Conference vicki::boats

Jim,
My guess is that the 3.5hp outboard is not enough power to put your
inflatable up on a plane.  In that case yes, there is a maximum
"terminal velocity" that a non-planing hull can achieve.  No additional
amount of power can help to increase that speed.  This is the reason
that trawlers have relatively low horsepower, because any additional
horsepower would just be a waste of fuel.  There is a formula for
figuring that speed based on water line length and a few other factors, 
but I don't have it handy.  You have probably reached the maximum non-
planing speed for your boat at 3/4 throttle, which is why the last 1/4 
turn doesn't increase the speed.

My guess is that the 9.9 or 15 will dramatically increase your speed,
becasue either one should have enough power to put the boat up on a 
plane.  As soon as that happens and the boat breaks free of the water,
the boat should really take off.  Several years ago I ran my old
Evinrude 9.5 on a friend's 10' inflatable, and it was quite a wild
ride.

Rick

The original note here asks 'where is that power going' when he increases the
throttle and the boat doesnt move faster- A question that can only be asked by a 
power boater that has never looked behind his boat. It goes into making a wake!
Huge mountains of water that take lots of energy to raise, then expend
it by dumping some sailboat, or maybe simply rocking every boat in the harbor,
or sometimes simply erroding the shoreline.

    Excuse me Ron.
    
    The reason I have a 3.5 hp motor to begin with is so I can go as
    quietly as possible to the nearest fishing spot and then shut it
    off.  The 195 hp motors everyone seems to have to have these days
    make more wake in neutral than I do at full throttle!
    
    

RE: .2 
>> then expend it by dumping some sailboat, or maybe simply rocking 
>> every boat in the harbor, or sometimes simply erroding the shoreline.


Funny, my boat makes its largest wake just above trolling speed.  Once
up on plane, the faster I go the smaller the wake.  Since this is the
*power* boats notes conference, I assume you just *forgot* to include
the smiley face, right?    8^)
 
Rick W.

    
    
    
    Jim,
    	Regarding the phenominon that you are experiencing, I agree with
    Reply .1, you are not planing. 
    	Regarding the max hp rating, the manufacturer arrives at the size 
    by considering such factors as the designed strength of the transom, 
    the weight of the boat and just how much power you can keep under 
    control. 
    	Overpowering the boat will result in higher speed but may
    result in an out of control situation. It will also result in voiding
    your insurance policy and opening up all kinds of liability should
    an accident occur. 
    Regards,
    Paul
    	
    P.S. I know you aren't planning to overpower your boat. I just thought 
    that the liability issue was germaine to the topic.  
        

    I see a couple of explainations that work ; So I'll add another.
    
    Displacement Hull speed is derived by the hull shape. As it is moved
    forward a drag is created which combined with the pressure wave
    in front of the boat; limit the hulls speed. As was said, adding
    additional horsepower will only consume more energy to accomplish
    the same ammount of work. A losing situation.
    
    A Planing hull is designed to ship accros the surface of the water,
    thereby avoiding the resistance and obtaining higher speeds.You
    rubber raft is made to plane. It does require a very large ammount
    of energy to break away from the water, due to gravity and suction
    on the hull. Thats the reason that you should give a MAX trottle
    to get on plane, then back down to operating speed.
    
    I have a 12ft rubbert raft with a 15 hp on it. It flies! But needs
    to jump on plane, otherwise it just plows water.
    
    Wake is a function of hull design. The more water that is displaced,
    the larger the wake will be. Speed alone does not produce the "Big
    Waves"
    
    Hope this helps.                           Walt
    

    Forget the boating and lets get down to basic Physics.  (Actually
    this is left over from an Applied Differential Equations class)
    
    At a given input horsepower the boat reaches an equilibrium velocitly.
    If the horsepower is increased the speed will go up, this does not
    mean that it will be noticable though.
    
       The relationship is:
    
               Energy In = Energy of Drag
                      
                Energy In is the HORSEPOWER you apply.
    
                Energy of Drag = (CONSTANT)*(MASS)*((VELOCITY)**2)
                                                   
                     The mass is a constant for a given loading of the
                     boat.  As you put more weight in it gets slower.
    
                     The CONSTANT is dependant upon the design of the
                     boat, and will be different if the boat is planing
                     on the water versus plowing through the water.
    
                     What all of this boils down to is that the Speed
                     increases porportionally with the square root of
                     the horsepower increase.  If you doubled the
                     horsepower the speed would only go up by a factor
                     of the square root of two (about 1.414 times)
    
                     Lets look at what this would mean taking some
                     arbitrary numbers:
    
                         Assume 1/8th max horsepower causes 2MPH, then:
                                      
                              HP           SPEED         
    
                               0           0.000 MPH          
                              1/8          2.000  "  
                              1/4          2.828  " 
                              3/8          3.464  "
                              1/2          4.000  "
                              5/8          4.721  "
                              3/4          4.899  "
                              7/8          5.292  "
                               1           5.657  "
                                                              
    The first 1/4 HP (turn) increase from 0.000 to 2.828  =  2.828 MPH
    The next  1/4 caused an increase from 2.828 to 4.000  =  1.172 MPH
    The next  1/4 caused an increase from 4.000 to 4.899  =   .899 MPH
    The final 1/4 caused an increase from 4.899 to 5.657  =   .365 MPH
    
    I beleive what is happening is that the speed is still increasing,
    it is that it is increasing at such a slow rate that you can't notice
    it.
    
    I hope this makes some sense.
    
    -Joe
     
    
    

    Joe .
    Your logic is souund assuming constants. In a planing hull
    configurationthe coeffecents of drag are changing in an assemetry
    with a medium transfer included between the transition from water
    to air. The angle of attack in both is also changing as the boat
    comes on plane, this will change drag also.
    
    Actually the best , most effecent answer is to row.
    
                                    Walt
    

    RE .8
    
    I recognize that the drag changes the further up the step you
    get on a plane (I don't know the correct boating terminology,
    transitioning to a plane is often called climbing the step in
    Sea Planes).  Anyway, I chose to ignore that and assume two
    basic configurations with seperate constants.
    
    If one can characterize the transition from fully plowing to fully
    planing in a mathamatical formular then determining the terminal
    velocity one level higher order of differential equation.
    
    But really, who cares?
    
    -Joe

    Yea Thats the ticket.//
    
    Now lets take a look at a displacement hull, Lets use a sailboat
    (Don't hit me) as an example. At rest the hull displaces a constant
    volume of water. If we were to go to a constant velocity at the
    same trim/level there would be a point where speed would max out,
    no mater how much additional energy is applied. Simply because drag
    would balance out any additional power applied. But in real life
    the angle of attack changes as the boat is forced forward. Like
    in your airplane "lift" is created which changes the profile of
    the hull as seen at the waterline. This also occurs as the boat
    heels over, exposing a larger surface area of lesser angle and
    therefore higher lift potential. This will allow more speed to be
    obtained.
    All this neat stuff is considered or should be when designing a
    hulls contours for maximum proformance
    
                                        Walt
    

    I would like to add my two cents to this conversation because some
    of the replys make me "see sickness" The max H.P rating is a result
    of the manufacturers design of the transom (thats where the motor
    fits) Did anyone ever think that just maybe a 15 H.P. engine has
    a different size propeller. (larger diameter and pitch) This will
    give you more push behind the boat = better performance. Also your
    9.9 H.P. might be fine on a perfect day but the additional H.P.
    might also help you out when you are going against the "WIND & TIDE"
    if you use it in costal waters, if not, delete TIDE.

    Getting back to the original subject of my modest 10' inflatable
    with the dinky 3.5 Tohatsu...
    
    I've used the boat most of the Summer and after some experimentation
    with the info about planing in the earlier notes, these are my observations.

    It doesn't matter whether I start slow or fast. I still get 
    more noise but no additional speed in the upper 1/3 of the throttle
    range, BUT...
    
    Phenomenon discovered completely by accident:  When alone in boat,
    boat normally rides very with bow fairly high (seems normal with
    my 200 lbs plus motor all at stern).  Notice some loose piece of
    gear forward so I get down on my knees in the middle of the boat
    to grab it and guess what?  The boat doubles its speed!  Upper
    throttle range now working!   Move back to sit down again on the rear seat
    - still smoking along!   But if I drop power and then restore it
    while sitting at stern we're back to "normal" (i.e. slow).
    
    Try repeating experiment by adding second person to boat and shifting
    the two of us around but can't reproduce. Still in slow mode.  Gotta
    be just one person with weight approx centered to get fast mode.    

    Any guesses as to what's going on?

    Seems to make sense to me...
    
    When you ride alone, shifting your wieght forward lets the boat
    get on plane, and the motor has enough power to keep you there when
    you return to aft.
    
    With the second person, the 3.5 just doesn't have enough power to
    get on plane - regardless of position.
    
    The only thing you may be able to do to get the boat to plane with
    two people is play with the trim angle of your motor. This also
    may help you get on plane without shift your wieght forward.
    
    Any other comments?
    

    It's basic physics. Acceleration of a mass to X velocity requires X
    force. To maintain the velocity as resistance is decreased requires
    less acceleration.
    
                        Vt=M(A/R)
    
    Thats about as simple as it gets.
    
    
    Rx: Buy a bigger engine.
    
                                 Walt