Prop Selection

[FR8TR]

I am working on a 28" scratch built model of a Coast Guard Pickett boat of the 1930's. I am taking over the project that was started by another builder and as such the stuffing tube has already been mounted in the keel. This has limited my choice of prop as far as diameter. Within the diameter I have a choice of 3, 4 or 5 blade props. My question is what would the advantage/disadvantage be of the different blade count? (I was previously told that there would be an increase in thrust of about 10% with each additional blade although no impact on boat speed.)
Thanks in advance for your input........

[Tony Oliver]

If it's a scale boat, why not pick a scale prop? From that choice, you can then select the motor which will suit that. You can always consider a geared motor if the diameter/number of blades is too high for a direct drive.

The advice you were given is right. The basic thing to remember is that a particular shape hull has a limit to the top speed and any extra power you supply is wasted once that speed is reached. The more blades, the more power for a given diameter, but also the energy to drive it goes up. The increase in blades makes up for a limit of diameter.