What thought process do you guys use to determine which motor size, prop size and prop pitch. How do those various factors impact performance of the ship, be it efficiency, acceleration or turning? There's a lot of threads out there asking what motor/prop should I use in ship "X", but not many that I could see that talk about how one goes about figuring it out yourself.
There are lots of ships already built that are going to be the same or close to the same as the one you want. Copy what is working best for others. In general ships have prop and motor set ups like this: DD, small convoys, 7lb CLs 1" 3 blade with a 380 motor and direct drive "heavy" CLs, CAs and most other convoys 1 1/4" 3 blade with a 380 motor and direct drive. the real big CAs like Des Moines, some small class 5 ships and most class 4 BCs 1 1/2" 4 blade with a 550 motor and a gear box. Almost all class 5, larger class 4 ships, most class 6 ships 1 3/4" 4 blade with a gear box. Those monster class 6, 7 & 8 2" 4 blade with a 550 or 650 motor and gear drive ships with a single prop, like Bismark 2 1/4" 4 or 5 blade and a 650 motor with a gear box. Putting a direct drive on ships larger then 12-13lbs does not work well. Even if you use an ESC you need the extra tourque the gears create for starting and stoping.
Right, copying works, but stifles innovation. What I'm curious about is WHY to chose a three blade prop instead of a four blade prop. Why to choose coarse vs fine pitch. How does shaft rpm impact acceleration? What's the importance of torque.
For smaller ships with direct drive I like 3 blade props because a 4 blade overloads the motor too much. For larger ships with a gear box the motors can handle a 4 blade prop. We are capped with our speed by our rules. You'll note the speed boat guys only use 2 blade props with a very high pitch. They do that because the more blades you have the more "solid" the prop is at high speed, creating more drag. The high pitch gives you a higher top end speed. We don't need speed so for us more blades are better. More blades creates better acceleration and deceleration. The extra blades create more drag, slowing your ship down at top speed. The lower pitch has less slip at a start or stop giving you more take off power. If more blades are better why not put 12 blades on a prop. The more blades you have the harder your motor has to work to move them. In the experimenting that has gone on over the years, there has been a ton of it. 4 blades seams to work best. The smaller cruisers get 3 blades because the motors can't take the 4th blade. I base the size of prop on the size of the rudder and the size of the ship. Heavy ships need a bigger prop to move around, they also can take the weight of bigger motors, gear boxes and batteries. If you put too big of a prop on a ship your rudder(s) will not cover it and you'll not turn as well. The shape of the prop also makes a big differance. Props like the speed boats use are only made to go forward, using them will give you very bad acceleration and speed in one direction. Props with a little direction to them, like the old Dumas ones or some of the Prop Shop types are nice. You can put them on backwards and move well forward and have a little extra going backwards. Most people use the bidirectional props like BC sells. The work the same in both directions, we battle lot in reverse so this works well for us. Higher RMP gives you better acceleration and throws more water over the rudders helping you turn. BUT too many RMPs means your motors are working too hard. In 07 at NATS one of the locals could not get to speed, he increased the gear teeth and started to get slower. I had him run his prop out of the water then put it in the water. You could hear the RMPs drop way down. The motors did not have the tourgue to drive the props. We changed gears to less teeth and made speed. RPMs and torque are inverted. More RMP, less torque. More torque less RPM. You need to find the balance where you make speed and your motors are not over loaded. Motors are not all the same either. Some have more torgue at the same RMP as another motor of the same size. Anything else I missed?
Good answer! You just never know when a better solution might be found, if nobody ever looks for one! I work in Big Gun, so I have a little different set of constraints to deal with. These may or may not apply to you, but since it's the thought process you're interested in rather than the "party line", I'll share some of my methods. Prop diameter, location, & blade count are dependent on the original, 1:1 ship. If the original had 3 blades, that's what I get. If the original had 4 blades, I get 4. An Iowa-class gets 2 @ 4 blades & 2 @ 5 blades, etc. Blade shape is up to me, but I don't usually get much choice so I go with what I can get. I am allowed to go oversize by a %-age, but I usually choose not to do so. Oversized props usually won't clear the hull in their scale location, & to get them to "fit" in all respects except distance from the hull requires some wacky shaft angles. Anyway, getting to legal speed isn't difficult even with 100% scale-sized props. I also have certain self-imposed constraints that I like to follow, just to keep my equipment consistent from 1 ship to another. This allows me to use the same batteries, motors, etc. for everything & only keep a few parts on hand. I'm also very familiar with all of my common components & can put together a good system pretty quickly & easily. The motors have a RPM where they're most efficient & happy. I try to set up so that they're running at that speed when the ship is at max. speed, so that becomes more-or-less a constant, too. So, the basic variables I have to deal with are rather few, which is a good thing. Getting the ship's speed where it needs to be from a motor turning at a defined speed, is a matter of drive ratio. It's the same thing in your car, where the objective might be to cruise along happily at 65 MPH with the motor humming along comfortably at 2000 RPM. Instead, maybe I've got a ship that needs to go 100' in 34 sec., with motors turning 12,000 RPM. After doing it a number of times, I've managed to get to the point where I can "eyeball" a setup & get remarkably close. I know that doesn't necessarily help you much. I got there mostly by trial & error, but the basic goals were to get Ship X to travel at Speed Y, with motors turning at Z RPM. I generally prefer a shallow (fine?) pitch over a steep (coarse?) pitch. A steep pitch at 10,000 RPM is going to produce a lot of violence at the business end, about 2" below the surface. The combination of high speed rotation & steep pitch is going to mix a lot of air in with the water, & that's something that the props are going to have trouble grabbing. Technically, this is not cavitation; cavitation occurs when the blades literally "pull away" from the water, creating bubbles of vacuum. The blades just can't get any bite on vacuum, so efficiency goes way down. True cavitation isn't going to happen so close to the center, with props this small, however the effect of churning up a frothy mix of air & water has the same practical effect. So, I choose a shallow-pitched propeller, so that I can turn them faster without whipping water & air together into a froth that the props can't grab hold of. The general principle is just like the need for car tires to grip the pavement. If your wheels are spinning, you won't go anywhere. Look at what happens to a drag racer when the tires lose their grip. Same thing off-road in mud, snow, ice, etc. If your tires lose their grip, you're stuck! Another consideration is handling weeds. I believe that props can more-effectively chop through weeds when they're turning at a higher speed than at a lower one. Shallow prop pitch has the same effect as a high gear ratio in reducing the motor's speed. At the same time speed is reduced, torque is increased. It's torque that lets you power through weeds, according to my theory, at least. I don't have any hard data to back that up, just my own gut feel based on what I've seen. Torque is force applied to rotating a shaft (1 turn), as opposed to the "normal" way of measuring force linearly. Power is force, applied over time. To increase power, torque has to be sacrificed, & vice versa (for a given set of conditions at either "end", i.e. powerplant & load). High power/low torque lets you cruise at high speed over time; low power/high torque lets you pull hard at a lower speed. Consider a dragster vs. a hill climber, that may use very similar blown big-block engines: The dragster needs to get high speed (300+ MPH) from 10,000 RPM; the hill climber might turn the same 10,000 RPM & only go 10-15 MPH, but needs gobs of torque to power himself up the hill. The difference comes down to drive ratios, to get the speed & torque at the wheels appropriate for the job. Prop pitch selection is one way of matching the input (motor) speed (where it's most efficient; produces most mechanical power from the electrical power used) to the output (prop) speed (where the ship is going at the correct speed). Another way of doing the same thing is by using a transmission. A transmission takes an shaft input at one speed & outputs to another shaft at a different speed. As speed changes, so does torque (inversely). Transmissions come in a few differnt types: Gears: Most efficient in terms of power lost; most difficult to set up properly, can be noisy & excessive/uneven wear if not set up properly; limited ratios available because center-to-center spacing gears allows only certain combinations to be used; strongest & most-resistant to slip/skip (when set up properly). Chain & sprockets: Nearly as efficient as gears; tolerant of misalignment; noisier than gears; lots of available ratios because center-to-center spacing of sprockets isn't critical; needs proper tension & can skip/jump if chain span is too long; chain can break if tension is too tight. Belt & pulleys (friction): Somewhat less efficiency that chain & sprockets due to slippage; very tolerant of misalignment; very quiet; pretty-much infinite ratios available; similar tension/span length considerations as with chain & sprockets. Belt & pulleys (Gilmer; also sometimes called "timing belts"): Similar considerations as chain & sprockets, but much quieter. I almost always include a transmission, & my preference is chain & sprockets. I like that I can choose a wide range of ratios, it's tolerant to misalignment so I don't have to be that critical, & I've got a good supplier of compact components that let me build what I need in some pretty small spaces at low cost. The noise is tolerable & reliability/toughness have been excellent. I could say the same about other types, too, & what I use is at least partly a function of what I have used in the past & feel comfortable with (as well as having a lot of parts on hand). I hope this is what you're looking for. I've tried to stick to general thoughts vs. specific instructions (don't think I mentioned a single specific product, in fact!). JM
Where is a good place to get the chains and sprockets? Or even gears for that matter? Also, if you have a ship with four shafts is it better to have the two inner shafts powered in an attempt to improve turning, or have all of the shafts powered to improve acceleration? Just speculating.
My personal favorites: http://www.servocity.com http://www.servolink.com The 1/8"-pitch Delrin chain & sprockets, as well as the Delrin gears are 100% compatible - what ServoCity sells is actually manufactured by ServoLink (I have confirmed this with ServoLink). The splined gears & sprockets (for mounting on servos) are only available from ServoCity; ServoLink offers a greater selection of plain-hub parts. http://www.sdp-si.com Get their catalog set, even if you never order a single item from the (the catalog set is free). It's a multi-volume set, with every drive component part you'll ever need. Each volume also has lots of great power transmission engineering data. JM
Well, how do i rig gears? I am planning to build/ almost ready to start building a USS Maryland and was wondering about that- where can i get gears, and how do i rig motors?
