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Go Back   SailNet Community > Skills and Seamanship > Learning to Sail > Course to steer at a given speed.
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Thread: Course to steer at a given speed. Reply to Thread
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Topic Review (Newest First)
12-07-2012 01:38 AM
jackdale
Re: Course to steer at a given speed.

Quote:
Originally Posted by chucklesR View Post
he said the magic word 'leeway'

What he didn't say was "oops, I can't steer that high, so I fell off 3 degrees, then had to tack 3 miles to starboard' because the on shore wind shifted.

Actually it can work the other way. In 2007 Van Isle 360, when we were coming back from about 55 miles offshore, we got this great lift that took us to within about three miles of the leg finish. Then the wind died - offshore breeze competing with the prevailing westerlies.
12-07-2012 01:10 AM
chucklesR
Re: Course to steer at a given speed.

Quote:
Originally Posted by jackdale View Post
One way to do the calculations

https://www.youtube.com/watch?v=7ahoGVGA1do
he said the magic word 'leeway'

What he didn't say was "oops, I can't steer that high, so I fell off 3 degrees, then had to tack 3 miles to starboard' because the on shore wind shifted.

12-07-2012 12:51 AM
jackdale
Re: Course to steer at a given speed.

One way to do the calculations

https://www.youtube.com/watch?v=7ahoGVGA1do
12-06-2012 06:06 PM
seanpatrick
Re: Course to steer at a given speed.

First, I'd like to say thanks for all of the responses. You guys have really given insight to someone who doesn't spend much (if any) time on the water. (Marine nav. is just a hobby of mine.)

Second, a big thanks to svHyLyte. Your explanation of Polars is exactly what I was missing.


Quote:
Originally Posted by svHyLyte View Post
In terms of your question seeking a specific time of arrival...
Sorry, I should've clarified this part a bit. It was a problem about keeping a schedule that got me thinking, but my question here was not ultimately about that. It was mainly about staying on course with a variable speed, such as in a sailboat. And as I said, you have resolved that problem for me.

Thanks, again!
-Sean
12-06-2012 05:45 PM
chucklesR
Re: Course to steer at a given speed.

Jack, JRD etc..

Quote:
Of course, this is predicated on the ability to point.
Or of course, not to point (so well).
I add 3-5 degrees of lee way as an assumption when I'm laying base course lines.

Absolutely correct - but, assuming the OP doesn't have a chartplotter and a autopilot to follow it, the exercise is a good one - to determine an approximate course to get one near his destination.

That curved course line looks a lot more like a snake every time I look at my wake.
12-06-2012 05:17 PM
jrd22
Re: Course to steer at a given speed.

Actually, if you use the bearing line on your chartplotter and change your heading to compensate for leeway, current, etc in order to keep the line on the destination you will travel a straight course to the destination. In your example Jack, if the current is ebbing in Georgia in order for the bearing line to be pointed at Silva B you will have to be pointing further and further N as you get into the main current and then less as you get out of the current as you approach destination. The bearing line gives you your direction traveled over the ground so it adjusts your heading if you keep the line on the destination. I do it all the time crossing Rosario in the power boat and end up with an arrow straight line regardless of the varying currents.
12-06-2012 01:32 PM
jrd22
Re: Course to steer at a given speed.

sv+ jack- note the smiley at the end? Tongue in cheek response to the OP's question about a straight line solution to his question. I too have been getting where I'm going for 40 years. Here in the PNW it's not uncommon to be crabbing 40-50 degrees one way and then the other a few minutes later while trying to maintain a course.
12-06-2012 01:29 PM
jackdale
Re: Course to steer at a given speed.

Quote:
Originally Posted by jrd22 View Post
Sailing isn't straight line, or constant speed. Your course will vary based on wind direction, speed through the water, etc so while your question is a good mental exercise it's not a practical application for sailing. In other words, line up the bearing line on your chartplotter with your destination and do your best to get there as soon as possible:-))
It is a fairly standard part of coastal navigation courses.

Your use of a bearing line will result in a curved course.

A example is crossing Georgia Strait from Vancouver to Silva Bay. On a strong ebb if you wish to minimize the time on course you will have to point further north than the bearing found on your chartplotter. If you simply use the bearing and maintain it you will up down by Thrasher Rock.

Of course, this is predicated on the ability to point.
12-06-2012 01:22 PM
jackdale
Re: Course to steer at a given speed.

Quote:
Originally Posted by seanpatrick View Post
A while back, I was reading the chapter in Bowditch dealing with dead reckoning. I was especially interested in the section dealing with course and speed made good. Well, last night on my lunch break (I work nights), I decided to solve a quick problem I made up:

"If I were on a ship which was 10nm due East of port, with a set of 360° and drift of 3 kts., and I needed to arrive in port in exactly one hour, what course and speed should I use?"

I figured the answer to be 253° at 10.5 kts. But then I started to wonder: That's all well and good for a vessel under engine power, but what about a sailboat? I know the way to figure course to steer at a given speed, but wouldn't turning a sailboat into the current (as in my example) actually slow the speed through the water enough to change the necessary course to steer? Or is this error so small as to be negligible? Or is there some other way to factor that in?

Regards,
Sean
You answered the questions as you should. In order to maintain a speed made of of 10 knots, you would need to have a boat speed (knotmeter) of 10.5 knots. In theory, it does not matter if you are a powerboat or a sail boat. In fact a powerboat on flat water can maintain a speed; a sailboat subject to wind speeds, helming ability, trim, sea conditions, etc.. is much more difficult.

BTW - am average boat speed 10.5 knots in a sailboat is really rather quick.

If you really want to get it right, you also need to account for leeway.
12-06-2012 01:21 PM
nolatom
Re: Course to steer at a given speed.

I'd do what my old man taught me..

"Put some 'money in the bank', head up higher than you think, it's easier go spend the money than it is to get more later on".

And I'd watch the shore once I could see it... is my bow "giving up land"? then head up. "eating land"? bear off. Steady bearing? then keep on. And it won't be my headstay that's heading toward port, it'd be some point on my stbd bow that's heading towards our course-made-good. So if the heading is 253 to make good 270, then the seabuoy will come up 17 degrees on the starboard bow.

That would get me there without a lot of course change. And I could do the hypotenuse calcs roughly in my head. And by pocket GPS would give me the speed made good, so I could figure out how many minutes to get there. But I couldn't necessarily make it come out 60 minutes, if we weren't making the speed. Speed along that hypotenuse is much trickier under sail than power. We sheet 'em in, and that's our speed. So we take what we can get, we don't control speed, and luffing for 10 miles (or more, once you know what 'C-squared' is) to slow down is hard on the sails.

So precise trackline-following for sailboats, and precise predicted-log speed, is more seat-of-the-pants than it is math for most of us (meaning me), this is especially true on a beat. So we'll reach port when we get there.


Also sailboats sideslip more than powerboats, it's our nature. So you have to allow for a crosswind even in still current.
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