Extra! Extra! Catalina 27 does 20 knots!!!!!

[Rob]

Okay, a while ago I posted a question regarding how fast a 27 could go if it was surfing. Hull speed is around 6.5 knots as I recall, and I got reading of 9 knots in flat water one day.How did it happen?The answer lies in the sensitivity of my GPS. It is just a standard Garmin GPS 12, about 140 bucks at West Marine. Once the unit is homed in tightly, you can get erroneous speed ratings simply by holding it in your hand and moving it quickly forward. Imagine throwing the unit forward, but never releasing it. This small motion is enough to throw the speed completely out of whack. The unit will register your boat speed and add the speed of your hand.I was surprised at how sensitive it was! I do have long arms, but I never thought something as small as a movement would be enough for the GPS to measure. At least I know where those crazy speeds were coming from.Fair Winds -Rob

 

[R. Young]

If only that were true

Well, the GPS is not sensitive enough to measure hand movement into your velocity calculation (unless you have very, very long arms). Even with WAAS, your accuracy is never greater then something around 20 feet. So, hand movements could not register into the velocity calculation. More likely, your unit is measuring instantaneous velocity rather then average velocity and is picking up the normal errors - resulting in occasional high readings.

 

[Rob R.]

Let's not split hairs

R. Young - I have to agree with you as to the specifics of GPS averages and instantaneous readings.However, being a former pilot, I am still amazed at the accuracy and consistancy of GPS. When I was a teen, the civilian market still had no access to GPS - we had to guestimate our position unless we had two VOR radios on board, and even then, we were subject to error.If satellites miles above the surface of the earth can help my handheld unit to tell the difference between my normal speed on a boat, and my speed when I am accelerating the unit over less than a fathom's length (or, in this instance, maybe less than a second)- well, that's pretty damned accurate IMHO.Rob