On the hard, or standing on tiptoes

[Roger]

The recent thread about climbing a mast while on the hard has led me to ask experienced sailors and scientists on the list about the stability of sailboats on the hard, and the physics therof. So... more than half the weight of the boat is in the keel (the last 30 inches maybe on my c27). The whole weight of the vessel is on a very few square inches in contact with the wood blocks. Jackstands are on either side, and are snug, but not truly weight bearing, that is, not relieveing much, if any of the weight on the keel. Or do they?So... how stable is the boat without the jacks? I assume the function of the jacks is to keep the boat from 'heeling', not holding the boat off the ground. If she does 'heel' where is the 'righting moment' with no water? In other words, without jacks, if it heeled five degrees would it fall onto its beam?I have no particular reason for asking, except that I have the boat in the driveway (with three jacks to a side- I insisted despite the fact that the boat hauler said two to a side was plenty) and I keep trying to figure out what is going on in terms of balance dynamics. She sure looks like "Fantasia"- the hippos on pointe shoes in the ballet sequence!We in New England have to pass the winter somehow.RogerC27#5012 & MabelCape Cod

 

[Bayard Gross]

Function of boat stands

Down here in southeastern Connecticut, we call boat stands poppets, although I know of no particular reason for doing so. Just one of those regional particularities.Nevertheless, to answer your question about how much a boat can “heel” on its keel while supported by poppets (jacks as you call them) I think it may be best to envision your vessel say beached somewhere. It is resting on its freeboard but with the keel still in contact with the ground while the mast is well above the ground although leaning almost parallel to the beach. Your atypical boat washed ashore say following a great storm.In this case, the boat’s heel would be greater than forty-five degrees, yet she still rests upon her freeboard, as the weight of the keel holds her down and from toppling over altogether.Hence, the same action is applicable to boat stands as well. You could theoretically heel your vessel over as much as you can onto the boat stands as long as the keel remains resting on the ground. However, unlike the broad area of a beach, the boat stands would concentrate a great deal of the vessel’s weight onto the rather small area of the boat stand tops. As this weight concentrates onto the relatively soft areas of the freeboard, there could be the danger of cracking the hull.As your boat now stands, the keel, which is the strongest part of the boat, supports almost all its weight. The purpose of the stands is merely to balance all this weight properly over the keel. Little, if any, pressure should apply to the boat stands. Further, to prevent the stands from being pushed apart by the slanting sides of the hull, chain should be run between opposing stands. The chain should be snug but not rub against the bottom of the vessel. Moreover, the tension on the chains should be examined every couple of weeks to assure the stands are remaining in place and if the chain’s tension has increased, then the stands should be repositioned. An easy task for you as you wisely use six stands.

 

[Brian Stevenson]

Approximation of force on stands

Roger--I would approximate the the area of the side of the boat that is subjected to the wind. We have used anywhere from 8 to 13 p.s.f. as a guide to wind pressure. There are also factors to take into account for shapes. This, times about 1.7 ( for negative pressure on the lee side) would give you the force that wants to overturn the boat. Assuming that the distance to the center of the force from the bottom of the keel is approximately the distance from the keel to the stand, this is the force that would have to be resisted by the stands. This is a crude estimate, but on the practical side, the area of the pads is more critical to resist the punching force that wants to go through the fiberglass. Short answer, 3 a side is better. The test-come spring- no damage to fibreglass--you did it right, guess you know that!!!

 

[W Surgenor]

just a thought

In my opinion you are 100% right on having three jacks per side. consider two jacks per side, the ovious points to place these are, one thrid in from bow and one thrid in from stern, but where is the widest beam dimention? Outside both of these jacks! So you would be saver with three per side. If you've got them use them

 

[Dennis Thomas]

Tippy-Toe

Without water the boat is standing on tippy-toe just like the hippos! If you don’t have something to lean against it’s going to fall over. If you didn’t have jack stands, just standing there balanced, you could tip it over just by moving your weight from the centerline to the outside. In areas with high tidal ranges you find lots of bilge keels and twin keels so that the boats don’t get upset when left high and dry. In your drive I like the idea of 3 jacks to a side so that you could reposition one and still have two supports.S/V Anodyne36’ Catalina