Centre of gravity

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Centre of gravity

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18 June 2008 at 20:40 #14290
Brian Fry
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@brianfry61832
I am in the process of building a hard chine style motor boat and will shortly be installing the radio and battery equipment.

Could anyone please advise where the centre of gravity should be on a design of this type?

thank you
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19 June 2008 at 10:42 #14299
Jake Knight
Participant
@jakeknight71345
the motor should go a little more to the bow (front of boat) from the middle of the boat so then you can get the battery in between the propshaft ans the rudder
19 June 2008 at 11:04 #14300
Brian Fry
Participant
@brianfry61832
Thank you Jake, I am sorry if my question was not clear.

What I wanted to know was where along the keel the boat should balance, e.g. a third of the way from the bow?, half way back? more?

thanks for your your reply
19 June 2008 at 11:11 #14301
Paul T
Participant
@pault84577
Brian

The positioning of batteries as ballast is subjective so the best way to check is put the model in the bath and then put the batteries in and move them around to achieve the best trim. If you wish to allow for the batteries whilst building then follow Jakes advice and as a rule of thumb the best position is just rear of the center of the model equally set along the axis of the keel.

Paul
19 June 2008 at 13:17 #14304
ashley needham
Participant
@ashleyneedham69188
Brian. BUT once its in the water and zooming along you may want to make some trim changes in order (for instance) to keep the nose up or down a bit etc etc so I would leave some adjustment in the battery department in case. Ashley
19 June 2008 at 21:35 #14308
Brian Fry
Participant
@brianfry61832
Thank you Paul and Ashley.

You see, I am an ex-aeromodeller and for an aircraft, a nominal centre of gravity is always calculated (usually just in front of the thickest part of the wing chord) so that the first flight has a reasonable chance of being successful.

I understand your points about moving the ballast (batteries) about on a trial and error basis, but I wouldn’t think a full size boat architect has the same luxury, and needs to calculate some sort of balance point before it goes into production.

OK, we’re talking models here – its just that I like to get things right from the start if possible.

That said, I have no argument with your advice and I guess a static test in the bath when the hull is painted is a good starting point.

Thanks again gents,

Brian

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20 June 2008 at 12:13 #14309
Paul T
Participant
@pault84577
Hi Brian

Now I understand why you wish to be so exact, it is easy enough to calculate the exact centre of gravity on a model boat but you need a lot of information on size of hull, size / output / weight of motor, size / thrust of prop, size / weight of battery (batteries) and expected performance of model.

Just like a model aircraft except that on an aircraft you have to include change of mass due to fuel consumption.

Paul
20 June 2008 at 16:01 #14311
Brian Fry
Participant
@brianfry61832
Thanks for the extra information Paul, – I had forgotten about the weight shift due to (liquid) fuel consumption!

But to be absolutely pedantic here (sorry!) we would only be at the unpowered gliding test stage of the aircraft.

Lets get off planes shall we? – this is a boating forum! it was only an analogy after all…………..

I have started to note the general layout of components in similar boats to the one I am building so, since I am no scholar, trial and error it is!

Regards

Brian
9 January 2009 at 19:20 #1628
Brian Fry
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@brianfry61832
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