The bathtub test will provide you with a different measure of pounds of thrust. On a boat, the pounds of thrust can be measured by a scale such as you posit. But the bathtub pounds of thrust is a measure of the weight of the water that spills out per minute. The easiest way is to measure the amount of water in the tub by placing gallon marks on the side with magic marker, run the motor for 20 minutes, see how much water was lost. The formula is gallons * 8 / 20 = bath_pounds_thrust.
From: John Green
Sent: Sunday, August 28, 2011 10:50 AM
Subject: [Electric Boats] Testing Prop Efficiency
Just a question.....or two.
If I wanted to test, say, one prop compared to another, without
involving a boat, but maybe in a home bathtub, could I just take the
thrust alone as a measure?
I am thinking of using a motor suspended over the tub, and set it up so
that everything is the same except the prop, (or duct around/behind it).
The wife will be away at the time.
Do I need to measure the amps. or just use, say, solely a horizontally
set up weighing scale to measure the pull/push?
I realise that there will be added drag once fitted to a boat.
Thanks
John
PS note to Myles - I hope that you are as proud of the brevity and
single topic of this post as I am!
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