Theory of Hull Speed
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06-12-2002, 02:06 PM
#13
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Here's an example of a power vs. speed graph with hull drag and engine HP both expressed in prop HP. Hull drag in this case includes all sources of drag mentioned by Jimboat and is based on observations and test results. This is a good method for predicting what your boat might do with more power when you do not alter the sources of drag in any way (like when you raise the drive). Predicting what a different hull design might do requires Jimboat's more detailed theoretical approach.
I compared two engines on the same hull, light and heavy. This is not exactly your specs but you can see how the more powerful blue engine is faster when the boat is light (yellow) but when the boat is heavy (green) both engine curves (red and blue) cross the green hull drag curve at the same point.
Better breathing normally aspirated engines generally need to rev higher to reach their HP potential. You have to decide what kind of load you normally carry and prop for that load.
I compared two engines on the same hull, light and heavy. This is not exactly your specs but you can see how the more powerful blue engine is faster when the boat is light (yellow) but when the boat is heavy (green) both engine curves (red and blue) cross the green hull drag curve at the same point.
Better breathing normally aspirated engines generally need to rev higher to reach their HP potential. You have to decide what kind of load you normally carry and prop for that load.
06-12-2002, 09:12 PM
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This is my theory for my boat. I have hit a wall. When I load the hull up with all the weight the drag on the hull is my limiting factor. If I could double my hp without increasing weight I would only get a modest increase in speed. A cat hull design would solve the problem.
If this is true there is no need to try some new props. I tried a couple of different props a few years ago and had very mixed results. Running empty I could gain alot of speed by increasing the pitch on the prop. Unfortunally my deep v runs too deep from the weight in the boat the drag is holding the boat back.
I look forward to trying some new props but i will wait until I damage the one on the boat before I buy a new one.
If my theory is right longer is better even for a planning hull.
Thanks for saving me money on trying new props.
If this is true there is no need to try some new props. I tried a couple of different props a few years ago and had very mixed results. Running empty I could gain alot of speed by increasing the pitch on the prop. Unfortunally my deep v runs too deep from the weight in the boat the drag is holding the boat back.
I look forward to trying some new props but i will wait until I damage the one on the boat before I buy a new one.
If my theory is right longer is better even for a planning hull.
Thanks for saving me money on trying new props.
