PHRF versus approx. passage times on dif. boats?

[Adelie]

Originally Posted by Adelie

Evans Starzinger and Beth Leonard published a formula for estimating daily passages based on SA/D & LWL. Using a database of boats including PHRF ratings I did a regression analysis and found their formula corresponds very well with PHRF ratings. The values their formula gave we’re about 2/3 to 3/4 of what the Rating gave.
I don’t have the formula handy but will look it up tomorrow.

The formula for PHRF is 360sec + rating in sec = sec/nm. Working that out for daily progress:
86,400sec/day / (360s/nm + rating) = progress in nm/d.

So the LeonardStarzinger Milage formula is
Daily Average Mileage = 24*(2.62 + 0.066*SA/D+ 0.051*LWL ).

Result of this are about 80% of what PHRF mileage is.

[hellosailor]

"Daily Average Mileage ="
Well, not so fast. If I had to plot a random run without knowing the winds or current, I'd start by taking the straight-line mileage and multiplying that by 1.4 to get the actual miles through the water that a vessel tacking all the way might have to cover.
Then with any luck refine the guestimates with some weather information. But one year I tried that with the historical records from buoys, which matched a forecast for 5 knots of tailwind. And got thunderstorms and headwinds on the next morning. Kinda threw that set of plans out right away.
Scheduling works better when you're running a railroad.(G)

[Adelie]

If you want to follow a schedule why are you on a cruising boat?

The formula is not good for figuring out how far you can sail tomorrow, too many random variables.
It might be OK estimating how far you can go next week.

What it’s good for is figuring out how far you can go over a long period when the random variables average out.

[wingssail]

Originally Posted by Adelie

If you want to follow a schedule why are you on a cruising boat?

The formula is not good for figuring out how far you can sail tomorrow, too many random variables.
It might be OK estimating how far you can go next week.

What it’s good for is figuring out how far you can go over a long period when the random variables average out.

Other than doing your own calculations there are products available which can do the passage planning you are discussing. Here are three I use plus one which I wrote. I am sure there are many others, and tools such as Navionics or the big MFD chart plotters probably do a lot of it as well.

1. A product I have used for many years, on the PC, is Visual Passage Planner II. from Digital Wave.

It has a basic VPP for many boat types and sizes built in plus pilot charts and all that that entails, wind speed and directions for percentages of the time, current information, percentage of gales, percentage of calms, for the whole world, all months of the year and a lot of other features, plus a world map/chart. Lots of reports such as predicted average speed, sea states, etc. It does great circle routing and weather optimizing (based on historical data only).

It has proven useful for long passage planning, less so for short trips. It is available here for $119 Digital Wave - Visual Passage Planner 2

2. I have my own software which I developed for short trips which takes into account current local conditions and can plan for routes of up to four legs, and is quite effective. I enter the true wind speed and sea state plus the heading of each leg, the boat type, length, and some additional factors about the boat and conditions, (most of that is constant data). It produces predicted apparent wind speeds and directions and predicted boat speeds plus gives me the best heading to sail for optimal VMG. We we use it for deciding which direction to go, for sail choices, or whether sailing is even worth it, maybe motoring is a better choice sometimes. It does not tell us when to tack or gybe or calculate durations.

3. The best tactical information we use is "Sail Racer" an Android app connected to our B&G sailing instruments via Blue tooth and a proprietary Bluetooth data server. I have it running on a daylight viewable (barely) rugged tablet. I can enter destinations, marks, courses, etc and it feeds back lots of tactical information for a racing sailboat such as time, distance, and bearing to a mark, When to tack or jibe, what the apparent wind conditions will be at the next mark, what the heading to the next mark after that will be, what the wind has actually been doing (shift trends etc,) as well as start line timing, favored ends, etc. This device requires a full time navigator to get the most out of it, and is useful mostly for racing.

4. We also use GRIB files, for several days in advance, and show them on our OpenCPN. With tool #2 above, we can then predict the distance we will go on a day. Then we advance our position along the path that amount and place a waypoint there and look at the GRIB for the next day, at that point. This has been very accurate on some passages, less so on others since the weather is often not as predicted on the GRIBs. OpenCPN has a lot of this functionality built in to it's plug ins.

Everyone has their own approach to cruising, navigating, and route planning. Some do none of the above and are happy that way. We use a lot of computing on our boat, but for us it does not detract from the fun of sailing, anyhow, passages can get boring, what else is there to do? We have not had major sea condition surprises during our travels except when the passage is more than about four days, even then we have some warning of changing conditions, and we rarely miss our ETA's by more than a few hours. We have not had a major grounding or wreck and that we account to robust passage planning and route planning. The computer helps.

Before the computerized tools we used, and still refer to them, British Admiralty Ocean Passages for the World, brilliant book, and Jimmy Cornel's World Cruising Guide and Ocean Routes of the World. There are online versions I believe.

For racing, these tools help us win.

[Don C L]

I won’t get any of that stuff cuz then I won’t be able to say “Oh yeah? well I’m sure I woulda beat ya if I’d had.......”
back at the bar.

[Hamish_ct]

If both boats are sailing the same course and you can sail them to their PHRF, the formula is pretty simple. 650/(550+PHRF). this will give you the Time on Time (ToT) adjustment multiplier. For a boat with a PHRF of 100, the factor is 1 for a boat with a PHRF of 160 the adjustment is 650/(550+160) = .91549. That means for every 1 hour of sailing time the boat with a 160 PHRF will be adjusted to 1 hour * 0.91549 = 54:56 or 5:04 mins. Depending on the wind and how long the sail is, for every hour of sailing time the boat with a 160 PHRF will be 5min 4 seconds behind. If you are racing, and the boat with a 100 PHRF finishes in 1 hour, the boat with the 160 PHRF could finish the race 5Min and 34 seconds behind and tie the other boat. (The 5:04 is based on if both boats finish in 1 hour, the 5M 34s is the amount of extra time based on the formula for the boats to tie).

Depending on the boat, it is very difficult to sail a boat to its PHRF rating unless the boat is perfectly tuned and you sail the boat perfectly.

Fair Winds

[roverhi]

It really depends on the route you're sailing. If you are going some place and encounter lightish winds on the nose, the PHRF rating may have some validity to predict time at sea. Sailing the typical trade wind routes, the PHRF ratings are useless. Heavy displacement boats will sail way above their ratings in the tradewind routes which are primarily reaching in stronger winds. Westsail 32's typically do very well in the Single Handed TransPac winning on corrected time at least once and usually finishing middle of the pack or higher boat for boat. If you are talking coastal sailing, PHRF has validity but out on the open ocean it's virtually worthless.

[Don C L]

I want to thank you all for encouraging me to join the sailing club and start racing my ancient PHRF 228 boat!

[Pizzazz]

The rating of the boat is less useful in estimating passage times, be it PHRF, hull length or even a polar diagram. Much more useful is to have a number of polar diagrams for your boat (or a similar design) for different sea states and different sail combinations. This is what we use in offshore racing and it can get very detailed. In reality, all you need to do is sail for a little bit in different sea conditions and different sail combinations, then just put all the observations in a polar diagram, specific for your boat, your level of effort and comfort. After having done all of this, you will come to the conclusion that miles per day is limited by the waterline length and and is typically less than the polar diagram. Very few boats can hit 200 nmpd consistently, and very few do less than 100 nmpd. I would say use 120 for 30 ft boat, 130 for 40 ft, 140 for 50 ft and 150 for 60 ft. You will not be far off. But if you have a good polar, you will be able to weather route and save a couple of days from a 20 day passage.

I believe the original poster is asking the question, does a larger boat dramatically reduce passage times. The answer is, typically not, unless there is enough wind or you have exactly the right sails for any wind condition that cruisers typically do not have. Last time I crossed the Atlantic, I run the routing before we started, and it gave me 17 days for the 58 ft boat I was crewing on and 20 days for my 31 ft Hunter. We crossed in exactly 18 days and I believe the Hunter would have crossed similarly to the prediction, may be a few days more. These are extremes (size wise) for light winds (15-18 knots). If you have the larger boat and the wind is 22-25 knots, you can probably average 200 nm per day. But trade winds are rarely like that.

[Adelie]

Originally Posted by Don C L

I want to thank you all for encouraging me to join the sailing club and start racing my ancient PHRF 228 boat!

Could be worse, mine’s 282 in NewEngland.

In the high wind Bay Area I would be 273

[Sailor 99]

Originally Posted by hellosailor

Just a reality check here.

If two 38' boats are planning to make the same longer passage, say 600 miles, and one boat has a PHRF of 100 while the other is rated 160 (or 80 vs. 140, pick your own pair) then that would mean the boats have relative speed ratings that are different by 60 seconds per mile sailed? SO, one minute per mile, or one hour per 60 miles sailed? So a back of the envelope figure would be that they boats would arrive 10 hours apart a the end of 600 miles, all else being equal and the one not holding back to keep with the slower?

Basically 1/2 day apart, dawn versus dusk, on a 600 mile run, and a full 24 hours apart on a 1200 mile passage?

Or do I have my PHRF ratings all mixed up again? (And yes, I know, those are of limited value for specific conditions only, but it beats "Gee, I think we can make six maybe seven knots" as a starting point.)

PHRF ratings work well when the numbers are close, say 9,12, 15,. But my experience tells me that by the time your at a difference of 30 they are not even close. Especially over a long distance.