How a Marine Propeller Pattern Is Made

The manufacturing process of marine propellers has remained virtually unchanged in the last 50 years. As the boat designs evolve, the shape and style of the propellers have changed, but they are still made the same way they were in the 1950's. We have over 30 years of experience making propeller patterns, so we thought we'd put together a short description of what goes into making one.

Follow up:

What is a propeller?
Propellers are generally viewed as a fan (for aviation applications), or a screw (for marine applications). In this case, we’ll be talking about a screw. Each blade can be considered a small section of the screw’s thread, with water being the medium in which the screw is used. The rotation of the screw creates force (remember Newton’s third law?).

As the thrust face of the blade turns one revolution, the distance traveled represents the pitch of the propeller. If the pitch angle is 100in, the blade will have travelled that distance in one revolution. By applying these fundamentals to the propeller, it easy to see why it is considered a screw. As the blade revolves around the hub, it will create force in a forward of aft direction depending on which way the blades are turning.

Calculating the pitch involves not only the geometry of the propeller, but it also must account for slippage in the water. If the blade travels 100in in one revolution, a slippage loss must be taken into account in figuring the speed of the vessel. However in our case, we can disregard that.

The part of the blade that represents the screw thread surface is called the face (or working surface, or thrust side, but we’ll stick with face). The back of the blade does not resemble the face, as it doesn’t actually do any work. It is designed to reduce friction and cavitations, and to offer the strength and rigidity needed for the blade.

Parts of a propeller

1. Hub
2. Leading Edge
3. Trailing Edge
4. Fillet (pronounced fill-it, not fill-ay)
5. Face
6. Back

Beginning with the Layout
As with most woodworking jobs, the layout is done first. The layout is the most important part of the job. It is impossible to make a propeller pattern without it. Propellers involve a lot of numbers (we had 5 pages of measurements for this set). It isn’t uncommon for larger layouts to take the better part of a day. The layout shows cross sections of the blades at chosen points. The cross sections include the angle, rake (if needed), thickness (plus a little more for machining), and length. All of this is done taking shrinkage into account for the metal casting process.

Once the general layout is complete, we use it to figure out how much wood will be needed in the construction of the propeller. We generally like to use jelutong, but in this case we used 4/4 clear pine. It is important that the wood be clear of knots, as a lot of carving is involved in the shaping process. From the layout we can determine how many layers (or cants) will be used. For these propellers, 19 layers were used for each.

A separate layout is then generated off the original layout to determine shape of the individual cants. Once both layouts have been completed and checked, then checked again, construction can begin.

Construction
Preparation of the wood is very important. As said before, clear wood is chosen first. The wood is planed down according to the layout, and then shaped accordingly. In some cases (like this one) the wood may need to be glued into wider planks.
The layers are cut on a bandsaw; but we keep the off cuts (you’ll see why later).

In order to guarantee the layers line up properly, a center hole is drilled in each layer. This center hole represents the center of the hub.

Using the layout, an offset is determined enabling the correct pitch of the blade. The layers are then marked and prepared for gluing.

The layers are glued together using the offset. We use screws to hold the layers together because it is very difficult to use clamps. It is now starting to look like a propeller blade.

Shaping
Now that it’s all glued up and starting to look like a propeller, we can start shaping the wood. The steps in the wood need to be taken down to make a smooth surface. Tools used for this can include everything from and axe, to a belt sander. You can start with an axe near the root of the blade to get the bulky wood down quickly. From there, tools are used in order of precision. It can typically go as follows:

Axe – Draw Knife – Power Plane – Angle Grinder – Spoke Shave – Block Plane – Sand Paper.

While making the surface smooth, the blade is checked often for pitch. Adjustments are made to get the correct measurements. The face of the blade is always done first because the back of the blade is measured off the face.

Once the face is complete, the back is layed out according to thickness. The back is taken down in the same fashion as the face. Once the blade is finished (or close to it), the hub can be built up.

Remember the offcuts from the cants? They are kept to build up the hub. Hubs can be different depending on the foundry’s needs, and the specifications of the vessel the props will used for. Typically a machining allowance is added on the diameter, as well as an addition in length on the forward end. The extra length allows for the propeller to be placed in a lathe’s chuck for machining.

Once the hub takes shape, and the blade is near completion, the fillet is made. The fillet is the radius where the hub meets the blade.

The fillet is created using composite filler. It can also be carved into the wood (which you’d have to allow for in the cant layout), but in our case, this way is much faster. The fillet not only adds strength to the blade, it also helps in the metal casting process. If a fillet size is not specified, we can use a calculation to determine the correct size. If the fillet is too small, the metal will freeze and won’t flow properly into the blade when casting. If the fillet is too big, it can create a hot spot during casting resulting in cavitations. This is particularly dangerous because you won’t know if that’s happened until the blade falls off in the water. This is why companies rely on experienced manufacturers. Once the fillet is finished, the pattern is ready for finishing.

Finishing
At this point, all the small holes, scratches and minor defect are filled with wood filler. The entire blade is sanded down with at least 80 grit sand paper. The scratches left by the 80 grit paper will be filled with paint.

A special pattern paint is used to finish the propeller. It can be sprayed, rolled, or brushed on. The paint will fill small scratches and become extremely hard once dry.

After painting, the pattern is now ready for molding in the foundry. The pattern can last decades, usually outlasting the boats design and becoming obsolete before actually wearing out.

Questions?

Why only one blade?
The pattern is made with one blade for a number of reasons. The first being strength. If the pattern were to be made (in wood) with more than one blade, the hub would actually only be connected to one blade and the others would have to be attached after the fact. The second reason is continuity. The pattern is made with one blade to ensure that when molded, all the blades are the same (because they’re all made from the same pattern). The third reason is price. Most of the price of the pattern is in the blade shaping. If a 5 blade pattern is needed, it would cost 5 times what a normal pattern would.

Why flat on the back of the hub?
Because of the way the pattern is molded, less than half the hub surface is used at a time. So a full hub is not necessary. A flat back on the hub also allows for easier pattern storage.

Why is the pattern painted yellow?
The color of the pattern usually indicates the type of metal being used. However a foundry may have their own reason for color. For the last 30 years, most marine patterns we’ve done (props, struts, anode’s etc.) have been yellow, regardless of foundry.

Can you modify an old propeller pattern?
Yes. It’s actually quite common to re-pitch old patterns, update the blade shape, or build up the hub diameter. As we said earlier, propeller patterns are always built per vessel. They have to take into account the boat size, engine specs, region, etc. So if an old pattern was built for an engine with less horse power than a new one, the pattern may have to be modified to allow for a stronger propeller.

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