Mr Krabs build

[Ub Hauled]

Slim,
since you are starting up on the sport, try not to reinvent the wheel, what I mean by that is, why would you rivet the turn fin on the sponson? that should not even be an idea... with time, there will be some slack created and the fin would become loser and loser... probably causing and accident before getting ripped off, use nuts and bolts, they have been proven. Also, buy a turn fin m8, that way you'll learn the mechanics faster and lose less money out of the gate.
About your CF prop... good for lower RPM boats, you need either stainless or Berillium, aluminum will not be strong enough for fast applications, CF will blow on you every time.
More Volts and Amps is not necessarily better, I am not sure if you will be racing on official clubs but there are restrictions in hull and cells (sometimes more stuff too), so be sure that you look it up b4 trying to shove too many cells in her. Personally, I think that you will not need more then 4 cells in that hull to go fast, done properly she'll be a hand full, imagine if not done properly.
Also, keep in mind that the RPM that we estimated is nominal Voltage with 100% efficiency, that being said, the motor has probably not even 85% eff, the batteries under load probably drop the voltage from 4.2 fresh to 3.3 or so and that's not counting the loss at the prop which is considerable... that brings you down to +- 43500 RPM or less. when you buy a better motor and better cells, be sure to get a better ESC as well... unless you like bonfires hehehhehe. This is the number of a really high end system running on 4 cells. What size prop are you running?
I am not telling you all this to put you down, I just want you to understand that there is a reason why the "pros" do it a certain way, if you wanna be out there going fast, some adjustment will be needed.

[Xzessperated]

Slim the guys above have given you some great advice.

In short...
Your turn fin is not big enough.
That motor should never run on anything more than a 4s lipo
Plastic props have no place on our type of boats

The guys here are trying to save you some money. I run my Mr. Krabs on 4s and it is doing a shade over 70 mph and the speed is improving with modifications. If you buy 4s now your motor and esc should give you good service and you can upgrade the boat as you can afford it. It will make your boat reliable and let you get some wheel time so you get to enjoy your boat and learn how good it really is.
My Mr Krabs is the best boat in my fleet in every way.

Please NO RIVETS

[slimshady]

Viking, Thank you so much for your words of wisdom and also substantiating the fact that you have measured the Amps from your ESC. To me this is a very comical affair at work, to be honest I agree totally with you, Work under load requires power i.e. Amps and volts therefore if the prop leaves the water the work reduces and also the Amps, he (electrical engineer) is still insistent that for a very brief moment / split second after the prop leaves the water the work increases and takes the Amps high. The other electrical engineer is still on the fence and cannot make his mind up, I do not think any of them have done proper work today because they keep asking me questions on the electrics of the boat.

We cannot decide if the theory of the motor moving backwards is correct because if you decelerate the prop it will still be moving in the same direction but it has to rotate slower than the amps that could drive it, which could cause the same thing, reversing the motor would cause this, my setup will not allow this, therefore if the brake is not on then no problem, it should simply slow down.

Ub, I am not aiming for the new speed record or entering it in races, not sure if a class has been invented yet but I do want to drag the boat and get a high speed (once again not sure if this is achievable) that’s why I am thinking of using a 5S but I still need to turn in the width of the pond. I understand that under load the volts will drop and your figures look good, thanks. I have ordered an Octura M945 – this is a 2 blade 45 x 1.6 pitch brass prop, I have also designed a balancer, but I have little experience in sharpening and balancing but the other engineers at work will help, and show me how to file it to the correct shape.

Mr Krabs was designed to use 16 – 20 Cells so I want to use them all. But I need to walk before I can run, on that note I need to control it and manage to turn it.

Paddy, you are right but only before I tested it, the turn fin is not adequate and it is sitting at the bottom of the pond. The motor is fine at the moment until I cook it (watch this space) and I am trying to get a bigger Lipo, better prop and turn fin.

I have sketched up a turn fin and waiting for some aluminium sheet to make a prototype, you are right, I used a test bit of CF and although it held strong, if it was subjected to side force it could loosen and pull out of the sponson, I took Mr Krabs into work today and we are stumped on how to fix the turn fin, although I have had a lot of suggestions.

[egneg]

How many engineers does it take to change a light bulb and how long will it take? Depends on the number of engineers involved! Ask the technicians as they are the ones that make the stuff work. Just my 2 cents.

[Viking]

Solid Water

Hi Slim;
Earlier you stated "We cannot decide if the theory of the motor moving backwards is correct because if you decelerate the prop it will still be moving in the same direction but it has to rotate slower than the amps that could drive it, which could cause the same thing, reversing the motor would cause this, my setup will not allow this, therefore if the brake is not on then no problem, it should simply slow down."---
Slim have you ever hit the water at 50+MPH ? Say water skiing?? I have and I have tried diving in to the water from over 25', both OUCH. at those speeds water is a solid, or pretty close to it. You fall off those water skiis and it is, Skip, Skip, Bump, and then splash.
So I have a boat weighing in at over 9 lbs, doing 50 Mph, when I cut the current to the motor that weighs 1 lb. The propeller becomes the blades of a turbine and in very short order has that motor stopped and spinning in reverse.
Can I prove any of this? Not right now my lake is really solid, Frozen you might say.
Viking

[Ub Hauled]

I have ordered an Octura M945 – this is a 2 blade 45 x 1.6 pitch brass prop......
Mr Krabs was designed to use 16 – 20 Cells so I want to use them all. But I need to walk before

Do you mean an M645 ?

You are right about the cell count, but keep in mind that they are Nixx cells, not Lipos.
Lipos output WAY more power and weight a fraction of Nixx. The Krabs was design for a specific type of race and a specific speed, us speed freaks always wanna go faster, just keep in mind that this hull will not run 6s... at least if done properly.

[Viking]

O shut my mouth.
The propeller is spinning in the same direction like you said. However it is turning the motor like a generator, causing a current to flow in the opposite direction than normal. Would any one like to apply a current to the out put of their ESC to see if it smokes???
Viking

[Xzessperated]

I took Mr Krabs into work today and we are stumped on how to fix the turn fin, although I have had a lot of suggestions.

Slim go back through the pictures on how I did my turn fin bracket. It does not matter if you cut an oversize hole in the back of the sponson as you can fill it with epoxy later.
Why not cut a slot in the back of the sponson and slide a piece of aluminum in there that has two threaded holes in it. Before you slide it in mark and drill where the bolts will have to pass through on the outside of the sponson, then slide the aluminum inside the sponson and carefully bolt it in place. Once bolted in place dribble some epoxy around the edges of the aluminum so that it glues it to the inside of the sponson and thus adds strength. This should not be hard for a few under worked engineers to do . Instead of aluminum you could use a piece of carbon fiber that has nuts glued to it if you are worried about weight. I would go with the CF option.
If you are not sure what I mean tell me and i will make some sketches. This is better than the way I did it but if I ever have a problem in this area I will do it this way next time. IMO if you have to add something to a boat try to do it in a way that it adds some strength to the overall package. Remember to leave some aluminum hanging out the back to bolt the turn fin on to. There is more than one way to skin a cat.

I have water skied and I know what Viking is talking about. Water can be like hitting a brick wall at speed.

Slim I have edited the picture below in to show you that I am not afraid to drill some holes in the boat and pour a little epoxy in to strengthen my work at the transom up . I have to sand these areas back and paint over it but that can wait until I paint the whole boat. When I had put the epoxy inside the boat where I wanted it, I let it dry and then forced some Kleenex tissue (brand does not matter) in the hole, so it made a platform for the expoxy to fill my drill holes to sit on.

[Xzessperated]

How many engineers does it take to change a light bulb and how long will it take? Depends on the number of engineers involved! Ask the technicians as they are the ones that make the stuff work. Just my 2 cents.

How many women does it take to wash a Volvo?

[andym]

One very big Sweedish one Paddy!!!!

[egneg]

And here she is asking me how many coats of wax I want on the volvo.

[Ub Hauled]

HAHAHAH, I have seen that shot, there are a few others as well, she was even on the news... that is one taaaaaall hot lassie!

[slimshady]

Thanks again Paddy, I am almost on your wavelength today I cut the Turn fin out of 1.5mm Aluminium and sharpened the blade. Tomorrow I was planning on bending it inwards and then cutting the back of the sponson to the inside and inserting a long piece of Aluminium (although CF will tap out to say M5 I am not sure how long it will last and how much force it will take before the thread strips) by drilling the sponson holes on the outside through the plate before inserting it I thought about using gasket sealant but I like your idea of using resin and using the setting time to screw a couple of STST button heads with penny washers resined to the outside of the sponson then resin up the back to seal it. Thread lock should seal the threads if the resin doesn’t so double bum covered. The other thing I thought of was drilling a small hole and injecting it with expanding builders foam which should also float but at the same time not take on water (could be a weight issue).

I am still waiting for the prop to be delivered so if its not been delivered by tomorrow then no runs this weekend.

Viking you are right it bloody hurts when diving off the top board at the swimming pool and bizarrely one of the electrical engineers said that he might have had things slightly the wrong way round. He now clams that.... (and I have to say in theory because I am still not sure that he knows for sure). “When the boat is in the water and the prop is almost submersed it will sustain a constant current but that current will cause heat in the electric system under the Amp load we are supplying and interestingly he also said that cooling was essential and he is not sure how we keep the boats electric systems cool enough to handle it, over time they will heat up and burn out very quickly. When the prop leaves the water the current demanded from the free running motor will consume a lot of amps for a very split second thus will drain the battery quicker putting the ESC under serious load and heating up the system more. When the prop is free from the water the load is less and the Amps will decrease but the heat will remain similar or slightly less when the prop hits the water once again the Amps will shoot up by lots and therefore the heat in the system will increase again. In short too much of this will kill something. The prop will always turn in the same direction unless you inadvertently do what I did and switch the brake on, this will stop the blade from turning and cause the boat to spin as mr krabs nearly did before the force became too much and ripped the whole prop shaft from the coupling.

egneg
It takes 6 engineers to change a light bulb; 1 to complain about poor under lit working conditions, 1 to completely reinvent the light bulb, 1 to read the manual 17 times and still complain that its written in German or Japanese and 3 to take the P1$$ out of the other 3 looking at nude calendars drinking 10 cups of tea, eventually one of them will finally pull out the old bulb out and screw in a new one. Job done, so why is everyone moaning and management analysing why it took 6 engineers 4 hour to reinvent the light bulb take a patient out and simply say F it and stay with the standard, beats me.


Joke of the day (duck joke just for you Paddy)

A duck walks into a bar.....

Duck “got any bread”
Barman “no sorry m8 we have run out”
Duck “got any bread”
Barman “no m8 I just told you we have run out”
Duck “got any bread”
Barman “M8 we have no bread”
Duck “got any bread”
Barman “no m8 we have no bread”
Duck “got any bread”
Barman “no bread”
Duck “got any bread”
Barman “no”
Duck “got any bread”
Barman “no”
Duck “got any bread”
Barman “no”
Duck “got any bread”
Barman “no”
Duck “got any bread”
Barman “no”
Duck “got any bread” “got any bread” “got any bread”
Barman “no” “no” “no” “ look you annoying little duck if you say “got any bread” one more time I am going to nail your F’ing beak to the bar”

The duck pauses for a second........

Duck “got any nails”
Barman “NO!!!!”
Duck “got any bread”


I still think the simple ones are the best


Keep it real

Slim

[Xzessperated]

LOL. I love the duck joke.
You should learn from the joke Slim. No nails and no RIVETS

Silastic is a good thing to stop water entering from the outside Slim. A nice wet finger will smooth it out but keep the finger wet. Andy and I had a laugh about how to keep the finger wet but I will not write that here.

[Xzessperated]

And here she is asking me how many coats of wax I want on the volvo.

Oh my, oh my, oh my. Does she do SAAB's? I have got a big one for her. Maybe I should explain that statement. I mean it is the 9000 Griffen model.