After posting my initial question about building a displacement hull boat for lake cruising I hadn't been back on in a while and missed Bill Hopen's response where he was kind enough to offer up a slide show of his custom made outboard. That really got me thinking-especially the fact that he's swinging an 18" prop. I started playing with some prop calculators online to see how large and slow I could go in order to get my desired speed (6-7 knots). For example, using the following calculator:
I was able to determine that with an 18x20 (18" diameter x 20" pitch) prop (allowing for 25% slippage) I could make 7 knots with the shaft turning at 500 RPM. Another calculator, shown below:
Calculates for HP based on desired speed, hull dimensions, etc... It told me I would need 5HP at the shaft to turn the 18x20 prop at 500 RPM/move a 1600lb displacement hull boat through the water at 6 knots. Does that sound right? It also told me I'd only need 3.5HP to travel at hull speed (about 5 knots). It's great to work all of this stuff out on paper, but I'd love to see some real world examples of the following:
- Prop diameter/pitch
- Max RPM
- Boat/payload weight
- Hull type
- Operating voltage
- Max runtime
The plan is to find the optimum combination of prop size, RPM, HP, etc...and build a custom outboard much like Bills.
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Posted by: Matthew Hall <yotatruck91@yahoo.com>
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