Panama to San Diego: Issues with Range under Power

2100 hours – Considering BEOWULF’s range under power.

2100 hours. The breeze has switched around the the south for most of the day. Mainly light, but it makes for a good motorsailing angle (right now we have a true wind angle of 140 degrees and an apparent wind angle of 45, with six knots of true wind speed – with working sails and engine we are averaging 10.5 knots at 2.5 gallons of diesel per hour).

The issue on a long passage like this is one of range under power. Winds are apt to be light, or contrary, and the risk of tropical storm systems makes it imperative to move quickly. Light air sailing, peaceful and usually pleasant, is a luxury we cannot afford right now.

The basic rules of hydrostatics control all boats in the same way. The faster you power, relative to your effective waterline length, the less efficient your fuel consumption. Speeds are considered in relationship to the square root of waterline length, and in a term called a speed length ratio. A speed length ratio of one would be the square root of your LWL – so for a 36′ waterline vessel this would be six knots – is usually the most efficient speed. You can typically power at a SLR of one for a third of the fuel required to push you at a SLR of 1.3.

Beowulf is very efficient at high SLRs – much more so than other boats which are heavier for their length. At an SLR of 1.0, in our case about 8.7 knots, we consume 2.5 gallons per hour. Kick that SLR up to 1.2, or 10.5 knots, and fuel consumption goes to close to four gallons per hour. In most cases you would say slow down, and make the fuel last. But on this passage the time/weather exposure issue makes the equation more complicated.

At the slower speed, assuming no headwinds and no sailing,we can make the entire 2850 miles in one shot. But with headwinds we will need fuel – and we’ve had some of those already. Add in the fact that we want to go faster to reduce our exposure time, and a fuel stop is beginning to look like a necessity. But then, if you get to stop for fuel, might as well push the speed up a notch and get there faster.

Over the years we’ve done these sorts of calculations with slide rules, then with handheld calculators, and even spreadsheets. Today we play “what if” games with the route planning and fuel curve features of MaxSea.

All of which is a complex way of saying we’ll be stopping somewhere in Mexico for fuel. All other things being equal, we’ll make that pit stop in Acapulco – where we can get in and out quickly (and handle clearance at the Yacht Club). If the present progress continues, we’ll be off Acapulco early Saturday AM. On the other hand, if this very favorable motorsailing wind angle continues, and there are no immediate weather threats, we’ll continue up the coast to Manzanillo or perhaps Puerta Vallarta before topping off.

Speaking of the weather, the current NMG faxes show the low pressure pattern continuing with favorable winds for the next 48 to 72 hours – that would be really nice if it happens. The tropical wave is still with us – the clearing we thought we had earlier has evaporated, and we’ve had a cloudy, semi-squally day. Just goes to show you, with all the satellite images and computers the National Hurricane Center has to use, you can still get a pretty good handle on what’s causing your weather by tracking clouds, pressure, and wind, all of which point to the low still being in the neighborhood.

Posted by Steve Dashew  (June 6, 2002)

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