To Bermuda 7: The Tortoise and the ....Tortoise fight it out in the Bermuda Triangle

For two successive nights now Bonny and Titti4 have made their scheduled VHF call at 1800. You may not think that particularly remarkable given the two boats left St Martin within 30 minutes of eachother and are both bound for Bermuda, but for a single hander talking to someone else other than one's self is a bit of a novelty. It's also a little strange given that by the end of day 2,  Titti4 had shown Bonny a clean pair of heals and we only managed to exchange garbled static, I also made out a couple of words from Jan but n. I could still 'see' Titti4 on my AIS display, she was about 10 miles ahead of Bonny. By day 3, she was no longer visible on the AIS and my 1800 call disappeared into the radio waves. Thanks to Starlink I can monitor her tracking website and by the end of day 4 I could see she was more than 15 miles ahead of Bonny. 

It was no more than I expected, Titti4 is a metre longer than Bonny which means she CAN go faster (I'll come back to why later). 

But by the 1800 on day 5 we were able to speak to eachother over the VHF. Her distance off had reduced to 9 miles! Yesterday at 1800 we spoke again when she was only 6 miles ahead of Bonny. Earlier this morning she was only 3 miles ahead and in plain sight. The gap has grown again since. As of 1120 she's nearly 4 miles off.

What's going on?

A number of things. I suspect Bonny reduced the gap initially when under power in the two periods of absolute calm which lasted about 6 hours each.

That's not the whole story though because up until recently we've been gaining under sail alone when we've been sailing on very similar headings.

So why, given Titt4 is a longer boat?

When I said she's faster, that wasn't completely accurate. She has a faster theoretical hull speed - that's, the fastest the laws of physics determine that a boat can go without planing!  

It's actually the boat's waterline length which is important, not her overall length. In addition when a boat heels its waterline length changes. Boats of the vintage of Bonny and Titti4 (i.e. old) are usually longer on deck than at the water line and so when they lean over the actual waterline length increases and therefore so does hull speed.  As the boat moves through the water it creates a wave. The longer the boat, the longer the wave it produces. But, at a given speed determined by the waterline length, that wave can't get any bigger and it doesn't matter how much force via the sails or engine is applied, the boat can't go any faster. UNLESS that is it planes over the top of the wave.  Neither Bonny or Titti4 can plane except under pretty extreme conditions - they're the wrong shape under water and too heavy. One wouldn't really want to be on board when there's consistently sufficient power in the wind to lift these heavy old girls off their waves!

'Of course', the shape of the boat matters too. Take two boats of the same water line measured from end to down the middle of the boat (which is how water line length is measured); one thin the other fat. The fat one has actually got more length in the water and will produce a longer wave and will therefore, under the right conditions, go faster than the thin boat of the same length.

Mind you, don't forget about drag! Oh I haven't mentioned that yet have I! Well 'drag' as you probably guessed, slows the boat down! As soon as a boat heels over, the drag on it increases compared with it being upright.

Which has most effect will depend on the circumstances. But, it's certainly true that there comes a point when, to maintain maximum speed, one needs to reduce sail area and therefore the angle of lean. That reduces drag. Keep up now!

Bonny's hull speed is about 6.5 knots. I'm guessing that Titti4's is perhaps a third to a half a knot faster. BUT, in relative terms, Titt4 is long and thin whereas Bonny is, if not fat, certainly more rounded! She's also lighter which is an advantage in light winds; which is what we've had during the entire period we've been gaining on Titt4.

It all adds up to a most unusual scenario. For two days Bonny has been sailing faster than a bigger boat.

However, the gap is growing again now (1100 on 15/5) as our courses diverge slightly while we look for the best route through the changeable winds ahead. Based on my weather info, I'm trying to stay to the west of a high pressure ridge that's due to produce even lighter airs than we have now. So far it's not working! 🤣

After typing that I noticed the wind had, already veered south and eased from light to very light, as a consequence we were heading south east! Not very helpful. 

So another major sail change from upwind slutter rig to a down-wind goosewinged rig with cruising chute. Like previously, it took the best part of two hours to complete the change which I finished around 1330. The only major difference this time was that I managed to lose one of my two mast head haliyards up the mast. Such a bloody silly, stupid, thing to do. I left the end carelessly untied on the deck. The gremlins needed no further invitation and hoiked it up the mast "without a by your leave". I toyed with the idea of going up the mast to retrieve it, but I can manage without it for now, so another job on the list for Bermuda.

The two tortoises remain locked in battle as they crawl in slightly different directions but both, generally heading northwards!

Stop Press: Tortoise race suspended at around 1615 when crews agreed to motor in light airs/calms in search of wind ahead and target of arrival Bermuda before dark on Friday. 

The wind will pick up late tonight and on Thursday will get quite blowy with a lull on Friday before another spell of less blowy weather. All from W/SW which is good. 

May have to 'heave to' (as near as one can get to stopping without anchoring/ mooring) over night Friday and wait for blow to pass over night and then enter harbour on Saturday.