Stability Test
2015-Nov-13
Stability Decay
2017-Feb-26
A resistance and powering study determines how much power it takes to push your ship through calm water. The key here is calm water. It does not normally include any added powering for seakeeping. Or if there is a seakeeping allowance, it is specified just as a percentage margin based on experience. The resistance study is the first step to deciding how big of an engine you need. Here is what you get out of it:
- Ship towed resistance
- Ship towed power
- A curve showing how power varies with ship speed. This is important because there will be sweet spots in the curve that you should aim for.
Notice that you only get a towed resistance. This is what it would take in terms of force coming out of your propeller. But the propeller has its own efficiency losses. That is a separate study. The resistance study just tells you how much force you need.
There are many different ways to do a resistance study depending on where you are in the design. The trade-off is accuracy vs cost. In reality, you will probably do several studies of increasing accuracy and cost. Here are the major methods, listed in terms of increasing cost.
- Parametric estimate – general size only.
- Standard series estimate – based on similar hull shapes to yours.
- Low order computational fluid dynamics (CFD) – specific to your hull, but has a fair margin for error.
- High order CFD – specific to your hull and accurate, +/- 2%
- Experimental towing tank – 1 hull only. Not always necessary.
Generally, a design will progress with methods 1, 2, and 4 at a minimum. Here is the thing to remember about resistance studies. You probably will not like the answers that come out the first time around. So plan a program of resistance studies that tell you both what the resistance is, and provides information on how to reduce resistance.
Deliverables
The output will be an engineering report. You should get a graph showing your resistance and power (called the effective power). And you should get a table listing the specific values. This report should also point out any important knuckles in the graph. Typically there is a hull speed. Above that speed, resistance goes up fast. More important than the report, focus on what went into the resistance estimate. Here is a list of things that should be considered for an accurate estimate. Maybe a few are left out in the initial stages. But when you get to engine selection, they should all be there.
- Wavemaking
- Hull friction
- Appendages
- Windage
- Seakeeping margin
Budget
Budget varies depending on the method of estimate. Here are some rough numbers for the analysis, based on each method.
- Parametric estimate: 2-4 hours
- Standard series estimate: 4-8 hours
- Low order computational fluid dynamics (CFD): 2-4 days
- High order CFD – specific to your hull and accurate: 5 – 10 days. Plus computing costs ($500 – $2000)
- Experimental towing tank: 10-15 days engineering time. Plus model construction ($10k – $20k). Plus towing tank time (from $2k – $20k per day).
In addition to that, you have the engineer’s time to write up the final report. Roughly 3-5 days.
Schedule
The schedule also depends on the method involved. The lower order methods can be done in a few hours. The more complicated stuff takes weeks. Especially consider how this task will interface with the rest of the vessel design. Normally resistance estimates are something that you will iterate on several times. And other people in the design team also desperately need this information. It can quickly become a critical path task.
Required Information
Most of the information will come from the engineers. One thing you should provide is a design speed that you want to achieve. And specify how much time the vessel will spend at its design speed. The engineer will try to optimize around that design speed. But not if the vessel spends half its life at a lower speed. Then the engineer might try to balance the two.
