TOPIC: All speedometers metered the same?

My latest acquisition has a 1994 Force (Mercury Marine)motor. The speedometer pitot is located on the leading edge of the gear case. I understand other OBs have this setup, including Mercury and Yamaha. Testing the speedo to see why it doesn't read, I found the gauge itself is dead. Won't respond to pressure applied with a hand pump. So, eventually I'll likely buy some new instruments. Wondering, if I get one that's made for a transom mounted pitot, but hook it to the original pitot on the motor, is it likely to read correctly?

Maybe one of you folks with a Physics background could tell me, would the exact size of the pitot hole matter, or would the air pressure in the line be the same as long as the amount of water pressure on the pitot opening is the same?

BTW, the gear case pitot is clear, no obstructions, and the factory line isn't cut or crimped.

Would not move with an hand pump...BAD I can blow 15 MPH with out drinking.

All speedo (for boats less it is a paddle wheel) work on water pressure. You can hook them up to any pitot tube. Tubes with small holes just take longer the react as where large holes will give you a bouncing reading in heavy seas

Thanks, that's good news. A speedometer isn't essential but it bugs me to see a dead gauge in the panel. Since my voltmeter is also way off kilter from the readings I've taken with my digital VOM, I figure I'll eventually pick up a set of nice looking replacements. They seem to be pretty affordable.

PS, I just sent you an email.

They do NOT read the same!!! Bass and Walleye boats magazine had an article on speedometers years ago that found they are pickup specific. One boat using a pitot speedo on a gearcase pickup had a 62 mph boat running 84 mph! Their advice was to use the pickup that the speedo manufacturer recommends. Outboard manufacturers usually sell gauges that match their motors and are pretty accurate. Use a Merc speedo with your gearcase pickup, or get an aftermarket speedo with a transom pickup.

I would buy the speedo you like and try it with the current pitot. check against gps and if the gap is small enough for your comfort use the existing pitot. if not mount the one that came with the speedo. You might like a boost of 15mph. just don't race anyone if you tell them you go fast.

Hold on a second. Does not a hose to the speedo work by water pressure "pressurizing air" in the hose to the speedo diaphragm?

Then it should not matter what pitot (or hose dia) is used. And I've never heard any speedo be pitot specific. Locating on the hull (water flow) does though.

Robby, you are sorta correct. In the mathematical calculation for pressure in a Pitot device there is a coefficient applied that is specific to the design of that particular Pitot. Theoretical speed is the same regardless of the size of the opening. Even though pressure is applied evenly throughout the system, there are losses on the walls of the tube (ing)and an equivalent pressure drop.

www.wmi.org/bassfish/bassboard/boats_motors/message.html?message_id=159925

All great info, thanks to all for the replies. When the time comes, I think I'll take Tomahawk's approach. The rig is blue and white, and this set would look pretty nice in there.

Interesting thread! Never to old to learn something!

I never really cared much mechanical speedo, as long at it worked, OK me..(long as I beat the other guy!)

I have no trouble getting my speedo needle to move in the right direction, but seems the gas gauge is a better indicator of how fast I'm going! Seems to move just as fast .."in the other direction!"