The Science of Towing
By kismet - Published January 14, 2011 - Viewed 1038 times
By Jim Favors
It’s been over 20 years since Lisa and I have had a trailerable boat, a 20-foot Four Winns runabout. We towed that boat all over northern Michigan looking for and exploring the inland lakes, camping over night while out on the hook… they were fun times. However, as I think back, I can’t recall ever giving much consideration to the vehicle we used to tow the boat with, I guess the boat was light enough that it didn’t make much difference. Well, that will change for us with the purchase of a trailerable trawler, which can weigh as much as 8,000 lbs.
First, let me set the stage by telling you how we’ll use our trailerable trawler, as this will become a critical component as to how it will be towed. We’ll need to choose a tow vehicle that will be capable of towing between 7,000 and 11,000 lbs, depending on the final boat choice. Our master plan is to use the boat close to home during the summer months. For example: one year we may tow the boat northeast so we can revisit Canada’s North Channel and or the Trent-Severn Waterway. Other summer destinations might include Lake Champlain in New York, the Rideau Canal or Montreal in Canada. Any of these destinations (and there are many more) are within one or two days drive from our home. Upon reaching the planned destination, we would slip the boat into the water and begin a week or two of cruising, then tow our boat back to Michigan.
Engines are important in the towing process, GM, Ford and Dodge all offer a standard, large V-8 (like in this photo) as well as an optional 6.6 or 6.7 diesel V-8 for more towing capability.
The fall and winter is when we see ourselves really venturing out for longer periods, maybe heading south to either revisit a favorite spot or one we’ve yet to explore. This time of year is also perfect to head west through the mountains to get to places like Lake Powell in Utah and Arizona, Yellowstone Lake in Wyoming, or we could head to the Pacific Northwest to revisit the San Juan Islands in Washington State – this is just for starters. I say for starters because this type of boating is new to us and I’m sure, just as the five years of liveaboard lifestyle brought us unexpected pleasures and surprises, so will the trailerable trawler lifestyle. With that stated we are, and will remain, open to what lies ahead, but first, we need to make sure we have the proper vehicle for towing our future boat.
The title of this log is “The Science of Towing” so lets get on to the nitty gritty of the subject. As a somewhat novice “trailering” person I have to say that I am just beginning to understand the elements that will be needed to tow our new boat, trailer and equipment. Not far into my research I found out there really is a bit more to it than taking a good guess at what is needed for a tow vehicle… it requires some understanding of the basics of towing, the terminology and how to apply it to your specific situation. I found out you really do need to pay attention to the details and gain understanding as to how GVWR, GCWR, payload, tongue weight and towing capacity each play a roll in safe trailering.
To get started I searched the Internet and was able to obtain definitions for many of the towing terms at www.howstuffworks.com. I found out that each vehicle manufacturer sets their own GVWR, GCWR, payload, and towing capacity. I also discovered that each vehicle has a GVWR number on a label, located just inside the driver’s doorframe and that there are more numbers to be found inside the owner’s manual. Each major truck manufacturing company also has a website that will allow you to spec out the truck of your dreams. When you’ve completed a form on the site you’ll receive a detailed spec sheet along with a capabilities and comparison report that not only shows you the GVWR, GCWR, payload, etc. of the truck you’ve spec’d out but that of the competition’s as well. I found these websites to be a very useful tool in helping me make sure that we get a tow truck that will actually do the job.
This label was attached to the factory installed trailer hitch of a GMC truck, note how it spells out the maximum capacities I outline below.
Here’s the definition I found on www.howstuffworks.com for the following terms we all need to understand if we plan on doing any kind of heavy towing:
1. Gross Combined Weight Rating (GCWR): “A vehicle’s GCWR is a specific weight determined by the manufacturer to be the maximum weight of a loaded tow vehicle and its attached loaded trailer.”
2. Gross Vehicle Weight Rating (GVWR): “You can think of the GVWR as a weight limit for your specific vehicle – a weight limit set by the automaker. Gross vehicle weight ratings take into account the base curb weight of the vehicle plus the weight of any optional accessories, cargo and passengers.”
3. Curb Weight: “is simply how much the vehicle weighs on its own, without any cargo or passengers. This measurement includes a full tank of gas and any other fluids that keep a car running.”
4. Payload Capacity: “a vehicle’s payload capacity - the amount of stuff it can safely carry after you’ve filled up the tank with gas and topped off all fluids – is just a matter of subtraction. Gross vehicle weight rating (GVWR) – curb weight= payload capacity.”
5. Towing Capacity: “sometimes called maximum towing capacity, is the maximum allowable weight that a vehicle can tow. Towing capacity is specified by the vehicle manufacturer.”
The GVWR definition I found on the How Stuff Works website helps understand how the label from this Ford F250 highlights the trucks 10,000lb. GVWR, plus additional information.
With the groundwork laid let’s apply it to a hypothetical situation. Lets take a boat that’s 26 feet long, weighs approximately 6,500 lbs, with a tandem axle trailer that weighs 1,700 lbs and an additional 1,000 lbs for gear and fuel (equaling 9,200 lbs). With this in mind, any tow vehicle we look at will have to be able to handle this weight with a maximum towing capacity ABOVE 9,200 lbs.
Next, I went onto the Ford Truck website for a comparison and selected a F250 4X4 XLT Crew cab with their standard V-8 engine in a 156” wb and I received the towing stats for the F250 along with a comparable Dodge, Chevy and GMC. With this truck selected, I found that the Ford had plenty of standard towing capacity at 10,000 lbs. and that Chevy and GMC, at 9,400 came in just above our hypothetical measurement. The standard Dodge truck number, according to the Ford report, at 8.500 lbs was well under our requirement. This is not good or bad news, its just information. It’s what one does with the information that makes a difference.
Here we see a label on a Chevy 2500HD that highlights the GVRW at 9,500 lbs, but also see how it refers to the owners manual for more details.
If we look a little further into the report I received from the Ford website, I found it also shows that by beefing up the truck with additional equipment the maximum towing capacities go up. Up to 15,800 lbs for Ford, 12,650 for Dodge, and 16,700 for Chevy and GMC. By adding a trailer towing package (which typically includes heavier suspension, limited slip differential and more), transmission oil cooler, larger V-8 gas or diesel engine and or larger tires the towing capacity along with a trucks payload capacity, GVWR or GCWR, all increased to numbers that are more than adequate to do the job of towing our hypothetical 9,200 lbs.
What I learned from my towing research was that each towing component, whether it be GCWR, GVWR, payload, or towing capacity, was critical in determining if the tow vehicle was properly equipped to handle the loaded trailer. It doesn’t matter if one was looking at a Ford, Dodge, Chevy or GMC each could be built to handle a specific towing situation. Each of their respective websites gave me the ability to build a truck and then look at the towing numbers to make sure I had what I needed to handle my future trailering needs.
The primary reason for the manufacturing companies to set GCWR, GVWR, payload and towing capacity is for safety. I can recall taking my current ½ ton pickup to get a yard of dirt and as soon as I started driving back home, I experienced how differently it made in the truck handle. The dirt was more than the payload was rated for therefore it caused the bed of the truck to squat in the rear thereby forcing the front suspension upwards. In my case I was putting hardship on the rear suspension, the brakes worked overtime when making stops and the steering was a lot less responsive than desired. The trip was only a short distance and everything worked out but I certainly wouldn’t want to do this type of unsafe driving across country.
A tandem axle trailer, like the one pictured with this Nordic Tug, is typical of what would be necessary for most trailerable trawlers.
If one overloads a tow vehicle and or a trailerable boat, I would think a lot of towing problems with be created. Not only do you have the brakes and transmission working too hard, they could overheat or fail mechanically, therefore putting you in harms way. The same applies to the tires, suspension, cooling system and engine and I have not even mentioned the trailer. Don’t forget that when trailering a rig potential problems would be compounded when towing beyond the maximum capacity rating. So the message I got from my towing research was to make sure your vehicle has enough GCWR, GVWR, payload and towing capacity to handle your trailer, boat and all of your equipment, preferably with enough margin left over for safety.
We’ve come to the conclusion that, just like there was a learning curve to the specifics of a 40-foot trawler before it became second nature, we think that learning the ins and outs of trailer trawlering will soon become old hat and the process for us started with learning the “Science of Towing.”
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