boat doesn't plane

[GregF]

These days I bet you can get a Cobra 100 or some other similar "dumb" hand held GPS for $10-20 on Ebay. I have 2 (Cobra and Magellan) and they generally suck on land but they are pretty good out in the boat (lock 7 or 8 satellites just about anywhere). You can set way points, get miles traveled and speed, along with Lat/Long (probably some other stuff I never use).

[spurhunter]

You might want to bring a GPS with you if possible. My boat is no speed demon but I know it's faster than the 4mph that my dash speedo says.

You can even bring a GPS unit from your car if you have one. It won't do any good for navigation, but the speed will be accurate.

Even a smart phone will do. You can download a speddo app, that will do the trick.

[RonKMiller]

The Ulysses speedometer app is excellent and free. I've got a busted speedo on my boat, and I seriously thought about taking the time, effort and money to fix it - by why bother... Ulysses even has a heads up display option that you can read in the windshield.