Measure the height of the hull bottom above the pavement at the transom, and the distance from transom to hitch.
Dwg credit: JWH
Getting your Boston Whaler (or any boat) off the trailer and into the water is usually accomplished with the aid of a launching ramp. It seems that no two ramps are quite the same in terms of their rate of descent. This can make a difference in how easily you are able to get the boat off the trailer and floating. Some ramps make you practically submerge the back end of your car to get the boat launched; others keep you high and dry while the boat floats free. Can you predict how a particular ramp is going to work before you back your car into the lake? Some basic trigonometry is needed to figure this problem out.
To calculate the minimum launching angle for your particular boat, you need to take two measurements. First, while the trailer is hitched up and on level ground, measure the height of the boat's bottom above the pavement. Second, measure the distance from the boat's transom to the trailer hitch.
Using my fifteen foot Boston Whaler on a Shoreland'r trailer as an example, I measure the bottom to be 25 inches above the pavement, while the transom is just about 18-feet (216 inches) from the hitch.
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Trailer on Level Ground Measure the height of the hull bottom above the pavement at the transom, and the distance from transom to hitch. Dwg credit: JWH |
To get the boat floating, assume that you will need to have the transom submerged in at least one foot of water. Using my boat as an example, it will actually float in less than a foot of water (8-inches with the outboard up). When launching on a ramp and at an angle, not all of the hull will be in the water and thus it is likely to have less buoyancy. The portion submerged will also have more weight bearing on it. The result will be that it may take more water to float the transom while entering at an angle than it does when the entire boat is in the water and at its static (level?) trim. But figure a foot for a starting point. You can refine this part of the equation some day by actually measuring the draft at the point of floating off the trailer. For sake of simplicity here, let's just use 12 inches as the amount of water needed to float the transom.
Adding 12 inches to the initial height of the transom, 25-inches, gives a total height needed of 37 inches. Now we know the dimension of our "launch triangle." It is a right triangle with an hypotenuse, in this case, of 216 inches. The side opposite the angle is 37 inches. Now we can find the angle:
Let x be the angle of the ramp
Sin[x] = 37 / 216
= 0.17
thus:
x = ArcSin[0.17]
x = 9.86 degrees
This case is illustrated below:
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Trailer on Ramp With a ramp that has adequate descent, the transom of the boat will be submerged at least one foot by the time the hitch reaches the edge of the water on the ramp, keeping your tow vehicle, its exhaust system, and its differential safely clear of the water. Will the boat be afloat at this point? Check your draft when floating free, ignoring the outboard. For most Boston Whaler boats it is usually less than one foot. Dwg credit: JWH |
It is a very simple extrapolation to see that the steeper the ramp, the sooner the boat is going to float free and the farther from the water your tow vehicle will remain. But the steeper the ramp, the greater the pull into the water on the hitch and the tow vehicle, and also the greater resistance when it is time to retrieve the boat and trailer and drive up the ramp. This can also be calculated quite simply.
When the boat is on the trailer on level ground, all the weight of the load is born by the wheels and axle of the trailer and by the tow vehicle through the tongue weight. Moving the trailer and boat only requires that the tow vehicle overcome the friction of the bearings, the wind drag of the boat and trailer, and inertia.
When the boat descends a ramp, however, a significant portion of the weight of the boat and trailer is transferred from the tires and axle to a pull on the hitch and tow vehicle. The steeper the descent, the more the load increases. This, too, varies with the descent angle.
Using the example above, the boat, motor, and trailer weight about 1400 pounds. At rest on level ground, the rig can be easily moved by hand. But on the ramp the load increases according to this relationship:
Let w be weight rig on level ground, and
let x be the angle of ramp descent
hitch pull = w * Sin[x]
For the case of a 1400 pound trailer on a 10-degree ramp:
hitch pull = 1400 * Sin[10]
= 1400 * 0.1736
= 243 pounds
"Well," you say, "that is not much of a load." Yes, it is not, but if the tow vehicle is on the same descent, then it is also experiencing a pull down the sloping ramp. You know this intuitively because you have your foot on the brakes! If the car or truck weighs 4500 pounds, it is creating its own pull down the ramp of 780 pounds. Keep your foot on the brakes.
So just attempting to resist the pull of the car, boat, and trailer down a 10-degree slope requires over 1,000 pounds of force to be exerted by the car. Compared to level ground, this is a huge increase in the load on the hitch and tow vehicle.
And when recovering, we have to consider the hydrodynamic resistance of pulling the trailer and boat through the water, as well as the extra weight of water temporarily aboard the trailer! It all adds up. Now to get this mass accelerated from rest we have to over come all the existing frictions, plus the new water related frictions, plus overcome the thousand pound pull into the lake! This will require much more horsepower and effort from the tow vehicle than when on level ground.
The ideal boat launching facility would be to have two ramps. One fairly steeply descending ramp for launching, and one less steep for recovery. On the steep launch ramp, you would be able to easily float the boat off the trailer, and with the weight of the boat removed, there would not be difficulty in pulling the trailer back up the slope. For recovery, a less steeply sloped ramp might be preferred. The boat could be winched onto the trailer, and then the load of pulling the complete rig up the ramp would be more easily handled on the less steeply pitched ramp.
If the level of the water is fairly constant, it would be possible to combine the two ramps into one. Make the slope of the ramp on the portion where the tow vehicle will operate rather gentle, say less than 10 degrees, but as soon as the ramp submerges below water, increase the slope to more like 15 degrees. This will get the boat floating faster, yet keep the tow vehicle from from having to climb Mt. Everest on the way up the ramp. This approach would be good on a lake whose level is controlled by a dam, but it would not work well on water whose level varies much.
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Copyright © 2000 by James W. Hebert. Unauthorized reproduction prohibited!
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Author: James W. Hebert
This article first appeared July 1, 2000.