how fast is a 1984 19' bayliner with 125 horse outboard

The Model 1950 with a 125 H.P. outboard would run 35mph. The same boat with the 120H.P I/O would run 38 mph. This was with a relatively light fuel load. It had more to do with the hull design than anything else. In fact the V-8 was only a few mph faster, in part do to the extra weight. Having said that, l noticed that 4 people and 3/4 tank of fuel was about the same, as the test boat.

Use your GPS and find out for yourself. Speed is dependant on the condition of the motor, The prop used, cleanliness of the boats' bottom and driver skill.

Richard is correct, my dad had an 84 with the volvo penta I/O and it would run at 40mph, according to the spedo. The outboard should be close.

boat speed is a mathmatical function of Propeller slipage, propeller pitch, and propshaft turning speed, do a search there are several various calculators that will estimate hulls speed if you know ho effiecent your hull is, but 35-40 is average.

Optimistically, 38 or so. Under ideal conditions.

some tips to improve performance:
Tune the motor up to get all the power it can make. Clean the carbs, decarb the cylinders, use the right spark plug with the right gap, use fresh 87-octane gasoline (unless u have a manual specifies otherwise), change the impeller every 2-3 years & gear oil every year. Add an ounce of seafoam per gallon of gas to every tankful (preserves the fuel & helps prevent carbon build-up).

Hang it so the the antiventilation plate is between 1 & 2 inches above the keel (outboard trimmed perpendicular to waterline.

Learn to use the power trim.

Get a prop that lets it run at the upper end of the recommended RPM range at WOT.

Jettison as much weight as possible from the boat.

Distribute the load evenly.

Check the boat for waterlogged floatation foam.

Keep the bilge empty with a float switch for the pump.

35 to 40, but since it is likely a Chrysler Force outboard, do not expect it to run long.

Mine does low 40 is with 3 people and a cooler of beer. Its a piece of shit FORCE 125 but its got a nice after market prop.