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Theoretical Hull Speed

6K views 14 replies 13 participants last post by  Classic30 
#1 ·
Yesterday was a great day. Not only did I make it out to the bay with some rather fancy maneuvering, I also got up to 7.1 mph, which is just shy of the 7.4 theoretical hull speed for my boat. This is, of course according to the formula.

Here's the question: is there something that happens to the boat when it reaches that speed? I don't mean like a sonic boom -- I'm not quite than dense. I'm wondering if there is a smoothing out of the ride or something else that changes the way the boat sails that can be felt in the tiller or something else.

One thing I did notice is that my swing keel, or the cable for the keel seemed to start humming at around 7 mph.

Any thoughts or ideas?
 
#2 · (Edited)
Noah,

Nothing really happens. But first I think you are mistaken about what hull speed means.

The hull speed formula we all use 1.34*srrt(lwl) is not a technical one, it's just a very rough guess. First the 1.34 is not really a constant, it is derived from the average leingth/width ratio of boats that were designed around 1900. It has little to do with modern boats.

Secondly it doesn't take into account displacement. Since a vessels displacement dictates how much energy is put into the bow wave, this lack means that the formula simply doesn't work with very light boats for their leingth. Which is why racing row boats can routinely exceed double their 'hull speed' in displacement mode. The same can be said of almost any long, lean, light vessel. Even without planeing the bow wave simply doesn't create enough parasitic drag to effect the top displacement speed.

The humming you are experiencing btw is most likely vortexes generates from the trailing edge of the fins. It's a good idea to hit one trailing side with a little sandpaper and bevel just one edge a bit. This keeps the vortexes from forming.

For a more detailed discussion http://www.wavetrain.net/boats-a-gear/168-crunching-numbers-a-better-way-to-estimate-hull-speed
 
#3 ·
Yep, it starts to dig a hole, and drop into it, then eventually rides up its own wake backward, so you'll surf down it, and start all over... Shortly after that you'll either continue to surf, or start breaking things :)

It takes a lot of power to get the boat past that arbitrary number, but it's doable, and usually is easiest off wind between 60 and 120 degrees. If you can throw up a kite, things get real interesting real fast!

I've seen double digits on my S2 once (and I'm just getting used to her)... on my Capri 25 I was able to do it a lot (lighter boat)... It also makes for some spectacular wipe-outs!
 
#8 ·
The keel cable on my boat hums sometimes too when I really get going. I can get rid of it by adjusting the tension on the keel winch.

The hull speed speed for my boat is 5.9 knots. A couple weekends ago I had it up at 6.5 knots for a while :) but then stuff started flying around the cabin and it was deemed time to put in a reef.
 
#9 ·
Actually, the theoretical hull speed for mine is 6.4 knots, though unless the wind is really light, I usually start off with a reef, primarily because it's harder for me to put in a reef when the wind builds than to shake one out if the wins is too light.

Can't imagine what it was like for stuff to start flying around your cabin. Yesterday, I noticed the container with all the life jackets had fallen on the cabin sole while I was speeding along -- I hadn't even realized when it happened!
 
#10 · (Edited)
Here's the question: is there something that happens to the boat when it reaches that speed?
Yes. And its weird some of the previous posters have made a joke or two about it and your question because I think its vital when cruising especially in heavy weather.

A displacement boat above hull speed (i agree with Stumble we dont know what it is exactly) becomes unstable. The waves it makes get larger, the boat tries to get itself up and plane but it can't. The more force you put on the boat, more sail, engine on full blast, wont make it go much faster, but you are adding stress to every part of the boat.
The most important thing is the instability. You've heard or read people saying they sailed at 12 knots when their hull speed was 8 knots? Well thats gotta be BS... What they are doing is surfing down waves. Fine in a nice, gentle storm, with nice gentle rollers with a good helmsman. But cruising on autopilot, or at night, with tired helmsmen, or when the niceness goes out of the storm and the weather really gets nasty that speed above hull speed increases that instability to dangerous, catastrophic levels.
In a split second your smart ass 12 knots is a disaster as you broach down a wave, throwing the boat's beam onto the wave as the people below get thrown form one side of the cabin to the other breaking arms and legs.

Even in a normal passage you can push the hull speed with too much sail. It destroys your mast, shrouds, rigging and sails. Thats why often times when you reef you dont slow down at all... You just remove the pressure off your boat.

My hull speed is meant to be 7.94 knots. So when I start getting up to those speeds I SLOW THE BOAT. Its safer and I don't have to replace so much broken geer when I arrive in port.

Treat your boat well at sea and it will love you for it.
Btw, its a good reason not to buy an ex race boat... They have been pushed at hull speed every race.

Mark
 
#11 ·
Just for the record, all of this discussion is pretty hypothetical without discussing the specifics of the boat in question. Most discussions of hull speed apply to pure displacement type hull forms. But there are other hull forms such as a semi-displacement hull form (mentioned above) which produces a very small bow wave reducing the energy required to exceed its theoretical hull speed, and planning hull forms, which routinely exceed their hull speeds by climbing up on their combined bow and stern waves.

While a pure displacement type hull does get pretty squirrelly at speeds much in excess of their theoretical hullspeeds, and will tend to broach at those kinds of speeds, semi-displacement hullforms and planning boats actually 'calm down' at speeds over their theoretical hull speeds. My boat has what I would consider a semi-displacement hull form with a theoretical hull speed around 7.6 knots.

On a number of occasions, I have been able to 'blast reach' (lots of wind slightly aft of abeam but apparent winds forward of abeam) at sustained speeds well over 9 knots with brief bursts up over 12 knots (turning down in big gusts and getting assistance of the wave action although we were moving faster than the wave train). At those speeds the boat became very steady, helm loads became very light and control felt surgically precise. The wake was very dramatic, flat and wide. I really did not get a sense that the boat was on the edge of a broach or wipe out. I have experienced a similar feeling in planning boats at speed.

Jeff
 
#12 ·
The OP's blog says it's a MacGregor 25. Reading Mark's comments of wipeouts did create some concern. The theoretical hull speed in a Freedom 38 (10 tons of displacement) is 8 knots, but I've been up to 14 knots downwind and surfing. It felt stable, but I was very active on the wheel. The AP would not have handled those conditions and a broach would have been likely. I'm still in learning mode, and to carry on with the OP's train of thought (if I may), I would be interested in hearing at what hull speed would be the theoretical time to reef under downwind sailing and surfing conditions.
 
#13 ·
I've hit up to about 8 knots with a 6.6 knot HS boat. 11 surfing as noted with a 6' wave in a 40 knot gust down wind with a full main and 110 up. Nobody from what I under stand held a spin up for too long with what I would call 26-30 semi typical IOR half ton style boats.....

As noted, a lot depends upon hull shape. MOST if NOT ALL of the old square rigged boats from Columbus's day forward, when caught in a hurricane, most sank as they were above hull speed, and they literally sailed into the bottom of the sea.

The humm is common on a CB boat. My step dads 21' boat, would humm around 4 knots. Even my current 28'r humms sometimes at certain speeds.

As far as stuff flying.....for me, that only happens when pushing upwind and heeling a LOT, IE over 30-40*. if under 20, or going down wind, nothing is usually flying. This assumes things are loose and not secured. When constantly over 20-30*, I usually find it is time to reef or reduce sail. Downwind, one can use/handle more SA than upwind.

marty
 
#14 · (Edited)
I don't really think it is the speed alone that makes a boat squirrely, but, when the boat is going really fast, a slight change in heading can cause it to heel and to suddenly become a handful. Also, the conditions, including wind speed and direction, wave size and direction, your sail choice and similar factors can make the boat either more or less stable. My boat once exceeded hull speed on jib and mainsail by about 2 - 2 1/2 kts on a beam reach in fairly smooth waters and about 25 kt winds for three solid hours, and the autopilot steered it all the way. I crewed on a boat that raced downwind under twin headsails in 18-25 kt winds, exceeding hull speed by 2 1/2 - 3 kts for hours, and it was very stable. By comparison, I have crewed on spinnaker boats running downwind in big winds that were very squirrely.

In these circumstances, I don't start to think about reefing at any given speed. Figuratively speaking, I listen to the boat. It has ways of "telling me" when it isn't "happy." I reduce sail area when the boat tells me it's unhappy. At the first sign that it is beginning to labor, or the helm becomes sluggish or unresponsive, it's time to reduce sail area.
 
#15 · (Edited)
Squirrely: I have to agree with SM6. In my experience with heavy-displacement boats, a lot of that depends on the boat's design - some are worse than others and some boats get downright dangerous.

My full-keeler has a theoretical HS of 7.5kts and I regularly get it to that in flat water with plenty of wind. At around that speed she slowly sinks (squats) lower in the water, but doesn't get really squirrely because the wide hull section allows her to do that without getting unstable.. although when a few waves get added and she starts surfing (sliding) down them on a run then things get mighty interesting.

The quickest I've ever gone in her was 2 weekends ago when we were reaching at 8.0kts in flat water in 25kt winds. At that point I've got the stern wave coming over the aft quarter to tell me loud and clear that we've hit the limit... Needless to say, we won that race! :D

FWIW, this is what hull speed looks like:
 
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