I have had several questions about recent posts so I lumped together my thoughts to open up a discussion about what makes a boat go faster.
If the boat stays the same, ie. Weight, hull profile (hook), water type and temp, prop blade thickness and material, the horsepower to push the boat at a given speed should stay the same whether supercharged or not. If the prop pitch is increased, since the HP required is the same, the RPMs drop but the same shaft HP is required to drive the boat at a the same speed. As the RPMs on the engine are lower, internal friction of the engine is less; Brake Specific HP (volumetric efficiency) may change a few factions of a percentage points. As speed increases, HP to push boat goes up. In a car, HP goes up at the second power of the speed. Probably about the same for a boat. If the non-supercharged engine produces 400 HP and went 65 MPH with the optimum prop, a supercharged engine is only producing 400 HP to push the boat at 65 MPH, same conditions. Change the prop pitch to reduced RPMs with the supercharged engine, we now have an additional 100 HP and are not against the RPM limit. This HP allows the boat to push past the old speed limit. By changing pitch of prop, the multiplication factor of shaft torque is changed. (Think of the old screw jacks, the finer the pitch, the easier to lift the load.) The optimum pitch allows the motor to exert maximum force to the water as the max RPMs are reached. Max RPMs means either running out of HP before hitting a limit. Too little pitch and HP drives the boat past/against the RPM limit. Too much pitch (less torque multiplication) and the reduced HP developed at the lower engine RPM does not allow RPMs to increase and develop maximum HP/speed.
A supercharged (or natural) engine running at part throttle, steady RPMs, restricts the flow of air (throttles) and therefore the reduced HP is developed. This restricts the boost that a supercharged engine can produce. An accelerating engine with wide open throttle allows the supercharger to flow as much air as the displacement or vane profile and RPM allows and as more air flows than can be crammed into the engine at one atmosphere, we get positive pressure. This is the HP we see on a dyno chart.
Labbing a prop reduces the parasitic losses of turning the prop thru the water. This allows additional shaft HP to be used in propelling the boat forward at a faster speed.
Acceleration is accomplished by the excessive power (some say torque) above what is required to perform the work (moving the boat). More power means faster acceleration. As HP drops off past the peak, some boats will continue to accelerate past the HP peak until the available but decreasing HP thru the prop matches the increasing HP required to propel the boat. The gain in speed past max HP RPMs should be slower than below the peak(?).
A truck pulling a big frontal area trailer (high drag, more like a boat than a car) will get better gas mileage at the lowest RPMs that allow enough HP to be efficiently developed. If it takes 100 hp to run down the road at 70 mph, that can be made at say 4000 rpm in 3rd, 3200 rpms in 4th or 2800 rpms in 5th. All three RPMs would reguire throttling air flow to only develop 100 HP. I would expect the best gas mileage would be in 5th. I am guessing that the increase in mileage is due to reduced internal friction of the engine. The assumption is that the net HP out of the engine is 100, not gross developed including internally used.
In 5th, not enough HP and torque multiplication would be available to pull much past say 4000 rpm. 4th probably would allow higher rpms and maybe a faster speed. .
We prop our boats so that we have enough push to get to some RPM limit. This is good for top end but probably causes cruise RPM to be higher than what is needed. What if a boat had an overdrive so that at cruise we could reduce RPMs and get better mileage?
I’m sure that my reasoning on one or more of these points can be argued to be incorrect. That’s why I posted this thread. Looking for other opinions.
Trishark
If the boat stays the same, ie. Weight, hull profile (hook), water type and temp, prop blade thickness and material, the horsepower to push the boat at a given speed should stay the same whether supercharged or not. If the prop pitch is increased, since the HP required is the same, the RPMs drop but the same shaft HP is required to drive the boat at a the same speed. As the RPMs on the engine are lower, internal friction of the engine is less; Brake Specific HP (volumetric efficiency) may change a few factions of a percentage points. As speed increases, HP to push boat goes up. In a car, HP goes up at the second power of the speed. Probably about the same for a boat. If the non-supercharged engine produces 400 HP and went 65 MPH with the optimum prop, a supercharged engine is only producing 400 HP to push the boat at 65 MPH, same conditions. Change the prop pitch to reduced RPMs with the supercharged engine, we now have an additional 100 HP and are not against the RPM limit. This HP allows the boat to push past the old speed limit. By changing pitch of prop, the multiplication factor of shaft torque is changed. (Think of the old screw jacks, the finer the pitch, the easier to lift the load.) The optimum pitch allows the motor to exert maximum force to the water as the max RPMs are reached. Max RPMs means either running out of HP before hitting a limit. Too little pitch and HP drives the boat past/against the RPM limit. Too much pitch (less torque multiplication) and the reduced HP developed at the lower engine RPM does not allow RPMs to increase and develop maximum HP/speed.
A supercharged (or natural) engine running at part throttle, steady RPMs, restricts the flow of air (throttles) and therefore the reduced HP is developed. This restricts the boost that a supercharged engine can produce. An accelerating engine with wide open throttle allows the supercharger to flow as much air as the displacement or vane profile and RPM allows and as more air flows than can be crammed into the engine at one atmosphere, we get positive pressure. This is the HP we see on a dyno chart.
Labbing a prop reduces the parasitic losses of turning the prop thru the water. This allows additional shaft HP to be used in propelling the boat forward at a faster speed.
Acceleration is accomplished by the excessive power (some say torque) above what is required to perform the work (moving the boat). More power means faster acceleration. As HP drops off past the peak, some boats will continue to accelerate past the HP peak until the available but decreasing HP thru the prop matches the increasing HP required to propel the boat. The gain in speed past max HP RPMs should be slower than below the peak(?).
A truck pulling a big frontal area trailer (high drag, more like a boat than a car) will get better gas mileage at the lowest RPMs that allow enough HP to be efficiently developed. If it takes 100 hp to run down the road at 70 mph, that can be made at say 4000 rpm in 3rd, 3200 rpms in 4th or 2800 rpms in 5th. All three RPMs would reguire throttling air flow to only develop 100 HP. I would expect the best gas mileage would be in 5th. I am guessing that the increase in mileage is due to reduced internal friction of the engine. The assumption is that the net HP out of the engine is 100, not gross developed including internally used.
In 5th, not enough HP and torque multiplication would be available to pull much past say 4000 rpm. 4th probably would allow higher rpms and maybe a faster speed. .
We prop our boats so that we have enough push to get to some RPM limit. This is good for top end but probably causes cruise RPM to be higher than what is needed. What if a boat had an overdrive so that at cruise we could reduce RPMs and get better mileage?
I’m sure that my reasoning on one or more of these points can be argued to be incorrect. That’s why I posted this thread. Looking for other opinions.
Trishark
