speed (6 knots) at half power from our auxilliary engine. There is no way we can measure the reduction in drag, but I guess it must be worth something. The original propeller, still perfectly serviceable, is now in the bottom of the forward cabin sole, under the bed. Let's hope we never need it...
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