<< > In the recent thread on how big a motor to put on a P-19 several folks
have
> mentioned that one should go to a higher horsepower motor if sailing in
an
> area where there are high currents. I'm confused by this, isn't a current
> just moving water? It seems to me that if the boat is equipped with a
motor
> that will drive it at hull speed there would be little benefit in using a
> bigger, heavier, more expensive one. A hull speed in still water of 5
kts.
> will get you five kts. over the bottom. Hull speed against a 3 kt
opposing
> current will get you 2 kts. over the bottom. How is more horsepower is
going
>
> to change that? What makes more sense to me is how motoring against a
head
> wind, towing something or being heavily laden could benefit from more
power.
>
> Thought I'm suspicious that with a propeller pitched so that hull speed
(or
> a
> little more) was reached at the peak power of the engine even heavy
weight
> or
> head winds wouldn't be an issue. When you see an 5 horse engine pushing a
> Zodiac along at 12 or 15 kts, you've got to wonder whether that same prop
is
>
> the right one to be pushing a heavy sailboat at 5.5 kts.
>
>
> Dave Kautz
> P-15 #!1632 Tilly Lucy
> Palo Alto, CA
>
There is an online calculator for hull speed (using Dave Gerr's formula for
displacment and semidisplacment hulls) and horse power at
http://www.yachts4sail.com/hullspeed.html
Accroding to the calculator, it takes 6.7 hp to drive a 2000 pounds (loaded)
P19 with an LWL of 16.25' at hull speed of 6.3 knots. This fits well with
Jerry's observation that his 5hp Nissan can't push the boat quite as fast as
he can sail it.
I have a 5 hp Nissan 4-stroke, and it does the job. Not to hull speed, but
good enought for SF Bay and Sacremento River Delta currents. With today's
technology, anything beefier than that would just be too heavy for me to
handle.
To make hull speed with an opposing wind, you'd need more than 6.7
horsepower. As you pointed out, an opposing current doesn't enter into the
pciture..
Best,
Judy B
>>
Ah! We revisit this question every year..
The law does not allow you 30 minutes to get out of way on a busy inlet (I
mean busy!) with bigger vessels restricted to same channel you are on/or
blocking. So you better have power to get out of way or transit quickly.
Otherwise you are responsible for damage to much larger vessels by blocking
their channel/inlet. A 5HP barely holds its own at tide full swing on any
narrow inlet transiting the lift bridge. A 6HP only manages 2 mph against
that current. There is also a question of prop pitch/size involved, the 6HP
having the larger prop/edge.
But then again you can stick to channel at full power making 1mph against
current
with a 5HP and incur a liability for a bigger vessel running aground simply
cause you are in the way and prevent them from attaining steerageway.
Or you can watch them towering 12' above you and 2' from your stern trying to
get under the bridge at inlet... NOT ME! I'm getting outa the way Buster..
Many times at inlet I've wished my puny 6hp were a 350 cubic inch
turbocharged V8..
SF