posted 03-30-2009 08:17 AM ET (US)        

My first-pass analysis of this situation focuses on the distribution of boat weight on the trailer. At this point the bow of the boat is not in the water, but typically some portion of the stern of the boat is still immersed. This means that the buoyancy of the hull is imparting a lifting force at the stern. The stern of the boat is not putting its full weight on the trailer. This has to change the weight distribution on the trailer. It would seem to imply that the weight on the tongue will tend to increase, or at least a greater percentage of the total weight would be carried on the trailer tongue.

As the boat is pulled from the water, the buoyant force of the hull is removed. The weight distribution on the trailer returns to its more normal division between axles and tongue. The dynamic change in trailer loading and weight as the boat moves up the ramp may be helpful. The buoyant forces on the hull reduce the overall weight of the boat and trailer, lessening the burden on the tow vehicle to pull them up the ramp. The shift in weight distribution forward increases the tongue weight, which helps transfer weight to the tow vehicle rear axle. More weight on the rear axle improves the traction.

Once the trailer begins moving up the ramp, the stern of the boat's hull is pulled from the water, and this buoyant force is removed. This causes the overall load on the tow vehicle to increase, and it also causes the weight distribution on the trailer to return to its normal state.

posted 03-30-2009 11:54 AM ET (US)            

The moment you begin your decent down the ramp, the dynamics of trailer and the loads change. At the transition point when the boat/trailer is on the ramp and the tow vehicle is on level ground, there could in fact actually be an upward force on the tongue depending on how the boat is loaded. This potential upward force is reduced as the tow vehicle descends onto the ramp with the trailer. At this new stage of equilibrium, the dynamics of the trailer/tongue weight do not return to what they were on level ground. Since the force of gravity is still straight down, and the tow vehicle/trailer are now on an incline, the weight distribution is transferred to the rear of the boat reducing the tongue weight as forces are not only pulling the boat down, but backwards off the trailer due to the incline. When the boat begins to float, all forced on the trailer are reduced. The reverse is true when loading the boat and hauling out. Now, its a bit more complex than that, but that's a readers digest version.

posted 03-30-2009 12:37 PM ET (US)            

Make sense?

To answer your question about traction, yes, there is less downward force pushing the wheels to the pavement, hence why some cars slip (not to mention the fact that generally it is wet and sometimes slimy from algae)

posted 03-30-2009 01:59 PM ET (US)            

With the boat on the ramp, the hitch load will be decreased - as the trailer wheels will be more directly under the CG of the boat - and hence carrying more of the load. Typically, on level ground, the CG is only a few inches forward of the trailer axle(s) and when on a slope, the CG is effectively more directly placed over the axle(s). This is, contrary to your thoughts, being on the ramp is not helpful as the hitch load is decreased, which decreeases traction of the tow vehicle's tires.

Further, the steeper the slope of the ramp, the more pronounced will be these effects. In fact, with a really steep slope, or a real light level hitch load, the hitch load when on the ramp could be upward.

When the stern is "buoyed" during the launch or retrival, the boat weight will be decreased - which will tend to decrease the hitch load, but also the "effective" CG location will be moved forward, as only the bow is supported by the trailer - which will then tend to increase the hitch load. Therefore, the answer to part of your question - during the launch (or retreival) is: it depends - on which (partial buoyancy and location of the effective cg) is more controlling.

One last point - with the vehicle and trailer on the ramp, the energy requirement is primarily that of potential energy (mass * distance), rather than kinetic that is of interest. That is, the magnitude and rate at which the total load is moved vertically up the slope. --- Jerry/Idaho

posted 03-30-2009 07:11 PM ET (US)            

Thanks to those who offered observations about retrieval of their boats, which when loaded on their trailers, had no hull immersed. Situations like that have no application to this discussion because there is no possibility of a buoyant force being created to affect the tongue weight. I cannot see anyway to draw an inference from your experience.

Jerry--Thank you for responding to my topic. I was hoping you would, since, because of your previous experience in trailer force analysis, your opinion was the one most sought.

I must be wearing good foul weather gear, because so far I don't have a drop on me, that is, I am not wet at all. I think everyone so far has agreed that the loading on the tongue weight changes on a ramp, and the buoyant force of the hull is an influence. That was my point. so thanks for confirming it.

I am not much concerned wit what happens as we descend the ramp, as most of the time there is an excess of braking force available to handle any situation. I am more concerned with what happens when we try to go up the ramp and the rear wheels spin.

posted 03-30-2009 11:45 PM ET (US)            

None of the influences that Fishgutz mentioned--a long list in which I think he mentioned every possible influence except the tire pressure and the tire tread--none of those really bear on the situation that I am presenting with regard to the influence of the buoyant forces of the hull. The buoyant forces of the hull are only affected by hull form and the amount of the hull that is immersed when the boat is fully loaded on the trailer. For a particular rig, those force remain fairly constant on a given ramp. As soon as you start up the ramp, the hull leaves the water, and that buoyant force disappears.

In the case of my boat, I would estimate that an area of about 8-feet by 1-feet by 8-feet, or 64-cubic feet of boat, in other words, the aft 8-feet of the hull, are still in the water when the boat is recovered onto the trailer. If we consider this is a v-hull, then we can assume the volume of the hull is about half of that or 32-cubic feet. With a water density of 64-lbs per cubic foot, that is a buoyant force of about 2,000-lbs that is lifting the stern of the boat up. If I am on a ramp with a 30-degree slope, I only have to pull ahead a couple of feet and, suddenly, the boat is out of the water and 2,000-lbs of weight are added to the rear of the trailer. This has to affect the tongue weight, and it can only tend to decrease it.

I believe this is why you see the rear wheels slip on some tow vehicles just as the boat comes out of the water. The tongue weight decreases then and the rear wheels lose some down force.

posted 03-31-2009 08:11 AM ET (US)            

I also like to imagine these instances and your scenarios in extremes. Take for instance a trailer with zero or negative tongue weight i.e. a floating tongue. You would start with a lot of forward weight as you go up the ramp to no weight and the trailer actually lifting the rear wheels of the vehicle off the ground as the boat/trailer combination gets high and dry. This will vary greatly with regard to the slope of the ramp. A very steep ramp will start with lots of tongue weight and decrease to negative tongue weight. A level ramp will go from no tongue weight (whole boat floating level and trailer level under water) to negative tongue weight.

Now imagine a trailer with its wheels at the very rear of the trailer. LOTS of tongue weight. At the ramp there is lots of tongue weight as the boat is winched up the trailer. The tongue weight will increase little if at all as you pull out.

Now with your theory, you have a properly set up trailer and use the same ramp. You will be somewhere in between these 2 extremes. Your starting tongue weight, the slope of the ramp, the position of the axle will be the main variables (constants in your situation). So yes, as you pull out the tongue weight will be heavier as you start until the total weight of the boat is on the trailer.

I would bet with the way I've seen people have their trailers set up that one would see some of these extremes (to some point). In my case the increase in tongue weight going up the ramp at my favorite launch is minimal. And in my case I have a front wheel drive vehicle so although it would put me at a disadvantage, my front wheels are usually on dry pavement or at least not in the slime.

posted 03-31-2009 07:47 PM ET (US)            

quote:

---

The moment you begin your decent down the ramp, the dynamics of trailer and the loads change. At the transition point when the boat/trailer is on the ramp and the tow vehicle is on level ground, there could in fact actually be an upward force on the tongue depending on how the boat is loaded. This potential upward force is reduced as the tow vehicle descends onto the ramp with the trailer.

---

Seems we are in agreement. But as you reminded us, this was not the subject of your thread when you wrote

quote:

---

Thanks to those who offered observations about retrieval of their boats, which when loaded on their trailers, had no hull immersed. Situations like that have no application to this discussion because there is no possibility of a buoyant force being created to affect the tongue weight. I cannot see anyway to draw an inference from your experience.

---

posted 03-31-2009 08:36 PM ET (US)            

The slippage of the rear wheels is probably the result of that is the first time during the retreival when the total weight of the tow vehicle and boat is moving under power via the tow vehicle's engines and driven tires.

Backing over the end of the ramp, perhaps enhanced by low water which many are faced with today, can be a real bummer as some have found out.

As mentioned above, the hitch load can, under certain circumstances, be upward - or "negative". Dynamic loading (bouncing/pitching) can cause negative loading - as can the vertical projection of the CG of the boat being behind the axle - as mentioned below.

In general, the steeper the ramp, the more weight that is put on the axle(s) and the less weight that is on the hitch - up to and perhaps giving an upward hitch load. The reason for this is that on the level, the CG of the boat will be slightly (2 - 8) inches in front of the axle. With the boat on a ramp, the vertical projection of the CG will move closer to the axle - which increases the load on the axle - and decreases the load on the trailer. And, with increased slopes, the vertical projection of the CG could be in back of the axle - which would give an upward (negative) hitch load.

And a couple of more comments. And here, I am out of my baliwick - as my 17 Outrage is not a big boat. But, those with a heavier/bigger boat could be using a lighter (around 5% of the total load) hitch load (less weight on the tow vehicle, easier to handle, et al.) - whereas those with the lighter/smaller boats could be using a heavier (around 10% of the total weight) hitch load.

posted 04-01-2009 10:09 AM ET (US)         

Lets for simplicity assume that the ramp is at 45 deg therefore any load applied to the vehicle axel via the tow hitch will be made up of 2 components ie 50% of the load acting via the tyres to the ground and 50% acting horizontally. The total resultant 100% is acting down the slope and along the trailer.

This load will vary as the buoyant vessel weight is applied and distributed to the trailer bunks.

Assuming that before moving forward the boat is winched hard against it’s winch stop and that the normal CoG of the fully loaded boat is 1 ft in front of the trailer axels and that there is 4ft of trailer bunks in front of this position and the hitch is another 4 feet in front of this.

As you move forward and the total buoyant load is gradually decreased and applied to the bunks load.

We could assume for simplicity that for every 1ft of contact this would equate to 25% of the boat weight applied.

This would support your thoughts that initial 25% load is applied 3’-6” forward of the 100% trailer loading CoG (4’-6” in front of axels) and progressively as the trailer is moved forward and more load is applied to a greater area of bunk the weight will shift progressively rearwards towards the axles.

Eg
For a 4000lb boat for every 1 foot of contact the applied load would look something like this:

1000lbs Axel 500lb Hitch 500lb
2000lbs Axel 1222lb Hitch 728lb
3000lbs Axel 2166lb Hitch 834lb
4000lbs Axel 3333lb Hitch 667lb

I tried a few different scenarios with different length bunks and distance to hitch etc what they all seem to shown that as you approach the 100% loading and CoG location there is a drop off of hitch weight.

Depending on your configuration of course and I have always assumed to give a hitch weight the normal loading CoG is in front of the axels.

As you can see in this simple case the hitch weight briefly increases before dropping off, this happens in most of the scenarios.

We could factor additional buoyancy of tyres and if your trailer is made of sealed hollow sections but I think this would be relatively small vs self weight of the whole trailer. Partially trapped water is likely to a bigger concern.

So yes I agree in reality ignoring friction and water resistance as the trailer is moved forward depending on your setup at near 100% loading point a noticeable decrease in hitch weight is experience, leading to possible loss of traction.