Calculating Passage Times on 1000nm+ Routes

[GordMay]

Originally Posted by Rhoel_Asia

... 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...

The classic 45°/Right triangle has two sides of 1 and a hypotenuse of root 2 (1.414)* , hence the ratio of approximately 0.707 (Sine) / 0.707 (Cosine) / 1.0 (Tangent)
* 1 / 1.414 = 0.707

I don't think you can do an accurate calculation, without empirical data regarding your boat's performance, under similar conditions, and under your command.

Many sailors, however, assume about 100nm/day PLUS about 15% (fudge factor), for planning purposes.

[swagman]

Google Digital Wave Passage Planning - or Visual Passage Planning Software.

We got this years back and it only cost nominal bucks but worked well - and easily gave us what we wanted.

It came holding the kind of historic wind and current data you find on passage charts, and you add in your own boat data. Like polar type info on boat speed at differing wind speeds and at various angles of sail etc. You can also input the amopunt of fuel you carry and consumption, even the minimum speed you'd sail at before you'd want to run on an engine etc, etc.

You then drew your planned course, told it when you were leavings, and it produced a pretty detailed passage plan in both chart and text formats.

Of course it's only as good as the data and that may well be outdated with current global effects, but if you do wish to review passage plans to the level of detail you seem to want and very easily get the outputs from multiple variations of input, you might think worth the investment.

Cheers
John

[zeehag]

for provisioning, i count my miles, and my average time as far as speed over ground and distance to be travelled and then i add a month to the time to be done--i tell my mom this last number and provision for this last number--so there are 2 sets of numbers for my passages-- mine and for my momma. always give momma the longest time figured.

[svcambria]

Rhoel, the more calculating you do, the more numbers you can crunch, the more accurate the estimate will be.

Well, actually "the more precise the estimate will be..."

Dealing with items like Pilot Charts and past experience are great, very recommended, but Pilot Charts are climate, not the weather you may actually face, and things that go wrong are rarely the things that went wrong before 'cuz you took care of them already. But the more experience the better...

Michael

[Rhoel_Asia]

Originally Posted by Pagos

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.

Thank you, the information would be very useful - as said, my previous boat was 25' and for me ideal. I am currently boatless in Cambodia and looking to make its replacement early 2012. Originally sailing out would have meant transiting the northern Indian ocean but the continuing Somali action closes the northern route. That leaves the southern route open, which is 2,500 max between ports.

Hence the question: By reverse engineering the boat length, speed and provisioning, I can confidently invest in another 25'. Unfortunately, the figures I have for my old Itchen Ferry 25 are useless as I rarely made any longer passages purely under sail - I had a reliable Yanmar and invariably used it to ensure daylight arrivals.

The discussion has produced some interesting comments, and I suspect will be useful to many planning longer passages.


Rhoel

[Dhillen]

While good for theoretical planning purposes, calculating passage making times too finely tends to forget the human factor. This is especially true if cruising with some one who doesn't like to be bashed around for that extra knot of speed: like my wife. We sail as fast as I am allowed to until it isn't comfortable. That ends up depending on sea state, wind direction and the current - the current mood of the Admiral, I might add.
Dhillen

[Adelie]

Originally Posted by barnakiel

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.

+1

Smaller boat loaded heavily, figure 100nm/d.