One over the sine of the slope
Everything on this page comes from a single number: how many feet along the ramp you travel for each foot of depth you gain. That is one divided by the sine of the ramp angle, and on the shallow slopes ramps are built at, it is a big multiplier.
| Grade | Angle | Ramp per foot of depth | 3 ft of tide moves the waterline |
|---|---|---|---|
| 6% | 3.4° | 16.7 ft | 50.1 ft |
| 8% | 4.6° | 12.5 ft | 37.6 ft |
| 10% | 5.7° | 10.0 ft | 30.1 ft |
| 12% | 6.8° | 8.4 ft | 25.2 ft |
| 15% | 8.5° | 6.7 ft | 20.2 ft |
| 20% | 11.3° | 5.1 ft | 15.3 ft |
Read the last column as the reason a ramp that works at one state of tide is a different place at another. On a gentle six percent ramp, three feet of tide is fifty feet of concrete — and if the good surface is only forty feet long below the high water line, half the tide range is unusable regardless of what the boat needs.
The three heights that stack up
The water depth the transom needs is not the draft of the boat. It is three things added together, and the middle one is the one people leave out:
Depth at the transom = hull bottom above the ramp + draft + your float-off allowance
The hull does not sit on the ramp. It sits on bunks or rollers, which sit on a frame, which sits on the axle. That stack is typically somewhere between a foot and two feet, and it has to be submerged before the water even reaches the hull. Add the draft on top of that and you are already asking for three feet of water in a case where the boat only draws eighteen inches.
The allowance on top is a judgement. Enough water and the boat lifts clear and swings free; too little and it drags on the bunks, which is where the winch strap gets loaded and where a boat sometimes comes off in a way nobody planned. How much is enough depends on the hull, the bunks and how much you trust the wind, and this page asks rather than assuming.
Where the tow vehicle ends up
Once the depth at the transom is fixed, everything else is subtraction along the rig. The trailer wheels sit forward of the transom by whatever that distance is, so they are that much less far down the ramp and in correspondingly less water. The tow vehicle rear wheels sit forward of the trailer wheels by the length of the coupling and the tow vehicle behind its rear axle.
Worked through with the defaults on this page — a 15 percent ramp, 1.5 feet of bunk height, 1.8 feet of draft and half a foot of allowance — the transom point is 25.6 feet down the ramp, the trailer wheels are 19.6 feet down in 2.9 feet of water, which is two feet over the hub centres, and the rear wheels of the tow vehicle are 5.6 feet down in ten inches of water.
Ten inches is not nothing. Hub-deep trailer bearings, wet brakes and a rear axle in salt water are the ordinary consequences of a routine launch on a shallow ramp, and they are the reason people with long tongues, extensions and drop hitches use them.
What the geometry quietly assumes
It assumes the ramp is a plane and the trailer is on it. Both are approximations. Most ramps have a break where the flat apron meets the slope, and while the rig is crossing it the tow vehicle is level and the trailer is tipping — the stern is lower than the flat-plane arithmetic says, which helps, and the tongue is higher, which does not.
It also assumes a constant slope. Plenty of ramps get steeper at the bottom, some flatten out onto silt, and a good number simply end. The number that decides a launch is not the grade on the sign; it is what is under the water at the end of the concrete, and no calculation reaches that.
And it takes the bunk height as a fixed measurement. A trailer with the suspension compressed under a loaded boat sits lower than the same trailer measured empty, so measure it loaded if the margin is thin.
Questions people ask
How deep does the water have to be to float a boat off a trailer?
Deeper than the draft, by the height of the trailer under the hull. The hull bottom sits on bunks or rollers on a frame, and that whole stack has to be submerged before the water reaches the boat at all — commonly a foot to two feet. Add the draft, then add however much extra you want above the point where the hull just lifts, and that total is the water depth needed at the transom. A boat drawing eighteen inches on a trailer with eighteen inches of bunk height needs three feet before it starts to float and more before it floats clear. The float-off allowance is a judgement about your hull and your bunks, which is why this page takes it as an input rather than choosing one.
Why does a small tide change move the waterline so far up the ramp?
Because the slope converts vertical into horizontal by a factor of one over the sine of the angle, and ramps are shallow. At 15 percent grade that factor is 6.7, so a three foot tide moves the waterline about twenty feet. At 8 percent it is 12.5, and the same tide moves it nearly thirty-eight feet. This is the single most useful number to know about a ramp you use regularly, because it tells you how much of the tide range the concrete actually covers. If forty feet of good surface runs below the high water line on an eight percent ramp, that is only about three feet of tide before you are launching off the end of it.
Will the tow vehicle rear wheels go in the water?
On a shallow ramp with a short tongue, usually yes. The arithmetic subtracts along the rig: the transom sits at the depth it needs, the trailer wheels are forward of it by the trailer length behind the axle, and the tow vehicle rear wheels are forward of those by the coupling and overhang. With the defaults here the rear wheels finish in about ten inches of water. Whether that matters is a question about your vehicle and your water, and salt is the part that decides it. Longer tongues and tongue extensions exist because of exactly this arithmetic, and they move the vehicle up the ramp by their own length one for one.
What ramp grade should I expect?
There is no figure this page will state as fact, because ramps are built to whatever the site allowed and to whatever the authority that built them specified, and those differ everywhere. What is worth knowing is the range you will meet in practice: a purpose-built concrete ramp is usually somewhere in the low teens as a percentage, gravel and natural ramps are often much flatter, and a few are steep enough to be difficult in the other direction. Measuring yours takes a tape and a level: the vertical drop over a measured length along the surface, and the grade is the drop divided by the horizontal run. It is worth doing once for a ramp you use often.
Does the trailer sitting on the slope change the numbers?
While the whole rig is on the ramp, not much — trailer and ramp are on the same plane, so a bunk height measured on level ground still applies. The place it does change is the break at the top, where the tow vehicle is still on the flat apron and the trailer has tipped down onto the slope. There the stern is lower than the flat-plane arithmetic says, which is why boats sometimes float earlier than expected during the transition, and the tongue is higher, which is why the coupling can bind. Suspension matters too: a loaded trailer sits lower than an empty one, so measure the bunk height with the boat on it if the margin is small.