Hi,
Yes, your description of when to use mini-keels is what I had in mind. I would rather not build them, but I am unclear as to what the minimum rudder area needed when sailing in light winds. Here is the scenario I have in mind:
In the chesapeake, or the bahamas, there are many spots that are 2-3 feet deep that I would like to glide along in relatively light winds. The waters are sheltered, and therefore flat, and the wind is likely to be less than 10 knots. When doing this, I would like to have a draft of 2' or less. I can't have my rudders extended to 4 or 5' depth to get the requisite 2% of sail area, so I thought that fixed minikeels, maybe 1' deep, would give the boat a 1.5' draft. That, plus the rudders raised to 18" draft would give me more foil area than the rudders alone. I would not be looking to sail at > 5 knots under these conditions, because i really don't want to hit anything.
In that scenario, with a boat about the size of a harry, or a harrigami, would I be able to steer with just the rudders (total submerged area of 3 sq ft, vs 400 sq ft of sail)? Would I be better off with an additional 3 sq ft of mini-keels?
My Maine Cat 30 had the mini-keels attached with 5200, for precisely the reason you mentioned.. in a severe collision, the keels were supposed to break off without ripping open the bottom of the hull. I have no idea if that would have worked or not, but it gave me peace of mind to believe it, so please don't contradict the idea <grin>.
- Gardner
On Mon, Jun 21, 2010 at 11:39 AM, Mike Crawford
<jmichael@gwi.net> wrote:
I'm not sure I'd go with mini keels, but let's say it's a good idea for the sake of discussion.
There is an argument for the keels in terms of keeping the hull off the bottom when beached carefully -- less to repair in terms of skin punctures and/or bottom paint if you beach where there are rocks or gravel. (Or you could always keep cradles on board for such an event).
Some people will build a shallow slot into the bottom of the hull and then fasten the mini keels with 5200, in which case they can be sacrificed without structural damage in the event of a grounding.
Finally, as Rudolph showed with Blind Date, two mini keels will help windward ability while taking some load off the rudders (and/or making them more responsive). Two mini keels will provide more lift than a single longer keel.
Minimum draft in the Chesepeake is also something that it's hard to argue against.
A compromise might work well. Absolute minimum draft would be just the bare hulls in the water, but the boat isn't going to sail or steer usefully at that depth. How about mini keels sized to the depth of minimum rudder required for sailing and steerage?
That way you could still keep your minimum working depth, while also getting some lift and having something semi-hard under water to protect the outboard and rudders.
- Mike
On 6/20/2010 7:27 AM, George Kuck wrote:
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Hello Gardner,
I would not use any keel other than a protective cap (or extra layers of glass/Kevlar on bottom) for beaching ,as you will be sailing on Chesapeake bay you should try to keep draft to a minimum. There are many places on bay that have shallow watter and small coves you can only get to with shallow draft. As it is I can only get in creek where I keep my boat at half tide, with my TT720 which only has abought 18" draft with centerboard up and rudder lifted part way. The shallow draft with zero rocker is one of the main reasons I like the Harryproa.
As far as twin keels for hull to rest on, any catamaran or proa has two hulls so it will not tilt like a mono hull so I do not think that is a valid reason.
Happy sailing,
George Kuck
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