Propellors
15/06/07 13:57 Filed in: Propulsion

So the new boat is definitely going to have a feathering prop.
One problem with a new propellor, especially on a new boat, is knowing what the pitch and diameter should be. With Two-Step we got it fairly close so we didn't need to change anything. Nevertheless, after a few years I decided we might reduce the diameter and try to reduce noise transmitted through the hull. Tip-clearance is the measurement from the tip of the prop to the hull at its closest point, and it is generally recommended that the clearance should be 15% of the diameter (or more). We were close and wanted to exceed that. With the Southerly it is an issue as well since there is a skeg that protects the prop when the boat is in very shallow water (less than 1 meter!). So planning a new propeller has meant we are in the same situation - trying to fit in that restricted space.

One thing I always wanted for Two-Step was to try a feathering prop – but I wasn't able to find one to fit (they mostly have a longer cone out front that is too long for the aperture between rudder and keel where the prop is). For the new boat I am very excited to find the Variprop that looks like it will solve all problems! It has a short cone so the smaller space isn't an issue and best of all it is available in a four-blade version that offers greater efficiency when you have had to choose a smaller diameter than normal. The actual business of ordering a propellor can seem daunting since there are a lot of questions to be answered. Maximum diameter and clearances around the prop are just the start! Engine horsepower, and transmission ratios are used to calculate diameter and pitch. Shaft diameter, taper and nut dimensions are obviously important to insure the prop just slides on. In the case of the Variprop we will be able to keep the basic prop as a spare since the Variprop will be made with the same dimensions as the original.
One of the best things about a feathering prop is that they are generally adjustable so if you do guess wrong and the pitch is not correct, you can reset it. With a fixed prop you would have to have it expensively re-pitched if it wasn't right. Most of the feathering props let you realign gears or adjust something and set the pitch. In most cases you have to haul the boat out, disassemble the prop and fix it. But the Variprop even allows you to re-pitch it in the water – saving a haul-out.
I only wish we had tried one on Two-Step so we could compare it to the performance of our fixed blade prop. But I will definitely report back on how it goes on the Southerly.
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