In this article I will review the different types of propulsion found in yachts and the benefits of each. This will help to understand why certain yachts have the powertrain they do.
1. Straight Shaft Inboard
2. V-Drive Inboard
3. Jet Drive
4. Surface Drive
5. Pod Drive
1. Straight Shaft Inboard
Illustration of a Straight Shaft Inboard Yacht
Straight Shaft inboards are the most traditional type of propulsion in use today. It is comprised of a rear-facing engine connected to a transmission which has a driveshaft going straight out of the bottom of the boat.
Running Gear of a Straight Shaft Inboard Yacht
See in the photo above a boat hauled out of the water the shaft coming out of the bottom of the boat. It is held in place by a strut right in front of the propeller. The thrust that is created from the propeller is directed by the rudder behind it.
Benefits of Straight Shaft Inboard:
- direct drivetrain results in a minimal loss of power from the engine
- the motor is placed in a more central location on the yacht, the central weight distribution results in a smoother plane and less bow rise
- very simple setup results in the lowest maintenance and high reliability
Drawbacks of Straight Shaft Inboard:
- In some configurations the necessity of placing the engines further forward results in a loss of potential cabin space
2. V-Drive Inboard
Illustration of a V-Drive Inboard Yacht
V-drive inboard yachts have forward facing motors with a transmission that re-directs the drive shaft towards the back. On the bottom of the boat the running gear looks the same as on a straight shaft inboard. You will see this application on cruising motoryachts that want to maximize the amount of interior cabin space, and where performance is not necessarily a top priority.
Benefits of a V-drive Inboard:
- With the engine being placed further back, you have more space for interior cabin room
Drawbacks of a V-Drive Inboard:
- More weight in the back of the yacht with further back engine placement, which may mean more bow rise on plane
- Some of the efficiency is lost from the motor to the prop with the direction change of the V-drive
3. Jet Drive
Illustration of a Jet Drive Propulsion System
The jet drive is very common today in runabouts and tenders, primarily because of the perceived safety of the propeller (called an impeller here) being on the inside of the boat and the shallow draft. Because there is no way to reverse the impeller, a cover goes over the jet nozzle to re-direct the thrust backward to move the yacht in reverse. In larger yachts jet propulsion is not nearly as common as in runabouts, you see it mostly on yachts that value speed. When in the yacht category, there are also other benefits…
Jet Drive on a 38R Hinckley
Benefits of Jet Drives:
- Shallow draft. When you are able to save 1-2ft. of draft on a 90ft yacht, going from what would be 6ft to 4ft, you can cruise around the Bahamas and Caribbean with much more confidence.
- Smoother Ride. The way a jet drive sucks in water through the small impeller that spins much faster than a traditional propeller results in a very smooth ride with little to no vibration.
- More Speed. because the jet drives are on the back of the boat instead of the bottom, there is no drag created by traditional running gear, resulting in more speed.
- More interior space. Engines are placed further back in the yacht.
Drawbacks of Jet Drives:
- Not as fuel efficient as traditional propulsion
- Maintenance cost is higher. There are a lot of small moving parts that need to be regularly serviced to prevent failure
- Less maneuverable at low speeds. There is no rudder, the yacht is steered by the thrust from the jet drive, so when at low speeds with less thrust, the steering is not as responsive. Some jet drive boats, like Hinckleys, are equipped with a docking joystick make docking a breeze
4. Surface Drives
Illustration of a Surface Drive
Surface drive yachts share a lot of similarities in function to the jet drives. The propulsion unit comes out of the back (transom) of the yacht instead of the bottom and the thrust can be directed with a number of hydraulic rams attached to the shaft. The main difference is the propeller is on the outside of the boat. They are called surface drives because the top half of the propeller is out of the water while underway, which reduces cavitation and kicks up a really cool rooster tail!
Profile of a 70 Pershing with Surface Drives
Picture of Surface Drives on the Back of a Yacht
Bottom line for surface drives- If you want the ultimate in performance and handling and do not care about increased maintenance and costs, a surface drive boat is for you…Pershing’s entire lineup is surface drive propelled and they are some of the most beautiful and fun to drive yachts out there today.
Benefits of Surface Drives:
- Shallow Draft. No running gear below the boat
- Fuel Efficient. Better thrust angles possible with hydraulic trim, no running gear below boat reduces drag
- High Performance. Less drag, direct thrust angles, big props
- More interior space. Engines placed further back in yacht.
- Really cool rooster tail!
Drawbacks of Surface Drives:
- Higher Maintenance. Lot of moving parts that need service regularly.
5. Pod Drive
Zeus Pod Drive and Transmission Mated to a Caterpillar Engine
Pod drives first hit the scene in 2006 and since then they have gained a very prominent place in the yachting industry. There are 3 manufacturers that are making pod drives: Volvo Penta (IPS Pods), Mercruiser (Zeus Pods) and ZF Marine (ZF Pods).
Volvo Penta IPS Pod Drives
Each manufacturer makes a few different size configurations, but they are available for engines ranging from around 300hp to about 1100hp…enabling them to work in boats from about 30ft to over 100ft depending on the number of pods and the style of yacht. For the system to work you need at least 2 pods, but some yachts, like the Lazzara 76 LSX quad, utilize 4 Volvo IPS pods.
The pods themselves are a very efficient form of propulsion that provide a great degree of handling and performance, with counter rotating propellers that direct the thrust and large gear housings that act as rudders…the yachts steer and handle like a much smaller sport boat. In addition to those benefits, the pods are also mated to a joystick maneuvering system that will independently actuate the pods to make for precise docking and low speed handling.
The final kicker is the position holding ability that pods have. Called the Skyhook on Zeus pods, it will hold the boat at a fixed location via GPS regardless of the wind and current…waiting for a bridge to open? No problem
Illustration of Zeus Pods on a Yacht
Benefits of Pod Drives:
- Fuel efficient. 10%-30% more than traditional shaft drives
- Great handling at speed. The pods turn, which enables direct thrust vectoring.
- Great docking ability. The joystick docking feature enables you to walk the boat sideways in a current, without bow and stern thrusters.
- Position holding. the pods, run by a computer will independently actuate to hold a position via GPS regardless of sea conditions.
- Less noise and vibration. The exhaust is let out underwater for less noise and the smaller counter-rotating props spinning at higher speed make for less vibration.
Drawbacks of Pod Drives:
- Expensive. These systems require not only a lot of complex mechanical parts, but also powerful computers…increasing the cost of a boat by up to 15%
- Higher maintenance/repair cost. As they are complex systems with a lot of moving parts, they require more TLC.
- Smaller service network. If you find yourself with a broker pod drive in St. Marten you are going to have to fly a tech in to do repairs…as well as fly in parts.
- Size limitations. As they only have applications up to 1100hp, this limits the size of boat they can go on, or they need to put 3 or 4 pods on a yacht for them to make sense…resulting in more added costs.
**For more information about pod drives, check out these additional articles: The Truth About Pod Drive Powered Boats and Yachts and Pod Drives in Yachts Volvo Penta IPS, Cummins Mercruiser Zeus and ZF Marine