Calculating nbr of people in boat design

Discussion in 'Boat Design' started by Raptor88, Aug 22, 2021.

  1. Raptor88
    Joined: Apr 2021
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    Raptor88 Junior Member

    There's a formula for calculating the number of people that a small boat can hold. Length x Width (in feet) divided by 15. I assume the width is measured at the top rails.

    So a 12' x 4' boat can hold 48/15 = 3.2 people.

    But take a flat bottom jon boat for example. Some jon boats will have sides that angle outward so the bottom width could be 3 feet and angle outward 6 inches on both sides for 4 feet at the top rails. If the bottom measurement is used then 12'x3'/15 = 2.4 people.

    On the other hand, take a jon boat that has vertical sides. The bottom is 4 feet wide and the top is 4 feet wide.

    Seems to me that the jon boat with vertical sides should have a different formula since more of the width will be displacing water than a jon boat with angled sides.

    Do jon boats 12 feet long have to be registered with the coast guard as a general rule? If so, will they calculate the maximum number of people differently if the boat has angled or vertical sides?
     
  2. TANSL
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    TANSL Senior Member

    Whichever formula you use to calculate the number of people on board, you must always show that the boat is stable enough to carry them.
     
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  3. DCockey
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    DCockey Senior Member

    Where did you find that formula? There are USCG regulations maximum passenger capacity which apply to many boats less than 20 feet in length, but the rules are more complicated than this simple formula.
    Boats which are not sailboats, inflatable boats, multihull boats, canoes or kayaks (based on the regulations) manufactured or imported for sale in the US need to have capacity plate and USCG regulations provide the formulas for determining the maximum capacity. For more information on these regulations see New Boatbuilders Home Page | Everything Boat Building https://newboatbuilders.com/
     
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  4. Raptor88
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    Raptor88 Junior Member

    Boat Capacity, Rules & Weight Calculator | BOATERexam.com® https://www.boaterexam.com/boating-resources/boat-capacity.aspx
    https://www.boat-ed.com/indiana/studyGuide/Calculating-Your-Boat-s-Capacity/10101602_35133/
    Boat Safety 101: Know Your Boat's Capacity - Williamson Source https://williamsonsource.com/boat-safety-101-know-boats-capacity/

    Thanks. I will visit that webpage this evening.
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    The best "formula" would be to err on the side of less people on board, especially if venturing out in less than calm conditions. That formula quoted in the OP would allow 6 people in a 15x6 feet boat, which seems double what would be sensible in less than protected waters.
     
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  6. TANSL
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    TANSL Senior Member

    One of the loading conditions that must be studied in passenger ships is the effect of the passage placed as far as possible from the center line. To study it, it is assumed that there can be up to 4 people per square meter. Perhaps a similar formula is the one the OP is applying. but that formula should be used as I said before, not at all, to determine the maximum number of people on board.
     
  7. Andrew Kirk
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    Andrew Kirk Junior Member

  8. Ike
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    Ike Senior Member

    here we go again. That formula was a rule of thumb published in the 1950's long before ABYC (in fact at the time ABYC didn't exist yet) or the Coast Guard got into the act. It was never intended to be a rule for determining the maximum persons capacity of recreational boats. It was solely intended as a guide for boat owners who had boats that didn't have a capacity label. Yes the old Boating Industry Association (BIA, long before NMMA) had capacity labels. Anyway, David Cockey gave you the link to my site that explains how the Coast Guard and ABYC determine persons capacity for recreational boats. Your question about different shapes of Jon Boats is a good one, because you are right, different shapes come up with different capacities. This is because the capacity is based on the displacement weight of the boat. Displacement weight is basically the amount of weight it would take to sink the boat. That is then divided by five to give a maximum safe weight. Then the weight of the outboard, portable fuel tank and gear is subtracted to give a persons weight. Then there is a formula using persons weigh to come up with number of persons.

    No Jon boats don't register with the Coast Guard, they are registered in the state of principle use. But if you are building boats for sale to the public you have to register your business with the Coast Guard and get a manufacturers identification code. see SafeAFloat.com https://safeafloat.com/ see boatbuilders tool kit.
     
  9. jehardiman
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    jehardiman Senior Member

    Hey Ike, what ever became of the proposal to amend 33 CFR subchapter S Part 183 Subpart C - Safe Loading after the ETHAN ALLEN sinking. Weren't they going to increase the weight per person from 141 to 174 or 185?

    Edit: Changed "165" to "174 or 185" after I checked the NTSB report.
     
  10. Raptor88
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    Raptor88 Junior Member

    Thanks for your overview of how capacity is calculated. Your overview makes more sense (though I haven't actually done a calculation yet) than the simple formula that I posted and is listed on several websites. You have an excellent website Ike. And thanks to D.Cockey for providing the link to it.

    Raptor88
     
  11. Ike
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    Ike Senior Member

    David, that never made it beyond some speculation. The number of persons on the capacity label was a concession by the Coast Guard to the boating industry because they felt the boating public would want to see a number stating the number of people the boat could hold. The actual requirement is the maximum persons weight. So it really doesn't matter what the maximum persons, in persons, is. You aren't supposed to exceed the maximum persons weight. So if you have a maximum persons weight of 600 lb and you have 700 lbs of people on board you have exceeded the weight of persons. Of course, a boarding officer (state or Coast Guard) isn't going to look at it that way. They're going to count the number of bodies and see if it exceeds the number on the label. Conversely, you can have One adult and a bunch of kids, which may exceed the number of persons, but doesn't exceed the maximum persons weight, so technically you haven't exceeded the maximum weight. Confusing? yes, but overall it works.

    The formula uses a figure of 141 which is not a weight. It is co-efficient, just the same as various coefficents used in design.

    For boats carrying passengers for hire, an actual weight is used because the determining factor is stability, not displacement. So they raised to weight to (IIRC) 180 lb. (I'll have to look it up.) So when doing the inclining test they now use 180 lb per person. That makes it harder to pass the stability portion of the test and effectively reduces the number of persons a T boat can carry.
     
  12. Raptor88
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    Raptor88 Junior Member

    Ike,

    Would it be correct to calculate the displacement weight to sink a 12 foot jon boat by calculating the internal volume of the boat and then getting the weight of water for that volume? IOW, one would have to add weight equal to the weight of water displaced to sink the boat up to the top rails.

    My 12 foot jon boat will be made out of marine plywood (not aluminum) so it should float when it's naked even though it's filled with water. I will add closed cell foam for flotation in the rear bench seat and in the bow to compensate for the weight of the outboard (55 lbs), 3 gallon gas tank, anchor & chain, and gear.

    Thanks.
     
  13. TANSL
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    TANSL Senior Member

    With much less weight, improperly positioned, you can get the boat to heel enough so that water begins to enter the top rail and the boat sinks.
     
  14. Ike
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    Ike Senior Member

    Actually that's the normal method. Most boat manufacturers do not have a test tank or enough weights on hand. Calculating it is fine. For very small boats the bucket method that Tansl mentioned is also a good way. Wood boats usually float well without built in flotation, until you start adding things, like outboards, batteries, coolers, gear etc. About 40 years ago I wrote research project on "why wooden boats fail level flotation". Mainly it's because of the outboard. So your plan is a good one.
     

  15. Raptor88
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    Raptor88 Junior Member

    Agreed.

    So my calculation will be mainly for the 55lb outboard and 3 gallon gas tank + gear stowed in the aft half to compensate for the boat tilting. The sides of my jon boat will be 17.5" high so it will displace a lot of water before the tilt angle lowers the transom in the back enough for water overflow. We'll be stowing most of the gear in the front half anyway to reduce clutter in the aft half.

    Will use best approximation of tilt before water overflows at the transom.
     
    Last edited: Aug 24, 2021
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