top speed estimator
04-02-2002, 08:36 AM
#25
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Join Date: May 2001
Location: St Charles MO, Lake Ozark MO
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V drive ski boat
the formula is not even close for a friends comp ski boat.
Would you account for the difference with the hull factor or HP loss with the drive?
Would you account for the difference with the hull factor or HP loss with the drive?
04-02-2002, 09:31 AM
#26
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The hull factor will be cignificantly different with a ski boat. the hull factor is a function of the wetted area of the boat.
The overall drag is due to three main contributors. The wetted surface and displacement of the hull, the aerodynamic drag ( ~10%) and the outdrive drag. Theoretically speaking ( physics) each contributor follows a power requirement as a function of velocity cubed (see my avatar) but the effect of lift from the aero and the pitch angle change as speed increase, significantly reducing the wetted area and thus the hull contributor. this overall efficiency (quasi hull efficiency) overall follows a squared function ( at least for cats, still looking to confirm for v hulls).
With a ski boat, the aero lift is insignificant, and the hull contribution is more significant that a fully planed deed V and a cat. (it's pitch never really changes, and it has a very large wetted area)
yada yada, i'm rambling....... the "hull factor" (ie quasi hull efficiency) is much different that a conventional V or cat
my $.03
Tomcat should be around any minute. He has good insite. He and I have had numerous discussions on this.
Tomcat. I guess I conceed on the fact that Cats follow an empirical v squared function ( my rationale is stated above. This allows me to still be kinda right regarding the physics of each contributor
)
i would still like to have some before/after performance for deep V's, preferably with the same prop.
The overall drag is due to three main contributors. The wetted surface and displacement of the hull, the aerodynamic drag ( ~10%) and the outdrive drag. Theoretically speaking ( physics) each contributor follows a power requirement as a function of velocity cubed (see my avatar) but the effect of lift from the aero and the pitch angle change as speed increase, significantly reducing the wetted area and thus the hull contributor. this overall efficiency (quasi hull efficiency) overall follows a squared function ( at least for cats, still looking to confirm for v hulls).
With a ski boat, the aero lift is insignificant, and the hull contribution is more significant that a fully planed deed V and a cat. (it's pitch never really changes, and it has a very large wetted area)
yada yada, i'm rambling....... the "hull factor" (ie quasi hull efficiency) is much different that a conventional V or cat
my $.03
Tomcat should be around any minute. He has good insite. He and I have had numerous discussions on this.
Tomcat. I guess I conceed on the fact that Cats follow an empirical v squared function ( my rationale is stated above. This allows me to still be kinda right regarding the physics of each contributor
)i would still like to have some before/after performance for deep V's, preferably with the same prop.
04-02-2002, 10:42 AM
#28
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Rambunctious - Are you going to the April 27th OSO meeting? I'm driving from north of Toronto and I hope to see you there!
I honestly don't know why power seems to be proportional to velocity squared in boats. In my business (industrial ventilation) we talk about the fan laws:
CFM varies directly with fan speed
Pressure varies as the square of fan speed
HP varies as the cube of fan speed
These "laws" are used to quickly estimate the new pressure and HP required when you want to increase the CFM through the system. You can see the cubed relationship with (fan) speed.
But these rules only apply where the pressure losses of each element in the system (elbows, contractions, expansions etc.) vary as the square of velocity of air through that element. That is the norm, but if you have an element, like a fabric filter to collect airborne contaminants, in which pressure loss varies directly with volume, you can't use these rules for a quick estimate; you have to recalculate system pressure losses.
This isn't a great analogy but is it possible that a planing hull has an element of drag which varies directly with speed? I guess that's what you're saying happens to the hull drag as wetted surface area decreases with speed.
The speed calculator on BAM's site is based on Mercury (squared)equations which seem to fit the data fairly well. I took these hull curve equations and added the ability to plot engine horsepower curves on top of them. This allows you to see how to prop the boat for your actual engine. The program has a prop optimizer button that selects the right pitch to make the engine curve cross the hull curve at maximum HP. See attached graph.
I honestly don't know why power seems to be proportional to velocity squared in boats. In my business (industrial ventilation) we talk about the fan laws:
CFM varies directly with fan speed
Pressure varies as the square of fan speed
HP varies as the cube of fan speed
These "laws" are used to quickly estimate the new pressure and HP required when you want to increase the CFM through the system. You can see the cubed relationship with (fan) speed.
But these rules only apply where the pressure losses of each element in the system (elbows, contractions, expansions etc.) vary as the square of velocity of air through that element. That is the norm, but if you have an element, like a fabric filter to collect airborne contaminants, in which pressure loss varies directly with volume, you can't use these rules for a quick estimate; you have to recalculate system pressure losses.
This isn't a great analogy but is it possible that a planing hull has an element of drag which varies directly with speed? I guess that's what you're saying happens to the hull drag as wetted surface area decreases with speed.
The speed calculator on BAM's site is based on Mercury (squared)equations which seem to fit the data fairly well. I took these hull curve equations and added the ability to plot engine horsepower curves on top of them. This allows you to see how to prop the boat for your actual engine. The program has a prop optimizer button that selects the right pitch to make the engine curve cross the hull curve at maximum HP. See attached graph.
04-02-2002, 11:11 AM
#29
'Squared or cubed', 'Horsepower or Torque' Here we go again!
Tomcat and Rambunctious your threads and comments are great as usual.
Tom did I read that you have that book on tunnels? Are there any formulas in there into which we can plug in our boats specific numbers- tunnel size, shape, etc. and get a ?tunnel number? And then can we see what we need to do to our hulls to make them better. Could this book predict a perfect 35' cat. I thought the bool was a bit pricy.
Tomcat and Rambunctious your threads and comments are great as usual.
Tom did I read that you have that book on tunnels? Are there any formulas in there into which we can plug in our boats specific numbers- tunnel size, shape, etc. and get a ?tunnel number? And then can we see what we need to do to our hulls to make them better. Could this book predict a perfect 35' cat. I thought the bool was a bit pricy.
04-02-2002, 02:14 PM
#30
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tomcat, unfortunately i can't make the party because
We will be in VEGAS!!!
our boat club here at work will hold our 2nd annual poker run fund raiser this August . 25 boats our first year!! shooting for 50 this summer. Maybe you can tow the boat down. or come and watch the GLSC series race in Grand Haven labor day with some of us.
We will be in VEGAS!!!
our boat club here at work will hold our 2nd annual poker run fund raiser this August . 25 boats our first year!! shooting for 50 this summer. Maybe you can tow the boat down. or come and watch the GLSC series race in Grand Haven labor day with some of us.
