Mr Krabs build

[slimshady]

New photos enclosed – 3 show my new proposed flexi drive prop shaft position and 1 shows my prototype turn fin for the front sponson’s (rough cut at the moment before I commit and make the final pair). I think the prop could do with being a bit lower it is only 8mm higher than the flattest part of the hull, flat to centre. What do you all think? Please comment if you can, and many thanks if you do.

Slim

[Xzessperated]

New photos enclosed – 3 show my new proposed flexi drive prop shaft position and 1 shows my prototype turn fin for the front sponson’s (rough cut at the moment before I commit and make the final pair). I think the prop could do with being a bit lower it is only 8mm higher than the flattest part of the hull, flat to centre. What do you all think? Please comment if you can, and many thanks if you do.

Slim

Slim I have a couple of concerns. I have included a picture of your strut bracket that appears to be bent. If it is make sure that the strut is running parallel to the sides of the boat. In other words make sure the prop is going the same way that the boat is or you will be running with some rudder added just to keep it in a straight line.


I need to take a picture of my own boat to deal with the other matter. I will post that a.s.a.p

[Xzessperated]

Slim,
First a picture of my stuffing tube and strut position. If you look above the CF strut bracket you will see I have scribed lines into the aluminum strut. The top mark is where I first started. Each time I raise the strut I am getting more speed. Because it is still a work in progress I can not give you a final measurement from the bottom of the main hull. I started at 33 mm from the bottom of the hull and I am now at 27 mm but I will keep raising the strut until I have decided that it is in the position for maximum speed.



Here you can see I am using a flat bottom strut...



The flat bottom is very useful for this next exercise. In this next picture you can see I am using a piece of aluminum angle to sit on the bottom of the strut and check to see if the strut angle and thus the prop angle is running parallel to the bottom of the boat. The problem with this boat is that it does not have a flat bottom but you can see I use an (un)educated guess to get it correct. I will play more with the strut angle when I get its depth correct.



The last picture is of my antenna. I have inserted the antenna wire into the plastic tube and it all just silasticed in place. You can see some wire comes out at the top but not enough to get caught in the prop.



I hope some of this helps.
Paddy

[slimshady]

Hi Paddy thanks for the info, always appreciated, I have not ignored your comments or photos, I simply haven’t had the time to post up recently with busy work and boat build.

I have nearly completed my build of Mr Krabs, and will post photos as soon as I can. I have made some major design changes including the bend of the stuffing tube and the mount, thanks for the photos back, that bend you pointed out was intentional now the prop runs directly in line with the front and sides of the hull.

I have pitched the drop similar to the photos of the other Mr Krabs models I have seen on the CD received with the hull, this way I intend to do the revere of what you have done by increasing the drop to see if this increases the speed or reduces it. You mention by reducing the drop i.e. moving it closer to the hull the speed increases, my starting point, but in the interest of science this should benefit us both.

I have now created a new bearing mount with an SKF 61801Z bearing after the motor mount to eliminate concentricity of the motor shaft, although I created a drawing of the coupling with a 5um tolerance for my coupling I was still not happy when spun up, the new metal geared servo is now at the stern and I have mounted the safety cut off line vertically due to space restrictions (always a problem with this hull). I have also replaced the 27Mhz system with a 2.4Ghz unit to eliminate frequency problems.

I currently have problems with my LiPo battery (only 14V) but after she sank last time I cannot get it to charge properly, could be a problem with the battery or the charger all I know is the NiMHs work ok but they are only 8.4V so useless to me and the ESC with under current setting. I want 18 – 30 V LiPos for my next adventure but why run when I haven’t walked yet. I need to get them to work especially as tomorrow might be the only day this weekend to get her on the water until next weekend.

I keep saying her is this correct as it is actually a Mr????? (Mr Krabs) What do you think??


As always, the very best regards, and thanks for your help


Slim

[Xzessperated]

Slim,
I was about to send out a search party for you. Good to hear you have been busy on that boat. I think you are starting at the right end with the strut depth. I was talking to a guy on the mainland that runs one and he measured his while I was talking on the phone with him and it was 17 mm below the bottom of the hull to the bottom of the strut. That does not say it will work on our boats but it will be close.
It is good to hear you got the 2.4 Ghz radio. I was really worried about you using the other one after you told me what it was doing. You get the benefit of a shorter antenna also. 27 Mhz is prone to interference. I hope you get that battery problem sorted so you can test it at the weekend. Due to the fact that my boss forced free beer down my throat I never got to test mine but I had 2 fastest ever runs in both of my Xzess 2's. If it stops raining and the wind dies Mr Krabs will be on the water in the morning.
I think for every Mr. Krabs there should be at least three Mrs. Krabs.
Good luck,
Paddy

[slimshady]

LOL, love it Paddy Also free beer, hang on how long do the flights take to get to the other side of the world for me, would love to join you, and hope it’s hot for you this time of year, (we have just had snow, not good for the pond).

When I have time I will post some pic’s

Slim

[slimshady]

nuts!!!! ----- and Props

Hi Paddy - Yet another rubbish day at the pond with Mr Krabs

I have spent every free evening fixing Mr Krabs the last couple of weeks, ready for today. All systems fine so I performed a range check, all ok or better than ok so I sealed the hatches and launched Mr Krabs into the pond, I realised the rudder is not big enough and the turn fins should be installed, but what the hell at least I could do a couple of drags up and down to see how fast she is.

Whapper-dooo now I am impressed with this hull, she literally launched out of the water and skimmed across it at a speed of over 50 - 60MPH and climbing (est), only problem was the fact that I couldn’t see her properly due to the spray and ran out of lake. I am using less than adequate controls 14V Lipo, 60A ESC and 3300 motor, (so should be rubbish) – 42mm hydro tri prop

She bounced to the left and looked like she was leaving the water so I throttled down but the grub screws connecting the flexi drive to the motor coupling had given up so the prop and drive were left standing – or pulled out, leaving them to the pond monster, unfortunately now due to having a big hole in the stern she took on water and sank, again, the sponsons kept her a-float and it was nearly a case of taking my jeans off and exposing my boxers to the passing spectators or cunningly use a reel of electrical tape and a bit of rope. Fortunately this worked and Mr krabs is now drying out.

So far the electrical gear is now dry and working but I have not tried the ESC yet and not sure about the LiPo or the motor.

All good fun down the pond................................. Not

[Flying Scotsman]

Slim, be very carefull with electronics that got wet...batteries, servos and the ESC are very questionable. Do you use Corrosion X or simillar.

Douggie

[Xzessperated]

Hi Paddy - Yet another rubbish day at the pond with Mr Krabs
Whapper-dooo now I am impressed with this hull, she literally launched out of the water and skimmed across it at a speed of over 50 - 60MPH and climbing (est), only problem was the fact that I couldn’t see her properly due to the spray and ran out of lake. I am using less than adequate controls 14V Lipo, 60A ESC and 3300 motor, (so should be rubbish) – 42mm hydro tri prop
All good fun down the pond................................. Not

This is great. We have a Scotsman and an Australian helping an Englishman. That is what forums are all about.
What Douggie said is quite correct. After everything is dry make sure you give the electricals a good test on land.
Slim do you use one of these connectors for your motor?



It is called a flex/hex. If not I suggest you get one. Grind a flat on your motor shaft (a Dremel is good for this) so that the grub screw can rest on it when you do it up tightly. Use a product like Loctite 222 on the grub screw so it will not undo itself. Do the flex/hex up TIGHT.
Always grease the flex shaft.
I take a rubber dingy to the lake with me to retrieve my boats.
Well you had a bit of bad and some good luck. Now you have got to see the beast you are dealing with and you have to tame him/her. It all takes time. You learned you need a turn fin and a longer rudder so it shows you already understand some of your problems.
I had some good luck with mine today too but I will not go into that just yet.

[slimshady]

Dilemma

Hi Paddy, thanks for posting back, Yes some good luck, 50 – 60 MPH with my rubbish set up but a sink all the same, the pond I use is a boating lake, i.e. raised concrete walls all round. Only 3 in the UK from what I understand, hence problems if I want more than 150M of constant speed.

I enclose a photo of my transmission, you can see the grub screws used but I did not think about using thread lock to secure them and I probably did not tighten them up properly, thought all the force was upstream and didn’t think the stopping force would pull it out – good idea using thread lock – wish I had thought of it before the incident.

I now think the problem was going too fast then stopping causing the prop to pull the flexi shaft out into the jaws of the pond monster.

The dingy is a good idea on a proper lake but on this boating lake waders or a fishing a rod with a large hook works just as well.

Dilemma: I need waders to rescue mr krabs in the future and I need a new flexi shaft – if I get some waders I might find the shaft and the prop, what should I do? Shaft is about £12, prop is about £3 wader are about £30

Damage update:
Servo dry and now ok
ESC dry and now ok
Receiver dry and now ok
LiPo damaged – it charges and shows volts but the ESC will not run with it anymore

Thanks again for the advice

Slim

[egneg]

Yep - waders and a good magnet mounted to a pole (who knows what treasures will be found).

[Xzessperated]

I enclose a photo of my transmission, you can see the grub screws used but I did not think about using thread lock to secure them and I probably did not tighten them up properly, thought all the force was upstream and didn’t think the stopping force would pull it out – good idea using thread lock – wish I had thought of it before the incident.

Dilemma: I need waders to rescue mr krabs in the future and I need a new flexi shaft – if I get some waders I might find the shaft and the prop, what should I do? Shaft is about £12, prop is about £3 wader are about £30

Damage update:
LiPo damaged – it charges and shows volts but the ESC will not run with it anymore
Slim

Slim,
I used much the same connection at first until I lost 2 props and shafts. Have a look at the link below and you will see connectors with two screws holding the coupler tight on the motor shaft and 4 screw holding the flex cable even tighter!. One grub screw at each end is not enough IMO.

http://www.offshoreelectrics.com/products.php?cat=68

I have never lost a prop or shaft with a flex/hex. I have had 2 break but that is par for the course.

I heard Engineers earn more than that in an hour over your way. Buy all three unless you have Scottish ancestors. In that case just have a Scotch Whisky and think about it. I really do not think that story about Scotchmen being frugal with their money is true. Douggies boats must have cost him a small fortune and he wears a dress, ooops I mean a kilt.

[andym]

Douggie are going to put up with that!! You go get him mate

[Viking]

Lost shaft and prop

Hi Slim;
I had the same problem with loosing the shaft and prop.
When you let off the throttle completely and the motor stops, the water spins the prop with the momentum of the boat. And it spins it the other way ! which will unscrew the prop nut and you might luck out and just loose the prop.
Some locktite on the prop nut is good. some people use a short length of fuel tubing behind the prop, it won't unscrew. Thus saving the prop!
On the shaft problem, I use a wheel collar ( sane as the airplane guys use) on the flex shaft between the end of the stuffing tube and the motor coupling. Photo enclosed
Viking

[slimshady]

Waffling on and on and on and on.....

Thanks chaps all advice good as usual, the problem I have with my setup is the concentricity of the motor (not good) I have designed and made a bearing mount after the motor mount to reduce this, you can see it in the last photo. The bearing is an SKF rated at 80,000 RPM with the conventional grease removed in favour of a light lube (fanny batter as us engineers call it) to help the motor spin up freely and not vibrate the hull to bits. Using this system I cannot use a hex flex unit otherwise it will undo the work of the bearing unless I completely redesign the unit to work after the hex flex unit, but it will experience efficiency losses. I think this will compromise the stability of the bearing unit, although saying that looking at Vikings photos he has achieved this (by the way Viking what is the black char grilled mess in the bottom of your hull, is it the remains of a burnt out ESC or LiPo?

Sorry drivelling on as usual, the point is, currently I need the coupling through the bearing to reduce motor vibration. The coupling actually has a through hole for all the grub screws and is subsequently screwed from both sides, in this case 4 grub screws were used, 2 on the motor and 2 on the flexi. Not tight enough and no loctite used, perhaps I should drill and tap at 90 degrees so I can use 4 grub screws on the flexi. I must admit I did not tighten them as much as I could because I thought if I crush the flexi then I would not be able to remove it again opppps. The other option is to drill a hole at 90 degrees and solder / weld the flexi to the coupling, only problem, it is irreversible and I will not be able to remove the flexi and oil it.

The other thing I thought of was mounting a pin after the prop sitting at the centre point to stop this happening again i.e. if the flexi drive pulls out the pin after the prop will stop it from moving further down and save it from stripping out, it might damage the plastic prop but I would rather this than lose the flexi and sink the boat. This could however compromise speed due to drag.

I am not so keen on using the manufactures flat spot on the motor shaft to secure the grub screw due to excessive vibration caused by offset inertia, the motor only has one flat and therefore with 2, or even one grub the offset in weight of the grub screw at 40,000 – 60,000 RPM will vibrate the transmission to bits (I have tried and failed hence the bearing mount) – I must admit I haven’t calculated the inertia inefficiency but from what I have seen it must be big, and will eventually cause catastrophic failure.

Apparently according to Allen at Astec the rudder should be big enough but I need to drop it down a peg, so I will try this before buying another, I have also mounted the pull arm closer to the rudder and I am now considering using my long arm aluminium arm by grinding off one of the arms. My CF turn fins should also help.

I can get a new flexi but what next?


Sorry for waffling on and on and on and on......


Slim