Miami Boat Show , Part III- Steering Wheel
One of the single most complicated parts of a boat is it’s propeller. Accordingly it’s also one of the most expensive, especially when you get into the ‘cleaver’ style props used for surface piercing outdrives/shaftdrives. Almost every feature and dimension on a propeller is responsible for a characteristic of how your boat and motor(s) perform from the amount of time it takes to get on plane, top RPM for your motor(s), top speed, handling and cornering, to acceleration If you have two or more motors, even the direction they turn in relation to each other can affect your boat’s handling.
It’s also one of the coolest looking parts of a boat, in my opinion, so it’s easy to see why the in-house designer at MTI decided to use it in the design of their new 52 foot catamaran. The first and most obvious part to adapt a propeller to is the steering wheel. Throw a rim around the blade tips, change the center a bit and boom. Custom steering wheel, right? Not quite.
A propeller is both wider in diameter and longer than a traditional steering wheel. Rather than scale a full size version I had previously, I decided to start from scratch. More on why later in the post. The first thing I did was measure the hub size and outer diameter of an existing steering wheel in the boats. Next, I measured the height of the steering wheel to make sure it wasn’t too shallow or too deep. Then came the spokes, or the blades in our case. The reason I didn’t used an existing propeller and scale it is because the pitch of most cleaver propellers is between 30″-42″. It’s hard to visualize but that means that there’s not much ‘twist’ in the blades. We wanted the same amount of ‘twist’, or rotation, in the blades, but in a shorter distance. This meant that the pitch had to change, an operation done easier in creating a new part rather than modify an exiting in my opinion.
After the initial blade feature was created, it was copied around the hub in a radial pattern four more times for a total of 5 blades. After seeing the steering wheel at this stage, assigning material properties to it, and analyzing it’s mass it was decided that reductions in weight were necessary even after a material change. It was decided that windows were going to be added to each blade to aid in weight reduction, as well as adding an additional uniqueness to it.
The blades on a traditional cleaver propeller are razor sharp, even sharper than those of a standard non-cavitating propeller. Obviously this does not lend itself very well to an object that is meant to be held, so the leading edges of the blades were modeled with large radii and smaller radii were applied to the rest of the part. My attention went back to the hub after the blades were finished and the bolt pattern was modeled along with a cover to hide them. If the customer had requested, I could have added grooves to the back of the rim for a little more grip.
Lastly part drawings were created and sent to MTI for confirmation and the wheel was analyzed to ensure it was feasible to machine such a part. Indeed it was and once the billet of aluminum arrived at the CNC shop it was go time. My fingers, as well as a few other pairs I’m sure, were crossed in hopes that the machining wouldn’t destroy the thin edges on the blades.