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[Subculture]

Is the inner shaft running in bearings/bushings or up against the full length of the outer shaft/tube?...

[Subculture]

BTW, if your tanks are pulling 3.5A at 12 volts- 42 watts for a free running tank, then something is definitely out of whack. An Engel tank draws about...

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Either will work.

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Okay so going back over the figures I realise I made in error in the first stage of gearing, and your final ratio is taller at around 10.8:1, which would...

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So you didn't mention the tooth count of the Emaxx gear, but a quick google throws up something around 65 teeth, so along with the other stage of reduction...

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I don’t know what pitch the gears are on the Emaxx. Most car gears in the 1/8-10th scale are usually metric pitched and somewhere in the region...

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Assuming your tanks are 70mm diameter like the Engel units, then something like these on the 1726RPM gearing, with an external 3:1 single stage to clear...

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Looks like a 919D, probably running on either an 11:1 or 6:1 ratio plus a further single stage reduction of around 3:1. Once again Norbert Bruggens book...

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Having two tanks half the length improves things quite a bit as you can see. Sadly another issue rears its ugly head if the tank is sealed- ensuring equal...

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If you have Norbert Bruggens book, there's a formula in there which enables you to calculate the reduction in metacentric height for a part filled ballast...

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Engel Typhoon is about the same size and construction as your boat, that has 1650ml of ballast tank volume split over two 825ml piston tanks. So I'd take...

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That is essentially how the Eden/Sheerline system works, except they tend to use a centrifugal pump, and the hose is checked by pinching it with a cam...

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All in the best possible taste.

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Also comes in snot green!

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So many of us have been there with the ‘gotta be a biggun’ desire, some never quit, but most downsize once they realise what a hassle dealing...

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Shouldn't build a big Typhoon if you want a light boat, regardless of dry weight they'll still be heavy to pull out of the drink. The snag with boats...

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Yes, the piston tanks. They look well built and I’m sure can be made to work well. Also cheapest solution as you already have them. Not familiar...

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One option, although you’d need pair for sensible dive times, even then it’ll be slow. Kavan pumps are another option, higher volume but more...

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It'll certainly motor along if you're running a 12v system, but the motors will be up to it.

Endcap material , acrylic will work fine, almost...

[Subculture]

You could use either tank arrangement, but I would use a single tank unless you have some sort of proportional feedback for ballast volume e.g. small...