Going upwind, how close to theoretical hull speed should you be able to get? Are theoretical hull speeds for nonexistent perfect world conditions and if so what about the real world?
Hull speed,fact or fiction?
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My Ericson 35 can come close to reaching hull speed when sailing about 32° apparent in about 15kts of wind. Boats can usually reach their hull speeds given the proper conditions. I have exceeded hull speeds on most points of sail. It is possible to ride up the trailing edge of your bow wake. You will be putting extreme forces on the boat and rig but it is possible. Some boats can ride over their bow wave and significantly exceed hull speed. This is called planing.
Hull speed is not an exact point
so much as a guide. That is why it is expressed as theoretical. It is also not a barrier so exceeding it is very possile, it's just that, past that point the amount of speed through the water requires a tremendous increase in propulsion for a very small gain in speed.
so much as a guide. That is why it is expressed as theoretical. It is also not a barrier so exceeding it is very possile, it's just that, past that point the amount of speed through the water requires a tremendous increase in propulsion for a very small gain in speed.
To amplify on the point Ross makes so well, if you had two boat of totally different designs (light and narrow beamed fin keeler vs a heavy wide beamed full keeler) , but the exact same waterline length, both would have the same theoretical hull speed. Their "real" hull speeds would be quite different, however. The constant 1.34 in the formula, which only considers LWL, needs to be adjusted up or down depending on your boat's design.
Or sailing a multihull.
Multihulls still have a "hull speed", it is just less distinct. On heavy multihulls, they are like monohulls and stop at hull speed.
Also, some multihulls (Stilettos) have very light flat hulls and can semi-plane, giving them a very odd set of rules indeed!
Hull speed is simply a guide and rule-of-thumb.
As Ross pointed out, different rules.
Multihulls still have a "hull speed", it is just less distinct. On heavy multihulls, they are like monohulls and stop at hull speed.
Also, some multihulls (Stilettos) have very light flat hulls and can semi-plane, giving them a very odd set of rules indeed!
Hull speed is simply a guide and rule-of-thumb.
Hull speed, fact or fiction?
The question specifically states "going upwind." If you look at polar diagrams for different sailboats you will see that the higher a boat points into the wind, the slower the boat speed will be. Therefore you will not reach maximum hull speed with a displacement hull while going upwind.
The question specifically states "going upwind." If you look at polar diagrams for different sailboats you will see that the higher a boat points into the wind, the slower the boat speed will be. Therefore you will not reach maximum hull speed with a displacement hull while going upwind.
In addition, I've read (forget the source) that effects of modern underbodies suggest that the 1.34 ratio could arguably be adjusted up to the 1.5 range.
Yes, LWL is still primary.
Yes, LWL is still primary.
Theoretical hull speed... the classic formula, 1.34x the square root of a displacement hull's waterline... is not applicable on anything other than a full displacement hull. I don't think the Hunter 260 qualifies.
Polars are a far better indicator of a boat's performance..... because they are based on actual experience... not theory.
Polars are a far better indicator of a boat's performance..... because they are based on actual experience... not theory.
Hull speed formula...
The 1.34 factor is an "ideal", and may not apply to a boat going upwind as that ususally isn't the fastest point of sail for most boats. I've tended to use the square root factor "straight" (1.0 times the sq. rt.) to get a realistic calculation for comparison purposes. So many factors come into play as Ross pointed out.
Case in point: a couple of weekends ago I was hitting 7.0 SOG with just my 110, partially rolled up, at 120 degrees of apparent wind in winds in the low twenties with gusts into the 30's. With a 28.25 foot LWL, the formula gives a 7.12 rating.
The 1.34 factor is an "ideal", and may not apply to a boat going upwind as that ususally isn't the fastest point of sail for most boats. I've tended to use the square root factor "straight" (1.0 times the sq. rt.) to get a realistic calculation for comparison purposes. So many factors come into play as Ross pointed out.
Case in point: a couple of weekends ago I was hitting 7.0 SOG with just my 110, partially rolled up, at 120 degrees of apparent wind in winds in the low twenties with gusts into the 30's. With a 28.25 foot LWL, the formula gives a 7.12 rating.
Not so much a function of point of sail rather one of drag
As a boat points higher into the wind the apparent wind speed increases so the speed of the boat will continue to increase until it is impeded by drag or in the case of a displacement hull the limitation of having to lift the boat up onto the bow wave. i have hit 12.8 knots in my 35 going dead downwind so you can certainly exceed hull speed.
Take an ice boat for example, they can hit speed up to 100 miles per hour going upwind. Another big issue for the displacement hulls is heeling. As windspeed increases the heeling force vector becomes a lot larger than the force vector providing forward motion. Another variable is the LWL changes as the boat heels due to overhangs on the hull. As the boat heels more length of the hull is in the water so the speed will increase a little.
As a boat points higher into the wind the apparent wind speed increases so the speed of the boat will continue to increase until it is impeded by drag or in the case of a displacement hull the limitation of having to lift the boat up onto the bow wave. i have hit 12.8 knots in my 35 going dead downwind so you can certainly exceed hull speed.
Take an ice boat for example, they can hit speed up to 100 miles per hour going upwind. Another big issue for the displacement hulls is heeling. As windspeed increases the heeling force vector becomes a lot larger than the force vector providing forward motion. Another variable is the LWL changes as the boat heels due to overhangs on the hull. As the boat heels more length of the hull is in the water so the speed will increase a little.
Hull speed:
The constants 1.34 for knots and feet, 1.25 for meters per second and meters and 2.43 for knots and meters relate to a specific construct. That is the purely theoretical two dimensional condition where a wave created by a point moving through the water at a specific speed will have a length equal to a "load water line". Thus: In deep water the speed of a water wave is:
.
Where the boat is traveling at the same speed as the wave it creates by substitution we get:
. This is hull speed. If we quit here there are no approximations.
Replacing the constants for gravity and pi with decimal approximations and moving them out of the root we get:
when using feet and knots and
when using meters and meters per second.
The theory is the same for any type of boat that operates near a fluid interface. The theory provides a motivation for why speed to length ratios (and Froude numbers) are useful scaling devices. The theory is not a speed limit on boats. It was never intended as such. I know I'm not answering the original question. I'm not trying to be difficult but I think the question as posed is unanswerable.
--Tom.
The constants 1.34 for knots and feet, 1.25 for meters per second and meters and 2.43 for knots and meters relate to a specific construct. That is the purely theoretical two dimensional condition where a wave created by a point moving through the water at a specific speed will have a length equal to a "load water line". Thus: In deep water the speed of a water wave is:
Where the boat is traveling at the same speed as the wave it creates by substitution we get:
Replacing the constants for gravity and pi with decimal approximations and moving them out of the root we get:
The theory is the same for any type of boat that operates near a fluid interface. The theory provides a motivation for why speed to length ratios (and Froude numbers) are useful scaling devices. The theory is not a speed limit on boats. It was never intended as such. I know I'm not answering the original question. I'm not trying to be difficult but I think the question as posed is unanswerable.
--Tom.
After being in and around a wide variety of boats all my life, I've concluded "hull-speed" is just a theory. I had a 14ft runabout that had a VERY stiff "wall" on transition to plane - but it was at about 10 knots (hull speed would be 4.6 knots). The 18ft runabout had a much gentler transition to plane - around 12-15 knots. I've been in 9ft dinghies doing 5 knots with an electric motor (NOT planing). And of course I've exceeded hullspeed in supposedly displacement sailboats. (some say you CANNOT plane without a "step" - a sharp angle between bottom and transom).
As for exceeding hullspeed UPWIND, there are many dinghies that can do that, but keelboats... I donno...
druid
As for exceeding hullspeed UPWIND, there are many dinghies that can do that, but keelboats... I donno...
druid
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