mikegarrison

I once saw exactly that scenario. Truck started slipping, and then once it was already slipping it was unable to resist being pulled into the water. Eventually the only thing keeping it from sinking was the bouyancy of the boat. Required three electric winches to pull the truck/boat combination back out of the water.

Not saying it is likely that you will screw up as badly, but I actually saw it so it must be possible.

Gernby

I understand what you are saying, but it seems that F is going to be the output of the engine and gearing. If the engine doesn't provide enough force to accellerate the car and trailer up the hill, then I won't be going anywhere but backwards. We know that the mass is constant, but the car has to be considered as part of the mass. The rate of accelleration is going to be dependant on F and M (A = F / M).

Gernby

I don't doubt what you are saying. I've heard several scary stories from my friend and brother about pulling boats up steep ramps. The reason I don't see this happening to me is that this ramp is really not very steep at all. I was serious when I say that the TL pulls this boat right up the ramp with only 20% throttle, and it takes very little brake pedal pressure to hold the car and boat still on the ramp. My friend's Expedition doesn't even slip on the algae when pulling the boat out.

The downside to having such a low incline ramp is that I will have to put the boat (and car) further out in the water before the boat will float. I wouldn't be surprised at all if my exhaust winds up being a bit submerged in the water. A steep boat ramp would allow my car to stay dry.

mikegarrison

This assumes you are pulling the boat out, and neglects the car's mass because that all happens on the other side of the hitch:

M= mass of (wet) boat plus mass of (wet) trailer
W= M*g
theta = slope of ramp

F (force on hitch) - drag of water - sin(theta)*W = M*A

Simple enough. The drag of the water and the slope of the ramp are going to reduce A or increase F (depending on whether you treat A as a constant or as a result). Drag is a function of velocity, but weight is not. So if you go slow enough and accerate slow enough, what you end up with is:

F just a little more than sin(theta)*W

How steep is your boat ramp? If you get impatient and start accerating too much and moving too fast through the water, the neglected terms are going to grow quite quickly. So you probably want to figure some factor of safety (FS) -- at least 2, maybe 3 or more. Your decision, your risk.

So F = sin(theta)*W*(FS)

Gernby

In the case of the boat, I expect that drag in the water will be compensated for by the bouyancy of the boat, so the force on the hitch will incease as the boat comes out of the water.

I am going to measure the slope of the ramp this weekend.

THANKS!

Elistan

Can you post pictures of how the hitch is attached to the frame?

Gernby

There used to be a DIY or FAQ thread with pictures in the library about installing the Da'Lan hitch. I can't find it now, and I didn't take any pictures.

BlitzSRM

gernby, mike is correct. the car is on the other side of the hitch. we are examining the connection between the boat and the car. if we include the car's mass, we can calculate the amount of force the tires must exert to move the car and boat (if/when you lose traction)

i was thinking last night about the force distribution. between the veritcal, and hoirzontal legs. the vertical legs will actually act like hinges, to a certian extent. imagine cutting of the hoirzontal legs. the veritcal legs will hang and give a little in the fwd and aft direction. not providing much force in the horizontal direction. The hitch eliminates bending moments.

We should calculate for the worst case. horizontal force is handled by the hoizontal legs only. makes it simpler too.

so gernby, is the horizontal legs attached to sheetmetal on the bottom of the trunk? I didn't get o measure the thickness yet.

Gernby

Mike and Blitz,

I agree that the mass of the car doesn't directly effect the force on the hitch, but it does indirectly. The total force available to move the car AND trailer up the ramp is whatever the engine will apply to the tires. If the mass of the car and trailer are equal (which it almost is), then the force acted on the trailer (and hitch) will be half of the total force the engine is providing to the tires.

What I'm getting at is that if my engine provides 100 ft-lbs of torque, then there will be about 1700 total lbs of force on the tires in the horizontal direction to move the boat and trailer up the ramp. If the boat and trailer have a TOTAL weight of 6000 lbs, and the incline of the ramp is 20 degrees, then it will take a force GREATER than ...

Wait a minute. When I calculate the horizontal force of a 6000 lbs weight on a 20 degree incline, it comes out to 2052 lbs! If the car only puts 1700 lbs of forward force to the wheels in 1st gear, then it isn't going to go forward at all. Please tell me I'm calculating it wrong.

Gernby

BTW, the horizontal legs of the hitch do bolt to the bottom of the trunk with 4 large bolts, nuts, and fender washers. Earlier version of this hitch only had 2 bolts, and had a tendency to rip through the sheet metal over time. I am pretty sure that the bolts were ripping through because the tire trailers that were being pulled (for many thousands of miles on interstate highways) had too little tongue weight (lifting UP on the hitch over bumps).