...But when backed down in the water, the weight of the van will be shifted to the back of the vehicle, with the weight of the boat pulling the rig backwards. I'd say give it a go and see how it does.
I've got an experience that validates this...forgetting the plug, full load of gas (both tanks), still attached to the trailer.
The reason we sat so long before releasing it from the winch was because of installation of new fuel lines, new water pump impeller (Merc 200hp V6), new tach, and checking my rebuild on the CMC jackplate pump system. I did all that work myself, and we just had a lot of systems to functionally check with the motor running, so I just kept it on the trailer (thankfully). Well, it came time to release, and that's when we noticed the problem.
"Hey, didn't you submerge the trailer fenders to float the boat off the bunks?"
"Yeah...but now it's sitting on the bunks...hey, there's water coming up from my aft floor drain...sh1t, the transom plug!"
All I know is that water is really heavy, especially if a couple hundred gallons made it onto the boat. Threw my van down to the lowest gear selection, and eased it up the ramp. In spite of probably an extra half ton of weight on a steep incline, first clutch did well (there's mechanical advantage helping here) with zero clutch slip, no wheel spin. Took a long time with both bilge pumps running before I could screw in the plug to back it all down the ramp and try again. It ran fine (both boat and van).
However, if you have any hills and do not select D3 (5-speed tranny) or D4 (6-speed tranny) while cruising along the roadways...there is the
HUGE potential problem that
manualman alludes to, and that is clutch slip.
If you are towing heavy, leave it in top gear, and ease into the gas while going up a slight upgrade, your mechanical advantage in the tranny is
against the clutch pack; it can slip
a lot. When it does, the RPM rise will be drastic, and you can destroy that gear's clutch pack within seconds if you don't immediately get off the throttle. This is most likely at lower RPM's, top gear, slight upgrade, and slowly easing in throttle angle on a Honda transmission. I've done it twice (once hauling my boat with my 2003 EX, the other time with a Casita travel trailer with the wife's 2002 EX). Both had ATF-Z1; switching to MaxLife, and later AmSOil ATF with their good static friction antislip additives solved that slip problem, but needless to say I pretty much avoided those low RPM top gear tranny clutch-slip situations by just selecting D3 if remembered to do it.
The Honda tranny is not a robust design; the designers are not confident with its strength while towing, so one gear down from top gear is
not user-selectable. This is why you are required to select D3 (or D4 for 6-speeds) while towing. Honda wants us to stay away from top gear while towing heavy, especially on an incline.
I still use top gear while towing, but if an incline comes up, I just don't use as much throttle angle, let the speed decay to a point where a D3 engagement isn't so jarring, and then do that and keep on truckin' until I can upshift again on a sustained slight downgrade or level ground. Lot of up and down rolling hills? I keep it in D3; I learned my lesson on that one.
OF