Lavro Dory Stern Chine Replacement

The Lavro Dory came from the factory with some slightly odd looking chine/rail extensions in the stern. They are attached with bolts, and are made from wood. Bartender boats have something similar, but their’s are more elegant, and run from the bow, angling down almost to the stern.

What Are They For?

Many people think they provide extra planing surface. Or that they are just spray rails. And they do a little of both. But from what I have read, the reason for them is because double ended boats do weird things when you make them into fast planing boats.

When George Calkins, of Bartender boats fame, was first figuring out his hulls, he discovered the boats would make some rather violent and sudden turns, almost tossing passengers out of the boat. They also would turn in other unpredictable ways, particularly when there were weight shifts side to side in the boat.

That led to the development of the chine/spray rails you see on Bartender boats, that stick out at the stern. In the picture below of a small Bartender, the rails start up high on the bow, and swoop down to the hull bottom. The primary purpose of the chine/rails is to provide some clean edges for more predictable turning at speed. They stabilize the boat. The folks at Lavro Dory discovered the same problems in their testing, and added their version to make the boat work.

What Is My Problem With Them?

There has always been something about the Lavro rails that look wrong to my untrained eye. They are rather short and are at a steep angle, and don’t have very sharp edges on the bottom side, which is important for cutting clean turns. And as you can imagine, the angle increases when you are trying to get up on plane, as the bow raises and the stern squats. At that angle the current chine/rails seem like big inefficient water plows to me.

I suspect the company was using that high angle to try and solve another issue of squatting, by forcing the bow down. The Lavro Dory has an open motor tunnel, which decreases the planing surface in the stern, compared to a Bartender, which likely contributes to the squatting.

I could not stop thinking that there is a better solution. My attitude towards them is not particularly fair. All of the owners I have talked to say the boat works. But since I am going to refurbish the entire boat, I felt I could come up with a solution that would allow the boat to get up on plane easier and earlier, and use less horsepower. Weight placement in the boat will be part of the solution.

Professional Help

While trying to understand this odd boat I talked to experienced boat designers, particularly folks with experience with Bartender boats. Local small boat designer Carl Chamberlin came by and looked at the boat, which I was very grateful for. He did some plans work for George Calkins on some of his Bartender boats. He actually thought I should just use them as is, and gave me lots to think about.

And Tad Roberts, the esteemed boat designer from British Columbia, also responded to my concerns about these chine/rails with a quick paragraph of suggestions that were pure gold. I also talked to the current owner of Bartender boat plans (also George). He looked at pictures of my boat, and we talked about what I was considering, based in part on Tad’s suggestions, he agreed it sounded like a good idea. I would have talked to the original manufacturer of the Lavro boat but could not find their contact information. The company has been out of business for years.

The Problems As I See Them

Talking to lots of owners of these boats, there are a few repeated issues with the boat.

1. The boat is a bit hard to get up on plane, requiring a lot of power and speed, and you have to have weight up in the bow. To me that hints that the boat is too heavy for its planing surface area, particularly in the stern. The normal response is to try and add more horsepower, but that is a downward spiral of additional weight in the wrong spot.
2. The boat tends to ride with a high bow, squatting in the rear. Again, maybe the boat does not have enough planing surface in the stern, and a weight imbalance issue?
3. Nearly all owners complain the boat is rather tender (tippy). This is due to a narrow waterline and the double ended hull. Coming from sailing, a bit of tenderness does not bother me. And there is so much hull flair all around that it picks up a huge amount of stability when it heels a bit. But some improvement in its initial stability would probably be welcome, for comfort when at rest.
4. The boat tends to steer itself around when weight shifts side to side. It is a quirk of a double-ender, and may also suffer from a bit of bow steering.

The Big Change

So here is what I am going to do. I have already removed the existing wood rails (read here). In their place I will be extending the existing chine flats on the bottom of the hull. These extensions will start all the way up at the bow, taper out to about 3″ wide mid ship, and end up being 14″ wide in the stern. This will effectively turn the bottom of the boat into something more like a standard planning hull.

Besides increasing the planing surface area, particularly in the stern, I think it will help stabilize the boat, giving it wide sharp edges for controlled turns. And it should make the boat less tender overall. Hopefully it will get the boat up on plane earlier, and with less power. And I believe it will keep the boat on plane with a lower angle of attack, further decreasing how much horsepower is required.

I am also going to be closing in a bit of the motor well tunnel, adding a little more planing surface. And if it seems necessary after testing I may add a plug behind the motor, for more planing surface, like many Bartenders boats use.

Combined with all of this I will be adjusting the weight placement of some heavy items. The helm station and main passenger seat are moving forward a couple of feet. The fuel tank will be right in the center below the passenger seat, with room under the helm seat for extra fuel for longer trips. That way as the fuel volume/weight changes the fore/aft trim does not change. The battery will also be central. Other heavy items can be moved around the boat to balance things out as needed, like fresh water jugs, anchors, coolers, etc.

What Are My Concerns?

I don’t have any concerns about building the extensions. But I am so far from an expert in hull dynamics that even attempting this is hubris. The design and function are a huge experiment, with a decent chance of it not working like I think, or it causing other problems. But I am fine with having to rework them if necessary. It is a fun project to learn from.

One unaddressed concern I have is regarding the steering issues on a double-ended planing hull. There is a train of thought that it is the water wrapping around the rounded stern that causes most of the steerage issues. And it has been suggested that interrupting the flow of that water around the stern is necessary. If you think about leaning a kayak on its edge to turn it, it is not hard to imagine why this boat turns when it leans over.

Some early power dories used to attach nearly vertical batons to the hull at the stern to break up the water flow to help these issues. My new chine extensions will not interrupt that water flow around the stern when at slower speeds, and the current chine/rails do. Then again, the chine/rails on Bartenders boats don’t really break up that flow either at slower speeds. I guess we will see.

Other concerns are that these boats like to bow steer a bit, in following seas. That may be due to a combination of what is mentioned above, and because the boat has a fairly sharp v-bow, and the lowest point of the boat is up in the bow. So I wonder if I am successful at lowering the planing angle if I will also be increase bow steering? Maybe the hull is designed to ride bow high, like a semi-displacement lobster boat? If that happens, I may be able add some vertical rails under the hull in the stern, for better directional stability.

These hull extensions may also not be great if the boat is broached in big waves, or even a big following wave. The chines might give a wave good purchase. If I intended to cross river bars regularly, heading out to the open ocean to fish, I don’t think I would add these chine extensions, or least I would not run them all the way to the stern. But that is not my kind of boating. In fact, if I never have to cross a river bar again, I will be happy. They freak me out.

How Will They Be Built?

These chine extensions will be made from a fairly high density foam core, with fiberglass skins. A local friend sold me a couple 1″ sheets of a structural foam made from recycled PET plastics, that he uses in boat construction. It is nice and dense, although not as lightweight as some other much more expensive foam cores.

I took some ideas for their shape and construction from an article that Eric Sponberg shared about adding some lifting spray rails to a big powerboat. You can read that PDF here. Of course the scantlings for those rails are massive compared to what I need, but it was a helpful article nonetheless.

I will shape the chines in foam, and then add about 30 ounces of fiberglass around it, blending it into the hull sides and bottom, using West System epoxy.

In order to get a good bond to the boat I first have to remove a thick layer of gel coat on the hull. Not a fun project, particularly on the underside. In fact, I expect it to be the worst part of this entire project.

After the hull work is done the boat will be painted. I am not messing around with trying to cover it in gel coat again.

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