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Old 04-09-2020, 11:48   #76
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Re: Centerboard Keels in Blue Water Vessels

I think IMOCA have up to three moving blades (now the new ones have 5 blades).


The center one is moving (side to side), the off center ones are daggerboard (up-down) style.


If they run in trouble, it is mainly when hitting stuff hard. But this is easy when going 20 knots night and day.


All OVNI boats have lifting keels too. These have sailed everywhere, including the Antarctic.


So this is my input. A centerboard as good as anything, just use it when there is a reason to have it (e.g. cruising shallow waters).


b.
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Old 04-09-2020, 18:15   #77
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Re: Centerboard Keels in Blue Water Vessels

Would I build a centerboard or lift keel boat for strenuous offshore sailing. No.
I like the French aluminum production boats. I like traditional wooden catboats.
Now I’ll tell you after building boats for 60 years I went with aluminum twin keels. Pure traditional centerboard boats do rely on form as they derived from fishing vessels which worked in shallow areas or great tides. The boards were relatively light and simple to repair as the boats were small. The advantages of centerline ballast placed low in the boat, evolved first into deep keel wood boats then gradually to fin keels with a lot of lead.
All the while, designers sought to please both markets. Go fast offshore, get skinny inshore. Centerboards gained weight, required more hull supporting structure and more powerful systems to lift what once was wood, now was ballast. There are a lot of wonderful centerboard / lift keel designs. The more weight which can be lowered, the stronger the internal support and the lifting mechanism must be. Internal support is weight above the waterline. Lift the keel and the stability curves move a lot. Failure to lift places the vessel in a potentially difficult position as it’s now full time deep draft. Stuck up and it’s tender because it’s form is not that of a catboat.
Twin keels are not perfect but neither is the centerboard nor the fin keel.
It has the advantage of let’s say half the draft and can self support on a sand bar. The weight remains low, not fin keel low. There are advantages in tracking in large seas. The interior remains open. There is no mechanism to fail. The disadvantage is a substantial increase in construction costs. For a commercial artic expedition vessel, initial costs are important and risks are balanced against these costs. In production boats, simple flat hull panels and centerboards are significantly less costly to build than foil twin keels.
Twin keels have a number of advantages and two significant issue which is why they are not as common in aluminum yachts. First, the keels are mirror images like your hands...a lot of care needed in the design and construction. Lastly, the cost of construction. As with all vessels, there is no perfect design, material nor method of construction.
Happy trails to you.
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Old 15-06-2024, 15:31   #78
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Re: Centerboard Keels in Blue Water Vessels

Many have touched on the salient points. I depends on the designer and the builder. One important term is "keel centerboarder" Many great offshore racers by Rhodes and S&S were keel centerboards, Carina and Finistere to start. On the Water boats">blue water boats the board is not considered in the ballast calculation. There is significant evidence the boats of moderate draft, like a keel centerboard with the board up, are less likely to be rolled. Dashew designed his keel to ventilated before the boats deck was submerged. This was a carefully tested principle not a bit of guesswork. Many owners of S&S designs report they only deploy the board close hauled. Board mechanisms can be simple or complex. Many have no pennetrations below the waterline for pins etc. Rhodes, Hoyt and Sheel all did designs like this. Hoyt did one with no pin at all. So if Shoal draft is of value to your plans, it can be a great option since unlike other shoal options it still gives you a deep foil for weatherly work. From my experience the horror stories come from boats that the simple routine maintenance was ignored. The same is true of most "sudden failures" And a good Keel Centerboard boat will continue to sail with or with out the board.
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Old 15-06-2024, 17:37   #79
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Re: Centerboard Keels in Blue Water Vessels

ROXY is a modified full keel, skeg rudder, swing CB. The original CB was damaged, unusable when we got her. We commissioned a new one. Minimum draft, 6’-8”. Board down draft is 13 feet. We have found the board to be only minimally effective so it hasn’t been down in 9 years. I also rebuilt the motor and lift including all reversing solenoids. I wish we’d spent the money eliminating the CB and adding 4” to the keel, sealing off the trunk. The boat sails very well without the CB.

The designer in later years remarked that the swing CB was the stupidest thing they did.
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Old 17-06-2024, 20:05   #80
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Re: Centerboard Keels in Blue Water Vessels

I've got experience with centerboard keel boats. The first was a Morgan 41 with a fiberglass board that increased the draft from around 4 feet to 10; my current boat is a Hinckley 48 with a 1200lb solid bronze board that takes draft from 5.5 to 12. I do use the board going to windward if I'm feeling motivated; but I use it more often to provide some additional stability and improve handling in rough conditions, and to reduce rolling at anchor.

The Morgan's board only weighed about 150 pounds but lowering it even halfway significantly reduces the boat's motion in rough weather and can really balance the boat depending on the point of sail. The Hinckley even more so as there is some real ballast there. In light air, however, the board can really slow things down--up to a knot or so if fully extended.

I did snap the pennant on the Morgan. This is the weak point on many CB boats because the pennant and fittings are constantly submerged; needs annual inspection. I replaced the pennant with Dyneema. The Hinckley is a bronze worm gear driven by an electric motor. There is another 48 up here in Maine that somehow managed to bend the solid 2 inch thick bronze board sideways so it wouldn't fully retract into the truck. They tried unsuccessfully to fix the board by heating and molding the board back to its original shape. I think they ultimately modified the trunk to accommodate the bent board.

Here's the Morgan on the travel lift hauled out to fix the pennant (straps maxed).
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