Question for all the Smart ones.

[formulaben]

5 minutes ago, tccombs said:

3  now  lets say you add 1000lbs to the boat .  The boat has more drag  now  because  it displaces more water.    so now the  boat is still at 5000 rpms..  but now the speed has dropped  a bit   to say  40 MPH.      so the prop is spinning at the same rate for less of a result  which means increased drag.. therefore more engine stress.  it is doing the same work for less result..

The first part of the last sentence is correct, the drag increased...but the engine has no idea if you've added ballast or not; if the RPM and throttle are the same then "doing the same for less result" is not true because you're hauling 1000lbs more than you were before.  The compromise is less speed, not more engine stress.

[tccombs]

8 minutes ago, formulaben said:

The first part of the last sentence is correct, the drag increased...but the engine has no idea if you've added ballast or not; if the RPM and throttle are the same then "doing the same for less result" is not true because you're hauling 1000lbs more than you were before.  The compromise is less speed, not more engine stress.

Okay  i'll agree your right  on that..

but i believe my overall conclusion to be accurate

[MadMan]

2 hours ago, tccombs said:

so with that in mind...  my definition  of "max stress" ... would be   "the point where you are using the most power  to achieving the slowest  speed."     

in other words     that point would have to be when you tie your boat so it cant move..  and floor it . 

 But for any practical  real life condition , then  I think that must be  " Surf configuration" for most of our boats 

 

It appears that you've created a definition to give the result you want.

[Woodski]

@tccombs:  You are making the assumption that prop slip has either no effect or increases drag.  An increase in prop slip can or will actually reduce the resistive torque on the prop shaft.  If you keep the same engine / shaft RPM and simply slow the boat down, by definition there is more prop slip. 

[justgary]

^^^ 3 and 4 above are the same thing, except that 3 can't happen.  Add weight, the rpm will drop.  When rpm drops, you are no longer at max power for the engine (rpm*torque), but might be closer to max torque (which probably occurs around 4200 rpm or so). 

Ideally, you should change the prop for each of these conditions so that you *can* get the engine near 5000 rpm at WOT.