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
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..