T&T: Hull speed theoretical

[John Baker]

You might need it to tow something or somebody.
 
You might need it to get yourself ungrounded.
 
You might need it to get someone else ungrounded.
 
You might need it to haul around all the extra stuff you and your spouse pile
onto the boat.
 
You might need it to get past all the friction caused by all the crap that
grows on the bottom of your boat that the theoretical formula does not account
for.  Your boat gets a little slower every minute it is sitting in the water.
 
You might need it to maneuver in tight quarters with heavy wind or
current(this is where current comes into play regardless of hullspeed...you
want the boat to change directions right NOW!!!).
 
You might need it to get your boat to hullspeed because the hull shape is not
what this magical equation assumes.
 
Phil, this is just some stuff I thought of right of hand.   Maybe someone else
can think of some others.  My main point here is that we live in reality and
not in theory.  The boat I own right now is semi-planing and has the power to
plane so it doesn't count in this discussion.  My previous boat was a
displacement cruiser.  1800rpm power put it right at hullspeed and that is
where I cruised it.  That yielded about 6.7kts thru the water(small boat).  As
time went on(months or even years), that speed would deteriorate so I could
either accept the slowdown or push the power up a little bit(or haul the boat
and clean it and/or get new paint).  WIth your theoretical boat/engine, you
can't really push the power up because you start straining the engine and
siginificantly increasing fuel burn.  Are you willing to accept the slow
degradation of speed mainly due to drag and weight?  Are you willing to cruise
it at 80 or 85% power as your perfect
 theoretical boat becomes not so perfect in reality?
 
Another thing to consider for power settings and calculations is that there is
a power CURVE and not a power LINE!  In other words, the power that your
engine produces is not linear....especailly if the engine is turbocharged. 
1800rpm/2600rpm equals 70% if we assume linear power.  My only point here is
unless you have the power curves/charts given by the manufacturer, your
estimation of percentage of power is a very rough guess.  My current engine
produces 190hp at 3100rpms and 240hp at 3300rpms.  That is 25hp per 100rpms. 
If that was a linear relationship(25hp per 100rpms), my little Yanmar would be
a 750hp engine at 3000rpms....you get the picture.
 
Trawler on,
John

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