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Old 07-04-2016, 08:24   #1
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Trimaran sailors..........

I have been curious about this for awhile. Wondering if there are some real world comments and experiences out there. Not really interested in the theory and the hypothetical............but more real life experiences.

Curious about the performance of mini keels on trimarans. Pros and cons.

Chris White has been an advocate of mini keels for some time. His big tri Juniper sported a mini keel as does his Explorer 44. Both are very nice designs in my mind. I believe his 39 footer "Kachina" sports a mini keel as well.

John Marples has designed quite a few of his CC boats with mini keel options. In my opinion they are also up there on the list of highly successful cruising tris.

And there are a number of older designs out there with mini keels. Some very well known and others not so much. So there are a lot of boats out there........... there must be quite a few experiences to share.

This could be a fun exchange. Thanks in advance.

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Old 07-04-2016, 08:37   #2
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Re: Trimaran sailors..........

Figured I would start off............

This is not first hand experience but..... Had a long discussion with an owner of a Marples CC 37 Fast Cruiser. Same design that won the 2005 OSTAR. His boat sported a mini keel. When she was built a sister ship was built along side her at the same time to save time, money, and effort. The sister ship sported a center board.

According to these guys there was no difference in the performance of the two boats. They basically pointed and sailed the same. One obviously more shoal draft than the other.

Not to suggest that a mini keel will compete with a deep dagger. But it appears to come close to competing with a swing center board. And according to Chris White in his book, The Cruising Multihull, a mini keel should point about 2 or 3 degrees less than a CB boat. Not bad if weighing the compromise between cost, maintenance, and performance......etc.
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Old 07-04-2016, 10:51   #3
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Re: Trimaran sailors..........

This doesn't relate to mini keels... But our trimaran had a daggerboard. About half of it snapped off after having a bit of a run in with a mango tree during a storm. While we were waiting on a replacement we sailed the boat without the stub (only about 1/2 of the projection). The only noticeable difference was in light air at low speeds where the boat tended to slide a good bit more. And under motor where she would skitter all over the place.

As boat speeds picked up the difference became more and more negligible. To the point that at around 10-12kn of boat speed I really couldn't tell a difference at all. Maybe there was one, but I couldn't tell.


I would point out that I think this is only true for strait board boats. More and more modern boats are moving to lifting foils, and those would have a vastly different answer. Not because of the leeway, but because of the extra RM.
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Old 07-04-2016, 11:16   #4
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Re: Trimaran sailors..........

Intersting experience. I figured that it would be pointing in light conditions where the mini keel, or in your case the stub left over, would be the least effective.

Curious ............ at the higher speeds were you able to point as well as before?

I am all for the deep dagger. There is no question as a foil that is the most efficient. But it is just your scenario of grounding or hitting something that hopefully will only shear off the bottom of the board and not do damage to the hull as well. Happens to us all. Newick grounded "Pat's Boat" in Maine waters and tore a hole in the bottom.

If the mini keel works well enough there might be a lot to recommend it.
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Old 07-04-2016, 11:37   #5
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Re: Trimaran sailors..........

I had a Piver AA31 that had the original fins on the amas (the whole boat was built to plan). One of the folks in NWMA had a book of NACA foil shapes. I bought the plans for a Cross keel and rudder, built them with some carefully selected foils. These modifications transformed the boat. It tacked and sailed beautifully. I remember John Marples, sailing aboard a very nice Cross 42, being quite impressed by my lowly old Piver

While I would have preferred daggerboards, our current Chris White Voyager 48 has LAR keels. We're still learning our way around the boat, but so far the keels appear to be satisfactory. Performance on our one major sail from Stuart, FL to the Chesapeake was quite pleasing with big smiles all around.

Cheers,
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Old 07-04-2016, 11:49   #6
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Re: Trimaran sailors..........

At high speeds the difference was zero, or so close to zero you couldn't tell. It was at low speeds where the difference was most pronounced. The boat would just skid all over the place.

My guess is that a couple of things are going on. The higher speeds mean the foil can generate more lift per inch, so fewer inches are needed. Secondly as speed increases the leeward ama depresses and adds to lateral resistance, while at low speeds it just kisses the water and doesn't do much.
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Old 07-04-2016, 13:28   #7
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Re: Trimaran sailors..........

Quote:
Originally Posted by jdazey View Post
I had a Piver AA31 that had the original fins on the amas (the whole boat was built to plan). One of the folks in NWMA had a book of NACA foil shapes. I bought the plans for a Cross keel and rudder, built them with some carefully selected foils. These modifications transformed the boat. It tacked and sailed beautifully. I remember John Marples, sailing aboard a very nice Cross 42, being quite impressed by my lowly old Piver

While I would have preferred daggerboards, our current Chris White Voyager 48 has LAR keels. We're still learning our way around the boat, but so far the keels appear to be satisfactory. Performance on our one major sail from Stuart, FL to the Chesapeake was quite pleasing with big smiles all around.

Cheers,
When you guys did your mod. Did you rebuild foils for the Ama keels? Or did you replace the Ama keels with a single foil.........mini keel just for the main hull?

And how much do you draw with the LAR keels?

Also curious how much draft with the Piver? And did you find the added draft to be an issue? Did she point well enough?

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Old 07-04-2016, 13:45   #8
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Re: Trimaran sailors..........

Quote:
Originally Posted by Stumble View Post
At high speeds the difference was zero, or so close to zero you couldn't tell. It was at low speeds where the difference was most pronounced. The boat would just skid all over the place.

My guess is that a couple of things are going on. The higher speeds mean the foil can generate more lift per inch, so fewer inches are needed. Secondly as speed increases the leeward ama depresses and adds to lateral resistance, while at low speeds it just kisses the water and doesn't do much.

Interesting about the dynamics. I agree especially about the Ama being depressed in strong conditions.

Interesting to note the difference on some of John Marples designs. He will design a center board with about 8.5 square feet. But a mini keel for the same boat will have 12 or 14 square feet.

I would guess that the lower aspect ratio of the mini keel requires a bit more surface area to do the job. I talked to John about this and he said it is really just a "judgment call" as to how much area. I have noticed that some of Chris Whites designs seem to have a bit less area for the mini keel. As a ratio to waterline for instance.
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Old 07-04-2016, 15:08   #9
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Re: Trimaran sailors..........

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Originally Posted by w32honu View Post
Interesting about the dynamics. I agree especially about the Ama being depressed in strong conditions.

Interesting to note the difference on some of John Marples designs. He will design a center board with about 8.5 square feet. But a mini keel for the same boat will have 12 or 14 square feet.

I would guess that the lower aspect ratio of the mini keel requires a bit more surface area to do the job. I talked to John about this and he said it is really just a "judgment call" as to how much area. I have noticed that some of Chris Whites designs seem to have a bit less area for the mini keel. As a ratio to waterline for instance.
At least for monohulls the keel size is more a function of sail area than it is waterline profile. Accounting for the amount of lead needed.

And yes I would guess that in switching to a mini-keel the size difference is related to aspect ratio. The higher the aspect the more efficient the keel is for the same surface area. Which is why full keel boats need so much area compared to a fin keel.
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Old 07-04-2016, 15:28   #10
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Re: Trimaran sailors..........

I spent years aboard a Chris White Voyager 45. She had two keels and one daggerboard, which ran vertically down through the starboard keel. Draft was 3'6" with the board up, 7'3" board down.

We rarely used the board except when racing or trying harder than usual to get to windward to clear an obstruction. The boat sailed very well either way, although the dagger did make a bit of difference, particularly at low speeds.

Nice to have the extra pointing ability of the dagger and still be able to get a 45' boat into 4' of water (or less).
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Old 08-04-2016, 10:47   #11
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Re: Trimaran sailors..........

Quote:
Originally Posted by w32honu View Post
When you guys did your mod. Did you rebuild foils for the Ama keels? Or did you replace the Ama keels with a single foil.........mini keel just for the main hull?

And how much do you draw with the LAR keels?

Also curious how much draft with the Piver? And did you find the added draft to be an issue? Did she point well enough?

I cut off the fins on the amas and built a keel on the main hull. As I recall, the draft ended up about 3'8" and was never a problem. I tacked through 90 degrees apparent according to the wind indicator at the mast head (old tech) and made good progress to weather. Seafire, a Brown 40, would point a little higher in light winds. Pointing was certainly the equal of most monos in the mid-70s. I would add that the foil shape is critical. A slab down there wouldn't help very much.

Our cat with LAR keels draws 4'. I wish my boat had the daggerboard, but not nearly enough to do anything about it. It's my highly prejudiced opinion that these Chris White Voyagers are very cool boats
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Old 08-04-2016, 13:04   #12
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Re: Trimaran sailors..........

jdazey..........Tx.

I agree, the CW boats are very kewl!!!

I have fallen for a recent Marples design. CC-40 FC trimaran. Its a sweet boat. Perfect live aboard for two with plenty of room for an occasional couple to visit for extended periods.......separate cabins. I would love to make it happen. Big project tho.

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Old 08-04-2016, 14:53   #13
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Re: Trimaran sailors..........

Me thing I would be very interested in is the effect of adding curved foils to the amas. A somewhat counter intuitive reality is that the heavier and slower a boat is the faster the lift from a lifting foil becomes valuable. The foils on the ORMA's for instance actually hurt until the boat is moving 20-25kn, while on slower heavier boats the cross over speed can be much lower say 5-10kn. In a big cruising tri there is at least an argument that the heavy weight would highly favor adding lifting foils, and since a boards are really not that expensive to make relative to strait boards it may be a very good deal.

I could very easily see a fast cruising tri with ama C boards, and winglets on the rudder as a very advantageous system. Allowing better speed, reduced liklyhood of pitch poling, and reduced pitching in a seaway. For not a major investment relative to the cost of theboat. Say in the $50,000 range for a 40' tri.
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Old 08-04-2016, 18:51   #14
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Re: Trimaran sailors..........

Hi,
I am building the prototype of my SCARAB 32 trimaran design. So far the floats and beams are complete (not joined yet) and the main hull is being completed inside before the deck goes on. My original idea was for a pivoting centreboard offset from the centreline and I have detailed that on the plans, but I got to thinking about simplicity etc. and decided to go for a LAR keel. The keel gives a draft of 950mm (just over 3 foot) but also means that I don't need a pivoting rudder, and as the outboard motor is mounted in a well forward of the rudder that is protected too. This is a cruising boat and I will sit her on the sand to clean the bottom occasionally so will just need a prop at each float while she sits on the keel. I have heavily stiffened up the bottom panel to allow for this and the keel will be attached to the hull with silicone bronze bolts. I can't give any performance figures yet but I believe the performance will be similar to the pivoting centreboard, a dagger board would be better but my main cruising ground can be a bit shallow at times. Another interesting departure from my published design is for a schooner rig (staysail schooner). This will reduce the size of the mainsail to a smaller sail that two (or one) old farts can manage easily. The main staysail, fore staysail and jib will all be on furlers so hopefully she will be a breeze to sail.
All the best
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Old 08-04-2016, 20:54   #15
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Re: Trimaran sailors..........

An endorsement of Fast Scarab's designs, I had a 9 metre tri of his and it was a great boat.
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