Conference vicki::boats

    JCL,
I've seen the same thing on my Johnson 88 hp outboard powered 
cuddy cabin.  With the prop shaft horizontal in the water the 
boat will do about 38 mph.  When I trim it out to the point that
the prop is almost starting to cavitate, the boat boat will do
41 mph.  

I once ran the boat with the interior cushions (about 40 lbs.),
anchors, and all other accessories removed from the cabin and
was able to hit 42 mph.  Lifting the bow of the boat definitely
seemed to help, but I don't think that adding weight to the stern
is the way to achieve that affect.  I would concentrate more on
lightening the bow first, either by removing unecessary equipment, 
or shifting the weight as far back as you can.
        
Rick W.

    Jan
    What you are seeing is the effect of reducing your wetted surface area
    on the hull. ie: Less hull in the water means less friction, hence more
    speed per rev. You will also note that at the same time your gaining
    RPM's as the trim angle increases. This will continue until the point
    where the prop looses it's bite and starts to cavitate.
    
    This is all perfectly normal. Not to worry.
    
    If you add trim tabs to your boat (mine gets a pair this winter) then
    you will increase your effective water line and be able to create more
    lift by changing there angle in conjunction with the engine trim.
    All in all the less load/friction  that the motor has to push = the
    less fuel consumption per RPM.
    
    Hope this helps.                            Walt
    

    re: .2
    
    Walt,
    
    Absolutely right about reducing wetted surface reducing drag and
    increasing speed, etc. And yes top speed is usually just 'below'
    the point of complete prop cavitation/with minimum wetted surface. 
    
    With some hulls thaty means bow up...others with pads or large lifting
    strakes ride more level but are still working to reduce wetted surface,
    just using a different approach.
    
    BTW - The trick when you mount trim tabs is to mount them slightly above
    the extended 'line' of your hull's bottom so that they only contact
    the water when positioned at a slight downward angle.
    
    You don't want them flush (or below the hull bottom)...constant
    drag that way.
    
    Mitchell
                              

    .3 YOUR REPLY MAKES SENSE, WHATS YOUR RECOMMEDATION FOR
    
    WHAT DISTANCE SHOULD THEY BE FROM THE "LINE" 1/4" LESS, MORE
    
    I'M GONNA PUT THEM ON NEXT SPRING. CAN YOU GIVE US MORE DETAIL.
    
    JIM.

    Thanks for the replys! 
    Good to hear that others have experienced the same.                                                         
                                              
    As to trim tabs on the boat I'm not so sure. I have them since this  
    spring and I don't feel they help in this context. (They are fine   
    otherwise and I wouldn't like to be without them.) My reason for  
    this: The trim tabs can only force the stern up, there is no way
    for them to directly force the bow up. Even if it were possible to
    angle the tabs upwards more than a few degrees they have nothing to 
    work against if one disregards the air behind the stern!
    A down-force on the stern has to come from either the prop or
    from the weight of the boat.
    The tabs may in fact just be a negative factor by increasing the length 
    of the wetted/planing area even lifted to maximum "bow up".                                                               
                                                                      
    Moving weight from the bow to the stern. Yes, this sounds very sensible
    but I have nothing to move if I dont start rebuilding the boat!   
    I still wonder if there is an optimum to be achived by adding weight
    at the stern. Might there be a stage where the rearwards movement 
    of the c/g outweights the increased total weight? Guess the only
    way to find out is to start testing!                
                                                                      
    My technical mind finds it difficult to accept that everything   
    (c/g, wetted area, angle of attack etc) is ok in a boat if maximum
    speed is achived with the prop axis far from horizontal. I can accept
    that this is the result of tradeoffs in the design but I'm curious
    to know what those tradeoffs are! Ride, handling, seaworthiness..?
         
    -jcl
         
        
        
        

    You can cahnge the amount of transom lift the boat has by the type of
    prop you use. A high rake prop will ift the transom and the bow where
    as a dog ear type of prop will give more bow lift with less efect on
    overal lift. Tabs will lift the transom to some degree but force the
    bow down in doing so, this is counter productive. You can try moving
    the motor back if you have an outboard by using a step jack , gill
    bracket or equivalent but if you have an I/O this is difficult unless
    you are using #4 speeddos and extension boxes. 
      If you boat is trailerable you can experiment with your center of
    gravity by balancing the boat on the trailer axle this of course is a
    crude measurement but a decent point of reference for determining the
    effects of different amounts of gas in the tanks etc.
      
        Have I added anything here or made this more complicated than it
    already is?
    
      Bill