Magmotor S28-400 with 8 inch prop direct drive.

The ends are just cones welded on. Believe it or not, adding foam adds a lot of weight relative to the rest.

For the weight you are suggesting you need big tubes. Probably 8" diameter by around 20ft long.

Might be easier to build from flat sheet. Like I say no more than about 20 hours work if you know what you are doing. Probably use two 1/32" sheets (0.8mm) 8X4 would do it. Add a couple of sheets of ply for a bridge deck.

Rick W.

 

Well going back to the roots of this thread, a s28-400 magmotor with 8 inch prop direct drive i've now realized is not realistic. Even a 5 inch prop is proving to not be realistic. For the past few days i've been in contact with magmotor getting efficiency/torque/everything data for the approx RPM's I was hitting and they match up exactly with my results. At approx 1500 RPM this motor is only around 30-40% efficient (at 36 volt). Basically without going into a lot of detail, the 3000 watts I saw pouring into the motor and only about 700-900 watts coming out, is about right with every efficiency calculation. The big one is that the motor simply is not efficient at that RPM, around 3500 it becomes a much better motor. So needless to say I think the S28-400 is not going to be my primary driving motor.

I'm now talking with them about their C40-500 motor with a special windings setup to obtain better performance. It still has about the same efficiency rating at lower RPM's, but with a smaller pitches prop and about 3 times the available power I should be able to hit the appropriate RPM's to output some serious power. Granted they are made to order and will take 6 weeks to get, then probably take me a few weeks to test, then it'll be winter time probably eheh.

They do offer some nice gearboxes that I may look into, since direct drive is looking to be more and more difficult.

How much more efficient is it to go with a gearbox and lets say a 8x8 prop as opposed to a 4x4 prop? The new motor could probably direct drive a 4x4, perhaps even a 5x5, (not being efficient until it starts moving a bit..) But would it just make more sense to use a large prop with a gearbox? I liked direct drive also because i'm trying to keep this compact, and a gearbox just adds to the unit.. Plus it is simplier.

Ross

 

Ross
You seem to be going to a lot of bother simply because you are stuck with a particular type/size prop.

Can you weld? Do you know someone who can weld? Literally 4 hours work to make a decent prop to match your existing motor.

Rick W.