The testing on this page came about after a number of discussions with various people on how fast a 1/12 model Severn should go and how fast we where going.
First there is the mathematics on the subject.
A 1/12 model should travel at the speed of the full size boat divided by the square root of the scale.
The square root of 12 is 3.46410
So 25(Knots) divided by 3.46410 = 7.21688
From this we can say that we need a model Severn to be doing 7.2 Knots
Now to check our models speed we need to do this over a short distance in this case 30 meters so we need to convert our speed in knots to meters per second.
There are 1852 meters in a nautical mile (1-Knot) so 7.2 multiplied by 1852 = 13334.4 meters per hour
To get meters per second we divided this by 3600
13334.4 divided by 3600 = 3.704
Therefore we need to cover 30 meters in 8.1 seconds.
The next factor we need to consider is the weight of the boat and getting it to float to its lines.
The weight of the model floating to its design lines (the ones on the plans) is 19.5Kg
After looking around at several Severns it was noteed that they were a little lower in the water at the stern this transferred to the model gave a model weight of 22.5 Kg.
So the weight of each part of the boat is Hull 14Kg Wheelhouse 4.6Kg and 6 volt 12 amp/hour batteries 2Kg each.
Upto the point of doing this testing the boat had only been run on 12 Volts after some testing it was proven that we could run the boat on 18 Volts.
First of all we needed to now what performance we had and how weight affected that performance.
The first run we did with the top off.
12V top off 18Kg 13sec 2.3M/sec 4.47Knots
The next run was how we have been running the boat.
12V top on 22.6Kg 14sec 2.14M/sec 4.16Knots (this is the performance that can be seen in the videos on this website)
It is interesting to note the difference in time due to the extra weight
Next we added an extra battery to get 18Volts.
18V top off 20Kg 7.4sec 4.05M/sec 7.87Knots
18V top on 24.6Kg 8.4sec 3.57M/sec 6.94Knots (the extra 2Kg puts the boat about 5mm below her lines.)
This is as close to the 7.2 knots of scale speed as we can get. It may be possible to get that extra 0.2 with a lighter battery load that got the boat back to its lines but this is close and steering is interesting at this speed.VIDEO HERE with the trim tabs up
The trim tabs have quite an effect with the extra speed.
It is interesting to note that for a 50% increase in volts we are getting a 65% increase in performance.
The problems with running at 18 volts are interesting first the first battery in the set seems to discharge more than the other two. While the speed looks great steering becomes interesting.
The performance at 12 volts looks good gives good control and a sustained running time for every day usage at shows. While it would be nice to run at 18 volts control becomes an issue with a lot of other boats on the water doing damage is a real issue and the extra power requirement would also reduce the available running time. It would be ok for a one off session to show the performance when you have the space to show it.
Update on running on 18 Volts
After some further testing and changing the configuration of the Batteries we have now got the boat doing over 20 mins on this performance power setup. With the addition of some clever electronics it is now also possible to run all the ancillary equipment that needs 12 volts (on board tv camera and sound system). Something’s like the bow thruster will run quite happily on 18 Volts and give an even better performance.
Improvements in the rudder setup have helped with the steering and this should be made much more controllable with new rudders of a slightly different design.
The new rudders have now been fitted and this has made the steering much Better at full speed.(20/9/05)Pictures of the Severn at Speed
Click on the picture