A few years a go I added a wash down pump with hoses to the front and back of the boat. This works great while on the water, but not at all on the trailer.
Sometimes when we put the boat on the trailer we have algae or other crud on the sides of the boat. I have thought at times that it would be nice to have a way to rinse the boat and trailer, but not every boat ramp has a wash down facility.
Today I tried something with very good results. I connected a 25 foot garden hose and nozzle to a 1/2 inch pipe to garden hose adapter fitting. Then I removed the transom handheld shower head from its hose and connected the above adapter and hose to the transom shower hose. With the water pump on and some of the left over water in the fresh water tank I was able to rinse the sides and bottom of the boat and the trailer. Just be sure to not lose the rubber O-Ring when you remove the shower head. Leave the O-Ring in the end of the hose and the adapter will seal against it. 3 or 4 feet of the shower hose and a 25 foot garden hose should reach just about all of the boat or trailer. A link to an example of the fitting is below. This fitting is brass, but a plastic fitting should also work.
http://www.homedepot.com/p/Everbilt-Lea ... /207176334I have also used this adapter with a hose to circulate RV antifreeze back into the water tank when winterizing the fresh water system or the bleach or vinegar solution in the spring when getting the boat ready for the season. It also works well to connect to a hose to run any solution such as the bleach or vinegar to a drain for disposal.