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: Measure Force and Convert to Speed
: Measure Force and Convert to Speed -- g.mod	Post Reply	Top of thread	Engineering Forum

Posted by: g.mod ® 03/06/2009, 17:18:24 Author Profile eMail author Edit

Thanks for all of the feedback and suggestions! Much thought and investigation required. Could someone suggest some entry level reading to further my knowledge.

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: Measure Force and Convert to Speed
: Measure Force and Convert to Speed -- g.mod	Post Reply	Top of thread	Engineering Forum

Posted by: jboggs ® 03/05/2009, 12:59:41 Author Profile eMail author Edit

OK, I'll bite. But I don't think I can do you much good. I'm not really clear on what you are trying to do here, but my gut tells me that you want to anchor a boat to a fixed object, start the motor and run it up to high RPM, measure the force generated, and then somehow extrapolate that information into some estimated speed over the water that the boat might achieve. There are many ways to measure the force. A load cell is one way. A simple spring loaded force gauge is another. If you can come up with an estimate of how much force you might expect to see, that will guide you as to your method of measurement. There are at least two theoretical problems with your approach: (1) I would think that the force generated by a propeller would vary significantly based on its actual speed through the water. In other words I would not expect a stationary propeller to provide the same force as a moving propeller. (2) The speed of a boat in the water is a result of MANY factors, and you are only measuring one. Other factors include drag (both water and air), loading, position of center of gravity with respect to center of bouyancy, and many more. If you can somehow estimate those other factors so that the only variable is propeller thrust, then you might be able to estimate the final top speed of the boat based on the measured thrust force. I would be curious to know how you proceed from here. Good luck!

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: : Measure Force and Convert to Speed
: : Measure Force and Convert to Speed -- jboggs	Post Reply	Top of thread	Engineering Forum

Posted by: rwolfejr ® 03/05/2009, 16:45:58 Author Profile eMail author Edit

Jboggs, I was guessing that was what he might have had in mind also. Sort of like a way to test a boat without a lake to run in. A Boat dyno... Another issue would be the less force the boat would apply to the post or dynamometer... the faster it would actually go. The pitch of the prop is a key factor in determining the speed the boat will travel. If the prop is flat it will pull hard. If it's layed out more it will go faster with less pull. Like changing the gear ratio in a rearend of a car. The amount of trim... which direction the winds coming from...hull design... how it planes out would all be a factor as well. So what if this is the quest? Say you develope knowns. You have xxx hp boat that gives xxxx lbs. pull and equipped with xx degree prop and have had it to the lake on a dead calm day and it went xx mph. Take the same boat and switch to a ?? degree prop and then measure pull and speed. do this with three or four pitches. At some point the pitch would be too shallow and the boat wouldn't plane out due to lack of pull. Maybe add the weight of the boat to the mix as well so you have a rough idea what your lifting for developing a correlation for differnet size boats. Yes maybe not nearly the same churning in a pond tied to a post but maybe enough information to get close to desired characteristics? I'm thinking you could eventually develop a close estimate of minimum pull required to make plane for a given weight and hull configuration. Come to think of it the ultimate prop would be one that could change it's pitch. Start flatter then kick back as you gain speed. Maybe there is such a beast already? But how could you do it? Like having a four speed prop!!

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: : : Measure Force and Convert to Speed
: : : Measure Force and Convert to Speed -- rwolfejr	Post Reply	Top of thread	Engineering Forum

Posted by: jboggs ® 03/05/2009, 17:16:19 Author Profile eMail author Edit

"Come to think of it the ultimate prop would be one that could change it's pitch." Airplanes have had them for 70 years.

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: : : : Measure Force and Convert to Speed
: : : : Measure Force and Convert to Speed -- jboggs	Post Reply	Top of thread	Engineering Forum

Posted by: Kelly Bramble ® 03/05/2009, 18:16:13 Author Profile eMail author Edit

"Airplanes have had them for 70 years." Hartzell variable pitch propeller - I think these where installed on DC-3's I read a technical editorial about Martec Engineering and thier variable pitch boat propeller. I don't think this technology is common in application.

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