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Old 11-09-2010, 09:28 PM   #1
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Phantom 25 - the optimal LCG( longitudinal Center of Gravity

Hi!

Have any of you Phantom 25 owners experimented and found the LCG (longitudinal Center of Gravity) that gives the best balance?

When I had a single Promax 300 with 7,5 inches setback, the LCG was just behind the front seat. Now with two engines, its more by the start of the rear seat. With the single engine I testet 11 inches of setback, but the boat did not run well. The trust line of the prop ended up too high in the boat, and I could not get it balanced in high seas. Running twin application without setback, but with more weight aft (and boat sitting much lower in the water - approx 3 inches), the balance of the boat is better. Despite the fact that a single engine was lighter.

Cheeers, ToffenG
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Old 13-09-2010, 09:25 AM   #2
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Longitudinal C of G

Hi there Toffen,

I would also be very interested to hear of anyones experiments and more importantly their conclusions.

In the coming weeks I/we will be fitting Twin 200 XS's on Gorilla jack plates (i.e. approximately 4 inch setback) to my P25.
So will let you know how it goes.

We have made various changes to the installation in an effort to move weight forward to counteract the additional weight of the lifters and sustain the neutral balance of the boat in flight.

The main changes are:

I . The fuel tank has been fitted further forward, approximately 450mm from the Transom.

2. The hydraulic pumps and batteries are being mounted in front of the kick panel underneath the dash.

3. The water ballast tank has been built into the hull at the very front of the bow, which enabled us to make it much shorter and wider. i.e. greater impact on the LCG.

Oh yes.....There was just one point in your post I didn't understand, so would you mind clarifying the following statement?
'The trust line of the prop ended up too high in the boat'.

Cheers

Steve
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Old 13-09-2010, 09:30 AM   #3
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Longitudinal C of G

Oops!

How do I delete a double post?
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Old 13-09-2010, 02:00 PM   #4
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Quote:
Originally Posted by TheMutzNuts View Post
Oh yes.....There was just one point in your post I didn't understand, so would you mind clarifying the following statement?
'The trust line of the prop ended up too high in the boat'.

Cheers

Steve
Thrust line?
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Old 13-09-2010, 06:37 PM   #5
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Phantom 25

Hi!

If I remember correct Steve Baker did not like setback very much. If you use a lot of setback, you can lift the propshaft higher. Moving the trust of the prop higher relative to the bottom of the boat. Using setback change the balance of the boat. You may get higher speed at wot, but the overall handling in seas may suffer as the attack changes. Even if you have the engine all way down.

What suprised me with twin engines was that the handling in high seas was so good, despite the fact that the weight aft been so much more. I am not speaking about handling in quartering seas. More like head on or head with the seas, where the counter rotating props does not mean so much. My theory is that using no setback, the trust from the props was more ideal to how Steve Baker constructed the boat (twin Xr2's) than running a single V6 on a setback plate.

I am sorry I am from Norway, and its not always easy to explain

Cheeers, Toffen
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Old 02-10-2010, 11:38 PM   #6
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LGC Phantom 25

Hi!

I have checked more into this. The LCG without any engine is 2,1 meter from the transom towards the steeing wheel. Just behind the front seats. A friend of mine who is an engineer, has made a calc scheme. So adding weight of the engine, setback, etc - we can figure how the LCG point changes.

With a Promax 300, CMC SS lift, hydraulic steering, etc the LCG will be 1,5 meter from the transom (7,5 inch setback in all). With two Merc 2,5 EFI SS, hydraulic steering, tiebars, etc the LCG will be 1,35 meters from the transom (built in 2 inch setback). I have also made added weight for the extra battery, catch tanks and pumps. The LCG with a Verado 350csi, hydraulic steering and Bobs Machine 4 inch setback bracket, the LCG will be 1,42 meter from the transom. I have counted in 2 inches more aft LCG for the 350 sci compared to PM 300, due to 4 stroke inline design.

But the question is where the optimal LCG will be. I will post the calculator whn I receive it.

Cheeers, Toffen
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Old 05-11-2010, 12:20 PM   #7
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COG Calculator

Quote:
Originally Posted by ToffenGunnufsen View Post

I will post the calculator whn I receive it.

Cheeers, Toffen
Hi again Toffen,

I was just wondering if you've received the COG calculator/spread sheet?

And if so, could I have a copy?

I can pm you my email addy.

Takk

Steve
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