K2FS18

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seaslug
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Re: K2FS18

Post by seaslug » Wed Mar 15, 2017 8:33 pm

The extra 26 lbs. would make so little difference you wouldn't even notice. My guess would be 1/4" more draft. That an uneducated guess I'll admit.



TomW1
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Re: K2FS18

Post by TomW1 » Wed Mar 15, 2017 9:55 pm

The PPI is 280 lbs at DWL so for the 26 lbs difference your looking at less than a 1/10 of an inch difference in draft. One thing I do question is the boat weight of 508lbs for the hull, it seems awful high from what other builders have reported and the design weight of the plan. What did you include other than the hull weight. I can see the fuel tank empty some wiring but is there anything else? To be 250lbs over Jacques design weight is difficult to make sense of, since his hull weight is for a finished hull and depending on the hull with basic accessories.
Restored Mirror Dingy, Bought OD18 built by CL, Westlawn School of Yacht Desghn courses.

seaslug
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Re: K2FS18

Post by seaslug » Wed Mar 15, 2017 10:02 pm

Either we're not understanding something, or your method of weighing the boat must not be accurate. Even a very heavy build would probably be no more than 350.

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Re: K2FS18

Post by Newt » Wed Mar 15, 2017 11:35 pm

jacquesmm wrote:
Wed Mar 15, 2017 11:12 am
Don't overthink.

You can calculate those centers to the 5th decimal but when you move your feet or tackle box, it will change everything.

You should keep weights in mind and not to go to any extreme weight distribution but plain common sense if enough.
That's why there is power trim. :D

Newt
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K2FS18
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Re: K2FS18

Post by K2FS18 » Thu Mar 16, 2017 5:07 pm

Well, I thought 508# was pretty good. Maybe not. This is the total weight of the completed boat minus engine and a few other small items. I think the scales are more than likely pretty close as I cannot come close to lifting the rear up, only the front.

TomW1
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Re: K2FS18

Post by TomW1 » Fri Mar 17, 2017 2:35 pm

Did you sum the rear scales and divide by 2. That would be the correct procedure, not to add all 3 weights together. Try this and see what you come up with. The 508lbs is only a 100lbs less than a much larger OD18 which has 5 sheets of 1/4", 7 sheets of 1/2" and 1 sheet of 3/4", vs the FS18 which has 6 sheets of 1/4" and 3 sheets of 3/8". That is why I am questioning the weight of your FS18. Try doing the averaging of the rear scales and see what you come up with.

Tom
Restored Mirror Dingy, Bought OD18 built by CL, Westlawn School of Yacht Desghn courses.

joe2700
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Re: K2FS18

Post by joe2700 » Fri Mar 17, 2017 5:35 pm

TomW1 wrote:
Fri Mar 17, 2017 2:35 pm
Did you sum the rear scales and divide by 2. That would be the correct procedure, not to add all 3 weights together.
Could you explain that. Doesn't make sense to me.

Edit: I'm almost certain that the sum of the 3 weights is the correct weight of the boat. The only condition is that all 3 scales are weighing perfectly vertically. If they are fighting each other that would be a source of error.

TomW1
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Re: K2FS18

Post by TomW1 » Sat Mar 18, 2017 2:09 pm

You got it correct in your edit. The weight on the rear scales you would think would measure only the part they are holding up are in fact weighing more as each point is a concentration of forces. The formula would be a+(b+c)/2 = w. The area that is being weighed is actually a semi-circle from the attachment point and doesn't cut off right in the middle of the stern, thus the need to sum and divide by 2.

The best way to have done it would have been to make a sling and weighed the stern from one point.
Restored Mirror Dingy, Bought OD18 built by CL, Westlawn School of Yacht Desghn courses.

joe2700
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Re: K2FS18

Post by joe2700 » Sat Mar 18, 2017 4:39 pm

TomW1 wrote:
Sat Mar 18, 2017 2:09 pm
You got it correct in your edit. The weight on the rear scales you would think would measure only the part they are holding up are in fact weighing more as each point is a concentration of forces. The formula would be a+(b+c)/2 = w. The area that is being weighed is actually a semi-circle from the attachment point and doesn't cut off right in the middle of the stern, thus the need to sum and divide by 2.

The best way to have done it would have been to make a sling and weighed the stern from one point.
Are you picturing the scales coming from a central point to the 3 corners? I was picturing one scale going straight up from each corner to the ceiling in which case you would simply sum the 3. I'm still now sure about that divide by two though. I don't see how you'd figure that out without knowing the angle between the scales.

K2FS18
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Re: K2FS18

Post by K2FS18 » Sat Mar 18, 2017 7:33 pm

This has gotten a little more complicated than I expected. College physics was way to long ago and I apparently forgot. When I weighed the boat, the scales where hanging from the roof trusses. The front scale was attached to the bow eye. The rear scales where placed on each side of the boat and a 2" strap supported the boat and each end was attached to a scale. The weights on the scales was 125# for front scale and 194# and 189# for the rear scales. The hanging points where not 100% vertical, so I can see how the weights would be more as the scales are slightly pulling on each other. I did not think of that.

Once the primer dries, I will redo the setup and post photo. Then I will re-rig and and come up with a way to hang boat from one scale. I am really curious about this.

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