Has anybody heard of, or have, a "rule of thumb" for the sizing of brushless ESCs? Am new to brushless motor powertrains and have been using ESCs that nominally exceed the current draw capacity of the motor. Have heard that they should substantially exceed the motor capacity and wonder, if that is the case, by what percentage?
Hi Doug, Certainly having fun and gaining more appreciation for the various drivetrain combinations available to us. My objective for this summer is to get a combination, using all three shafts, that works properly at all speeds. This coming winter will refine by using items such as you recommend. That item would help, so will keep the details on one side. The program would be assisted enormously if a supplier (located in Hong Kong) was able to meet back-order commitments!
Hi Doug, Am undecided how to use the centre shaft. Have had conflicting suggestions; one to use it for "boost", the other for maneuvering. Have components on order which will let me do either, once can install them think will try and see what works best. Rowen
Whilst waiting for the new motors and ESCs, reviewed videos of the vessel under power and noted that as the speed increases, the bow lifts towards a plane. However, as she gathers speed the transom flaps become effective, forcing the bow down in a cloud of spray. At this point the plane has been lost and the model becomes almost uncontrollable. Decided to temporarily ballast the hull to simulate the new motors and ESCs, then try to establish the optimum flap angle using just the centre propeller and shaft. This is the original 2838 brushless motor installation with a 30 mm propeller. With this simulated drivetrain it would also be an opportunity to determine the best battery locations for both 2 and 3S Li-Po batteries. Made up an angle template with a spirit level to get the correct deck inclination with the vessel floating at rest. From this located each type of battery statically - somewhere close to the mid-point of the hull. Which also seemed as good a place to start as any! Somewhere in the research for this model found a reference to the transom flap angle. This was at a 2 degree -ve (pointing downwards) angle. Installed the 2S battery and tried the model. The bow dug in at speed. Adjusted the flap to a straight and level position and tried again. The bow still wanted to dig in, but to a reduced extent. Readjusted the angle to 2 + ve and repeated. The bow now lifted so the forefoot just cleared the water and then remained in that position. Replaced the 2S battery with the 3S. The extra power obviously increased speed and the bow lifted slightly further. The spray was deflected by the chine rails and a level plane established. The conclusion is that the transom flap angle is critical to the correct planing of this model and that it should not be negative. Until the new motors and ESCs are fitted will leave the transom flap and battery locations as is. Once these components are installed, intend to repeat the test. Am confident that with some fine tuning the model can be now made to plane properly at a scale speed. Interesting to note that the model will just about plane with only one propeller operating – wonder what it will be like with all three?
Thanks Doug, Good suggestions. Have dropped a line to the Yorkshire Belle 2 organization and will let you know what transpires. From the picture the vessel looks very much like the one I sailed in too many years ago to want to describe! Rowen
Just been reading a book on the pleasure steamers that used to ply the Yorkshire coast from resorts such as Scarborough, Whitby and Bridlington. The Coronia and Yorkshire Belle bring back memories and am considering building a model of one. Has anybody any idea where I could find any plans?
Hi Doug - she does have navigation lights and a working radar. Did not turn either on for the pictures, was so please to see her sail! The stepper has a maximum capacity of 3 amps. As am using a lightly loaded 500 size motor all seems well. Must get around the checking properly though. Would not want try a higher current draw.
Water trials have been delayed by trying to resolve the challenges with my HMS Brave Borderer project drivetrain. As those efforts have been halted, waiting for new components, decided to complete the Teakwood water trails. First battery trails used 8 x 1.2 Ni-Mh cells is series, giving 9.6 volts. This was unsuccessful as the battery life was only minutes. Lesson from this is not to buy cheap Ni-Mh cells from an Oriental source. Tried 2 x 7.4 Ni-Mh packs is series giving 14.8 volts. Vessel was far too fast and throttle control poorly modulated. Was thinking of fitting a voltage reducer with this battery layout so could adjust the top speed. Have done this before and it works well. When searching in my box of bits found a voltage step up increaser. Never used this before so rewired the Ni-Mh packs to parallel and adjusted the output voltage to 10 volts to see how it worked. Result is a nicely performing model with an usable top speed potential for emergency, Duration of the first run comfortable exceeded 1 ½ hours, which am satisfied with. The cells are positioned around mid hull to supplement the fitted ballast. From the pictures the model has all the grace and style of the original vessel and sails well. In conclusion, a successful project that justifies the many hours spent in building and refining the model
Hi. Thanks for your comments. Before I started the BB did some canvassing of the net to find other builders. The drivetrain remarks were particularly interesting. The consensus seems to be that building three screws, as is scale and as I am determined to do, is the most complex and that for performance it is better using either single or two. Once deviating from scale bigger propellers also work better. I have rather limited my options with being determined to capture the original layout though. The vessel is being scratch built on a hull from MTBHulls, of which I am well pleased. The HK source is HobbyKing, often find their products are on backorder, but usually only take a few days to arrive. In this case have been advised it will be rather longer.
Doug, Thanks for the comments, you are probably right on all counts. The loose connection was more than likely caused by moving the wire to get it lined up once installed and the movement released it slightly. Rather like the terminal block idea though as it holds the wires in place, can now guard against looseness and corrosion by careful assembly. However, am mulling over the gold connectors suggestion, probably have some weeks to come to a conclusion! They are much lighter than the block too. The motors are on back order, am now resigned to waiting for them. It does give the opportunity to park the Brave and get some running time on my Teakwood, which has hardy moved this year. Rowen
Decided to retry with the 2S battery and the original scale style propellers. Concluded that the speed is fine, especially in the windy conditions encountered and in a small pool that limited acceleration. The model had a very flat plane, must adjust the transom flap angle to see what effect that has. The forefoot did not rise much from the water surface. Was frustrated by the “stutter” referred to in the last blog, noticed this occurred on the two out shafts only and when the starboard was operated after the port was running. Swopped the starboard motor over with the centre one to see what effect it would have. As started to remove the motor noted that a connector was not tight and that the screw had corroded. Exchanged motors, removed all connectors then cleaned and refitted using a water resistant lubricant. The stutter seemed cured. Another lesson learnt, when dealing with these high currents every connection is tested and all defects exposed. The opportunity to retest using a 3S battery arose so installed it, all worked fine on the bank. Put the model in the water and a major short occurred. 2 fuses blew and about 6” of wiring melted and burnt through the insulation. At least there was no hull damage! Did an inquest and, apart from the damage described, also found the starboard ESC and motor had failed. These were the ones where the “stutter” originated, but cannot see any correlation between the two problems. Discussed the model with some of fellow scale modelers and concluded that the 4500kV motors are unsuitable for the scale propellers used. Every suggestion points toward motors in the 1 – 2000kV range. As now needed to obtain a new motor and ESC, decided to reequip both outer shafts with 2000kv motors and water cooled ESCs. Felt modifying these outer shafts would allow assessment of this new drivetrain combination, could then decide what approach to take with the centre shaft. Due to the mounting and driveshaft arrangement, the choice of motors was restricted to 28mm O/D with a 1/8” shaft size. Unfortunately, suitable items are on back-order from Hong Kong, so there will be no further updates for a while.
Now the spray has settled have assessed these first tests; have also reviewed various pictures and U Tube videos of the Brave and Pekasas in operation. The actual vessels look to plane rather like mine, whilst some model bows lift up until a significant length of keel is exposed. Anyway, have been able to draw some conclusions: 1) Moved the battery towards the stern and, at speed, the forefoot lifts slightly clear of the water. The plane is now almost flat. The battery is not well positioned when near the bow. 2) The 2S battery used was a 4000mAh 30C; suspect this battery does not have the capacity to operate the model. Every motor will run up smoothly until a second one is operated. The first motor then “stutters” and a fuse might blow, this could be indicative of a power surge. Any comments from the electronic experts among the group would be appreciated. 3) The 3S battery was 10,500mAh and 40C; with this battery all three motors can be run at full speed together and fuses do not blow. It was also very heavy at 1700g, holding the model down. 4) The motors are 4500 kV. On refection, think a slower motor around, perhaps 2000 kV would have been a better choice. 5) Would concur with comments by others that a simple single or two bladed propeller layout for this model is probably best - that is unless you want to capture the true scale layout. The centre propeller seems to have little effect on overall performance, although it will power the model quite nicely when operating by itself. Have had several suggestions about how best to use the centre propeller. Will think about them and decide later how to do this when I start to finish the model. 6) The 2 blade Hi – speed propellers both increased performance and current draw. The model is more than fast enough with the original scale layout. 7) Will purchase a lighter, 3 S battery as that seems the best choice for performance and weight. 8) Testing using the bare hull with a minimum of detail worked well. For a models with a complex power train, this is a good approach as access to the internals can be gained easily. Nothing worse that finishing a boat carefully just to find the performance disappointing. Then having to to rip it apart to make major modifications or adjustments!
itted 2 x two bladed 35 mm “hi-speed” propellers to the outer shafts only as these are the easiest to change. Can also use the centre shaft measurements as a check of the previous figures as it is unchanged. With these propellers the current draw and bollard pull both increased. Subjectively, think she was also slightly faster, but the speed exceeds scale speed anyway. The increased load on one of the 2 bladed props wiped the blades off and several 30 A fuses on various motor circuits blew. Originally, the battery was fitted as far forward as possible to hold the bow down – some thing it seems to have achieved! Decided to remove the forward battery location frame and replace it with one which will allow the battery to be positioned anywhere between the bow and the centre of the model. The battery can now be located where the best plane is achieved. Once the correct battery is fitted the final location will be determined. This frame movement will also allow adjustments for any weight gained during final finishing. Whilst the idea of using a load cell and ammeter/wattmeter to measure bollard pull and motors loads sounds logical, it is fraught with challenges. The vessel both bucks and the readings fluctuate wildly under load making getting steady, consistent results difficult. Off now to cogitate over the results and decide a path forward.
Looks like everything is set for the first open water test. Sun is shining, ice has gone and water smooth. Intention is to start the open water test program with a repeat of the pool test, except this time with everything wired correctly; the load cell positioned so the “pull” is more horizontal and ballast available to hold the propellers underwater if necessary. Hope these improvements help reading stability. To modify the “pull” arrangements, wrapped a light cord around the propeller shaft struts and fed the loose end above the transom shelf and out over the stern. The load cell was hooked into this and then tied to a fixed grating on the pond side. Started by measuring the electrical requirements for each of the three motors and the propeller bollard pull, using the 2 S battery. Found the bollard pull was up slightly at almost 3 lbs per propeller. Probably because they were now held at a greater depth in the water. Also blew several 20A fuses, so fitted 30, which seem to work. A series of runs showed adequate performance with plenty of spray, although the bow did not lift much onto the plane. The forefoot did raise almost above the water surface. Then tried a 3S battery. Although this was much heavier, the performance improved dramatically. The bollard pull was up to almost 18 lbs per shaft. The bow still did not lift much to a plane, although the forefoot was almost clear of the water at full speed. The battery was located just back from the bow, so it is suspected that it held the bow down. The impact of the transom flap down angle could also hold the bow down, but have decided to leave as is for the time being and avoid the temptation of making too many adjustment at once. Whilst it is still too early to draw definite conclusions, it seems as if a 3S battery will be required. The model sustained some slight damage due to the test arrangements, so will repair that and also fit the 2 bladed Hi Speed propellers. Will then repeat the program and report. Should be able to draw some definite conclusions then on the best power train. Neither of the batteries used, neither the 2 S nor the 3S are ones I would choose for this model. As a result the capacities and weights are not ideal. That must also be remembered in future deliberations.
From the brief pool test, had decided that the motors could be susceptible to overheating, so connected up the water jacket cooling system and powered it with a small pump. Did not leave enough space to fit a scoop behind a propeller anyway, but prefer the positive action of a pump though. From feeling the ESCs, was also concerned they could overheat within a confined space such as the hull. Mounted a couple of small fans in a bridge structure above the ESCs, along with the ESC switches. Not sure either of these cooling modifications are really required, but erred on the side of caution. Final weight of the hull, with all electrics (apart from battery) comes to 5.05 lbs. Looks like will not achieve the target weight of 6 lbs, but am hopeful will be able to get close to it.. Built the deck up with gun mount bases and a removable decking over the engine area. This limits access to the internals; so will not fit it permanently until the test program is complete and all modifications incorporated. Have now reached a point where any further work will be to start finishing the model, unless drivetrain modifications are required. Have thus decided to leave it until after the first open water test date. This will be in late May as am away until then.
Thanks Doug. Always thought, my old mathematics text books would come in useful! Was hoping for was a simple chart of speed vs thrust, recognizing there are many factors that influence them. Even a max speed and efficiency rating would help. R
Does anybody have any performance charts or operating information for Raboesch propellers? Am looking for the rpm vs thrust relationship for the under 45m/m 3 blade sizes particularly, but anything would help. My traditional supplier cannot help and neither does the web site. Am trying to plan a future models and would like to be rather more scientific than the "if it fits it will work" approach or just using the motor O.D. as a guide to the propeller diameter.
An unexpected opportunity arose to try the unfinished hull in a small pool. Whilst the performance envelope could not be explored, was able to try and measure operating parameters and get a “feel” for the model. Used an electronic scale and a combination voltmeter/ammeter/wattmeter to measure propeller thrust /bollard pull and motor power requirements. If it is necessary to fit different drivetrain components, or a 3S cell this will serve as the baseline. The model floated levelly and well above the waterline. At about 8 volts the motors drew around 20 amps each at full speed; so only about 35% of the potential output capacity was being used. Tested each motor individually and measured the bollard pull at just over 2 lbs. A considerable amount of spray and wash was created making stable readings difficult. For further testing, will add ballast at the stern to hold the propellers further underwater. Should help reading stability. Currently using 20 A fuses; which as one failed seem marginal. For sustained use think 25 or 30 Amp better. With these high-speed, low torque motors establishing the “dry” propeller rotation is deceptive. Found one motor to be reversed! Nevertheless, the model accelerates quickly and is sensitive to engine speed movements. Left the pool with a list of modifications to make before assessing the installation properly on an adequate body of water. Some conclusions can be made though. If it is necessary to add a second cell this needs to be located around midships, not in the bow or stern. Still hoping a 3S cell will not be necessary and that 2S may be adequate. The suggestion to do testing using the bare hull with a minimum of detail was a good one. For a models with a sophisticated power train think this is a good approach. Nothing worse that finishing a boat just to find the performance disappointing, then have to to rip it apart to make major modifications or adjustments!
Whilst waiting for the ice to melt, decided to make up the deck and transom flaps. The deck was made from styrene sheet, again for lightness. Made the deck beams out of square styrene sections to avoid traditional, heavy, full width bulkheads. Hoped the stiff MTBH hull would resist twisting without bulkheads. First impressions are that this is the case and when the deck is finally bonded to the hull, should be even better.. The transom flap was made from thin aluminium plate and added simulated stiffener ribs in styrene. Understand that about a 2 degree flap down inclination works best on this model. My original plan was to operate the flap using a servo with another radio channel, however once the best plane is achieved it is unlikely the flaps will need further adjustment. Unlike the real vessel, the operating weight will remain fairly constant. So, abandoned the servo idea to use adjustable bottle-screws instead. The flap angle can still be adjusted, but not in motion. These screws are much simpler, lighter and cheaper than a servo. One challenge was to make the very small hinges required for an adjustable flap. After much thinking and investigation, decided the simplest and neatest way would be to use thin, self adhesive aluminium tape, as used on forced air heating ducts. Would stick the self adhesive surface to the underside of the flap and then onto the inside face of another thin aluminium sheet, which could then be fitted to the transom using double sided tape and small screws. This seems to work so far, it also avoids drilling through holes into the transom .