well my boat has been mentioned, so you might aswell know whats going on, HS93 is up to speed with the build and has commented accurately, just for the record, I am no expert, but have spent over 2 years developing the boat with significant testing, so hopefully this will help, however mine is 42" and weighs with batteries 7kg, single screw.
I started with an align 1620kv which was excellent, direct drive, and various gearbox ratios, to get a balance of speed, run time and efficiency. I tested witn over 15 props, cleavers, steam, 3 and 4 blade, and a selection of cheap "x" props. The best was a 52.5mm 2 blade "x" prop, on 2.5:1 ratio, 22mph, 20 mins running, the brass props where slower and offered no advantage given their price . As brushless technology has come on leaps and bounds in large scale boats over the last 2 years, its apparent that a lower kv motor will produce better efficiency enabling a larger prop, but.... I think going as low as under 400kv will not produce enough speed.
Just testing a cheap turnigy outrunner, 880kv. 2.5:1 ratio was very slow, 11mph, then changed ratio to 1.25:1, and speed now up to 24mph, 2027watts, , wait for it.......107.92 max amps!!, so be carefull with the esc, You cannot have overkill, buy the best you can afford, I have wasted so much money and time buying cheap inferior over stated products from china! The prop that gave best speed was a 48mm 3 blade brass cleaver, the 52.5mm " x" that was fastest on the other setup was now 21mph, and way down the list. Most efficient, and still fast, was a "x" prop 55mm, 23.10mph, acceptable 73.74amps, 1516 watts, so a sacrifice on speed to get a more efficient setup.
batteries are 2 x 5000mah 35c 3 cell lipos, runs to test where very short so batteries didn't go off. Motor was a bit too warm on the brass prop, but cold on the "x" prop. Its so important to use some sort of data logging, I have eagletree data recorder with lcd monitor and gps acquiring average of seven satellites, so the information given is accurate, not just opinion or hearsay
In summary, don't disregard the plastic "x" props, whilst propshop are far superior, when testing, at a quid a go, its far cheaper to get a load of props, and start with small ones gradually getting bigger.
Tomorrow I might be able to comment further, as I now have a 1:1 ratio fitted and will be running the boat
Hope that helps, wish I knew how to upload the data its very intersting to see all the readings
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Topic: motor help 6ft boat (Read 7023 times)