View Full Version : Engineering question
10-30-2003, 09:24 PM
Last week we had record rains and a flood. My boat is moored in the main river. The river current has been estimated at 8-9 knots during the ebb. Lots of logs and trees floating around.
My toilets are affixed to the deck with four hold down bolts topped with decorative caps for appearance. All the caps have been thrown to the deck. They are very light china probably less than an ounce. The question is " what force of impact is required to dislodge the caps"?
Vessel displacement is appx. 120,000 lb, current to 9 knots, and caps have to be raised at least 1/4" to be dislodged.
10-31-2003, 10:44 AM
Not sure I understand the question, but - It's probably a safe bet that the whoile 120,000 lb vessel didn't move enough to jump out from under the nuts. If that had happened, you would have more problems than loose toilet nut caps. It's more likely that there was some local "drumhead" type harmonic vibration in that part of the deck that reached the 1 g necessary to launch the nuts. Either that or they floated off and the water dried up.
10-31-2003, 11:04 AM
I noodled this one over for a few minutes, but I think I'm missing a component. Perhaps a high school physics teacher can chime in to set me straight.
So far I have:
acceleration required to bump the one ounce cap up 1/4" is .5 feet per second squared, plus gravity's 32 f/s/s.
Therefore, force required to accelerate the 120,000 vessel at 32.5 f/s/s is 3,900,000 pounds.
I think that if your question is "how big was the log that bumped into my hull?" we could only guess, because precise determination requires knowing how much the tree decelerated upon striking the hull. Plus there's a vector problem because the force of that log strike wouldn't be directly upward. But here's what I have on that...
Assuming that the log is moving 8kts (10.13 ft/s) before the strike, and that it loses a quarter of its velocity to the strike (delta-V of 3.37 ft/s)....
Assuming that period of impact is .25seconds, the deceleration of the log is 13.5 ft/s/s.... mass of log is therefore 28,903 pounds.
But that assumes a direct upward strike. After pumping that data through a vector diagram whereby only a small fraction of the total energy is vertical.....
Safe to say it was a pretty big friggin log.
I used to LOVE this stuff. I just haven't used it in forever.
10-31-2003, 11:14 AM
oops! nobody caught my mistake.
Force on a 1 gram cap is ~1.0e-2 N
A simple Newtonian mechanics problem.
Otherwise the hydrodynamic load on a hull created by an impact would require Finite Element Analysis to solve.
what's a slug (dyne)?
[ 10-31-2003, 12:56 PM: Message edited by: popeye ]
slug is the fps unit of mass. SI's kilogram.
dyne is the cgs unit of force. SI's Newton.
In some ways, it's astonishing that engineers can talk to each other!
10-31-2003, 02:41 PM
I don't suggest doing experiments with logs until you find the solution.
I think your assumption of how the caps came off is wrong.
If your boat is slack on the anchor lines and then coomes tight, you might get enough sideways acceleration to push the cap up a ramp, the inside of the cap.
10-31-2003, 03:04 PM
Nah, 'tis simple really: the mice where looking for rain caps ;)
11-02-2003, 04:40 AM
Thanks gentlemen. I don't know the size of the culprit log but what I saw and felt when I was able to get aboard was enough to give you a heart attack. I thought we were going to lose the boat. Actually there was a 55 footer that sunk a couple of hundred yards up stream. We here are wondering what happened to another boat that sank several months ago. It's an 85 foot converted ASR. It was grounded on the dike at high tide and tied off. It has always been visable even at highest tide. We have not seen it since the flood.
Peter Malcolm Jardine
11-02-2003, 05:15 AM
What kind of boat is this Ken? :eek:
11-02-2003, 08:23 PM
I just can't imagine a boat with little decorative porcelain caps on bolts holding the toilet(s) to the deck. What happens when you take the boat out for a run? What about cleaning, do you take each little cap off, clean, and replace? Jeez!!
11-03-2003, 08:19 AM
I'm beginning to think more along the lines of a Loch Ness monster type incident.....
11-03-2003, 02:38 PM
Originally posted by Phil Young:
I just can't imagine a boat with little decorative porcelain caps on bolts holding the toilet(s) to the deck. What happens when you take the boat out for a run? What about cleaning, do you take each little cap off, clean, and replace? Jeez!!I just have hard time imagining a boat with toilets on deck...it just seems like you wouldn't get the kinds of privacy you usually want for your Personal Me Time™...and the library would likely get soggy. :D
11-03-2003, 09:27 PM
Nicholas Nicholas Nicholas. I don't know what your boating experience is but consider this. On every boat that has an installed toilet you will find that the toilet is affixed to either the deck or the bulkhead. In 48 years on boats I have never found one to be swinging from the overhead.
Phil. Toilets are usually sold with some device or another to hide the hold down bolts or at least make them more attractive. The bolts in any form of construction (house or boat) are not considered objects of beauty. Toilets are ugly enough as it is so you do what you can to improve the appearance. It is common practice. You need only look at your own toilet for proof.At $1,100 a pop for these toilets I will set the caps on the bolts. At some future date when I have finished rebuilding this boat I will affix the caps more securely.
I think it is safe to assume that harmonics and frequency are not the culprits here. There is little doubt that the boat was hit very hard by something coming downstream. The caps were displaced on both toilets. They are located on the same deck level but 30 feet apart. This area has 3 seperate compartments and the decking is seperate for each compartment. The deck beams are 2" x 3" on 12" centers and the decking is 1 1/8" tongue and groove. The deck beams are supported at the outboard ends as well as with supports to the floor timbers at intermediate points. The beams in the midships head are also supported on steel keelsons which are 52' long and run all the way to the transom. The keelsons also serve as the inboard stringers for the main engines.
Hello Peter. The boat is an 83' ex. Coast Guard patrol boat. The ex. CG 83451 to be exact. I have been working on this thing for years and probably will never get it done, but it is paid for so I will keep on keeping on.
It is entirely possible that she was hit twice, each hit popping off one set of caps.
Meer got it...mice. What kind of goo do you have on the bolts to prevent corrosion? Any other visible damage on board?
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