Exceeding Hull Speed, Discussion?

[mr-canada]

Highseas: yes - the ebb should have given us about a 1 kt boost at peak as the chart described. Be were about 7 nm away fro the river so I don't think that had much effect, the river isn't raging or anything as most of the snowpack has already melted.

The goal was really to scream out of there as fast as we could, I figured if we could make keats island by 10am (we left at 630am) then we might make an attempt to make it to powell river as originally planned anyways. Unfortunately we slowed to 5-6 kts by anvil and the wind died at gambier entirely, we didn't make keats until 130pm so rather than trying to race up the coast we just enjoyed a vacation in the outer sound.

[Jim Cate]

G'Day Mr Canada,

Here's my take on your exciting ride:

You report '10-20 knots" of wind from astern. You were towing a dinghy. Your had only a 150% Genoa for sail area. Your boat was heavily laden. None of these factors are conducive to generating great speeds, and I greatly doubt that your speed through the water could have exceeded hull speed by much, except briefly whilst surfing.

On the other hand, your knowledge of the velocity of the current is pretty sketchy. The speeds shown in tidal current charts are estimates of average speeds under windless conditions. As others have said, current flows have large anomalies from eddies, and these can easily increase or decrease the speed over the ground locally by considerable amounts. Further, surface currents can be generated by sustained winds, adding to the potential errors in the predicted velocity.

So, my guess is that while you were indeed having a great sail and enjoying the occasional surf, your speed through the water was not in great excess of your hull speed.

Cheers,

Jim

[MarkSF]

Originally Posted by djmarchand

Hull speed, generally calculated at 1.34*sqrt(LWL) is just an engineering definition. That defintion is based on the theoretical speed of the bow wave. At hull speed the boat catches up and a trough forms. It takes a lot of horsepower to get out of that trough.

Any boat can exceed hull speed; all it takes is horsepower. But I would be surprised if a sailboat could exceed hull speed by 10% other than in ideal circumstances- ie 20 kts of wind on a beam reach.

David

Yes, 10% is what I can routinely exceed hull speed by, in a beam reach. At that point the boat is sitting in a big hole in the water.

[rognvald]

Originally Posted by SailtheWind

Last Spetember while sailing from St Augustine FL to Charleston SC we were in the dark of the night (new moon) sailing in 26Kts wind on a broad reach (135% jib fully deployed and the main sail reefed as we always do at night) and our speed indicator showed 10.2kts which exceeds the hull spped of a Caliber 38 (7.81kts) the water was nearly over the side rail. This ride lasted about 10 minutes then we were back to 6.9 kts which we keep for a few hours.
I have to say it was both thrilling and worrisome to be going this fast in complete darkness. We were 54 miles offshore so not much to worry about other than the occasional freighter.
The fastest we have ever been on our boat is 12.9kts while me wife was at the helm she has reminded me that she holds the record speed for our boat.

10.2-12.9 knots in a Caliber 38? Were you sailing in the Gulfstream when you reached those speeds?

[David M]

High performance cats like Hobies and Tornados frequently exceed hull speed without much effort in relatively low wind speeds, The same applies to sailboards and kite boards.

All hull speed means is that you have a bow wave, a trough in the middle and a stern wave. It's a physical description which can be estimated with a mathematical formula.

[S/V Alchemy]

Originally Posted by mr-canada

Yeah that was it exactly the feeling. It was a surreal feeling and then heard the "sound".

I've heard it described as a half-floor ride in a fast elevator, but the sounds is always described as "ssssshhhhhhhhhhh".

[barnakiel]

We sailed at 655 knots (or was it 665?). Hull speed 6.7 or thereabout. So I think the hull speed thing is grossly overrated.

b.

[denverd0n]

Originally Posted by micah719

Hullspeed isn't an unbreachable limit, it is just the theoretical maximum beyond which going faster costs disproportionate energy.

BINGO!

Almost all sailboats, with a modern hull design, can easily exceed hull speed given the correct conditions. Many can exceed it by quite a lot. That doesn't mean that their hull speed somehow isn't "real." Or that the formula doesn't get the "correct" hull speed for them. It just means that hull speed is not the hard limit that way too many people seem to believe it is.

"Hull speed" is simply the maximum speed at which the boat is still displacing a volume of water that represents the boats full weight. To get beyond that speed the boat needs to climb up onto its bow wave to some degree, will then be displacing a volume of water that is somewhat less than its actual weight, and will be in at least a "semi-planing" mode. This is just a matter of getting enough energy from the sails. Nothing magical about it.

[bill352]

As others have pointed out, the hull speed formula is more a guideline than a hard and fast rule. But "hull speed" means speed through the water, and if you're taking your speed off the GPS, you really don't know what that is. You need an accurate knotmeter to talk about hull speed.

Just to take an extreme example, if you put your boat on a trailer, you can probably get 50 Kts or better on the GPS, while your speed in relation to the trailer is (one devoutly hopes) zero.

[mr-canada]

By my original math as described we topped out at 7.8 kts and were easily doing 7.5 kts. With 1 kt of current deducted that would be 6.5-6.8 kts ; the hull speed of the boat is 6.2 kts, so raw "by the numbers" we were exceeding hull speed by 0.3-0.6 kts. As pointed out that was based on SOG off the GPS, not via a knotometer giving actual speed through the water.

We were heavily laden, and we were towing a dinghy. No rooster tail. Some surfing going on (awesome :-) ) Winds at that point were likely 20 kts+, seas were 2-3 ft with all waves breaking. Fetch couldnt have been more than 5 nm due to Watts Point. Ebb tide was falling about 10 ft, we were about an hour into the ebb tide as we left right at high tide to time the current. River is a negligible issue as it was 7-8 nm behind us, the snowpack has already melted, and the river was also around Watts Point.

We have done 7.5 kts in the boat before when I was out with the sailing instructor; although in that scenario we were close hauled pointing into the wind in 25+ kt winds with a double reef in the main and the 150% genoa. Tides in that scenario were a non issue as we were sailing perpendicular to any current flow ripping around in random directions as we were doing a lesson. (No dinghy that time) As previously stated the "calculated" hull speed of a Columbia 26 Mk II is 6.2 kts.

Surfing is an interesting way to exceed hull speed as I have learned recently on my trip (and here on CF). I guess because you are surfing a wave (in my case in following seas it was also pushing us) you are displacing less water so you can therefore go faster.

Raw power I guess is another route. As some are describing here you get enough power to push the bow over it's own wave, forcing the boat to displace less water and therefore exceed hull speed, but it takes exponentially more power to increase speed over this limit (? correct me if I am wrong).

Call me a novice, as I am one (3 months sailing); but I would say a realistic top speed for my Columbia 26 Mk 2 is about 7.5 knots with the current sail arrangement (150% Genoa plus standard mainsail). Partly as I did it with the wind behind (when I hit 7.5 - 7.8 kts, surfing as described) and also into the wind (with the sailing instructor, double reefed main, 150% genoa, close hauled).

Obviously hull speed is proportional to boat length, is this a good speed in relation to other monohulls in the 26 foot range? As my experience sailing is minimal, I dont really have much to compare against, as to whether this is a relatively fast boat for it's size or not.

[denverd0n]

Originally Posted by mr-canada

As some are describing here you get enough power to push the bow over it's own wave, forcing the boat to displace less water and therefore exceed hull speed, but it takes exponentially more power to increase speed over this limit (? correct me if I am wrong).

That's pretty much right, although just how much extra power it takes varies quite a bit.

Small, flat-bottomed sailing dinghies can climb up on a plane and exceed hull speed by quite a bit, without requiring all that much extra power from the sails. Most modern cruising boats, with relatively flat-bottomed hulls, can also exceed hull speed pretty easily, at least by a little bit. It does take more and more power to get them to go further and further above hull speed.

On the other hand, if you want to get a Westsail to exceed hull speed, it is going to take a lot of canvas and a lot of wind pushing on that canvas.

[Dockhead]

Most fin keel boats, especially if they are relatively light (D/LWL of say around 200 or less), will easily surf downwind. You need to know your speed through the water to know whether or not you were exceeding hull speed, but I wouldn't be at all surprised. You will not necessarily be throwing a rooster tail. My boat does not make a rooster tail when surfing, but rather throws up a fine, high wall of spray from the bow.

I have sailed at 12 - 13 knots for hours at a time in my boat -- downwind in a strong wind (40+ knots) -- although my hull speed is about 9.3 knots. Although my boat displaces 25 or 26 tons loaded, that is not actually all that much over a 46 foot waterline. The cored Kevlar hull keeps things relatively light.

I also was able to maintain a speed of over 10 knots for more than an hour in a chartered Salona 45 in Croatia some years ago -- a light cruiser/racer -- downwind in 30 knots or so.

So enjoy it, if you were in fact surfing. It's important to be very attentive at the helm when doing that -- a lot of boats will have a tendency to broach when they're surfing. I like to take the mainsail down altogether to move the center of effort forward, to reduce any such tendency.

[carstenb]

This past summer I blew across the baltic with my Jeanneau 40.3 Sun Fast in 35-38 knot winds with 45 knots gusts. Only the Genua set. Knotmeter showed 9,5 to 10,5. Hull speed (theory) 8.5 knots. Proof of planing: We covered the 37 NM from Falsterbo to Ystad in just under 4 hours, including the time spent setting and taking down sails and motoring into the harbor. Average speed. just over 9 knots. Knock off about 1/2 hour for sails and motoring into harbor, then the average speed is 10.5 knots.

I have to say we really did fly across the baltic. The next day we did it again on a 48 NM stretch - just a mite slower.

[Oliver L.]

David hit it pretty much on the head - "hull speed" really has nothing to do with maximum speed, but simply is the speed at which the wave that the boat creates has the same wavelength as the length of the boat.

Water has a "dispersion" quality - longer waves travel faster, shorter waves are slower. As long as you are at or below hull speed, the resistance of the water is reasonable. At hull speed your rear end surfs down a wave, while your bow has to push up a hill. Essentially they're both same size, so it's pretty much a wash.

When you go faster than hull speed, your boat-created wave gets longer than the boat. This means that you are increasingly sailing "uphill", while the nice downslope moves further and further aft. If you've ever been on a runabout, you will have noticed that when you hit the pedal, first the nose comes up and the boat feels like it is stuck, until it starts planing, at which time the nose comes down again. This "nose up" is what you are doing if your sails create enough power.

The phenomenon is exactly the same when you surf down a big wave, except that the power for that is created by the gravity of your boat and less by the sails. And no, I've never looked behind to see the tail end of my wave - the look was too scary

Oliver

[capn_billl]

I'm with the above the hull speed is more of a guidline than a rule. Why aren't boats round and flat instead of ...well, boat shaped?

We know that long thin hulls take less effort to push through the water than lets say a flat board. The famous hull speed equation does NOT take this into account.

It gives you the location of the bend in the power/speed curve, not the slope of the curve.

Modern flat bottom hulls with thin fin keels taker a lot less energy than old designs to push higher on the slope of the curve.

So exceeding hull speed by 10% in a modern sailboat, very doable, 20% or more, not unless you are in a Macgregor 26M.