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Hull Speed?

6K views 38 replies 16 participants last post by  MeanzBeanz 
#1 ·
Can someone explain hull speed to me? I thought I understood it but a thought occured to me as I was talking over a "Dark and Stormy" that caused me to question my understanding.

Does hull speed mean the maximum speed a boat (displacement hull) can travel while under power or the max speed under sail?

My 32' boat motors about 5.5 to 6 knots full power from it's 18 horse aux, but I've seen higher speeds (even touching 8 knots for a second)on the knot meter while sailing and the KM speed was consistent with my GPS.

I'm thinking when I hear the term "hull speed" it applies to the theoretical max speed while under power and not heeled over resulting in a longer water line allowing a higher speed.
 
#2 · (Edited)
Midliefesailor

Yes, its correct, you can get higher speeds than theoretical hull speed, (which is a calculation based on hull lenght at water line, based on bow wave height and separation between the bow and aft waves), if your hull starts planing, meaning it rode the wave and is now "surfing" on it, so the wave is no longer an obstacle, or if as you well said, when healing, the water line increases due to the overhangs, but so does drag and other limitations.

Please check these threads bellow

HULL SPEED

GZ

and this site. In this site if you place the cursor over the rectangles, it explains what is what.

SAILCALCULATOR

Hope it helps
 
#3 ·
I was going to say that Alex or Jeff would be along shortly, but Alex beat me to it.

Alex, I get error messages on all the sites. Is it me?

I was looking this up the other day. I know that the theoretical hull speed on my C34 is in the mid to high 7's. I also know that I have surfed it at much higher speeds. I need to do a little more looking and I know that your references will probably give me some answers.
 
#4 ·
A displacement hull does not plane... that is a sailboat, It can slide down a wave or surf at more than hull speed.

The notion is that the displacement hull creates a bow wave as it pushes the water aside. The wave gets steeper and longer as it moves faster. At some point the vessel is in a trough and it would be trying to go uphill. I don't think a powered sailboat will create a longer and longer wave... steeper and steeper as more power is applied to drive it up the back of the wave.

I think the hull speed is 1.34 x square root of the LWL.. which does change a bit depending on the hull shape. So a 36' LWL would have a hull speed of 8 knots... 25' would be 6.7.

jef
sv shiva
 
#11 · (Edited)
A displacement hull does not plane...
Yeah they do, just not very well and for very short periods of time.

All multihulls displace water just like any other hull and plane when pushed harder than "displacement speed" just like any other hull. The thing is that the dynamics of the event start to alter as the beam to length ratio decreases (and the wetted surface area, weight etc etc alters). I have had it explained to me and it gets complicated, so I am not the one to explain, but my understanding is that the relationship or "multiplier" constantly alters... i.e. its a curve not a fixed ratio and it is far more efficient at the narrow beam end of the scale.

Try asking at www.boatdesign.net there are plenty of designers there that will explain if you ask nicely.
 
#5 ·
Any formula you use will give you a "theoretical" hull speed. As some boat designer named Bob Perry told me once, the 1.34 x the sq. root etc. will get you in the ball park for most cruising boats.
 
#10 ·
Note also that very narrow hulls can exceed the hull speed defined by the formula 1.34 X sq. root of LWL. High performance multihulls are a good example of this.

Destroyers are another example. The WW II HMCS Haida had a maximum speed of 36.5 knots - LOA is 377 feet (estimate LWL as say 20 ft. less) and you get a ratio of 1.93.

http://hmcshaida.ca/tourintr.html
With two steam turbines generating 44,000 hp I certainly hope the Haida can exceed the 1.34 multiplier. I don't think that is not the kind of boat this discussion is about.

As for high performance multi-hulls, they most certainly do plane.
 
#9 ·
I think the reason we can sail or motor faster than hull speed can be accounted for largely by considering the speed at which the current is moving us at.

I don't know if the asymmetrical shape of a heeled boat affects the formula or if it is only the waterline length, but I think its the latter only.
 
#13 · (Edited)
I think that one thing to consider with performance cats and modern dinghies etc is that they displace relatively little so they are starting the process of planing at much lower speeds and with much less resistance. I remember the first "Sports Boat" that I sailed slipped onto the plane so quickly and easily I was not sure it had happened... its a fantastic sensation. If you are a heavy displacement sailor and never done it I'd recommend getting your tail on one of these things for a ride at least once and talk to the guys about "how they sail"... the rules change a little with these things (more like a stable dinghy!).
 
#16 ·
Destroyers are not exempt from the principles of naval architecture. While they and other warships are capable of a good turn of speed, and btw the older one's were faster-speed is no longer the priority it once was, they do so at considerable expense. They will not effectively plane, and the stern squats substantially, often on the order of twelve feet. Once the stern begins to submerge all extra horsepower is of little use, in fact, it is counter productive. "Real" figures on naval vessel top speeds is mostly conjectural, but the naval architect has as good a guess as any since hydrodynamics will always have it's day.
 
#17 ·
No catamarans that I know of plane.

The general formula of 1.34 x SQRT (LWL) doesn't generally apply to most multihulls, including the "non-performance" cruising multihulls.

Most cruising catamarans have a waterline-to-beam ratio of 6:1 or so. Most trimarans have amas with a hull ratio of around 12:1, and main hulls of around 8:1 or so. While they are technically displacement hulls, the high waterline-to-beam ratio often means that they are not subject to the 1.34 * SQRT (LWL) limit, as keelboats generally are. In many cases, they have a hull speed formula of 2.0 * SQRT (LWL) or higher, depending on their exact hull design.

On my 28' trimaran, I've seen speeds of over 15 knots under sail, and generally we can run down most of the monohull sailboats that sail in the area around us.

Monohull keel boats can often exceed the calculated hull speed due to several reason. First, when they are sailing down the face of a wave, or surfing, they can exceed hull speed. Second, when they are being pushed by a following current, they can exceed hull speed as their Speed Over Ground, but may not be exceeding their calculated hull speed relative to the water. Finally, some of the smaller keelboats can plane...but not too many of the larger keelboats can or will.
 
#18 ·
Yeah but....

No catamarans that I know of plane.
Mine does... :D

Think about it! Anything doing that speed must build up pressure under the hull and generate lift. Cats are light, high powered (well some are :)) and their hulls are narrow, the combination means that you are planing more easy (no noticable "speed hump")... so much so that its hard to notice the transition (some sports boats are that good U hardly know it). When I look off my transom's at 15 knts+ it looks just like looking off the back of any other high speed planing vessel. You might not be generating similar lift to a mono hull at that speed but then you have not got the drag either.

My old Paper Tiger (hard chine cat) planed more noticeable than the round bottom boats I sailed against but at some point they generated lift. We had a big cat (60'? Plywood? :eek: ) locally called "Big Bandicoot" that actually had V type planing hulls like the Paper Tiger (more power boat style)...

I suppose the question is what do you call planing? How much lift is planing? Sitting right on top of the water? or merely exceeding the hulls displacement speed capacity.

Just like all hulls displace water there is a speed at which all hulls will generate enough lift to rise up out of the water. Physics demands it! The only questions are things like...

At what speed?
How much force does it take?
Can the hull take it?
How stable is the hull form at that speed?

A multihull's speed comes from several factors. Multihulls plane on the water rather than plowing through it like most monohulled sailboats. Multihulls also use the wind in their sails more efficiently because the extra hulls keep the boat from heeling over excessively. Multihulls are lighter than comparable monohulls, so their lighter weight also increases their overall speed.
Chris White, excellent multihull designer

2c MBz
 
#20 ·
MeanzBeanz-

Planing is generally when the hull has generated enough lift and is mostly out of the water... which is generally not the case with most multihulls. Which Catamaran do you have??

BTW, citing boats that compete in the Sydney-Hobart race that are over 60' LOA and the Volvo 60's is relatively irrelevant IMHO, since these are mostly pure racing machines, and in most cases far out of the budgets of most sailors. I seriously doubt that anyone cruises on one of these designs, since they are not really designed for comfort to any degree.
 
#22 · (Edited)
Its only a Seawind 24'

Planing is generally when the hull has generated enough lift and is mostly out of the water
Thats a fuzzy definition! So what is it when you are going faster than displacement hull speed but not mostly out of the water?

My Seawind 24 can pull upto 20 knots on a hard reach... there is no way that you can claim that is a "displacement speed" for a 24' boat of any sort. It simply must be generating lift at that speed and therefore riding up on the water to some degree (the amount of spray supports that! oih! she is wet!) That in my book is planing as is surfing, the only difference is the source of the power i.e. Sail vs Sail and Gravity (wave action).

In my book when the boat is displacing less water than its gross weight it is starting to ride up onto the surface of the water. The rest is semantics about degree, which relies on hull form, weight etc etc. Therefore surfing is planing, big heavy monohulls don't do it very well or for very long I grant that and they often need the force of gravity to make it happen (a v big wave) but happen it can. We have had our old Brolga 33 (heavy) planing at 14 knots (well 10 to 14 and back very quickly). You can see the mass of green water she shoves aside as she rides up on it....but alas it scares the poor old girl and she will only do it for 10 seconds at a time and then she wants to turn around and go right back home. You ended up with tiller up round the ears as she falls off her bow wave and the following wave starts to catch her. If you didn't anticipate it, it was ugly, wet, flappy, noisy and the water got very close to the helmsman.... fortunately she was built like a ...... , well nothing I have seen since.

I'm gunna go with Chris White on this one, multihulls plane, very efficiently with minimal fuss but they do it. Thats why I keep talking about sports boats, they plane so easily that it makes the fuss that goes on getting say a J24 up look like a circus. That however does not mean they are not doing it :)

Have you seen Mr Whites "Hammerhead 54'" .... now thats a cruising tri ! 20 knots in that :eek: !

Cheers
MBz
 
#26 ·
Ok I think I have it, but have one other question. When looking at a boats specs is the measurement given for LWL taken when the boat is sitting upright or is it taken assuming X degrees of heel?

Using the formula given, a boat with a 26' 8" LWL would have a theoretical hull speed of about 7 knots. Would that figure take the longer water line resulting from heeling into consideration or is the theoretical speed actually slightly higher when the boat is heeled?

Thanks for all the great feed back and discussion.
 
#28 · (Edited)
My understanding is at rest, so yes level, and fully loaded to the designs maximum capacity (Load Water Line). In a light load condition you will find that the LWL can be well above the actual waterline.

To get closer to real world you would have to calculate the waterline at the optimum angle of heel in a typical load state. Often designers will give the ideal angle of heel to sail the boat at for maximum efficiency, a couple of Farr designs I sailed it was 18 degree's, Farr reckoned above that you where slowing down and it was time to depower and flatten out the boat. "Flat is fast" was the motto! I guess at the end of the day it requires an intimate knowledge of your boat and then its a best guess calculation because sea state, freeboard windage etc etc (all the other unique features of the design and your boat) will impact the result. Not to mention the fact that is an approximation to begin with :)

Is the fog gone? Or just sitting around the ankles ..... :D LOL

So there you go, we don't really know for sure.... well ain't that just dandy :D

Cheers
MBz
 
#29 ·
As a hull heels the the waterline plans becomes assymetrical, the lifting from keel starts to diminish and the hulls slips and makes more and more leeway. It's all very complex and why heeling excessively is a bad idea.

And the wife complains more too.

jef
sv shiva
 
#35 ·
So far, our best speed is 9.5 in a 10 to 12 kt breeze. We need more wind before we will realize her top speed. When we purchased her, we were told that in a few races, in perfect conditions, they had her up to 17kts. OMG! I would love to see that, however, when we brought her home and Skip started tearing her down, we found an OLD kt meter, and subsequently think maybe the meter was not reading right. I am still hopeful one day we will get out of here and see some real wind to sail her on and discover for ourselves just what top speed is.
Kathleen
aboard
Schooner MISTRESS
 
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