How small of a guy wire base can I get away with?

[esc]

You can read the full story about my portable wind mill tower project here:

http://www.fieldlines.com/story/2008/11/11/213333/08

Look at the last post for a picture.

My question is, can I get away with 3 guy wires on a 21' 1.5" schedule 40 pipe.  1 six feet in front the other two 6' back and 8' apart?  The pipe will have a 2x2, steel, braced, support for the first 5'.

I can go with a larger base and 5 guy wires (1 10' in front, 2 4' to each side, 2 10' back and 8' apart), but there are several drawbacks to doing so.

Thoughts?

[SparWeb]

Be safe.

You plan to pull this trailer on public roads.

If you can't prove that something is adequate, don't do it.

What are we supposed to say?  "Ya it's okay, maybe try some baling twine, too"?

Whatever you do decide to build, and I expect you'll find a way to build a fairly safe rig in the end, give it a realistic test before heading out on the road.  Hitch the trailer to a truck in gear and in park.  Hook a chain onto the top of the tower and HEAVE-HO!

If you break the mast or flip the trailer, be very glad it didn't happen on the road.

Trailer mounts have been tried, and some members have reported their experiences.  Search the board and you should find at least 2 or 3.  One of them flipped over.

[esc]

The tower will never be extended while driving, only when stationary.

It will have about 8lbs on the top (Air 403) and I will be using stainless wire rated at 1540 lbs.  SWP says the tower should be able to support a 150 lb lateral load to survive a 110 mph wind.  I doubt it will ever see half of that.

I'm not asking for a definite answer.  I just would like someone with more experience that me to make an estimate.  Is it:

A) a death trap waiting for a victim

B) it might be OK

C) It will probably be OK

[esc]

Did a search (not the first) and didn't find much of anything.

Can you supply a link or recommend search terms?

I will test it as you suggested, but if the consensus is that it will be a death trap, I will alter my plans accordingly before putting all the effort in to constructing it.

[tmcmurran]

I really would not want to have that raised while the trailer is in motion.  But you should be looking into what laws may also need to be followed.  I would be that a rig like that would just be a magnet for being pulled over every few miles, so I doubt you would even be able to gain much since you might be sitting on the side of the hiway more often then driving.

[Ungrounded Lightning Rod]

Give us the dimensions (width between wheel centers, axle-to-tongue, axle-to-rear-frame-member, location of base of mast, width and length at corners suitable for guy anchors) and weight of the trailer-plus-genny-and-mast and we can figure it out.

If the sideforce on the genny (plus that on the trailer, mast, and guys) isn't enough to tip the trailer over sideways when resting on the wheels it also won't be enough to tip it over fore/aft.  (Note that you need some margin because tipping it over toward 11:00 or 1:00 is a tad easier than toward 9:00 or 3:00.)

If that's the case you can just anchor the guys to the trailer and put chocks, rear stabilizer jacks (or blocks), and a hitch pylon or tongue jack with a "foot" on it and you'll be fine.  (One travel trailer I had used two stabilizer jacks on the back plus the hitch jack to make a super-easy setup:  Chock it, unhitch it, tilt it slightly nose-down, drop the rear jacks, then jack the tongue a few inches to level it - and transfer maybe 10% of the weight from the wheels to the three jacks.)

The trailer should be strong enough to form a rigid structure with the mast and guys if you don't pretension the guys excessively (like to a half-ton or more each).  You WILL want to pretension them a tad so the mast can't start swinging back-and-forth and then knock the trailer over when it fetches up against the guys.

[Ungrounded Lightning Rod]

Give us the dimensions (width between wheel centers, axle-to-tongue, axle-to-rear-frame-member, location of base of mast, width and length at corners suitable for guy anchors)

And most importantly:  Heights of the mast and the floor of the trailer.

[dnix71]

The real issue is what happens to the trailer if the guy wires hold fast in an unexpected  gale.

Even 15 pounds at the top of a long pole may be enough to tip the trailer, with the wind pushing on the boat, too.

Maybe you should consider a break safe style mount.

The breakpoint hinge should be well above the middle, so the falling mill won't hit the ground. You could also make your guys wires with a breakaway link and hinge the gin pole.

Test your guy wires by placing tying a long rope to the top of the mast and pulling. If you can pull the trailer over, then the wind may be able to do it to. Remember to use a long rope, so keep it as close to parallel to the ground.

[esc]

I do not intend to move the trailer while the tower is up.

It will be hitched to my truck and stationary any time the tower is up.

[esc]

I just wrote a long reply and it didn't save right...so I'll try again.  

The pipe I'm using for the tower is a single 21' piece of 1.5" schedule 40 galvanized pipe.  It will be topped by an Air 403 that weighs less than 8 lbs.  Southwest Wind Power recommends that the tower be able to handle a 150 lb lateral load in order to survive 110 mph winds.  I doubt the tower will ever see half of that wind speed.

The trailer is 20' over all.  The axle is ~60" wheel center to wheel center and ~70" outer sidewall to outer sidewall.  It is located 5' from the back of the trailer.  That is right in the middle of the 10' rectangular section that makes up the back half of the trailer.  The floor of the trailer is about 18" off the ground.  The trailer also has a 5' high triangulated mast support made of 2" galvanized box section that was originally designed to carry the mast of a Hobie 17 as well as the mounting point for the winch.  The top of that support will be the pivot point for the pipe.  The 21' pipe will seat on the floor of the trailer.

The base of the tower will be 6.5' from the front of the tongue.  1 Guy wire will go to the front of the tongue.  The 2 rear guy wires will be mounted 6.5' back and 8' apart.  That will place them a little more than 7' from the tower base, in a straight line.  I anticipate attaching the guy wires 3' from the top of the pipe, which will be 18' above the floor of the trailer.

The empty weight of the trailer is about 450 lbs the loaded weight will be about 900 lbs.

trailer 450 lbs

Boat 125 lbs

Pipe 75 lbs

Battery/gear box 200 lbs (just behind the tower footing)

Lawnmower generator and solar panel ~50 lbs

Please note that the intended use for this rig is camping out at the beach.  The tower will always be in the down position any time the trailer is moved.  The trailer will always be hitched to a truck any time the tower is up.  The tower will not be left in the up position without supervision.  It is unlikely (but not impossible) that the tower will be in the up position during sever weather (especially lightning), as severe weather does not lend its self to camping out at the beach.

OK, I think I answered all the questions.  If anything is not clear let me know and I'll try to do a better job describing it.

Thanks,

Eric

[Mary B]

Add outriggers on all four corners that fold in along the length of the trailer. Make sure you add some type foot (trailer jack maybe?) on the end of each one. That would add a lot of stability. That's a very common setup on emergency communication tower trailers.

[esc]

Out riggers would work, but they are heavy and a lot of work to build.  I think that if I have stability issues with the trailer I'll probably look at using tie downs that drill in to the sand (I forget what they are called).

But back to my original question:

If the guy wires are attached 18' up and 6' out, the leverage of the tower should triple the force applied to the guy wire, right?  So 450 lbs.  Now the force will actually originate 3' above the 18' mark so it will be somewhat larger than 150 lbs at the 18' mark.  

WARNING: Math problem ahead!

Given:

Lateral load of 150 lbs at top of 21' tower,

Guy wires attached 18' up and 6' out.

150. lbs * 21' / 18' = 175lbs of lateral load at the point where the guy wires attach.
     
18. ' / 6' = 3 (leverage presented by tower/guy wire geometry)
    
175. lbs * 3 = 525 lbs (tension on guy wire)
     
     

Would this be the correct way to figure out the stress on the cable?

If my formula is correct it should be fine with the 1540 lb cable.

This assumes that the stress will be directly opposite the shortest guy wire.  What if the lateral load is between 2 guy wires?  How would the stress be figured out then?

Would I draw an imaginary line between the two guy wire anchors opposite the force and measure the distance from the tower base, in the direction that the force was coming from, to that imaginary line?  Then use that measurement for the base leg of the triangle to compute the leverage that the tower would have on the cables?

Then how would the force be distributed between the 2 guy wires?  Would it be proportional to the relative distance, from the point that the force vector crossed the imaginary line, to the 2 anchors at either end of the imaginary line?

So I have a 2 part math problem and I'm not sure if I'm setting it up right.  Since this is a computation that should probably be performed on any home made tower, I hope someone here will know if I am doing it right.

Thanks,

Eric

[TomW]

Eric;

Its early so maybe I missed it but your biggest loading concern is wind induced thrust, not the turbine mass. Is that included and I missed it? Thats the force that will tip the mast. Just a minor point you [or I] seem to have missed??

Tom

[esc]

Southwest Wind Power says that a tower designed to hold an Air 403 should be built to withstand 150 lbs of lateral loading based on their stated survival wind speed of 110 MPH.

So the 150 lb figure I am using is the largest wind induced thrust that I can expect.  Actually it will be MUCH less, since I generally don't camp on the beach during hurricanes.

[imsmooth]

You calculate the tension using trig.  If the lateral load is 175lbs you first need to calculate the angle, which is theta = arctan(18/6).  The tension, T, on the cable is:

Load/T = Cos(theta).

T = Load/Cos(theta)

T = 175/Cos(arctan(3))

I don't have a calculator on me, but I think it is something like:

T = 175/0.3 = 583lbs

[esc]

That being the case it is still well within the safety margin for the cable I intend to use.

Thanks,

[thefinis]

I have not checked the strength of 1.5 schedule 40 iron pipe or its flex but if flex strength is a problem a second set of guys lower on the mast tied to the same points on the trailer should fix it. If you are concerned about the guying and its weakest spots it will be when there is a wind directly from the direction of one of the back two guys(guy to mast wind) as they have the least amount of angle ie the most stress and the two other guys give no support for that wind direction. If the trailer mast and guy wire ties are built strong enough to handle the stresses then your math problem becomes more like a freestanding tower than a guyed tower. This assumes that you are not going to anchor the trailer and all that holds the trailer is its own weight plus the weight of what is on the trailer. Two other factors come into play and they are that it will be hitched to your truck and that probably the boat will not be on the trailer while the turbine and mast are extended. I have not done the math on your guys mast etc because they are not the parts I think will fail. It would take more details on trailer strength and how the guy wire ties are attached to trailer frame to really analyze and long before you break most cables the trailer will flip over.  

Here is a link to math on figuring forces on a freestanding tower.

http://www.thebackshed.com/Windmill/FORUM1/forum_posts.asp?TID=252&PN=15

You will have three main points to use for figuring forces they are the hitch point and both tires. The hitch point will be the best tied(?) due to truck weight and length of trailer tongue. The tire points will be very dependent on weight distribution and width between them because location of weight and distance between tires factors heavily into forces needed for tipping the trailer. The more the weight is distributed over the axle and to the outer edges the better.(for the turbine forces not hauling) The trailer tongue takes some of the weight off the axle how much depends on how the weight is distributed.

If it was me I would point my truck to the water(prevailing wind direction) and use two screw in anchors tied to the back corners of the trailer if needed. Hope this helps. A little trial and error will give you most of the info you need.

Finis

[TomW]

esc;

OK, I should not post before coffee, I guess!

Be sure to keep the mast from trying to play sidewinder by guying it midway so it cant flex in the middle.

A seasoned D.I.Y.wind turbine installer once told me "working load is not breaking load". He will remain anonymous. Having said that I much prefer to go beefier, where possible.

Good Luck with it.

Tom

[esc]

As far as the 1.5" schedule 40 pipe flexing...I'm inclined to think that it won't be a problem.  If it is and a second set of guy wires are in order it will not be a problem.

Are the guy wires built strong enough?  That was my first question, but I have now convinced myself that they are.

Is the trailer strong enough?  I think so.  If it can bounce around on rough roads with close to 1000 lbs on it it should be able to hold the 500 or 600 lbs of force we are talking about applying.

All in all, I'm starting to think that I should go forward with the 3 guy wire plan.

Eric

[ghurd]

I didn't wreck my car last month, but I paid the insurance.

ghurd math.

What does it need, then add $20...

If something needs #14 wire, usually $20 more gets #12.

If this needs 3/16", $20 more should get 5/16".

Quickie (crappy multi-site cross-referance numbers) look shows far less than $20 more would go from 1/16" (480lb) to 5/16" (9000lb).

Not worth $20 in asprine to worry about it!

G-

[TomW]

Eric;

Don't neglect the suspension travel. If it is in high winds the springs may relax on one side and compress on the other vastly changing the load characteristics of the rig. Jack stands or blocking could eliminate this issue. Just looking at it from the what could go wrong angle

Tom

[esc]

Those are both good points.

The wire I am considering is this:

http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=97605

It is thin (3.2mm) but has a good working load rating, 1540 lbs.  It is also stainless steel which is good for something that will spend a lot of time on the beach.  The facts that it is inexpensive and available locally didn't escape my attention.

I would not object to spending an extra $20.00 on heavier wire if I could find it locally in stainless or some other rust resistant (coated?) form.  But this should have close to a 3x safety margin, which is enough for me feel that it is adequately "Murphy resistant".

The suspension travel is definitely a factor, but I don't think that it will be a deal breaker.  The suspension will be about 9 feet behind the tower base and the truck will also be there to help hold things down.  If it does show a tendency to tip, it will be a good excuse for me to add more batteries.  

Also, I had good luck with my previous attempt at this project.  The tower was only 12', but even unhitched it never tried to tip over.  The goal with that tower was to get it high enough to keep from chopping off peoples fingers.  Now I want to get it as high as I can to get cleaner wind.  So tipping the trailer is trice as likely, I'll just have to see how much pull it takes.  If extra batteries doesn't make it stable enough, I may resort to shortening the tower...but I don't want to even talk about that yet.