boat speedometers? how do they work?

[pcrussell50]

i know gps is the real way to get an accurate speed, but still...

i just bought a pristine 1985 boston whaler, 13 footer with a 40hp oil injected yamaha of the same year. everything, and i mean everything on this boat is exactly like new, except the yamaha "speedometer", which reads in kilometers per hour, btw. in any event, i'm interested in getting it working just because everything else on the boat works, but i have no idea how these things work to begin with. where should i start in the troubleshooting process?

apologies if this belongs in the yamaha forum, but i'm 0 for 2 over there. two questions asked, none answered, none attempted. besides, i don't know if there is a sensor on the boat that is not neccessarily a yamaha part to begin with.

-peter

 

[AZSenza]

Re: boat speedometers? how do they work?

Thre is a "Pitot" or pickup either on the hull or the drive (Engine), it sends water pressure to the speedometer. The amount of pressure varies with the speed, therefore it induces a corresponding speed to the gauge. If its not working at all (Nothing idicated) there is usually a piece of line disconnected or in some cases, especially if it reads the same speed all the time, the pitot is clogged and just needs to be cleaned out. NEVER blow from the pickup it wil do BAD things to the bellows on the gauge, instead disconnect it from the gauge and blow back towards the pickup. Maybe someone with the same=similar set up can steer you towards the right places but thats the basics of how it works.

 

[IVAZ]

Re: boat speedometers? how do they work?

Thre is a "Pitot" or pickup either on the hull or the drive (Engine), it sends water pressure to the speedometer. The amount of pressure varies with the speed, therefore it induces a corresponding speed to the gauge. If its not working at all (Nothing idicated) there is usually a piece of line disconnected or in some cases, especially if it reads the same speed all the time, the pitot is clogged and just needs to be cleaned out. NEVER blow from the pickup it wil do BAD things to the bellows on the gauge, instead disconnect it from the gauge and blow back towards the pickup. Maybe someone with the same=similar set up can steer you towards the right places but thats the basics of how it works.

They send air pressure.
They were first put on airplanes.

 

[QC]

Re: boat speedometers? how do they work?

Since there still might be some confusion The speedometer is actually an air pressure gauge. Water jams into the "pitot" as speed increases which pushes on the air in the tube causing a corresponding rise in air pressure . . . This is a typical boat mounted version that would be hanging off of your transom, but yours might be integral to the Yamaha's underwater portion of the gear case as AZS indicated:

 

[Blue Crabber]

Re: boat speedometers? how do they work?

They send air pressure.
They were first put on airplanes.

You are both correct.

The pilot picks up water as you are moving. There is air in the hose from the pilot to the guage. The water pushes against the air in the hose giving the air the same pressure as the water.

The guage should NOT see any water, just air.

This is my understanding of it anyway.

As far as getting it fixed goes, AZsensa has the right idea. Follow what he said and hopefully that will help.

(EDIT: Sorry, looks like QC types faster than me!)

 

[Nico2112]

Re: boat speedometers? how do they work?

I got this pic from the Yamaha forum. It's the lower unit speedometer pickup hole. It's below the cavitation plate. Someone having the same problem, just cleaned this hole, and his speedometer worked again.

Hope this helps.

 

[pcrussell50]

Re: boat speedometers? how do they work?

now THIS has been a meaty, tech-rich set of responses, gang. thanks.

-peter

 

[esobofh]

Re: boat speedometers? how do they work?

With regards to determining your speed.. a GPS can only tell you what your speed over land is, not your speed in the water.

If you are pushing an indicated 10 knots by pitot guage, in a 5 knot current - that's 5 knots over-land by the GPS. If you're pushing 10 knots by GPS in a 5 knot current, it would be an indicated 15 knots by pitot guage.

When measuring speeds by GPS, it's standard practice to take a speed reading over a fixed distance in two directions, and then average the two readings to take out wind/current factors.

-eso

 

[IVAZ]

Re: boat speedometers? how do they work?

With regards to determining your speed.. a GPS can only tell you what your speed over land is, not your speed in the water.

If you are pushing an indicated 10 knots by pitot guage, in a 5 knot current - that's 5 knots over-land by the GPS. If you're pushing 10 knots by GPS in a 5 knot current, it would be an indicated 15 knots by pitot guage.

When measuring speeds by GPS, it's standard practice to take a speed reading over a fixed distance in two directions, and then average the two readings to take out wind/current factors.
-eso

Why would GPS be affected by wind or currents?

 

[MrBigStuff]

Re: boat speedometers? how do they work?

The speed of the water current is relative to the observer.

Imagine two boats side by side and a person standing on the shore. If the boats are drifting in a 5mph current, then the person on shore sees both boats going past at 5mph. If the people on the boats look at each other, they are traveling at the same speed so it looks like they are not moving. The pitot tube sees "normal" pressure and so registers 0mph.

Now anchor with the bow facing into the current. The pitot tube now sees pressure equivalent to 5mph even though the boat is stationary (relative to land.

Start the engine and go fast enough against the current to read 10mph on the speedometer. Relative to the shore, you are going 5mph forward and this is what your GPS would read.

GPS cannot account for the effects of wind or current, it is always relative to stationary land observation.

BTW- my boats typically have two speed measuring devices, a pressure gauge and a paddle wheel. They work equally well IMO. Occassionally, mud daubers plug up the pitot while weeds can affect the paddle wheel operation...

 

[QC]

Re: boat speedometers? how do they work?

Better way to put it might be that in a boat, GPS gives you the speed over the bottom . . .

 

[esobofh]

Re: boat speedometers? how do they work?

Why would GPS be affected by wind or currents?

The GPS isn't affected.. your speed is.

 

[IVAZ]

Re: boat speedometers? how do they work?

The GPS isn't affected.. your speed is.

If I re-read your first post I can see what you mean :redface:.
What you describe is no different than the way they determine speed runs at the Bonneville salt flats. Had to pull my head from my rear to see the point you were making.

 

[esobofh]

Re: boat speedometers? how do they work?

well, to be fair, we were discussing real-time speed determinations, and I switched the topic there to time-trial/tests over specified distances.. such as when you are testing prop/trim changes etc..

 

[QC]

Re: boat speedometers? how do they work?

How fast does a GPS read if you are traveling straight up at Mach 4? Straight down?

 

[esobofh]

Re: boat speedometers? how do they work?

How fast does a GPS read if you are traveling straight up at Mach 4? Straight down?

Zero!

 

[QC]

Re: boat speedometers? how do they work?

Yeah, I agree, except that I believe if the unit was set to indicate rate of climb it could do that (aeronautics unit?). Triangulation and response time with the Satellites could still indicate a speed up if that was what you wanted to know. I am not sure why anybody would want to know decent speed in a marine application unless it was a Submarine . . . Although . . . I am curious at what rate the Titanic got to on her trip the bottom . . .

 

[IVAZ]

Re: boat speedometers? how do they work?

Yeah, I agree, except that I believe if the unit was set to indicate rate of climb it could do that (aeronautics unit?). Triangulation and response time with the Satellites could still indicate a speed up if that was what you wanted to know. I am not sure why anybody would want to know decent speed in a marine application unless it was a Submarine . . . Although . . . I am curious at what rate the Titanic got to on her trip the bottom . . .

Coming from a land surveying background I can tell you that GPS is not accurate to determine elevation. Even though most of the time its the most accurate way to measure speed under certain conditions which can happen everyday, its not that accurate. It has to do with the angle the satellites can be in relation to the receiving unit. For high accuracy tasks we would measure one location for 45 minutes in the morning and another 45 minutes in the afternoon and see what the discrepancy was. There was always a discrepancy but on the good days it might only be hundreds. on the bad days it could be as much as ten feet. These were $80,000 Leica units. I dont think we can always trust the few hundred dollar units we use on our boats.

 

[t_brown1986]

Re: boat speedometers? how do they work?

IM gonna borrow your post for a sec.. I have a evinrude 115. i can blow in PITOT and it works... but once it hooks into that 45degree on back of foot it wont work.. my question is where does it go inside of foot.. what does it do?