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Author Topic: C of G where should it be?  (Read 6089 times)

Davenotdone

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C of G where should it be?
« on: September 23, 2014, 02:09:14 pm »

Hi, building a model boat at the moment. 24 inches long, V shaped  ( planing ) type hull. Speed 400 motor, 7.2v stick battery. It ran fine the first time i took it out, over an hour and a half at a reasonable speed. It sits a little tail heavy ( the battery is sat towards the back of the boat ).  I could fit it approx half way ( across the hull, under the motor coupling ) or do i just add more weight to the front? I presume that levelling off the boat would increase the speed as it would also put the prop at a shallower angle?  All thoughts and ideas welcome,      regards, Dave.
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Jerry C

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Re: C of G where should it be?
« Reply #1 on: September 23, 2014, 02:50:48 pm »

I can't tell you where G should be but I can tell you that weight is your enemy. Adding weight is bad. Moving weight is good. Moving a weight longitudinally is twice as effective as loading a weight. Try to avoid raising the weight. Close to keel is good.
Jerry.

malcolmfrary

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Re: C of G where should it be?
« Reply #2 on: September 23, 2014, 05:38:58 pm »

Its not so much how it sits at rest (within reason) but how it sits running.  If you want a boat to run fast, don't add weight.  Do as Jerry suggests and move the heavy component.  Leveling its attitude when running will help with getting forward thrust, but adding more weight to shove along just takes more power than you gain.
Keeping the weight low is good for stability as long as the battery is easily accessible.
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Davenotdone

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Re: C of G where should it be?
« Reply #3 on: September 23, 2014, 08:14:38 pm »

Thank you :-)) , all good thoughts to ponder over. Regards, Dave.
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Jonty

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Re: C of G where should it be?
« Reply #4 on: September 23, 2014, 09:04:46 pm »

  I think I've read that 1/3 of the length (waterline, presumably) measured from the transom is about right for a planing hull. Certainly, full size racing boats seem to have the engines and other major weight well back.
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jarvo

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Re: C of G where should it be?
« Reply #5 on: September 23, 2014, 10:09:04 pm »

HI Dave


Most of the advice is great, Jonty mentioned real racing boats have virtually all the weight at the stern, very true but, they use huge ballast tanks to even the boat at speed.


Mark
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Neil

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Re: C of G where should it be?
« Reply #6 on: September 24, 2014, 12:18:31 am »

C of G where should it be??

Now if you'd have been a member of Fleetwood Model Yacht and Power Boat club, in the early 1980's .........you'd know exactly where it was..................that's if you'd have been able to stay awake throughout one of our illustrious members 31/2 hour lectures, non stop, without breaking for breath, on that exact subject.............mind you, very few of us were able to stay awake..........so sadly the mystery still eludes me as to where it is, {-) {-) %% %% %% {:-{ {:-{ {:-{ {:-{
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jarvo

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Re: C of G where should it be?
« Reply #7 on: September 24, 2014, 11:24:37 am »

HI Neil


HA, HA, know what you mean, if someone taped it you could edit it, i.e., remove all the gobbledy gook, and give us the meat!!!


Shame cause those guys lived their subject, we didn't listen now we have lost it!!!!


Mark
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raflaunches

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Re: C of G where should it be?
« Reply #8 on: September 24, 2014, 07:52:57 pm »

Just been reading up on this subject, surprising how it works. However the book I'm reading is about a horrific incident called 'Black Night off Finisterre regarding the loss of HMS Captain. Truly terrible but involves the rolling motion rather than the balance longitudinally which we are looking at here.


I agree with Jonty, about a third along the hull from the transom looks right and works on all the fast boats we have in our fleet. The only one that breaks the rule is the CMB which balances on the step half way along the hull and apparently this is right for this type of hydroplane.


Just one point, not all v-shaped hull fast boats are planing hull vessels, compare the early RAF ASRLs and Royal Navy MTBs with a modern fast attack boat. The early boats had flatter hulls to 'plane' across the surface whilst V-shaped hulls cut their way through. Both have their advantages and disadvantages,
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Davenotdone

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Re: C of G where should it be?
« Reply #9 on: September 24, 2014, 08:55:46 pm »

Thanks to you  all again for your help :-)) , regards, Dave.
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derekwarner

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Re: C of G where should it be?
« Reply #10 on: September 24, 2014, 10:47:22 pm »

  :} .....& Colin says....."Pliss spik Inglish.... I fort you wus a teecher" ....well I think he was a teacher of how to make sawdust & wood shavings {-) .... Derek
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RAAArtyGunner

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Re: C of G where should it be?
« Reply #11 on: September 24, 2014, 11:34:32 pm »

  :} .....& Colin says....."Pliss spik Inglish.... I fort you wus a teecher" ....well I think he was a teacher of how to make sawdust & wood shavings {-) .... Derek

Derek,
 :o :o :o I tink Neil waz rite coz da speeker waz goin flat out like a lizard on fire an da speeker diden even brake an anyway we tort Neil  proper Inglish while he waz  ere in OZ, good ta see he asnt forgotten da lingo   %) %)
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canabus

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Re: C of G where should it be?
« Reply #12 on: March 27, 2015, 08:41:31 am »

I have a 24 shallow vee self build boat with a Hobbyking F3 Sprint outboard on the stern.
I started with 1/3 the length of the boat from the stern.
I moved to CG a bit more forward with sliding the 3S 2650mah battery to the front(a bit at a time).
I also tilted the prop inwards a bit a time.
The boat sits flat at full speed(minus the hull on the water).
So it's change one thing at a time and test.
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derekwarner

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Re: C of G where should it be?
« Reply #13 on: March 27, 2015, 12:28:40 pm »

Well....canabus....[which is a strange name for an OZ member >>:-(] the thought process of ......."it's change one thing at a time and test"  is sound O0

So let us know how you progress :-))..... Derek
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radiojoe

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Re: C of G where should it be?
« Reply #14 on: March 28, 2015, 10:10:02 am »

Surely the C of G is determined by the designed waterline of any vessel, of course you want the ballast as low as possible, but the vessel should sit level on the designated water line when ballasted and trimmed, the design of the hull and the power of the drive determine the sailing characteristics.
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Perkasaman2

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Re: C of G where should it be?
« Reply #15 on: March 28, 2015, 02:39:22 pm »

Hi Dave. Power enables a planing  hull to transition from being in the water to being more on it as speed increases. The bow wave gathers under the forward hull and raises the bow cushioning the hull on a wedge of water reducing water resistance on the hull. The planing hull can achieve  far higher speeds and displaces less water than a displacement hull. The above  describes a semi planing hull rather than a full planing hull where the hull is virtually clear of the water and only the stern and drive is immersed while at full power, e.g. a hydroplane. Extreme power and full planing hull forms enables considerable speed. When we talk of hulls planing it is often a question of degree. Heavier boats such as MTB's were very heavy and required huge amounts of power and fuel to make 40knots.
The degree to which a hull can plane is dictated by it power, weight, size and hull form. I would suggest that the c. of g.  is'nt useful in considering the distribution of weight in a hull. I have a always found it more useful to view the hull as a see saw and find where the balance point occurs. (We want to avoid snow ploughing caused by too much weight at the stern and where the rear hull half is trapped and displacing a large amount of water while trying to make forward progress.) ideally the bow should be just clear of the surface at speed and there is minimal water resistance along the hull length.)
Moving the battery further forward is, as many members mention, the correct remedy. Place a pencil under the hull as a fulcrum to give you a guide to find the hull's balance point. Bear in mind that there is a reducing amount of bearing surface toward the bow. Weight should be distributed in the rear 2/3 of the hull as a guide and starting point. High speed trialling will give feedback. Moving weight further forward  can give surprising gains in speed.
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canabus

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Re: C of G where should it be?
« Reply #16 on: March 29, 2015, 11:09:06 am »

Hi All
I had a look on you tube about the old full size offshore power boats.
You can see how nicely they were set up to run at high speed in very large swells.
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Arrow5

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Re: C of G where should it be?
« Reply #17 on: March 29, 2015, 11:41:02 am »

Don't forget that the full-size craft had trim tabs (adjustable, mostly ashore) and adjustable trim on the outboards during the race.  I don't know the regulations about the tabs.
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Martin (Admin)

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Re: C of G where should it be?
« Reply #18 on: March 29, 2015, 02:21:48 pm »


This might be of interest:  http://tinyurl.com/oza996a

  Pages 97 - 100
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bj

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Re: C of G where should it be?
« Reply #19 on: April 05, 2015, 10:27:26 am »


This might be of interest: 
http://tinyurl.com/oza996a
Pages 97 - 100


On a similar vein, there was an interesting article in the January 2015 issue of Marine Modelling International magazine on page19 entitled 'C of G For planing Hull'
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