Dear Group,
In an earlier post i mentioned my project being a pontoon boat. I
have run into conflicting information about the shape of the pontoon
versus bouyancy of the pontoon. Can someone enlighten me on this
subject? Large flat bottomed pontoons versus round pontoons? Thanks
folks...


M Russon

MRusson ) writes:
Dear Group,
In an earlier post i mentioned my project being a pontoon boat. I
have run into conflicting information about the shape of the pontoon
versus bouyancy of the pontoon. Can someone enlighten me on this
subject? Large flat bottomed pontoons versus round pontoons? Thanks
folks...

And the conflicting information is?

You use less material in a round shape because a circle encompases the
most area with the shortest perimeter. You get more strength from a circle
because an arc distributes stress.

Hope that helps.

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What exactly is the confusion or conflict ?

heres some info as I understand the issue.


Bouyancy can be calculated by the amount of water displaced by the
hull. (as a rough figure, ignoring salinity etc)

For a cylinder it is pi * radius * radius * length with all figures in
feet. (pi is 3.14 for rough calculation)

A cubic foot of water weighs about 62 pounds.

If your pontoons were square or rectangular cross section the
calculation would be

width * height * length

(* = multiply)

square pontoon 1 foot square and 10 feet long = 1 * 1 * 10 = 10 cubic
feet
10 * 62 = 620 pounds load.
that is 620 pounds including the pontoon will have the water level at
the top of the pontoon.

since you are not going to have the top of the pontoon level with the
water surface the actual load carrying capacity is less than the figure
obtained.

I would divide by 2 for half of pontoon in the water.

The load figure must include the weight of the pontoons.

Square section pontoons carry more load for a given size, but other
structural issues come into play. The square section might need to be
heavier to deal with the stresses.

Also a round section pontoon will sink further under initial loads and
then sink less as the wetted area increases.

Calculation is per pontoon. Double for two pontoons on a boat.

regards.

On 2 Sep 2005 11:29:22 -0700, wrote:




Square section pontoons carry more load for a given size, but other
structural issues come into play. The square section might need to be
heavier to deal with the stresses.

Such as increased bulkhead sections and heavier material?

Also a round section pontoon will sink further under initial loads and
then sink less as the wetted area increases.

As the water level rises up the radius, more surface area is
encompassed and it tends to resist sinking more?

Calculation is per pontoon. Double for two pontoons on a boat.


So in a nutshell, a square or rectangular pontoon will be the best
for bouyancy due to its increased surface area, but will require a bit
beefier construction since it will be exposed to hull stresses that a
cylindrical pontoon could absorb. Also, the square or rectangular
pontoon would have to be slightly larger in order to accomodate the
increased weight and offset that weight to add more bouyancy. Am i
understanding that correctly? Thanks a ton!!!!


MRusson

MRusson ) writes:

So in a nutshell, a square or rectangular pontoon will be the best
for bouyancy due to its increased surface area, but will require a bit
beefier construction since it will be exposed to hull stresses that a
cylindrical pontoon could absorb. Also, the square or rectangular
pontoon would have to be slightly larger in order to accomodate the
increased weight and offset that weight to add more bouyancy. Am i
understanding that correctly? Thanks a ton!!!!

That's it. Another thing to remember about pontoons is that each pontoon
should be able to float close to the maximum load. You want to be able to
walk right over to the edge of the boat carrying bar-b-que and cooler, and
have it remain level.

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William R Watt National Capital FreeNet Ottawa's free community network
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One other point on shapes. Put the front platform rail a ways in from the
bow. We rented a pontoon boat on Kentucky Lake and set off with four
aboard. The other three took their lawn chairs as far from the motor as
possible, while I manned the helm. At drifting speed it was fine, but once
we left the marina and I opened the throttle, it became a submarine.

Dive, Dive, Dive.

The water only came half way back along the deck before I closed the
throttle and let the bow bob back up.

Roger

http://home.earthlink.net/~derbyrm
"William R. Watt" wrote in message
...

MRusson ) writes:

So in a nutshell, a square or rectangular pontoon will be the best
for bouyancy due to its increased surface area, but will require a bit
beefier construction since it will be exposed to hull stresses that a
cylindrical pontoon could absorb. Also, the square or rectangular
pontoon would have to be slightly larger in order to accomodate the
increased weight and offset that weight to add more bouyancy. Am i
understanding that correctly? Thanks a ton!!!!

That's it. Another thing to remember about pontoons is that each pontoon
should be able to float close to the maximum load. You want to be able to
walk right over to the edge of the boat carrying bar-b-que and cooler, and
have it remain level.

--
------------------------------------------------------------------------------
William R Watt National Capital FreeNet Ottawa's free community
network
homepage: www.ncf.ca/~ag384/top.htm
warning: non-FreeNet email must have "notspam" in subject or it's returned

On 3 Sep 2005 15:09:04 GMT, (William R.
Watt) wrote:


MRusson ) writes:

So in a nutshell, a square or rectangular pontoon will be the best
for bouyancy due to its increased surface area, but will require a bit
beefier construction since it will be exposed to hull stresses that a
cylindrical pontoon could absorb. Also, the square or rectangular
pontoon would have to be slightly larger in order to accomodate the
increased weight and offset that weight to add more bouyancy. Am i
understanding that correctly? Thanks a ton!!!!

That's it. Another thing to remember about pontoons is that each pontoon
should be able to float close to the maximum load. You want to be able to
walk right over to the edge of the boat carrying bar-b-que and cooler, and
have it remain level.

Thanks for the insight William. I appreciate the feedback very much. I
am new to this whole pontoon game, and i really want to get it right
the first time.

regards,

M Russon

On Sat, 03 Sep 2005 17:14:25 GMT, "Roger Derby"
wrote:


That's it. Another thing to remember about pontoons is that each pontoon
should be able to float close to the maximum load. You want to be able to
walk right over to the edge of the boat carrying bar-b-que and cooler, and
have it remain level.


Roger,
Did the boat emit a sonar sound just before submersing?.....)
I would probably have dropped a letter to that manufacturer and asked
them just how deep the pontoon boat can submerge before it's
considered a diving vessel.....lol. I bet they would not think it was
funny though. Thanks for your replies. I will take what i have learned
from you and Mr. Watt and get some figures put together. Then i'll
post a few numbers along with sizes, and see what everyone thinks.
Thanks again.

regards,

M Russon

This discussion needs to be considered in simplistic terms.

If you build a square section with flat front you will need a lot of
power to push it through the water.

This is why the commercial pontoons are tapered at the front in such
a way as to provide a smooth water flow and reduced resistance.

You can build a pontoon out of plywood by using a parabolic section
base and flat top.
(like a U section)

A thin ply can be bent around the bulkheads and the spaces filled with
flotation foam.

There is another thread on the rec.boats forum concerning attachment
loads for skinned structures. Simply stated you need to spead the
stresses out at attachment points or you rip out the plywood.


There are a number of old boat plans for pontoon boats. Maybe your
library has something.

Correction.

I meant to say this discussion has been stated in somewhat simplistic
terms.

There is a lot about hull shape , directional stability etc to be
considered.

For example, you could build a pontoon boat that will carry tem people
evenly distributed over the boat. If someone shouts ' come look at the
pretty birdy' and all ten rush to the left side rail you have a
situation. The requirement that the boat stay flat in that situation
changes the design.

Weight and balance left/right and front/back need to be considered. (I
suppose I should have said port/starboard and fore/aft but you know
what I meant - and which way is port again ?)