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Getting Under the Trailer Bunks to Clean the Boat Bottom


Jim Smith Asked:

How do I get between the trailer bunks to clean & polish my boat bottom. I want to apply your VS721 Bottom Coat on the bottom of my boat, but it’s on a trailer and I need to get at the bottom that is supported by the bunks so I can clean it and apply the VS721 Bottom Coat. How can I do this without renting a crane?



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When cleaning the boat bottom with ALGAE STRIP, GREEN GENIE, WATERLINE STAIN REMOVER or BOAT SCRUB and protecting the bottom with VS721 it’s important to get the boat up off the bunks or rollers so that you can clean and protect the entire boat bottom, otherwise you will be left with unsightly patches that can negatively effect the boats performance and will result in poor performance and future damage to those sections of the boat hull.

When cleaning your inflatable boat or RIB with FOUL OFF, INFLATABLE BOAT CLEANER, INFLATABLE BOTTOM SPRAY, or SPEED CLEAN and protecting the bottom with POLY GUARD and REPELIN, it’s important to get the boat up off the bunks so that you can clean and protect the entire boat bottom, otherwise you will be left with unsightly patches that can negatively effect the boats performance and will result in poor performance and future damage to those sections of the boat hull.


  1. Hydraulic Bottle Jack of a capacity that meets or exceeds the weight of your boat.
  2. 9 to 12 cinder blocks or wooden blocks 8″ x 8″ x 16″
  3. 2” x 8” planks cut to 16” length pieces. You will need a minimum 3 pieces, but more may be required.
  4. Wooden shims as required

CAUTION: This is a safe procedure when done correctly and with care. If you do not feel confident in your abilities or equipment to handle heavy weights, do not lift your boat in this way, get professional help.

These illustrations are not to scale.

fiberglass boat on trailer picture drawing - aurora marine

1. Place the boat and trailer on a firm, level surface.

fiberglass boat on trailer - front lowered

2. Lower the tongue of the trailer to the ground to elevate the transom.. You may need a friend to help keep the tongue down.

fiberglass boat on trailer lift back with blocks - support transom

3. Block the transom using cinder or wooden blocks. Make sure that there is a 2”x8”x16″ wooden plank between the boat and the blocks to prevent damage to the boat. If the blocks and wood to not fit between the ground and the boat, you may have to use additional pieces of 2”x8”x16″ wooden plank to compensate for the difference. If the bottom of your boat is angled, use scrap lumber shims as required between the blocks and the hull.

fiberglass boat on trailer - lifting front with hydraulic bottle jack

4. Place a hydraulic bottle jack under the tongue of the trailer and raise the front of the trailer. When the jack extends as high as possible, you may have to place blocks and wooden plates under the keel (at the forward bulkhead location), to support the boat. Then lower the jack and use wooden plates under the jack to give you additional height. Continue jacking up the trailer. This procedure may have to be repeated several times to gain the necessary height.

fiberglass boat on trailer - support keel under forward bulkhead

5. Continue raising the front of the trailer until the boat is level or slightly bow high. Place blocks with a 2”x8”x16″ wooden plank on top, under the keel at the forward bulkhead to support the boat. Start lowering the jack until the weight of the boat is supported on the three columns of blocks and plates. Make sure that the boat is well supported on each column and is stable.

fiberglass boat with trailer - supported with cinder blocks

proper use of cinder blocks - do not use cracked blocks

6. Lower the bottle jack. Extend the trailer jack to support the tongue and remove the bottle jack. The boat should be well supported and stable on all three columns and the bunks of the trailer should be about 6 to 8 inches lower than the bottom of the boat. This will give you clearance to clean and polish the areas normally supported by the bunks.

7. Do not remove the trailer. It will act as a safety device to catch the boat in the event that it slips off the blocks or if one of the blocks breaks or the ground becomes infirm and the blocks sink.

8. Do not proceed to work on the boat until it is well supported and stable.

9. To reseat the boat on the trailer, reverse the above procedure.


Raising your boat off the bunks or rollers allows you to gain access to the complete bottom so that you can remove growth with Algae Strip, Green Genie, Waterline Stain Remover or Algex, clean and brighten the bottom with Boat Scrub or Alumabrite CBX and to protect it against marine growth, Zebra mussels and osmosis with VS721.

It’s important to clean and protect the entire bottom. Working around the bunks and rollers will make any marine growth worse and will affect the performance of your boat. Leaving parts of your boat bottom unprotected can compromise the integrity of your boat.

This procedure is proven safe for boats up to 40 ft. providing that you follow the instructions and make sure that the ground is firm and level, that your trailer and equipment is sound and the blocks and wooden planks and shims are in good conditions. You may have to modify this procedure slightly or adjust your boat on the trailer depending on the style of your boat, for example pontoon, catamaran or monohull. This procedure is NOT recommended for sailboats with a fixed keel.


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