speedodometer calibration?

[akita]

please explain how a speedodometer works on boat.

 

[JB]

Re: speedodometer calibration?

There are three types that I know of, Akita.<br /><br />The pitot tube type works on the water pressure into a sensor tube mounted on the transom. They need to be calibrated to give accurate readings.<br /><br />The water wheel type has a small water wheel mounted on the transom. The meter measures it's rate of spin. Also needs calibration.<br /><br />Finally, almost any gps unit gives very accurate readings and can be set up to read ft/sec, mph, kph (kilometers). Some can be set to read nautical mph.

 

[magster65]

Re: speedodometer calibration?

Some of the newer depth sounder/fish finders have speed indication and it appears they get the info. from the transducer. I guess that'd be sonar.(?)

 

[akita]

Re: speedodometer calibration?

on the pilot tube type.how is calibration acheived?

 

[DBenja4148]

Re: speedodometer calibration?

If you have a GPS which I di (Garmin 182C) I would use that which is more accurate than a speedometer especially in a 6mph wake zone. I've seen the police get boaters in the Baltimore Harbor doing slightly over that so accuracy is important.

 

[DJ]

Re: speedodometer calibration?

akita,<br /><br />No pitot tube style speedo is going to be accurate at low speeds.<br /><br />The fishfinders, that have a speedo feature, use a paddlewheel.<br /><br />The only two types, I would trust, at displacement speeds are the paddlewheel type and GPS.

 

[lakeman1999]

Re: speedodometer calibration?

Neither the pitot tube, or the paddle wheel will be accurate, at any speed. because neither one has a hard no slip connection to the governing surface (water), there will always be slippage, plus currants (yes lakes have currants), plus waves, plus flotsam. Gps and sonar "MAY" be accurate, if you have the right outfits, and they are accurate.

 

[DJ]

Re: speedodometer calibration?

Lakeman is correct. I was not figuring in "currents".

 

[Solittle]

Re: speedodometer calibration?

Kinda depends on what you intend for speed to mean. If it is relative to the water you are on current does not come into play as you are moving on it. If it is relative to the bottom then current and the other things that lakeman mentions come into play.<br /><br />For example a Piper Cub may be flying at 50 miles per hour relative to the air it is flying in. Throw in a 50 mile per hour head wind and he is going 0 mph relative to the ground and if a tail wind he is going 100 mph.

 

[miloman]

Re: speedodometer calibration?

i have always relied on my gps to give me an accurate speed and you are right about no wake zones I got hit one by the cops in one lucky I was able to say no no officer my gps said 9 mph

 

[lakeman1999]

Re: speedodometer calibration?

For example a Piper Cub may be flying at 50 miles per hour relative to the air it is flying in. Throw in a 50 mile per hour head wind and he is going 0 mph relative to the ground

Yes, and that is the point, you have to judge speed as to are you getting anywhere. Headwind, you ain't moving, tailwind, look out you are going places.

 

[airman]

Re: speedodometer calibration?

Sorry Lakeman but I really have to take issue with your statements. Pitot speedometers can be dead accurate. If you doubt that, take a look at the requirements for accuracy on a commercial jet's pitot-static system. The same goes for the paddle wheel type units. Waves will not affect them if they are installed correctly. Currents will never affect them. It is important to note of course that they do not even attempt to indicate the boat's speed over ground. They only indicate speed through the water, which is the number to watch if you are gauging performance. As for the police issue, a boats wake is determined solely by its speed through the water and has nothing to do with its speed over ground (ie GPS speed). As for junk in the water, if it clogs the pitot tube or paddlewheel the gauge will usually read zero at all speeds rather than giving an inaccurate reading. The problem with the pitot units is that they are usually of low quality and there are no provisions for calibration. Paddlewheels read low speeds better but it is possible to build a pitot to read low speeds too, it's just expensive. As for calibration, the only answer for most available units is to make up an error sheet for your installation and correct your indicated speed to a true speed (through the water!). To get the accurate speed you need to run 180 degree courses to cancel out current effects and get the average speed (over ground) via time/distance or GPS. My point is that the instruments can be accurate, but you need to understand what they are telling you.

 

[lakeman1999]

Re: speedodometer calibration?

I am sorry airman, but you are wrong, you may be moving in comparison with the water/air stream, but if your top speed is 50MPH, and you are bucking a 50 MPH wind/currant, you ain't gettin nowhere. As Bubba used to say the faster I go the behinder I get.

 

[lakeman1999]

Re: speedodometer calibration?

PS!!!! By the way, if you are out there in a boat, you can be bucking both wind and currant, that will effect your progress, or direction. If you doubt me on that, get a pontoon with a top on it in a crosswind and/or crosscurrant.<br />!! 'hAAANG ON bUBBBA".

 

[boat doc]

Re: speedodometer calibration?

I have a few more things to add. If you are using a digital type sytem with pitot type pickups, these feed into a transducer, the transducer sends an ohm reading to a gateway box, the gateway box usually averages the speed of the pickups (if you have more than one pickup). If you have one pickup it takes that reading. The best way to calibrate a speedo is using GPS, here is the second best way, if you do not want to purchase GPS. Get some beer, and a good buddy, before you start drinking (so you can read the speedometer) get an aircompresssor and an air chuck. Disconect your speedo air line from the back of the guage (on air driven analog guages), and blow out the lines backwords. This will remove any debris from your lines. Hook the airline back up to the guage, and disconect your line from the pickup (outside the boat), regulate your air pressure down to 12 p.s.i. with the air chuck button pressed (so the air chuck is letting out air) now the buddy comes in, set your buddy in the boat to calibrate your speedo, and blow 12 psi into your speedo line, adjust your speedo to 36 mph. This is not 100 % accurate, but maybe 99%. Do not blow more than lets say 15 psi into your air line, this will blow up the diaphram in your speedo.

 

[airman]

Re: speedodometer calibration?

Lakeman, you've missed the point entirely. A pitot or paddlewheel speedo does NOT attempt to indicate speed over ground. It indicates speed thru the water and is the only instrument which will tell you that accurately. A GPS, while useful for navigation, will falsely indicate improved or decreased performance because of currents. To use your example of a 50mph current, the GPS will indicate that "you ain't goin' nowhere" but that doesn't mean that you should overhaul your engine or look for a new prop. On the other hand, if you normally get 32mph at 3000rpm and your (pitot) speedo indacates only 27mph you should be checking your prop for dings or rope. To restate the point again, a pitot or paddlewheel can be very consistent and an excellent indicator but you have to know what it's telling you.

 

[lakeman1999]

Re: speedodometer calibration?

Not being a speed demon, or not in a race (although I used to race motorcycles all over the country), the only use I have for a speedometer is to let me know of my progress in reaching my destination. In this event, and the actual purpose of the instrument, it will never be accurate.