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Author Topic:   Crouch's Hull Factor

davidestel posted 12-26-2005 02:17 PM ET (US)
[How is the HULL FACTOR calculated as used in Crouch's formula for predicted planing speed of monohulled boats?]
Thanks Davide
Peter posted 12-26-2005 03:25 PM ET (US)
Crouch's forumula for predicted speed is
Predicted Speed = (HP/Weight)^(0.5)* Hull Factor
In the case of the classic Outrage, the Hull Factor that typically works well, at least for all of my Whalers, is 180.
If you know the Actual Speed, HP and Weight, you can solve for the Hull Factor. See below
Hull Factor = Actual Speed /((HP/Weight)^(0.5))
jimh posted 12-26-2005 06:17 PM ET (US)
Crouch's equation for predicting the performance of monhulls is presented in the REFERENCE section. See:
http://continuouswave.com/whaler/reference/prop2.html#crouch
If you measure the other parameters involved, you can deduce the hull factor constant C.
elaelap posted 12-27-2005 11:13 AM ET (US)
God knows I'm no mathetician, but let's look logically at Crouch's formula: Seems to me that the critical element is the so-called 'hull factor', and this is where I sense a lack of logic in this entire exercise. The 'shape' of a boat's hull is constantly variable...it's affected by each second's chaotic interaction of many forces--each small wave, the current, the depth of the water, the wind (speed and direction), the temperature of the water, the temperature of the air, the density of the air and water, whether the water is salt or fresh, the attitude (angle) of the hull to the horizon and its attitude to each and every small wave and larger swell, weight distribution aboard, wind resistance/topside windage which increases geometrically as speed increases, bottom paint or lack thereof, motor mounting height and drag of the motor's bottom end, prop size, pitch, material, smoothness, drag or lack thereof caused by everything from transducers to barnacles and other sea critters to wax on the hull, and probably dozens of other factors I haven't considered.
Ah mankind; ah science--always attempting to quantify everything in sight. I suspect that the only way Crouch's formula has any reliability at all is when the formula is calculated backwards; i.e., when each boat's owner first tests his/her craft's speed in certain conditions and then does the math in reverse to determine the exact 'hull factor' for those specific conditions. I can't see the thing working in the other direction, other than in its broadest, most general terms. I'd bet an experienced motorboat operator/racer like CW member Sal DiMercurio could estimate the top speed of a hull/outboard motor combination with at least as much accuracy as could be provided by this formula, probably much more.
This is obviously just my very humble, too-many-cups-of-coffee, too-early-in-the-morning uneducated opinion...hope it generates some interesting responses 'couched' in language even I can understand ;-)
Tony
jimh posted 12-27-2005 11:33 AM ET (US)
Ah, mankind; ah lawyers. Always attempting to introduce "reasonable doubt" into everything.
Estimates of speed such as given by experienced boaters are formed from their experience. If one attempts to quantify those predictions, I am certain that the result would be very similar to Crouch's formula. I am a believer in the general rule that you can't really claim to be able to describe something unless you can express it in some mathematical terms. This is what Crouch has done.
My experience is that Crouch's formula is quite accurate in predicting performance. As you say, the greatest determinant is the value chosen for C, the hull factor or constant in the equation. Fortunately, we have quite a bit of test data for Boston Whaler hulls which can provide guidance.
elaelap posted 12-27-2005 11:45 AM ET (US)
"I am a believer in the general rule that you can't really claim to be able to describe something unless you can express it in some mathematical terms." --jimh
As many here may have guessed, I'm philosophically somewhat a materialist. That is, I usually find myself strongly endorsing an assertion such as jimh's above over a 'spiritual' or 'religious' world view. On the other hand, there are times when a materialistic/scientific world view strikes me as just as absurd as some of the beautiful, fantastic, irrational visions of the mystics. For me, chaos theory has come to the rescue; its hands-thrown-into-the-air acknowledgement of the oftentimes impossibility of understanding many physical occurences seems to harmonize man's heroic, Promethian stiving for total rational comprehension of his universe with the ineffable, never-to-be-resolved mysteries of existence. The 'hull factor' critical to Crouch's formula seems to me to be so absolutely dependent upon a chaotic jumble of disparate events so as to make it an easy prey to chaos theory's axe.
Tony
davidestel posted 12-27-2005 12:12 PM ET (US)
Thanks Jim, I was only trying to calculate the possible performance of my boat. I have just finished to repower it with an Evinrude 225 VRO, and I want to know the possible max speed .

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Author	Topic: Crouch's Hull Factor
davidestel	posted 12-26-2005 02:17 PM ET (US) [How is the HULL FACTOR calculated as used in Crouch's formula for predicted planing speed of monohulled boats?] Thanks Davide
Peter	posted 12-26-2005 03:25 PM ET (US) Crouch's forumula for predicted speed is Predicted Speed = (HP/Weight)^(0.5)* Hull Factor In the case of the classic Outrage, the Hull Factor that typically works well, at least for all of my Whalers, is 180. If you know the Actual Speed, HP and Weight, you can solve for the Hull Factor. See below Hull Factor = Actual Speed /((HP/Weight)^(0.5))
jimh	posted 12-26-2005 06:17 PM ET (US) Crouch's equation for predicting the performance of monhulls is presented in the REFERENCE section. See: http://continuouswave.com/whaler/reference/prop2.html#crouch If you measure the other parameters involved, you can deduce the hull factor constant C.
elaelap	posted 12-27-2005 11:13 AM ET (US) God knows I'm no mathetician, but let's look logically at Crouch's formula: Seems to me that the critical element is the so-called 'hull factor', and this is where I sense a lack of logic in this entire exercise. The 'shape' of a boat's hull is constantly variable...it's affected by each second's chaotic interaction of many forces--each small wave, the current, the depth of the water, the wind (speed and direction), the temperature of the water, the temperature of the air, the density of the air and water, whether the water is salt or fresh, the attitude (angle) of the hull to the horizon and its attitude to each and every small wave and larger swell, weight distribution aboard, wind resistance/topside windage which increases geometrically as speed increases, bottom paint or lack thereof, motor mounting height and drag of the motor's bottom end, prop size, pitch, material, smoothness, drag or lack thereof caused by everything from transducers to barnacles and other sea critters to wax on the hull, and probably dozens of other factors I haven't considered. Ah mankind; ah science--always attempting to quantify everything in sight. I suspect that the only way Crouch's formula has any reliability at all is when the formula is calculated backwards; i.e., when each boat's owner first tests his/her craft's speed in certain conditions and then does the math in reverse to determine the exact 'hull factor' for those specific conditions. I can't see the thing working in the other direction, other than in its broadest, most general terms. I'd bet an experienced motorboat operator/racer like CW member Sal DiMercurio could estimate the top speed of a hull/outboard motor combination with at least as much accuracy as could be provided by this formula, probably much more. This is obviously just my very humble, too-many-cups-of-coffee, too-early-in-the-morning uneducated opinion...hope it generates some interesting responses 'couched' in language even I can understand ;-) Tony
jimh	posted 12-27-2005 11:33 AM ET (US) Ah, mankind; ah lawyers. Always attempting to introduce "reasonable doubt" into everything. Estimates of speed such as given by experienced boaters are formed from their experience. If one attempts to quantify those predictions, I am certain that the result would be very similar to Crouch's formula. I am a believer in the general rule that you can't really claim to be able to describe something unless you can express it in some mathematical terms. This is what Crouch has done. My experience is that Crouch's formula is quite accurate in predicting performance. As you say, the greatest determinant is the value chosen for C, the hull factor or constant in the equation. Fortunately, we have quite a bit of test data for Boston Whaler hulls which can provide guidance.
elaelap	posted 12-27-2005 11:45 AM ET (US) "I am a believer in the general rule that you can't really claim to be able to describe something unless you can express it in some mathematical terms." --jimh As many here may have guessed, I'm philosophically somewhat a materialist. That is, I usually find myself strongly endorsing an assertion such as jimh's above over a 'spiritual' or 'religious' world view. On the other hand, there are times when a materialistic/scientific world view strikes me as just as absurd as some of the beautiful, fantastic, irrational visions of the mystics. For me, chaos theory has come to the rescue; its hands-thrown-into-the-air acknowledgement of the oftentimes impossibility of understanding many physical occurences seems to harmonize man's heroic, Promethian stiving for total rational comprehension of his universe with the ineffable, never-to-be-resolved mysteries of existence. The 'hull factor' critical to Crouch's formula seems to me to be so absolutely dependent upon a chaotic jumble of disparate events so as to make it an easy prey to chaos theory's axe. Tony
davidestel	posted 12-27-2005 12:12 PM ET (US) Thanks Jim, I was only trying to calculate the possible performance of my boat. I have just finished to repower it with an Evinrude 225 VRO, and I want to know the possible max speed .