Multihull manuverability

Hi Marc

I've posted a pic of my tri in Delane's "Hobie/J24 trimaran conversion, page 7".
This season it has a taller mast (old one broke, too short).
I'm an experienced sailor but not a designer so not qualified to say whether this tri is a successful conversion but it's right for me.

Sorry about the off-topic.

Harry

 

No need to apologize harry. Your comments were very insightful. Thanks for posting them!

 

G'day Richard. Have a great New Year cobber. Each and everyone of us has benefited from your shared knowledge & experience. I take this time to Thank-you very much.

You'll be pleased to know - that Jan 1, 2012 - will happen & it's a grand day for sure. That's cause it's noon - the 1st here in Far North Queensland, Australia - & we'll wait for you-all to catch-up. The 'barbie's-on', the slab's on ice, prawns & bara - garlic & herbs at the ready & 'sundowners' - will always be ready (as they are today) when you arrive - wish that could happen - maybe 1 day we can meet 1/2 way. That would be a grand happening. Sure we could get 'a cast of thousands' to come, real people & not a rent-a-crowd.


Thanks again for all your help, we all appreciate. Ciao, james - & Hoo-Roo from down-under.

 

newer designs?

Messabout, point taken- I was thinking of the more "modern?" sporty multis- cats and tris. The designs have matured to the point that most of the lighter craft will tack-but some of the older designs and quite a few of the current "cruising" and charter cats have to start an engine in most conditions
My old Buc will tack, but it probably does have too much rocker in the main hull, and not enough volume aft. I have not sailed a "stock" 24, but I think my taller mast must dampen the hobby-horsing some, and provides a little more punch in light air so I am not as effected by short wave patterns. Certain size motor boat wakes CAN beat me up- crash, bang, splash I think some of the newer tris are not as sensitive. I do see some questionable crew work on racing multis that should know better, but sometimes, having crew at all is better than waking them up every time the wind shifts a little. You do what you can with what you have
Happy New Year Everybody! B

 

Hi Harrygee, what's your centreboard like, thick or thin?

 

I can only comment from my own experience.
I have built, cruised and raced 7 multihulls of my own, and the only one in which you could hold the tiller down on any point of sail and it would rotate through 360 deg without hesitation, was a Piver Nugget with a LAR keel and a self tacking jib.
Don't ask me why, but it was just magic.
Other than that the fastest tacking Tri I owned was the Bucc 24---hands down. I'm sure that was because of the rounded keel line, the vertical daggerboard and the fact that as the boat came head to wind, the floats cleared the water.

 

There was a similar report in an AYRS publication - Design for Fast Sailing, where a smallish daggerboard was changed to a LAR of much larger size and moved farther forward. Apparently the change was dramatic as far being able to tack.

 

My centre-board is cut from a pine bed-head, about 1.25" thick (including the two layers of glass), 20" by 40" deep, shaped by eye. The original soling keel was about 40" square, so I've halved the area. That must help when doing donuts - she spins faster than any other boat I've sailed. If I want to spin her before a race start, (if there's no-one nearby), I need to warn the crew because things happen pretty fast.
Sailing single-handed, I can't easily get to the board so I leave it down, which doesn't affect the speed much but it vibrates. The case is stronger than the board. I think.
The rudder is of similar construction, transom hung so it can kick up simply, about the same area as the soling rudder.
I'll try to organize a video when I can find someone with a bit of tech-sense.

Harry

 

I don't suppose you remember when that was published? I would be interested in picking that up for the personal library

 

Harrygee, I was expecting you to say you had a thick board actually.

I think a poorly tacking multi can be cured with the right board. The question is, when you're on the new tack going at less than half your upwind speed and you try sheeting on, is your board already stalled? If so you're screwed.

Mono's don't stall their boards cause they're thicker, and yes they have more momentum, so they're going a bit quicker too.

A LAR board? Lawrence Astro Radiation board? Doesn't stalling have to with thickness and AOA and not aspect ratio?
Maybe the LAR board works cause when the board is all ready stalled and we're travelling at a very large AOA, say 20+ deg, the square profile creates more resistance (drag as opposed to lift) than a normal board.

And why does low aspect ratio board get its own acronym? Its old tech from the good old days before carbon?

 

Rapscallion,

Google AYRS (a Brit group) and look in their publications. I think around '66. The hardbound volume is still available for a reasonable price.

LAR - low aspect ratio. I first heard it from Norm Cross publications and understand that either he or Jim Brown adapted them to Pivers. However I might not really know who was on first, or second because the same shape board was shown in the book reference above. Stalling has to do with board shape, particularly the leading edge and how much AOA in a given situation. In the days when I heard of LAR, Carbon was restricted by price to military aerospace of space.

Idkfa, I can't make sense of your next to the last sentence. You seem to be confusing several different situations at once. Why would it help to be stalled with more drag to tack? One of the typical issues with multihull taking is that it does take more time to get the boat around a circle and people try to push it by pushing the tiller over too far, stalling the rudder and stopping the boat. Finding the right tiller position is a significant learning curve for most, especially a monohull sailor use to jamming it hard over and letting the inertia take care of completing the tack.

A thick properly shaped board will resist stalling very well, but might be slower in the straight.

Marc

 

The main is out, the jib is being eased on, we're heading 55deg on the new tack,,, but we have stalled boards, cause not enough speed/poor board thickness, leeway is 20deg, pic on the left, true heading 75deg.

We have lots of resistance to our motion, but it is also resistance to increased slip, larger leeway angle, which would be even more resistance... The jib is pulling us, slowly, and as the speed builds our leeway angle falls, resistance falls, eventually the board starts making lift again and we sheet the main in and finally we really are on the new tack.

I'm suggesting that a LAR board will make more drag at 20deg than a HAR board at 20deg? (the sim thickness & profile) by virtue of it being square.

Harrygee, maybe the soling27 would be better with a thinner keel too! Just a thought, maybe monos could do with thinner keels, they do have the momentum.

 

I'll offer, as a data point, my experience with fairly big, well-shaped rudders. As drawn, Slider uses NACA 0012 rudders of modest aspect ratio. She tacks without a board as easily as with. Like a mono, you just slam the rudders over and around she goes, in any conditions, with any sails up, even with the daggerboard completely up.

One of my builders did not shape his rudders as carefully as I shaped the prototype's and his boat is not as easy to tack as the prototype. I think that big, reasonably thick, low-drag rudders that do not stall as early in the tack as thin rudders contribute a lot to a well-behaved cat.

I think it's probably true that rudders like Slider's are slower at high speed than rudders that are thinner and smaller. But to me, good behavior was more important than top speed.

 

Ray . I don't think that rudders with 0012 section are too thick. And I don't think you will have any observable slowness from them. However with that section they shouldn't stall early either.

 

what section do the A and C cats use?