'95 Force 120hp/ Tachometer

[Dave K.]

Good Morning Gentleman, Is it possible to understand how fast I'm going using a tach instead of a speedometer? For example, if my tach reads 3500, appoximately how fast is the boat going? The boat weighs 1745lbs if that helps.
I understand other variables are probably at work: prop, total weight, conditions etc. I'm looking for a general idea.
Ty, Dave

 

[pnwboat]

On my boat 18ft Bayliner open bow w 125HP motor, I think I'm usually in the 20 - 25 mph range at that RPM. Can't remember exactly off the top of my head. Like you mentioned, a lot of variables come into play.

 

[GA_Boater]

Unlike a road vehicle with a solid connection of tires on pavement, boat speed and motor RPM have very little in common. Current, passengers/weight, water conditions, wind, prop and condition, etc. affect the fluid connection of prop to water. The best you can do, short of having a speedo or GPS, is go by feel and guess.

On your question - Are you driving a pontoon, a flat bottom jon, a v-hull? These all enter the equation as well.

 

[midcarolina]

You can get a theoretical speed using a prop calculator input rpm,prop slip and prop pitch.........but for actual speed you could get a buddy to run at same pace as you and get a reasonable idea........ of coarse as was said conditions, load , trim angle will play a roll in how accurate the numbers is in future trips......

If you have a smart phone there are plenty of gps apps you can get.......also Navionics marine and lakes has a good built in gps

 

[eavega]

With some numbers, you can take a stab at what speed you're SUPPOSED to be moving. You will need to know your gear ratio (i.e. how many times your prop spins for each spin of the eingine), your propeller pitch, and your RPM. There are online calculators that will give you THEORETICAL speed with those three numbers (which probably doesn't mean much by itself). What you won't know until you actually put a speedo on board (GPS or otherwise) is what the SLIP is on your rig, and slip can vary with weight, conditions, etc. I believe typical slip can vary anywhere from 10%-20%, depending again on conditions, they type of hull, condition of boat, weight, etc.

Rgds

Eric

 

[Jiggz]

Got a GPS? Use it and it is about the best and accurate measurement you can use. Don't have a GPS? Use your iPhone instead.

 

[Dave K.]

Thanks everyone. I have a Bayliner 1700LS, 17' boat total weight 1745lbs. Add me and 20 gallons of gas and now it's a good ton, I'm about 185.

I'm the 2nd owner. The original owner used the speedo set up to run a fish/depth finder and runs the tach to the engine. I know because I just got under there to hook up a new Hummingbird. I have the original speedo and it's damn near new.

I like the GPS idea and have it on my phone. I wouldn't mind hooking one up to the boat and probably will.

TY again, Dave

 

[jerryjerry05]

The pitch and WOT RPM's. Load and if the motors running right.
The prop's probably a 17 maybe a 19.
The number 17 is the distance 1 revolution of the prop as it goes forward.
So 17p is 17inches forward motion.
Extrapolate from there