Stressing (bending) the mast

[Tom Hoffman]

I have built my own motor mount and until this past weekend it has not really worked as designed. I could not get enough lifting power to get the motor out of the water when sailing or at the dock. My rigging was not the best. The prototype was crude and did not have good blocks or enough of them.

Finally this weekend, I rigged a 6:1 lifting system. The cable goes to the top of the mast and is separate from the the back stay I got it the same time as I got my new fore stay. Well, I hooked everything up and tried to hoist away and nothing happened. I shifted the position of my upper blocks to be directly above the eye bolts and hoisted away again. Success...

Now after all that, here is the issue. The motor weighs around 80 lbs, and even with 6:1 it seemed very heavy and seemed to put an awful lot of tension on the mast. The back stay bowed loose.

Is using the mast in this way to lift 80 lbs to much pressure? I don't have a mast post. The mast is cabin top stepped.

Here is a pic of how is should go. Mine has blocks for the lines to go through rather than over the bars like these do. This is not a picture of my boat. I forgot to take any yesterday. It was to darn HOT!!!

What say you???

 

[Mike B1026]

Very unusual design.

 

[Tom Hoffman]

Motor Mount

I made the mount because I got tired of heading for the dock with my butt in the air and my head down trying to operate the motor controls. I also mounted a power tiller on my tiller handle so now I can maneuver while looking forward standing up. The motor stays straight forward and does not turn. Much like a fixed prop shaft thru the hull.

While the motor is in the water, it works fine and now, because I have finally gotten the raising and lowering to work all seems well. I am just concerned about to much tension on the mast.

Tom...

 

[THECUSCUS]

That is really cool idea for a lifting device.Maybe you could use some big springs to help in the lifting. Saw someone use old springs from a washing machine to help lift an outboard. If you put some type of locking mechanism on it so once it's in the raised position you could let the pressure off the line to the mast.

 

[Tom Hoffman]

Locking Mechanism

THECUSCUS; You have my exact idea. Once the motor is up and is going to be up for a while. I just put a couple of band clamps on the up right tubes just below the motor mount block. My original reason for doing so was so I could take my halyard and blocks off and store them when I was not on the boat. Problem with that is I have to use my cordless screw driver out over the water when I put the clamps on. As dropsy as I have become in my old age, I can just see my nice De Walt Screw Driver hitting the water.

I wonder if there is a 1" dia. toggle clamp that might be available with a thumb screw. I will have to look.

Thanks,

Tom...

 

[THECUSCUS]

Tom, why not just drill a hole in the tube and insert a pin to hold the block up? or drill a hole through the block and insert the pin through both?

 

[tkanzler]

You didn't say what you're using for the sliders as far as bearings - plain, roller with plain bearings, roller with rolling element bearings (ball, roller, needle, etc.). Sliders like those shown in the pic typically have friction as their worst enemy. The OB's center of gravity being outboard of the lifting plane puts the top of the slider in tension (pulling outboard), and the bottom in compression (pushing against the hull), and the closer the top is to the bottom, the worse the resulting forces are between the slider and the tubes.

You can mitigate that by putting the lifting points directly in plane with the OB's CG. It sounds like you did that to some degree by moving the eyebolts aft. It will get better and better as you move them further out, to a point at least, as the guide sheaves are still forward. That puts some horizontal force component forward, against the tube, which counters the aft component.

But it would be better still if the lead sheaves were moved aft also, so the actual lifting line of action is dead vertical, and in plane with the OB's CG. That way, the sliders don't really do anything other than keep things steady. But like anything in engineering, you don't necessarily have to be perfect - just close enough, wherever close enough ends up. Some angle to the lifting lines may be perfectly acceptable. Try moving the eyes on the moving part further aft and see what happens. Eventually, you'll start working against yourself, and you'll know then where the sweet spot is.

There are also plastic bushing mades specifically for such dry sliding mechanisms.

Now after all that, here is the issue. The motor weighs around 80 lbs, and even with 6:1 it seemed very heavy and seemed to put an awful lot of tension on the mast. The back stay bowed loose.

Is using the mast in this way to lift 80 lbs to much pressure? I don't have a mast post. The mast is cabin top stepped.

80 lb vertical load on the mast is nothing, but with the sliding friction, I suspect you're putting a lot more than 80 lb of load on that halyard. The shrouds are yanking down on that mast with hundreds, and potentially thousands, of pounds of force when the rigging is heavily loaded. The bend induced by the halyard would be concerning to me, though. Without knowing how much bending stress is actually on the mast, there's no way to know how close to damaging it you're getting, though if the backstay is only getting slack a little, it's probably not as bad as it looks.

How much slack is actually developing? If it's about as much as you'd shorten it to intentionally induce bend into the mast, then it's no different than doing so with an adjustable backstay.

If you have the room, you can cut the force on the mast in half by using small blocks on the lifting eyes, and terminating the dead end on the motor mount fixed portion. That way, you're pulling up on the other end of a 2:1 purchase, and voila you've halved the force to the mast. If you can move the lead blocks aft on your structure, you can also terminate the dead end of this 2-part system to the same mount - if there's room for fairleads, there's room for a termination.

Or get really creative, and go with 3- or 4-part rigging at the mount, so the halyard is really lightly loaded.

Incorporate some sort of hinged hard stop to hold it there, or even a pin (or pair of them) through the tubes, with some sort of tethered hairpin or other keeper (ring-dings are great, but not conducive to quick installation).

Just some random thoughts.

 

[Mike B1026]

I may be missing something, sorry. I don't see an advantage to that design. Why not use a traditional motor mount? It has springs and a handle built in to help you lift and lower the motor. Also because of it's size, I would worry about damaging it or the motor if it came into contact with a dock or piling.

With the motor mounted farther back, it seems like you would still have issues reaching the controls. (I assume they are located on the outboard tiller?

Since mine has a cutout in the transom, I may not be seeing the advantage of that design in your situation. (I don't use a motor mount.)

 

[Tom Hoffman]

I may be missing something, sorry. I don't see an advantage to that design. Why not use a traditional motor mount? It has springs and a handle built in to help you lift and lower the motor. Also because of it's size, I would worry about damaging it or the motor if it came into contact with a dock or piling.

With the motor mounted farther back, it seems like you would still have issues reaching the controls. (I assume they are located on the outboard tiller?

Since mine has a cutout in the transom, I may not be seeing the advantage of that design in your situation. (I don't use a motor mount.)

Hi Mike, My reasoning was that my old off side motor mount was almost trash when I bought the boat. I did not trust is and it was way to the port corner and down a ways. It was OK to get the motor up out of the water, but the problem still existed that I had to stand on my head to operate the controls and was always crashing into the dock with my butt in the air and my SWAMBO yelling at me etc. Not a good situation at all. You may have missed it, but I have added a power tiller that allows me to control the motor from my tiller arm and not have to deal with the motor until it is time to start it or shut it down.

I found this motor mount idea on line and made a copy of it.

TKANZLER, I think you hit the nail on the head, there is way to much torque friction on the tube by the CG of the motor being out board and when the lift starts, it binds fore and aft.

I did not have any way to make a sliding block with any kind of bearing in it. I use 2 pieces of White Oak 1" thick and then routed a half round in each to give me a circle somewhat larger than the 1" tube. So yes the friction is a lot. I will explore the option of moving the attachment points to the motor it self or to the motor clamps. There by moving the pressure further aft. That should help some. The only thing I know about engineering is that they drive trains don't they???

Thank you.

 

[Mike B1026]

Perhaps rout the half round a bit larger and install some type of material with less friction like a bushing? A nylon or plastic material, similar to the stuff they use on bunk boards? (or even a bronze bushing?) Then lube with Sailkote or something similar?

 

[watercolors II]

Additional idea to setup

If the lift was straight up then you would be lifting just the motor weight without any friction applied to the slides.

Mount a turning block right above the motor so the line is straight up at the motor then to the mast or better yet add another turning block beneath the top one level with the stern and run you line up from the motor and then down to stern height and then to the headsail sheeting winch where you can use a winch and handle to do all the lifting.