6ft Fast Electric Mono

[Amp User]

I want to build a 6ft shallow V mono out of door skins and square stock. I plan on using a outrunner electric 3phase motor, its output is around 2800w or 3.7hp max amps 90. I would like to buy a small surface prop around 5in in dia. I hope to run the prop at 3000rpm/2100w/65amps/34v at cruise speeds of 16mph whats that in knots?

What props are out there?????
I have had bad luck finding anything suitable.

Ther are some brass and steel hobby props up to 130mm or 5in roughly. They are submerged props that would require cutting rakeing cupping. I dont mind low thrust and long legs to get the boat up to plane and max speed. I do want a high lift surface running boat like a Cracker Box. Its going tobe a one seater with tight room for two when needed. I would have to keep the loads down with two but hope there is enough mid range thrust to low plane the hual at 70% throttle with two people aboard.
What do you guys think is this possible with a 3.7hp electric motor. I plan to keep the hual very light no glass just fill and paint and epoxy clear coat. My guess the boat will be 50pounds bare. And a mere 30pounds extra giver or take a few using a 782g brushless motor and li Ion cells. So 80 pound plus me a 190pound person + another 10 pounds in rudder/prop/stuffing box/axle/controls. So all up weight goal is 275 to 290pounds. The hual will be 6'X3.5''

Any thoughts on this please feel free.

[Amp User]

I have a Hunter ceiling fan motor that is going tobe coverted into a 3phase brushless motor, this one has enough iron to produce an easy 4000w at 70A 60V system. Im shooting for a 100k/V and will terminate in wye with 4 strands of 19g mag wire. I need a custom bell with 40X15x5 N50 neo's. Im going to use lipoly cells in a aluminum protection box with a vent and blower. The vent tube will be off to the side and pointing down towards the water.
The packs I plan to use can put out 100A for approx 45mins. The packs will look like these.

[Amp User]

Each cell is rated at 150 amps, so the 14S8P pack could in theory put out 1200 amps at about 46 volts - 55,200 watts or 74hp I would imagine its now possible to achieve safe and fast running on the water with long duration possible. If I had a choice a all carbon mono perhaps 5.5X 3.8 with 25hp 1.5hr full throttle duration.

I found some 7.4v 5000mah (30C 150amp) lipo's for 126.oo per pack, I take 18cells to produce 22v 15000mah bursts to 1350A 29700w 40hp . I should run it at 150A for 1.3hrs / 4.5hp should get me around 18 mph. I will run for short sprints for fun at times drawing 300A for 9hp. and near 30mph.

[marshmat]

16 mph = 13.90362 kts
You seem to know your electrics pretty well. I would advise caution with the LiPos, though; they're amazingly powerful batteries but also amazingly easy to blow up or permanently destroy. Are you designing your own BPS or buying a stock unit? Each cell may be rated at 150 A, but trust me, you don't want to let them do that
Make sure the intake side of the batt cooling is well protected from water and debris. Inside the boat's forepeak would strike me as a good place for the inlet filter.
I'm not sure about the feasibility of the propeller you describe. Surface props work best when they're big, trimmable, and going 50-plus mph. A small fixed one would be tricky to get going. The huge torque of a BLDC motor would, imho, be better used to turn a submerged prop designed for your speed range.
What are you using as a motor controller? This is often an afterthought in such projects and should be considered from the get-go.

[Amp User]

Hi Marshmat, ty for your input. Im going to use a 150amp esc built for record setting SAWs class racing riggers they are water cooled. Option 2 would be any number of the high amp controlers made for electric go carts.
Yes your right on care with lipo's. The aluminum box to hold them will have cool air drawn through where you sugested and a vent off the left side of the hual and pointing down into the water incase of flame/heat/smoke. I will have the batteries off the floor of the safty box. When in storage I will keep them in ammo boxes. Each cell can put out 3.4v at 110amp continuous this is a safe 110 of about 20C the pack can handle peaks of 150 but short 30 second bursts for fun to play it safe. I have found a few props able to handle up to 10k most are much less sub 5k. I can mod props by adding cupping rake and cutting the lower pitch out. I have built many RC riggers and monos with fixed surface drive set ups. The 1man mono wont plane as high out of the water so im going to have to raise the stuffing box out the transom higher to allow enough hual in the water to suport me at lower speeds and still allow the prop to breath and break the surface. I may suffer performance but the rooster tail will be worth it. I want everything small and light with high output. 30pounds of batteries that act like 100pounds worth of sealed batteries. A light hual and only strong enough to handle its purpose. I will insure floatation incase of a breach.

[Amp User]

Here is a fast drawing of the size shape of the boat I drew out full scall on a large sheet of brown paper. I measured myself out and drew myself in the boat and this kinda shows the proportions of the full size plan. The low center of gravity should be a plus. CG should be around 35% from the transom. Batteries behind me and motor infront between my legs.

[Amp User]

Ok found my prop 5.25in 3Blade will spin this up to 3500rpm. Prop sales for $56.25, not a bad price really. I would rather have this in steel that can handle 5k.

[Amp User]

42 tooth 4Kg (8lb) 250mm (10") stator, 14 ferrite magnets in the 2kg (4lb) rotor. Taken off the bottom of a direct drive washing machine at the tip recycler for just $10. Sure enough it has 3 wires in and is wired in star (Y). Im thinking two of these ganged up direct drive should be very powerful with neo magnets.

[gonzo]

You drew something like a PWC, which is a planing hull that requires at least 30HP to plane. For a power of 3.5 HP you need a longer, lean hull.

[Amp User]

Gonzo what if the hual bottom was wider then you guessed.. as I plan on taking on a passager from time to time for tooling about near the shore line a 3.5ft wide bench seat is tobe used little tight but just doable. The bottom is almost 4ft wide at the rear, the extra surface area was to help a single person plane on 3.5hp. Will this work?

[gonzo]

Wide hulls need a lot of power to get up on plane even if the speed potential is high. Four feet of beam on that length is exagerated. You need a long lean hull to move it with so little power. I have a 9'6" dinghy with a 3.5 HP outboard that has a much better power to weight ratio and it planes.

[Amp User]

Ok will have to redesign the hual type completly. I can push to 7'x3.5' and have a near flat bottom and just keep it away from rough water.

[Tim B]

I did a feasability study for Electric Trimarans some time ago. The two biggest problems were Hull resistance and weight (weight mainly). I suggest that you use a Savitsky power prediction (There's an Excel version on the forums), then talk to a few propellor manufacturers, remember, the best prop efficiency you can expect to see is about 60% to 65%. Usually required power is estimated as ( Resistance*Speed )/0.5. This works in a bit of a margin. I think you'll be surprised how much power you'll need.

Tim B.

[messabout]

Gonzo is right about making the boat longer. Build it 12 feet LOA at least. The 6 or 7 footer, if you ever succeed in getting it to plane, will behave like a motocrosser. Not only will it pound you silly but it will need more power than you are anticipating. Longer boats are a more practical solution. Doorskins are cheap but that is the only thing that recommends them. They have core voids, are not stiff, and they soak up a scandalous amount of paint or epoxy. The boat will be lighter and stronger if you buy some BS1088 okoume. When you consider all the work you'll do in building the boat, trash material does not make good sense.

[Quicksilver]

please bring this topic here, we can help tremendously

http://www.rcuniverse.com/forum/forumid_229/tt.htm

edit: opps you want to ride in it. has to be much bigger like previously said. I dont know very heavy