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Old 04-01-2016, 07:08 AM   #101
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I remember calculating set and drift in Captain school, but have since forgotten most of it. I passed the course so it must not be to difficult. Like Brittania I plan to do some blue water cruising some day so knowing this stuff is important to me. I can understand that cruising closer to populated places would not require a person to think about it to much. Our problem is one of physics, our boat size doesnt allow a large enough fuel capacity to make it a non issue. So we worry
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Old 04-01-2016, 07:33 AM   #102
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Your numbers above:
Up the river T = 10 nm / (10 kn - 2 kn) = 1.25 hrs = 75 min
Down the river T = 10 nm / (10 kn + 2 kn) = 0.833 hrs = 50 min
Wadden explained how to do the calculations, but Baker asked the more important question of "why?"

RT asks "where did the other 5 minutes go?" which wadden then explained in that the 2 kt difference is spread over different distances in the two examples, which is true.

I look at it another way. In the example of being set by 2 kt current, the effect on your travel time is going to be equal to the percentage of that current to your final SOG. In this case 25%. 2/8=25%. Add 25% to what your travel time would be if you weren't fighting the current, or 15 minutes in this case.

In the case where you are being carried by a current, it is the same thing. The effect on your travel time will be the percentage of the current compared to your SOG. I this case 16.6%. 2/12= 16.6% You reduce your travel time by 16.6% over what it would be without a current. 16.6% of an hour is 10 minutes.

Conceptually, think of it this way, a contrary current is a bigger percentage of your SOG than a helpful current is.

Not to be too pedantic, but... yeah I am being too pendantic.

Use the same example only a 4 knot current.
Boat speed = 10kt
Current = 4kt
Distance = 10nm
Affect on time to destination is current/SOG

Against the current: 4/6= 67%, 67% of 60 minutes is 40 minutes longer to make the trip.

With the current: 4/14= 28%, 28% of 60 minutes is about 17 minutes less time to make the trip.

Bottom line is that current hurts you much more than it helps you if you are looking at not only time, but fuel consumption.

Learning to sail in a region with strong currents on boats with unreliable motors makes you very aware of the effect of currents. It also makes you aware of where the current is. If you are in a boat with a top speed of 5 knots, a 4-6 knot current is a big deal!
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Old 04-01-2016, 08:01 AM   #103
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RT, to put it simply and briefly, you are exposed to the foul current longer than your are exposed to the fair current. They made that explanation in numbers but that is what it boils down to.

Brittania, I understand there are a few that plan to cross oceans. Maybe I will one day. We just get back to the basics of time/fuel/distance. If I were doing it, I would have a day tank...which has been discussed already.
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Old 04-01-2016, 08:05 AM   #104
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RT, to put it simply and briefly, you are exposed to the foul current longer than your are exposed to the fair current.
Yeah, well of course that explanation works too.
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Old 04-01-2016, 08:14 AM   #105
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Thanks guys. Good explanations and the second cup of coffee helped as well...
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Old 04-01-2016, 08:14 AM   #106
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Not sure whether my "2nd cup of coffee " post made it to the forum:
Down the river we travel 8.3 nm through the water - 17 % less than over ground. Up the river we have to go 12.5 nm through the water - 25 % more than over ground.
This explains the lost 5 minutes.


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Old 04-01-2016, 08:56 AM   #107
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However, current is not always on the nose or on the stern, cross currents need to be accounted for also, even in degrees of cross currents. Its possible, and at one time neccessary, to know those things before setting out on a crossing, be it the sound or the ocean. I do recall it being interesting and even enjoyable once I got it figured out. Determining a course to steer and a speed to make to arrive at a point several hundred miles away, given current speed and direction (set) and wind speeds on average (drift) is satisfying in its own.
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Old 04-01-2016, 09:20 AM   #108
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Originally Posted by kulas44 View Post
I remember calculating set and drift in Captain school, but have since forgotten most of it. I passed the course so it must not be to difficult. Like Brittania I plan to do some blue water cruising some day so knowing this stuff is important to me. I can understand that cruising closer to populated places would not require a person to think about it to much. Our problem is one of physics, our boat size doesnt allow a large enough fuel capacity to make it a non issue. So we worry
Go fly a kite, to pull your boat.
Best if the kite has a positive buoyancy, so it floats in the air all the time.
That way it is ready for the wind when it blows.
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Old 04-01-2016, 09:40 AM   #109
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"Loose" 5 min here and 15 min there. I'm still at the helm look'in around smiling and I can't remember not getting where I want to go. "Loose" time? Ridiculous. just because you plan something it dos'nt mean it's bad for it not to become true. Very often things turn out better than planed. Sometimes not though. But to get upset if current slows you down while cruising is counter to enjoyment.

Of course if it's really important you can get tide current information make a plan that will "probably" work as planned. But usually in my neck of the woods there's enough time and space and anchorages to make adjustments to plans (almost always tentative for me) that are as good as the original and frequently better.

But some people aren't built that way. Time spent other than as planed is time wasted to many as the plan is perfect or as perfect as possible ... that's why it's the plan. Perfection is ... well perfect .. and anything else is imperfect or flawed and to be avoided.

We get in our habits by striving to do good or get as close to perfection as possible. Just because of who we are we may strongly gravitate toward that or to go in the other direction. This is about P type people and J type people. I am a P type and we like things undecided. J types are uncomfortable until things ARE decided. Where I'm going to anchor at night is frequently extremely flexible. I may change my plans several times during the day. J types like to form their plans quickly and stick to them. Changing plans upsets them. A P type may say "OK let's go to Danny Cove instead" immediately embracing it and looking fwd to it perceiving it as a surprise adventure. J types may respond to a change of plans like when their favorite team looses the game. A bummer.

So leaning toward extravagant planning and sticktuativeness or being a "What me Worry?" drifter is not something we can easily change. Being a drifter is a joy to people oriented that way and a nightmare to those not. We should enjoy who we are and like it but be able to lean one way or the other (more than a little at times) for safety or benefit to oneself or others. So I should say to Baker that using the expression "up tight" was at least a poor choice of words and possibly from me being a bit up tight. Sorry John.
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Old 04-01-2016, 09:48 AM   #110
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Current matters or it doesn't. All depends. Most aren't often in extreme currents that are enough to put them in risk of running out of fuel. Now, if you were making a long trip south along the east coast running off shore then the gulf stream would materially impact your fuel range. If your boat's normal speed was 6 or 7 knots, it could easily be a factor of requiring 20-30% more fuel. I've even known boats to run out of fuel between the Bahamas and South Florida because they failed to consider the impact of the gulf stream. This was a combination of running against it and also being pushed into a longer course.

Accuracy of fuel consumption either matters or it doesn't. If none of your cruises ever use more than 25% of your fuel, it doesn't matter a bit. We're heading up the east coast to NYC soon and then going to proceed on the Loop. Our trip up the coast is just to position the boat so more a delivery cruise. Based on 90% our fuel range at 27-28 knots (cruise) is about 260 nm, at 20 knots about 330 nm, at 15 knots about 380 nm. Looking at it another way, at 28 knots it's 9.2 hours, at 20 knots it's 16.5 hours. Now that's a huge difference and it impacts every day's run and where we'll stop or if we wanted to run longer, the need to come into shore during the day and get more fuel. On the trip up the coast we will fuel at each marina we stop for the night except Atlantic City as from Ocean City to Atlantic City to NYC is the only stretch for which we have enough fuel capacity for two days of run. It will also impact us on the loop.

So there's no universal answer of how important either of these are. It's simply a matter of each person's circumstances. As we do cruise long distances knowing fuel burn at all speeds is important to us. As we cruise generally at higher speeds, the impact of current is important occasionally but far less so than if we cruised at 6 knots.
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Old 04-01-2016, 10:46 AM   #111
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However, current is not always on the nose or on the stern, cross currents need to be accounted for also, even in degrees of cross currents. Its possible, and at one time neccessary, to know those things before setting out on a crossing, be it the sound or the ocean. I do recall it being interesting and even enjoyable once I got it figured out. Determining a course to steer and a speed to make to arrive at a point several hundred miles away, given current speed and direction (set) and wind speeds on average (drift) is satisfying in its own.
I remember doing those calculations when learning to fly. I still probably have that old flight calculator in my flight bag which I haven't used in 30 years. Back before GPS, those were actually very helpful.
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Old 04-01-2016, 11:50 AM   #112
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I remember doing those calculations when learning to fly. I still probably have that old flight calculator in my flight bag which I haven't used in 30 years. Back before GPS, those were actually very helpful.
Yea using a 'whiz wheel' was mandatory when we sat behind an 'early pinball machine' panel back in the day.

On the boat, I actually do still use a Weems & Plath 105 from time to time . . . . . and paper charts, and dividers . . . . . . it's fun, and I'm a dinosaur

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Old 04-01-2016, 01:18 PM   #113
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I've read on line that sailboats can get 8MPG under power(no sail) ~ at low speed like 5-6knots.

Obviously power boats have very different engine, hull design etc. I wonder in general what kind of MPG can a power boat get at low speed? Or could you tell me for your boat, what max MPG can you extract? At what speed and what kind of engine? Thanks!
We used to get about 8 nautical MPG at 7 kts in our Jeanneau Sun Odyssey 35. It had a Yanmar 3YM 30 engine.

Our present boat is a Helmsman 38 with a Cummins QSB5.9 common rail diesel, rated at 380 HP. At 7-1 /2 kts it burns 2.5 GPH. The boat weighs 32,000 lbs empty, and carries 400 gal of fuel and 140 gal of water.
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Old 04-01-2016, 02:28 PM   #114
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Yea using a 'whiz wheel' was mandatory when we sat behind an 'early pinball machine' panel back in the day.

On the boat, I actually do still use a Weems & Plath 105 from time to time . . . . . and paper charts, and dividers . . . . . . it's fun, and I'm a dinosaur

Wifey B: I think they are so cool. There are some other really weird looking old navigation slide rules but this one seemed to work the best. We have a friends who has a really old one. I don't think it's worth a lot even as an antique but it looks cool with his sketches and models and everything nautical. I love his old ship's wheel and bell and horn and compass and all. Without that though I wouldn't have even known this circular slide rule existed.
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Old 04-01-2016, 02:57 PM   #115
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However, current is not always on the nose or on the stern, cross currents need to be accounted for also, even in degrees of cross currents. Its possible, and at one time neccessary, to know those things before setting out on a crossing, be it the sound or the ocean. ...
Yup. Last week we crossed from Rodriquez Key to Bimini. We set our course about 23 degrees below our destination. We a fun trip riding the stream north diagonally. With just a little adjustment when we got close, we hit the entrance.
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Old 04-01-2016, 02:58 PM   #116
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Current matters or it doesn't. All depends. Most aren't often in extreme currents that are enough to put them in risk of running out of fuel...
Here’s our trip summary from several years ago when we crossed from Bonaire to Trinidad.

We had contrary current of at least 1 knot and as much as 2.5 knots for the entire trip. The first half we had lighter head winds and less current. We typically cruise at 6-6.5 knots, 3.8 mpg with light winds and no current.

First 224 miles: 4.9 knot average SOG, 3.2 mpg
Last 201 miles: 4.0 knot average SOG, 2.0 mpg

We weren't going to run out of fuel but with the slow pace it affected the forecasting with a weather system heading our way and a night time arrival.
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Old 04-01-2016, 05:01 PM   #117
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Heres our trip summary from several years ago when we crossed from Bonaire to Trinidad.

We had contrary current of at least 1 knot and as much as 2.5 knots for the entire trip. The first half we had lighter head winds and less current. We typically cruise at 6-6.5 knots, 3.8 mpg with light winds and no current.

First 224 miles: 4.9 knot average SOG, 3.2 mpg
Last 201 miles: 4.0 knot average SOG, 2.0 mpg

We weren't going to run out of fuel but with the slow pace it affected the forecasting with a weather system heading our way and a night time arrival.
Exactly....different people care about or have to worry about different things. Start trying to tell others to follow just one philosophy shows how out of touch with others one can be.

For those using slide rules...I still use my 1970's flight school one to switch back and forth between knots and statute and give ETAs....I can do it faster than people trying to punch the numbers into a calculator.
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Old 04-01-2016, 07:03 PM   #118
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"Loose" 5 min here and 15 min there. I'm still at the helm look'in around smiling and I can't remember not getting where I want to go. "Loose" time? Ridiculous. just because you plan something it dos'nt mean it's bad for it not to become true. Very often things turn out better than planed. Sometimes not though. But to get upset if current slows you down while cruising is counter to enjoyment.

Of course if it's really important you can get tide current information make a plan that will "probably" work as planned. But usually in my neck of the woods there's enough time and space and anchorages to make adjustments to plans (almost always tentative for me) that are as good as the original and frequently better.

But some people aren't built that way. Time spent other than as planed is time wasted to many as the plan is perfect or as perfect as possible ... that's why it's the plan. Perfection is ... well perfect .. and anything else is imperfect or flawed and to be avoided.

We get in our habits by striving to do good or get as close to perfection as possible. Just because of who we are we may strongly gravitate toward that or to go in the other direction. This is about P type people and J type people. I am a P type and we like things undecided. J types are uncomfortable until things ARE decided. Where I'm going to anchor at night is frequently extremely flexible. I may change my plans several times during the day. J types like to form their plans quickly and stick to them. Changing plans upsets them. A P type may say "OK let's go to Danny Cove instead" immediately embracing it and looking fwd to it perceiving it as a surprise adventure. J types may respond to a change of plans like when their favorite team looses the game. A bummer.

So leaning toward extravagant planning and sticktuativeness or being a "What me Worry?" drifter is not something we can easily change. Being a drifter is a joy to people oriented that way and a nightmare to those not. We should enjoy who we are and like it but be able to lean one way or the other (more than a little at times) for safety or benefit to oneself or others. So I should say to Baker that using the expression "up tight" was at least a poor choice of words and possibly from me being a bit up tight. Sorry John.
No worries at all Eric. And in some respects, I am guilty as charged. That is my job...to strive for perfection. But it doesn't invade my personal life as much as you may think. In fact I surprise myself how unplanned I am. But I still feel the sting of lost performance due to current. Just can't help that one!!!...

Ps...sorry for the entire quote. On phone and don't have the time to trim it down.
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Old 04-01-2016, 07:25 PM   #119
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What is important is thinking....not just letting the rest of the day hit you in the face.

We see it all the time.

A good example of tide planning is the Delaware River run upstream.

In a 6 knot boat, running the 60 miles or so to the next reasonable anchorage or marina against a 1.5 knot tide on average will take 13.3 hours and will burn 27 gallons of fuel in my Lehman.

If I catch the tide, I am now making 7.5 knots and make the same trip in 8 hrs and burn 16 gallons.

So at dinner time in Chesapeake City, I have refreshed, had a cocktail, made a reasonably early dinner and have an additional $27 to spend on dinner. Only if I was wise enough to plan the tides.

Sure flexibility is good, sure relaxing instead of extensive planning is good, but thinking ahead probably never hurt anyone when it came to making smart decisions.

P or J......at some point they overlap....just like type A or B...
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Old 04-01-2016, 08:00 PM   #120
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