I started this blog entry with the belief that the information I wanted to share could be contained in one blog entry. As I began writing, I quickly discovered that a quick read would not be of real value to anyone who seriously wanted to take advantage of my knowledge and experience. Therefore, because of the extensive nature of the information, I have decided to make this a multi-part series.
If you happened to reach this blog entry and have not read part one, I highly encourage you to read “Part 1: Boat Storage.” Based on my experience, Boat Storage is the top consideration when deciding to buy a boat.
As I mentioned in Part 1, this series’ main purpose is lessening my overall effort, time, and complexity of answering someone’s questions that is not serious; or not the right state of mind to comprehend anything I say. If after one reads this article and wants to have a more serious conversation, I am always happy to do so.
Part 2 Propulsion
There are 4 main types of boat propulsion: inboard, outboard, stern drive, and jet drive. As with boat storage, there are positives and negatives to each boat propulsion type.
INBOARDS
The inboard motor’s power plant (the motor) is connected to a drive shaft that extends from the engine, through the boat’s hull, and connected to a propeller at the end. As the propeller spins, water is pushed away by the rotating blades.
Since the shaft is fixed the prop wash is bidirectional, meaning the propeller pushes water in only two directions, forward and backwards. The water is then directed over a flat surface, known as a rudder. As the water strikes the rudder, force is increased on one side and decreased on the other side. The rudder, which is connected to the hull, moves in the direction of decreased water pressure. However, since this blog is about buying a boat and not about rudder physics there are few important take aways regarding inboard configuration.
The lowest point of a boat is the propeller. An important consideration of an inboard and its inability to have any up- or down-level adjustment of the propeller and rudder. Most inboards are not designed for speed; and a primary consideration if you are boating in shallow waters such as coastal Florida.
Blades are designed as either a right or left rotating propeller. The pitch and angle of the propeller blades when the propeller shaft turns left or right will move the boat forward and slightly in the direction opposite for the propeller rotation. When the propeller starts to turn, there is an inherent force known as “side-force” created from the propeller blades' pitch as the blades push the water. This rotational side force will push the boat’s direction slightly left or right as the boat begins to move forward. The boat’s direction is always opposite of the propeller’s rotation. While this is an advantage with boats that have two opposing rotation propellers, single engine inboards have a longer learning curve to master.
Boats are designed to move forward. The unidirectional characteristic an inboard propellers' water flow over a plain (rudder) loses significant steering capabilities when the propeller pushes the water in the opposite direction, going in reverse. This is another inherent characteristic of an inboard that takes longer to master.
Inboard’s innate design is not meant for fast moving or high-performance boats. It is very uncommon to find recreational boats with a single inboard engine; but they do exist.
Bottom line, inboard propulsion is best used when a boat is consistently in deeper water and boat speed is not a consideration. Which is why many sailboats have an inboard motor.
PROS
Excellent fuel efficiency
Quieter operation and longevity
Engine is isolated from weather
Superior torque and power
CONS
Cannot operate in shallower water
Lower speed performance
Requires a hole in your hull
Cramped compartment for maintenance and repair
Shaft and propeller require underwater inspection or boat pulled from water
STERN DRIVES
The stern drive motor’s power plant (the motor, often the size of a car motor) is connected to an “outdrive” which is mounted on the outside of the boat’s transom. The engine’s location of a stern drive, like an inboard, is found inside the boat at the back (or stern), adjacent to the transom. While an inboard’s engine is connected directly to the propeller shaft, a stern drive engine connects to the outdrive through the transom via a short shaft and universal joint.
The shaft and universal joint from the engine connect to a vertical shaft on the outdrive. The vertical shaft connects to the horizontal prop shaft, and thus the propeller. A sterndrive internal gear ratios allow the outdrive to be matched with a variety of engines.
Unlike an inboard’s unidirectional thrust from the propellers, a stern drive design allows the outdrive and propeller the ability to direct the pushed water either left or right, negating a need for a rudder. Additionally, a outdrive can be tilted up and down to aid with forward and aft weight distribution. This weight distribution capability better aids the boat when traveling at greater speeds.
Because the outdrive can be connected to a variety of engines, this allows for greater engine horsepower, as well as a fully intact transom, which is why they are often found on larger and heavier boats, boats with cabins and associated amenities; and, where performance and speed are desired characteristics.
Unlike an inboard, there is a limited up and down tilt ability of the outdrive. The main purpose of the outdrive-tilt is for forward and aft weight distribution at higher speeds. However, the ability to tilt the outdrive does allow the ability to run in shallower water unlike an inboard configuration. What this really equates to is the boat’s ability to maneuver in shallower water. Boating in shallower water deserves some consideration if one wants to anchor near beaches and sandbars where the waters are very shallow.
Like an inboard, the entire outdrive sits in the water. Again, this is a consideration when looking for boat storage. Boats that sit in saltwater, e.g., a wet slip, require additional maintenance considerations. Sterndrive boats, which are typically larger boats that go fast, usually cannot be accommodated by a typical boat lift. These larger boats are most often needed to constantly sit in saltwater must have extra care of the outdrive and all the malleable parts that create the watertight seal at the transom. Which means, the owner should be aware that proper maintenance would require the boat to come out of the water for practical cleaning and maintenance.
PROS
Multiple engine compatibility, choices in gear ratios; enhancing prop spin RPM‘s and/or torque
Faster speeds for larger heavier boats
Engine is isolated from weather and outside elements
Can accommodate different boat configurations such as swim decks
CONS
Limited operability in shallower water; near beaches and sand bars
Higher maintenance requirements; more difficult to work on engines due to limited space in engine compartment
More difficult to find out-of-water storage
Outdrive and propeller require underwater inspection or boat pulled from water
OUTBOARDS
Outboard boat configurations are the most popular choice for small recreational passenger vessels; especially in coastal Florida where the waters are typically shallow.
The engine and lower unit (much like the outdrive of a stern drive configuration) are assembled as one unit; and the entire unit is mounted on the transom. In the simplest explanation, the only differences between a sterndrive and outboard operational concept are the engine design, how it connects to the drive, and how the engine and drive are mounted. Like the outdrive (sterndrive), the thrust of the propeller is multidirectional; and does not need a rudder to steer. One unique difference with engine tilt, and why an outboard engine configuration is popular, is the tilt capability of raising the entire lower unit completely (in most cases) out of the water. Admittedly, I have a general bias toward boats with outboards. The positives in the types of boat propulsion that are important to me are found with outboards.
Another unique feature that sets outboard engines apart from all other propulsion types is easily access for maintenance, repair, and replacement. In a word, they are “easier” to work on. And because the motor is not contained in an engine compartment, the common smell of fuel and oil is not existent; moreover, the dangers of gas fumes is far less than with a boat that has an engine room or engine compartment.
However, I would be careless as an author not to speak about the dangers of fumes; and have done so in a previous blog called We Need to Clear the Air!
Outboard motors when comparing HP to HP are much more expensive when compared to other propulsion types. While this does not work in all cases, the general rule-of-thumb is to take the engines HP rating (usually stenciled or painted on the powerhead) and add two zeros. For example, a new 250 HP engine estimated cost would be $25,000.
When the HP goes above 250 you can add a minimum of 15% for the larger size. For example, 350 HP estimated cost would be 35,000 times 1.15 or $40,250.
Along with the higher unit cost compared to inboard and stern drive engines of the same horsepower, outboards are not as fuel efficient and require higher quality fuel. This results in a higher overall operating cost. Outboards also lack fire suppression systems typically found with inboard and stern drive propulsion systems.
Another consideration is the ease of replacing the motor. While this an added benefit for repair and maintenance, it is a liability and consideration when it comes to motor theft.
Registering Your Outboard
Assuming that the reader is a first-time boat buyer, in Florida, if you buy a “used” boat with outboard engines from a dealer, they should make the buyer aware of a tax benefit found when buying a used boat “with outboard engine(s).” However, if you purchase from a private seller, the seller may not be aware. In Florida, you may exclude the cost of the outboard engines when determine the taxable value when it is time to register the boat and pay sales tax.
For example, you purchase a boat with a 150 HP for $25,000.00. The current sales tax rate for a boat in Florida is 6%. That means, you owe the State of Florida $1,500.00 for sales tax. However, because the outboard engines are not a permanent part of the boat, the law allows the buyer to deduct the cost of the outboard engine. Therefore, you should ensure the bill of sale separates the value of the outboard engine on the bill of sale. The seller is typically the one that establishing the value of the outboard; however, you can certainly discuss the value of the boat and the separate value of the outboard engine with the seller. For example, the 25,000.00 boat has $12,000.00 (or more) valued for the cost of the outboard engine; making the total sales value of the boat $13,000.00 for which $780.00 is now the taxable amount, not $1,500.00.
PROS
Easier to maintain and work on
Can operate in shallow waters
Can reduce tax cost when purchasing a used boat
CONS
HP to HP more expensive
Not as fuel efficient and higher cost in fuel to operate
Does not have good fire suppression capabilities
Security considerations
Propellers
The type of propeller installed on an engine will have a significant impact on the overall performance of the engine and the boat.
More propeller blades equal more torque (thrust); therefore, a three-bladed prop will have a higher top speed than propellers with four or more blades. More blades equal more hauling power or "umph" at low RPM’s. This an important consideration if the boat is sluggish or takes a long time to get on plain to reach its cruising speed.
The pitch of the propeller is also an important characteristic. Propeller pitch is the distance the propeller would move forward in one rotation.
If the pitch is on the low side, your engine could exceed its suggested WOT (wide open throttle) RPM’s and cause engine damage. A pitch too high could put undue stress on the propeller system and shorten the life of gear case components. Typically, two inches of propeller pitch equates to about 400 RPM off your WOT.
Stainless Steel props are about three times more expensive; but stainless are stronger, made thinner and can keep their shape while spinning so you don't lose performance at higher RPM’s.
Jet Drive
Jet drive boats are a newer propulsion system for boats; first introduced by Yamaha in 1991. In the same fashion as other propulsion solutions, jet drives have their distinct advantages and disadvantages.
If you are familiar with the physics and working mechanics of a jet ski, the jet drive boat operates in the same manner. Unlike the other boat propulsion technologies, a jet drive has no propeller. The jet drive engine uses an impeller. the impeller of a jet boat as an internal propeller; however, the blades are much smaller and typically encased. Because of the smaller blade size, the impeller’s rotation, measure in RPM’s (rotations per minute), is much greater. This design and greater RPM forces water through a housing and tapered housing creating a high velocity of focused water flow that has enough energy to move the boat forward. The lower pressure behind the impeller continues to pull water into the impeller through an intake and screen. The jet drive nozzle mounting is at the transom, below the waterline; and like a sterndrive and outboard, has directional thrust. However, jet drive engines require a more horsepower than other propulsion types to equal the same performance characteristics.
Jet drive engines do not have a mechanical reverse. Unlike a traditional propeller that can rotate in the opposite direction to create reverse thrust, a jet drive must use a shroud (cover) that redirects the flow of water in the opposite direction from the outgoing flow at the nozzle.
A jet drive vessel (this includes jet skis) is inherently difficult to maneuver at slow speeds. When the jet drive engine is running, the drive does not have neutral; meaning, the ability to have the engine running, producing no thrust, where the boat can remain stationary. Once a jet drive engine starts, water immediate flows from the nozzle and the vessel starts to move.
There have been advances in jet drive technology to minimize the flow of water at idle; however, this feature typically exists on newer and more expensive boats. Additionally, the only way to turn a jet drive vessel is the use of the water’s flow and directional force; meaning, you need to accelerate to turn. This can be tricky and counter intuitive; making the jet drive vessel difficult to control at slow speeds.
Jet boat manufacturers have introduced innovative technologies that with a push of a button, sets the boat into minimum-wake mode. Not only does this keep the boat from going too fast in a no or minimal wake zone, but the technology behind this feature also allows for better control at slow speeds.
With other propulsion technologies, a rudder, an outdrive, a lower unit can function as a rudder as the boat moves through water. This will allow the boat to turn even if the engine is off. With a jet drive, turning only occurs when thrust from the nozzle is pushing water and positioned with steering. When the engine is off (or not producing thrust) steering will have no effect in turning the vessel. For the beginner, this type of steering control, or lack thereof, is counter intuitive and difficult to master.
An advantage to jet drives is that the engines typically have a lower maintenance cost in relation to other propulsion types with similar HP. Another advantage with jet drives is the absence of an exterior propeller and the dangers of spinning sharp blades. Even so, the Jet Drive does eject a powerful and focused water flow at higher RPM’s and has the potential to cause serious injury.
Jet Drive boats can easily operate in shallow water. Unlike, the dangers of damaging propellers in shallow water, Jet Drives challenges deal with sucking up debris (grass, trash, silt, etc.) and clogging the intake or impeller. The placement of the engine is like a stern drive boat, which can allow for a more open deck layout, including swim decks, lounge seats, or other configurations; and this type of configuration makes a jet boat easier to get on and off a trailer. However, because the way the nozzle is mounted at the stern, there is no engine tilt to assist with weight distribution between the bow and the stern.
Jet drive boats can perform in rough, choppy water. However, the freeboard (the side of the boat from the water line to the gunwale (the upper edge on the side of a boat) can be lower to the waterline comparted to other types of boats. While the manufacture might disagree with my opinion, unless you have a large multi-engine jet drive boat, the typical jet drive boat does best in calm sheltered waters such as lakes, rivers, and small bays. When the water chop gets above 2 feet, a serpentine approach to navigate is highly recommended.
PROS
More HP for less money
Advances in jet drive technology make it easier to operate a slow speed
Fairly easy to maintain and good operability
Can operate in shallower water
CONS
Difficult to control at slow speeds
No reverse, no neutral and does not stay stationary at idle
Does not perform well in rough water
Can be loud at higher RPM’s and poor fuel efficiency
Now that you have a better understanding of the main propulsion types, you should have a much better idea on what propulsion system is best for you.
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