Crownline 242
Member
- Joined
- Jun 12, 2006
- RO Number
- 21744
- Messages
- 391
Just to add to the confusion: to divide by the square root of X (using FORTAN) = X**-1/2
Hull Speed Question
- Thread starter seabug
- Start date
Just to add to the confusion: to divide by the square root of X (using FORTAN) = X**-1/2
- Joined
- Jan 1, 2000
- RO Number
- 824
- Messages
- 2,407
Didn't know you could do those calcs on an abacus...
I don't have three degrees, well I don't have any degrees. But the question was where does the 1.34 constant come from???
In a nut shell!!! It basiclly has to do with wave motion and velocity as given by the equation..
V=((g*lwl)/2pi)^1/2 where
V= velocity
g= acceleration due to gravity or 32 ft/sec^2
lwl= Lenght of the water line.
Thus V=((32*LWL)/2*3.14))^1/2
Or V=(32/2*3.14)^1/2*lwl^1/2
Solving we get (5.09^1/2)*lwl^1/2
reducing we get 2.25*lwl^1/2 or a constant of 2.25
But wait a minute, we want it in knots and not Feet/sec so to convert ft/sec to knots we multiply by .59 resulting in.....
(2.25*.59)*lwl^1/2 or about 1.34*lwl^1/2 giving us a 1.34 constant in KNOTS
Hope that helps
Dave
In a nut shell!!! It basiclly has to do with wave motion and velocity as given by the equation..
V=((g*lwl)/2pi)^1/2 where
V= velocity
g= acceleration due to gravity or 32 ft/sec^2
lwl= Lenght of the water line.
Thus V=((32*LWL)/2*3.14))^1/2
Or V=(32/2*3.14)^1/2*lwl^1/2
Solving we get (5.09^1/2)*lwl^1/2
reducing we get 2.25*lwl^1/2 or a constant of 2.25
But wait a minute, we want it in knots and not Feet/sec so to convert ft/sec to knots we multiply by .59 resulting in.....
(2.25*.59)*lwl^1/2 or about 1.34*lwl^1/2 giving us a 1.34 constant in KNOTS
Hope that helps
Dave
- Joined
- Jan 1, 2000
- RO Number
- 3238
- Messages
- 4,831
"I can't wrap my head around the concept that one pretty precise hull speed formula applies to all hull configurations" -- Sandy
Well, you shouldn't worry then, because it doesn't apply to all hull configurations. It applies to full displacement hull types. And to make it even further confusing, "hull speed" is a contrived concept! Yes, there is a speed which cannot be ( normally ) exceeded by a full displacement type. And it is that theoretical speed that the formula references. All hull types can become a planing hull if you apply enough power ( picture skipping a rock ), so that is why you add the word "normally" to the above.
All hull typed might be considered variations/refinements of a full displacement type, and all hulls, at low speed operate in the "full displacement realm", so the formula is useful in all cases, though it's accuracy varies w/ hull type. In non-displacement hull types, once you apply enough power, the hull rises up out of the water, and exits the "full displacement realm". In a full displacement type, you just dig a deeper hole, until you pop up out of the water and skip like a rock ( but you never reallt become a planing hull... )
Mostly, the formula is useful only for racing sailors who win races in the courtroom vs on the water. For all others, is becomes a rule of thumb, accepting that it is going to heve some error, for determining the theoretical hull speed ( ie: for max range calculations ) Otherwise it is good food for conversation at the bar, but of no specific purpose.
Computational Fluid Dynamics, however... [
ropeller]
Well, you shouldn't worry then, because it doesn't apply to all hull configurations. It applies to full displacement hull types. And to make it even further confusing, "hull speed" is a contrived concept! Yes, there is a speed which cannot be ( normally ) exceeded by a full displacement type. And it is that theoretical speed that the formula references. All hull types can become a planing hull if you apply enough power ( picture skipping a rock ), so that is why you add the word "normally" to the above.
All hull typed might be considered variations/refinements of a full displacement type, and all hulls, at low speed operate in the "full displacement realm", so the formula is useful in all cases, though it's accuracy varies w/ hull type. In non-displacement hull types, once you apply enough power, the hull rises up out of the water, and exits the "full displacement realm". In a full displacement type, you just dig a deeper hole, until you pop up out of the water and skip like a rock ( but you never reallt become a planing hull... )
Mostly, the formula is useful only for racing sailors who win races in the courtroom vs on the water. For all others, is becomes a rule of thumb, accepting that it is going to heve some error, for determining the theoretical hull speed ( ie: for max range calculations ) Otherwise it is good food for conversation at the bar, but of no specific purpose.
Computational Fluid Dynamics, however... [
ropeller]Crownline 242
Member
- Joined
- Jun 12, 2006
- RO Number
- 21744
- Messages
- 391
Delete double post
- Joined
- Jan 1, 2000
- RO Number
- 824
- Messages
- 2,407
Bill, has one other use - comes up a lot in my line of work. Our doctrine holds that max safe towing speed is theoretical hull speed -10% safety factor. Id est, (1.34 x square root of LWL) - 10% safety factor. And it's an equation that comes up again and again and again in written tests, boards, and servicewide exams. But, again, this formula is theoretical. Know how to determine the actual safe towing speed of a displacement hull? When your main engine RPM's stay the same, but you suddenly gain a few knots, and that sailboat you're towing looks like it's planing on it's hull-side - you're exceeding it. Slow down.
As for it's relevance to different hull forms? I have an e-mail in to Archimedes, but he hasn't gotten back to me yet.
As for it's relevance to different hull forms? I have an e-mail in to Archimedes, but he hasn't gotten back to me yet.
Thank you Radio Bill. That makes more sense. And checking back, I just noted you did indeed make it very clear in the previous hull speed thread that the formula is truly intended for true "displacement hulls". It seems to be fairly commonly misappplied.
I'm just glad Dave fount a way to get that 2Pi into the discussion. Any technical discussion about real physical situations not containing 2Pi is always cause for alarm.
In my experience people usually overestimate their displacement mode speed. The formula is a good guide line but for economy you will want to be below that max speed. If you are making much of a wake you are above displacement speed. Round hull boats such as sailboats and true displacement trawlers will form a bow wave and a stern wave with a trough in the middle at max displacement speed. They cant go faster than that without climbing on top of the bow wave but that isn’t possible with their shape. Powerboats usually have some form flatter sections providing lift if enough power is available. The waves at the bow and especially stern may not be as obvious but the bow lifting is. The stern sinks into the trough as the bow tries to climb the wave.
All that bow lifting and wake making is consuming a lot of power and fuel
In my experience people usually overestimate their displacement mode speed. The formula is a good guide line but for economy you will want to be below that max speed. If you are making much of a wake you are above displacement speed. Round hull boats such as sailboats and true displacement trawlers will form a bow wave and a stern wave with a trough in the middle at max displacement speed. They cant go faster than that without climbing on top of the bow wave but that isn’t possible with their shape. Powerboats usually have some form flatter sections providing lift if enough power is available. The waves at the bow and especially stern may not be as obvious but the bow lifting is. The stern sinks into the trough as the bow tries to climb the wave.
All that bow lifting and wake making is consuming a lot of power and fuel