Need advice: Very thick fairing over cast iron keel

[Adelie]

Originally Posted by boatbrain

You did not ask this question and please don't be offended but I think you should carefully consider if it makes sense to put a lot of money and time in the boat when you can get many small boats for free down in San Diego and LA areas - I am sure many would need work before being ready to race but starting with the keel seems like you might do a lot of work, spend a lot of money and then not be able to sell it if you wanted to.

I'm not starting with the keel, I'm researching for two haul outs into the future.

The goal is to put some money into this. I'm figuring about 8 liters of filler. As a guess I'm going to spend $100 which really means about $250.

The time is a learning experience I am happy spend.


Sent from my iPhone using Cruisers Sailing Forum

[Cadence]

Originally Posted by Adelie

I'm not starting with the keel, I'm researching for two haul outs into the future.

The goal is to put some money into this. I'm figuring about 8 liters of filler. As a guess I'm going to spend $100 which really means about $250.

The time is a learning experience I am happy spend.


Sent from my iPhone using Cruisers Sailing Forum

If I sound crass or like an ass, sorry. I think you should move to the lee rail so you are not pissing up wind. Best of luck.

[IolantheSF]

Think it through. Do you really want the additional buoyancy of light foam, or wood down low? Remember what the keel weight is for?

How about epoxy with structural filler with layers of fiberglass to reinforce, then a couple of layers wrapping the outside when you're done. If you can get away with the weight, just bolt some cheap iron or steel plates (lead if you're flush) to the side of the keel to build it up, then epoxy/filler/glass the whole thing to make it watertight. In any case, nothing less dense than water.

[Adelie]

Originally Posted by IolantheSF

Think it through. Do you really want the additional buoyancy of light foam, or wood down low? Remember what the keel weight is for?

How about epoxy with structural filler with layers of fiberglass to reinforce, then a couple of layers wrapping the outside when you're done. If you can get away with the weight, just bolt some cheap iron or steel plates (lead if you're flush) to the side of the keel to build it up, then epoxy/filler/glass the whole thing to make it watertight. In any case, nothing less dense than water.

No I don't want that buoyancy there but building in foam and covering with glass would not be that detrimental.

At worst I expect to add about 3/4 to 1 cf of volume to the fin, probably less. Let's say it is 1cf and I faired with vacuum, that would be 64lb of buoyancy. In the real world foam would be closer to 55lb of buoyancy. The righting arm for the foam is going to be about 24". The fin is less than 300# but let's use that number anyway and it has the same arm as the foam. That means the bulb is 600lb with an arm of 36". So righting moment is 600# * 3' + 300# * 2' = 2400ft-lb vs 55# * 2' = 110 ft-lb. That's about a 4.8% loss. Accounting for the buoyancy of the steel it would a 5.5% loss. The boat is very stiff to begin with so it can afford to lose that amount.

On the whole though it's not really a 5.5% loss because initially stability comes from form rather than ballast, the extra buoyancy does not affect initial stability at all, that is a whole separate set of calculations. Final stability will actually be aided, at 45-60deg heel I figure the top of the keel to start coming out of the water so the added weight of foam and sheathing will actually contribute to righting moment to a small degree.

The place the buoyancy will be detrimental is the peak of the righting moment curve. Given that form stability is the bigger contributor I expect to lose about 2% off the peak.

"Well" you then say, that's 2% loss of capsize resistance. Wrong. Maximum righting moment is tenuously related to capsize resistance. Vastly more important is roll moment of inertia. Since I am going to upsize the wires, and add a radio antenna, cabling and nav lights this should significantly increase my roll moment of intertia and therefore my capsize resistance.

In reality I don't know if I want to use foam, but the issues of placing thick masses of glass and polyester make me hesitant. The only conclusion I have come to is that Bondo is out due to its problems when continually submerged.

I have a lot think about for the next couple years.

[IolantheSF]

You've gone far beyond me in thinking this through. Do let us know how it turns out.