Question of aerodynamics?

Ray

Answer is simple

There is less drag with the tinnie on top of the car. The wind deflects off the windscreen and then on to the upturned bow of the tinnie which causes a streamlined effect.

With the tinnie on the trailer the deflected wind curls down at the back of the Pajero and then bounces on to the tinnie from above ( even though you have a deflector ) dragging the whole towed mass down and thereby creating more wind drag.

My thoughts. No doubt some hi tech luminary will state otherwise...lol

Cheers

i also have been thinking about this very thing, am going to make a custom wind deflector like on the big trucks for the front of the van.(am a mould maker by trade) will also put one on the roof of the patrol. i notice a big improvement in fuel with the tinnie on top. comments please?

As per Willem's comments about drag.
(Could you try mounting the boat on the trailer closer to the back of the car so the air flows more smoothly off the back of the car and onto the boat?)

But with camper trailer in tow, you also are lugging an extra 500 to 1000kg up the hills.
If you are doing the same speeds around the 250k loop, you must be using more throttle getting up the hills with the camper trailer.

How many hills and how steep? If a few, part of the answer.

I find that with my camper trailer on the back that I am about 0.5 to 1l/100k worse off than having the roof rack on and no camper trailer. I try and use the same throttle settings, so end going up the hills slower with the CT.

Hi Ray

You don't make it quite clear , but were you towing trailer in both cases ?

Unless there is abnormally bad turbulence from your trailer then as you suppose there will always be less from boat on trailer than roof.

Either way some other factor is at play unless you drive very very fast because you have stated a 23% increase in fuel consumption alone at 100 and while air resistance dominates at 100k its still usually only 60/40, hence implying a true 30%+ wind resistance increase which isn't realistic.

I have seen previously figures that were wrong because someone has added weight to a trailer and not pumped up tyres proportionaly and has had increased tyre drag as a factor, espically because when you try and drive at 100 if you look at say "moving average " on a trip meter you usually only average 90.

I'd record more figures

Robin Miller

Geez Ray, As you know Des and I weighed one of my campers Saturday and it came out at 720kg, I really thought yours would have been a tad more than 85Kg LOL Then again you did have to spend time with Uncle Des and that is enough to confuse the best of us....