MacGregorSailors.com

Bill,
Did a bit of rough estimating. I’m going to take some ketchup on my crow, please. Looks like the braking/bump loading on the shrouds could be almost double the load while sailing (wind pressure only). I didn’t bother trying to add shock loads due to chop to it, because the number of assumptions I’d have to make would reduce the estimate to a wild guess.

Based on what I could determine, maximum deceleration is going to be a smidge under 3 G. So, if your mast, Genoa furler, boom (if attached), main (if in a bag on the boom), etc, weighs in at 100 lbs; you’d be looking at 300 lbs while bumping. Should have checked the shroud loads, but ran out of curiosity for today.

There is one caveat, however. If the boat does not stop moving in the trailer before the vehicle and trailer come to a complete stop; that load - and more significantly the load at the v-block as the boat slams into it could be considerably higher.

I don’t think that will ever be the case if the boat only has to move inches; but hold my beer…

Jimmy,
Those numbers line up with my rough estimates as well, including that troublesome situation where the vehicle and trailer have stopped but the boat can still move.

For those of you following along at home, kinetic energy goes up with the square of the velocity. In a perfect situation all of that energy is will be used to overcome the friction between the bottom of the boat and the bunks with a tiny bit left over to ever-so-slightly deform the rubber roller as your boat gently nudges itself against it.

However, if the boat has not used up all of that energy before it gets to the front roller it still has to go somewhere. That energy will be used to do a number of things, some potentially destructive, in a relatively short period of time and that's where the G forces go up, perhaps beyond the point where your rigging or fittings were designed to operate.

Nice summary. My takeaway is to be very cautious doing a "Mac dump" with the mast up.

I like "Liquid roller"

Be Free wrote: ↑Sun May 07, 2023 8:59 am Jimmy,
Those numbers line up with my rough estimates as well, including that troublesome situation where the vehicle and trailer have stopped but the boat can still move.

For those of you following along at home, kinetic energy goes up with the square of the velocity. In a perfect situation all of that energy is will be used to overcome the friction between the bottom of the boat and the bunks with a tiny bit left over to ever-so-slightly deform the rubber roller as your boat gently nudges itself against it.

However, if the boat has not used up all of that energy before it gets to the front roller it still has to go somewhere. That energy will be used to do a number of things, some potentially destructive, in a relatively short period of time and that's where the G forces go up, perhaps beyond the point where your rigging or fittings were designed to operate.

thank you for all the replies i have done the bump a couple of times and can tell you 10mph is way too fast. 2-5 works fine. I broke my bow roller and now need to find a replacement. the boat is ok though.

Here are the results of a miscalculation: like, the Goldilocks effect.

Of course, anything may be fixed in time--including the ugly dent in the trailing vehicle and the aluminum trailer itself. And it has.

Rather than the MacBump, I now prefer to back up again, push the boat back after I have emptied the ballast tank, crank again. Do it again, if need be.

Takes five extra minutes--a humbler approach, to be sure, which doesn't even have a moniker, but I find it safer