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View Full Version : Center of gravity question ?????????



Gary S.
09-24-2002, 04:29 PM
A few posts have got me thinking on this subject. Such as Chris stating as to how the C/G needs to be shifted with the Arneson to maintian bow lift,,,would it not help anyone who raised their engine and outdrive to also move them back closer to the transom and or move the fuel tanks aft ??? I'm asking because if I do not go Arneson and stay with my Volvo I do want to raise the X, and if I were to say raise the engine and drive 3'',, make an extension for the drive and move it aft maybe 6'', and I can easily fit two 25 gallon saddle tanks on either side of the hull under the engine hatch , my thoughts are I'll allready be bow high and can use tabs to push it down. Am I thinking on the right track here ??? My thoughts are it's harder to bring the bow up than to push it over. Also,,, has anyone tried or heard of extending the outdrive? I would think that doing all three would raise the preformance by first,, bringing the prop closer to the surface, second,, by moving the powerplant and fuel aft creat a bow high attitude easily corrected with tabs, and third by extending the drive farther from the transom you would give it more leverage. Whats everyone's thoughts ???? I don't mind playing around with the idea of extension boxes,,,, I can easily make them at work , if anyone else want to try something like this all I need are mounting flange templets and how much room to put between them.

RH
09-24-2002, 04:40 PM
Gary, with the Hornet that you are restoring, another option for you would be to go with a newer Volvo Duoprop set-up and big block power. You may want to talk with the Dennis Weaver. He powered his bench seat Hornet with a new 454 Mercruiser and Bravo drive. He repowered from the original Volvo drive and the 351 Ford set up.
Hope that you are making good progress!
RH

BERTRAM BOY
09-24-2002, 04:51 PM
Gary,
If you raise the X dimension in an 18 to 3 inches over stock, you have to cut holes in the hatch to clear the flame arrester, the exhaust risers and possibly the distributor....... How do I know this?
BERTRAM BOY :D :D :D

Gary S.
09-24-2002, 04:54 PM
I forgot to add my question is about my Hornet,,,,, I can go pretty high,,, I have about 36'' from the hull to the hatch.
Royce,,,,,I think I'm making progress,,,, I have a fab shop to lift the deck,,, lots of space and 6 10 ton over head cranes will help not to mention maintaining 55 degrees all winter long !!!!

BERTRAM BOY
09-24-2002, 04:58 PM
Pardon me for not thinking..... I just assume everyone is working on an 18 like I am.

HyperDonzi
09-24-2002, 05:27 PM
the C.G. is approx. 29% of the total boat length from the transom. on an 18, this is 5feet and a little more than an inch from your transom. you want most of your weight here. as for using the tabs to fix a badly balanced hull, everywhere i have read actually says not to do this.

RH
09-24-2002, 06:18 PM
Gary, glad to hear that the confines of a work space will be at 55 degress this winter! That is darn right hot in Iowa! The proper tools make the job alot easier for sure. Good luck and keep us informed. It is nice to live vicariously through others... alot cheaper too!
RH

Woodsy
09-25-2002, 06:33 AM
Gary....

Some of your ideas, are kinda well... wrong.... you want to keep your weight aft, which is good. However, you want to keep the weight about the centerline (Keel) of the boat. By putting aprroximately 2, 190lb weights off centerline of the boat, you will create a SERIOUSLY dangerous condition... your boat will snap roll and could seriously injure you.

The Hornet, by its very design (weight & beam), unless you use alot of HP, will not be a seriously fast boat. I would go with a stock Merc setup, 454MAG or 502, Bravo Gimbal in the stock location and use an IMCO shorty lower unit... turn key cool....

Woodsy Von Donzi :D :D

Gary S.
09-25-2002, 03:25 PM
Woodsy,,, if I change from Volvo I will certainly go Arneson,,,I'm curious to see if you can reap some of the surface drive benifits by repositioning the drive, if I'm wrong so be it, I'd like to know what the therory's are behind how and why these things work.

Woodsy
09-26-2002, 06:45 AM
Gary...

I didn't mean it as a slam. I deal with weight & balances all day long on things that fly faster & farther than any Donzi here. While you are worried about your CG, what you really have to worry about in a boat is your RC, Roll center, pilots call it pitch & yaw. If you think in 3-D, (XYZ), you can grasp what I was trying to tell you. Two saddle tanks on the rear of your boat, off the centerline will create a serious off axis balance. This WILL cause the boat to handle badly, could very easily cause the boat to snap roll if the nose should grab. Ask the 22zx guys what thats like, they have a similar condition caused by the aggressive step in that hull, and you will be 375-400lbs heavier in the rear because of the saddle tanks, and that weight will be off axis.
VERY DANGEROUS!!

That being said, go Arneson... you will not be dissapointed with the performance of that setup. As the Hornet never really had a lot of bow lift to begin with, a set of the Arneson ROcker Plates would definitely help to air the nose out a little... without adding weight.

Lightness is Rightness!!

Woodsy Von Donzi :D :D