A Question of Balance

Scott T-Bird · Mar 26, 2017

Benny and Aaron pretty much summed it up nicely. Aaron is exactly right that the weight on the tongue is not the same as the weight on the rear axle and the front axle does get unloaded. This is where the lever arm stuff comes in (the weight on the rear axle would logically be greater than the weight on the tongue, due to the lever arm). The fact that the summation of the truck alone and the summation of the truck plus trailer load do not add up exactly is also a point of confusion. I'm not familiar with grain scales. Do I assume that you get independent weights for each axle? If so, I would throw out the totals and calculate totals from the axle weights alone.
In that case, the truck weighs 6,080 pounds and the weight with trailer is 10,060 pounds. Simple math tells us the trailer weight is 3,980 pounds. (That sounds about right for a C22 and trailer, no?). The loads on front axle, rear axle and trailer axle are 3,300, 3,460, and 3,300 pounds, respectively. That sounds pretty well balanced. By simple subtraction, the weight on the tongue is 680 pounds (3,980 - 3,300). There is no need for lever arm calculations to determine weight on the tongue. The lever arm calculations would be used to find out how much is loaded on the rear axle and how much is unloaded from the front axle ... but we already have the axle weights so what would be the point of doing the calculations? Theoretically, the distance between front and rear axle would need to be 4.85 times longer than the distance from the tongue to the rear axle for these weights to be right. If the wheelbase of the truck is 10' then the extension of the bumper and tongue is a little more than 2'. Again, this sounds reasonable.
The only troubling aspect is that the tongue weight is about 17% of the total, which is a little high, I think we all agree. Perhaps moving the boat back several inches wouldn't be a bad idea.
OTOH, if the axle weights are off and the totals are more correct, this would throw off the calculations above and perhaps indicate a lighter tongue weight by about 300 pounds. In that case the tongue weight might be in a good range. This can only explain the rear axle weight being 800 pounds greater if the lever arm from axle to tongue is longer.
I think you need to find out why there is such a large error in the weights and it would also help do a check on calculations if you knew the distance between truck axles and distance between the ball and the rear axle. These (and correct weights) are all you need to calculate the additional load on rear axle and reduced load on front axle.
If the trailer axle is rated for 3,500 pounds, why is this a problem? The axle weighed in at 3,300 pounds when it is loaded on the truck, so I don't see a problem there.
I'm not familiar with trailer extensions, but the fact that the extension has been repetitively loaded and unloaded without any permanent deflection seems to indicate that the flex you see isn't a problem. If the stress resulted in strain, then I'd be more worried. I-beams are designed to take a load with limited deflection. Square tubes are not. They are going to flex. It doesn't mean that it is going to bend. Maybe sometime down the road, but if you are sliding that thing in and out without any unexpected friction, what is the worry? If you feel you need to stiffen it up, why not just insert some bar stock inside the tube and call it a day? Bar stock on the bottom would be most helpful, but fastening it would certainly be a problem to solve. There must be some practical solutions.

Rick Webb · Mar 26, 2017

I think your fine as far as weights go

boat and trailer weigh 3640#
trailer axel weight 3300#
tounge weight 340# should be another 20 or 30 pounds (probably changes when all the gear is loaded)

Your absolutely fine with your tires and axle with a 3500# capacity

As far as the deflection of the tounge extension is concerned I don't think it is that extreme. To mitigate concerns over it use the tounge jack to support some of the weight of the tounge and as a safety back up attach a line from the hitch to the trailer jut in case it fails catastrophically. I am taking it for granted what you are describing is an extended bar to launch or retreive the boat at a ramp.

How many times has the setup been used? If it is more than a few times and shows no evidence of failing just keep an eye on it. If it deflects and does not straighten out when the weight is removed then it is time to look at a change.

Scott T-Bird · Mar 27, 2017

To understand if the extension tube is overloaded, it shouldn't be too difficult to calculate the anticipated bending stress and check it against the allowable bending stress that would be published for that particular square tube, but you would need some information to have that done. You need to measure where the load points are (it's obvious that one end is the weight on the ball). The other is the support tube where the extension tube enters the fixed trailer frame. I'd call the ball weight a point load and the frame connection would be a fixed load (edit) actually, it will be a fixed end moment (it creates bending moment where the tube enters the frame). The other point load is at the trailer axle. The maximum bending stress may be somewhere between the center of the extension tube and the location where the tube enters the frame (it may be right at the frame connection - I'm a little fuzzy on that detail).
You'll need to know the accurate weights ... there are discrepancies in your reported weights that need to be resolved. You will also need to know the distance from the trailer axle to the point where the tube enters the frame, because that load is all bending stress that would need to be calculated. When you find your max bending stress on the extension tube, you can compare it to published allowables for the steel. You'll need to know the grade of steel but it is likely to be the lowest one published. To start with I'd assume the lowest allowable bending stress for the dimensional tube that you should be able to identify simply by measuring the dimensions of the tube. After going thru all of that, my prediction is that the actual stress will be within the allowable.

Scott T-Bird · Mar 27, 2017

With regard to your trailer, if the trailer capacity is 3,500 pounds, you are indeed slightly overloaded and you would be more so with the boat loaded up. The tongue weight implies that almost the entire boat load is carried by the rear axle. How much does the trailer by itself weigh with tires ... maybe 800 pounds? That load is likely to be much more evenly distributed than the boat load is. Based on your numbers and dimensions I did a rough calculation that suggests that the boat contributes only about 60 pounds to the tongue weight. That suggests that the boat is pretty much balanced over the rear axle and doesn't contribute much weight at all over the extension bar. Intuitively, I would say that is about right. I had an inboard ski boat that was very similar in length, weight and distribution of weight and I know that it was difficult to lift the tongue of the trailer with the boat off and just a bit more difficult to lift the tongue with the boat on (maybe 60 pounds worth).
I looked at your metal geek results and don't agree so much with the assessment. First, the deflection appears to read less than an inch so I don't know where you read 8". I don't know what the "decimal gauge" refers to. The length between supports is actually 297" (over 24 feet) based on your drawing. The total beam is the length between the rear axle and the ball hitch. Young's modulus (elasticity) is correct. Moment of Inertia (I) is only about 1/2 what it should be - don't know what that is about (probably related to a "decimal gauge" that isn't correct). If you are standing on the extension, your weight isn't centered on the beam. The weight on the beam, I estimate to actually be centered, which means that the center is actually over the trailer frame, not the extension.
I have not done structural calculations for a long time so I used an Engineering calculator website to do a very simple representation (but not with your weight considered) with a square tube steel extension with 3/16" wall thickness. I found deflection to be just under 2". If it were a small I-beam, deflection would be less than 0.1". This seems to correlate with the photographs that CaptDon shows for his Cat 22, which indicates no problem at all for this set-up (even less deflection than I would anticipate). The fact that the frame should be taking the greatest bending moment, even with the extension fully employed, indicates that the tube extension isn't the critical component and it shouldn't be bending very significantly.
If you have 8" of deflection and a bent tube extension, something is indeed out of wack. I suppose I would start by looking for a trailer that is designed for the boat, rather than making modifications on this one. Personally, I don't think the problem is with the extension. If you believe you can economically make all the necessary changes, go for it, and maybe the extension has failed for other reasons. I'd start by not standing on the extension (but I don't even think that should be an issue)
BTW, the 5' dimension from ball hitch to rear axle does pretty much explain the truck axle weight changes when the truck is loaded with a 360 pound tongue weight. Based on your figures and measurements, I would conclude that your wheelbase is longer than 10'. 360 pounds on the tongue would unload about just 180 pounds on the front axle at a 2:1 ratio (5' on one arm and 10' on the other arm over the rear axle). You appear to be unloading about 120 pounds so the wheel base must be longer.

Scott T-Bird · Mar 27, 2017

I just did another calculation with your weight on the extension at 7' from the ball hitch (ignoring the weight from the boat). Your weight alone shows a 5" deflection. I think that possibly you shouldn't be standing on the extension. A 5" deflection suggests permanent damage to the tube, in my opinion. Working that winch could be putting much greater point loads in bursts of activity as well. Probably not a good idea. Do folks with trailer extensions typically do this as a matter of practice and what is their experience?

CloudDiver · Mar 27, 2017

You would be amazed at how much stronger a 1/4" wall 2x2 is over a 3/16" wall. I would just get the steel and upgrade your trailer springs.

Rick Webb · Mar 27, 2017

Your OK on the axle, just so but not so close that is an issue. The axle only cares about the weight on the axle not the total weight of the trailer.

Something else to consider is if you increase the capacity of the trailer to more than 3000# you will need to add brakes in most states. The requirement for brakes is based on capacity not on actual load. Just something to keep in mind.

It does now seem apparent that your extension needs to be beefier. Run it over to a metal shop or a trailer place and see how they can get you a stiffer extension.

pclarksurf · Mar 27, 2017

Just a point of reference: all the stock Trail-Rite axles and couplers are rated at #3500 -- and mine is original 1973 and still has the rating tags. Doesn't make it right-- but just for reference.

pclarksurf · Mar 27, 2017

I have priced out a complete wheel to wheel replacement of my existing original trailer including the axle, leaf springs, hangers etc.... I come out around 400 ish pre tax and shipping and the assumption is that I'm doing the work myself ( sourced from online retailers). Typically if you are having trades labor on your behalf you can double the hardware cost and add 10 to 20 percent. Sounds like you're in the ballpark depending on your marketplace.

Rick Webb · Mar 27, 2017

Sounds like you've decided on your answer but let me throw this out there. Have you considered adding another axle? you could even go with four smaller tires and lower the trailer thus making it easier to launch vs going to a larger tire an having to go deeper on the ramp than you currently do.

Scott T-Bird · Mar 28, 2017

$1,500 is a very cheap price for peace of mind when it comes to routinely trailering a 4,000 (plus or minus) rig. If you are not comfortable with the rig as it is, and if you're not comfortable with the price of making it right for you, you may never be comfortable with boat ownership. Parting with your cash is all just part of the game.
It sounds like a good price for a new axle, breaks, rims, tires. I've easily spent that much for the same treatment on my ski boat (I had 2 axles).

Rick Webb · Mar 28, 2017

My trailer had welded on spring hangers and fender mounts as well. it took no time at all with a grinder to remove them. I then used bolt on dual axle hangers to convert it to a double axle trailer. Alternatively you can stay with a single axle and use a drop axle to get it a bit lower.

Daveinet · Mar 29, 2017

I would suggest looking into an axle with torsion springs instead of leaf springs. The reason is that not only will that give you a drop axle, but also the axle pipe that goes across the bottom of the trailer does not move. This means it can be mounted so it is even with the cross members. With the trailer sitting that much lower, you may not need any extension at all.

AaronD · Mar 29, 2017

LakeShark said:
...can I just bed it with Butyl tape or should I use a Marine sealant meant to be used below the waterline. ...

I think you'll see both opinions. I asked @Maine Sail about the motor mount awhile back for a similar reason. He said butyl tape was fine (https://forums.sailboatowners.com/i...hardware-with-bed-it-butyl-tape.117172/page-7). I have a note in my maintenance list saying he approved butyl for gudgeons too, but I can't find that reference, so I'm not 100% certain of that.

I just bedded my new gudgeons with butyl - hoping it works... (but my crew's also smaller and lighter than yours - just 4 of us, 2 of whom are under 10 years old; so our stern probably rides a little higher).