Speedometer on boat

[Deano]

My neighbor has a new boat (to him) actually an 88 fiberglass with an 80 merc on the back. He was out riding and noticed the speedo on the boat was way off compared to his GPS. He asked if I new how it worked, the only speedo's I ever seen were with the piece that went on the back that was spring loaded and moved up further the faster you went. But he doesn't have that. So the question is how does it work? is it off the tach and can it be calibrated. Any insight would help.

[SlowPoke]

Some have what you described attached to the boat while other have an inlet on the motor. Same principal, air pressure is sent through a pitot tube to the speedometer. They're never dead accurate but should be +/- 3-5 mph at cruising speed. Usually a horribly inaccurate reading is a result of a leaking pitot tube.

-Brian

[Randy]

I measure speed 3 ways, the pitot tube which is connected to the speedometer in the dash, a paddle wheel which is connected to my FF and GPS. For accurate measurements most people rely on the GPS.

[misfish]

GPS is the easiest and most simplest way of measuring.

[Terry]

and the gps is way more accurate

 

20 years ago most of the guys running bass boat thought they were running between 70 and 85 mph according to their Speedometer...gps with speed display were not available at that time

they had a radar gun at a tourney and it turned out most were doing between 52 and 68 mph....

 

the Speedometers don't do well at real slow speeds too...maybe they have gotten better the last few years

[Deano]

Thanks for the replies, he will be using the GPS for speed, but he was, as I, curious to how they worked cause i was not familiar with them. Now I know the basics.

[aplumma]

Hey Deano look at the back of the speedometer and see if it is a hollow tube or a few wires going in to it.

if it has wires check the colors and see were they go inside of the engine cowl. If it is the hollow tubing then unplug it and blow smoke thru it and see were it comes out of the engine. Either way their is very little you can do to calibrate them most of the time it is due to the age of the rubber diaphrams or corrosion that takes them out of calibration.

 

 

Art

[Tybo]

I'm actually surprised how accurate mine is between 30 and 40.

Also run a gps.

 

My performance boat ran 87mph on the gun in 3 to 4 foot rollers.

the water pick up type of speedo would read any where between 55 and 100,

until the water tore the blade off.It was the stainless steel type.

[POLLIWOGG]

The vacuum operated speedo and wheel type on the ff. give the apparent speed, like when you anchor in a river with a 10mph current the speedo reads 10 mph and your going nowhere or where the log on the ff. clocks 20 miles when you know you have traveled 15, you went 5 miles up and down the waves. The gps will read actual speed over land. Use the speedo for trolling and gps for navigation and the faster one for bragging.

[irishfield]

Use the speedo for trolling and gps for navigation and the faster one for bragging.

 

All right then...my 2150 goes 80.2 miles an hour.... according to the piece of junk Lowrance speed sensor paddle wheel on the back of my boat, when we were motoring to change spots Monday! While at the same time the paddle wheel right beside it for the Merc smartcraft gauge was showing 50 and the GPS showed 52.7

[POLLIWOGG]

Wow does that thing ever go........

[Headhunter]

Hey Wanye, have you considered putting wings on that boat... sure would make to rough days on Temagami easier!

HH