Originally Posted by Plumper
Quite the dissertation. I'd have to argue with the 80% power going backwards. That seems excessive to me unless your boat is way under powered. Some boats could drag their anchors around all day with 80% astern power.
We'll have to agree to disagree then on this point. If you can drag your anchor with 80% sailboat aux power you'll most likely have issues in anything over 25-30 knots.. I can't budge my anchor even swinging a big ol' fixed three blade on a 2003 four cylinder 44hp Westerbeke and that's the way it should be. If I could drag it I'd buy a bigger anchor.. Keep in mind many reversing gears are NOT the same ratio in reverse as in forward..
It's interesting to note what the West Marine/Sail Mag test boat used as an engine and prop to try and drag anchors intended for 35-40 foot boats!!!!!! They did not drag them around the bay with a sailboat aux engine!!
"We were able to apply and accurately record how the anchors reacted to up to 5,000 pounds of load, thanks to the 52-foot, 92,000-pound research vessel Shana Rae, which we chartered to do the pulling. Equipped with a 375-horsepower diesel engine and a 40-inch-diameter propeller, Shana Rae allowed us to apply more force on each anchor than it would ever experience in “normal conditions.”
Converting HP to thrust is a complicated science, with all the variables, and far to complicated to go into here. Suffice it to say that depending upon prop and other variables thrust is anywhere from 20lbs to 35lbs. per hp.
So let's take a look at a Universal M-25, a very common sailboat aux engine. At 3200 RPM it develops 21 hp however max RPM is rated at 3000 RPM. At 80% of rated RPM it will develop about 18hp. Now if you figure you have a two blade sailor prop with a 45% efficiency, as many do, and a reduction gear that is half the speed in reverse as forward the equation would look something like this.
18 hp X 25lbs thrust = 450lbs Now subtract your prop efficiency loss and reduction gear X% of forward for a given transmission and you're likely down to a thrust of bellow 200lbs.. If you can drag an anchor around on less than 200lbs of thrust than as I said you have some serious issues..!
BTW my old Catalina 36 had an M-25 in it. In 42 knots of wind that boat will be pulling between 1800 and 2100 lbs... Even in 30 knots it's pulling between 900 & 1100 lbs...