Model Boat Mayhem
The Shipyard ( Dry Dock ): Builds & Questions => Pleasure boats, Sports, Race, Power and Leisure Boats: => Topic started by: Dave J on September 18, 2016, 10:01:36 pm
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I have been running a KING OF SHAVES catamaran model which when on full power does not respond to the rudder unless the power is reduced. I increased the rudder width which gave improvement and also added two side rudders, but it will not turn until the power is off, I wondered how can I resolve this problem?
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Dave....a little difficult to see from the images, but it appears that the outer rudder blades are knife profiled on the leading edge and outboard side only
You note that the main [central] blade been extended in length [fore & aft]
Again, very difficult to see, but is the depth of the central rudder blade as manufactured & supplied?......the image from the WEB site questions this
When we think of the dynamics of the movement of the water, the original central rudder blade is the only one in direct flow of the accelerated water from the propeller. The outer two are only creating an angular resistance to the speed of the hull through the water
From studies too many years ago, & if the memory bank is in gear.....the issue may be too much turbulence at high RPM, and the propeller is being subjected to & causing cavitation so hence the lack of steerage at high RPM...... another part of the equation is the distance of the rudder blade from the propeller & to complicate things further many high speed drives have the rudder blade offset to the axis of the propeller [relative to direction of the drive hand of rotation]
I am sure one of the high speed drive members will offer a more concise answer
Derek
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Derek,
Thank for your comments, the main rudder has only been extended on the back edge the length as original. I agree the cavitation from the propeller must be part of the problem, I could try and increase the the distance between the prop and the rudder. I have also been looking at the underwater stern recess distance to the stern hull end, could that influence the problem? On the plane this area would be above the water?. Do other catamaran hulls have this turning problem?
Dave
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A cat only needs one rudder,sounds to me like your servo or pushrods are not doing their job properly ?,if anything at speed a boat needs much less rudder input to turn than when running slow.Its possible with the central rudder you are putting a lot of strain on your servo and it may not have the power to do the job,most people don't use rudders directly behind the prop on surface drive boats especially when using large props,they cause too much prop wash for the rudder to work well .Adding the two outer rudders isnt the solution here,you need a high torque servo and good strong pushrods if sticking with the original rudder in centre. .
My advice would be try a stronger servo on the central rudder and remove the others and if this doesn't work then go with one single offset rudder on the right like most use,then you wont have any problems turning either way and your boat will gain some speed also as the rudder behind the prop will slow you down more than you realise.
Also you have too much up angle on your prop that isn't helping either,set it so it is level so all the thrust is directed where it needs to go rather than up in the air.It also looks like its quite high up ? when the boats sat on a flat surface with the rudders hanging over the end set the strut so the bottom of the strut hub is level with it then raise it maybe 1/8" and you should find it runs better there.
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Thank you for the suggestions, I will try and re position the rudder along side the prop. looking at a side view of your system is the rudder in front, level or behind the prop, which would you suggest as the best position? I will change the servo and upgrade the pushrod setup. I can fit the original rudder bracket to the rear transom but depending on position I may have to add a suitable distance piece.
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On a cat most people seem to go alongside the prop some just behind but some cats like them sometimes closer to the transom ,having it too far back on a cat can sometimes create strange handling .Pic isnt of my boat but a customers as i build these cats.
Heres another pic of this same style cat its a Thunderbolt but a similar setup should work on most cats.Personally i like it as in this pic it seems to give the best results on a cat .
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Can I ask why it has the 3 rudders?
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The hull originally had one rudder positioned behind the prop which would not respond to the tx stick with power on, I then added 15mm to the back of the rudder which did not have any effect. (unless the throttle was low and the hull was off plane and without thrust.) Thoughts along the lines that it also needed side control outside the central thrust line led to fitting the two linked side rudders.
This rudder arrangement was a experiment which did not correct the problem, it did give more movement but was not as successful as I expected. That is the background to my Question. I have had good advice and I am now in the process of fitting one side rudder and different servo and I hope getting the balance right.
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Dave just as heads up you need a metal geared servo of at least 12-13 kg torque,normal sized will do just so long as it has enough torque,the ones i use are 16 kg and i use them on my 58" Apaches and the 50" cat .
Hope this solves your problem for you .
Another thing to check is possible signal interference ,are you using a resistor plug on the engine ? ie number should begin with an R this stops any chance the engine interferes with your radio signal . Might also be worth a check to rule out that possibility .
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I have just ordered a Hitec servo HS-5585MH 17kg standard size, the one in the hull at present is a Futaba S3003. the Zenoah engine has a Champion plug RZ7C.
I have been using Futaba radio 2.4 and have not experienced any electrical interference up to now.
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Hi Dave while the Champion plug is a resistor plug they are renowned for breaking and parts going inside and destroying engines so please throw it away and get yourself an NGK plug .Glad i mentioned it now as id hate to see you ruin a good motor,its the first part most of us throw away when we by a zenoah engine .
http://www.ebay.co.uk/itm/NGK-CMR7H-3066-Standard-Spark-Plug-Replaces-RZ7C-/361558368234?hash=item542e8edfea:g:-qUAAOSwdV1XNyk8 (http://www.ebay.co.uk/itm/NGK-CMR7H-3066-Standard-Spark-Plug-Replaces-RZ7C-/361558368234?hash=item542e8edfea:g:-qUAAOSwdV1XNyk8)
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I have not had a problem with the RZ7C plug but I will change it for the type you advised.
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Hi Dave,
Just seen your article and l think that the issue has already been answered by other contributors, but l would have said straight away that the servo isn't power full enough and you are stalling it. I have experienced the same issue with a mono hull and l was advised that you need a servo with at least 20kg thrust so you may find that the new Futaba servo that you have ordered may still be marginal. I am now using a Trackstar servo from HK running on a 2 cell lipo which for £28 gives you 30kg of thrust. I made life complicated for myself as although the RX can also work on high voltage the throttle servo was a standard one and l fitted a MKS in-line BEC unit to reduce the voltage to this one servo.
I hope that my advice will resolve your turning issues.
Michael
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Dave,
Make sure that there isn't any flex in the pushrod and use a 3mm pushrod.
Michael
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Ive been using 16 kg servos in my 58" Apaches without any issues but more kg wont hurt ,that said a cat doesnt need much to turn it like a large mono hull does and much less rudder throws unless you want to spin out a lot lol .17 kg should be plenty,the one he has in it only has a tad over 4 kg torque so theres the culprit right there. .
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The rudders are now off the hull and I do intend to upgrade the servo and pushrod etc as suggested. When I look at the design of the stern of this hull (see picture oo2 on first posting) the last step on the cat hulls is at least 8 inches from the original rudder position, the hulls on other cats appear to have the last step nearer to the rudder, although the design can not be changed could this also effect the steering?
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Yes the sponson transom is well short of where your prop is at so this can and will effect things but that said there are a few of these cats out there and while they aren't the best handling of cats it should still turn with a decent servo etc just not sharply .For comparison here's the bottom of one of my Thunderbolt hulls .Prop is much closer to the sponson transoms than on yours as you say.
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I have now re positioned the steering post bracket to the right hand side. I had to fit a 5mm thick Ali plate to the inside of the hull to strengthen the transom this was then drilled and tapped to support the steering post. With the transom not being square I also had to put a tapered distance piece in to position the rudder so the leading edge will be at 90 degrees to the hull. ( The rudder still has the distance piece added) The push rod and servo have still to be positioned.
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Should work once you get your new servo and pushrods fitted,one thing ill say is try and level your prop angle so theres no up angle,if anything on a cat you might need a tad down to keep her on the water at speed as cats create a lot of lift but start level .
Is the prop sharpened and balanced ? if not id try and do that if you know how ? or buy one thats done for you.
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At present the servo has not arrived and the linkage will be on hold until I can line it up correctly. I will reline up the prop to the level position and I had balanced it for earlier runs.
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Another good idea at some point would be to get a brass prop tube instead of just the teflon tube to avoid any chance of the cable whipping and destroying your hardware or damaging the motor .I see that setup on a lot of these Chinese rtr boats and its just not the best idea to be honest .
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Another good point, at present the brass outer tube only goes from inside the hull by the coupling through to the entry out of the hull. I have enough thin wall brass tubing the correct o/d to cover the teflon tube from the hull outlet to the prop skeg. The tube will need a slight bend in it, what would be the best way to bend it to suit the prop angle etc?
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Hi Dave all i do is install the flex into the brass tube or a spare one if you have one ? then bend it by hand using gentle thumb pressure the flex should stop it from kinking but just do it carefully and you should be fine.Wondering if you do it as you suggest that the tube has somewhere to lock into ie will it fit inside the other brass tube or is it the same diam as it needs something to hold it from been able to move .Might be best to redo the whole thing in one piece at a later date ?.What i do is once im happy with the bend of my tube is i take the next size up cut a 4" piece and use that as the through hull fitting,i then epoxy this in place using some plumbers epoxy putty,this way if you ever need the longer tube out it will slide right out.
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Mart, I have decided to replace the complete tube, the tube I have is 11mm OD and is a close fit over the teflon liner. The tube will not move it is rigid so I have tried to soften it with heat buI have not tried to bend it yet. I have also tried to drift out the tube in the hull but that was not successful, I will have to remove some of the fibre glass under the radio box inside the hull to release it. I could not fit a smaller dia tube at the prop skeg end because the teflon is a neat fit, the tube should give enough strength when supported at the hull end through to the inside.
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You should be able to loosen the tube from the hull by dremeling away some of the material holding it in place once youve done that give it a light tap with a hammer and a block of wood and it should come out , dont go too mad as you dont want to damage the hull as these hulls are quite thin as you know. As regards the strut if you get 5/16 od brass tube and do away with the liner the tube should fit into the strut as mine do ,might need to use some emery cloth on the end of the tube to let it slide in but it should work.You dont need a liner just use plenty of grease and your good to go.
http://www.ebay.co.uk/itm/STUFFING-TUBE-for-36-x-1-4-6-35mm-flexishaft-rc-model-boat-brass-8mm-/391244889514 (http://www.ebay.co.uk/itm/STUFFING-TUBE-for-36-x-1-4-6-35mm-flexishaft-rc-model-boat-brass-8mm-/391244889514)
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Mart I have removed the tube from the hull, 11mm i.D. which was made from a thick wall aluminium this meant my tube needed an additional outer tube to make up the difference. This was alright because it gave extra support in the hull. I had spoken to a Coppersmith friend who told me after softening the brass tube, block off the end with a wood dowel then to fill the tube with fine dry sand (See pic 018) compress it and after sealing the end off, the thin wall tube can be bent without the tube diameter being distorted. This I did and the bent tube now lines up between the hull and skeg. (see pics) I thought you meant to retain the teflon liner and I have put it inside the new brass tube, The skeg has two bearings inside and with the wall being thick enough (on the skeg) I have been able to bore it out to suit the new tube. Now I have got it lined up and needs fixing in the hull (pic 021). What started out as minor mod has turned into big overhaul, but should cover for any shaft problems.
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Looks a lot better than it was mate ,usually we only use thin walled brass but at least yours will be a lot stronger than it was so its all good :D .
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The shaft assembly has now been fibre glassed inside the hull and on the outside the tube has been profiled with P38. The skeg end has enough movement to allow for adjustment of the prop angle. With this section nearly complete I can now go back to the rudder arrangement. I have got the new higher torque servo now and with a heavier gauge push rod it should make a difference. When I had the three rudders linked together, I think the servo must have stalled and been ineffective with the water pressure.
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Nice job :-)) one thing i notice is you dont have much of a gap between drive dog and strut,you need at least 1/8" to allow for cable shrinkage under load when using a round collet that grips the shaft .Also at the other end put a small zip tie round the flex between tube and collet and trim it off this will save your shaft and prop in case the collet ever lets go.
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Thanks Mart that is good point, I will make sure I leave a gap between the Drive dog and strut. On this shaft arrangement the prop shaft has a shoulder which would prevent it coming out of the strut and if the flex drive came loose at the drive end it should be recoverable. When I take the flex out I have to take the prop assembly and strut off and draw the flex out of the tube, then I can dismantle the the prop drive dog etc.
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Sounds like it was originally set up for a square ended flex and collet with those you dont need the gap as the flex floats if you get me .If using the round collet that grips the flex then the gap is needed or else things bind up and thats when things can get ugly .
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I do not know what the original assembly arrangement was, but like you mentioned the early flex drives on the electric boats I came across used the square ended flex and did float in the squared socket end of the drive shaft. I have taken a photo of the dismantled assembly on this boat and you can see the flex shaft 6mm is brazed to the 6mm stub shaft which is then stepped down to the 5mm section of the shaft which is the i/D of the two internal bearings. I understood the original prop was two bladed and had a bore to suit the stub shaft 5mm. The three bladed prop I have on has a 6mm I/D and has a brass tube distance piece. when assembled. I will still make sure the flex shaft has clearance between collet and the strut body which should allow it to move
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If it were me id do away with that strut and shaft and get a new 1/4 "flex cable with welded shaft with drive dog but thats just me :} .Id get a skegged strut too as that will also help the cat in the turns.
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If I have to change the present drive arrangement i would go for the system you have. I did have a similar arrangement on another boat and the weld failed on the joint and I lost the stub shaft and propeller. The replacement shaft (See photo) is loose in the strut and the plain bearing? must have failed? and together with the strut will need replacing. If the joint fails on the present system on the flex to stub shaft I will still have the prop.
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Back on the steering, the radio box with the new steering servo is back in the hull and fitted with a 2.65mm dia push rod. The push rod connection on the rudder arm had to be changed to take the push rod dia. The steering system is rigid now and I do not think it will flex under load.
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October-November, I have had a number of runs now with the new set up and the boat turns a lot better to the right as expected. As suggested the main problem was the servo, lack of kg thrust. I also had cooling problems I cracked the cylinder block through lack of coolant and had to replace the barrel, the piston etc was ok. I have now replaced the single water supply rudder with a double one and separated the water jacket and manifold to two individual supplies which now leave the side of the hull where I can observe them.
Attached pictures show the improved turn.
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Well done Dave,
You are now making good progress gradually sorting out the issues as they develop.
Keep going in the right direction but make sure that you keep the engine cool enough.
Regards,
Michael
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Hi Dave sorry Ive not been around lately had a few health issues going on and a lot of boats to build so its been hectic .Good to hear the steering is now sorted.
As regards your cooling setup be sure to go in at the bottom of the cylinder and out the top this avoids getting any air which will create a hot spot and cause big problems.
I always use separate cooling for the motor and the exhaust,i go in the cylinder and out into the cooling flange then out of the boat and the tuned pipe is completely separate .
Mart
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I have been using the supply from the rudder to supply separately the water jacket and the manifold, the vertical transom tube to supply the tuned pipe. On the water on the right hand turns no water comes out until the boat is on the straight, I have now tried to extend and modify the rear supply tube as per picture and use this supply for the water jacket and use the two supplies from the rudder for the manifold and the exhaust pipe. (these connections as you mentioned Mart to the lower input stubs).
I also found the hull difficult to handle (15" wide and 6" deep) and is awkward to transport, I had a stand made of plastic tube but this came apart and i accidentally let the boat fall. Now making use of a light weight luggage folding trolley and a old walking frame I have now a have trolley/stand which I can store the hull in the upright position. The pictures show the basic parts, the finished trolley with protected pipe insulation and the use of thick cushion foam on the end, the boat on the trolley and also in the vertical position.
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Im looking at that pickup tube and thinking thats going to cause lots of drag and could effect handling ?.I only ever use the rudder pickups on my boats,sometimes i open the holes a tad and always use large diam tubing 4 mm int diam not sure what your using now mate .
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I use a round file to extend the inlet holes on the rudder right up to the leading edge.
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I have been using 4mm bore tubing, but I had not thought of opening up the lead holes on the rudder, a good idea.
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You could extend the hull back to reduce the length of the step it needs to be about 1/2 the length/ I did this on a boat that wouldn't plane but it did afterwards . Good luck John
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John,
Do you mean add trim tabs? the hull appears to plane alright the prop angle makes a big difference to the way the hull planes.
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I meant build the bottom further back. But no matter if all OK {-)
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With the changes to the water pick up, I have had a few good runs on the lake the spray made the water outlets difficult to see clearly but the engine appeared to be cool enough. Then I hit a submerged item, (a branch may be?) the rudder was kicked back but I was able to bring the boat back to the jetty. I re-positioned the rudder and re launched the boat after half a lap the power became reduced and the boat stopped. When I eventually got it back to the bank I noticed the engine was still very hot-- no cooling, I had not examined my curved inlet when I hit the submerged item only the rudder. (the tube had bent back kinked the tube which closed the water supply off) The cylinder barrel was cracked and had to be replaced. As previously suggested I have opened out the two holes in the rudder which now supplies the water jacket and manifold. The curved water inlet has been replaced with the original straight tube. I have had a run today with the rebuilt engine and looks good for further sailing when the weather improves (bitterly cold and windy today)
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Sorry to hear about your mishap :D ,regarding your cooling the two in the rudder should be enough,i use one for the motor and header then out and the other just cools the pipe . Merry Christmas ;)