Well, now that the Crash Tender is pretty well finished, it seems like a good time to consider if it has served its purpose of demonstrating a simple, cheap boat build suitable for kids....
The plan was drawn up by eye, from a profile drawing on the web, and a number of inaccuracies became apparent as the build progressed. Major ones are:
The bow plan shape is too pointy
For the front superstructure, the front roof is too small and the rear roof is too large
The front chine former (F1) is 1/4” too small
Former F2 is 1/8” out of position
These are corrected on an error page
You can't quite fit a small 6v gel cell across the battery compartment
The hook mount inside the rear cockpit was a bit of an afterthought, and the former could have been moved back an inch here.
Access to the motor area is limited
The front superstructure is fiddly, complex and tricky to glaze
There is a lot of complex sanding and finishing
I would like to address some of these on a new plan, but the latter three issues are likely to remain. In practice, once a motor is installed in a small boat it is rarely removed, but it is poor practice to make this difficult! I really need to rethink the front...
Though the hull is simple, there is rather more work than I hoped, particularly in finishing. There are 17 windows, and, apart from the deck, 7 separate horizontal levels. A lot of sanding!
The boat looks good, and has many opportunities for adding fittings, but it needs a medium level of skill to complete. I think that this should be a second or third boat rather than a first one, so we still need a very easy introduction model...
The basic hull achieved the aim of being low-cost – around £3.50 in balsa. However, there were considerable extra costs which need to be pushed down.
Shafts, rudders and props are expensive, about £5 upwards for each. The Hobbies prop shaft at £2.60 would be ideal (with propeller!) were it not for the poor shaft quality. I replaced it with 2mm piano wire, which required a 2mm thread cutting. Die cost - £2. However, the problem of a coupling cost of £5.50 still remains. We must see if we can address this. SHG do a plastic coupling range for £2.50, but postage costs....
Motor and batteries.
A cheap brushed motor and NiMh batteries cost £5 and gives reasonable performance
A Graupner Speed + NiMh batteries cost £ 12 and gives good performance
A brushless motor +Lipos + charger cost £15 (£22 with the ESC) and gives excellent performance (but only for 15 mins!)
I think these are reasonable costs (particularly if the motor could be upgraded later).
It's not really part of the boat build, but a modern radio can be obtained from around £30, and the brushed speed controllers I use come at £2 each from Hong Kong. A brushless motor must have a speed controller, so that puts the price up for the motor, but down for the radio....
Glues and Paints
This is the big one.
A tube of balsa glue cost £5
Five Halfords spray cans (four colours and a clear lacquer) and a tin of sanding sealer cost £39!
Of course, I did not use all the paint in these cans. Still, that's a huge cost. I suspect that using thinned polyurethane varnish for initial surface preparation might keep the price down, and with access to a spray gun cheaper paint can be bought. But I need to pay more attention to this aspect. Cutting down on the number of colours would be a start....
Working in balsa, total build time can be very short for an experienced builder. There is time to make a few of these for Christmas!
This is probably a bit complex for a small start model. We need a simpler hull. With only one roof...but still exciting... We will have a few more boats coming which should match this specification. Watch this space...