



Help!
Propulsion and Weight
Folks,
I am in the process of building a scale model of the USS Nevada in its December 1941 configuration. Because its going to be a relatively large model (1/48th scale), I need to know what kind of propulsion system to plan for. I do have several options available in terms of hull construction which relate to the amount of affordable power I can put in the ship. Based on my calculations, a 27,500 ton battleship in 1/48th scale should weight something in the vicinity of 550 lbs. The model itself is likely to weigh something like 100 lbs max. This would require building a false bottom and allowing the hull to flood fairly close to the waterline. The other option is to build something like a waterline model. I really prefer to take the first option. So, here is my question. The model will use 2 3" screws. Based on the weights indicated above; 100 lbs or 550 lbs, what kind of power would I need to push it at double the scale speed (about 4 knots)? I realize there are several variables at work here but is there a general rule of thumb that I can use  something like X number of Watts can be expected to push Y number of pounds at a speed of 1 Knot? Rob 




You're going to need a pair of motors rated around .13 hp each. Running on 12 VDC, you'll be drawing around 16 Amps.
If you do an advanced search for "HP Formulas", you'll get the information you need. Basic formula is: Scale of model: 1:48 Scale Factor (the Cube Rule): 110592 = (48x48x48) Proto Shaft HP: 24800 Proto Watts: 18500800 Model Watts: 167.3 (0.22 hp) Model Watts + 15% scaling factor: 192.4 (0.25 hp) 



Bob,
You are the best! Thanks so much for the info! That really helps. So it is practical for me to build a full size scale hull. I have a couple of other questions you may be able to help with. Its obvious from the formula that the calculations would drive the boat at scale speed. Nevada had a speed of about 20 knots. I assume the scale speed would be a linear function of the scale times the protype knots such that a 1/48th scale model of the Nevada would move at about .4 knots which under normal circumstances is exactly what I want. However, she will be sailing at a lake and I will have to deal with fishermen in boats and other full scale inconvienences. So, on occasion, I might need to be able to move at a faster speed. Lets say I wanted to double that speed. What would I need to consider in terms of watts? The next question I have concerns ideal rpms for the screws. Any ideas. Thanks again. Rob 


Joined Dec 2004
4,519 Posts

KAMPFLEIGER For what you have stated as to what you re going to run for props with a 12 volt supply for power, I would suggest using two of the M.A.C.K. Products motorgear box setup #204130 which come with a 3 to 1 ratio. You will have no problem swinging a 3” diameter, four blade propeller with a amp draw of about 5 amps per motor. You can see the motorgear box set ups at http://www.mackproductsrc.com/Power%20p2.htm




Frank 
Thanks for the info on the M.A.C.K. motor. But, even with twin motors, There is not enough power to drive Nevada based on the calculations. Using a 12 volt system, I will need at least 20 amps combined. The M.A.C.K. motor is rated at 5 amps so the two will only provide 120 watts of power. I need something over 200 watts minimum. I was thinking about using two surplus scooter type motors or similar. Of course that would bump me up to a 24 volt system but I would think that would be OK assuming I could get a speed controller that would support that kind of a configuration. Having stated all that, I am a total novice when it comes to powering model ships, so I will need alot of help from you guys in that area. Tachikaze  1/72nd scale was an option, but I chose 1/48th scale for several reasons. First, I live in Kansas and the only place I have to sail the ship is on one of our lakes around the Kansas City area. It tends to be pretty windy here especially in the spring and fall. So, I wanted to build the ship with the size such that it would not be bobbing around like a cork in the swells on our lakes  not to mention the wakes caused by fishing boats. That was the major factor in going to 1/48th scale. Based on my calculations, the boat will weight in (when balasted) at around 550 lbs. With a length of a little over 12 feet and beam of just under 24 inches it should be rock solid in the water. 1/72nd might be good enough to achieve that same objective but I definitely know 1/48th will. Second, at 1/48th scale, I can just as easily manage transport and all the other logistics as I could with 1/72nd scale. Third, building scale parts  in particular the secondary ordinance  will be a little easier in the larger scale. Right now I am building 10  5in 51 caliber casement guns that I know would be more difficult in the smaller scale. Finally, I like the presence the larger model will have in the water. Now, in terms of transport, its a consideration, but not a concern. I am a member of the Kansas City rowing club and compete regularly in the midwest. This requires transporting racing shells all over the place on the car and naturally, they are a heck of alot longer than 12 feet (my racing double is just over 30 feet). I have a roof rack for my van that the boats are transported on. All I need to do is build a cradle to support Nevada inside the rack. Whats nice about these racks is that there is a bit of spring in the mounts themselves that greatly reduces the stress on the boats from road vibration. One thing, the ship itself is likely to weigh less than 100 pounds when fully built (the other 450 pounds will be water balast) so weight will not be a significant consideration. Moreover, the entire superstructure (from the boat deck up) will be removable so I won't have to worry about the masts being hammered in the wind on the highway. I'll try to post a picture of the rack once if figure out how to do that. Rob 



Going to place a video camera in it?
The Sainted Cliff Shaw placed a camera on the bridge and one in the A turrent of a Deutschland Class, then used a screen that one would wear like a visor. This was really pretty neat, it really gave a whole new prospective on running your boat, especially one that you will be able to take a long way out from shore. 



Tachikaze  Thats an interesting thought. Lets see how far I get with the build first. Rob




This gives you a general idea of the kind of rack I am refering to. Mine is a little different.
Rob 



Hey Rob... With a powered model that big on a public lake, you ought not be surprised if the state of Kansas makes you put legalsize registration numbers on it....... no joke! Like the fine print says:..... " Check your local laws"..........



USA, FL, Vero Beach
Joined Sep 2006
107 Posts

Motor Power
Kampfleiger, The Mack Motors with 3:1 gear reduction will positively turn the 3" propellers at about 3000 RPM at 12 Volts. They will be very efficient and won't get hot. The 3000 RPM max is about right for 3" props and will give you the scale speed you are looking for. When you use a gear reduction you don't need a big high performance motor. I hope this helps. Captain Bill



Blackpool, Lancs
Joined Feb 2006
3,421 Posts

Quote:
Its a big ship with relatively low power, so dont expect acceleration like a rescue launch. With that amount of mass, slowing down will need lots of forward planning as well. (Thinking of a guy with a 1/72 Forestall who decided to stop it by interposing his hand between boat and poolside. It just kept pushing.) 




Quote:
No question about it! Wyandotte County, I'm sure will require it  $25.... Rob 




Quote:
Thats great information to know! I would never have thought of it. Certainly seems counter intuitive. Its also good news as that scale speed will give me more thust than I was anticipating. I do plan though to have more power to spare in order to more easily sail around fishing boats, etc. I am also a little concerned about the turning radius of the ship as our dock is in a cove. I plan to take a launch out with Nevada the first couple of times on the water just it case I need to employ some full scale human intervention I also know exactly what you are talking about in terms of the ships mass having had to help several novice rowers in our club manage docking quads and eights! Thanks 



United States, ID, Rexburg
Joined Sep 2008
6,599 Posts

I am facing the same registration issue(s) with a large BB I am working on (sit down scale 1/35th) and the local sheriff has promised to work with me to try and get the hull number into the registration number (if possible).
Foo 