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1 Tonner Dobroth Keel Draft Reduction


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Ahoy sailors,

I'm in the process of modifying a 1985 Lager 40 on a Laser sailors budget, lol. She was refitted in 1998 with a Brendan Dobroth designed stainless steel keel with a lead triangular ball and matching rudder. She also received a new rig and chain plates to carry more sail. She originally drew 7 feet, and now draws about 8 feet 6 inches. Apparently this actually netted a significant reduction in overall weight getting rid of the solid lead fin keel and moving to the lead torpedo. They compensated by added lead to the sole of the boat to maintain its weight and waterline length for going to windward.

Fast forward now a few years and she has been sitting on the hard on the north shore of Lake Ontario. Far too deep of a draft to use practically in these waters. We have begun the process of reducing the draft back down to 7 feet by cutting the stainless blade, removing a 16 inch section and welding the lead ball back on with a new total draft of around 7 feet.

My question to you sailing masters has to do with adding additional lead weight to the existing torpedo to make up righting action for the shortened keel blade. I also noticed the blade, once cut so I could see the cross section, seemed to not have much of a proper foil shape to it, and appeared to almost taper off equally in the front as the back does. This and the strange shape of the torpedo with a triangular cross section apparently from my digging acts also like a foil and assists with going to windward rather then a perfectly round torpedo which is just ballast going along for the ride.... 

I weighed the ball once it was chopped off, which came in at just under 4000lbs, my crude math showed me a 8 and half foot draft dropped down to 7 feet, would need 800lbs added to the 4000lbs already there to "make up for" the leverage lost from shortening the keel.

I was considering rounding out the torpedo with additional lead to make the ballast heaver and more like a modern looking racing yacht's round ballast bulb. But don't want to ruin its function as a foil, if thats what the triangularization is doing. That or maybe make a lead shoe to bolt on the bottom. Or Mars keel, which is local to me and manufactures keels for Catalina and C&C said to potentially use thin sheet lead, and epoxy it on one layer at a time, building it up and forming it to the original shape just fatter, until I've added the needed weight.

Can anyone back me up if my math seems right with the amount of lead that should be added or net the same leverage/righting motion? With the lead out of the floor she is already under weight as I said so its no sweat adding lead as needed. Round the bulb? extend the ends forward and aft? add lead wings? Shoe on the bottom?

All of the work, lifting the boat off the keel, handling the bulb, removing the engine, its all being done by me and whoever is around the yard to help out at the time with 4x4s and chain hoists, so it needs to be a backyard friendly solution! :)

Thanks friends!

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Well, I hope you've got a lot of money left for a proper load analysis. Those 'I' beam sections inside the keel profile supply most of the strength. How are you going to re-weld (?) them back the beams in the bulb? Those beams were engineered for a specific load analysis - even with a shorter length, adding more weight on the bottom combined with loss of strength re-connecting the beams may still leave the structure under strength. Do you have the Dobroth design plans/calculations? Or just hire him again.

You're committed now, hope you can finish it without endangering lives.

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I concur with longy. Who recommended cutting the shark tooth? It’s obviously to increase the area of the cross section weld. Looks very extreme. Also very hard to accurately reassemble.  Welding is pretty good at replacing the original strength with a bit more added material in critical places. How are you going to connect the internal webbing?

You should have taken the length out of the top or bottom not the middle. 
 

One of my best friends is a marine surveyor and an engineer. There is no fucking way he would sign off on an insurance survey without a pro engineer  providing a shite load of docs and all of the liability. You may have trouble getting insurance. 

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What's your plan for the internal I-beams?
I don't see anyway to weld them once you reassemble the keel & I don't know if I'd trust such a weld anyway.

From the photos it looks like the I-Beams are bolted through the side of the keel skin, it would seem to make more sense to replace them completely.
And +1 to everything Longy said above about load analysis, this isn't something you want to guess.

 

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MIddayGun - good call on beam to skin connection - looks like plug welds at forward and aft edges of the vertical beams. It's a strong structure as designed and built. Now?

Cutting a $$$ custom keel in half before you've figured out how to safely put it back together, if that's even possible, was ill advised. It's done now but can it be salvaged? You still have the righting moment to figure out, let alone how you will make the structure sound. 4,500 or so pounds is a lot of weight to manage on the end of the fin. I am torn between trying to conjecture how it could be done safely and just saying "WTF were you thinking???"

ALSO: has everyone has forgotten something?

T or STFU

 

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Righting moment is a torque. So the product of weight and distance would be kept equal. 800# sounds like too much. However we don't have the accurate measurements of center of buoyancy, center of masses and masses.

Ballast in the floor sounds just silly. Especially so with all the effort of fitting a bulb. Adding weight to slightly increase waterline length, really? Only if there is somehow an extreme surplus of sail power. But what do I know?

Not being able to completely weld that internal I-beam is probably not a concern. The stress will be carried in the skin, like it or not, which is presumably connected to the I-beam. How thick is the skin? Tin can or plate?

The stress down low is far less than the stress up near the hull. So with a good weld...

A pointy leading edge on a foil seems ill-advised. Will stall at low angles of attack. But maybe someone figured that was okay.

A lifting shape on a bulb is likely pointless because it's span is so small.

 

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Another thought - the inside of your keel doesn't seem to have any corrosion, so they got it watertight when they built it.

Stainless will corrode once it has used up all the oxygen about and there's moisture present, so you're gonna have to get it reliably watertight !

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8 hours ago, CaptainAhab said:

One of my best friends is a marine surveyor and an engineer. There is no fucking way he would sign off on an insurance survey without a pro engineer  providing a shite load of docs and all of the liability. You may have trouble getting insurance. 

When was the last time an insurance company asked you questions about the keel on your boat?

If it was me, I'd talk to Mars Metal - they know as much about keels as anyone on the planet - and they are close to the OP.

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10 hours ago, SloopJonB said:

When was the last time an insurance company asked you questions about the keel on your boat?

After the marine survey says that the keel was shortened. Or after the keel falls off. 

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12 hours ago, El Boracho said:

Not being able to completely weld that internal I-beam is probably not a concern. The stress will be carried in the skin, like it or not, which is presumably connected to the I-beam. How thick is the skin? Tin can or plate?
 

 

 

Let’s do a hypothetical. You build the same skeg out of typical high quality composite. Biax/Uni carbon epoxy, corecell foam. You use two pieces of Corecell. You get distracted. You layup over the core that now has a 1/4” clean gap between the two pieces. The core is discontinuous. The skins are taking 100% of the load across the entire structure in the middle of the span. 
 

What’s going to happen? 
 

What is the existing gap between the internal webs where he cut them in half?  Is this a typical situation with an i-beam where you cut out the web mid span where the flanges can handle the shear?

The internal flanges are attached to the skin with 1/4-20 flat head machine screws. You can see them in the last pic. 

I’d ask somebody way smarter and more educated than myself. 

 

 

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22 hours ago, SloopJonB said:

When was the last time an insurance company asked you questions about the keel on your boat?

I sail mostly multi, so never... but if it is anything like car insurance, they ask few questions up front, but when there is a claim, they ask however many questions are required to deny the claim.

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I hear that sort of talk about insurance companies all the time.

I've been buying insurance for 50 years - it's probably one of my biggest expenditures now what with multiple everythings - and I've never had a claim denied or anything remotely near it.

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On 11/6/2020 at 6:36 PM, System_Error_117 said:

But don't want to ruin its function as a foil, if thats what the triangularization is doing.

It's most likely triangular to get the center of gravity ever so slightly lower or reduce draft. Hydrodynamic effects are probably close to zero.

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Seems like a better way to shorten the keel would have been to cut the strut to length, then modify the bulb, or cast a new one so that it surrounds the strut and gets through bolted from the sides capturing the I beam flanges. 

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14 hours ago, mathystuff said:

It's most likely triangular to get the center of gravity ever so slightly lower or reduce draft. Hydrodynamic effects are probably close to zero.

Well, there's that.

But IIRC, it was also an IMS thing.  Plus a kind of end plate effect as it tends to shovel the flow (for lack of a better term) back up toward the foil.

But in itself, like El Boracho said the span is too short for the bulb to act as a lifting body.

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On 11/8/2020 at 10:19 AM, SloopJonB said:

I hear that sort of talk about insurance companies all the time.... I've never had a claim denied or anything remotely near it.

My significant other works in risk management at a corporate scale. It does happen.

Usually insurance companies will simply refuse to renew if they don't like your usage/risk profile.

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So the keel has been welded back together, we lifted and re bedded it last night. Now the shaping can begin! I employed a professional experienced stainless steel welder who was able to back fill the stainless all the way up to the plate surface from the internal I beams based on my angular grinding that allowed the weld to begin at the center and work its way out. Lets just say I cleared our local welding shop out for stainless welding rod, lol. Also after measuring the cross section of the blade, determined it was nearly 3/4 of an inch too narrow based on its width for a proper foil shape, so instead of piling on the filler like was done before (and all fell out shortly after), we used the left cut out discarded section of keel blade to fashion some reinforcing plates to tack on the outside and aligned with the internal beams to provide even more strength.

Note the photo was early on in the welding as we were tacking things into place.... the plates were totally wrapped in 2 layers of weld, as well the larger triangular voids behind them totally back filled with rod. After drilling but before refilling the internal void with spray foam I pressurized the void which held 10 PSI for over an hour, so I think its nice a watertight.

The main job now is figuring out the weight to be added, and the method of how to go about it. I've begun collecting lead shot incase I have to just epoxy cast some parts to bolt on the side, or if I go with a shoe on the bottom, I could cast that in a plywood mold and bolt it up.....

Just looking for some advice on doing the math for the correct weight to add, and tips on where/how to add it based on the triangular bulb. Right now I'm leaning towards bolting on some custom formed lead halves that would effectively turn the triangular profile into a round bullet. But I'm open to suggestions!

 

 

 

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On 11/7/2020 at 11:10 AM, El Boracho said:

Righting moment is a torque. So the product of weight and distance would be kept equal. 800# sounds like too much. However we don't have the accurate measurements of center of buoyancy, center of masses and masses.

Ballast in the floor sounds just silly. Especially so with all the effort of fitting a bulb. Adding weight to slightly increase waterline length, really? Only if there is somehow an extreme surplus of sail power. But what do I know?

Not being able to completely weld that internal I-beam is probably not a concern. The stress will be carried in the skin, like it or not, which is presumably connected to the I-beam. How thick is the skin? Tin can or plate?

The stress down low is far less than the stress up near the hull. So with a good weld...

A pointy leading edge on a foil seems ill-advised. Will stall at low angles of attack. But maybe someone figured that was okay.

A lifting shape on a bulb is likely pointless because it's span is so small.

 

I agree, the weight added in the floor seemed odd. There was a diagram that showed about 20 bricks, but when I lifted the floor up I only found 5, so someone already might have thrown those over board for the same reasons your saying!

A friend did the 800 pound calculation based on the current 4000lb ballast, original 8 and a half foot draft and the 16 inches or so removed. I'd assume we would just need to look at the reduction of lever length, and adjust weight for the same work done on the "load" or in this case "boat". I know there is a bit more to it with center of buoyancy etc., but that math has already been done by Brandon. We are just reducing the lever length....

The foil shape is quite odd indeed. Nearly symmetric angles front and back, very pointy in the front. It seemed like it had over an inch of filler on each side at one point, but it had long fallen out, and was sailed with just the raw stainless blade for the last few years until it was left on the hard. This is why I opted for reinforcing plates on the outside, figured I had the room for them with the filler and shaping I'd be doing later. 

As for the Torpedo, I agree if the shape doesn't help, then I'll turn it into a perfectly round torpedo, adding cast lead or lead mixed with epoxy to fatten it up. Then couple layers of encasing glass to hold it all together if she ever had a sharp grounding that could fracture the connection points. I've been told as well the right angle that a modern foil meets the top of a perfectly rounded bulb acts as a end plate, forcing water to stay on the foil longer when heeled over going to windward instead of washing off the bottom like a it might on a classic fin keel...

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You need to blunt the front of the keel.

Sharp leading edges have a very narrow "groove"

Examples of the sharp-edged foils are the ogival that have been used by may hydrofoil designers. They have the advantage of being simple to construct and have low drag within their design range of angles of attack. The problem with sharp leading edges is they only have a small range of angles of attack at which the flow is attached. Outside that range, they experience leading edge separation. This can lead to sudden ventilation – a charactersitic that has bedeviled many craft that use these sections. You can find section data for ogival sections published in the literature and in books like Hoern’er’s “Fluid Dynamic Drag”.

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So I found this pretty slick righting moment calculator on the Selden website. Its all in mm and kg, but converting is a simple task. Using the original factory numbers for the boat from the sailboat data website, combined with what I know about the modified keel, I was able to calculate what the righting moment before and what it is now, as well as what weight would be needed to get the boat back to that number.

https://support.seldenmast.com/en/services/calculators/rm_calculator.html 
I used the following; Beam: 3860 mm, Draft: 2641 mm (before cutting), Draft: 2133 mm (after cutting), Displacement: 4626 kg, Ballast: 2086 kg 

Therefor:
Righting moment before cutting: 33.7 kNm
Righting moment after cutting: 31.4 kNm

That's still 93.2% of the original righting moment. Adjusting the ballast up until I hit 33.7kNm with the shortened keel number entered, it seems I only need just over 400lbs of additional weight. Also to put it into perspective, they allow you to add crew to the rail to play with your RM number. By adding 2 crew members to the rail made up perfectly for the lost RM from the draft reduction.

At this point, for only a 6.8 percent decrease in righting moment, on a boat that apparently sailed like a lighthouse even in a gale, might be worth sailing it the first season as is and making a decision after that!

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On 11/15/2020 at 11:36 AM, System_Error_117 said:

The foil shape is quite odd indeed. Nearly symmetric angles front and back, very pointy in the front. It seemed like it had over an inch of filler on each side at one point, but it had long fallen out, and was sailed with just the raw stainless blade for the last few years until it was left on the hard. This is why I opted for reinforcing plates on the outside, figured I had the room for them with the filler and shaping I'd be doing later. 

As for the Torpedo, I agree if the shape doesn't help, then I'll turn it into a perfectly round torpedo, adding cast lead or lead mixed with epoxy to fatten it up. Then couple layers of encasing glass to hold it all together if she ever had a sharp grounding that could fracture the connection points. I've been told as well the right angle that a modern foil meets the top of a perfectly rounded bulb acts as a end plate, forcing water to stay on the foil longer when heeled over going to windward instead of washing off the bottom like a it might on a classic fin keel...

I suspect the odd "diamond" section was done for ease of fabrication - then fairing compound slathered on and faired to proper shape.  Probably the fairing compound didn't adhere well and started coming off - so previous owner just stripped it all off.

Definitely want to redo with fairing compound and proper foil shape.  I assume the boat will be sailed on a lake, so fouling not a big issue so I would go with a laminar shape like a NACA 6 series - 63xxx or 64xxx.  These have a rounded but sharper leading edge than a NACA 4 digit.

As SJB said, these have an earlier stall angle than a 4 digit, but have the drag bucket for offwind.

Careful reshaping the bulb. Simply creating a round section may ruin it.   Generally speaking, a bulb should be: long, thin, flattish in section with a fine nose and maximum thickness aft of 50%

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On 11/16/2020 at 3:41 PM, 12 metre said:

Careful reshaping the bulb. Simply creating a round section may ruin it.   Generally speaking, a bulb should be: long, thin, flattish in section with a fine nose and maximum thickness aft of 50%

Interesting! Based on my little research it seemed like the triangular bulbs were tempting to generate lift or act as a bit of a bulb wing, where as a true rounded bulb is just the most efficient way of getting the most lead down low as possible with the least amount of drag, and therefore less drag then a triangular cross sectioned bulb per given volume.

Like everything in life, its seems there is good and bad to just about any way of doing anything!

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1 hour ago, System_Error_117 said:

Interesting! Based on my little research it seemed like the triangular bulbs were tempting to generate lift or act as a bit of a bulb wing, where as a true rounded bulb is just the most efficient way of getting the most lead down low as possible with the least amount of drag, and therefore less drag then a triangular cross sectioned bulb per given volume.

Like everything in life, its seems there is good and bad to just about any way of doing anything!

I consider a bulb with a round cross section to be primitive and is about the most inefficient shape.  Most modern bulbs are squished into more of an elliptical cross section which lowers the VCG a touch and at the same time also allows for a slightly longer keel span - given a fixed amount of draft.

Here's a paper on the testing of 4 different keel types: http://publications.lib.chalmers.se/records/fulltext/148387.pdf

CFD and wind tunnel testing was used

The keels are 1. Fin

                         2. Integrated (similar to the Dobroth keel)

                         3. L-Keel

                         4. T-keel

They all yielded fairly similar results IMO, but overall, keel 4 gave the best results, especially in stronger wind.

Interesting that their original keel 4 tested poorly, but it was an "old timey" bulb (fig 8) thick with a forward point of max thickness and a quite bluff nose.  The revised keel 4 is more along the lines of what I described in an above post. - longer, thinner sharper nose, with max thickness well aft .  Actually max thickness coincides with the trailing edge of the keel.  Which I think is something T. Speers recognizes as well as raising the y unit ordinates to the 1.5 power when translating foil co-ordinates to a bulb (to generate a sharper nose).

Also, the paper has an illustration that the bulb portion of keel 2 somewhat surprisingly does actually add to lift, although the tradeoff seems to be that unsurprisingly it also generates more tip vortices.

Anyways, there is a lot more to good bulb design than simply slinging a lead torpedo onto the bottom of a keel.

 

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