Conference vicki::boats

    I doubt you'll find any 20% slip boats. That would be too good.
    Using your formula, it says my 31 footer should do 56.8 MPH at 3000RPM
    with a pitch of 20. It is really doing about 20 MPH at 3000 RPM
    - a slip of over 50%. 
    

    RE: .0
    
    How are you measuring your shaft RPM? If you are measuring it using the
    engine tach, you forgot to factor in the gear reduction. For example,
    most small V8 Mercruiser's are 1.50 : 1. I wrote a basic program last
    year and offered it in this file, but no one seemed interested. I made
    some boat weight vs slip assumptions that are only rough guesses. I
    will post it here if someone wants it or can send it by mail. Its in
    Vax Basic. All I ask is some feedback on whether or not is seems close
    to your application. 
    
    Scott

re                       <<< Note 575.2 by ATE030::MALCOLM >>>
>                      -< Don't forget gear reduction.... >-

>    How are you measuring your shaft RPM? If you are measuring it using the
>    engine tach, you forgot to factor in the gear reduction. For example,
>    most small V8 Mercruiser's are 1.50 : 1. I wrote a basic program last
>    year and offered it in this file, but no one seemed interested. I made
>    some boat weight vs slip assumptions that are only rough guesses. I
>    will post it here if someone wants it or can send it by mail. Its in
>    Vax Basic. All I ask is some feedback on whether or not is seems close
>    to your application. 
    
>    Scott

	I was assuming the reduction in the box, somewhere in .0 I 
used the term  "shaft RPM".  I had played around with the numbers in a 
spread sheet but hadn't made a print listing to include in the note, 
so I was working from memory.   Errrr, I'm doing it again - anyway the 
engine does 4100 RPM and that feeds a 1.23:1 box(transmission) and I 
think that comes out to 3333.33333333333333 (or so).  Yes, that boat 
does footin' speed with a 16 inch pitch prop, so its 40'ish.

	Reg

	{More accurate measurements will be made following (or during) 
	the spring thaw (-: }

RE: Note 575.1  

>>  I doubt you'll find any 20% slip boats. That would be too good.
>>  Using your formula, it says my 31 footer should do 56.8 MPH at 3000RPM
>>  with a pitch of 20. It is really doing about 20 MPH at 3000 RPM
>>  - a slip of over 50%. 

Comparing the prop slippage of a 31 foot cruiser to a smaller more efficient 
hull is like comparing apples and oranges.  Most relatively small planing 
hulls not only do better than 20%, they can do better than 10% slippage. 
Let me show you by using my boat as an example.

Using Reg's formula and based on the following for my 17'4" Sunbird cuddy:

prop:       17" pitch
top speed:  41 mph  (by my speedo and Trailer Boats magazine's test results)
engine rpm: 5300    (by the factory OMC tachometer)
gear ratio: 13:26   (.5)

shaft rpm   x pitch / 1056
(5300 x .5) x  17   / 1056 = 42.661 mph

The theoretical speed is 42.661 mph, the observed speed is 41 mph.  This 
comes out to 96.1% efficiency, or 3.9% slippage. A "tad" better than 20% 
slippage.  Even allowing for some error in the tach and speedo, I'd say 
the prop is still running at less than 10% slippage.

While we're on the subject of efficiency and prop slippage, has any ever
compared the efficiency of a prop driven boat to a jet drive?  I had the
chance to do just that at one of our September gumballs in a very 
"unscientific" drag race with our friend Jeanne Decarolis as a witness.  
I raced my 17'4 Sunbird with 88hp outboard against my friend's 17' Chrysler 
with 260hp jet drive.  My boat blew his away, both off the line and on top 
speed.  The outboard also beats the inboard jet drive on noise, weight, and 
fuel economy.

Rick W.

    Rick, in your specs you said 13:26 ratio.  Doesn't that mean 13 revs in
    and 26 revs out.  So your ratio is 2 instead of .5?
    
    Rich

    Here is some data I took last summer because I was curious as to how
    RPMs correlated to MPH:
      Boat is a '87 Malibu Skier w/ Chevy 350 C.I.D.(Indmar). Transmission
    is a Borg Warner Velvet Drive 1:1 ratio w/ 13X13 Nibrel bronze prop:
      Test course was an Accufloat slalom course (875') and instruments
    were electronic stopwatch and 2-Airguide tournament speedometers
    calibrated to AWSA specs...
    
    RPM           Time        Speed(indicated)
    2000          28.44        22
    2500          20.42        29
    3000          16.8         36.5
    3500          14.7         41
    4000          13.38        46
    
    The above numbers prove two things:
    1) tournament speedos aren't accurate accross the whole range, only the
    speed they are calibrated to. (consult your AWSA speed/time chart)
    2) this was not a very scientific test, as I only did one pass at each
    RPM(indicated) using a handheld stopwatch, viewing indicated speed.
    Therefore, to much redundency here.
    
    Doug_FWLIW (for what little it's worth)

    
    RE: .5
    
    Rich,
    You had me thinking about that for a second, until I worked the
    numbers.  A 2:1 gear ratio, with 5300 engine rpm and 17" prop 
    gives a theoretical top speed of over 170 mph!  I wish it were
    possible!
    
    The engine tech sheet says:  13:26 (.5)
    
    What it means is that there are 13 teeth on the input shaft, and
    26 on the output shaft, giving a .5:1 ratio.
    
    Rick W.

    A few more chicken scratchings.....
    
>      Boat is a '87 Malibu Skier w/ Chevy 350 C.I.D.(Indmar). Transmission
>    is a Borg Warner Velvet Drive 1:1 ratio w/ 13X13 Nibrel bronze prop:
>      Test course was an Accufloat slalom course (875') and instruments
>    were electronic stopwatch and 2-Airguide tournament speedometers
>    calibrated to AWSA specs...
    
>    Doug_FWLIW (for what little it's worth)

FPM Feet per Minute  (RPM X Pitch)/12
Time for 875' course (875/FPM) X 60

    RPM           Time        Speed(ind)  Speed(calc)	FPM	Time(calc)
    2000          28.44        22		24.62	2166.66 24.23
    2500          20.42        29		30.77	2708.33 19.38
    3000          16.8         36.5		36.93	3250.00 16.15
    3500          14.7         41		43.08	3791.66 13.84
    4000          13.38        46		49.24	4333.33 12.11

Interesting......

Rick    

re        <<< Note 575.7 by NRADM::WILSON "A man's place is on his boat" >>>
>                          -< World's fastest cuddy? >-

	You have it right, your prop turns much closer to 2,650 than 
10,600.



	Re Doug, geeze - looks pretty good, assuming,,,duhhh, etc.

	Lookit, see this here spread sheet stuff:-


 Reduction      1.00

     Pitch     13.00
                                                                  Prop slip,
                     |---------  Theoretical  -------|  Observed  percent of
                     inches per   inches per Miles Per Miles per theoretical
Engine RPM Shaft RPM     minute         hour      Hour      Hour

  2,000.00  2,000.00  26,000.00 1,560,000.00     24.62     22.00      10.65%
  2,500.00  2,500.00  32,500.00 1,950,000.00     30.78     29.00       5.77%
  3,000.00  3,000.00  39,000.00 2,340,000.00     36.93     36.50       1.17%
  3,500.00  3,500.00  45,500.00 2,730,000.00     43.09     41.00       4.84%
  4,000.00  4,000.00  52,000.00 3,120,000.00     49.24     46.00       6.58%

	I know its only the speedo readings and if I took the trouble 
I could convert your times to speeds, etc, etc,  but it does kinda 
look as though your rig has its least prop slip right around where 
you'd expect to run a ski boat  -  *_SOMEONE_*  knows what they're 
doing, wun'cha say ?

	Reg

re Doug's numbers:

	Using the timed runs instead of the speedo readings comes out 
a little worse, but it still looks as if you have minimum prop slip 
right around tournament ski speeds - no surprise I guess, probably a 
design goal (-:   Wind, current, course length, tach error and 
the observer's ability with a stop watch remain ?;  bah, they're 
awash, though I'll probably set up some groups of runs in each 
direction next year - partly to calibrate things, partly for curiosity 
now that I've started this.  Though once the lakes and rivers thaw I 
just might wanna play on the water instead of on THIS tube (yeah, I'm 
a closet tube'r too).


 Reduction      1.00

     Pitch     13.00
                                                                  Prop slip,
                     |---------  Theoretical  -------|     Timed  percent of
                     inches per   inches per Miles Per Speed MPH theoretical
Engine RPM Shaft RPM     minute         hour      Hour

  2,000.00  2,000.00  26,000.00 1,560,000.00     24.62     20.37      17.26%
  2,500.00  2,500.00  32,500.00 1,950,000.00     30.78     28.37       7.81%
  3,000.00  3,000.00  39,000.00 2,340,000.00     36.93     34.49       6.62%
  3,500.00  3,500.00  45,500.00 2,730,000.00     43.09     39.41       8.53%
  4,000.00  4,000.00  52,000.00 3,120,000.00     49.24     43.30      12.07%


re my original numbers:  I was working from memory and had run the 
boat with uncalibrated speedos (new boat, first time out, etc.), i.e. 
these were not numbers from any kind of a controlled experiment.

    Yeh, the efficiency numbers seem close based on the boats
    characteristics. Around 20 MPH, the boats pushing alot of water;i.e.
    less efficient. At 36 mph the boat is at optimum towing speed, on plane
    and is pushing less water (as indicated by the small [non-existent?
    ;^)] wake.)  The best fuel economy is also achieved around this (3000
    rpm) speed. After that, up around say 4000 rpm, even though there is a
    slight decrease in wake size, (i.e. pushing less water) the increase in
    hull drag results in more prop slip/less efficiency.
    
    Makes sense to me.
    
    Doug
    BTW: The time trials I did were not in the same direction each time,
    therefore wind/current did play a factor. (as well as
    stopwatch/speedo/tach redundencies.) 
    We'll have to do a *Real* study in the spring...

re                        <<< Note 575.6 by SETH::WHYNOT >>>
>               -< Here's the numbers, now fill in the equation. >-

>      Test course was an Accufloat slalom course (875') and instruments
>    were electronic stopwatch and 2-Airguide tournament speedometers
>    calibrated to AWSA specs...

	Errr, ummm,  875 feet ?   

	I had assumed a metric spec course of 259 metres, gate to gate.

	Anyway, springtime is THE time - lets ajourn ?

	R

    oops, gate to gate should be 850'. (The 875' is overall length)
    There goes the numbers...
    Yeh, lets wait 'til spring and start over...
    DW

               -     o     skier bouys       o                       o
               |                                                    /
              38'                                                  /
               |                                                  /       
 o -    -o     |     o           o  boat     o           o       /   o   end o
   8'   7'     -                     guide                    154'       gates
 o -    -o     |     o           o    bouys  o           o   /       o       o 
               |                                            /  
              38'                                          /  
               |                                          /  
         o     -                 o       skier bouys     o


 |-88.5'-|--134.5'---|--134.5'---|--134.5'---|--134.5'---|--134.5'---|-88.5'-|

	Let's see, my 13 X 13 Nibral prop theoretically should push my boat at
51.7 MPH, it actually pushes it at 42 MPH, that's 19% slippage. The 14 X 16 
stainless steel prop I picked up over the weekend theoretically should push
my boat at 63.6 MPH. Using 19% slippage, actual speed should be 51.5. Gee, it
would be big fun to hit 50 MPH in my boat, then immediately remove the 16 pitch
and put the 13 back on so slalom skiing doesn't suffer.

	Of course, I have no idea what the 13" vs. 14" prop diameter will do
to the equation. Any ideas?

	Also, what about over/under propping? I know the phenomenon exists,
but was at a loss to explain it to my father this past weekend. Higher pitch
props can cause overevving while lower pitch can cause lugging? Is this
correct? Why?

Rick

ps. You won't believe the story behind this SS prop....

	Drawing on Rick W.'s expertise in note 254, apparently Over Propped
means the prop pitch is so high that the engine doesn't have enough power to
spin it at the engine's max RPMs. (Max RPMs might be 4400, but can only attain
4000).

	Under propped, so Rick says, means too low of a pitch in the prop.
Apparently, not enough force/drag is exerted on the engine by the lower pitch
prop so it allows the engine to spin above the recommended max RPMs.


Rick

    So are you going to let us know your results after you try this new
    prop?  I would be vary interested ... maybe even buy one myself.  Mine
    does about 46 MPH with no load and drops back to 43-44 MPH with a 240+
    pound barefooter.  Barely fast enough for two feet, not near fast
    enough for one.
    
    /Steve
    

    
    	Had a chance to try out the Stainless Steel 14X16 prop on the
    Nautique this past weekend, in place of it's factory 13X13 nibral
    wheel.
    
    	The results were very interesting....
    
    	The Good....
    
    		Out of the hole performance suffered only slightly.
    
    		32 MPH is now 2200 rpms, not 3100 rpms (engine's
    			working less and much quieter)
    
    		The WAKE! I hadn't even thought of this aspect, but
    		the wake became considerably smaller! I assume this is
    		because the prop is turning slower so prop wash is
    		diminished and the wake has more time to flatten out
    		before it reaches the skier. This change was very noticable
    		on both a slalom ski (wonderful!) and a kneeboard (not so
    		wonderful for wake jumps).
    
    
    	The Bad...
    
    		RPMS... A 351 V8 of 240 HP and 1100 hours would only push
    			the 14X16 prop to 3100 rpms. Top speed of.... 42 MPH,
    			same as the 13X13 turning at 4200 rpms.
    
    	The Ugly....
    
    		It's for sale!
    
    		I think this prop would be awesome on a boat that simply
    		has the ability to turn it up to the engine's max RPMs.
    		Either by a gear reduction (mine's 1-to-1) or just more
    		HP.
    
    		I'd also bet the 14X16 would do wonders for reducing the wake
    		on Sport Nautiques for instance. Since Sports have a gear
    		reduction and possibly more HP, it might be the ideal
    		setup.
    
    
    	Rick
    		It_sure_was_nice_skiing_on_Saturday!
    				_especially_considering_that_its_November!