Speedo confusion

[DYLBOSS]

This is probably a stupid newbie question, but I'm wondering how my speedo is actually measuring speed? I don't have any sensors penetrating the hull besides the transducer for the Hummingbird depth finder.

 

[q5ka]

Re: Speedo confusion

On the transom (back), you possibly have a white plastic item with a tube attached to it. That tube will continue to your speedometer and your speedometer measures the pressure that is in the tube. So as you move in the water, the plastic pickup will try to push water up the tube. The faster you travel, the harder it pushes, the higher the pressure. That is how your speedo is possibly working.

That or you got a sweet GPS speedo as these are the only two methods I am aware of. Please correct me if I am wrong anyone.

 

[boat1010]

Re: Speedo confusion

I have a bravo stern drive and mine is the small hole down near the bottom of the stern drive, on the rounded part. the speedometer is moved with pressure. The faster you go the more pressure there is up to the speedometer. They are not very accurate but will usually give you a ball park speed. If it quits working use something small and see if the orifice is plugged. Hope that helps...

 

[boat1010]

Re: Speedo confusion

On the transom (back), you possibly have a white plastic item with a tube attached to it. That tube will continue to your speedometer and your speedometer measures the pressure that is in the tube. So as you move in the water, the plastic pickup will try to push water up the tube. The faster you travel, the harder it pushes, the higher the pressure. That is how your speedo is possibly working.

That or you got a sweet GPS speedo as these are the only two methods I am aware of. Please correct me if I am wrong anyone.

Some have little paddle wheels also. Those are electric.

 

[180shabah]

Re: Speedo confusion

You have a GenII, on the leading edge of the drive, above the "bullet" there is a small hole. That is the pickup for your speedo. there is a plastic tube that runs through the transom shield and up to the dash.

Your speedo is really just a pressure gauge. As you move forward, water is forced into the hole and pressureizes the air in the tube, moving the needle on the gauge. X pressure = Y speed. simple and effective, but they don't tend to be very accurate.

 

[DYLBOSS]

Re: Speedo confusion

Aha! Thanks for the great info. I would have never found that. Makes perfect sense. And no, I don't have a cool GPS style, but maybe in the future

 

[PitchFork]

Re: Speedo confusion

Check this thread out it shows exactly where the hole is.
http://forums.iboats.com/showthread.php?t=388546&highlight=Wood

 

[cr2k]

Re: Speedo confusion

BTW a functioning pitot type speedo (water pressure) is fairly accurate. I have GPSed min more than a time or two and have always been within a few MPH.

A GPS can be as far off as the water current as it measures land speed. So if you are going 50 on the GPS down stream in a 5 mph current it will be 5 MPH off of water speed and 10 mph off going up current.

The little paddle wheel (Hall Effect) type are the most accurate for speed relative to the water.

So the title of this thread is Speedo confusion, now I hope I have added to that.

 

[bhammer]

Re: Speedo confusion

A GPS can be as far off as the water current as it measures land speed. So if you are going 50 on the GPS down stream in a 5 mph current it will be 5 MPH off of water speed and 10 mph off going up current.

Very true. It's why I can fly a sail plane backwards without stalling. My gound speed is -10 my airspeed is 51Kts..... It was a windy day.

 

[smokeonthewater]

Re: Speedo confusion

This is probably a stupid newbie question, but I'm wondering how my speedo is actually measuring speed?

It's a seat of the pants measurement...... at about 100 mph it will start flapping..... now trunks will give you a reading at only 10 mph LOL