Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length.

[class37]

Hi all,
 not sure if this is the right place for this, but I need some advice on where to put various bits in my new build.

this is a 1/72 platform supply vessel. and on the original the engine room is in the second quarter of the length of the hull, ie bow quarter is accomodation, 2nd quarter is engine room, and then the rest of the hull is internal tanks, with the main deck above.

is this a good position for the motors in a model, or should they be more central ?

the advantages to having the motors where the engine room is on the original ship are that the rx and possibly esc can be mounted higher up in the forward hull, in line with the focsle deck and so well above any water, hopefully, and the rudder servo could also be mounted above the motors, but the disadvantages are long propshafts [about 20"] and long linkages to the rudders.

any constructive comments welcomed,

cheers

alan

[malcolmfrary]

The major contributor to the centre of gravity is the battery.  Once you know roughly where that goes, it more of less settles where the rest goes.
For a nice simple reliable layout, direct drive is good.  Its even better if it is possible to arrange this with the inner end of the prop shaft above the waterline (stops leakage up the shaft).  Top of the class is putting all the working bits where there is access for maintenance/repair.
This may not leave room for the required battery in a convenient place, and may lead to thoughts of a stick of batteries each side.  Much depends on the boat.

[tigertiger]

As Malcolm said
It is useful to be able to get to everything (or as much as possible) for service and repair.

It is important to have easy access to the things you will need to connect/switch on every time you go out. Things like the battery if you remove it for charging.

[FullLeatherJacket]

Can't add much to what Mark and the Doc have said, except that the wiring between the battery/ESC/motors should be as short as possible. If you are using two batteries then it's a good idea to fit them in line-astern along the keel, to reduce any tendency of the model to porpoise in choppy conditions.
FLJ

[Umi_Ryuzuki]

Playing combat with model destroyers, maneuverablity is a plus.
For years, a friend has had a twin to my ship, and for years mine has
always out turned his boat. For years he tried different rudder configurations
longer, taller, deeper... My ship could alway turn quicker than his.

He would come over and check the center of gravity, and (of course), they
would always match within 25mm. Finally, I had an epiphany... his battery placement,
and Co2 tank placement were all set out at the far ends of the ship. My battery and tank
were almost centered in the ship... The "center of momentum" was  different.

His ship was turning llike an ice skater with her arms out, and my ship was turning like
an ice skater with her arms tucked in. My ship turns faster...

This is something to consider if maneuverablity is one of your goals. 

[class37]

Hi,
many thanks for those comments.

it would appear that I should be able to keep the engine room virtually where it is on the plan, but maybe move the rear bulkhead and the moters slightly further back.

this will still let me put all the radio gear up in the base of the superstructure, hopefully out of the way of the water.
the batteries can then go in the next compartmemt back, on their own, and flat on the bottom of the hull, with their own hatch.

as another question, is there normally any problem with charging gel cells in the boat ?

I was thinking of a socket wired straight to the battery terminals, and then just plugging the charger into that.

would I need to seperate this from the main leads or not ?
would it be better to take the two leads from the battery to a d/p-d/t switch with one set of leads from this going to the charging socket, and the other pair to the electrics. that way the electrics would be isolated from the battery while charging.

cheers

alan

[so much relevant knowledge on mayhem -- so little in my head]

[Colin Bishop]

would it be better to take the two leads from the battery to a d/p-d/t switch with one set of leads from this going to the charging socket, and the other pair to the electrics. that way the electrics would be isolated from the battery while charging.

Yes, that is good practice. On my boats I always fit a "control panel" underneath a hatch or some suitable bit of superstructure as shown below. The switches isolate the main power from the battery (M); the radio receiver (R) and the onboard lighting (L). The two sockets connect directly to the main and radio batteries respectively and are used for charging - I don't use BECs. The two LED indicator lamps show whether the main battery and Receiver circuits are live or not. Obviously you don't need one for the lighting! I don't like removing the superstructure from my boats at the pondside - you tend to break things! Just turn on the transmitter, lift the hatch, flick the switches, replace the hatch and off you go.

Colin

[tonyH]

One idea I used ages ago followed the principle of the cargo loading balances. Just a piece of wood to suit the balance length of the model, hung from the centre, with the batteries, motors, etc. hung from it in turn so it stayed level.
This gave the fore/aft layout from the centre of balance and it also meant that where there were fixed points for access etc. you could hang the right items at the right point and just move the others to suit, bearing in mind what Umi said about turnability.

Tony

[class37]

thanks for all the suggestions, especially like to idea of a main control panel, easy to get to and no need to remove superstructure.

for the loading ballance, is the length the overall length, or the waterline length, or something elsa.

stupid question I know.

cheers

alan

[Bryan Young]

managed to do the first "under power" trials of "Northumbrian" today. Because (for once) I got most of the ballast weight roughly at the centre of momentum she steered beautifully. With "Baroda" the hull form meant I had to put a lot of ballast well forward. This has made turning the thing a bit difficult as the rudder has a lot more strain on it. One lives and learns!. BY.

[Umi_Ryuzuki]

Reading these again, I think I want to say Center of inertia...

 

[GG]

Hummm...... cannot help but think we are mixing up "Moment of Inertia" and "Center of Mass (or Gravity)".  A bodies Mass alone can be used when considering translational motion but how the mass is distributed (i.e. a bodies Moment of Inertia) controls how it behaves when rotated.
Consider holding a ladder with its long axis horizontal (usual way to move it).  Starting to walk forwards, backwards or even sideways is relatively easy.  But, stand still and starting to rotate the ladder in a horizontal circle takes a lot more effort, this is very noticeable when you try to stop the end of the ladder hitting a window!
Now try the same thing holding a more compact body of the same mass, say a bag of cement.  Getting the bag of cement to move in a straight line should take the same effort as with the ladder.  But now starting to rotatie with the bag is much easier than when holding the ladder.
Both the bag and the ladder have their Centres of Mass at the same position (where you are holding it) but the ladders mass is spread over a greater distance from the axis of rotation. Although the ladder and bag of cement have the same Mass, the ladder has a larger "Moment of Inertia".
This ties in with Umi's observation that a model with the internal masses spread out along its length takes more effort to start or stop a turn compared with a similar one with the masses concentrated around the Centre of Mass.

GlynnG

[tonyH]

Not a daft question,

As far as I remember it, I used the length from the centre of buoyancy shown on the plan to the transom and doubled the figure for the length of the balance. This wasn't totally accurate but it worked for me, especially since I didn't have a lot of leeway on the displacement figure.

There was a small amount of juggling with the final ballasting but I'm sure that it was a lot less than there would have been.

I'm sure someone on the forum can give you chapter and verse.

Tony