To assess the performance, the engine GEAR RATIO must be known. Please give the engine gear ratio as indicated in the owner's manual.
If the gear ratio were known, then a PROPELLER CALCULATOR (such as the excellent
propeller calculator available on continuousWave) could be used to assess the propeller.
For example, assuming the GEAR RATIO was 2:1, then your perfromance data would be entered into the
propeller calculator in order to compute a value for SLIP:
RPM=5800 (as mentioned as the usual engine speed employed)
RATIO=2 (assumed)
PITCH=18 (stated pitch)
MPH=34 (stated speed)
This produces a SLIP value of 31.
A value for SLIP of 31 at a boat speed of 34-MPH is very unusual. The propeller SLIP should be about 10 when the boat is on plane and running well. There several explanations for this:
- the engine trim is much too high and the propeller thrust is being spoiled by airy water
- the propeller is badly damaged, which would be clearly visible by bent blades or other obvious damage
- the propeller hub is slipping
- the engine speed is incorrect, probably due to an error in the tachometer or in its calibration.
Among those possible explanations, a slipping propeller hub or a bad tachometer are perhaps the most likely to be the actual cause.
Regarding your expectation that the boat should be able to accelerate to 40-MPH: generally to have a reasonable expectation of top speed for a moderate V-hull planing boat, the boat weight and horsepower are needed, along with a coefficient for the hull design. With those data a method developed by naval architect George Crouch can be used to predict boat speed.
The method is embodied in another calculator, which I have created and called
Crouch's Calculator.
Entering the following data into the
Crouch's Calculator:
POWER = 70 (stated)
COEFFICIENT = 190 (based on experience with Boston Whaler hulls)
SPEED = 40 (expected)
The expected hull weight is calculated: 1,579-lbs.
For a SUPER SPORT 15 hull with a relatively light 70-HP engine and one person, the total weight should not be more than about 1,600-lbs. On that basis a boat speed of 40-MPH is expected.
ASIDE: I used to have a SPORT 15 with a 50-HP engine. It would reach 34-MPH. Using Crouch's Calculator to find the weight, the computed weight is 1,560-lbs. On that basis, I would say that 40-MPH with a 70-HP is in the ballpark of expected speed.