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Old 31-10-2011, 09:16   #1
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Calculating Passage Times on 1000nm+ Routes

I am sure this has been discussed before but the search engine is failing me.

What is the most reliable method for calculating the downwind passage time for any given boat length. My interest is for long passages,where a 10% margin of error would have serious implications. For example, on a trip from Darwin to Capetown the leg between Cocos Keeling to Reunion is approximately 2,500nm. If your hull speed is 5 knots, and you could average that, then elapsed time would be 20 days.

If instead, you took a route average of 3.5 to allow for becalmed conditions, sailing in storms under bare poles, or the sailing off-track to make life on-board more comfortable, then the route would take 29 days. On a small craft, that additional 9-10 passage time has serious implications for supplies.

I have been using a formula of hull speed x 0.707 to get the downwind passage time. The origin of the 0.707 is long lost but its familiar to my mathematics brain as this number magically re-appears in many solutions ... its probably why I remember it.

My question is "Is this calculation a reasonable one?, is there one better one which is more reliable?"

My thanks is advance for any contributions.

Rhoel
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Old 31-10-2011, 09:25   #2
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Re: Calculationg passage times on 1000nm+ routes

Having kept track of all my offshore passage-making times and distances, my overall average has worked out to be about 80% of hull speed. That's for a total of about 50 days at sea in the North Atlantic. For a much longer, Pacific style passage, I'd tend to be more conservative and plan for maybe 70%.

If not mistaken, I think the 0.707 factor is to estimate VMG when tacking upwind.
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Old 31-10-2011, 09:27   #3
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I'm sorry but if you have to calculate your passage so finely you should stay well tied up in a marina where you will be safe. The calculation really depends on your water length and whether you will put on you're engine when the boat drops below a certain speed. We have a 38' sloop and allow 100nm per 24 hours, I have to say on some trips we don't seem to make this. We're not purists but on a long trip with a family it is the most awesome time without outside interference after all it is our home and were in it.



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Old 31-10-2011, 09:45   #4
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Re: Calculationg passage times on 1000nm+ routes

One of our CF advisers, Evans Starzinger, researched this question and published an article on it in 2005 in Cruising World.

His formula was : IMS rated speed in knots = 2.62 + 0.066*SA/D ratio + 0.051*LWL in feet.

Evans' article was called "200-mile days" and can be found on his website here: Articles

The bottom line is you want to develop a fairly good way to figure out what kind of average day to realistically anticipate on your boat given your sailing style.
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Old 31-10-2011, 09:48   #5
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Re: Calculationg passage times on 1000nm+ routes

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Originally Posted by Hud3 View Post
Having kept track of all my offshore passage-making times and distances, my overall average has worked out to be about 80% of hull speed. That's for a total of about 50 days at sea in the North Atlantic. For a much longer, Pacific style passage, I'd tend to be more conservative and plan for maybe 70%.

If not mistaken, I think the 0.707 factor is to estimate VMG when tacking upwind.
Many thanks... good to know the 0.707 calc puts more safety in than needed.

Pagos - I understand your reasoning but my previous boat was 25' lwl. I am currently evaluating its replacement: Personally, even if I could afford it, I wouldn't sail a 38 as for me, it would be unsafe. I sail solo and micro-cruisers are by far the best option for me: 20-25' means I need to consider the optimal loading of the boat and match routes accordingly.

Rhoel
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Old 31-10-2011, 10:23   #6
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Re: Calculationg passage times on 1000nm+ routes

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Originally Posted by Rhoel_Asia View Post
My question is "Is this calculation a reasonable one?, is there one better one which is more reliable?"
To address the second part of your question as regards Evans' formula, when I work his calculations for my boat, I come up with an average speed of 5.85 kts, which would give me an average day of just over 140 nm. In reality, I use 150 nmpd as my usual planning figure, with fairly good success. (This may simply be because I tend to have fresh sails and such go-fast features as a folding prop.)

Alternately, using the .707 calculation, I get an average day prediction of 151 nm.

My conclusion would be that Evans' formula is probably a bit conservative, which probably makes it more reliable for planning purposes, especially in terms of provisioning.
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Old 31-10-2011, 10:25   #7
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We have some friends that are sailing in a 25' boat and are in Indonesia heading to New Zealand direct there engine is an 8hp outboard . I will ask them and get back to you if this is any help? They have currently done over 30 thousand miles.
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Old 31-10-2011, 10:36   #8
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Re: Calculating Passage Times on 1000nm+ Routes

i generally figure my max hull speed is 5 kts. if i am going to sail 1500 miles, i figure that time is 4 kts max per hour, divide the total by the MAX of 4 kts and watch as i am proved wrong by mother nature. i can make 120 miles in one 24 hour period, IFF mommy nature is obedient---er, cooperative---sometimes i get a boost by a huge wind and break 8 kts--but that is extremely rare in my formosa. i figure time off shore, as a safety, as one 4 hour watch-- more than 20 miles, minimum-- for safety, and i prefer 100 miles out. (reduces chop)
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Old 31-10-2011, 10:39   #9
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Re: Calculating Passage Times on 1000nm+ Routes

The 0.707 is the sine (and in this one instance, the cosine) of a 45 degree angle - so it would probably be trotted out when tacking.
Considering one will not know the weather more than a few days in advance of starting, it is all but impossible to get a strict handle on which direction you may go, at what speed, and for how long before being able to point directly at your destination. The OP can get a minimum amount of time, but the maximum is somewhat open-ended and he must just plan accordingly.
Even the people who did commercial packet work during the "Age of Sail" couldn't get it right very often - one of the reasons powering was so attractive...

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Old 31-10-2011, 10:40   #10
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Re: Calculationg passage times on 1000nm+ routes

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Originally Posted by Rhoel_Asia View Post
(...)

1) the most reliable method for calculating the downwind passage time for any given boat length.

2) on long passages (...) a 10% margin of error would have serious implications.

3) on a trip approximately 2,500nm. If your hull speed is 5 knots, and you could average that, then elapsed time would be 20 days.

4) On a small craft, that additional 9-10 passage time has serious implications for supplies.

5) I have been using a formula of hull speed x 0.707 to get the downwind passage time. The origin of the 0.707 is long lost but its familiar to my mathematics brain as this number magically re-appears in many solutions ... its probably why I remember it.
Hi, Rhoel,

Your query escapes me sorts of so I divided your post and will try to address single items below:

1) Look up the wind, current data for the passage, month. Then look up your boat's polar and combine the data. Add 50% as a safety margin. There are computer programs that will calculate the THEORETICAL passage times for you.

2) what implications do you have in mind?

3) no way a boat with hull speed of 5 knots will average her hull speed on a passage, and especially so downwind. Downwind tends to be the slowest, the least comfortable, point of sail too.

4) ??? Why should this be? You want to carry water and food for the planned passage AND a safe margin. The only excuse not to do so I can think of is when you are participating in some sort of extreme sports event. If so, please make sure your insurance covers rescue costs from mid-ocean. Sailing with exact, no margin, amounts of food and water is a asking trouble.

5) a) you do not have to sail downwind if the destination is downwind, you may elect broad reaching which may result in better VMG, b) take your boat on a shorter downwind passage, fully loaded and measure her speed over 24 hours, allow for the current. This will be a good starting point to estimate the possible passage times. c) look up passage times on the same route by boats with similar characteristics.

Now, you must also allow for the fact that stat data (meteo and boat's) may and will considerably vary each side of the average. Allow for that.

Make sure your wind-pilot and auto-pilot are there for you. You can only keep up good speed if the boat is sailed. Lay a-hull to get some sleep and the averages drop way down.

From my limited experience and minding the limitations of our smallish boat I can tell you that a boat with 6.5 knots of hull speed can average 3.8-4.8 knots on an extended downwind, tradewind passage - a small boat (23' LWL), loaded with supplies and gear, sailed well, but conservatively. Bigger boats may reach their upper speeds easier as they are less loaded and stand up to wind and sea better.

PS The Cocos - Rodriguez is a bad example as this stretch of the ocean is usually way more windy than other offshore downwind passages. Many boats sail in the uppers of their potential for most of the Indian passage, ban the take off and the landfall.

Cheers,
b.
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Old 31-10-2011, 10:41   #11
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Re: Calculating Passage Times on 1000nm+ Routes

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Originally Posted by zeehag View Post
i generally figure my max hull speed is 5 kts. if i am going to sail 1500 miles, i figure that time is 4 kts max per hour, divide the total by the MAX of 4 kts and watch as i am proved wrong by mother nature. i can make 120 miles in one 24 hour period, IFF mommy nature is obedient---er, cooperative---sometimes i get a boost by a huge wind and break 8 kts--but that is extremely rare in my formosa. i figure time off shore, as a safety, as one 4 hour watch-- more than 20 miles, minimum-- for safety, and i prefer 100 miles out. (reduces chop)
Zee, your boat has a LWL of 31.75 feet and therefore a hull speed of 7.55 knots.
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Old 31-10-2011, 10:44   #12
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Re: Calculating Passage Times on 1000nm+ Routes

dockhead-- formosas need a big wind to push them consistently at that hull speed--i found mine likes to travel at bout 4 with normal breezes, and 8+ in 60 kt winds......downhill...uphill, i dont want to find out on west coast.. but is comfortable at the cruising speed of 4 kts. i can count on htat number so is he one i use. more--happy dance. less--engine........
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Old 31-10-2011, 11:06   #13
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pirate Re: Calculating Passage Times on 1000nm+ Routes

As a small boat owner/sailor when it comes to something like a non-stop UK-N.Spain non stop... around 500miles....
I take an average of 4kts which would be 125hrs then triple it for water and supplies...
I've found it often takes longer than the statistics say....
Personally I 'heave to' if the wind goes above 6 in my 21ftr... and have been that way for several days at a time....
I've low freeboard and see little point in getting battered for little gain when I can be below warm and dry and the boats riding easy...
One December crossing of the Biscay in a 22ftr took 3 weeks from Poole to Viviero... 80% of the time in the Bay I was 'hove to' as the gales swung between SW and W...
If your going offshore carry plenty... it won;t go to waste...
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Old 31-10-2011, 11:39   #14
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Re: Calculating Passage Times on 1000nm+ Routes

Rhoel, the more calculating you do, the more numbers you can crunch, the more accurate the estimate will be. There is supposed to be some high-end race navigation/routing sotware that will take into account known currents, weather predictions, your own polars, etc. to give a good estimate but I've never worked with anything in that league.

In more "civilized" waters you can sometimes get previous weather/sea data from the logs kept from bouys. Otherwise you need to try to find historical or predicted weather data, which can often be flat out wrong. I've seen "well it has averaged five knots consistently for five years" and met 15 knots of wind instead, which do you predict for?

And of course if the wind shifts, you need to allow for foreseeable shifts and the effect that will have on your boatspeed. When you mention a downwind run, you also have to ask how that boat will handle downwind. Some are pigs and any following sea will gybe the boat. Some few actually run wing-and-wing very nicely dead downwind, despite the "fact" that modern boats always are faster gybing back and forth and not running dead downwind. Depends on the boat.

You really need to play "what if" and start out with the range of possible wind, weather, performance information, and you might want to use a spreadsheet for that rather than calculate it all manually. Then you refine your starting data by saying "OK, this is the best/worst I may be after 24 hours. Restarting from each of those locations, what would the next 24 hours bring?" and you repeat the process, refining it each time. Some of that high-priced software will do iterations every hour or less, to really refine it down.

Sometimes "one to three weeks, probably two" is as close as you can get. Look at the widely varying times from year to year for the Bermuda races to get some idea what variations can be caused by simply playing the wrong side of a weather front (in heavily forecasted waters) or getting caught without wind.
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Old 31-10-2011, 11:52   #15
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Re: Calculating Passage Times on 1000nm+ Routes

Keep track of your passage time in a log from before pulling the anchor to having it set. Over time this will be your best measure. When you are coomfortable with your data, you will be ready for that 20 day trip. If you need to arrive in daylight give yourself several hours extra on a long passage, or a couple hours on a day or two passage.
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