Rigging Tuning on trailer vs afloat -or- Am I losing My Mind?

[wes]

OK, friends, I need to know if I'm losing my mind.

I tuned my rigging last weekend with Bella on the trailer at the ramp. Got things just perfect, I thought, then locked down all the turnbuckles and went sailing.

While out on the water I chanced to grab a shroud and was surprised at how loose it was. All of them were loose and woobly in fact, and this was in a dead calm. Out came the pliers, and I re-tweaked the tension all around until it seemed perfect again. Sailed all afternoon with no problems - firm on the weather side, looser on the leeward side, just like the book says.

Pulled the boat out at the ramp at the end of the day, and the rigging was scary tight - just guitar string twanging taut all around, doing its best to drive the compression post thrugh the bottom of the boat. Yikes! Out came the pliers again and I loosened things up.

Other than the possibility that I'm suffering from early-onset Alzheimers, my working theory is that on the trailer, where most of the weight of the boat is concentrated on the bottom of the keel, the hull flexes downward a little under the force of gravity, but the mast doesn't move since it's locked to the keel via the compression post, and as a result the rigging is pulled tighter. In the water, the weight of the boat is supported by the water more broadly over the entire hull, and therefore the deck shifts back upward to its more natural position and the rigging gets looser.

Has anybody else noticed this? I have never seen it mentioned either on this forum or anywhere else.

Wes

[brackish]

I haven't noticed it because I've never tuned on the trailer, but I think your theory is correct.  Another factor might be differential expansion rates changing because the hull is cooled off in the water.

[CaptRon28]

Your theory is correct. The hull has a slightly different shape while on the trailer vs when in the water. Not much, but enough to increase the tension in the wires. Most of the sail boats that I've owned were significantly larger than a 19 to 23 foot Compac, and I usually had to wait maybe 24 to 48 hours before I could tune the rig after a spring launch. The in-water rig  tune is more accurate than the on-trailer tune, and since the mast is probably down when trailering it, why even bother doing it?

[Billy]

Interesting....

I agree the the hull is pulled down by gravity. Plus when it is in the water and the hull is supported, but the keel pulls down, possibly pulling the mast down as well .Making the rigging even looser.

Just my thoughts. Never noticed that. Next time I go out I'll be sure to check on the trailer and on the water.

[MacGyver]

I tune mine when in the water, and now just loosen the backstay then push forward on the mast and then the wife pops the pin on the forestay and presto the mast is ready to be let down. The side stays I dont touch.
I set the tension under sail, after some has been done at the dock.

When I go to pull her I plan to take the rig down at that time then pull it out.
I did put the rig up then launch the first time, but tuned in the water.

still not sure what the exact awesome way is but I figure guitar string tight is not all that good, LOL

Mac

[skip1930]

The windward side goes tight, the lee side goes limp. That's my throw away line.

I'm think it's not the hull as much as the temperature of the aluminium mast. She grows as she warms. The coefficient of expansion of aluminum is far greater than steel, by the way the rigging and chainplates is steel wire and bar. It grows at temperature but not as much as aluminium.

Wire. Wire cable stretches under load, and contracts when the load goes away. At BayShip Building, we never picked up even light loads of 16 or so ton with a chain. Always use wire rope. Our max was a pilot house for a thousand foot freighter at 270 ton...on wire. The stretch and relax is the reason why our boats should be tied to the dock with three strand nylon line sized for the load. Smaller dia is better for a more relaxed 'feel' at the dockside.

FiberGLASS is pretty stable, as glass goes it's not a great 'expander' with heat.

Palmer Johnson aluminium 104 ton yachts loose three inches in length for each 150 foot of sold boat length, so we saw cut it apart and jack, pull, and push that three inches back into it's length. So we weld-up the boat three times as she shrinks during welding.

My dad's grandpa was a bridge and trestle engineer during the steam engine railroad day's. He had to factor with his slide ruler the growth of the steel as the sun first heated up first one side, than later the other side of the structure, properly placed expansion joints keep the train on the tracks. The contact patch of the wheel to rail is less than that of a dime. Per wheel. It's hard to stop a train for this reason. Aside from weight.

skip.

[Bob23]

Wes:
   Don't listen to all those other guys....you are loosing your mind!
bob23

[Koinonia]

tighten the rig in the water, thats what racers do that trailer from one club to the next.  Alot of them though wont check the next day which is good to do since the boat will usually flex and settle into its stressed position.