How does my speedometer work?

[izoomie]

I have a 2007 Legend aluminum boat and it has a speedometer on the dash. On older boats I've seen a plastic hose run down to a plastic arm on the transom that drags in the water. I think vacuum controlled it. I don't have that plastic arm on the back of this boat.

How do the newer speedometers work?

 

[Slip Away]

Re: How does my speedometer work?

Speedo pickup is in the lower gearcase I believe

 

[crabby captain john]

Re: How does my speedometer work?

A few Merc OBs have a pickup in the LU. The rest are as you described. Check out the store here I bet they have them. None are very accurate but give you an idea ~~~ approximate.

 

[dingbat]

Re: How does my speedometer work?

I think vacuum controlled it.

How do the newer speedometers work?

Pitot speedometers work on the stagnation pressure of the water in a dead headed tube. i.e measuing the rate of compression (pressure) of the air trapped in the end of the tube.

Most new Speedometers are GPS based

 

[haulnazz15]

Re: How does my speedometer work?

Pitot speedometers work on the stagnation pressure of the water in a dead headed tube. i.e measuing the rate of compression (pressure) of the air trapped in the end of the tube.

Most new Speedometers are GPS based

Agreed. The OP's speedo could be GPS controlled, or from a pitot pickup on the nose of the outdrive.

 

[izoomie]

Re: How does my speedometer work?

Agreed. The OP's speedo could be GPS controlled, or from a pitot pickup on the nose of the outdrive.

I should have mentioned in the original post that it's a 2007 90hp merc 4stroke efi. I'll look closer on the lower unit to see if I can see something.

I think the gauges are teleflex so it could be GPS I guess, that would be more accurate.

 

[H20Rat]

Re: How does my speedometer work?

I think the gauges are teleflex so it could be GPS I guess, that would be more accurate.

Depends... If you boat in a river with any substantial current, GPS speedo's are less accurate than a pitot based system.

 

[izoomie]

Re: How does my speedometer work?

Depends... If you boat in a river with any substantial current, GPS speedo's are less accurate than a pitot based system.

More accurate than the tube, is what I should have said,

 

[haulnazz15]

Re: How does my speedometer work?

Depends... If you boat in a river with any substantial current, GPS speedo's are less accurate than a pitot based system.

Depending on what type of "speed" you are trying to measure. If you are measuring miles traveled per hour, then GPS is about as accurate as it gets. If you are concerned about velocity through the water, then the pitot system (or better yet the paddle-wheel style) would possibly be better depending on it's accuracy at low water speeds.

 

[limitout]

Re: How does my speedometer work?

I have a 2007 Legend aluminum boat and it has a speedometer on the dash. On older boats I've seen a plastic hose run down to a plastic arm on the transom that drags in the water. I think vacuum controlled it. I don't have that plastic arm on the back of this boat.

How do the newer speedometers work?

close but its water pressure forced against it that controls it.

most newer speedos are gps controlled so they are accurate, the pressure controlled speedos are never accurate just in the ballpark area of giving you the true speed

 

[QC]

Re: How does my speedometer work?

GPS speedos show speed over the bottom. Pitot speedos show speed over the water. Both are valuable. In my experience Pitot speedos can be quite accurate. I have had two Merc system speedos, 1 Alpha, and 1 Bravo that were within 2% of GPS with no current. In fact they are an air pressure gauge graduated for MPH. The water pushes on air in the tube and the gauge reads that pressure. No different than one you use for a tire. That year Merc has a tiny hole in the leading edge just above the "bullet" on the lower unit it leads to a tube that runs all of the way to the speedo head. Unplug it and you will get wet as the water will push and there will be no resistance. It will pee all over you, but it is still an air pressure gauge when it is hooked up properly.

Edit: Sorry for the repeated info, but it seemed like it needed a summary.

 

[UncleWillie]

Re: How does my speedometer work?

This may help in finding the Pitot.
That "Ding" in the Lower Unit is intentional!

 

[izoomie]

Re: How does my speedometer work?

Edit: Sorry for the repeated info, but it seemed like it needed a summary.

Nice summary QC and thanks for the pic Willie. I never would have found that without having it pointed out.