With 100% efficiency, speed would be 32.396 mph. You can compote your own efficiency by comparing to actual speed.
With 100% efficiency, speed would be 32.396 mph. You can compote your own efficiency by comparing to actual speed.
First off the formula is based on screw propellor theory that is based on a completly submerged propellor and with out cavitation or ventilation. The high speed boats run water piercing props that have a good portion of the blades above water and are ventilated (not cavitating), thus the formula was not based on these conditions. However, I have used the above formula to check against my hydro speeds both with gps and water pickup speedometer and all are within a few mph of each other, I run the prop shaft about 3/4" below the bottom so it is surface piercing. As noted in the calc the key features with boat speed determination are rpm, gear ratio, pitch and slip. RPM s and gear ratio are easy to measure. Pitch is measurable and very dependent on where and how measured, by what equipment, and who does it. The supposed standard is somewhere near the 70% radius at the midpoint of the blade cord. But there are other ways to measure: average across the variable pitch blade at a certain radius, average of many points on the blade at different radius, blade contour mapping, etc. Slip on the other hand is not directly measurable with a tool like pitch. It must be calculated somehow from measured results of speed, rpm and pitch. The usual way is to use the formula above.
When people report pitch most do not know if it is accurate or how is was determined other than the stamp on the blade (if there is one). So that is an important part of the speed and must be accurate. If there are reports of very different speeds than what the above calculation gives than I would suspect a problem with the pitch number they claim that is being used in the calculation.
So with any rig the real answer is speed is what ever you measure with the equipment at hand. Myself absolute speed is not as important as differences in speed with changes made in setup and props. To be the last word on absolute speed just mark off a measured distance and run threw it and record the time with a stop watch and calculate the average speed over that distance like they do at kilo runs or Bonniville.
prop calcalations will only get you in the ball park not a exact science after racing boats for 30 years more than one time I have tested 3 props right out of the box. same prop same pitch same boat same water props new from mercury test run back to back with in a short time. and one prop has been as much as 4MPH faster. do the same test the next day with same results.in my opinion its impossible to tell somebody exactly how fast a boat will run by prop cal. if you could their would be no reason to race.just saying.
Strictly business
Obviously, the result can be no more accurate than the input GIGO
I have raced boats for a great number of years. One thing experience has taught me. Never go by an instruction manual, and never believe a theory. If we all went by the books and theories, where would the world be today ? Think Mr. K listened to the critics when he developed an outboard ?
One huge thing to think about, what works on one persons rig might not work well on another. There are way to many foctors at play to calculate anything accurately. I would say add or subtract ten MPH from those sums. Those formulas get you to a starting point, after that it is up to the individual to tweak and achieve the desired result. In addition, those formuals do not account for certain modifications some do internally. Like I said, way to many things to account for each one. Testing is key.
sounds good to me...the problem some have they cheat themselves,
out front again
Ahhh yes, i agree with the testing
but theory when given figures can only be right in the maximum possible.
to get to that figure is where the testing comes into play.
all the testing in the world cannot get you past the maximum possible attainable speed. !!!! the testing is trying to get as close as possible to this speed is what its all about.
we all like to think our boats and cars are farster than they are, ive raced car down the drag strip years ago.
and the amount of times i beat cars that were supposed to be sub 9 second cars was unbelievable.
I never got to maximum theory, neither did donald cambell, neither has anyone who has broken the
landspeed record or air speed record.
No boat has ever gone past or even too its maximum mathematical speed, !!! this is due to many factors.
the boat being the main factor in this case, the absolute perfect boat with zero drag/slip has yet to be invented.
there is no boat anywhere in the world that dont have a certain amount of drag.
And there has not yet been a propeller invented that is 100% perfect, they all have a certain amount of slip and drag to them.
but given a set if figures like revolutions per minute, and a gear ratio and a given size propeller, there is a
maximum speed, there is not getting around it with testing and raising the engine, lowering the the engine or filling
the boat up with helium,the max speed is max speed and that is the target, it cannot possibly be passed.
now anyone who thinks there boat is faster than the maximum calculated speed given the figures of rpm's, gear ratio, and prop pitch is a fool who is lying to himself.
and he is not fooling anyone.
at the top i gave figures, the figures are easy to calculate or to use on a formular, the speed reached can never ever possibly be faster....its just a fact.
test as much as you like with a gps, but dont take ya gps as gosple, no world records rely on gps they use a set length
and a timer, its far more accurate than gps.
with gps you are relying on the satelites , gps is great for pin pointing, but even then not 100%.
gps is a signal sent from a device to a set of satelites and then bounced back, this again takes time.
but in this particular case, the gps is way off target by around 20%.... or the figures quoted of pitch,rpm and gear ratio.
the gear ratio is not suspect in this case as its not going to be different.
33mph with a very good tail wind (hurricane strength) COULD be possible on a race boat.
but what a boat that would be
phill
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