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"I need a nerd" Trailering question
http://www.hobie.com/au/en/forums/viewtopic.php?f=9&t=8464		 Page 1 of 2
Author:  buzzman2 [ Tue Dec 18, 2007 11:43 pm ]
Post subject:  "I need a nerd" Trailering question
I don't have an engineering degree, but I could have according to my college academic advisor, something about effort as an inhibiter.


Is trailering a 750 lb. total weight (boat & trailer), the same as loading three of my 250 lb. friends in the car plus wind resistance?

Can the wind resistance of the average trailered cat from the average car be converted to additional horsepower needed to maintain a constant speed on a flat surface?

How do elevation changes effect necessary horsepower with this given weight and wind resistance?

No offense to the nerds, I need you guys. :D
Author:  Skipshot [ Wed Dec 19, 2007 1:10 am ]
Post subject:  Re: "I need a nerd" Trailering question
buzzman2 wrote:
Is trailering a 750 lb. total weight (boat & trailer), the same as loading three of my 250 lb. friends in the car plus wind resistance?

I'd say no, because the weight is distributed over three axles instead of two. Others will need to chime in on the other questions. By the way, where are you going with this? Is this a homework assignment?
Author:  MBounds [ Wed Dec 19, 2007 6:07 am ]
Post subject: 
Quote:
Can the wind resistance of the average trailered cat from the average car be converted to additional horsepower needed to maintain a constant speed on a flat surface?


Wind resistance (drag) increases as the square of the velocity - i.e. there's four times as much drag at 60 mph than 30 mph.

There's a lot of other factors to consider, though. 4-stroke gasoline engines have a power "sweet spot" at a particular RPM that maximizes fuel efficiency and power. The trick is to be going the speed you want, with the engine running at its sweet spot. A lot depends on the transmission.

Cars are optimized for highway driving without consideration for towing.

Where are you going with this?
Author:  Karl Brogger [ Wed Dec 19, 2007 6:10 am ]
Post subject:  Re: "I need a nerd" Trailering question
buzzman2 wrote:
Is trailering a 750 lb. total weight (boat & trailer), the same as loading three of my 250 lb. friends in the car plus wind resistance?


No, because you also have the additional drag of a third axle. Not just spinning the extra tires but also the third axle hitting bumps and slowing you down. A negligable abount but its there

buzzman2 wrote:
Can the wind resistance of the average trailered cat from the average car be converted to additional horsepower needed to maintain a constant speed on a flat surface?


You need alot of drag before you start struggling to maintain normal speeds.

buzzman2 wrote:
How do elevation changes effect necessary horsepower with this given weight and wind resistance?


What are you towing with, a Tonka Truck? 750lbs is nothing for all but the smallest of vehicles.
Author:  srm [ Wed Dec 19, 2007 6:56 am ]
Post subject: 
Two big factors that must be overcome when driving at highway speed on flat ground would be the air resistance and the rolling resistance.

When you attach a cat trailer to your car, the rolling resistance doesn't go up a whole lot. The weight is distributed through the trailer wheels and the rear wheels of the car. This is noticable in that it is fairly easy to roll a trailer by hand. However the air drag is increased.

When you add weight to just the car, the air drag obviously doesn't change because the shape of the car hasn't changed. However, the rolling resistance increases due to the axles and tires being overloaded. Also, the handling of the car will be affected.

My guess is that it would be about the same either way. At lower speeds, the trailer would probably be more efficient.

Also, the wind resistance could be used to calculate additional horsepower required. (Don't ask me to do it). The wind resistance for a trailer at a given speed is a constant force. Knowing that force and the velocity, you can calculate power (P=F*v).

sm
Author:  iresq [ Wed Dec 19, 2007 7:22 am ]
Post subject: 
Air and rolling resistance has been covered. You also ask about elevation. Do they have mountains in Columbus? Higher elevations rob engine horsepower. You'd probably never notice it towing 750lbs unless your tow vehicle was a Yugo and you were traveling through Colorado.

Not a real nerdy answer.
Author:  MBounds [ Wed Dec 19, 2007 8:38 am ]
Post subject: 
There are two effects of mountains - elevation and grade.

The higher the elevation, the thinner the air: less oxygen = less horsepower.

The steeper the grade, the more horsepower you need to maintain a constant speed.

Columbus is only a couple of hours from mountains - mountains with a steep enough grade to make a difference.

I was coming back from a regatta in Delaware with my 17 on the Trailex - about 700 lbs - and I had to slow down in the mountains of Maryland / Pennsylvania to avoid overheating my Volvo C70 (200 HP). "Slowing down" meant that I was doing something closer to the speed limit.
Author:  Adrio [ Wed Dec 19, 2007 10:40 am ]
Post subject: 
Remember it is all about drag and change in speed and elevation.

1) If you are towing at a constant speed and not changing elevation, then all your power required is to overcome drag (rolling resistance plus wind resistance) - Newton's first law

2) If you are changing speed (as in accelerating) then your power requirements are a function of how fast you wish to change your speed. The quicker you want to go from speed X to speed X+Y the more power you need - Newton's second law

3) If you are climbing a hill then the slope of the hill combined with the speed you are travelling (or accelerating) combine to give you a vertical acceleration (even if you are maintaining a constant speed over the sloped ground). So you have in essence the same situation as in 2) above and the second law applies.

All that said (and it was too much), I was able to tow a cat of about that weight from Canada to Florida and back through the hills of Pensilvania with only a little Fiat 124 (which might have 60 to 80 HP in its little four banger engine). I still tow my H18 with a four banger Honda and have no trouble. So unless you are talking about a vintage VW bug I imagine you will be fine.
Author:  tjp [ Wed Dec 19, 2007 11:59 am ]
Post subject: 
The thing you most have to worry about with modern small cars and towing is transmission. Towing puts additional strain on the transmission at speed (see air resistance above). While the engine might be fine with it, a modern transmission is not designed with a lot of margin - beefing it up costs money. Too much strain, especially on the tiny overdrive gears (especially on manuals), as well as overheating of the transmission fluid (especially on automatics) can cause transmission failure.

Older small cars are not as vulnerable to this - before computer modelling and such, a much bigger margin of strength had to be put into place with the design - thus why you can use a Volkswagen transmission originally designed in the 1930's for a 30 hp car on a 150-hp drag car without failure (or at least less than might be expected).

Probably not what you were asking but hey...
Author:  buzzman2 [ Wed Dec 19, 2007 12:29 pm ]
Post subject: 
Is there a engineer translater in the room? :lol: You guys lost me almost immediately, just kidding, but I should be more careful what I wish for... :D

This is what I'm getting at: I plan on going to pick up my H20 with galvanized trailer with a 2003 Acura TL-S (260hp). I just put the hitch on and am just being a little paranoid. It's 450 miles with some mountains.
I may have been a little low on my weight estimate and correct me if I'm wrong. I know not to use the overdrive and can't seem to locate transmission oil coolers for this model. I wanted to shoot down to Wrightsville for a few days right when I got it, next week.(I realize this might not be a great idea in itself). Anyway solo driving three 450 mile trips, two with trailer within 5 or 6 days mostly flat but some mountains. Concerns? Nuts? I am good for about ten straight hours of driving before being alerted by others to call it a day.
Author:  srm [ Wed Dec 19, 2007 12:43 pm ]
Post subject: 
Any chance the owner's manual lists anything about towing.

Personally, I would just do it. Plenty of people have used cars to tow Hobies. The weight would not be significantly different if the car was loaded with 4 passengers and a trunk full of luggage. Keep your eye on the gauges and go a little slower if necessary.

sm
Author:  J_Eaton [ Wed Dec 19, 2007 1:09 pm ]
Post subject: 
Buzz,

Finally going to pick her up, eh? The key to towing is horsepower and you've got it covered. Skip the transcooler idea and get the fluid changed more often you'll be fine. I don't recall exactly what that trailer looked like but I would guess you're close guessing 750#s. As stated the wind resistance is the big thing. We've pulled our 20 and heavy trailer (1K#'s) all over heck and back with two different vehicles under 200hp.

If the transmission does a lot off shifting in and out of OD (bad) you have two choices
1. switch it off
2. speed up :) trust me it works

Have a safe trip, have a fun maiden voyage, and make it over to Kansas this summer for a regatta :wink:
Author:  MBounds [ Wed Dec 19, 2007 1:29 pm ]
Post subject: 
Quote:
Anyway solo driving three 450 mile trips, two with trailer within 5 or 6 days mostly flat but some mountains.


Sissy. :lol:

I've done two 1000+ mile trips in 4 days (that included sailing for two days) in the Volvo coupe, mountains in KY and TN on the way to FL - with a double stack. No special towing equipment.

Car's got 170K miles on it now and still going strong, although the downshift on the transaxle is getting harsh.

I do keep an eye on the guages in hot weather / hilly terrain. If she starts to heat up, slow down. In my case, that means bringing it down to the speed limit.
Author:  Karl Brogger [ Wed Dec 19, 2007 4:22 pm ]
Post subject: 
buzzman your fine. If you were using a Aspire, a Metro, or the original cooper that would warrant caution. Probably add anything made by Renault, crappiest car I ever owned.
Author:  Sail Revolution [ Wed Dec 19, 2007 7:07 pm ]
Post subject: 
Hey, Watch it with the small car bashing Karl :lol: :lol: :lol:


Image

I have to bust this one out whenever I can. We have towed 2 Getaways 2 trailers and several kayaks with this rig. 1000 miles round trip. We towed an aclass to Texas last summer with it. Almost 5000 miles round trip.
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