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Effect on Towed MPG of Weight Saved by Aluminum Trailer

Posted: Fri Dec 04, 2020 8:31 pm
by BamaBoater
I have the standard steel trailer for my 170 MONTAUK. If one did a lot of towing of a 170 MONTAUK boat on a trailer, a lighter aluminum trailer would save a lot in fuel costs.

Re: Effect of Towed MPG of Weight Saved by Aluminum Trailer

Posted: Sat Dec 05, 2020 6:58 am
by jimh
The dry weight of a 170 MONTAUK according to Boston Whaler is 2,050-lbs. Assume that fuel and gear aboard the boat add 200-lbs. The weight on the trailer is then about 2,250-lbs.

A 2,400-lbs capacity steel trailer might weigh 550-lbs. The total towed weight of boat and trailer will then be [2250 + 550 =] 2,800-lbs. [Fixed math error in that calculation as pointed out by readers, along with a couple of others, and I added bracketed figures to show the operations involved.] If the trailer frame were aluminum, how much weight would be saved? Even with an aluminum frame, there are still many steel components in a trailer: the hitch, the springs, the axle, the wheels, the bearings, the winch, and many steel fasteners. There are also non-metal components in the the trailer: the bunks, the carpet, the rollers,the tires, the winch strap, the plastic lamps. Let's say half the weight is in the frame, or 275-lbs. Steel weighs 2.5-times more than aluminum, but generally steel is stronger, so for the same strength, more aluminum is needed. Let's say you would save half the frame weight with aluminum, or 137-lbs.

The use of an aluminum-frame trailer then would reduce the total towed weight to [2800 - 137 =] 2663-lbs. Exactly what sort of improvement in fuel economy would be expected to occur if the total towed weight of the trailer and boat goes to 2663-lbs from 2,800-lbs? I think you'd be hard pressed to see an improvement of 1-MPG. What would be a reasonable fuel economy for a towing vehicle? Hmm, maybe about 15-MPG. So perhaps it could improved to 16-MPG.

Now we tow the boat 10,000-miles. At 15-MPG we need 667-gallons of fuel. At 16-MPG we need 625-gallons of fuel. The saved fuel amounts to 42-gallons. Fuel in the USA is selling for about $2-per-gallon, so you just saved $84. In order to save the $84 you had to tow 10,000-miles. Using a cost figure of $0.50-per mile to include depreciation of the towing vehicle, your towing expense was then $5,000. The $84-savings represents a reduction in the cost to tow 10,000-miles of 1.7-percent. Quite a savings.

I suspect the aluminum trailer sells for a lot more than $84 more than the steel trailer.

By the way, if you were really keen to save 137-lbs in the total weight being towed, just always tow the boat without any fuel. Having a 24-gallon fuel tank is quite typical for a 170 MONTAUK. Gasoline weighs about 6.25-lbs-per-gallon, so you could save the same 137-lbs if you always towed with just 3-gallons of fuel in the tank instead of a full tank.

I doubt there is much opportunity for fuel savings in reduced towing weight from an aluminum trailer to ever pay back the added cost of the aluminum trailer. The reason people buy aluminum trailers is for reduced corrosion when immersing the trailer in saltwater, not to save fuel cost when towing. And who tows their 170 MONTAUK 10,000-miles?

Here is another approach to analyzing the change in fuel economy for the towing vehicle: we look at the total weight of the towing vehicle and the trailer. Let's assume the towing vehicle has a 6,000-lbs tow rating, a reasonable choice if you are going to tow 10,000-miles. For an example, let's use a Jeep Grand Cherokee V6. It is rated for 6,200-lbs towing, and it would be comfortable towing the 2,800-lbs of boat trailer.

The curb weight of a Jeep Grand Cherokee V6 is about 4,550-lbs. Now we add the driver and a passenger at a total of 450-lbs; 24-gallons of fuel at 150-lbs, some luggage and boat gear at 200-lbs. The tow vehicle total weight is now about [4550 + 450 = 150 + 200 =] 5,350-lbs.

Hook up the trailer at 2,800-lbs, and the total weight truck and trailer is now [5350 + 2800 =] 8,150-lbs.

If we shave off 137-lbs of trailer weight, the in terms of the total weight of the truck and trailer, a reduction of 137-lbs from a 8,150-lbs total weight is a reduction of [137 / 8150 =] 1.7-percent in weight. What difference will a 1.7-percent change in weight make in the fuel mileage of the truck? I don't have a good analysis for this, but I would not expect the relationship to be strictly proportional, that is, decreasing weight by 1.7-percent won't boost total fuel economy by 1.7-percent. I base this on what I see regarding fuel economy when towing versus not towing.

In this example, adding the trailer at 2,800-lbs to the 5,350-lbs of truck is an increase in weight of (2800 / -5350 =]52.3-percent. But the MPG would not decrease by 52.3-percent. The Jeep should get about 24-MPG highway without the trailer. If the MPG decreased by 52.3-percent, it would be getting [24 x .477 =] 11.4-MPG with the trailer. I would expect--from experience--that the Jeep MPG would drop only to about 16-MPG. On that assumption, we see an 52.3-percent increase in weight has reduced MPG by only 33-percent. So the proportionality is 33:52.3 or 0.63. Now we apply that to the 1.7-percent reduction in towed weight: 1.7-percent x 0.63 --> 1.01-percent increase in MPG, or 16-MPG x 1.0101 = 16.19-MPG.

Now we can figure saved fuel. For 10,000-miles at 16-MPG we used 625.5-gallons. For 10,000-miles at 16.19-MPG = 617.7-gallons. We just saved [625.5 - 617.7 =] 7.8-gallons, and that is over 10,000-miles of towing. At today's price for gasoline at $2-per-gallon, we saved $15.60 by buying the aluminum trailer option.

I do not think that buying an aluminum trailer option has any possibility of a payback in reduced fuel expenses while towing compared to the steel trailer.

Re: Effect on Towed MPG of Weight Saved by Aluminum Trailer

Posted: Tue Dec 08, 2020 7:50 am
by jimh
Many thanks to Nick for pointing out a math error that threw off many following calculations. I see the posting time of my original article was rather early in the morning--I should have had more coffee.

I corrected the error and all the subsequent calculations that were based on the error. The amount of money saved in towing 10,000-miles increased by less than $10, and the "savings" remained only a small fraction of the added cost of the aluminum trailer.

Again, I seriously doubt anyone could ever recover the added cost of an aluminum trailer in preference to a steel trailer by saving fuel costs in the towing vehicle. Are there readers who think that conclusion is in error?

Re: Effect on Towed MPG of Weight Saved by Aluminum Trailer

Posted: Tue Dec 08, 2020 11:32 am
by biggiefl
I agree Jim. There is no way you could save money with a couple hundred pound savings.

I buy aluminum trailers because they look better, are lighter, flex more, and don't turn white if acid drips on it when cleaning the hull. I have a 1982 Cox galvanized trailer under my 13' which came with it new. There is no rust and you can stand on the metal fenders still. They do not make quality galvanized trailers like that anymore in my opinion.