15/06/07 13:57 Filed in: Propulsion
As we are sailing right now in the Tyrrhenian Sea off Italy, we have had many days I thought it would be great to have a feathering prop. For the past two weeks there have been quite light winds and flat seas. Yesterday on the way to Tropea (a gorgeous gem if you get a chance to visit!!) there was just 5-9 knots of wind most of the way. We sailed as best we could and just enjoyed the flat seas and spectacular coastline. But I kept thinking we would be able to make more of the light winds if we had a prop with less resistance. If we are sailing at 4 knots and our 3-bladed prop is spinning – we will slow down to almost 3.5 if I put it in gear. General consensus is that a feathering or folding prop would give you in the area of one extra knot under sail. And its often those very calm days when that would make the difference between sailing and having to switch the engine on. The difference between sailing with a feathered prop and towing a fixed-blade one through the water has been described as similar to dragging a bucket around. And who wants to do that?!?
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.
Two-Step has an aperture for the prop
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.