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Center of gravity for lift placement

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eric.von eckartsberg
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Center of gravity for lift placement

Post by eric.von eckartsberg » Mon Jul 20, 2020 8:26 am

Trying to figure out center of gravity on a 17 ft '53 Rocket with KBL for proper placement on lift with fore to aft wood bunks under stringers. Is it generally true that a midpoint between the lifting rings will be close to actual center of gravity? From the other discussions on lifting ring use, it seemed like that would be the case but I don't know if CC moved the lifting ring placement based on which engine was installed as that would certainly change things. Boat is in the water so I don't have any way of actually weighing it front and rear. Tried just looking at the tilt of the boat in the water as a person shifts weight front to back, but it's very hard to tell much from that. Any rules of thumb that would get me close? The lift has a healthy reserve of capacity, but I'd like it to be as close to balanced as possible.

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mbigpops
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Re: Center of gravity for lift placement

Post by mbigpops » Mon Jul 20, 2020 2:24 pm

Hi Eric,

Welcome to the Boat Buzz forum !

What is your hull number ?

I have a 1953 Rocket Hull number D-17-098. I am curious on yours as there are not many left. Mine has a KL.

From what I understand the boat alone weighs around 900 lbs which would be about evenly distributed. Maybe slightly more at the widest point which is mid hull at the two cockpits. The engine is the real driver at 600 lbs sitting behind the rear cockpit.

As far as mid way between the lifting rings - front being mid bow and rear at the transom - that would be around the middle of the rear cockpit. That could be the COG or it could be further aft directly under the engine. The only way to tell for sure would be to model this information in a CAD program that predicts COG.

One other item is your comment on bunks being under the stringers. This is true for the forward bunks they are under the stringers but the aft bunks are located under the chines. This assumes you can configure four bunks on your lift.

Mark
1953 CC Rocket Runabout "Rocket Man"

eric.von eckartsberg
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Re: Center of gravity for lift placement

Post by eric.von eckartsberg » Mon Jul 20, 2020 3:36 pm

Hi Mark. I'll have to pull the hull number tomorrow when I'm back home. I think you meant 1900 lbs, which is roughly the dry weight that I've seen for the boat. 1800 - 1900 lbs according to the specs, and we had an on trailer weight of around 2600 lbs, so it makes sense. We have the KBL, no sealing boards inside, but do have a 5200 bottom, so maybe a little bit heavier.

The reason I was thinking mid-point between the rings is because I saw another post showing a lifting brace design where boats could be lifted by the lifting rings up to a spreader that then lifts the whole thing from one point. Seems like that would only work if the rings were spaced evenly out from the center of gravity. But then again, maybe they only got the lifting rings close enough for the center of gravity to be somewhere near the middle, not exact, since different engines have different weights.

The Hewitt lift currently has two bunks running under the stringers which our restorer has always told us should run all the way back to just slightly beyond the transom so they support both the weight of the engine and the transom. Our trailer has four bunks, two under the stringers, and two slightly further out for stability, but our restorer said it's most important to have them under the engine load bearing stringers all the way back on the lift where you don't need the stability of the outer bunks since it's just sitting the there. We've had the boat for 22 years, two restorations, and never had bunks under the chines.

eric.von eckartsberg
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Re: Center of gravity for lift placement

Post by eric.von eckartsberg » Thu Jul 23, 2020 8:49 am

Mark -- Our boat is D17208, sold directly to Deep Creek Lake, MD, where it still sits, when it was made in 1953. We're the third owner and have actually had it for about 22 years. Major restore back in '99 with CPES and 5200 bottom, a few new ribs etc., rebuilt engine, full refastenin, the works. Another cosmetic restore last year down to the wood again, and another full rebuild on the engine (we use the boat a lot). Sat in a slip for many years, but it's much happier up on the lift now out of the waves, and the triple carb KBL running perfectly now after many years of hesitation issues.

I re-read my old copy of Dannenberg's second book and he says bunks should be under the engine stringers all the way back to the transom, which is always what we've done. He also says it's good to have bunks under the chines as well, but this boat is so small, there's not much distance out to the chine from the existing bunks, and no way to put a second set of bunks on this lift since the space is taken up by bow guides. Our trailer has outer bunks close to the chines, but our restorer said the ones under the engine stringers and to the transom are the most important. Hull has not deformed in 22 years, so it seems to work.

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