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PROPEXPERT - I have never used the "Based on similar vessel" speed prediction option. What is this for and should I be using it?

First, let me comment of the importance of a good speed/power relationship in propeller sizing. Traditional propeller sizing techniques simply look at a "design" condition. In other words, the propeller was sized to utilize a given power at a particular RPM and speed. There was little else involved and it was assumed that the design power and speed were realistically matched.

Let's say for a moment that the hull's actual demand on the propulsion system was greater than expected. You would then encounter a vessel that cannot make speed and a propeller size for a higher speed than attainable. The vessel would have an over-pitched propeller and the engine would not reach RPM. By having a good understanding of the true speed/power relationship, we can then make size decisions that lead to a successful installation. Basing the speed prediction on trial performance - or on the trial of a similar vessel - can help insure a proper sizing.

PropExpert makes it easy to find and use trial data. When doing a repower or considering a new propeller, it is recommended to base the speed prediction on a prior trial. This tells the analysis not what the theoretical speed/power should be, but what the speed/power actually is. How does this relate to using a trial of a similar vessel? Well, when we have a credible vessel trial with good match between all components, you can save this data into the PropExpert vessel database for future use.

There is a relationship between vessel size, speed and power that conforms to what is commonly known as a geosim rule. Basically this means that for vessels of the same shape, the speed/power relationship is a mathematical scaling of one vessel size to another. You can find the speed/power of one boat by scaling a non-dimensional version of another boat's speed/power data. The closer the size, shape and speed range between the two, the better the results.

Many users typically do the same type of hulls over and over. In southern Florida, for example, users will perform many sizings for twin-screw motor yachts from the same builder. The hulls are very similar, but of different length and weight. Basing the speed prediction on a past trial of one of these vessels is a very successful technique if you do not have actual trial data in hand..

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