When you think of angular momentum, or gyroscopic power, the easiest way to understand it is to compare it to horsepower for an engine. Angular momentum is the measuring stick for how much stabilizing power any given gyroscope can have, similar to how you know that a 60HP engine is going to provide far less power than a 250HP engine.
So, why’s it so hard to explain? Two reasons:
To put it simply, angular momentum is the most accurate measure of gyroscopic power, but many times you’ll see simply “torque” listed instead. Let’s talk about what those are and why one is more accurate to rate performance than the other.
Angular momentum, measured in Newton-meter-seconds, or N-m-s, determines the total amount of torque available over time. Time is the biggest differentiating factor between angular momentum and torque, so keep that in mind. Greater angular momentum indicates a higher roll reduction potential for gyroscopic stabilizers. The Seakeeper 18 (18,000 N-m-s of angular momentum) is more powerful than the Seakeeper 1 (1,000 N-m-s). (P.S. this is also how we name our units!)
The faster a gyroscope precesses (or if you’ve watched a Seakeeper in action, that’s the sphere tilting fore and aft), the higher its peak torque value – stay with me here. However, a faster precession speed significantly reduces the amount of time the peak torque can be applied. If you’ve been on a boat that’s rolling, you’ll know that a wave doesn’t happen instantaneously – it rolls – just like your boat on top of it. Those rolls, depending on the boat, usually happen in a timeframe of 2-10 seconds. Those are called wave periods, and the time it takes for your boat to make one full roll over that wave is the vessel’s roll period. Because of this, the most effective use of a gyroscope’s angular momentum is to spread the torque over a period of time that matches the wave period. There’s that word time again!
When you see “torque” listed for any given gyro stabilizer, it means the maximum torque output, measured in N-m (see, no time). So, what’s wrong with that? Well, nothing, but it’s misleading as a performance measurement because it doesn’t account for the wave period or the natural roll period of the vessel. This is important to account for because increases in wave period or a vessel’s roll period can cause a significant decrease in peak torque values.
Let’s look at one example:
The Seakeeper 6 has 6,000 N-m-s of angular momentum. That’s a fixed number – never going to change. What will change, is the natural roll period. Let’s say a boat has a natural roll period of 2.5 seconds. The peak torque is a whopping 15,690 N-m. But if we use a boat with a 4-second natural roll period, that peak torque drops to 9,870 N-m. The stabilizer is the same, so why would a changing figure be used to show its power?
The natural roll period of a boat varies anywhere from 2.3 seconds for smaller boats and 7 seconds for some mega yachts (don’t confuse this with the wave periods discussed above). In order to eliminate boat roll, a gyroscope must be able to provide stabilizing torque throughout the entire roll cycle (that requires time again!). Using the example above for a gyroscope rated at 6,000 N-m-s of angular momentum, quoting peak torque values above 15,690 N-m is misleading because boats in this unit’s size range (~40 to 49 ft.) rarely exhibit a natural roll period below 2.5 seconds.
We agree! Forces you can’t see stopping boat roll? Now that’s cool. What’s not cool is trying to understand why you can’t compare two competitive products or why you can’t get a straight answer about a gyroscope’s performance. And we’re saying, “time’s up!” to those representing gyroscopic power with just “torque.” It’s time someone set the record straight.
If you’ve got questions or want to put Seakeeper to the test – drop us a line and let us know. We’re ready to nerd-out with you as much as you’ll let us! Want to see inside a Seakeeper to see which parts make all that angular momentum? Check out our How It Works page and we’ll walk you through it!