Boat Speed

Moderator: GreenLake

Boat Speed

Postby talbot » Tue Apr 29, 2014 12:28 am

As a reward for spending the winter repairing my hull, I bought myself a GPS, and am now frolicking in data.
I may even learn to sail.

Discovery 1: With a double-reefed main and full jib, sailing alone, I can point at about 60 degrees to the wind. Closer than that and the boat slowed way down. Lots of foam and dramatic heeling, but little speed. (Wind gauge battery was dead, but from forecast and water surface, It was probably about 14kt).

Discovery 2: Hull speed is not that big a deal. The boat usually sails at hull speed, given some wind the proper sail set up. (Theoretical hull speed estimated at 1.34kt x 4 = 5.34, with 4 being the square root of the approximate waterline length of 16'). Again, I was sailing with a drastically reduced rig in whitecapped but nonextreme conditions, and reached 5.4 kt. easily.

Discovery 3: Going faster than hull speed IS a big deal when you are beating into the wind. Fiddle all you want with sail trim, centerboard, and weight distribution. Sail right into a line squall so the wind is crying in the wires. You can't -- well, I couldn't--get above 5.6kt.

Discovery 4: Off the wind with the board up, theoretical hull speed is almost irrelevant. I could make my 5.4kt under jib alone, and reached 7.5kt with my double-reefed main. There is a thread on this forum about whether the DS can plane. My data says that off the wind, it almost always planes, at least if you define planing as getting high enough on the bow wave to exceed theoretical displacement speed. That happens long before the boat actually feels fast.

So what? Racers probably know all this by heart, but for a cruiser, it means you should plan on nothing faster than 5.34 kt into the wind, regardless of sails or conditions. And consider that if aren't pointing higher than 60-degrees, your distance over the water is 2x the distance to your destination. So yes, setting out for that campsite three miles upwind a half hour before dark will put you off shore a half hour after dark. This explains so much . . . .
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Re: Boat Speed

Postby Scott Mulford » Tue Apr 29, 2014 7:00 am

Gps records your course over ground which includes lee way. Your are pointing closer to the wind than 60 if you are going by gps alone.

I am a cruiser that races everything I meet on the lake. I agree with your numbers.

I wonder what the hotshot racers get going upwind. I shoot for 90 degree tack angles as read on my gps but I rarely get them.
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Re: Boat Speed

Postby talbot » Tue Apr 29, 2014 12:59 pm

Thanks for the comment. I actually estimated tack angles with my steering compass.
I used the GPS for speed only. I could have double checked with the GPS, but ran into some biomechanical issues.

That is, holding the stupid electronics was getting in the way of keeping the boat upright. So I hung the GPS on the cabin hatch, but then the numbers were hard to read, so I switched to a mode that has the speed in big numerals, but no compass. Good thing the wind gauge was dead. I wonder if I could learn to operate it with my toes.

Something I learned racing on a Thistle crew a couple of years ago is that for a planing hull, speed is paradoxically related to crew weight, at least in moderate winds and above. Heaver is faster, because it takes several hundred pounds to keep the boat at an efficient angle. I imagine that with another crew member to supplement my meager size, the boat would have pointed higher and gotten onto a plane closer to the wind.
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Re: Boat Speed

Postby GreenLake » Wed Apr 30, 2014 12:17 am

So, what does the GPS tell you about your leeway angles? Curious minds would like to know. Since you are on a lake you shouldn't have any currents to interfere with your measurements - on a really long lake there may be wind-induced surface currents when the wind builds, but I don't think that was your scenario.

For cruising (daysailing) in moderate winds I rarely average better than about 3.x knts point-to-point (includes the effect of going the extra distance when going upwind). That number is surprisingly constant, and usually also spans variations in wind speed over the course (and usually not as brisk as you report for the duration).

Especially on the downwind legs I rarely get to the 5+ range, as the wind can't keep up :( The slightly oversized spinnaker I have been flying helps, but for longer trips, it can be used only for some part of the time and then the effect on the average isn't as big - but it's fun while it lasts.
~ green ~ lake ~ ~
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Re: Boat Speed

Postby talbot » Wed Apr 30, 2014 12:32 pm

I'll check it out this afternoon on our Olympic circle. (8 buoys at compass points around a circle with 1 mi diameter. I think the GPS can estimate course made good between any two waypoints, but I have to learn to use it first.
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Re: Boat Speed

Postby GreenLake » Wed Apr 30, 2014 9:16 pm

Well, you are probably still on the water - but when your have a circle like that, the leeway angle can be measured by a compass (not a GPS). As you round one of the marks, you sight the next one and confirm its compass bearing. Then you sail so you can just lay the next mark -- that means you will have to head upwind of it or you won't make it (because of leeway). When you have that dialed in so that your actual course is on collision with the mark, then you read the compass heading for where your bow is pointing. The difference between that and the bearing that you took earlier should be your leeway angle.

Assuming you can set up a steady state fairly quickly without sailing far off the line between the two marks.

Alternate method: trim and sail upwind. Once the boat is steady, note its heading. Then check which spot on shore you would reach if you continued. Ideally, you would be close enough to shore that points forward of the point you would "hit" would be moving slightly forward and points aft would be moving slightly aft, and only the selected spot would have "constant bearing". The difference between that constant bearing and your heading is the leeway angle.

Leeway angle, if not excessive, should be only a few degrees - that makes measuring it tricky.

Third method, check your wake. The angle between it and the reverse of your heading should be the leeway angle.

Let us know what you find out.
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Re: Boat Speed

Postby talbot » Sun May 04, 2014 7:50 pm

As a researcher I know that a big problem is the failure of subjects to follow the study protocol.
Mea culpa. That day last week was great sailing, and instead of conducting my investingation, I just tried to see how fast the boat would go.
8.4 knots. I might have been able to go faster, but I couldn't hold the boat down by myself.

So I guess I did learn one thing: The advice to "reef early and often" is usually cited as a safety measure. But it looks to me like there is no benefit in carrying any more sail across the wind than you can trim. If you have to put the rail in the water or luff the main to spill air, you might as well reef. Even if you are planing along at way over hull speed, as soon as you tip beyond about 15 degrees, you become a displacement hull again and the speed immediately drops back to about 5.5kt-6.0kt. It is much less work and just as fast to reduce sail to where you can keep the boat trimmed and stable. There isn't as much foam and drama, but your progress is better.

I know there are exceptions. If the wind is decreasing, you might want to stay under full sail to anticipate the change. Or the only way to reduce sail might be to drop the jib, which will really cut your speed and pointing ability. But if you have a smaller jib or unused reef point in the main, it probably pays to use them. Other sailors may call you a wussie, but they have to catch you first.
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Re: Boat Speed

Postby GreenLake » Sun May 04, 2014 10:59 pm

I think you are on to something, but it makes a big difference whether the wind is steady or not, and whether you are racing or not. For cruising, you might want to set your sail area so that gusts don't overpower you and there's less hassle/ work. For racing, I doubt you'd do well if you reef so that you are underpowered in the lulls, or in the average wind.

Sailed with a newbie this weekend, even 8-10kts of wind were too much. I could hold the boat, but I couldn't rely on that he wouldn't do something "drastic" with no warning, so I was holding both sheets in my hand, and not hiking too hard, and for that, the pull on the jib sheet was uncomfortable. But then, I couldn't have reefed the jib anyway ...

We had all the usual troubles for a season opening sail, such as a flat trailer tire, etc. so we didn't spend much time on the water, but the GPS gave readings up to and a bit beyond hull speed, despite spilling wind liberally.
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