Bottom painting while on the trailer

[waternwaves]

AS ALX and others have pointed out...... properly loaded......there is very!!!!!!!!!!!!!!!! little chance a concrete block would ever fail...... look at the load on a 40 or ft. high block wall,,,,,,the bottom blocks are not crumbling. properly loaded with no side load........ you have a better chance of being injured in an earthquake.
As he said.....keep the loads vertical on the web, dont use them on a slope, spread the load contact point with wood, and I would put a thin wood board between vertical levels of adjacent blocks just for better loading, as well as a piece of plywood on the ground to prevent point loading on a stone/concrete or rocky substrate.

Below is a couple of measured facts about some blocks.....

8x8x16 Gray Concrete Block 8x8x16 Gray Concrete Block
Complete Description
Product Description: A. Load bearing smooth face (light, medium, normal) weight concrete masonry units shall comply with ASTM C-90-96, UBC-21-4 (Type I or Type II). Provide required shapes such as corner, end units, radius bases, bond beams, lintels, etc. Provide units open one or both ends, as required. Maximum linear shrinkage shall be .065% from standard oven dry condition. Concrete masonry units shall have a minimum net compressive strength of 1900 psi.


Properties of Concrete Blocks — Strength
In the use of concrete blocks for the walls of buildings, the stress to which they are subjected is almost entirely one of compression. In compressive strength well-made concrete does not differ greatly from ordinary building stone. It is difficult to find reliable records of tests of sand and gravel concrete, 1 to 4 and 1 to 5, such as is used in making blocks; the following figures show strength of concrete of approximately this richness, also the average of several samples each of well-known building stones, as stated by the authorities named:

Limestone, Bedford, Ind.< (Indiana Geographical Survey).............7,792 pounds

Limestone, Marblehead, Ohio (Q. A. Gillmore) 7,393 pounds

Sandstone, N. Amherst, Ohio (Q. A. Gillmore) ..............5,831 pounds

Gravel concrete, 1:1.6:2.8, at 1 year (Candlot)............... 5,500 pounds

Gravel concrete, 1:1.6:3.7, at 1 year (Candlot)................5,050 pounds

Stone concrete, 1:2:4 at 1 year (Boston El. R. R.)..............3,904 pounds

Actual tests of compression strength of hollow concrete blocks are difficult to make, because it is almost impossible to apply the load uniformly over the whole surface, and also because a block 16 inches long and 8 inches wide will bear a load of 150,000 to 200,000 pounds, or more than the capacity of any but the largest testing machines.

Three one-quarter blocks, 8 inches long, 8 inches wide, and 9 inches high, with hollow space equal to one-third of the surface, tested at the Case School of Science, showed strengths of 1,805, 2,000, and 1,530 pounds per square inch, respectively, when 10 weeks old.

[Russ]

AS ALX and others have pointed out...... properly loaded......there is very!!!!!!!!!!!!!!!! little chance a concrete block would ever fail.

The point should be "properly loaded".
If you don't know how to properly load a block, don't try. They are very strong when done correctly and can crumble when not.


--Russ

[Kelly Hanson East]

Without trying to mother hen you guys to death...

the pictures shown above show patently BAD practice using concrete blocks.

As Russ points out, the blocks need to be properly loaded. In a wall, the blocks are aligned so that the compressive force is evenly spread out on the entire edge of the block profile (like a digital number 8 )

In the pictures above, for example. the blocks are crossed, and now all the load is bearing on about 1/10, maybe even 1/20 of the block edge area - e.g. the compressive forces are now 20x higher than if they were aligned. If the edge fails, the crack will propagate up and the block will completely collapse.

[Win]

Without trying to mother hen you guys to death...

the pictures shown above show patently BAD practice using concrete blocks.

As Russ points out, the blocks need to be properly loaded. In a wall, the blocks are aligned so that the compressive force is evenly spread out on the entire edge of the block profile (like a digital number

In the pictures above, for example. the blocks are crossed, and now all the load is bearing on about 1/10, maybe even 1/20 of the block edge area - e.g. the compressive forces are now 20x higher than if they were aligned. If the edge fails, the crack will propagate up and the block will completely collapse.

Would placing a piece of plywood between the crossed blocks evenly distribute the load and thereby eliminate the above weakness?

[ALX357]

The boat has been sitting on these blocks for about 6 months now. There have been high-wind storms, a freeze and thaw cycle, and the motor has been run on water-ears numerous times, plus I have been on and in the boat countless times as it sits. The most massive part of the concrete blocks is the center web, and then the corners, and if you look at the photos, you will see the webs are stacked on top of each other. If there were no sheer strength in the concrete blocks, yes the forces would be sitting on less concrete, but the concrete has cohesive strength, and holds itself together sideways as well, not just compressive forces as passing straight downward. If concrete did not have this strength, most structures that incorporate cantilever design could not be made of concrete, but you see bridges, and interstate piers all the time. Even the steel re-bar inside could not stop cracking if the concrete did not have cohesive force.
Three places of support under the boat, if divided evenly, come to about 1000 pounds on each stack. Probably there is more on the front than each of the rear block stacks. There are numerous places of contact between the blocks' sides and web, as stacked. If you are looking at 1500# on each stack, that is nothing for the strength of concrete blocks, even stacked as they are.

The danger EVEN IF ONE OR MORE STACKS collapsed comletely, is the boat would drop 5 inches onto the trailer. If I had my arm, hand or whatever between the bunks and the boat, at the same time of a collapse, it would be crushed, trapped. At other times, I could fall off the boat, the motor could catch fire and cook me, the boom could crack my head open. We sailors live a dangerous life.

I would say that any Mac owner (me included) is taking a huge risk to tow his boat on the original Rogered trailer at hiway speeds, the flimsy twisty thing that it is. Probably has no safety margin strength, just the minimum. Some have stated it should be considered packing material, good for supporting the boat from the factory into the container, and as far as off the dealer lot. Yet is is enough, and most of us deal with it as is.

Whatever the doom aspect, the point of the post was to show that one did need to build anything complicated to support the boat, nor to move it onto the supports. The trailer jack is strong enough to position the boat like shown, ( if using a plate for the jack instead of the wheel, ) without using anything but blocks, and sticks.

[beene]

I would never use those blocks myself.

I use would use 6x6 fence posts framed up to spec.

G

[Bluecrab]

I'm beginning my bottom painting prep now, I have a couple of questions for those who did multiple coats of 2000e and anti-foul.

How much of the 2000E did you mix each time to do one coat on the bottom?

How long did you wait before lowering onto the trailer to do the areas where the blocking was?

Did you do multiple coats with it blocked and then lower it an do the same process of multiple coats to the areas where it was blocked? Seems like a lot of mixing or is the pot life long enough to use the same batch between coats?

Thanks,
Wally

[Matt19020]

Wally I sent you a PM with more details if you do not get it ,let me know and I will try again....

More product specifics here:
https://secure.international-coatings.c ... 000186.htm

I believe it was about 5 hours between coats and put 6 coats on
More info here:
http://www.macgregorsailors.com/forum/v ... f=8&t=8080

[seacatcapt]

Now for a report on the boat painting project - did it all while on the trailer. Washed the hull with cleaner and water rinse. De-waxed the hull with Interprotect Solvent Wash 202 (wear a respirator), sanded with 80 grit, and cleaned again with Solvent Wash 202 (charcoal filters are a good thing ). Masked the paint line with 1-1/2 inch 3M Painters Tape #2090 (great stuff). Hired two college-age friends for the painting and using a car mechanic's creeper (another good thing), applied 4 coats of Interprotect 2000E (alternating between grey and white) with 3/8 inch nap rollers (don't go with cheap rollers - the fibers come off). Then applied two coats of Interlux Micron Extra with Biolux (16 hours apart for the Micron Extra). When the initial painting was complete and dry, backed the boat off the trailer about 9 inches, applied a tie-down strap to the bow (VERY important as the boat is stern heavy on the trailer), and repeated the above painting process for the now exposed bunk areas. Hull is completely painted, and I'm hoping that between the epoxy and the the anti-fouling, I don't have to worry about blisters or slime . Pictures of the process are available at http://s984.photobucket.com/albums/ae32 ... 0Painting/. All in all, not too bad, particularly with strong, hired help!
-Randy

[elvatoli]

How can you pull the boat back on the trailer ?
How can you move the boat back 9 inches while sitting on the trailer ?

Thank you
Hector
RGV, TX

[seacatcapt]

Hector,
Both moving back 9 inches and then back on the trailer was accomplished after the paint was dry by taking the boat to the launch and floating it back/forth as though launching the boat. Sorry for the delay in replying. I missed the post!
-Randy

[elvatoli]

On the photo No. 11 you are showing that the back is 9 inches back sitting on the trailer.
How you pulled the boat back while sitting on the trailer ?

Thank you
Hector
RGV, TX

[seacatcapt]

UPDATE - First, sorry for the S -L - O - W reply, Hector but after painting around the bunks and letting dry, took the boat back to the launch, backed in, and pulled it back up on the trailer.
Now for an unsolicited endorsement. As posted earlier this spring, I painted the hull with epoxy sealer and then two coats of Interlux Micron Extra with Biolux. I pulled the boat out of the slip for the season this past weekend (having been in since May) and ... not a bit of green slimey goo anywhere! A little cleaning along the stripe line (above the antifouling) and I was done! I'm very pleased with the Interlux performance. Will put on a new overcoat (no, not me - the boat) this coming spring.

[ALX357]

Just an update on the safety aspect of concrete blocks supporting the boat, for those that have been apprehensive of the strength of concrete blocks used for boat supports.

Now it has been THREE YEARS since the boat has been on these concrete block stacks. ( financial difficulties - make me an offer ) I have entered the boat uncountable times, sometimes with two other people, thru the transom and into the cabin, stood on every part of the deck, run the motor on water ears regularly, pressure washed the hull, cockpit, and decks, and raised the mast and lowered it several times, all while it's sitting on these same supports. 'Numerous freeze/thaw cycles, and some ground settling, ( three-point support, no problem ) but no cracking or crumbling of the conrete blocks.
Note again, if somehow the blocks suddenly vaporized, the boat would only be able to drop five or six inches onto the trailer, and only if a hand or arm was between the boat and the bunks, at that exact time, would there be any physical danger.