Theoretical Maximum Boat Speed?

KdaP

New Member
#21
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...
 
#24
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. . . ;-)
 
#25
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.
 
#27
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 ?
 
#28
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.
 
#29
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.
 
#32
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 ???
 
#34
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 ?
 
#35
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.
 
#36
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.
 

Attachments

#38
Did anyone see the on-line, real time ISAF laser worlds at www.traxu.com ?? All the boats were fitted with GPS transmitters so we could watch their heading, speed, tracks etc. They were regularly doing over 6knots upwind and about 9-10 downwind. I don't know what the wind speed was. Interesting to see they mostly went pretty well straight downwind and not much curving. Maybe they were constantly planing or not many waves ?
Also interesting to see a few stuff ups - CHN woman in radials in second place at top mark then suddenly went haywire on the bear away - dropped the mainsheet ?? tangle ? Even the experts eh ?
 

Wavedancer

Upside down?
Staff member
#40
They don't seem to archive the older races, but if you're quick you can watch the last (Finn) race here ....http://tracking.traxu.com/raceplayer/player.aspx
As of this (Thu) morning, you can still see the two Laser races:
http://www.traxu.com/?s=isaftracking

The interface works well and the races are quite entertaining with many lead changes. Towards the end of the Radial race you can 'see' the last place boat capsize close to the finish (my interpretation).

As has been pointed out, it's also interesting to see the different strategies.
 
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