Marine Engines

For those without the time, energy of confidence to row, paddle, sail or pedal, some kind of engine is clearly going to appeal. At the highest level this going to be either a water jet or propeller drive.
 

This introduction discusses:

• Waterjets
• Inboards & Z-Drives
• Outboards - Petrol
• Outboards - Electric

A water jet is safe for people in the water and can give exceptional performance. They are commonly found on jet-skis and water skiing boats. However they are built into the craft so the purchaser doesn’t really have a choice. This article is about choices of motor so water jets are not considered further. For propeller drive, there are three basic configuration – Inboard, Z-Drive, and Outboard. Where there is more than one propeller it is a good idea to have them counter rotating to reduce the effect on the steering.

Inboard & Z-Drive Marine Engines

The inboard is again built into the boat and a drive shaft comes out the stern (back) or the bottom of the hull. The boat is steered with a rudder and for larger boats there will be one or more diesel engines driving through a gearbox. Additionally there may be thrusters for pushing the boat sideways to assist with docking as rudders don't work unless the boat is moving a speed which is not compatible with trying to dock!

For smaller boats that don’t have enough depth in the water for a full inboard the a Z-Drive is popular. Here the drive shaft exits through the stern, then turns down to achieve greater depth before turning back, in the form of a ‘Z’. The drive incorporates steering so the direction of propeller thrust can be changed making this a highly manoeuvrable design. As it is possible to have a very large engine with the weight well forward this is the preferred design for powerboats. Again this is an engine arrangement that tends to come with the boat so it is not discussed further here. The final and most versatile choice is the outboard. Broadly these fall into Electric and Petrol engine models.

Petrol Engine Outboard Motors

The petrol engine models can be grouped into 4-stroke, 2-stroke and the very latest Direct Injection 2-strokes designed to meet stringent new emission standards (EPA 2006). The 4-stroke and 2-strokes are available with either electronic fuel injection (EFI) or naturally aspirated (with a carburettor). 2-strokes require a petrol-oil mixture and are lighter for the power they output. They can generally be laid on the floor without problem. 4-strokes are quieter, more fuel-efficient and less polluting. Unfortunately they are heavier for a given power output, must be kept upright and in saltwater are said to wear out three times as quickly. This is because the salty water aerosols mixed with petrol that has no lubricating oil evaporate on contact with the hot inlet valve assembly to leave gritty salt that is both drawn up into the valve guides and flushed into the cylinder.

Petrol outboards clamp onto the back (transom) of the boat or a motor mount. They can be turned from side to side and tilted so the propeller lifts out the water - handy for clearing weeds.  Steering can be via cables linked to an inboard steering wheel (remote) or via a bar, called a tiller, attached to the engine. Some models have an integral fuel tank, and some have an electric start with or without in integral battery; otherwise starting is via a pull-cord.

Weight is the most important consideration when choosing an outboard for a boat, especially if it has tiller steering that requires the helmsman (driver) to sit at the back of the boat. The thrust from the motor will also tend to lift the bow (front) out of the water so with all the weight at the back, flipping is a real possibility – even before hitting a wave. Boats designed for larger outboards will have the transom recessed so there is floatation projecting back on either side to support the engine.

Petrol outboards have power ratings quoted in Horsepower. The very popular Mariner range can be used to illustrate. Direct Injection is currently only available for the larger engines and the smallest engines are only available in the 2-stroke range. 1 Horse Power (hp) is about 1.3 Kilowatts or 1 KW is about ¾ of a horse power.

Mariner have many models in addition to the above but it is worth noting that the smallest 2-stroke is the 2.5hp which is equivalent to 1.9KW output. Compare this to the largest Minn Kota electric outboard giving 101 LB thrust drawing 37 amps at 36 volts being just 1.33 KW.

Electric Outboards

Electric outboards are quiet and run off a battery. Whereas petrol outboards have the engine mounted on the back of the boat with a drive shaft going down to the propeller, electric outboards have the motor mounted behind the propeller under water with just a small light control unit above the water. When you tilt a petrol outboard the engine weight counter balances the weight of the shaft and propeller. Tilting an electric outboard result in all the weight at the end of the shaft putting quite a bit of torque on the transom or motor mount.

Speed control is via a twist switch on the tiller or various optional remote control units. Depending on the model you will have either continuously variable control or discrete speed settings – typically five forwards and three reverse. As a rough estimate, an electric outboard will draw 1 amp/hour for each 1 LB of thrust. The Endura 36 at full power with a 105 Ampere hour battery would therefore run for about 3 hour with an outputs of 430 Watts or 1/3 Horse Power.  The Maxxum model has clever circuitry claimed to increase running time by up to five times as long. You can read an FBS test report of the Endura 36 on this website. Sufficient to say here that at full thrust it propelled my Colorado canoe at a good jogging-pace along my local canal. Certainly above the 4mph speed restriction - you would need a bicycle to keep up with me and I felt a noticeable jolt switching from speed 4 to speed 5.

Don’t use a car battery. You need a leisure battery – preferably deep cycle. A car battery, referred to as a cranking battery, is designed to give a lot of current over a short time to start your car when it is then immediately charged up again. If allowed to go flat it is invariably damaged. A leisure battery is designed to give a lower current over a long time. It can go from fully charged to totally flat and be recharged again without significant harm. Other features to look for are an integral charge meter and non-spill.

Minn Kota offers a choice of bow-mount or transom-mount. Being bow mounted requires that the thrust is directed in the same direction as the tiller. These are often called trolling motors. A larger stern mounted petrol outboard being used to get to and from the fishing ground and then the bow-mounted electric outboard being used to quietly move around the fishing ground towing a lour.

For family boating purposes we have selected three models for stock but all models can be obtained so please email your requirements. Models selected are:

• Endura 30 – 55 LB
• Maxxum 40 - 74 LB Extended Battery Life
• RipTide 40 – 101 LB Salt Water

See our product test report on using a motor mount with a Minn Kota Endura 36 on a Pelican Colorado Canadian Canoe.