It should be comforting that TP45 has declared that global warming isn’t happening and if it is man has no affect on it. However none of that comforts me about the projected path and strength of Hurricane Irma. She is a big category 4 storm barreling down on the Northern part of the Eastern Caribbean.
Prior to this Irma threat my only exposure to an Irma was a joke Captain Ricky tells. This particular joke can’t be repeated here… facial contortions are required.
The Saffir-Simpson scale for measuring hurricanes is shown at left. Some examples of Hurricanes at those levels are shown in the right hand column of the chart. Andrew made landfall in Florida in 1992 as a Category 5 storm. Forty four people died in Florida with massive damage from wind and surge.
Everyone hopes that Irma does not intensify to Category 5 but it is possible. If her path does not hit the DR or Cuba she could intensify further. The mountains and land mass can reduce the energy of a storm. The downside of that is the energy is transferred into damage on the land involved.
As sailors we use the wind to power our vessels and cool our vessels at anchor. We need to remember that the power of the wind is not a linear relationship with the wind speed. This means that a wind at 20 kts is not twice as powerful as a wind at 10 kts. This chart has the wind speed in meters per second. The conversion to knots is about a factor of, 5 m/s is almost 10 kts of wind. The ability of the wind to exert pressure is more than twice when the velocity (speed) is doubled. The equation for dynamic pressure is: dynamic pressure=density of the air (kgs/m)*velocity²(m/s).
This dynamic pressure has to create a force on some object that tries to impede the flow. That’s why we strip off everything on a boat that might restrict airflow, hence increase the wind load on the boat. So, if air density is constant and the stuff on our boat exposed is constant the wind load on the boat at 100 kts of wind (50 m/s) is 1500. At 20 kts of wind or 10 m/s the wind load is about 85. The wind speed has increased 5 times while the load from the wind has increased about 16 or 17 times. I don’t know if I’m teaching or just getting my mind around some first year physics (Waterloo 1972), perhaps D. Doug Becks?
Irma is bearing down on lots of folks we have met and friends who have boats in the path of Irma. Gunky Lump’s boat is on the hard in Antigua. His boat should be ok, he’s in a reasonably well protected hole. Captain Ricky’s boat is in the water on a hurricane mooring at Penn’s landing in Fat Hogs Bay on Tortola. We have to hope that the eye of the storm passes far enough north to keep Grand Cru safe.
All we can do is wait and hope that preparations made will be sufficient. Fracas awaits in Florida.