The Offshore Voyaging Reference Site

How I Know Drag Is Bad On An Offshore Sailboat

We just published another article on the importance of keeping drag low on our offshore sailboats.

Some may wonder why we make so much of this? Here are two (of many) reasons, which got cut from the article to keep the length reasonable:

#1 Fixed Props Suck

Back when I had my Fastnet 45 I had a great crew and we were pretty competitive racing inshore “around the cans”.

For one race we were in the process of having a new MaxProp hub bored for the somewhat idiosyncratic shaft, so I had taken off the old folding prop so the machinist could measure it, and substituted a spare fixed three-blade prop. We were granted the standard PHRF 12 seconds/mile adjustment.

We went from near the top of the fleet to DFL¹. We simply could not get out of our own way or sail to our handicap, even with the adjustment for the fixed prop. Not even close.

Being at the helm and trying to keep the boat moving as boat after boat sailed over us was heart breaking—I can still vividly remember the feeling 35 years later.

Yes, feathering props are expensive, but they also have one of the best cost benefit ratios of any piece of gear I can think of.

#2 A Clean Underbody is Vital

The summer we voyaged to Svalbard from arctic Norway we had been in the water for over a year and there was no suitable yard to haul the boat, so there was a thin layer of slime on the bottom (no shell).

On the way north across the notoriously bumpy Barents Sea I was horrified by how slow we were, to the point that before the return trip we spent hours building a scrubbing device to get the worst of it off while standing in the dinghy, and I even went over the side to check the cleaning and touch up (in a dry suit).

Lucky we did, given it was a five day stone-beat to windward across the cold, foggy, and bumpy five hundred miles back to Norway. I shudder to think how much longer that would have taken without the makeshift bottom scrubbing, even though we motor-sailed for much of it—you don’t hang around in the Barents Sea in late summer.

Just a thin layer of slime slows a boat a lot more than would seem logical.

Two Other Thoughts

In both cases the problem was not so much straight-line speed but it was the excessive deceleration in the lulls, slow acceleration in the puffs, and the loss of pointing ability that was so horrible.

And keep in mind that in each case there was only one slowing factor—fixed prop on the first, slimy bottom on the second—on otherwise optimized boats!


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Dick Stevenson

Hi John and all,
There are numerous reasons to get comfortable going into cold water and staying in for lengths of time.
For those who wander off the beaten path, to me, it is a safety issue to be able to get and stay in the water for at least the 10 minutes it might take to cut off a bad wrap. And, for my 40-foot boat, I was looking at 30 minutes to clean the bottom, clean the prop and shaft and add another 10 minutes to do the zincs. And that was without coming up for air (scuba tank on deck with a long hose).
Down to 12 degrees C/54 deg F I was good in a double layer of wet suits (shortie and full), but I was really very cold after 15 minutes of work and starting to get sloppy in my work. The dry suit with a really warm base layer enabled to stay in 5 deg C/40 deg F water as long as was necessary. It was wonderful, but be advised: there is a learning curve to using a dry suit that might benefit from a professional consult.  
My best, Dick Stevenson, s/v Alchemy

Dick Stevenson

Hi John,
I have used one of the Hookah breather rigs and would agree they are very creative way to stay in (and below) the water for long periods doing boat work. Were I not to carry full scuba gear, that would be the way to go: and it certainly is cheaper and takes less space/weight that full scuba gear.
My best, Dick Stevenson, s/v alchemy

Stein Varjord

Hi John and all,

In case somebody here thinks John is exaggerating, “a bit of slime can’t possibly mean much”, or “my heavily laden sedate cruiser keeps about the same slowness no matter what I do”. Well, John doesn’t exaggerate the slightest little bit!

A very slow boat probably also has other issues that need attention too, like a poor rig trim, but very few items will compete with the influence and frequency of the two issues discussed here. Many decades of excessive attention to sailing speed has proven this to me, beyond any doubt.

The first to prove it to me: In the seventies I sailed in the Yngling class, mini Soling, open keel boat for 3 people, which later became Olympic, as its bigger sister. It was easily the most competitive class in Norway at the time. I grew up next to the wharf that built them, so our club had the highest local competitive level and participant numbers.

Several of us were quite ambitious, so we’d sail close to every day, the whole year. With several teams going all in on hours spent and attention to every detail on their boats, those teams become extremely even. None of the best boats used antifouling, but would rather lift the boat and scrub and polish it when needed. Southern Norway has warm water in the summer, so at least once a week was preferable, but most did it every second week or so.

Since I lived next door to the club and the wharf, with free access to the crane, I could more easily keep the boat cleaner than the others. I always lifted the evening before any race. I know for a fact that at least two local teams were much better than us (also older and wiser, as I was a late teen), but we won close to every race in the local series. In more important races they did the same and usually beat us.

A one week layer of slime makes enough difference to drop you from a potential winner to a no chance. One week of growth! I’ve seen the same many times later with very tightly matched boats.

With antifouling, the same difference takes far more time, but beware that even two weeks on the mentioned boats, they usually looked completely clean, as if newly polished. You’d see absolutely zero growth. With a sensitive hand, though, you could feel it. What we call slime isn’t really slime. It’s lots of thin “hairs” that affect the boundary layer of the hull significantly. If you can see or feel ANYTHING but smooth antifouling on your boat bottom, it’s a big deal!

I have limited similar parallel experience with fixed versus folding or feathering propellers, but our own cat is one. When we bought it, it had fixed props. Our speed alone is at least (!) 20% faster with our folding props than with our fixed ones. Our pointing ability is also affected similarly. In good wind offshore mostly downwind, conservatively cruising, we averaged 7 knots over a couple of days. Now at the same, we usually average 9-10 knots. That makes hours per day…

A cat has two props and a significant speed capacity, so don’t expect + 20% difference at any angle on a cruising monohull, but also rarely very much less. The propeller issue is dramatic.