[Public-List] Yamar 2cyl 15hp

[John Riley]

Mike Lehman wrote:
> But it you need the extra HP to go through an inlet, or bridge, or
> fight some strong current, you can turn of the ignition switch, which
> will turn off the tach and the alternator and give you the extra power
> you may need. 

Mike,

I'm sorry, but I'm going to have to call "garbage" on you seminar presenter.

Speed through the water is speed through the water and it makes no
difference if the water is moving or not...the HP to the prop is
immaterial to whether you are going through an inlet, under a bridge or
fighting a strong current, unless you are in the habit of motoring at
far, far under the rated horsepower of your engine.  The power curves
tend to be fairly flat near their maximum efficient RPM's.

If we assume the engine can push the boat at hull speed at something
less than her rated horsepower anyway...and I guess most of the diesels
on A-30's can...NONE of this other stuff matters.

My point is that if while charging the batteries the boat can go 5 knots
through the water in still water, she will go 5 knots through the water
in a current.  If the water is moving, the speed over ground of course
changes...and therein lies the problem and why I think this incorrect
idea of "increasing power to motor through a current" persists.

We boaters have grown so accustomed to reading speed off the GPS that I
guess we often forget it measures speed over ground...a different
animal, of course.  So, when the little number on the GPS decreases, we
think "more power to the shaft is needed....", but in general, in cannot
help, unless you are grossly underpowered to begin with (and I question 
the loss of 3 HP as being "grossly underpowered").

And, it may hurt.

There are situations where burying the stern into the stern wave makes
the boat unsafe...closer to the 'edge' of a handling stability curve. 
There are a LOT of reasons to motor at 5 knots through the water (number
chosen basically at random...ideally, this should be your 'max efficient
motoring speed') and be happy with it even if the GPS registers slower
speed over ground.

I'll stop now.  This is just one of my pet peeves...the A-30 is not a
motorboat than can just power it's way through currents.


-- 
John S. Riley
S/V Gaelic Sea
1972 Alberg 30 #521


 1331847224.0