Calculating nbr of people in boat design

[Raptor88]

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.

Thanks Ike. It will be interesting what the state says the max people will be compared to my calculations.
Ain't designing boats fun .....

 

[TANSL]

Will use best approximation of tilt before water overflows at the transom.

The point of flooding, when the ship heels, might not be in the transom.

 

[Raptor88]

The point of flooding, when the ship heels, might not be in the transom.

Could you clarify where you mean the overflow might happen if not at the transom?
Thanks.

 

[BlueBell]

In the interest of timing,
I am willing to bet TANSL meant the place where the water first pours in may not be at the transom.
It may be at midships, or even at the bow, depending.
Please correct me if I am wrong TANSL.

BB

 

[Ike]

Read the page on Level Flotation. Boat Building Regulations | Level Flotation https://newboatbuilders.com/pages/flot2.html There is a stability test that involves putting part of the persons weight on one side and measuring the heel angle. It only allows a heel angle of 30 deg in that test.

What HP are you planning on? For small boats 2 HP or less, the rule allows more persons weight and is easier to comply with. Larger motors (55 lb is not a 2 HP, probably 5) but if you want this to meet Coast Guard standards you have to use the standard weight table Boat Building Regulations | Outboard Engine Weights Table 4 | 33 CFR 183 https://newboatbuilders.com/pages/table4.html rather than the weight of the specific outboard you are using.

 

[Ike]

TANSL's comment is very apt. If the boat's sheer has any curvature to it, the point of entry can be at the lowest point along the gunwale. The regs talk about the static float plane, meaning a level plane intersecting the point of entry of water.

 

[TANSL]

Please correct me if I am wrong TANSL.

Yes, that is exactly what I wanted to point out.
Perhaps the image clarifies the idea.
In addition, when listing the boat it is possible that it also acquires negative trim so that the transom will rise with respect to the mid-section.
The Ike images also clearly explain other cases that may occur.

 

[Raptor88]

Read the page on Level Flotation. Boat Building Regulations | Level Flotation https://newboatbuilders.com/pages/flot2.html There is a stability test that involves putting part of the persons weight on one side and measuring the heel angle. It only allows a heel angle of 30 deg in that test.

What HP are you planning on? For small boats 2 HP or less, the rule allows more persons weight and is easier to comply with. Larger motors (55 lb is not a 2 HP, probably 5) but if you want this to meet Coast Guard standards you have to use the standard weight table Boat Building Regulations | Outboard Engine Weights Table 4 | 33 CFR 183 https://newboatbuilders.com/pages/table4.html rather than the weight of the specific outboard you are using.

Very good links Ike. I will use a 6 HP outboard on the 12 foot jon boat that I will build.

 

[Raptor88]

TANSL's comment is very apt. If the boat's sheer has any curvature to it, the point of entry can be at the lowest point along the gunwale. The regs talk about the static float plane, meaning a level plane intersecting the point of entry of water.

Thanks for those pics. Very helpful to see the different configurations. My jon boat will have a straight flat inwale with no curves as shown in the upper picture.

The sides will be 17.5 inches high and will be vertical (not tapered outward). I need the vertical sides because the only way I can store the boat on our property is to store it on a side on some 2x4's on a concrete walkway next to a hollow tile wall. I will cartop the boat and not have a trailer because there's no space for a trailer.

The width of the boat will be 47" wide in the rear 8 feet, tapering about 6" on each side in the front 4 feet to the bow plate.

 

[DCockey]

Read the page on Level Flotation. Boat Building Regulations | Level Flotation https://newboatbuilders.com/pages/flot2.html There is a stability test that involves putting part of the persons weight on one side and measuring the heel angle. It only allows a heel angle of 30 deg in that test.

That stability requirement is for a swamped/flooded boat and is a test of the floatation. The USCG regulations for recreational boats do not have a stability requirement for recreational boats which are not flooded or swamped.