gybing center boards

Gooday Brian. Why would you want to have 'asymmetrical' boards on a mid-range - - cruising racing tri Or cat - for that matter. How would a small crew (in size & number of) ever manage to raise & lower 2 'asymmetrical' boards at the right - effieient time - while trying to cook, eat, relax & sail in a comfortable manner & that my friend is what life 'should be' all about. 2 boards plus 2 cases plus twice as many control lines - I can't get my head around this complex situation.

Please explain how 'asymmetrical' boards could be handled in a KISS manner. Thanks - looking forward to learning. As I've said - I sure do have a lot to learn. Ciao, james

 

Gooday 'raps' Tell us more about how it works & what you really notice - with that - - mini-rocket-ship - - of yours. We could all learn a lot - if you'd fill in the 'spaces' - Thanks bloke ! ! ! How's the prep going for the up-coming - Lake Mich - races ??? Ciao, james

 

In first place there are no 'cases', it is just 2 boards,... one board split in half

The controls would mimic a comman elevator in that the act of raising one pulls the other down. This would need to be done at the time of tacking to avoid additional winching.

Many years ago in a racing series known as SORC there was a twin dagger board boat that performed VERY well. It had aysemmetrical boards. It got 'rated' out of existance because it performed so well.

I remembered that for years, so when I was drawing up this Dynarig cat for a design contest, I thought why not include such a feature as twin aysemmtrical boards, particularly as they do NOT require any 'holes' in the hulls, and they could be serviced without hauling the vessel.

There are tradeoffs in all designs. I chose to consider this one for a number of positive reasons....some negatives may surface as well.

 

Gooday Brian. Thanks for the explanation. Who do I build them ??? Love to have a go - but can't quite see the full-colour picture in my head. ??? Please explain in minute detail. Sure sounds like it would be worth a go.

Thanks ever so much , james

 

To me it appears that there are three main approaches to lateral resistance, asymmetrical center boards, articulated center board and gybing center board. Then you are trying to introduce an angle of attack to further reduce lee way.
At What point would you consider using a articulated center board over an gybing center board, Doug mentioned forty foot earlier. By articulated I mean using a flap on it.

 

Hi Brian your Idea sounds interesting, please feel welcome expand on it if you wish the idea of this thread is to discuss all forms of lateral resistance.
I only heard of gybing center boards on this forum.

 

First off, you turkeys (sorry, joking, not serious) a centreboard is a pivoting board and a daggerboard is a dagger-like board and is what you see on a large percentage of sailing dinghies and multihulls - plus also on the latest monohulls like the Open 60's and VO70's and the new proposed VO65's for leeway resistance (because canting keels are near useless at it).
So we've got that straight? Cheers.
Brian is talking about the Bruce King designed Terrorist, a One Tonner which had a lifting asymmetric (check the spelling, since I'm being pedantic, but correct) asymmetric DAGGER board at the turn in each bilge - and she carried all her ballast internally and he is right, Terrorist went to windward like the proverbial witch (but lost the series late with a broken mast). There was a mechanism that lifted the windward board when the leeward, working one went down ... and they could lift both halfway for offwind work. Terrorist had a pot bellied look below waterline, to carry ballast low, plenty of rocker, and yet it was demolishing the One Ton fleet of the time. Outlawed immediately after the series. A more modern, flatter hull version would be a rocket - well, were talking about Open 60's and the like - (but without canting keels) - someone should try it.

 

The leeway angle will always be equal to the AoA needed for the foil(s) to equal the lateral forces from the sails and windage. I have a feeling that a foil large enough to work at AoA lower than 2-3° would be very large. The added draft and area would reduce the benefit. 3° is well within the low drag bucket for NACA xxxx sections and (IMO) there is little benefit from trying to reduce the loading with increased area to reduce the AoA.

On a very well instrumented boat, it would be great fun to have a trim tab or flap on the daggerboard so you could measure and evaluate the effects of what becomes a cambered foil.

I am *not* saying there is no benefit. I am saying that it is relatively small and in most cases not worth the effort. Getting it wrong *will* hurt performance.

I look at reports of 6° higher pointing with a high degree of scepticism. The boat would have to bee very bad to start with to see this from a gybed board.

 

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I was thinking 2 degrees......but I would measure the actual leeway before I tried to correct it.

 

Gybing boards are banned in many dinghy classes. Presumably not because they make the boat slower. Where they are allowed (505, Hornet for example) most people fit them

If a board operates at 2deg of leeway it will be very high aspect ratio and thus prone to stall in big waves and after tacks. I've tried boards like that and it is very irritating to tack and then sag off to lee for a boatlength until the board works properly again

I have also tried boards with trim tabs. Obviously they have to be left down all the time. I could point higher, much higher, with the tab adjusted. But I would slow down. It's the same in a plane, max camber for take off but flatter wings for speed once airborne. So VMG is worse

The only time the the trim tab was useful was when caught to lee of another boat. I could put on the tab and sail sideways and so get to windward of him. Worth going slower for a few minutes to do that

Mind you that was in a fast boat. I suspect that on a slow boat, like a 12m, where pointing is king, not boat speed, it's a different story.

I haven't tried asymmetric boards as they have to be adjusted at each tack and only one can be down at a time. I don't think the efficiency gain will offset the weight of a board and case that isn't being used. I don't know if any AC45 has been fitted with regular boards to prove which is faster

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

We tried gybing daggerboards in Nacra F18 cats. there were a few times in flat water when they 'bit' and extra performance upwind was noticeable.
In any sort of waves they didn't get a chance to work

They moved about in the cases and were a constant source of maintainance

They idea was eventually dropped

 

My dagger boards don't gybe if they are partially retracted. They have to be 100% deployed in order to gybe. When they can gybe, I can point considerably higher than i could if they were fixed and 95% deployed. In fact, at about 9 knots, I start to develop lee helm. At 11 knots and above, i pull up at least half of the board in order to balance the boat.

Marchaj talks about the degrees of leeway usually seen with a fixed board, as well as the advantages seen with a gybing board. Needless to say, Marchaj reported results different than what was discussed in this thread. I can say from personal experience, gybing boards are very effective.

 

Gary, let's not get too far afield.

Terrorist was not very competitive in the 1974 SORC, coming 24th in the One Ton class (won by Hood's Robin).

In the '74 NAs she finished 3rd. It would have been 4th, but the winning Peterson Magic Twanger was assessed a 4 place penalty, letting Robin win that as well.

In the '74 OTC she won the first race, was out of the top 10 in the second, lost her rig in the 3rd, and was 4th in the final. Competitive, but not demolishing.

Neither the boat nor the boards were outlawed. She was assessed a rating hit of about 0.2 feet when the MAF was installed into the rule that November.

She went on to place 4th in the "Above One Ton but Below Two Ton" class at the '75 SORC. It wasn't the most competitive class, being mostly production racer/cruisers. But she did do well.

Her larger sister, the Two Tonner Aggressive II won the NAs in '75 but lost the Worlds to a Peterson Pintail. The even larger (48') Hawkeye won the big boat class at Big Boat Series that year as well.

So even with the MAF those boats could still be competitive, not outlawed. If the rule was not changed and more modern boats from Peterson and Holland had the same type of appendages added it would have typeformed the IOR to something the owners of the time did not want.

 

Paul - can you remember what 'put-off' the designers & owners so much - back then - that they stopped using a 'trim-tab' on the back of keels in both 1/2 tonners & up to Admiral Cup size monos ??? I do remember that the 'trim-tabs' did make a considerable difference to the performance of several of the then top boats - then they got 'glassed-over' & that seemed to be the end of that move forward in design progress of keels - at least for some period of time - as I recall. Thanks for any light on this subject - appreciate your in-put. Ciao, james

 

I don't want to get too far off topic here.

Better hull shapes and more efficient keel designs ( the Peterson/Holland U-Shaped sections and Keels) made boats without moveable appendages point very high to begin with.

By using a trim tab you get more lift per unit of wetted area. But once you are at the limit of useful lift, getting more only adds drag. So the ideal situation at that point is to reduce the size of the appendage to take advantage of the tab.

However, within the IOR of the time you needed a certain volume to hold the required amount of lead. It wan't so easy to reduce the size of the keel and keep your stability. So having a trim tab on the big IOR keels wasn't buying much, if anything. More lift that wasn't helping most of the time, and more drag.

In fact I think after the IOR became really competitive (after Peterson, Farr, and Holland appeared in '73) the use of trim tabs was a thing of the past. Then the MAF really put the final stake through their heart.


Back on topic. As far as using tabs or gybing appendages on faster types (skiffs and catamarans) I think the benefit is marginal, if it exists at all. David Hollom has written a couple of good articles about this. The I14s try them from time to time, but always seem to conclude they aren't worth it (current thinking).