What factors are included in max weight capacity?

[nateo]

I'm looking at a '06 21' sea ray 205 sport with 305 mpi and '04 20' crownline with 350 mpi. Sea ray is only rated for 1200 lbs whereas this crownline that I'm looking at is rated for 1700 lbs, even though it's a smaller boat. Does fiberglass thickness play a factor in the weight rating or is it probably the added horsepower? 1200 seems a little low for a 21' boat. Any thoughts or advice?

 

[Woodonglass]

Calculating Your Boat?s Capacity

On boats less than 20 feet in length with no capacity plate, use the following rule of thumb to calculate the number of persons (weighing 150 lbs. each, on average) the vessel can carry safely in good weather conditions.

Number of people = vessel length (ft.) x vessel width (ft.) ? 15​

For example, for a vessel 18 feet long by 6 feet wide, the number of persons is 18 times 6 (or 108) divided by 15, which equals seven 150-lb. persons (or a total person weight of 7 x 150, or 1050 lbs.).

IF the Sea Ray is Approx. 6 ft wide then the Weight capacity, according to the above formula, is spot on. Not sure what size the Crownline is but if they are about the same size I can't account for the large difference. It should be noted this formula IS for boats 20' of less. I believe I heard they raised the 150 lb per person to 200 lbs due to the "Fattening of America" so that might account for the weight increase.

There is no formula/requirement for the larger boats so the mfg can assign whatever they believe is safe for the hull.???

 

[tpenfield]

There is a whole testing method for buoyancy and capacity determination that has been established by the USCG for boat manufacturers to determine their 'approved' capacity ratings. The difference that you noted in similar sized boats is likely due to a difference in the amount of flotation built into the boat versus its weight.

The Crownline most likely has more flotation built in and achieved a higher rating as a result of the certification testing. Hull (fiberglass) thickness would have some effect as it relates to the overall weight of the boat, but would not be a day & night difference between the 2 boats.

 

[Woodonglass]

The USCG Rating ONLY applies for boats 20' and less. At least that's what my research tells me. I can't find anything to the contrary.:noidea:

 

[tpenfield]

Yea, I cannot find anything specific to 20 feet and over, either. But . . . My 24 footer had the label and obviously the OP's boats under consideration have them.

Might be an ABYC thing, which tends to extend many of the practices to boats 26 feet or less. I'll keep digging, since I will be thinking about it indefinitely

 

[tpenfield]

Or an NMMA thing . . . http://legcon.nmma.org/certification/about/chart.asp

 

[Woodonglass]

Sounds like you got the same problem I do!!!! This kind of thing bugs me to no end!!! I really think it's left up to the MFG!!!

 

[tpenfield]

The only thing is if they want ABYC and NMMA certifications, which most major brands want . .

 

[nateo]

There is a whole testing method for buoyancy and capacity determination that has been established by the USCG for boat manufacturers to determine their 'approved' capacity ratings. The difference that you noted in similar sized boats is likely due to a difference in the amount of flotation built into the boat versus its weight.

The Crownline most likely has more flotation built in and achieved a higher rating as a result of the certification testing. Hull (fiberglass) thickness would have some effect as it relates to the overall weight of the boat, but would not be a day & night difference between the 2 boats.

Only thing I can think of that they could add that would increase buoyancy would be either length,width, or height. Boats look about the same roughly. Crownline might ride a tad bit higher.

When I was a little kid I asked my dad how is it that large ships made of metal are able to float. His answer was pretty short and sweet. "Water displacement"

 

[bruceb58]

One manufacturer's 21 foot boat may mean something completely different than another. For example, one may have a longer swim step but the wetted length of the shorter boat is actually longer.

Also, what is the beam width of both boats?

 

[Bondo]

Ayuh,..... There's more to hull design than just length,....

Floatin' more weight means displacin' more water within it's foot print,.....
A higher free-board displaces more water,...

Added floatation is just added weight, til it's sunk,....

 

[jakedaawg]

Added floatation is just added weight, til it's sunk,....

so true