I'm asking this question so that I can get some sleep. It is really a question of little significance, but is one of those questions that pops into your head at 2 AM for no reason at all and you can't shake loose of and get back to sleep. The question is: When the displacement of a boat is calculated, does that include the inboard engine?? My CD27 displaces 7500 lbs. But different models have different engines, from 8-12 HP. I know the weight difference is not significant but does the 7500 pounds include any engine at all? I'm really sorry for this. But I need the sleep and I know I won't get any unless I find the answer to this stupid question.
Warren Kaplan
Setsail728@aol.com
Displacement Calculation
Moderator: Jim Walsh
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john doyle
Re: Displacement Calculation
I believe it includes everything installed in the boat & may also include fuel & water.Warren Kaplan wrote: I'm asking this question so that I can get some sleep. It is really a question of little significance, but is one of those questions that pops into your head at 2 AM for no reason at all and you can't shake loose of and get back to sleep. The question is: When the displacement of a boat is calculated, does that include the inboard engine?? My CD27 displaces 7500 lbs. But different models have different engines, from 8-12 HP. I know the weight difference is not significant but does the 7500 pounds include any engine at all? I'm really sorry for this. But I need the sleep and I know I won't get any unless I find the answer to this stupid question.
Warren Kaplan wrote: Warren Kaplan
redzeplin@yahoo.com
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Bruce Barber
Re: Displacement Calculation
Warren,
There's the displacement calculation, and then there's the weight calculation. For a floating object, the displacement (weight of water displaced, determined from a hull volume calculation) equals the weight of the object, or the sum of ALL its parts). For a hypothetical floating object, put in any engine you want; the weight changes but the equality still exists. The variable, of course, is the draft.
I suspect that boatbuilders generally publish the displacement (=weight) with the typical fixed equipment they provide, including standard inboards. However, for non-fixed equipment like outboards, sails, etc, or when several engine options are available, this would be a question worth asking.
The answer may not be available, though, because the basic question is what did the designer have in mind. Certainly he did a weight estimate (and a hull volume calc) so that he had some faith in the published draft being more or less accurate. He selected an engine weight to include, along with everything else, and may have even thrown in estimates of provisions and a margin for error. Unfortunately, too often, he's probably no longer with us to fill us in.
NOW let's see you get some sleep!
Bruce Barber
25D Nancy Dawson
bruce.barber@att.net
There's the displacement calculation, and then there's the weight calculation. For a floating object, the displacement (weight of water displaced, determined from a hull volume calculation) equals the weight of the object, or the sum of ALL its parts). For a hypothetical floating object, put in any engine you want; the weight changes but the equality still exists. The variable, of course, is the draft.
I suspect that boatbuilders generally publish the displacement (=weight) with the typical fixed equipment they provide, including standard inboards. However, for non-fixed equipment like outboards, sails, etc, or when several engine options are available, this would be a question worth asking.
The answer may not be available, though, because the basic question is what did the designer have in mind. Certainly he did a weight estimate (and a hull volume calc) so that he had some faith in the published draft being more or less accurate. He selected an engine weight to include, along with everything else, and may have even thrown in estimates of provisions and a margin for error. Unfortunately, too often, he's probably no longer with us to fill us in.
NOW let's see you get some sleep!
Bruce Barber
25D Nancy Dawson
bruce.barber@att.net
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Warren Kaplan
Re: Displacement Calculation
Bruce,Bruce Barber wrote: NOW let's see you get some sleep!
Thanks for that answer. I think it will do the trick and now I can stop counting sheep ( or sailboats). I just never thought about displacement vs. weight.
Thanks again,
Warren Kaplan
S/V SINE QUA NON... just put on the hard today!
CD27
Setsail728@aol.com
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Boyd
Re: Displacement Calculation
Next time she is hanging on the slings at haulout time ask the operator how much she weighs.... you will sleep better... until you try and figure out what the true weight is from the two sling scales.
What does it matter anyway?
Boyd
Tern30@aol.com
What does it matter anyway?
Boyd
Warren Kaplan wrote: I'm asking this question so that I can get some sleep. It is really a question of little significance, but is one of those questions that pops into your head at 2 AM for no reason at all and you can't shake loose of and get back to sleep. The question is: When the displacement of a boat is calculated, does that include the inboard engine?? My CD27 displaces 7500 lbs. But different models have different engines, from 8-12 HP. I know the weight difference is not significant but does the 7500 pounds include any engine at all? I'm really sorry for this. But I need the sleep and I know I won't get any unless I find the answer to this stupid question.
Warren Kaplan
Tern30@aol.com
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Ken Coit
Re: Displacement Calculation
Boyd,
Why isn't the "true weight" the sum of the two sling scale readings?
Ken
parfait@nc.rr.com
Why isn't the "true weight" the sum of the two sling scale readings?
Ken
Boyd wrote: Next time she is hanging on the slings at haulout time ask the operator how much she weighs.... you will sleep better... until you try and figure out what the true weight is from the two sling scales.
What does it matter anyway?
Boyd
Warren Kaplan wrote: I'm asking this question so that I can get some sleep. It is really a question of little significance, but is one of those questions that pops into your head at 2 AM for no reason at all and you can't shake loose of and get back to sleep. The question is: When the displacement of a boat is calculated, does that include the inboard engine?? My CD27 displaces 7500 lbs. But different models have different engines, from 8-12 HP. I know the weight difference is not significant but does the 7500 pounds include any engine at all? I'm really sorry for this. But I need the sleep and I know I won't get any unless I find the answer to this stupid question.
Warren Kaplan
parfait@nc.rr.com
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Boyd
Re: Displacement Calculation
Ken...
Its because the slings are suspended from the crane and the calculation becomes a hanging cable supporting a uniform load type problem instead of a simple beam. It would be simple and the sum of the weights if the boat was suspened by rigid hangers. In other words the scales on the crane I am familiar with measure the tension in the cables. The tension in the cable is not the same as the weight hanging from it. I think they are there mostly to keep the operator from overloading the crane cables.
I am assuming we are talking about a typical traveling crane found in boat yards. If the boat were suspended by cables all intersecting at one point at the top then the tension on the one cable such as the case of a construction crane would be correct. I havent done the math myself on the boatyard traveling crane to find out how close the crane tension readings are to the real weight. I can fax the example from my old statics text book for anyone who has trouble sleeping.
Boyd
Tern30@aol.com
Its because the slings are suspended from the crane and the calculation becomes a hanging cable supporting a uniform load type problem instead of a simple beam. It would be simple and the sum of the weights if the boat was suspened by rigid hangers. In other words the scales on the crane I am familiar with measure the tension in the cables. The tension in the cable is not the same as the weight hanging from it. I think they are there mostly to keep the operator from overloading the crane cables.
I am assuming we are talking about a typical traveling crane found in boat yards. If the boat were suspended by cables all intersecting at one point at the top then the tension on the one cable such as the case of a construction crane would be correct. I havent done the math myself on the boatyard traveling crane to find out how close the crane tension readings are to the real weight. I can fax the example from my old statics text book for anyone who has trouble sleeping.
Boyd
Ken Coit wrote: Boyd,
Why isn't the "true weight" the sum of the two sling scale readings?
Ken
Boyd wrote: Next time she is hanging on the slings at haulout time ask the operator how much she weighs.... you will sleep better... until you try and figure out what the true weight is from the two sling scales.
What does it matter anyway?
Boyd
Warren Kaplan wrote: I'm asking this question so that I can get some sleep. It is really a question of little significance, but is one of those questions that pops into your head at 2 AM for no reason at all and you can't shake loose of and get back to sleep. The question is: When the displacement of a boat is calculated, does that include the inboard engine?? My CD27 displaces 7500 lbs. But different models have different engines, from 8-12 HP. I know the weight difference is not significant but does the 7500 pounds include any engine at all? I'm really sorry for this. But I need the sleep and I know I won't get any unless I find the answer to this stupid question.
Warren Kaplan
Tern30@aol.com
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Mark Yashinsky
Re: Displacement Calculation
Is this a crane or a travelift??? Where are these "scales" in relation to the "load"??? As you said, this is for the operator to have an idea of the weight, so they know if they are within limits at various boom angles and extensions (if a crane). These "scales" should usually just give an approx. load (weight) of what it is lifting. They do not want the operator to have to stop and run numbers to figure if it safe or not. Also, this scale may not have the granularity nor accuracy you are looking for.
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Neil Gordon
Re: Displacement Calculation
>>I just never thought about displacement vs. weight.<<
An object will displace water equal to its weight or its volume, whichever is less. For boats other than submarines, it's best that the water displaced by weight is less than what would be displaced by volume.
Regards, Neil
s/v LIQUIDITY
Cape Dory 28 #167
cdory28@aol.com
An object will displace water equal to its weight or its volume, whichever is less. For boats other than submarines, it's best that the water displaced by weight is less than what would be displaced by volume.
Regards, Neil
s/v LIQUIDITY
Cape Dory 28 #167
cdory28@aol.com
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Ken Coit
Re: Displacement Calculation
Boyd,
I see. You are assuming that the straps don't hang straight down to the rails and that there is a catenary to worry about. I was assuming that for all practical purposes, the straps do hang straight down to the rails. You are right, but I wonder how much it matters.
So a little trig will get you the answer more accurately if there is a significant angle between the straps and the vertical. As was pointed out in another post, the strain gauge may not be all that accurate either. It is also possible that it only measures the vertical component, depending on whether it is mounted in a component of the strap or a bolt that only moves vertically.
Assuming the gauge measures the tension in the strap, what we are looking for is the vertical component of that tension. That ought to be equal to the tension in the strap X the cosine of the angle of the strap to the vertical. If the strap is vertical, they are identical, if it is horizontal, the vertical component is zero (nearly impossible). Add them up and you have the weight of the bateau if the gauges are accurate. If the angle is 30 degrees, then the vertical component is about 87% of the tension in the straps.
Gee this is fun. Am I right? What does your statics book say?
Ken
parfait@nc.rr.com
I see. You are assuming that the straps don't hang straight down to the rails and that there is a catenary to worry about. I was assuming that for all practical purposes, the straps do hang straight down to the rails. You are right, but I wonder how much it matters.
So a little trig will get you the answer more accurately if there is a significant angle between the straps and the vertical. As was pointed out in another post, the strain gauge may not be all that accurate either. It is also possible that it only measures the vertical component, depending on whether it is mounted in a component of the strap or a bolt that only moves vertically.
Assuming the gauge measures the tension in the strap, what we are looking for is the vertical component of that tension. That ought to be equal to the tension in the strap X the cosine of the angle of the strap to the vertical. If the strap is vertical, they are identical, if it is horizontal, the vertical component is zero (nearly impossible). Add them up and you have the weight of the bateau if the gauges are accurate. If the angle is 30 degrees, then the vertical component is about 87% of the tension in the straps.
Gee this is fun. Am I right? What does your statics book say?
Ken
Boyd wrote: Ken...
Its because the slings are suspended from the crane and the calculation becomes a hanging cable supporting a uniform load type problem instead of a simple beam. It would be simple and the sum of the weights if the boat was suspened by rigid hangers. In other words the scales on the crane I am familiar with measure the tension in the cables. The tension in the cable is not the same as the weight hanging from it. I think they are there mostly to keep the operator from overloading the crane cables.
I am assuming we are talking about a typical traveling crane found in boat yards. If the boat were suspended by cables all intersecting at one point at the top then the tension on the one cable such as the case of a construction crane would be correct. I havent done the math myself on the boatyard traveling crane to find out how close the crane tension readings are to the real weight. I can fax the example from my old statics text book for anyone who has trouble sleeping.
Boyd
Ken Coit wrote: Boyd,
Why isn't the "true weight" the sum of the two sling scale readings?
Ken
Boyd wrote: Next time she is hanging on the slings at haulout time ask the operator how much she weighs.... you will sleep better... until you try and figure out what the true weight is from the two sling scales.
What does it matter anyway?
Boyd
parfait@nc.rr.com