The rate of turn indicator is helpful in many ways on ships, especially during channeling or significant course alterations.
When I was a helmsman, I always took time to watch how my fellow ABs countered or stopped a ship’s turn.
One of my methods in using the rate of turn indicator, or ROTI, is to observe a particular turn and see how much rudder was applied to stop the vessel from swinging!
I would love to show you how we use this navigational tool on board.
But let’s discuss some regulations behind it first and the formulas if you want to know some manual calculations.
- The rate of turn indicator measures the speed of the ship’s turn in degrees per minute and direction- port left/red, starboard right/green.
- SOLAS mandates ROTI for ships over 50,000 GT; IMO sets the performance standards.
- The formula for calculating the rate of turn is, Rate of Turn (degrees per minute) = 57.3 x V / R x 60.
- If possible, use mast observation for a reliable “natural” rate of turn indicator.
What is a Rate Of Turn Indicator?
A rate of turn indicator is a device that measures and displays the rate at which a ship is turning to starboard or the port side.
When a vessel turns to the port side, the ROTI’s pointer moves to the left side, which is colored red. If turning to starboard, the pointer moves to the right side, which is green in color.
The pointer rests in the center or zero if there is no movement. The left and right direction has calibrations to determine the vessel’s swing in degrees per minute.
Lastly, the color coding is taken from the vessel’s side lights– green for starboard and red for port, to avoid confusion.
Where can we find the ROTI?
In most ships’ bridges, you can find the rate of turn indicator in many places, including within the modern navigational tools we use today.
You can actually find ROTIs very easily.
First, ROTIs are installed on the ship’s ceiling directly facing the steering wheel. It is beside other equipment such as the speed indicator, course repeater, wind indicator, rpm indicator, and ship’s clock.
You can also find it on the ship’s conning display, just in front of the steering wheel.
Lastly, there is a more advanced feature in ECDIS or the Electronic Chart Display and Information System showing the ship’s predicted movement applying the vessel’s rate of turn.
There are two regulations governing the installation of the rate of turn indicators on board different types of merchant vessels.
I think you already know one of them.
1. Safety of Life at Sea
Like all the other equipment used on board, ROTI is not exempt from mentioning it as a SLOAS requirement.
SOLAS Chapter V, Regulation 19 Paragraph 2.9.1 states that:
All ships of 50,000 gross tonnage and upwards shall, in addition to meeting the requirements of paragraph 2.8, have a rate-of-turn indicator or other means to determine and display the rate of turn.
Though its installation mentions specific vessel sizes, that doesn’t mean smaller ones can’t have it.
I sailed with vessels as small as 11,000 gross tonnage, and we have this device on board.
2. IMO Performance Standards
The International Maritime Organization (IMO) defines specific standards for every rate of turn indicator installed on board.
Specifically, IMO Resolution A.526(13), adopted on 17 November 1983, lays out performance standards for Rate-of-Turn Indicators.
You can find specifications here, like the length of scale, sensitivity, linear range scale, accuracy, operation, and design.
Rate of Turn Formula
The formula for finding the Rate of Turn (RoT) varies, but the one I’m showing you is straightforward. Here is the formula below with explanations.
I promise this to be very easy. So here is the formula:
Rate of Turn (degrees per minute) = 57.3 x V / R x 60
V is the velocity (Speed) of the vessel, and R is the radius of the turn.
57.3 is the conversion factor from radians to degrees. Since the formula calculates the turn rate in degrees per minute, 57.3 converts the angular velocity from radians per second to degrees per minute.
The factor of 60 in the formula is for the conversion from degrees per hour to degrees per minute.
Check out the image below to understand more about this formula.
In this case, if the ship’s speed is 12.5 knots and the turning radius is 1.0 nautical miles, you will have a rate of turn of 11.9375 degrees or simply 12.0 degrees.
You can also apply the rate of turn when finding your wheel-over position during course alteration.
Real cases in using the Rate of Turn Indicator
I used this tool always when I was steering the vessel.
Observing and comparing the readings here with other navigational tools often provides better situational awareness. Doing this would also make you a better helmsman.
Instances in which I use this tool while steering.
- Steering during restricted visibility
- Steering during nighttime without any lights ahead
- Marking the ship’s manageable turn rate so I can stop her quickly
- Managing the vessel’s swing so as not to overshoot
- Steering to keep the ship as steady as possible
- Steering while having a glare or without fixed markers on the horizon
- Mentally marking the vessel’s max ROT and applying that knowledge when channeling
- Steering during bad weather
- During helm orders, verifying if the ship is turning to starboard after I applied starboard rudder.
While this equipment can be handy, it still has limitations and can be prone to error. That’s why it’s important to rely on something other than this.
Instead, you can cross-check its readings with other navigational equipment. The best case for this is to observe the mast against the fixed objects on the horizon.
That would be the best rate of turn indicator that you can rely on any ship.
May the winds be in your favor.