Mr Krabs build

[slimshady]

No rivets today, but this is what I have achieved, the prototype fin is extremely large in an attempt to turn Mr Krabs in the pond tomorrow, if it hasn’t frozen.

As said this is a prototype and the final fin will be adjusted in size, eventually.

The brass prop still has not appeared as expected from the rubbish online store Astec, so I will have to use one of my plastic props to try it, and be careful on the revs.

Slim

[Xzessperated]

No rivets today, but this is what I have achieved, the prototype fin is extremely large in an attempt to turn Mr Krabs in the pond tomorrow, if it hasn’t frozen.

As said this is a prototype and the final fin will be adjusted in size, eventually.

The brass prop still has not appeared as expected from the rubbish online store Astec, so I will have to use one of my plastic props to try it, and be careful on the revs.

Slim

Slim, unless my eyes fail me that turn fin looks about right for size and you should find your boat performing like a Chinese gymnast. For a bunch of engineers you have done well.
Unless you have a "buy British" thing you will have to start buying off OSE. 7 days from east coast USA to me testing my parts here in Tasmania. If Steven has a problem supplying a part you know about it within a few hours. I seldom buy anywhere else now as the service at OSE is the best there is. BTW I am not on Stevens payroll, just telling ya like it is.

[andym]

Slim just remember that with the bend in the bottom of that fin you deed to be able to adjust the forward - aft angle. To low at the back and the curve will lift the sponson out of the water like a wing and tohigh at the back will drive the rear of the sponson into the water. I set mine so as to be just about level and then if the pond is calm will change to loosen it and if a bit of chop will move the other way to tighten up the handling. It takes as little as 1 deg to make these changes. Hope I have explained this well. Oh I agree with Paddy that it looks about the right size, nice work.

[Xzessperated]

Slim, I know you worry about the sponson being waterproof so I thought I would share a tip taught to me by Andym.
Take the sponson off the boat. Get a container big enough to submerge the sponson in (bath, sink, baby bath etc.) and put hot water in the container. Next, submerge the sponson in the hot water. The hot water will expand the air inside the sponson and if you have a leak (the sponson, not you personally) you will soon see where it is because of the hot air bubbles coming from the sponson. If you personally have a leak you will notice this by the wet patch on the front of your trousers. I suggest you see a doctor about that problem as you should have grown out of it by now. If you have air bubbles coming from you personally I do not want to know about it.
When I say hot water I mean water that will not burn your hand but is quite warm as you want to get some air expansion going inside that sponson.
Because I had to use the word doctor in the above text a small fee of just 20 English pounds is required and the bill is in the mail..

[Ub Hauled]

Nice work!
I agree with Andy about the adjustability of the fin, you need to be able to do so,
although Andy, looking at the pictures, there is not much of a curve, but still...
I also think it is on the ballpark of the size required, you may wanna shave of a little
as you think needed as it will reduce drag. Find a happy medium between the slide on the
turns and a happy revving prop.

[andym]

Jan even with this amount of bend at 60 + MPH it can make a big difference

[slimshady]

Crashed Mr Krabs

Another interesting but disappointing day at the pond.

Got down the pond this afternoon and tried a range check, all ok so I went out for a run.

The turn fin worked brilliantly, at low speed I could turn in a 2 – 3 m radius, so I went for a higher speed run about 1/3 throttle but building up to this she started to snake about and it was a job to keep her in a straight line, after getting the hang of the correction I put more power down and at about half throttle I had to use almost full left hand rudder. At this point she flipped over onto her back and it was a job getting to the side. I changed the angle of the fin and then I tried a couple more times with the same result and ended up bending the turn fin back to an almost straight profile. Once again I took her out and although I managed just higher than ½ throttle I needed to increasingly correct the rudder as the speed increased, before she flipped again.

I then decided to remove the fin and use the remaining power from the Lipo to do a couple of drags, although harder to turn, but better than last week I managed to build the power up over 10 or so runs until I finally reached full throttle. Last week I stripped the blades off two props but this week I used a smaller prop and noticed that she was a bit slower and a bit skittish at the rear, suggesting that I need more weight at the front forcing the tail to even up a bit (not sure). My penultimate run was unusual, after decelerating I turned ready for the next run but when powering up again nothing happened so I powered down and tried again, this time the power went down and she was off again. Her final run was a bit disastrous, she reached full speed, again a bit skittish but not a bad speed lets say 45 – 50 MPH (Lipo drawing to an end and small prop) but when I powered off she dropped the speed a bit and continued on with the motor running, the final second I realised her consequence and switched the transmitter off hoping it would shut her down. She carried on and hit the wall, fortunately powering off the transmitter stopped her from powering on again but the bash was very very hard about 35 – 40 MPH and a very loud bash as she hit the wall. Fearing the worst I ran to the end to try and save anything sinking. When I arrived at the crash scene she looked ok but after pulling her out I realised the force had broken the 8mm and 10mm carbon rods attaching the sponsons to the hull on the left side, also a very slight mark to one of the sponsons, Mr Krabs is a tough cookie to crack, The ESC was very unhappy playing unusual tunes similar to the programming sequence but very different.

Went to my local model shop and ordered a meter length of each, dilemma now, the 8mm shaft is about 540mm long and I can only buy the carbon rod in 1M lengths should I shorten the width of the sponsons by 40mm (i.e. 20mm on each side) and keep a spare set in case I have another accident or should I cut them to size and waste 460mm of carbon rod (not cheap)? – other option is to make them wider – not sure if this will help, I am sure if H&M designers could read this they would be spinning saying “V designed das boat to optimum performance vi do you vish to change it”

I think the reason she crashed was that I had not waterproofed the receiver and it was sitting bear at the bottom of the hull and she took on water after flipping, or the water is coming up the prop shaft, it was quite damp anyway.

I love the idea of submersing boat bits in hot water to see if the air is leaking, I tested the right sponson in the bath but used cold water and shook it about afterwards to see if water had seeped in, that sounds a lot better way to test for leaks.

As for the increasing adjustment in the rudder with the turn fin and then flipping, the only thing I can think of is using a mirrored turn fin on the left sponson, I thought the theory of curling the turn fin inwards was to make the boat grip the water not flip (clockwise I think, can’t remember because it happened so fast) but all the same is the turn fin the correct size and should it be curved, should I have a mirrored one on the left sponson? Sorry too many questions.

Once again if you haven’t gone into a coma after reading this and can offer any advice then please please do comment however positive or more importantly negative.

Slim

Oh by the way if you type “Mr Krabs rc boat” into google (.co.uk version) this forum is the second hit – COOL!!!

Also in the photo, I am pulling the back of the sponson just to show that the rods are broken.

[Xzessperated]

Slim,
Sorry to hear about the crash. I agree that the likely cause was water on the receiver.
When you say it was snaking, was it changing direction from left to right without your help?
You said you ended up with full left rudder and this suggests it was not snaking but pulling to the right. If it was pulling to the right it means the turn fin is offset and needs to be corrected so that it is traveling in the same direction as the main hull. Get a good straight edge and test to see if the fin is traveling parallel to the hull and thus making the boat travel straight in the water. I had a pulling to the right problem with one of my riggers and when I tested it with a straight edge I found the offset was about 5mm over the length of the boat and thus was pulling to the right in the water. Because it is pulling to the right and you use a lot of left rudder it slows the boat by several miles per hour. A fast boat has everything going in the same direction.
I would replace the CF tubes with ones of the same length.
Does you mobile/cell phone do video. If so get some video of your boat running and it will help us see some of the "on water" problems.
Paddy

I will edit this in. I noted that you seem to be running your Lipo until it is flat. If this is the case it will not recharge. I hope you have the auto cut off set on your esc so the battery does not go flat. I hope I just did not understand you because if a lipo is drained of power below a certain level it becomes a dead lipo. That reminds me of John Cleese and the dead parrot.

Talking about dead parrots here is a video (courtesy of Chuck) you will all enjoy.

http://au.youtube.com/watch?v=CprQ295dxVw&feature=email

[andym]

If there is ever such thing as a personal anthem then that vidio is yours Paddy, the man who ruts anything, anytime.

[Xzessperated]

If there is ever such thing as a persolnal anthem then that vidio is yours Paddy, the man who ruts anything, anytime.

And what, may I ask, is wrong with that young Andy?

[andym]

Nothing, nothing at all my good Man!!!

[Ub Hauled]

I have another possible vu on the pulling right deal...
the fin may be too deep... after you check alignment, if it is
straight consider making the fin shorter and/or narrower...
just an idea.

[Xzessperated]

Slim,
Here are some pics of my favourite Mr Krabs turn fin that might help. I placed a ruler there so you can see the measurements...





The one below shows shape rolled into it to keep it planted in the water



The last one shows that the outside edge is sharpened



I notice yours is very similar but this might help you get the measurements close to what works really well for me. I am running a much shorter fin at the moment as I am after speed and not turning ability. Before anyone comments, yes I know they are faster without turn fins but my lake is small and the ducks are not frightened of the boat so I need some turning ability at speed. Slim you will find a boat without a turn fin turns quite well if you let it settle back into the water and turn it at slow speed.

[slimshady]

I think mine is very fractionally larger but not by much , the curve is very similar except for the front to rear curve, sorry I am not explaining that properly, if you look at the photo showing your fin end on I noticed the front is higher up in the photo than the rear i.e. curving downward. I spent a very long time making sure mine was totally straight IMO if the fin is bent backward (as yours is shown) then the flow would lift her up, if the reverse was to occur then I assumed the boat would pull down. These are only my thoughts and as seen yours works and mine did not, so what do I know.

Although the rods are damaged I used a steel rule and noticed a fraction less than 1mm over 60mm in a rightward angle about 1° not sure if this would affect the steering and the lift, too many things to calculate – surface area of the fin verses surface area of the rudder over the length between them, and I don’t think the CAD software will simulate this, and I can’t be arsed to do it but I am sure it’s not right and next time I will check it during rebuild.

When I say it was snaking I meant I am a C£&P driver and it took a while for me to get used to the build up of left correction, in other words frantically flipping left to right to control the situation, hence the snaking. Once used to it I realised that the problem was a build up and increasing the rudder to compensate helped keep it in a straight line, until it flipped, several times.

Yes I aint fik or nuffin – I know the LiPo should never be run down completely and I do programme the SC to cut off when low, but I can tell when the best part is up and time to come in, generally because she is S^!t at speed.

I think you are right, I will replace the rods with identical sized ones. So if anyone needs 460mm of 10mm carbon tube it will be on ebay soon.

I will try to take some vid of mine running for you

Love the Vid by the way true class

[Ub Hauled]

So Paddy, turn fins can be bought in the cell phone store down under?