Theoretical Maximum Boat Speed?

drjoefo

New Member
Is there a maximum speed a hull can travel at? If so, what is it limited by? E.g Length of waterline:rolleyes:

P.s. Theoretical is possibly spelt wrong
 
Maximum hull speed is 1.34 X square root of the length of the waterline. The Laser is 13'9" length overall so say about 13' 6" at the waterline. That gives you a theoretical maximum hull speed of 4.9234743 knots.
Hull Speed Calculator

That's why planing is good since it gets you up and over the bow wave and frees you from that constraint. Essentially a boat's displacement mode (ie hull speed) is limited by the bow and quarter waves which are determined by the waterline length. Good, basic references like Chapman's and Annapolis Book of Seamanship (among others) have better and more detailed explanations.
 
so the rule only applies to displacement hulls and not dinghys that can plane ?
- what is the definition of planing ? when your boat speed exceeds that of the bow wave ?

thanks a bunch.
 
so the rule only applies to displacement hulls and not dinghys that can plane ?
- what is the definition of planing ? when your boat speed exceeds that of the bow wave ?

thanks a bunch.

A planing hull is in displacement mode when it is not planing, so hull speed is limited then. Once the boat increases speed enough to climb up and over its bow wave then it begins planing.

The simple dictionary definition of planing is "skimming across the water", but your definition is probably just as good for the task at hand. The definition below might help make more sense:

From Chapman's Glossary p902 64th ed.:
Planing Hull A hull designed so that forward speed creates water lift, reducing friction and increasing speed.
More from Chapman's . . .
 
I don't think there is a simple mathematical way to determine your max speed when planing. Would there be a point when the skin friction becomes too great and can't be overcome? Hmmm.... I'll let you know if I ever come across anything in my studies. I'd guess someone somewhere figured out the equations but I have no doubt that it's wicked complex.
 
The theoretical speed of a planing boat with unlimited wind and waves and perfect crew work is only limited by the equipment, you can go faster untill it breaks =P
 
The theoretical speed of a planing boat with unlimited wind and waves and perfect crew work is only limited by the equipment, you can go faster untill it breaks =P

Does that mean that the boat can go so fast as to leave the bow wave all togeather?
 
Does that mean that the boat can go so fast as to leave the bow wave all togeather?

Sure. When Michael Blackburn crossed the Bass Strait in a Laser his max speed was 19.7 knots. And he averaged 8.6 knots for 117 miles. He esitimated that he caught a wave every 10 seconds for 13.5 hours.
 
although it sounds silly, i have to ask:
why dont they make boats with a really big sail and really big racks for trapezing off ?
 
Isn't is impossible to leave the bow wave all together without being a WIG? Or maybe hydrofoils maybe, but anyways... I think of it as you climb over the bow wave, it simply moves farther and farther aft. You can't stop creating one...
 
Isn't is impossible to leave the bow wave all together without being a WIG? Or maybe hydrofoils maybe, but anyways... I think of it as you climb over the bow wave, it simply moves farther and farther aft. You can't stop creating one...

Thats exactley what I was thinking.
 
Isn't is impossible to leave the bow wave all together without being a WIG? Or maybe hydrofoils maybe, but anyways... I think of it as you climb over the bow wave, it simply moves farther and farther aft. You can't stop creating one...
Well you'll always make some kind of wave but it won't really be the same as the bow wave made by a boat in displacement mode.
 
bigger bigger bigger :D
i know im exaggerating a bit, but what stops you from having a sail the size of a yacht and some racks the size of well, something big...

kp
 
There is a dude who hit 48. something knots on a stinkin' windsurfer. He's a big guy with a lot of sail on a windy day.
 
it dawned upon me yesterday why its not possible :
if the racks are to big, as soon as you heel over they would dig in and slow you down, thats why someone stuck a hull at either end and called it a trimaran :p
- thats my theory anyway...
 
Has anyone heard about this?
San Francisco Speed Sailing -- www.sanfranciscospeed.com/index.htm

I thought it might be kind of fun to be the one shmoe in a Laser setting the benchmark speed for everyone to come (even if it's only 4.8 knots or so). It could be the only time I'd have a shot at a record other than something like number of capsizes in a race or something. . . ;-)
 
bigger bigger bigger :D
i know im exaggerating a bit, but what stops you from having a sail the size of a yacht and some racks the size of well, something big...

kp

You're not exaggerating. In the late 1980s, the 18 Foot Skiffs were unrestricted and had 22 foot wide wings the crew could trap off. Masts were up to 43 feet tall (actually one had a 45 footer) on a carbon fibre hull about 78kg from memory.

When they had the "worlds" in a location known for steady winds, they extended the racks/wings until Brownie got Bradmill's wings measuring 30 feet (9m) from side to side. Then of course they were trapping off that. The main and spinnaker were similar in size to a boat around 35-40 feet long. He won.

As Phosphor says, they became impractical. On more resrticted waters, the big wings made the boat so hard to sial and slow to tack they didn't work. The boats were so expensive the class almost died. Rule changes were brought in, restricting rig size, wing width and hull designs and a smaller, skinnier boat started to dominate. As Geoff says, much of this is in High Performance Sailing but that leaves out the fact that the Bethwaite boats were the only permitted hulls that fitted the restrictions that were introduced.

The moral is that the idea that making boats faster will make them more popular is rubbish.....it's been proven many times before but people just don't like to look at the facts. That's why the simple, cheap and humble little Laser is such a huge class.
 
because it is affordable and you dont need to be superhuman to sail one ?
- i can see why that makes it more popular :D
 
i know its a bit of an offshoot, but (in order to increase max speed) why isnt the boom built so it extends off the back of the boat, increasing sail size ?
 
Balance of the rig versus the underwater shape I'd imagine. If you put more area back there without moving the mast forward will move the Center of Effort aft more.
 
i know its a bit of an offshoot, but (in order to increase max speed) why isnt the boom built so it extends off the back of the boat, increasing sail size ?

Making a sail larger but lower in aspect ratio (which is the ratio between the width and the height) is very innefficient.

I'm no aerodynamacist but I've been through books on the physics of sailing etc and corresponded with a sailor who is also an aerodynamacist for Boeing. There's lots of different ways of explaining how sails work (see the NASA site if you're interested) but basically they work by changing the direction of the wind flowing over them, from front to back.

Now the thing is, by the time the wind has got to the back part of the sail - the part you'd extend by making the boom longer - the sail has already turned the wind. The wind is already flowing in line with the sail. Adding more sail out the back isn't going to turn the wind any more so it won't create any more power.

The exception is square running when the wind isn't flowing over the sail, and then low-aspect sails are actually efficient. But then you also have to drag them upwind.
 
but then couldnt you increase the height of the mast so that the aspect ration stays the same ?
 
i can see this going around in circles :D
another question : on which point of sail are you most likely to reach maximum boat speed ???
 
right, cos someone (not on here) said that a broad reach was fastest cos then you had the "wing effect" of the sail as well as the wind pushing you along.
does it vary for different boats ?
 
I've been taught (and learned from experience) that the beam reach is the fastest. It has a 50:50 mix of lift from the sail and pushing from the wind. Broad reaches are too close to a run, the slowest point of sail.
 
i know its a bit of an offshoot, but (in order to increase max speed) why isnt the boom built so it extends off the back of the boat, increasing sail size ?

In addition to the other reason(s) pointed out already, it would be more effective to move the sail area up as opposed to back (More wind velocity the higher you go) and moving towards a higher aspect ratio. Next would be to work on the most efficient sail plan shape, which the America's Cup boats are showing is not a pointy head, but a fat head shape.
 

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