Mac 'bump' and silicone

[Jeremy K]

I know there're other posts about this, so mods delete and/or move this if you see fit. Just a quick note to say that after getting the bottom painted on our elderly M25 we noted as others have that the Mac 'bump' no longer worked, eliciting weeping and wailing and gnashing of teeth, and necessitating a winching with much wincing to get the bow into the notch so the rig didn't sway all over the road on our way down to Charleston SC from KY. But I bought a couple of cans of silicone lube spray from WalMart based on info I found on this most valuable and prized forum , and sprayed the bunks while the boat was in the water for a week down there. Pulling it out we did the bump and it slid like a charm right into the notch, best it's ever been! So for this and many other advices, thanks all!

[Tomfoolery]

Liquid Rollers, which is (AFAIK) silicone spray. Works like a charm for me, and I usually drive down a relatively steep hill at my local Erie Canal ramp on the way back to the parking area and gently hit the brakes. Two or three times, and it's snugly in the notch, without having to hit the brakes hard enough to break it free and slam it into the bumper like it does without lube.

An alternative would be to rearrange the winch, or add a fairlead for the winch line, but I have better places to put effort at this time, so Liquid Rollers it is.

[Russ]

Good tip.

Liquid Rollers or Silicone, sounds like they work well.
My fear has always been they work too well and allow the boat to slide off while driving down the road.

[Tomfoolery]

My fear has always been they work too well and allow the boat to slide off while driving down the road.

And don't think that hasn't occurred to me as I pull up the steep ramp and hill with the boat on it.

But that's what the winch, and a separate safety chain, plus the stern strap, are all for. Any one of them, including the stern strap, should hold the boat on a hill, as a last resort.

[Wind Chime]

side note on Mac-Bump:

To avoid having to use the Mac-bump, all that is required is to keep the bow of the boat tight aginst the winch stand's V-bumper when removing from the water. The problem is the angle of the strap because of where the wratched winch is located on the MacGregor tailer.

To solve this problem, we use a web cinch-strap from the winch stand to the bow pad-eye.
- We attach and tighten the strap while the boat is on the trailer and still in the water.
- This strap with a second angle seems to keep the bow snug against the v-bumber most of the time.

Works for us, so might work for others as well.

[Tomfoolery]

Here's a sketch I did a while back to illustrate the phenomenon. It's not to scale, but the principle is demonstrated accurately enough.

The winch line is the same length in both orientations, and in actuality, it gets slightly longer as the trailer comes up, though that's far from the dominant variable.

[BOAT]

Yes YES! Mr. Tom Foolery you have defined and illustrated a HUGE PROBLEM TO ME! I know this is not a big deal to many of you but for me this a HUGE issue because I pull 'boat' WITH A MOTORHOME!

Imagine all your glassware and scotch and sofa pillows and whatever is laying on the counter flying across the place every time you need to pull the boat out of the water!! IT SUCKS!! I almost broke a 32 dollar bottle of scotch that was in the sink one day trying to get 'boat' to move that last 2 inches on the trailer!! ARRRGH!

I was NOT happy.

I wish I was smart enough to picture what wind chime said in my mind but I just can't see it without a picture - I can't see how to overcome this issue because just like in toms picture the boat will always be short of the trailer V bunk when the trailer goes back level under the boat! ARHG! I HATE THAT!

Is there any way to prevent that? I suspect not due to physics and geometry (all things I don't understand anyways) but I would appreciate someone helping me with a solution because these "bumps" are tearing up my motorhome interior.

[Wind Chime]

Great sketch Tomfoolery,

Our second cinch-strap attaches near the base (or about half way down) of the winch stand, and then attached to the bow pad-eye. So about a 45 degree angle (maybe less) compared to the winch strap.

After the boat is out of the water and on flat ground, we can tighten the winch again a few clicks on the rachet.

I am not sure of the how the angles and lengths of this second strap works out, but it is somehow able to keep the bow tight against the V-bumber, and it seems to work. I am a “management engineer” not a “design engineer”, so I cannot explain the dynamic engineering forces at play here of the reciprocating parabolic coefficient factor as it pertains to the resistance of the flux capacitors triginomical strain on the negative angle of the winch verses positve angle of the bows reciprical leverage demad.

[Tomfoolery]

Is there any way to prevent that? I suspect not due to physics and geometry (all things I don't understand anyways) but I would appreciate someone helping me with a solution because these "bumps" are tearing up my motorhome interior.

Since I can't ever just leave something alone ( ), I once winched it tight, pulled forward a couple of feet, winched again, pulled forward a couple more feet, etc., until the stern was clear of the water. As the trailer comes up and less of the boat is supported by the water, winching becomes harder, but with some lubrication on the bunks, it was hardly much of a chore. Easy peasy, though a bit of a pain to keep getting out and winching. But it worked just fine.

A second person cranking in between moving forward would have sped the process up, but I let the ballast drain on the ramp anyway, so no big deal to keep stopping. I'd caution against having someone winch while pulling forward with an RV, as you can't see the other person, and a mishap could run him or her over.

An electric winch would solve that problem, though, as a tethered remote control could be held by the operator while outside the width of the RV and boat trailer, within view of the mirror. A little finesse in winching, so you don't pull the bow eye off, would probably go a long way, too. The sound of the winch motor is probably a decent feedback mechanism, as it will naturally slow as the load increases.

But electric winches aren't cheap, so the stop and winch method is by far the most cost effective way to do this, while giving extra time to drain the ballast. In my experience, at least.

[Tomfoolery]

. . . the dynamic engineering forces at play here of the reciprocating parabolic coefficient factor as it pertains to the resistance of the flux capacitors triginomical strain on the negative angle of the winch verses positve angle of the bows reciprical leverage demad.

Aaaahh, now you're talking MY language. Here's a brief explanation of equipment design common in MY world. I do a lot of power transmission design, including high-hp hydraulic systems, as well as gear train and support structures (large winches, in other words), and this gentleman's talking points are music to my ears.

https://www.youtube.com/watch?v=Ac7G7xOG2Ag

[EZ]

side note on Mac-Bump:

To avoid having to use the Mac-bump, all that is required is to keep the bow of the boat tight aginst the winch stand's V-bumper when removing from the water. The problem is the angle of the strap because of where the wratched winch is located on the MacGregor tailer.

To solve this problem, we use a web cinch-strap from the winch stand to the bow pad-eye.
- We attach and tighten the strap while the boat is on the trailer and still in the water.
- This strap with a second angle seems to keep the bow snug against the v-bumber most of the time.

Works for us, so might work for others as well.

Can you post a picture?

[Russ]

Here's a sketch I did a while back to illustrate the phenomenon. It's not to scale, but the principle is demonstrated accurately enough.

The winch line is the same length in both orientations, and in actuality, it gets slightly longer as the trailer comes up, though that's far from the dominant variable.

Love this sketch. Illustrates the problem so well. Should be a sticky with "Mac Bump" or some other group of How To's on this forum. I've seen other forums that have sections called "How To" with common help that newbies always ask about. Whatcha think moderators?


BOAT, how about coating the bunks with silicone as the OP mentioned. Then winch it up once on flat ground.


--Russ

[raycarlson]

I've used regular CRC silicone spray and Liquid Rollers, I have to say the Liquid Rollers product is waaayyyyyyy slipperyier then regular silicone. The mac bump is no longer a traumatic whiplash event, just a simple downhill firm stop does the trick with the rollers product.

[BOAT]

Thanks greatly Ray, it sounds like you may have found the answer to my problem. Where can I get these slippery rollers products? How is it applied?

thanking you again (You get some of my scotch now for saving my bottle if it works)

[RobertB]

Buy at WM. Spray on when boat is off the trailer. Store can so it does not start spraying in your car/truck while you are driving.