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Old 12-01-2012, 05:40   #16
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Re: French Centerboarders

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Originally Posted by phollings View Post
MacG --

What's the difference between a centerboard and lifting keel?

Thanks!
We are actually a lifting keel - but can be also classed as a centreboard.

A real centerboard is a keel that is in a trunk that slids vertically up and down. A lifting keel like ours has a pivot pin in the front of the keel and it lowers on that pivot. There are other variations of this basic theme as well.

The advantage of a lifting keel over centerboard is that substantial weight can be put into a lifting keel so that when down it can lower your center of gravity even further.

Two exceptionally important considerations in a boat of anytype with a "variable keel" are

1. There is no silver bullet in a monohull that will get you the shallow draft of a Cat without sacraficing blue water suitability. The boat will still have draft - ours for example still pulls 1.4 meters all keels up. This draft combined with an enormous amount of encapsualted lead in the bilge keel (the keel that is part of the hull not the lifting keel) give us exceptional righting moment and a stability angle of vastly greater than the 140degree minimum to prevent us from ever being locked upside down should we have a serious knockdown

2. If you have a lifting keel you must make sure that it is designed so that should you "go beyond 90" the keel will not slam back up into the trunk and potentially cause serious damage and or hole the boat. Many lifting keels are via a strap or wire and are held by gravity. Unless they have a cable that also holds them down or are hydaulically held into place they are not offshore suitable.

hope that helps
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Old 12-01-2012, 07:03   #17
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Re: French Centerboarders

I was thinking of anchoring at a depth and drying out as the tide falls. Wave and current action would seem to cause some motion as the hull grounded resulting in abrasion. (This would be the case even if there was no surf or large rocks which should be avoided because of the possibility of excessive stress on the hull.) So, I was wondering if this abrasion caused a problem -- particularly because I understand that pinholes in painted Aluminum can lead to corrosion.
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Old 12-01-2012, 07:37   #18
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Re: French Centerboarders

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Originally Posted by phollings View Post
(...) abrasion (...)
I believe you will look for sand or mud bottom to dry such a boat. But with a daggerboard or a lifting keel you will most likely find such a a bottom quite easily.

In another thread on the forum I was educated to know that aluminum does not like being abraded at all.

b.
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Old 12-01-2012, 10:27   #19
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Re: French Centerboarders

No hull material likes abrasion, no sensible skipper would ever knowingly subject his hull to it and would definitely NOT dry out on surf or a rocky beach. That being said if you are keen to get off the bottom after having been dried out you may have a few moments of possible abrasion if you are on a hard sandy bottom. For me I haven't seen any signs of it and I dry out several times a year to scrub the slime of.
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Old 12-01-2012, 19:51   #20
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Re: French Centerboarders

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No hull material likes abrasion, no sensible skipper would ever knowingly subject his hull to it and would definitely NOT dry out on surf or a rocky beach. That being said if you are keen to get off the bottom after having been dried out you may have a few moments of possible abrasion if you are on a hard sandy bottom. For me I haven't seen any signs of it and I dry out several times a year to scrub the slime of.
Thanks. That's exactly what I wanted to know: that beaching a boat several times a year under gentle conditions did not abrade sufficiently to cause pinhole leaks in the bottom paint. The other question, is why paint the bottom at all if you are prepared to beach several times a year to "scrub off the slime".
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Old 12-01-2012, 20:30   #21
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Re: French Centerboarders

@Foolishsailor

Quote
A real centerboard is a keel that is in a trunk that slids vertically up and down. A lifting keel like ours has a pivot pin in the front of the keel and it lowers on that pivot. There are other variations of this basic theme as well.
unquote

Actually, it is the other way round. A liftkeel slides up and down, partially or till against the bottom; a CB pivots on an axle and slides out of a trunk and can be lowered at will. The latter one carries the entire ballast internally or partly in the trunk.
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Old 13-01-2012, 06:00   #22
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Re: French Centerboarders

Thanks for the good discussion! Another question: what are the requirements for a beachable boat? Variants I've seen are boats that can stand on twin keels, or twin keels plus a skeg-hung rudder. Another solution are vertical poles lashed to the chainplates that function as portable pilings. These do not seem attractive solutions to me: the former because of wetted surface, the latter because of the hassle of rigging it, the need to monitor it when the tide comes in, etc. Another solution I've read about is a "bilge keel". How is such a keel defined so as to permit a boat with one to dry out upright? Are there other solutions? Thanks!
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Old 13-01-2012, 06:13   #23
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pirate Re: French Centerboarders

Quote:
Originally Posted by phollings View Post
Thanks for the good discussion! Another question: what are the requirements for a beachable boat? Variants I've seen are boats that can stand on twin keels, or twin keels plus a skeg-hung rudder. Another solution are vertical poles lashed to the chainplates that function as portable pilings. These do not seem attractive solutions to me: the former because of wetted surface, the latter because of the hassle of rigging it, the need to monitor it when the tide comes in, etc. Another solution I've read about is a "bilge keel". How is such a keel defined so as to permit a boat with one to dry out upright? Are there other solutions? Thanks!
A Bilge Keel is the English version of the Twin Keel...
Also... One does not have to monitor the 'Beaching Legs'.. you fix em on while floating.. then take em of once re-floated.. but one only use's legs in well sheltered waters..
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Old 13-01-2012, 06:52   #24
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Re: French Centerboarders

There is a German twinkeel design that claims to have the advantages of the twinkeel minus the disadvantages it normally has. So far I have only one yacht seen equipped with it. I might have a look if I can find details.
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Old 13-01-2012, 07:46   #25
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Re: French Centerboarders

Many sharpies are beach-able; if you want bigger check out a Herreshoff meadowlark.
Hake yachts makes a" beach-able" mono hull and of course multi-hulls would fit the bill.
Remember there are trade offs in any design ,so be aware what you are giving up in order to get this somewhat unusual feature. I found the beach-able feature of my sharpie allowed me to cruise without a dingy in places like Lake Champlain with minimal tidal ranges.
If I lived near the shoal waters of the Florida keys or Cape Cod
Come to think of it a Cape Cod style cat boat might be a cool choice.
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Old 13-01-2012, 08:31   #26
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Re: French Centerboarders

Go to youtube and type in bretan.flv to see how not to beach a french centerboard sailboat. In surfing talk the guy on the bow was hanging ten. All as I can say is Ovni will sell a boat to anyone.
Always look forward to seeing posts on centerboard ocean cruising boats. My wife and I are really excited about sailing our new French centerboard boat when she is completed. We will start out in France and sail her back to the S. Pacific.
Cheers
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Old 13-01-2012, 08:36   #27
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Re: French Centerboarders

Quote:
Originally Posted by MacG View Post
@Foolishsailor

Quote
A real centerboard is a keel that is in a trunk that slids vertically up and down. A lifting keel like ours has a pivot pin in the front of the keel and it lowers on that pivot. There are other variations of this basic theme as well.
unquote

Actually, it is the other way round. A liftkeel slides up and down, partially or till against the bottom; a CB pivots on an axle and slides out of a trunk and can be lowered at will. The latter one carries the entire ballast internally or partly in the trunk.
you're right...I am mixing the terms "dagger board" and "centreboard" for some reason...

one other thing not mentioned here is that some boats, like ours, have a small lifting "trim keel" aft and in line with the main keel. It can be used to balance the boat going upwind for better pointing angle and also when going down wind the primary keel can be fully lifted and the trim keel lowered. The boat goes so fast its unreal, and as the lifting keel acts a bit like the fletching on an arrow she is very easy for the autopilot and less sensitive to seas on the aft quarters...
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Old 13-01-2012, 09:13   #28
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Re: French Centerboarders

@ Foolish sailor

The CB is also called "Swingkeel" - your description as per below:

Quote
1. There is no silver bullet in a monohull that will get you the shallow draft of a Cat without sacraficing blue water suitability. The boat will still have draft - ours for example still pulls 1.4 meters all keels up. This draft combined with an enormous amount of encapsualted lead in the bilge keel (the keel that is part of the hull not the lifting keel) give us exceptional righting moment and a stability angle of vastly greater than the 140degree minimum to prevent us from ever being locked upside down should we have a serious knockdown.
unquote

To make it clear I post here a typical centreboard design From Dick Koopmans Jr.
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Old 13-01-2012, 09:43   #29
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Re: French Centerboarders

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Originally Posted by phollings View Post
Variants I've seen are boats that can stand on twin keels, or twin keels plus a skeg-hung rudder. Another solution are vertical poles lashed to the chainplates that function as portable pilings. These do not seem attractive solutions to me: the former because of wetted surface, the latter because of the hassle of rigging it,
The difference in surface area of identical modern twin keeled yachts and the fin version is minor in comparison to the overall surface area. If you want to sail on a race track then you might notice the difference, but back in the real world the condition of the sails or the bottom paint and the skill of the owner will have a much bigger effect on performance. A thin layer of slime can take a knot off our speed easily.

We discussed it recently:

Twin-Keels for Cruising?

Pete
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