Calculating Speed of a Boat for Dumbo - Help Needed

[Tai]

Hi everyone,

I'm hoping for some help in calculating the speed of a boat I am planning to build, I'm not looking for a precise speed just an indication of what it is likely to be to ensure the pumps I would like to use are powerfull enough.

The boat is a catamaran hull with the hull dimensions 150mmx100mmx620mm (per hull), the remainder of the boat will be above the water line. Overall the weight will be 8.5 Kilos, the propulsion system I would like to use is made up of two bilge pumps, each bigle pump is capable of moving 42 litres a minute so combined 84 litres per minute. The pump output pipe is 19mm in diameter, which i guess fits into the equation somewhere...

Is someone able to tell me an approximate speed in either kph or mph? I don't even have a clue where to begin

Thanks in advance,

Tai

 

[Speed]

You need to determine how the drag varies with speed. Then for a first approximation, you relate that to the power output of your engines:

$$P = \frac{1}{2} \rho v^3 LW Cd$$

Bear in mind that your drag coefficient would have to include viscous, wave and profile drag terms.

 

[ravenprp]

Hi Tai,

Calculating the speed of a boat can be a bit tricky, but I'll try my best to help you out. The key factors that affect a boat's speed are its hull design, weight, and propulsion system.

Based on the information you provided, your boat's hull dimensions and weight seem to be within a typical range for a catamaran. However, the propulsion system you plan to use may not be powerful enough to achieve high speeds. Let's break down the calculations step by step.

First, we need to convert the weight of your boat from kilograms to pounds. This can be done by multiplying 8.5 kilograms by 2.20462, which gives us a total weight of approximately 18.74 pounds.

Next, we need to calculate the total thrust of your propulsion system. Since you have two bilge pumps with a combined output of 84 liters per minute, we can convert this to gallons per minute by dividing it by 3.78541. This gives us a total output of approximately 22.18 gallons per minute.

Now, let's calculate the total thrust of your propulsion system. To do this, we need to multiply the output in gallons per minute by the weight of the boat in pounds. This gives us a total thrust of approximately 416 pounds.

Finally, we can use this thrust value to estimate the boat's speed. According to the basic formula for calculating boat speed, the speed in knots can be approximated by taking the square root of the thrust in pounds and multiplying it by 1.34. This gives us a speed of approximately 18.7 knots, which is equivalent to 21.5 miles per hour or 34.6 kilometers per hour.

Keep in mind that this is just an estimate and the actual speed of your boat may vary depending on other factors such as water conditions and wind speed. I hope this helps you get a better idea of the potential speed of your boat. Good luck with your project!