It is difficult to assess the ground clearance shown in Figure 1 because the camera is shooting downward. The clearance does look rather close.
DarrylR wrote:When parking the trailer in my driveway I had to remove the license plate to keep it from scraping the ground as I backed it up over the gutter.
When towing a boat trailer you will soon learn to be alert for road elevation changes in which there is a sharp descent followed by a sharp ascent. When the trailer transits such a change in elevation there is a very great chance the trailer will bottom out--or even worse, that the skeg of the outboard engine will bottom out. If you want to be certain the trailer or engine skeg will never bottom out, you need to increase the clearance above the roadway, and always tow with the engine in a tilted-up and locked position.
As a general rule, having the trailer frame set low to the ground will put the boat low to the ground. This will be useful when at a launching ramp that has little water depth at the end of the submerged roadway portion, or just in general at any ramp, as the faster the boat immerses into the water, the less distance you have to back in the trailer. The less distance you have to back in the trailer, the farther from the water your towing vehicle will be.
I have seen situations on some launch ramps with some boats and trailer where the towing vehicle rear tires are in the water, and even on occasion where the exhaust pipe of the towing vehicle is underwater and bubbling out exhaust. The lower the boat is on the trailer and the lower the trailer to the ground, the less likely these situations will occur.
Of course, there is a limit to how low the trailer can be to the ground and not become a nuisance with bottoming out in normal driving.
In Figure 3 there appears to be about 10-inches clearance between the road and the bottom of the rear cross member of the trailer. I assume that when this was measured the trailer frame was aligned to be level with the ground.
The trailer should be towed with the trailer frame level to the ground. I frequently see trailers being towed that are not level, but those instances are usually yahoos towing jet-ski boats or U-Haul trailers. The hitch height on the towing vehicle should be adjusted so the trailer tows level to the ground.
Q3: If you transposed the position of the axle and spring, how much increase in height above the pavement would the trailer frame rise?
By the way, I have never measured any clearance on my trailer. Using the metric of the license plate scraping on the pavement, I know that was occurring with my trailer set up. I discovered this when I noticed that the license plate was gone. There was just a bit of one corner of the plate left, firmly bolted to the trailer, but all the rest of the plate was gone. I was concerned I was going to have to pay $75 for a new trailer license plate, but the Secretary of State office kindly gave me a replacement plate for $4.
- Fig. 4. Trailer clearance. The lowest object on my boat trailer is the bottom edge of the keel roller brackets. In this view I would guess there is about 10-inches clearance, at most.
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