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Twin-Engine SpeedMatch Indicator For Boats

Avoid unnecessary noise and vibration in twin-engine boats by using this Twin Engine SpeedMatch Indicator. It comprises a meter that is centred when both motors are running at the same speed. When the motors are not matched in revs, the meter shows which motor is running faster and by how much.

By John Clarke


Power Consumption: 12V at 20mA

Tacho Input Range: 0-6000 RPM

Display Range: typically set to ±200 RPM

Tacho Input voltage: 0.83V to 350VAC

Most power boats over eight metres long have two engines, typically in-line 4-stroke diesels or petrol V8s, each driving its own prop-
ellor via a shaft or stern drive. Normally both motors should run at exactly the same speed unless the boat is manoeuvring up to a jetty or mooring, in which case the propellers may run at differing speeds and direction.

Click for larger image
Fig.1: the basic set-up of the Twin Engine SpeedMatch Indicator. It compares the tachometer signals from each motor and displays the difference in RPM on a centre-zero meter.

All boat-owners know how important it is to have the motors running at exactly the same speed. If the motors don’t run at the same speed, there can be excessive noise and vibration and the motors will be far less efficient as one prop tries to pull the boat harder and the other produces more drag. At the same time, having the motors running at slightly different speeds means that you have to provide correction with the rudder to maintain a straight course and that causes further drag.

In fact, a speed difference between motors of as little as 15 RPM can cause lots of vibration that can radiate through the whole boat – most unpleasant.

To explain further, with V8 motors a difference of 15 RPM will cause a beat note of 1Hz. This is because V8s have four firing strokes per revolution so 15 RPM is equivalent to 60 pulses per minute or 1Hz. Apart from being most unpleasant to those on board, such low frequency vibration also causes lots of wear in the engines, gearboxes and shafts. So synchronisation of motors is highly desirable.

In fact, late model up-market boats often do have a facility for synchronisation while there are also electromechanical synchronisers available for older boats although these can be difficult and expensive to fit.

Click for larger image
Fig.2: each tacho signal is fed to a frequency-to-voltage converter. The resulting outputs are then buffered and fed to a differential amplifier which drives the meter.

So most boat owners equalise the motor speeds as well as possible by watching the tacho readings and listening for the beat frequency. Trouble is, most boat tachos are not very accurate (typically ±3% or worse at mid scale) and they can also be subject to wavering readings. Furthermore, if you are driving the boat from the flybridge in bad weather, it can be very difficult to clearly hear the engine exhausts, meaning that it is even more difficult to listen for “beat” notes.

And if your hearing is not the best (very common with older drivers), the difficulty is compounded.

Clearly, an electronic beat indicator is required. In setting out to produce a suitable design, we thought about an indicator based on a LED bargraph. When it was centred, the motors would be in sync. However, trying to see LEDs on a bright sunny day when driving on the flybridge is next to impossible and that goes for almost any electronic indicator. That is why most boats have conventional analog meters – they are easy to see!

Hence we decided to base our design on a good old-fashioned analog meter movement. When the motors are running at the same speed, the meter will be centred and if not, it will show the difference at up to 200 RPM (or whatever you decide to set). It is then easy to adjust the throttles so that the meter is centred.

The basic set-up of the Twin Engine SpeedMatch Indicator is shown in Fig.1. It compares the tachometer signals from each motor and the difference in RPM is shown on the panel meter. The panel meter needle is centred when the motor speeds are identical. If the port (left) motor is running faster than the starboard (right) motor, then the needle will move left.

Similarly, if the starboard motor is running faster, the needle will move to the right.

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