# Math - speed gain needed to close a seconds-per-mile gap



## paulmcquillan (Jan 4, 2002)

Someone with a higher order math background may be able to shed some light on this one...

Trying to convert a seconds-per-mile difference between two boats into a delta expressed in knots.

If another competetor wins on corrected time by about 35 seconds-per-mile, how much faster does my boat speed need to be in order to beat him?


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## Sailormon6 (May 9, 2002)

You can beat him to the finish line even though his average boatspeed is higher than yours. A sailboat race is not just a *speed* race. It is a race of *time* and *distance*. The objective in a yacht race is to sail around a specified course in the shortest period of time. Even though your opponent''s average speed is higher than yours, you can shorten the *distance* that you have to sail to get around the course by finding favorable windshifts, and by tacking correctly on wind shifts. You can shorten your *time* by finding stronger winds than your opponent, and by being more efficient than your opponent in terms of sail handling, boat control, helmsmanship, etc., and by being more resourceful in the choice of tactics and your application of the racing rules. 

There are opportunities all around the race course to shorten your time and your distance, and to impede your opponent''s progress around the course (by forcing him to tack for clear air, for example), and the best racers are not only concerned with basic boatspeed and boat handling, they are constantly looking for those opportunities.


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## paulmcquillan (Jan 4, 2002)

I''ve got all that. Last race we finished first on actual time after catching and passing every other boat. We also beat some boats with the same rating or similar LWL by anywhere from 20 min to about two hours.

I''m specifically trying to identifiy a _____ knot increase in our net speed will be needed to close the seconds per mile gap against a particular boat.


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## geohan (Mar 8, 2000)

There is no one-sized "delta" to fit all cases. The increase in speed to overtake depends upon the distance left to go and the distance to make up. 0.01 knots would be enough if the distance-to-go was large whereas 10 knots wouldn''t be enough if the distance-to-go was small.
Solve each case by: S2(you)= S1(him)X D2/D1, Then delta = S2-S1 

Cheers, George


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## TSOJOURNER (Dec 16, 1999)

easy 
tell us the distance of the race and the time and all the phrf numbers in the fleet....oh O forgot ... you also need to know what phrf numbers your fleet is actually running on ( different locales are rtunning different numbers ) 
and if the RC is using tthese phrf numbers for the complete fleet, or if he makes good guesses for boats which are not listed in the region ...

do you have allotments in ohrf for roller furler, differnt props and so on... also important to know ... 

but most important is the mean distance sailed by all competitors and the mean time for all boatsand so forth 

if I take all of this in consoderation, I would say you need to sail 0.2 knots faster on the downwind leg of the olympic triangle course you raced last week .. ( note. not the course from 2 weeks ago ! ) 

Impossible in other words . Just get with it and have fun

Thor


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## TSOJOURNER (Dec 16, 1999)

Stated another way, the answer you are looking for is speed dependent.

Do the math, dude. Suppose your competitor''s average speed was 5 miles per hour.

5 mph -> 1/5 hours per mile X 3600 sec./hour = 720 seconds per mile.

So if you are giving up ten seconds per mile on corrected time, your corrected time if you want to beat them by one second would be

709 seconds/mile X 1 hour/3600 sec. = 0.196944444 etc. hrs. per mile, invert to get 
5.07757 miles per hour. So you need to increase speed about 1.5 percent (0.078 mph) to stay even on corrected time.

Suppose your competitor, RHH (ridiculously high handicap) averages 4 miles per hour:

1/4 hours per mile X 1/3600 sec./hour = 900 sec./mile.

You need a corrected time of 889 seconds for each mile:

889/3600 = .2469444etc. invert: 4.04949 MPH is how fast you have to go, or a 1.2 percent increase (.05 mph) in speed.

As you can see, this change is not constant expressed as either delta v (velocity) or as a percentage. But roughly speaking, you''re looking at needing to do about one to two percent better speedwise depending on wind conditions. Clean your bottom and you''ll be halfway there. The other half we''ll let you find with your sail trimmers and helm.

Allen Flanigan
Alexandria, VA


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## sabre32sailor (Jun 1, 2005)

Sailormon6 said it best.

It is time that counts.

Nobody runs the same tacks upwind nor has the same wind, esspecially if racing PHRF. 
This is not NASCAR.
I have a heavy cruiser with a centerboard. As such, until the wind gets to 12 knots or so, I am footing the boat and subsequently sailing a lot further. VMG may help make sense in such a case, but then you need to ask the other guy what his VMG was.

In the downwind leg, nobody should be going straight downwind as it is just plain slow. You need to be sailing apparent wind angles and using the extra speed to cancel out the additional distance. Again VMG (this is where a GPS earns it''s keep provided you remember to keep looking for wind and not staring at the instruments).

And finally, boat speed difference will tell you zip about what you really need to know. What you need to know is how many seconds you were behind the lead boat at each turning mark. Then you know where you made time or lost time. At which point you know where you have to invest some effort to get better.

If you are 25 seconds behind crossing the starting line (don''t laugh, most boats average 20-40 seconds off the line as seen from the RC boat), 15 seconds behind at the windward mark and 30 seconds out of the money at the end, you know you need to pratice being on the line correctly, and need to review what happened on the downwind leg (different gybe angles, you sailed into lighter wind, you went to the wrong end of the pin etc).

Someone on board ought to be keeping track of this (bonus is you can see which direction they are heading on the next leg) and then you can talk about it with the crew on the ride back to the dock. Identifying weaknesses and correcting them is how you move from the back of the fleet to the front.

john


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## chris_gee (May 31, 2006)

You need to sail 1.12% faster measured in knots. The time you need to go faster is 35 sec per mile. In nautical miles that is 35 x 2025/1760 or 40.3 seconds. As a knot is a nautical mile per hour to convert to hours this is 40.3 x100/60x60 = 1.12%. The speed is irrelevant because you have to increase it by the same percentage. Although others point out that you can make up for a lack of speed by sailing smarter and reducing distance, or time taken, assuming both you and your competitor continue to sail in the same way, to simply answer your question you need to increase speed by 1.12%After an hour at 4 knots a 1% difference is only 90 odd yards to put it in perspective. Chris.


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## mikehoyt (Nov 27, 2000)

I am guessing you are referring to only a few specific boats.

In our racing there are 3 other boats I am usually worried about. 1 is ahead and 2 behind. I use the time on distance approach and determine for every 10 minutes of race time how much time I need to be ahead or behind of each to win. Then at marks along the way we determine how many seconds ahead or behind we are of teh other three boats and how many minutes we have sailed. Then we know how we are doing at that particular point in time.

This is not exactly what you asked but shoudl help.

We use ASPN (Atlantic Speed Potential Number) which is Time on Time rather than Time on Distance. However most clubs using PHRF at some point convert the PHRF number to a time correction factor for each boat and then use that to multiply by actual time to get corrected time. ASPN is a bit easier to use because our number is a percentage of ACTUAL time - but the same thing. In one series we race PHRF and have a TCF of 0.9246 whereas in ASPN we have a rating of 107 (multiply actual by 1.07).

Racing we know that 1 point ASPN (approx 6 sec/mile for a boat rated 180PHRF) is roughly 1 seconds per 100 seconds or in 10 minutes roughly 6 seconds. The boats in question are rated at TCF of 1.00, 1.03, 1.07, 1.08. For every 10 minutues elapsed on the 1.07 boat I must be 42 seconds ahead of the 1.00 boat, 26 seconds ahead of the 1.03 boat and no more than 6 seconds behind the 1.08 boat. In ASPN these are actual handicaps of 100, 103, 107 and 108 which makes it easier.

I guess the point is that after a race and before teh next I find out teh TCF used for each boat I measure myself against and come up with how many seconds ahead or behind that boat I must be for 10 minutes of racing. With this knowledge in my head I calculate if I am ahead or behind at each mark (if I have time to do any calculations ...)

OK - So you asked. Although I didn't answer what you asked exactly it is jsut as useful or useless....

oh .. and I am the handicapper in my area so I know all the boats handicaps as should every racer in a race ...

Mike

Full Tilt 2


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## Beowulf (Jun 17, 2016)

Do you have a folding prop? If not, get one.


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## JimsCAL (May 23, 2007)

Beowulf said:


> Do you have a folding prop? If not, get one.


Your first post, and you reply to a 10 year old thread like this. Your contributions can only get better.


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