Boat off trailer

A forum for discussing issues relating to trailers and towing MacGregor sailboats.

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kurz
Captain
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Sailboat: MacGregor 26M
Location: Zürich, Switzerland, Europe

Re: Boat off trailer

Post by kurz » Sat Jan 30, 2016 6:08 am

Who knows this?

What a weight has the bow?

If you want to put the bow on wooden bar to tow the trailer out of the boat, what diemsion must the woodden be?

Or what dimension of you put a pipe of metall under the bow?

After the trailer is away you would sustain the bow with blocks. So some little "bounce" would be ok. But I have no idea how much weight sits on the bow and how to calculate a bar on a length of 300cm.

Capt Smitty
Deckhand
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Sailboat: MacGregor 26X
Location: Everett, Wa , 2002 26X , Nissan TLDI 50hp

Re: Boat off trailer

Post by Capt Smitty » Sat Jan 30, 2016 9:36 am

Don't make a big deal out of this, it's easier than you think. I concur with Tomfoolery and Don T, floor jack and wood blocks. Your boat, loaded, only weighs about as much as a mid sized car. How hard is it to jack up and safely support your car? I suggest lifting only a couple inches above the trailer pads, install your blocks, then try to rock the boat off of the blocks. If the blocks fail, you're only a inch or two above the trailer. That gave me the confidence to remove the trailer. See this link for my method.

http://www.macgregorsailors.com/modt/in ... ?view=1895

I made my blocks stronger than was necessary, out of fear of the unknown. Search this site and you'll find other constructions. Someone made support structures out of plywood!
Better to be safe than sorry. Always think safety.
Smitty

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Tomfoolery
Admiral
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Re: Boat off trailer

Post by Tomfoolery » Sat Jan 30, 2016 10:37 am

There's a lot more to that question than you realize, but I'll take a (conservative) shot.

Here's the boat on its trailer (photo copied from a post on this forum).

Image

The center of mass of the boat is just a little bit forward of the vertical line between the trailer axles. It probably weighs a little more than 3000 lb [1360 kg] with engine and 'stuff' inside.

The force on the two support points, one aft, and the other forward, depends on where the blocking is. If you block the stern about 2 meters back from that line, close to the OB, and block the bow twice that far forward of the line, then the stern blocking will take 2/3 of the vertical load, and the bow blocking will take 1/3. So the stern blocking will take about 2000 lb [900 kg], and the bow blocking or cross member will hold about 1000 lb [450 kg]. Since I (and you) don't know exactly how much your boat weighs, and where the CG is, it would be prudent to round up and even add some fudge if you think it's loaded a bit heavy.

This is where it gets a little dicey. For the cross member in the front, you need a wood or steel member that can resist the bending force of 1000 lb [450 kg] applied at the center of the span of 3m, . The ability of a member to support that load depends on the dimensions of the member, and the properties of the material it's made of.

I don't design in wood, so I'll stick with metal. The bending moment (a measure of how much bending 'force' the member has to resist) for a beam loaded at a point in the middle is simply one-half of the point load of 450 kg multiplied by the distance from the point load (the middle of the beam) to the support point at either end, which I'm assuming is one-half the length of the beam, or 59 inches [1.5 meters]. So the bending moment is 500 lb * 59 in = 29,500 lb-in (sorry, but gotta drop the metric here, as material properties in metric is not something I work with).

To resist that bending moment, the easiest way is to use a property of a member (a beam, in this case) called the section modulus, usually denoted as "S" in tables, which is published for common beams, tubes, and pipes. Assuming plain-vanilla steel with a yield stress of 36,000 lb/square inch, and applying a factor of safety of 2 (minimum, since there's a LOT of things about this that I don't know first-hand and for sure), you size the member to the bending moment divided by the material yield strength, which is 0.82 in^3. Applying a FoS of 2 gives you a required minimum section modulus of 1.7 in^3. Use a larger FoS if you think people will be on the boat, or if it may be heavier, or if you're uncomfortable in any way.

You can look up the properties of common structural members in various tables (including online). For schedule 40 pipe (don't know what you have available, though), a 3" pipe has a section modulus of 1.72 in^3, so would be adequate. But pipe, though readily available, is not a very efficient shape for this sort of loading. A square, or even better, rectangular tube, has more material where it's needed for this sort of loading. A 3 in x 3 in x 3/16 in wall square tube (hollow structural section, or HSS), which is what the pole tongue of the standard trailer for the X is made of, is just a shade light at 1.64 in^3, but it's close, though if you include it's own weight, it's not as close.

If at all uncomfortable with this, there is nothing wrong with upsizing a member. Weight of the member is not a consideration here for what you're doing (it's not an airplane, or a crane boom) other than to know that it has weight, which adds to the bending, so fudge on the safe side.

http://www.engineersedge.com/fluid_flow ... ule-40.htm
http://www.engineeringtoolbox.com/squar ... _1478.html

Keep in mind that I'm doing this sitting in front of a computer, without even a pencil and paper, and eyeball-estimating the location of the C of G or your boat, but I'll look closer at this if you want during the week.

And as always, don't get under a lifted load without proper support. Stick some blocking or a floor jack under the cross member when you don't need it clear. And if it were me, I'd bounce the load up and down to see if I could bend the support (with blocking under it with some clearance), just to be sure it's not marginal, before getting under it.

korn_kid_12
Chief Steward
Posts: 78
Joined: Sat Sep 17, 2011 1:27 pm
Sailboat: MacGregor 26X

Re: Boat off trailer

Post by korn_kid_12 » Sat Jan 30, 2016 9:31 pm

Capt Smitty wrote:Don't make a big deal out of this, it's easier than you think. I concur with Tomfoolery and Don T, floor jack and wood blocks. Your boat, loaded, only weighs about as much as a mid sized car. How hard is it to jack up and safely support your car? I suggest lifting only a couple inches above the trailer pads, install your blocks, then try to rock the boat off of the blocks. If the blocks fail, you're only a inch or two above the trailer. That gave me the confidence to remove the trailer. See this link for my method.

http://www.macgregorsailors.com/modt/in ... ?view=1895

I made my blocks stronger than was necessary, out of fear of the unknown. Search this site and you'll find other constructions. Someone made support structures out of plywood!
Better to be safe than sorry. Always think safety.
Smitty
I like this design for the rear!

Jeremysxx
Just Enlisted
Posts: 22
Joined: Sat Jan 16, 2016 8:47 pm
Sailboat: MacGregor 25

Re: Boat off trailer

Post by Jeremysxx » Sat Jan 30, 2016 10:26 pm

im strapping mine to a tree and cumalong, ill post pics when its done

Jeremysxx
Just Enlisted
Posts: 22
Joined: Sat Jan 16, 2016 8:47 pm
Sailboat: MacGregor 25

Re: Boat off trailer

Post by Jeremysxx » Sun Jan 31, 2016 5:45 pm

did this today on a 900lb boat to get to the keel

Image

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