|
|
|
|
|||||
|
||||||
From: Mike Maurice (no email)
Date: Wed Mar 01 2006 - 12:30:43 EST
I have had more than enough time to think about the wind recently,
having ridden out two overnight blows while tied to the dock on the
Oregon Coast. The first storm had short gusts to about 90 knots(100
mph) and the second was a little one with sustained stuff of about 50
mph. You get a lot more sense of wind power while in a boat, as the
weather service records wind gusts using a 60 second average, whereas
we boaters care about and feel the effects of even short gusts of 5
or 10 seconds.
In case you have never tried to think of wind power in terms of
horsepower here are my shorthand calculation formulas that you can
work in your head. Most sailboats will reach hull speed with a wind
of about 17 knots assuming the main and jib are full up. If the wind
speed goes up by 50% say to 25 knots then the power will double. In
other words if the boat requires say 40 hp to reach hull speed then
it takes 40 hp to make an equivalent wind power at 17 knots. It will
take 80 hp to equate to 25 knots, it will take 320 hp to equate to 50
knots and it will take 1280 hp at 100 knots. All this was estimated
on the original assumption of a boat of about 40 feet in length
needing 40 hp to reach hull speed. Remember the power goes up by the
square of the wind speed.
You can estimate the wind pressure on a power boat with no sails by
taking the lateral area EXPOSED to the wind, which would be the
entire side, or might be just the exposed transom area and using the
square footage as if it were a sail and running it through the
formula for calculating the wind force. There is no quick and dirty
way to relate the wind force on the power boat to speed through the
water and the equivalent horsepower like the sailboat example above.
But, you can quickly see that given the amount of windage that most
power boats have, at elevated wind speeds it can require a fantastic
amount of horsepower to maneuver in any direction except downwind.
(This compounded by the usual lack of a serious keel). The 40 hp to
hull speed is a pretty conservative number as most power boats will
require some to a great deal more just to reach hull speed although
they have less sail area than the average sailboat with sails up,
they most likely have more "sail" area if the sailboat has it's sails down.
This analysis is only a rough and dirty way of considering the forces
involved and is not meant to be an accurate way of describing the
forces at play.
Mike
Capt. Mike Maurice
Tualatin(Portland), Oregon
_______________________________________________
http://lists.samurai.com/mailman/listinfo/trawlers-and-trawlering
To unsubscribe send email to
with the word
UNSUBSCRIBE and nothing else in the subject or body of the message.
Trawlers & Trawlering and T&T are trademarks of Water World
Productions. Unauthorized use is prohibited.
|