Ruminations on Bolger's Storm Petrel

Don Elliott on Storm Petrel continued

"Wed Feb 7, 2001 6:49am
STORM PETREL STUDY NO 3

In the last article on Storm Petrel we saw that sometimes designers don't agree on where boat floatation should be located. In this article we'll compare those different locations and see what affect they have on the boat.

We decided in the last article that the best way to evaluate the different locations is to see how these locations affect the boat. We then made a list of what we want the flotation to do for us. Here's the list from the last article on what we would like to have.

1. What we want is the flotation material not to encroach on our needed space.
2. If capsized we want the boat to right itself and not be stuck upside down.
3. If holed or flooded we want to be able to plug the hole and bail the boat.
4. We want the boat to be able to sail or motor on in any condition.
5. We want to correct the problem in the least amount of time or better yet not to have it occur at all.

Is it possible to have all of these things? That will be the subject of our next article on Storm Petrel.

For item number 1: placing the flotation at the rear on the bottom is no problem but in the cabin area it would as there is very little space even to turn over in your sleep (about 24 inches (60 cm) of height in the cabin). If flotation is placed on the floor it would intrude on the cabin space, we don't want that.

See the following sketch for item number 2. On that sketch you can see that if the boat capsized completely and didn't right itself on its own it would be much harder to turn the boat back upright if the flotation were located on the bottom. If it were at the deck level (view A) wave action alone would most likely right the boat, with no effort on your part. If it didn't right on it own, with the flotation at deck level it would be a snap to roll it upright.



Item 3, this is a condition where for one reason or another the boat become flooded due to a hull puncture or simply water somehow entering the cabin. This is a more complex problem and will require a separate article in itself. We'll cover this later.

Item 4 the ability to sail in any condition is no problem; both locations will allow us to do that; even if Storm Petrel was unlucky enough to be flooded completely.

Item 5; correct the problem quickly or preventing it in the first place is closely tied to item number 3, so it will be discussed at the same time, that is, in the next article.

The last question; can we have everything in the list without compromising anything else? In the final article on flotation you will see that with careful design we can. Don"

 

I see now that my files had Don's posts slightly out of order in but they are all still understandable and the dates at the beginning of each post can help sort them out. I hope this is useful stuff, I know it has me thinking. Cheers, Matthew

 

IIRC, Bolger designed this boat for a client. I wish I could remember the client's SOR, but I think it was to have a boat that could be sailed in cold conditions by a young sailor who had a tendency to take risks.

With what I have learned about small boat stability, by doing my own math experiments, makes it easier for me to understand the design reasoning.

What you don't want on a small boat, which is expected to right itself, is too much initial stability. The reason for this is that initial stability upright usually translates to initial stability upside down. The higher the initial stability, the lower the likely hood that the boat can self right.

The was an Open 60 race boat that capsized with its bulb keel intact. It lost its rig but continued to float upside down for months, in the great southern Ocean, where it most likely encountered its share of breaking waves. One would think that any of those that hit her would surely knock her back upright. But the high AR keel and flared sides offered little for the breakers to push against. And the bulb, now over a dozen feet in the air, provided substantial roll inertia.

So, in survival conditions, one does not want a lot of initial stability which can defeat altimate stability. But one also doesn't want a boat that is so heavy it will hardly move with the amount of SA the altimate stability requires.

The solution is to make the boat heavier when it needs to be, much lighter when it's sailing.

This is accomplished by the free-flooding bow and the flood able cockpit. The free-floading bow offers no support buoyancy when the boat sailing in smooth water. But it offers almost the full amount of lifting buoyancy when the boat encounters a wave. All this buoyancy is lost once the boat capsizes. And most of the buoyancy of the stern is lost too, leaving just the sealed midsection of the boat to provide all the buoyancy. With all this lost buoyancy, the boat will sink deeper in the water and have far less initial stability.

Now, here's where the long keel comes in. It provides not only considerable weight higher up to make the boat less stable upside down, but it also provides a broad target for breaking waves to hit.

With the floatation installed high enough in the hull, it will tend to make the boat even less stable upside down.

As far as self-rescue from swamping goes, the center section can be sealed off and the water can be first pumped or bailed our of it, then the cockpit could be bailed out.

The floatation installed higher up will tend to keep the boat from capsizing while this is being done.

The likely reason the Lateen rig was chosen is because it comes with the shortest mast. If the sailor notices a storm coming, he can quickly lower the sail, and maybe even lash the boom and the yard down, before ducking into the mid-section and sealing it off for cover. With a fractional sloop rig, the tall mast would be left standing. And it's high center of gravity would require a heavier keel to counter balance.

So. I think 'modernizing' this well thought out design could well defeat its original purpose.

 

Personally, I like original Bolger design just fine in most aspects, but I think Don Elliott made some good points, especially on the impracticality of the loose box hatch top. His suggestion of a flush hatch top with a snorkel could work though it would make the cabin feel more like a coffin. I would probably go with the original box hatch top but hinged not loose or something like the fixed spray hood and aft hatch that Don showed, which is reminiscent of what Bolger himself did on other designs like "Singlehander Catamaran" in Boats with an Open Mind. I would also want additional ventilation that could be closed up watertight if need be, maybe screw-in inspection ports in the cockpit benches that would be removed when not underway.

On the flotation questions, one thing I would do is remove the foam from the cabin roof and replace it with an equivalent volume in the forward well, just to gain some space in the cabin. On the rest, major damage to and flooding of the central cabin area is unlikely and it would just have to be repaired and pumped out if it were to happen, though the truck inner tube and bicycle pump trick could be a handy emergency measure.

For the rest, the only other point I would make is ensuring the rudder and tiller are easy and quick to unship (or alternatively giving the rudder a folding blade to come right up out of the water) as I think the boat would be a lot handier under power if steered directly with the outboard motor tiller without the rudder's resistance.

PS--One additional change I would consider would be using tack-and-tape construction (fillets and tape, so stitch-and-glue without the wires) as much as possible. That would eliminate the external chine logs and much of the framing, though likely require moving up to at least 1/2" plywood for the transom, midships bulkhead, and forward bulkhead.

 

Rounding out this Bolger Storm Petrel thread, here are some images of Bruce Hallman's great Storm Petrel 3D renderings for future reference.

 

And a few more....