overweight fairmile

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27 June 2013 at 22:08 #41897
steve h
Participant
@steveh76883
Hello all, got a weight problem with the fairmile, I put her in the test tank yesterday and shes an inch over the waterline!!. Spent rest of the day drilling lightening holes in everything. I managed to lose nearly a pound in weight ,but its not enough.Only solution is to change the batteries from the two 6v gel cels to nimhs. Any ideas would be greatly appreciated. steve.
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27 June 2013 at 22:08 #4003
steve h
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@steveh76883
27 June 2013 at 22:36 #41898
Colin Bishop
Moderator
@colinbishop34627
You have already identified the solution – you will need to change the gel cells to NiMH. Component Shop have a good,reasonably priced range. LiPo cells would be even lighter but will need a more expensive charger.

Component Shop list weights for their battery packs. So if you remove the gel cells and ballast the model down to the correct waterline, the weight of the ballast will tell you how much weight you can allocate to the new batteries.

Colin

Edited By Colin Bishop, Website Editor on 27/06/2013 22:49:35
28 June 2013 at 06:36 #41901
Paul T
Participant
@pault84577
Steve

Going for a lighter battery pack and drilling holes in the frame are the primary ways of reducing hull weight.

Take care that these weight reductions don't change the boats centre of gravity by making the model top heavy.

Paul
28 June 2013 at 17:36 #41909
steve h
Participant
@steveh76883
Thanks guys. I looked at the nimh cells at component shop, cant decide , the D cell packs, 7.2v 9000mah x 2 , or four 7.2v 5000 c cell packs. similar price, similar weight. Im using four johnson 600 motors. dont know what the run time will be.? Help. steve.
28 June 2013 at 18:14 #41912
Colin Bishop
Moderator
@colinbishop34627
The weight needed to get down to the correct waterline will be the deciding factor, just get as many amp hours you can for that. Run time is essentially a separate issue and will depend on your motor/prop combinations. If you run the boat in the bath with your existing gel cells (useful for testing as per my other post!) you can measure the amps drawn at different throttle settings. The current drawn when the boat is tethered in the bath will be slightly higher than when running free on the pond at the same throttle setting.

Comparing the amps drawn at full throttle with the capacity of your batteries will give a rough idea of the run time at full speed but of course on the pond a model boat spends quite a bit of its time throttled back or stopped which will extend your 'time on the water' time!

Colin
28 June 2013 at 18:31 #41913
steve h
Participant
@steveh76883
Cheers Colin. ill probably go for the two 9000 packs, theyre the highest capacity ive found and they save 4 pounds in weight, if thats not enough weight off then ill take the deck off and do some more drilling!!…. she wont go in the bath, too big, had to build a tank in the garden. steve.
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