lemonader83

Hi all,

I decided its time to bring my hobby back to the ground after losing so very many dollars in the air this summer....but I love the sound of EDF's so hoping to find the best of both worlds, and maybe even have something I can use in the snow.

So, where I need help is figuring out how much thrust I'll need to power the boat I'd like to start with. This is not going to be your typical air boat, so my apologies for posting here to start, but I figured this is where the most help/interest would be.

the boat I'd like to use is linked below...
http://www.hobbyking.com/hobbyking/s...30mm_ARR_.html

Since its a medium sized boat, I was thinking I could easily make it scream with twin 64mm edf fans. My thoughts were to grab a nacelle like whats linked below, and mount it on the top of the cabin area.
http://www.motionrc.com/dynam-a-10-edf-housing-green/

next, since that part is foam, molding it to fit like a glove on top, gluing it down and then using the stock rudder....or perhaps use one similar to an airboats(behind the outgoing air) to steer it, maybe even both.

Do you think that two 64mm edf fans could make that boat go fairly fast? I know I'll never have the power of a true prop in the water, but trying to make something different that might still get me cruising along pretty good.

thoughts?

thanks!!!!

crispyspa

I say give it a go. It will be interesting and fun and if you go with air rudder, you don't have to worry about weeds.

lemonader83

Thanks for the advice, I think I'm going to do that...and since I last posted I've had some ideas run through my head ad new questions pop up.

-thrust to weight ratio? Is there a recommended amount...

-decided to use the nacelles linked above with a pair of 4500kv edf motors on 3s, 30a esc to each and should come out somewhere in the neighborhood of 2kg thrust.

-boat idea is out, I'm now looking at a pair of hobbyzone super cub floats which I plan to affix to the nacelle using a carbon fiber panel(2mm), then place one battery/esc in each float for perfect COG, and the twin edfs dead center with about a 5 degree forward down angle

-total weight should be slightly under 1000 grams, so I should have a little over 2:1 thrust to weight

-planning to run two park size servos (one in the back of each float) to run an air rudder behind each edf, this will also be able to be switched to an under air water/ice rudder if air isn't working

-am I making any crucial or slight oversights?

I spent the better part of my day yesterday contemplating how to do this, and then I happened to be in my hobby room and there was my jet sitting next to my cub floats and the light build went on.... Seems like a good, and cheap way to make a catamaran???

keep any and all advice coming and thank you!!!!

lemonader83

Oh also....on the angle of the edf, I've been reading down is a must to avoid skipping....what about placement? I was thinking center, but open to more experienced ideas.....

Got RPM

Realize that what you are used to with EDFs in the air is not going to happen with a boat, not enough speed to make the fans unload and become fully efficient. They will push the boat, but will not be as fast as a similar-powered prop drive. Several others have done what you propose and I would definitely try it, just be realistic in your expectations.

lemonader83

I appreciate your input! My expectations are pretty much blank on this as the feedback in the dozens of threads I've read is all over the boards. I've watched every video I can find and one minute you see a twin edf pushing a mini boat sluggishly going a max of 15mph, and another video you see a single edf zipping a 30+ inch cat around like a rocket.

That's why I was hoping you guys might be able to steer me in the direction of the most speed possible, it's pretty obvious that some folks are making them go much faster than other with very similar "concepts". I figured after all the failures, we'd be able to figure out a way to get a win for this project as it seems to be a non stop(but not 100% of the time) let down for those who have tried.

And I present exhibit A: https://www.youtube.com/watch?v=iUjMlkFFhKM

In my opinion, that's fast.

So, if that's a low kv 90mm EDF on 6s, my assumption is the nacelle used is what gives it the extra push...or is it because he's powering it with a very large motor in comparison to the typical outrunner most use?? I've seen plenty of 90mm edf boat videos going dog slow compared to that thing. I happen to have a 2000 watt 4200kv 2860 inrunner that's looking for a home.......maybe try that instead of the little 15mm length motors? If the issue is getting it up on plane and unloading...we need more power to get it there one would at least think????

And the confusion continues when you bump up into higher sized batteries and larger set ups as eventually the weight counteracts the extra thrust and things seem to plateau. Maybe if speed is my goal....sigh...I just buy a boat with a prop and juice it up. Had to dream a little though....

Got RPM

The biggest cause of slow speeds with airboats is excess drag caused by too much hull in the water. A catamaran can have less drag than a flatty, but the best would be an outrigger design. Next would be a twin hull like my air-prop-driven twin airplane float airboat. That gets low-30s on 4S with low amp draw. Less hull in the water means higher speed, period.

With an EDF weight is the enemy too. Use of smaller capacity/lighter packs can increase speed...the downside is less run time. Someday I will build an EDF airboat......

http://www.rcuniverse.com/forum/rc-s...-r-c-boat.html

crispyspa

Do whatever makes you happy, seriously. If you're dreaming of an EDF, build an EDF. The water prop boats are fast, but they are not unique.

lemonader83

Well gents, you just answered my questions. I was going to use super cub floats and there you have one that does 40....so that's settled, using my floats.

as far as the power source goes.... Monster 70mm powered edf, or two very well powered 64mm edfs?
-only draw back to the twins is a lot more amp draw....and probably going to need more battery.

thank you!!

lemonader83

the nacelles off of an A-10 just landed from Motion RC. All is remaining within the weights forecasted, and they look HUGE sitting on top of Super Cub floats. Once I figure out how to attach them I'll start posting pics in a build thread....

Got RPM

Twins don't automatically mean more amp draw, for the same speed the total amp draw will be the same with a single or a twin. An advantage of twins is 'less' amp draw per motor meaning a cheaper speed controller....or what I prefer, more headroom. Can't wait to see your boat!



.

lemonader83

So, one esc or two? The motors are rated for 50 amps, so i was originally planning on two escs, at 60a a piece. It seems like a safer bet for better linearity? Same batt, same motor, same esc, same everything..... Wouldn't that do better, I'd rather run the single esc for weight considerations...although I've read a lot of bad things about long term use splitting two brushless motors off a single brushless esc.

per usual, thanks for the advice and keep it coming!!

Got RPM

It is generally a bad idea to run two motors off a single ESC. Some higher-end controllers can do it 'okay', but I'd council to use two. The biggest issue with offset thrust and linearity in boats is differences in the motors, not the ESCs. The ESCs are solid state electronics while the electric motors have lengths of wire wound around the stator. Can't speak for EDF planes, but very very few twin brushless boats run a single ESC.


.

lemonader83

Ok, two escs then, it's settled....thanks again!

new question, super cub plane floats have a two part hull. I was thinking it would be a good idea to make the goal to have it plane off on the front bit of the floats rather than both, so it would make most sense to out the thrust up towards the front. Since you did something similar, I was curious how it went with the hull being not 100% smooth on the bottom? To me it could be a nice thing as I could effectively cut the water drag in half once on plane. What were your results like?

Got RPM

I found that if I tried to run on the front step only the boat porpoised terribly and would flip if I kept in the throttle too long. This was accomplished by moving the CG, jet thrust vectoring alone will not be enough. I was constrained by the semi-scale design, you will not be.

A float plane rides on the front step by using wing lift to stabilize the attitude, which we do not have. Getting a balance between CG and thrust vector to keep the boat on the front step only will be a challenge. Float designs vary so yours may be more - or less - forgiving than mine.

The upside is that running on both the front step and the transom is still a huge reduction in wetted area and drag over a solid bottom, or especially compared to a flatty. While it would be great to be able to run on the front step alone, it is probably not too practical especially for a prototype. I suspect that you will have a fine running EDF boat with the CG behind the step.

Did you see the videos? I will try to post more.

https://m.youtube.com/watch?v=C65C7C...ature=youtu.be
https://m.youtube.com/watch?feature=...&v=QxzdlF5GPN0



.

lemonader83

That's awesome, I see what you mean about the front being hard to plane off on. I'll probably mount my fan on slides and then I can play with it a bit to see what will work. If I could put a horizontal rudder behind the fan, it would be controllable to a certain extent...

im now thinking, for weight purposes, I'm going to run a single 90mm fan, it makes more sense and has far more thrust.

Got RPM

I found that if I tried to run on the front step only the boat porpoised terribly and would flip if I kept in the throttle too long. This was accomplished by moving the CG, jet thrust vectoring alone will not be enough. I was constrained by the semi-scale design, you will not be.

A float plane rides on the front step by using wing lift to stabilize the attitude, which we do not have. Getting a balance between CG and thrust vector to keep the boat on the front step only will be a challenge. Float designs vary so yours may be more - or less - forgiving than mine.

The upside is that running on both the front step and the transom is still a huge reduction in wetted area and drag over a solid bottom, or especially compared to a flatty. While it would be great to be able to run on the front step alone, it is probably not too practical especially for a prototype. I suspect that you will have a fine running EDF boat with the CG behind the step.



.

lemonader83

Still toying with platform ideas. The motor/fan I bought pushes 3.2kg of thrust, so now the goal is 3:1 thrust to weight ratio. I could go 8s for more, but seems like it might be overkill and weight will quickly become an issue. So anyways....I found a few F1 hull kits that are pretty light, and only 24" long. Also, I ran across a cheap one in eBay called the NQD PX-16....check it out and let me know what you think. For drag purposes and a better building "base" that could be a good option. The floats seem good, but a purpose built cat may take some minor engineering flaws out on my end you know?

this should be interesting....a car guy and a heli guy swapping ideas for a jet powered boat. Let's make the goal 60mph and see what happens!!!

tim.fuggdesign

Where did everyone get to with this? I've been toying with the idea of an EDF boat ever since a bet to make a fast prop air boat a few years back.

https://www.rcgroups.com/forums/member.php?u=324856

https://www.rcgroups.com/forums/show...boat-test-runs


Was debating 90mm vs 70mm but from what I learnt previously weight is key so stupid 70mm set up will be used. Only concerned with how fast I can get this thing to go so will aim to carry just enough juice for about 60 seconds of run time, 130c 6s set up. Main concern will be the fan tearing its self apart.

Rough Plans so far, time to break out the balsa and carbon!