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Thread: first scratch build at 1:16 scale motor questions

  1. #1

    Default first scratch build at 1:16 scale motor questions

    hello i'm new here and have built scratch before but static and now i would like to install an r/c setup in a pt 109 dimensions are 61 inches long by 16 wide and going to guess 24 to 28 lbs
    would like to run either dual or triple screw on it
    like ideas on either gas as in a 26 cc weed eater or 2
    and electric option with 2 or 3 screw and what would b a fair to good set up
    best screw ideas also welcome 2 or 3 blade
    and best way to insur plenty of air to get inside ... had an old lind kit 109 and had nothing but problems

    thank you in advance
    Mai

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    Welcome Mai you are in the right place to get good info. from many good guys here.As your boat is a scale one you don't need a fast combo but you might consider the size and weight of your boat.I undestood you meant with "screw" props Am I correct? Anyway for electric you will need a big motor with low kvs on 8 or 10S batts.For esc I have seen good results with the Sf 240 or maybe the 300A unit.I'm helping a friend now to build a 53"Cris Craft using a SSS 56104 motor with 500kv on 10S with a Sf 240A...batts from Revolectric.hope this gives you an idea from what to use with your boat.
    Gill
    GO FAST AND TURN RIGHT !
    www.grsboats.com.br

  3. #3

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    Mai to get the "scale" look then you will need at least two motors and more prototypically three props from memory. Twenty five years ago I radio control modded an old big Lindberg kit but alas used a 6 volt Dumas motor driving through a splitter gearbox (plastic gears) to two shafts with the then stock 1/8" driveshaft and stock props. It ran reasonably but never really did anything but raise a bow wave.

    For your boat as suggested at least one 56XX series motor, but in reality to get up on plane more likely at least two such motors with large capacity ESC's and batteries (at least 8s). I would not go with three drives as your drives no doubt would be submerged and cause considerable drag. Ideally, but not prototypical would be two 1/4" drive Speedmaster (or other brand) nitro stingers (or gas stingers if you went with gas motors) mounted on the transom. If you use the conventional submerged drive for that prototypical look then expect a lot of amperage and reduced performance.

    The prototype caused a lot of wake behind it so the stingers would not look out of place with the boat running although static would appear a little strange. You would need to decide between proto looks or practicality.

    Due the rather narrow confines of the prototype superstructure then if choosing a gas motor then perhaps an inline twin 58 cc model centre mounted with single drive. I feel that this would be the cheaper option as a twin or triple electric drive would be very, very expensive. That said I know little if anything at all about gas motors and their performance so take this info with a grain of salt.

    I always wanted to build a large PT-109 and the Dumas model would regularly tempt me. An Australian manufacturer has a large fibreglass PT boat and deck available (no superstructure) for a relatively cheap price so I am once again tempted (see link).

    https://www.gumtree.com.au/s-ad/beth...0mm/1176274452

  4. #4

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    thank you for the impute but comes to electric this is my first attempt at it so bear with stupid questions
    that is a nice looking fiberglass and roghly same size i'm working on but shipping would be insane for that
    i can run dual propellers easy enough and i can probably save some weight if i fiberglass and run kevlar stringers with ply formers and be more semi scale fun than serious competition, everything will be internal to the hull except the drive shafts and rudders
    now as for motors you mentioned a 56xx or sss 56104 motors and 8s or 10s batteries and 500kv could you explain a little more as sounds lik a better option to run on more choices of lakes
    thank you guys
    Mai

  5. #5

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    I made reference to a 56XX series of motors (the XX referring to the different length motors available and 56 the diameter minus the cooling jacket in millimetres). These are 56 series motors from many of the manufacturers. SSS, Leopard Hobby and TP Power all make decent motors. Check out the Motors section on the OSE store website for models available. Also Hobbyking do a T20 motor which is a 56 series with a choice of either 1280 kv or 730 kv depending on how you wire them, either delta (D wind) or star (Y wind). This motor is cheaper than the other brands and comes complete with cooling jacket plus all the wires, fittings and shrinkwrap needed. It is a version of the Proteus T20 motor (much more expensive). The Y wind 730 kv motor may be suitable on 10s batteries for you and work out less expensive than a motor from the other brands.

    When calculating the kv required you take as a starting point your desired rpm (nominally between 25,000 - 30,000) and divide by your battery voltage. For example, a 10s battery would be 10 times a nominal 3.7 volts per cell, so 37 volts, giving an unloaded rpm of 27,010 rpm. A loaded rpm, i.e., under load in the boat, would be around 15% less than this figure.

    An electric boat this size cannot be built on a budget as big motors require quality drive components and expensive ESC and batteries. A project I have onhold is a 60" Expresscraft Supercat. I have the two Hobbyking T20 motors, mounts and driveline components, but still need to get the ESC's. My son purchased the hull inexpensively and in the next couple of weeks I will be carrying out a carbon fibre inlay to reinforce the hull to take the power. In fact the hull is almost the least expensive part of the build.

  6. #6
    Join Date
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    Default

    What speeds and run times are you looking for?
    Also what is your budget?
    There are viable setups at several different investment levels.

    This info will help us offer some good setups that you could choose from.
    "Look good doin' it"
    See the fleet

  7. #7

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    One thing to consider is that this is submerged, not surface. I don't think the racing idea of 25k-35k rpm would really be necessary. Consider a 4 blade with rpm down in the teens.

    Also considering the type of boat, I think he should be focusing more on efficiency over power. I know it's a big boat, but to get 10min out of your scale boat would suck. He should have just a little more than enough to get on plane. I'm thinking low discharge high capacity batteries (think endurance multirotor).

    But as was stated, we need more info. One big factor would be desired prop size if you want to keep a certain scale aspect.


    That's not supposed to be any guiding advice, just something for everyone to think about.

  8. #8
    Join Date
    Aug 2008
    Location
    CO
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    5,758

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    Good call on the “how scale do you want this”.
    prop size limitations is a good variable to know going forward.
    "Look good doin' it"
    See the fleet

  9. #9

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    well basically like to get on plane plus a little extra not into racing but would put a couple braggarts to shame with their arf speed boats lol
    but i could go probably 300 usd give or take in a dual setup one month for 1 power system and another 300 following month but a 3 or 4 blade was already in my plans to start with so if i can plane under 25,000 as you mentioned that would be best i think and the prop looks like it will scale out around 3.75 to 3.85 give or take even fit a 4 inch if have to
    run times hmmm i guess 10 to 20 min max and if i have a 2nd set batteries then charge a set while running a 2nd
    so hop that helps in giving every one a good start
    eventually this project will be my brothers boy his 8th birthday gift lol in about 3 years
    Last edited by Mai_t; 06-14-2018 at 12:43 AM.

  10. #10

  11. #11

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    Mai,
    following are examples of four-bladed propellors, however, the scale sizes you mention I believe are far too big for electric propulsion and even for gas power. Even the ones linked to are too big for electric and more inline with what is used with a gas motor. These are just an example and at 1.7 pitch are rather agressive. I stand to be corrected but I believe around 60 - 62 mm is about the limit for a brushless motor. Perhaps sacrificing scale and opting for smaller diameter two or three bladed props would be more suitable to limit amp draw.


    https://www.ebay.com.au/itm/1-pair-p...0AAOSwbiFZOfjz

    https://www.ebay.com.au/itm/6717-4-B...UAAOSwPWRZOfl~

    Also I assume you will be using lipo batteries so your recharge time is too optimistic. Fast charging a lipo not only limits its life but to my mind is dangerous.

  12. #12

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    i do like those but dont post to the u.s. but i can look elsewhere and i dont mind running smaller since i can swap out when static and if i get say 20 min per battery set thats more than enough
    thank you

  13. #13

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    Quote Originally Posted by Old School View Post
    Mai,
    following are examples of four-bladed propellors, however, the scale sizes you mention I believe are far too big for electric propulsion and even for gas power. Even the ones linked to are too big for electric and more inline with what is used with a gas motor. These are just an example and at 1.7 pitch are rather agressive. I stand to be corrected but I believe around 60 - 62 mm is about the limit for a brushless motor. Perhaps sacrificing scale and opting for smaller diameter two or three bladed props would be more suitable to limit amp draw.


    https://www.ebay.com.au/itm/1-pair-p...0AAOSwbiFZOfjz

    https://www.ebay.com.au/itm/6717-4-B...UAAOSwPWRZOfl~

    Also I assume you will be using lipo batteries so your recharge time is too optimistic. Fast charging a lipo not only limits its life but to my mind is dangerous.
    I understand that they are just examples, but the props you linked wouldn't be my first second or fifth choice. Not only are those surface piercing props, but they are also very high pitch. Unfortunately I don't have a source for a more suitable prop.

    As for the size, there's always a motor big enough, but after a certain point, there comes price or power limitations or whatever. That's when gearboxes begin to shine.

    HTB1pC4hntzJ8KJjSspkq6zF7VXak.jpg

  14. #14

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    i'm not adverse to building a gear box either

  15. #15

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    Mai,
    My suggestion on props would be around 50-55 diameter and 1.2-1.4 pitch. For speed in a scale sense you would be flying at 35mph I would think. With 56 series motors you only need to go 92 mm in length, the longer cans will just draw more amps. You may get away with 40mm cans with the right props, = less cost! Seaking HV 130 ESC's will do the trick but you probably only need 6-8s as well. 8S gives just under 15K rpm unloaded, x 2 screws will be plenty of speed. Hobbyking have Multistar batteries in 4 & 6S at 20000mah. I use these in my endurance offshore boat for runtimes of 20 mins (max). The lower you get your amp draw the longer the runtimes. You will be looking to keep the amps down around 40-50 amps for decent runtime. These are all suggestions as unless you can find someone who has built a similar setup then you have no actual starting point. You will need to be prepared to test and try different props, Octura have a range of 1.2 and 1.3 props but you will probably have to settle for 3 blades, bearing in mind that the more blades equals more current draw, two blades may also be sufficient.
    NZMPBA 2013, 2016 Open Electric Champion. NZMPBA 2016 P Offshore Champion.
    2016 SUHA Q Sport Hydro Hi Points Champion.
    BOPMPBC Open Mono, Open Electric Champion.

  16. #16

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    thank you as thats pretty much what i needed to know for a reasonable starting point

  17. #17

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    Mai_t.
    I've built a few scale models in my time. You don't need as much power or revs as you might think to get a big PT boat up on the plane.
    Power required, is the original boats power, divided by the cube of the scale.
    So,
    3 x 1500hp Packard V12 engines = 4500hp.
    1:16 scale cubed is 4096.
    4500/4096=1.02hp.

    We usually add 25% for losses, but since you want to go a bit faster let's add 50%
    So that's 1.53 hp total.

    At 746watts per hp we end up with a total of 1141 watts.
    With 3 motors that's only about 380watts per motor.

    So 3 x 3674 size motors with a Seaking 60A ESC each will be more than enough. BUT. Getting them in a low enough KV rating will be the hard part.
    Even a 4074 size doesn't have a low enough kv that I can find.

    The correct props for the boat would be the scale props from Raboesch, Rivabo, or Prop Shop. The 50mm to 55mm 3 blade props only require around 7000rpm for best efficiency under load.
    http://www.loyalhannadockyard.com/PROPS.htm

    So lets allow for 10000rpm unloaded. (That may even be a bit much.)
    If you want to run 6s lipo, then a kv of around 450kv is required.
    On 4s a kv of around 675 kv would work.

    You may like to look at some outrunners with those kv ratings. Three of these will easily drive the boat.
    https://hobbyking.com/en_us/turnigy-...ner-motor.html

    Scroll through this build thread of Pat's. He is a master builder. The motor discussion starts on page 6
    He used 3 x 3674 1200kv motors on 8.4 volts with 45mm props. His model is only 58" not 61" like yours, but you get the picture.
    https://www.rcgroups.com/forums/show...-16-Elco-77-PT

    I hope this helps a bit.

  18. #18

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    thats awsome and what i was looking for thank you
    pat makes my poor boat look like something from the reject factory but i dont have all his fancy tools but if it floats and moves i'll be happy and our size differance is he is working off a 78' vs my 80'

  19. #19

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    Yes, Pat's skill & craftsmanship leave a lot of us wanting when it comes to fine scale modelling. When you have a bit of spare time do a search on R/C Groups for some of his other builds. They are truly inspiring.
    Just a note on the prop charts. I've always found the Raboesch "A" type 3 blade props to work very well. But at the bottom of the page there are some nice looking WWII 3 blade props from Prop Shop that might work well too. I've never tried them though.

  20. #20

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    ok great thank you

  21. #21

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    think i'll go with pats setup with the 3 blade and i dont mind being off scale that much so long as it um ah looks right :) and thank you for all that
    now to show my ignorance to radios i'm trying to find the proper charger for a futaba 7c transmitter with looks like a nicad 9.6 volt 1100 sries sanyo kr1100aau battery with 110ma 14-16 hr charge and looks like a 4 cell since i can feel 4 batteries in the pack and a futaba r606fs 6 channel 2.4 ghz fasst receiver and no idea which battery pack goes with that
    thank you if you could help
    Mai

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