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JERRY SCOTT'S IMB
Contents:
Contact info:
- Jim Michalak
- 118 E Randall, Lebanon, IL 62254
- Send $1 for info on 20 boats.
Jim Michalak's Boat Designs
118 E Randall, Lebanon, IL 62254
A page of boat designs and essays.
(15JUL98) This issue will continue the sail discussion with the workings of ropes. Next issue, about 1 August, will continue with the workings of sailcloth.
WEB SITE ALERT...Click on over to the TEXAS HISTORICAL COMMISSION if you live on the Texas Gulf Coast and are a boating history nut. The Texas historical commission has been digging up large bits of LaSalle's failed expedition including very well preserved ships and even a sailor's skull with a brain inside (sounds like a good start of a horror movie). I know it's not small homebuilt boats.
MESSABOUT ALERT....Craig O'Donnell (who is sort of responsible for the startup of my own web page) has a messabout scheduled for July 24 through July 26 at the Dublin Dock Restaurant and Pub in Betterton, Maryland.. Click on over to Cheap Pages to get the latest details.
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YOU CAN'T PUSH A ROPE!.....
I don't know how many times I heard that statement when I worked in the aero industry. It is worth repeating. Ropes are pure tension elements, all you can do is pull on them. The same would be true with chains and wires. "But," you say, "I push sideways on ropes all the time..."
POP QUIZ...
Question: Look at figure 1. A rope 20' has a 50# load placed in its center. How much tension must be placed on the rope to make sure there is no deflection at the load?
Answer: It can't be done. The tension would have to be infinite. The deflection itself is the only possible mechanism that can allow the rope to balance the load.
Now look at figure 2A:
Here is a way that the rope could balance the load. Here I have shown the rope deflecting 2' under the load. To do so the rope must stretch on each half from 10' to a length of 10.19'. The rope now slopes at a 2:10 ratio and, since the rope can only develop a force along its own length in tension, it now has a small vertical component that can resist the weight. With the weight in the center as shown, half of the 50 load is resisted by each half of the rope, or 25 pounds per side. But that 25 pounds is just one component of the force in the rope. With a slope of 2:10, the rope must have a another component of 125 pounds to maintain the 2:10 slope. The total tension in the rope as you see must be 127 pounds if the 50 pound load is to be balanced with this geometry.
But as you look at the other examples in figure 2 you will see there are lots of other solutions. The greater the deflection allowed, the less the tension needs to be in the rope to balance the load. We mentioned one extreme in the pop quiz where infinite tension is required for zero deflection. The other extreme case will be when the defection is a full 10' and the load is supported by the two rope halves pulling only straight sideways at 25 pounds each.
So which solution is correct? It depends on the stiffness of the rope. The stiffer the rope, the less it will deflect and the higher will be the tension in the rope.
I dug out a 1995 copy of a famous marine catalog and gleaned from it some strength and stiffness data on 1/4" ropes. Ropes of greater diameter are of course a lot stronger and stiffer.
TYPE ROPE................... STRENGTH...........STRETCH PER 100 POUNDS
3 strand nylon....................2000#............................................7%
braided nylon.......................?..................................................3.5%
3 strand poly......................1350#............................................2%
braided poly.......................1200#............................................?
3 strand dacron..................2000#...........................................2%
braided dacron...................2500#...........................................1%
The 3 strand nylon is actually quite strong but very stretchy which is why it is used a lot for anchor and tow lines. I didn't find the strength of braided nylon but from looking at the other ropes it doesn't look like braiding changes the strength in a big way. Braided rope looks to be about twice as stiff as 3 strand. Braided dacron is hands down winner as far as low stretch. I thought the low stretch of braided poly was a bit of a surprize since it costs a fraction of braided dacron and is available everywhere. It is more prone to sun rot. I have tried cheap braided poly as halyard on my Twixt. It does work well, stiffer than nylon, but is unpleasant to work with. It has a memory and once you tie a knot or bend it around a cleat, it seems to retain the kinks and bends of the action. It is also slick to the touch, not a good quality.
So if we push the nylon braided rope sideways with 50# how far does it stretch? It turns out that if the rope deflects 2.6' sideways (which is 31"), the rope will stretch to 10.33' long (or a stretch of 4" per side over the original length), that produces a load in the nylon braided rope of about 99 pounds and all is in balance. It's the only solution that works with the 1/4" nylon braided rope.
If we push a braided dacron rope sideways with 50#, how far does it stretch. It turns out that if the rope deflects sideways 1.7' (about 20"), the dacron rope stretches to 10.15' long (about 2" per side over the original length), that produces a load in the dacron braided rope of about 145 pounds and all is in balance. It's the only solution that works for this rope.
So the general results of the comparison is that the dacron rope, which is by nature 3-1/2 times as stiff as the nylon rope, reduces the sideways deflection about 30% and the tension in the rope and the reaction loads at the ends of the rope are increased about 45%.
And that is general is the way that rigging works. The stiffer you try to make everything the greater become the tensions and forces in the rig. It's not a linear effect either in that if you try to reduce the rig deflections by a factor of two, for example, the tensions and loads in the rig may increase by a factor of four or six or ??. You may quickly reach a point where the results aren't worth the effort. In extreme racing sailboats, I think there is no rope or wire at all in the standing rigging. The masts are held up with stays made of solid steel rod.
PRETENSION....
One effective way of reducing the flexings and stretchings in a sail rig is to pretension the ropes. For example look at figure 3.
Here we have the same solution for braided nylon 1/4" rope that we had before except someone has yanked on the ends of the rope and pulled them outward with a force of 150 pounds. The rope stretches out in length from10.33' to 10.53' per side, the end supports pull apart a bit such that there is no longer 20' between them, but 20.78'. And the 50 pound load which used to cause a deflection of 2.6' is drawn inward so that the deflection is now 1.7', a 35% reduction. I've also figured the case where the rope is loaded to 500 pounds of tension. Deflection of the 50 pound load is reduced to 1.2', a reduction of 54% over the original case. There is a price of course. Internal loading in the structure is now 500 pounds where the original was 99 pounds!
Now look at the effect of preloading a halyard. Sailor 1 hoists his yard to the top of his 10' mast and cleats it off with no preload. The wind comes by and applies a 50 pound load to the yard which transfers it to the halyard. The 10' halyard stretches 1.75% or 2" under the load and a gap of 2" will appear between yard and masthead.
Sailor 2 hoists his yard and yanks on the halyard while cleating it with a 50 pound force. In doing so he forces the yard against the masthead with a 50 pound force and that prestretches the halyard 2". As with Sailor 1 the wind pipes up and applies the same 50# load. Now how much does the pretensioned halyard stretch? Answer: Until the yard load exceeds 50 pounds the halyard won't stretch at all beyond its original prestretch and no gap will appear between yard and masthead. What happens is that the preload force between the yard and the masthead will reduce to zero and the force in the halyard stays at 50 pounds. As the yard load exceeds 50 pounds, say to 100 pounds, the halyard load will now increase from 50 pounds to 100 pounds and a gap between yard and mast will open up to 2" where with a similar 100 pound load the gap of sailor 1's rig will open to 4".
Actually almost any type of sail rig will benefit from lots of preload in the halyard. If you have a simple rig made of stout materials you can hardly overdo it. You can often preload a sail's halyard most easily by pulling down on the tack line since that line is usually quite short and can be made a multi part tackle. In the extreme you can preload the rig to the point of destruction even with no wind load on the sail!
Here is another thought. Clearly in all these cases the total amount of rope stretch is about directly related to the length of the line. So one way of reducing rig deflections is simply to keep the ropes short. Thus some Bolger boats like Birdwatcher have their sail tops tied directly to the top of the mast, there is no halyard. Similarly some racing boats have gizmos on their mastheads where you hook the halyard after hoisting sail, effectively eliminating the halyard stretch. Both of these schemes have the added benefit of eliminating half of the preload compression in the mast. And one might reconsider plans to lead lines that set the sail long ways into the cockpit for convenience. All the extra lengths add to the sail rig deflections.
NEXT TIME...
Next time we'll look at the sailcloth itself. After all, cloth is in many ways a collection of tiny ropes woven together .
DEANSBOX
SAILING BEACHBOAT, 16' X 5', 300 POUNDS EMPTY
Someday I may get to put my full catalog on the net. For now I'll put one design in each issue.
Deansbox
Deansbox was designed for Dean McClure down in Key West. Dean had an interesting background in boats including a Dovekie and a Chapelle sharpie (junk rigged, to boot). He was looking for a daysailing boat for his shallow area, small enough for easy handling and yet big enough to not require perfect balance on the part of the helmsman.
I've lost track of Dean but a few years ago he had this boat built by a pro down there. I never got a photo of the completed boat or a test report. So if you live down there and have seen it, I'd like to know how it's doing.
Dean was very specific about needing, but not wanting, a small motor. It was more or less required by the docking situation down there, he said. I can believe it because there are no marinas around here that allow you to sail in and out of a slip. (But I do recall a visit to Panama City, Fla, to a man who was operating a 30 or 40 foot ketch out of the public marina there with no motor.) So Deansbox got a real motor well. One or two horsepower is plenty for a boat like this. Usually the best plan is to start the motor and steer with the rudder, although that won't do in crowded areas like a marina where quick response to the gear shift or throttle are required. And that was a problem with the boat, as I recall. They cut down the aft deck a bit to make it something more of a motor boat. It's a fact that even the smallest motor will quickly dominate a sailboat. The stern gets very crowded and heavy, especially when you sit aft to work the motor.
The hull shape is the best box I could think of, side and bottom curve matching per Bolger's theory to minimize swirling around the chine. My older Pencilbox design is quite similar and that boat sailed very well. With the ends high out of the water, these boats will maneuver like sports cars if the waves aren't too steep. The hull has big 8' long benches on each side for comfy seating and buoyancy/storage boxes fore and aft. It uses the simplest of nail and glue instant construction requiring six sheets of 1/4" ply and two sheets of 1/2" ply.
Dean wanted a mizzen for steadiness (although I'm pretty sure a daysailer might be better without). The main is a 92 square foot balanced lug, one of my favorite sails, and one which Dean wanted to try. The proportions of the main shown are what I feel now to be the overall best. In particular I like to hoist the yard at about 40% aft instead of the usual 33% which is what most old manuals will recommend. The more "balanced" the less the sail will twist and the closer it should point. But you can overdo it. Actually the old 33% rule is fairly good except everything conspires to pull the sail aft in reality. So if you hoist at 40% , the yard might really cross the mast at 33%.
Blueprints for Deansbox are $20.
Prototype News
Some of you may know that in addition to the one buck catalog which now contains 20 "done" boats, I offer another catalog of 20 unbuilt prototypes. The buck catalog has on its last page a list and brief description of the boats currently in the Catalog of Prototypes. That catalog also contains some articles that I wrote for Messing About In Boats and Boatbuilder magazines. The Catalog of Prototypes costs $3. The both together amount to 50 pages for $4, an offer you may have seen in Woodenboat ads. (If you order a catalog from an internet page you might state that in your letter so I can get an idea of how effective this medium is.) Payment must be in US funds. The banks here won't accept anything else. (I've got a little stash of foreign currancy that I can admire but not spend.) I'm way too small for credit cards.
Anyway..... Anytime a design from the Catalog of Prototypes starts getting built I pull it and replace it with another prototype. So that boat goes into limbo until the builder finishes and sends a test report and a photo. Here are some boats in that catagory (although I been leaving most in the prototypes catalog).
Sneakerbox: Here is a boat that was in the prototypes catalog only a short while so most folks won't know about it. I'll try to feature it soon. It's an "instant" version of the 12' garvey box that appears on page 65 of Chapelle's American Small Sailing Craft, a book you should have in your collection. A builder in Pennsylvania has one to the point where he's ready to hammer on the bottom.
Sportdory: Word from Dave Burdecki in California was that he had his completed for a 4th of July row but have no more details. You can check out John Bell's Sportdory progress by clicking here .
Skat - the prototype Skat builder is Roger Palaski. Up until now he was a mystery man know to me only as "Roger", his Skat plans bought for him by someone else. Then I find out he has his own web page with the Skat construction photos on it! Skat is a small somewhat traditional 12' cat boat with a gaff rig. Even has a centerboard, the only boat I've ever designed with one! See Roger's progress by clicking here.
Nothing new on the Texas IMB. Click here to visit Tim Webber's page and see some photos of the IMB as of a few weeks ago. Then poke around Tim's web page a bit.
Shantyboats (archived copy)
Herb builds AF3 (archived copy)
