How does speedo work?

[Simoniz]

Just curious really,

How does the speedometer and rev counter on Bayliner 175 Merc 3.0l Alpha 1 Gen 2 work?

There is no wheel type speed measurement pick up and whenever the boat's been out of the water, Ive bever seen anything that looks as if it could have something to do with measuring the speed.

Also, the rev counter. I guess its electronic (counts the sparks) but occasionally the rev counter indications will wander around when the boat is at a constant speed. Its not cavitation or slippage as there is no chan ge in the engine note. How can that occur?

 

[ken_23434]

Re: How does speedo work?

There is a pitot tube that measures water pressure and this pressure causes the speedo to move. It's a very small pressure. It is probably mounted on the bottom edge of the transome to the side of the outdrive.

The Tach usually is picking up a pulse from the discharge of the coil. I would suspect a tach with a "jumping" needle would have a loose connection somewhere.

 

[cooter2506]

Re: How does speedo work?

Your outdrive actually has a small hole in the front of the lower unit that thepressure is created at to run the speedo.

 

[Simoniz]

Re: How does speedo work?

Ah thanks.

As a one time aircraft pilot, I should have guessed that it involved pitot pressure somehow.

I keep meaning to take a GPS on board to see if its accurate, but haven't got round to it yet. How accurate are boat speedos generally?

 

[ken_23434]

Re: How does speedo work?

I have the gen I Alpha which does not have the pitot built in (or, mine isn't hooked up at least).

I hoped I did not send you looking at your transome for something that wasn't there. :redface:

 

[Mkos1980]

Re: How does speedo work?

Some are WAY off, but mine is very close. At WOT 65ish MPH My speedo is right on 64. Good enough for me, but at those speeds sometimes it flips up causing no MPH read.

 

[Simoniz]

Re: How does speedo work?

I have the gen I Alpha which does not have the pitot built in (or, mine isn't hooked up at least).

I hoped I did not send you looking at your transome for something that wasn't there. :redface:

No worries, the boat is in the water and Im too interested in driving it during the summer to worry about poking about on the sterndrive.

Some are WAY off, but mine is very close. At WOT 65ish MPH My speedo is right on 64. Good enough for me, but at those speeds sometimes it flips up causing no MPH read.

Christ, doesnt the air pressure at that speed cause a small reading?

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