Strength of winch and running rigging

I'm looking into the possibility to get electric winches for my sun odessey 51. This because I'm going to be single handling the boat, and it is time to replace the running rigging (including blocks and some of the winches).

I notice when searching for electric winches for sailboat that none of them are using drums. They all are like the manual winches, just with a motor. I can see why as the sheets on my boat is 16mm (5/8 in) and the drums would have been huge to hold several meters of this size. I have the feeling the lines on my boat is significantly oversized. I don't know the specs for the current lines and winches, but looking up specs for lines that seems to be similar shows they should handle loads of 6-9 tons. I doubt neither the clutches, blocks nor the current winches would be able to hold such forces.

My question is then; How strong does the lines actually need to be? From this also follows how strong does the winches need to be?

 

Atilan, welcome to the forums...

<sigh...> There really isn't a good answer to your question. Sails should fail before lines, and lines should fail before winches and rigs. But...every turn in a line reduces the strength of the line, and lines have to be handled by people. So...the minimum diameter of a handled line needs to be ~3/8" and the rated load of a rigging system needs to be ~200% of the line. Often this means that the entire weight of a smallish (~30 ft or less) vessel can be supported by the jib and main sheets...alone. Notice that this is not like Club Mediterranée in the 1976 OSTAR where particular sails were expected to blow out to ease the load on the single crew. Generally, the sail area and sailcloth weight specified for any arrangement in any wind is designed to be less than the mast and halyard/sheet loading. The maximum of these sail conditions sets the line, mast, and winch loads as well as the righting ability....and the crew is expected to keep these requirements or a loss of mast or vessel is expected.

 

Captive reel type winches are used on some superyachts and they where relatively common on normal sized yachts when wire halyards where used. There are several reasons you don't see them: bulk, speed, complexity, redundancy.
Line size isn't a problem, 8-10mm of dyneema can handle 6-9t. Sheets are indeed oversized because human hands can't adequately hold small diameter line under high loads.

 

Je, may I ask why you say that sails should fail before lines? Lines are far cheaper and easier to replace, and a blown-out sail can cause windage issues and be very difficult to dowse. I'd much prefer to snap a line, although I also have no interest in powered winches as I feel that they can do too much damage when a manual operator would notice excessive loads caused by a problem.

 

I feel that once you're on a big boat where the sheets are held by clutches for hours at a time, the feel you get for the excessive load by manual handling of the sheet, compared to a motor winch is negligible. On a smaller boat, where you actually can hold the main sheet I totally agree with you.

I'm thinking that if I get to the point of installing electric winches, I'll also include a display to show the load on each winch. I've never seen such simple thing used on a sailboat, but it really is just a $3 current sensor on the power cable to each winch, connected to a MCU (i.e. a $5 arduino) and a display by the helm. Knowing the power consumption vs force on each winch (to be found by a few tests on each winch) the MCU will be able to tell the load while pulling.

I'm thinking that if I go for electric winches with drums, I'll locate them where they are needed. I.e. my main sheet is fed into the boom, down the mast, out to the side deck, before it finally goes back to the cockpit. An electric winch, with a drum, could be mounted on the boom, near the mast. This will save a lot of turns that the current line is passing, and thereby reducing the friction the winch has to overcome.

Thinking about the sizing of winch and line... Looking at the largest manual winches I find in online stores, none of them has gear ratio of more than 13:1. Counting for a winch handle being 5x the radius of the winch, the total ratio becomes 65:1. Assumed my strongest winch is similar to that (which I doubt is the case) and I can't think of any time winching in lowest gear has been heavy, I'm guessing I've put 10-15kg of force on the handle at the max ever, that is 650-975kg on the line
I've never been out sailing in bad conditions though. I consider my boat as a house with sails. Under bad conditions I would depower, easing loads. But what about gust winds arriving before I manage to depower? Say they are 3x the max load I've ever experienced while winching. That is 3 tons. If my current lines are of the weakest kind I've found for their diameter - 6 tons - they are just double of the gust wind load thought out above. Maybe they aren't that much oversized as I thought?
(I should count the gear ratio of my winches to figure out how much off the above guessing is.)

 

I've sailed maxis and racing 60s and 50s a bit, and even at those sizes there's still feedback on the loads from those on the winches or the grinders. Even when you have someone tailing a winch and a couple of people grinding, if they know their job they will normally be aware of excessive loads.

The load display sounds like a good idea. One wonders whether there could also be a default to turn the winch off if loads reached a certain point, plus an over-ride switch.

I'm no expert on line loadings but I think you'll find that shock loadings in heavy air are the biggest problems. When a sail flogs and then fills with wind in 35 knots the loads spike pretty severely before the boat can heel and reduce them.

I understand that the loads on a 50+ footer are quite high for inshore pleasure sailing (that's a major reason we abandoned our plans to move up into a bigger boat and get a 52-55 footer) so can understand you looking for ways to handle them. Good luck with the project.

 

An overload shutoff with override switch is easy. All you need to add to the above mentioned electronic is a mosfet (for each winch) controlled by the MCU, and an override button - either a button for each winch, or an common override that enables override for the next winch to start. It does need an extra cable to the winch though. So, the cable you use for load sensor should be 3 core rather than 2 core. (All though load sensor can do with single core cable it should use 2 cores TP or coax due to noise on the sensor signal.) I like to use shielded cat 6 ethernet cables for any low current connection. They are cheaper than most single core cables you find, and gives you 8 cores + shield/ground.

I did some more thought on the loads.... If one assumes my keel is 7 tons, with center of gravity 1.5m under water. Then assume center of area of the sail is 1/3 up the mast - that is 6m above the boom, 2m above water. Then the mast will become horizontal with a force on the sail equal to 1.3125 tons. If I so assume that the force that causes the the boat to lean over so much that the mast becomes horizontal is the highest force to ever hit the main sheet, I then get that 1/3 of that load hits the rear end of the boom, and as my sheet is near half way out the boom, this load doubles. So main sheet max load is 1.3125/3*2=0.875kg. Then this sheet goes 6 times up/down from the traveler, so only 146kg is left on the sheet.
Doing the same estimation on the head sail sheet, I end up with 1.3125 tons /3 = 438kg.

 

Something isn't right here. My Catalina 38 had a 590 sqft genoa - similar in size to yours. When single handing, the tack procedure was to get about 150# of pull on an extra-long handle on a 50:1 Lewmar to get about 5000 pounds or so on the sheet as it came through the wind. Then fall off until the wind pulled the sail mostly off the spreader tips. Normal sheet loads on a pleasant day were 5000 - 7000 pounds when beating. But working through the Christmas trades in the Gulf Stream with 25 knots on the nose, and catching a barrel of seawater in the sail from time to time, would double that. My cruising genny was a tough old thing - all 90 pounds of it.

 

And you need to think about the force on the main sheet that is needed to move the main, not hold it. On a 6 part main, that works out to be about double the static holding force using good, name-brand blocks. To that, add in any geometry issues like the sheet loads being distributed along the boom. For a powered setup, I would look into simplifying to mainsheet to a 2:1 system. The mainsheet starts outboard of the traveler track, runs in to a big sheave, turns up to the boom, back down to a second sheave and outboard to a winch. If the mounting point on the boom is correct, the main sheet can be left alone when the traveler car is adjusted, but it takes more traveler power.

 

I tried wind load calculator.
Wind Load vs. Wind Speed https://www.engineeringtoolbox.com/wind-load-d_1775.html

If wind of 20m/s (about 38knots) hits 90 deg a 55m^2 flat surface, it's a load is 13500N. If one assumes that surface is a triangle held in each corner with equal load distribution, which I think is close to the conditions of a headsail, that should pull 4500N on the sheet. That is 450kg.
Backing the guessing I did above.
How can you get 7000pounds on the sheet in nearly half the wind? Maybe I should have a look at air foil lift calculations....

 

Atlanta

Wind loads have two components
-drag wich is parallel to the wind direction
-lift with is perpendicular to the cord of the sail

The sheets are rarely in-line with either of these directions. Therefore the fotce of the sheets applies to the sail does not directly counter the wind forces on the sail.

There are trigametric formulas to determine the difference between applied force and actual work done by lines at a angle to resistance. But it has been awhile since I have done the math required to explain better.

At zero degrees applied force and work done equal. As the angle approaches 90 the effort requires approaches infinity.

The sheeting angle while close hauling is often more than 80* off the the cord angle. Close hauled sheet loads are ten or more times those of reaching

 

Blueknarr, I see your point about lift being able to generate a larger force than the direct wind impact, and the principle of force required to pull a line straight is way more than the force required to put a curve to it. However, if you pull the line+sail straight, you don't get much of an air foil shape. You will let out a bit more line to have a curve. The sheet on a headsail will always be pulled forward and a bit to the side. If it has an angle of 60 degrees to the imaginary line from its attachment point to the nearest point at the forestay, the line has double force of a line that holds at 90 degrees to the sail. If you tighten much more than that, your sail won't have much of a foil shape. I'm no expert at sailing, but I've noticed several times that boat speed increases when I let my sails curve more than I initially though.

Playing with the Foilsim
Student Airfoil Interactive | Glenn Research Center | NASA https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/foilsimstudent/

I can't find a way to make the foil triangular, so it's rectangular.
Span (mast height) = 16m
Chord = 3.4m (makes area 54.4m2)
Wind speed = 72km/h = 20m/s =~38knots
Chord thickness = 1 (the thinnest possible)

With this set, I played around with Camber and angle of attack to find the worst case scenario where Lift + Drag = the highest possible number. This should make the highest total load on the lines.

The worst I found was:
Camber = 20%
Angle of attack = 11.6 deg
This makes a lot of turbulence.
Lift is then calculated to 38kN. Drag 10.2kN. Total force 48.2kN = 4900 kg.

1/3 of that on the sheet: 1633kg = ~3600lbs.
Thats 3.6 times my 450kg, half of Philsweets 7000lbs at almost double of your windspeed. I think Philsweets got the numbers quite a bit too high.
Reducing wind speed to 25knots, as Philsweet uses.. That is 46.3km/h. Looking for the angle of attack that puts max Lift+Drag... That is 12.3degree. Then the Lift=15.79kN and Drag=4.26kN. Total 20.05kN. Giving sheet load of 6.68kN = 681kg.

Note that when playing with foilsim, the camber calculations expects the sail to stretch into a curve, keeping the same projected area seen from wind direction, while in reality curve to the sail will reduce projected sail area.

The more ways I look at this, the more confident I get that 1 ton winch will do the job.

 

Atilan, You need to calculate the tension on the sail membrane. Lets say you pull your jib halyard down to the base of your shrouds. Now look up and figure what the greatest lateral distance is between the halyard and the cap shroud. Maybe it's a meter. And the length of the halyard to the block is about 16 meters. So the sail membrane has a draft of 1/16 vertically. If you just assume an arc, the foot of the sail will lie at an angle of 20 degrees to the halyard. So you need downward pull of 2.75 times the side load, and a sheet tension of nearly three times the supported load. That's just for twist control and leach flattening. Now you have to pull the foot of the sail aft and set the fullness of the genny's fore-aft camber. Lets say there's some draft cut into the sail, and the actual tension needed equates to a draft of 6% on a flat sheet. Now you need about 2.75 times the tension pulling aft. So the sheet tension is the SQRT(1^2 +2.75^2 + 2.75^2) times the side load on the sheet. That's right at 4 times the side load (wind pressure load) at the clew.

Now in practice, when close hauled, my 160% genny sheet pulled down much harder than it pulled aft - three or four times as hard - and the leach had very little sag in it. So in my case, the sheet had 9 to 12 times the side load. Then throw a barrel of sea water in it for fun.

 

Sorry I've been away for awhile. Anyway, after having seen the damage and chaos caused by a parted line and water dropped sail vs what happens when a sail blows out, I'd much rather have the latter, easier, mess to clean up...especially in a blow. YMMV.