Need some "engineers/or people smarter than me" help.

K

Ken Scheffler

New member
So I am trying to design my own layout boat. . . I am wasting away in Iraq and have nothing else to do with my time. So i got to thinking about the dimentions that I want and need to design the boat so the lip rides just below the surface. Here is my dilema, how much water displacement do I need to float the boat and me (prospected total weight of 300 lbs). Is there some sort of equation that I can follow for water displacement?

My theory was this, if you fill a gallon milk jug half full of water and set it in the water, the water lines will be equal right? So if a gallon of water weighs approx 8.5 lbs and a gallon of saltwater closer to 9. The cubic inces in a gallon of water are 231. . . so it all equates to roughly 7700 cubic inches of water displacement i would need to float the boat? Right?

Anybody confirm or deny my theory. . .or have a better one. . . I would love to hear it.

Thanks, Ken
 
Basic princible:
An object in a fluid experiences an upward force equal to the weight of the fluid displaced by the object. So if a boat weighs 1,000 pounds (or kilograms), it will sink into the water until it has displaced 1,000 pounds (or kilograms) of water. Provided that the boat displaces 1,000 pounds of water before the whole thing is submerged, the boat floats.

Although the density of water varies somewhat with temperature and pressure, roughly, salt water weighs 64 lbs/cu. ft., as opposed to fresh water, which is about 62.2 lbs/cu. ft. There are 7.4805 gallons in a cubic foot. So a gallon of salt water weighs 8.556 lbs.

The above info is enough to figure out how much water the layout needs to displace to achieve the desired float lines of the boat.
 
Last edited by a moderator:
Something to remember is that the water displaced is what is 'under' the water so if you figure the whole boat weighs "X" lbs with you in it and design for that then the whole boat will be at/under the water. If it were me I would design it with the box and maybe a half inch or an inch under water with you in it. Seems obvious but an easy mistake to make. Thanks for your service.
 
Ken,

Let me see if I can help by leading you on a goose chase.
Google Simplicity Boats.
From there go to links.
somewhere there is a design page with the equations you need.
 
Thanks for your service and stay safe.

To throw you one more variable to consider, it is illegal to hunt from a floating device that conceals your body below the surface of the water.

from 50 CFR 20.21

Migratory birds on which open seasons are prescribed in this part may be taken by any method except those prohibited in this section. No persons shall take migratory game birds:

(c) From or by means, aid, or use of a sinkbox or any other type of low floating device, having a depression affording the hunter a means of concealment beneath the surface of the water;
 
Thanks fella's, that is exactly what I needed to know.

Brad, if my calculations are right then there will be no need to worry about being below the surface. . . if they're wrong then I better wear a wetsuit and watch out for the wardens :) ha ha ha
 
Thinking about ducks and water is better than looking at sand any day. Your calculations for displacement will effect the amount of box below the boat and the shape of it. The box will have to be wider at the back from your butt to your head and narrowing down to your feet. You should calculate the boats weight in sections for box displacement. Only about 20% of your body weight is from your butt to your feet. The shape an depth of the box should be considered so the boat will ride and float correctly. Try not to crowd your shoulder and make it so it is not a struggle to get in and out of with heavy clothes on. Make it to withstand heavier wave action than normal hunting conditions. Fresh water is 8.34 lbs. per gallon an 7.48 gallons per cubic foot. The weight of the water being displaced by the box should be equal to weight of the boat plus average weight of the persons using it to make it float right and keep the edges of the boat in the water. Dream Away!

.
 
Figure about 60 lbs per cubic foot of displacement at the designed water line. It's really something like 62.305 or something like that but it won't matter, just use 60.

Ed.
 
Oh yeah and I would encourage you to make a scale model to test your design so that there's no surprises when the full scale boat is finally complete. I built several models when designing the Peregrine. I'm glad I didn't go with my first idea which ended when I saw the performance of the model. When you build a model the weight of the model needs to be the cube of the scale, so for instance the weight you need to add to a 1/4 scale model would be to make it 1/64 the weight of the full scale boat, motor, and payload.

Ed.
 
You must log in or register to reply here.
Back
Top