Thread: Start Building Another Dutch GIS

Is the new edge boom hollow or is it simply a lamination? The middle picture looks to me like it has a capping piece on the top, hence why I'm asking.

The discussion about the rig is great and Ralph's comparison of how the Finn rig works compared to the balanced lug seems spot on to me. Ultimately we seem to need to achieve the same end result with the GIS!

I recently read a rather heated discussion on another forum about whether a loose footed sail compared to sail lashed to the boom changes a boom's dynamics to any significant degree. The discussion was a bit nasty and the combatants eventually peeled off agreeing to disagree. Anyway, the discussion (it was really an argument!) centered around whether the forces on the boom in each case were any different, and so was a stiff boom really necessary for a loose footed sail?

My initial thoughts are that yes, a stiff boom and loose footed sail IS preferable as otherwise the booms bend would alter the distance between clew and leech foot, and induce unwanted extra curve into the sail shape.

I'd be interested to hear what your thoughts are? At least I know the discussions around here are most cordial all the time

haha,

That revisits the argument I had with my sailmakers over the same point - they were ready to fight to the death. But when the standard GIS booms have been rigged loose footed they seem to bend a lot.

Try it and see what you think! It is not tooooo bad with the goat upwind because two falls of the mainsheet are near the end of the boom

MIK

Originally Posted by woodeneye

Is the new edge boom hollow or is it simply a lamination? The middle picture looks to me like it has a capping piece on the top, hence why I'm asking.

The discussion about the rig is great and Ralph's comparison of how the Finn rig works compared to the balanced lug seems spot on to me. Ultimately we seem to need to achieve the same end result with the GIS!

I recently read a rather heated discussion on another forum about whether a loose footed sail compared to sail lashed to the boom changes a boom's dynamics to any significant degree. The discussion was a bit nasty and the combatants eventually peeled off agreeing to disagree. Anyway, the discussion (it was really an argument!) centered around whether the forces on the boom in each case were any different, and so was a stiff boom really necessary for a loose footed sail?

My initial thoughts are that yes, a stiff boom and loose footed sail IS preferable as otherwise the booms bend would alter the distance between clew and leech foot, and induce unwanted extra curve into the sail shape.

I'd be interested to hear what your thoughts are? At least I know the discussions around here are most cordial all the time

Now it's two 18mm fir planks glued together. I did that to be able to plane as far as I needed to plane to get the right feeling in flex. 18 mm is because I had that available, laminated because that's the only way to get it straight in my experience and 36 mm ( bit less after sanding ) seems fine comparing with original 40 round boom, since you ad significant material in the height.

I am actually thinking about a hollow version if current one works out with Joost using it. I found good quality 12 x 69 fir planks at 360 cm length. With a 18x18 straight stave in the top and another one following the bottum taper it would be an easy and light construction with greater stiffnes. Sizes mentioned are standard fir sizes locally, you could narrow top and bottum staves a bit.

Regarding my Finn comparison;
GIS could be a nice home build " one-design " race dinghy ! with restrictions in construction ( wood-epoxy only, no full size glas laminates ) and Dacron for sails you can play with all kinds of things to improve performance without too much costs. And as MIK say's with probably only a few percent variation in performance...great for one on one racing, but hardly influence in cruising.

Different from a Finn, one could row and motor a GIS and take extra people ( even for racing? would be nice to compare 2 handed and single handed GIS in different wind conditions ) at a very reasanoble budget.

In my experience a boom need to be rigid ! I totally agree with your thought that a bending boom increases the depth of a sail at the wrong moment. And even with lashed boom ( or grooved ) I would not like too much flex, since you loose control over your leach tension and twist of your sail.........but that's a pretty technical issue

1 With a plank-on-edge boom you basically have two fixed points at your bottum leach;
The front end and aft end. In a gust this forces the top of mast ( or yard ) to take all the flex and to flatten the top part of the sail reducing power. That's where you want it depowered for greatest effect on stability.

2 If you really need to depower the bottumpart too ( bigger gust ) you want to have a bigger angle of your sail with the boat ( since appearant wind angle is increasing) , so you let your mainsheet go a little ( or traveler in a high tech dinghy or yacht )
I like MIK's idea about a balanced luff in this aspect; you don't need a kicking strap and you don't need a adjustable traveler. Luff tension is balancing leach tension
I need to see if it works perfect, but I am a believer already ! It's just simple without all the ropes and stuff.

3 If you want to depower even more ( increasing wind ) you could flatten total depth of the sail by adding tension on foot leach and/or put a reef. In both cases stop the boat and pull a few ropes. In my opinion it's better to put a reef and keep some depth ( power ) in stead of making a sail dead-flat.

I all 3 circumstances you want a boom that's rigid.......1 and 2 you don't want to flatten the bottum part, in 3 you better put a reef.
NB with a reef the sail is basically loose-footed anyway, your reef eyes are normally not constructed by a sailmaker to lash your sail to the boom, but only to bind the rest of your sail together.

Convincing ?

Best Regards
Ralph

Originally Posted by Boatmik

haha,

That revisits the argument I had with my sailmakers over the same point - they were ready to fight to the death. But when the standard GIS booms have been rigged loose footed they seem to bend a lot.

Try it and see what you think! It is not tooooo bad with the goat upwind because two falls of the mainsheet are near the end of the boom

MIK

You are probably right that it is not too bad, but for my new GIS it's easier to make it a bit overspecified. Anyway; building it from standard fir planks does not cost a fortune...........lots of work, but that's fits with our "wood building desease"

In a standard gaff main you really do not need a very strong boom for a loose footed sail, if you have your main sheet at the end and no kickingstrap there is not much that pushes any curve in it. But in GIS's balanced lug you always have your downhaul pulling a curve in your boom, that curve is reinforced by leach tension and some of the mainsheet tension.

Attached a few pictures of a Dutch 12ft dinghy class, standing lug. You see it's not a very strong boom, but it holds well. Sails are not loose footed, but have great depth and do not really support the boom halfway.

Regards
Ralph

Originally Posted by Watermaat

Convincing ?

Best Regards
Ralph

Well you convinced me

The current rig is directed as much towards prettiness and consistency of method than anything else.

One of the thoughts was to keep the sail area quite large to overcome the lack of the ability to tune the sail much and reef if overpowered. So the assumption is that the sail will end up a bit flatter than a racing sailor would consider more of the time.

I suspect having the ability to shape the sail is probably not more effective than the extra 10 or 15 square feet of sail the Goat has compared to many other similar size boats.

However, I am really happy for people to play with the boat too. Looking forward to doing some vicarious learning!

When designing I was pretty confident I (with help from the builders in making nice foils) could keep her well balanced and nice to handle even when she is heeling. Thie hgh topsides (I didn't realise how high while designing ... just wanted to use up the maximum width of the ply) help too.

Best wishes
MIK

Howdy Ralph,

The Dutch class looks great - what is the name of the type?

MIK

Hello MIK,

It is actually an internal class with strong historical following in the Netherlands:

The International Twelve Foot Dinghy Class

International website available here: http://www.12footdinghy.org/

It was an Olympic boat back in 1920 and 1928. They are very pretty boats, build of timber rather than plywood. New boats are pretty expensive, decent second hand ones seem to sell for around EUR 7000.

Hope this helps.

Joost

Originally Posted by Watermaat

Convincing ?

Best Regards
Ralph

Wow, nice post Ralph! No arguments from me. All that you say aligns with my thinking.

I do also think that while MIK has the Goat nicely optimised for most builders, he knows deep down that this boat still has a lot of potential, if a few things are tweaked, and I get the vibe that he's not averse to a bit of that. On the other hand a Moth-type situation is undesirable and a Goat on foils would be too over the top

About now, I'm realising that these posts probably should go onto the GIS section

Anyway, I can't help thinking that it would be nice to give the crew something to do other than just lazing there in a bikini and passing beers to the skipper, so a smallish jib on a bowsprit might be nice to have when sailing two up. Rather than have two mast positions, maybe a longer daggerboard case (or perhaps one with two slots) could work? Could the unstayed mast handle this I wonder?

If you like MIK, feel free to bump this post over to the GIS forum!

By the way, I've attached an artist's impression of the crew I'm looking for. I can dream can't I?

Hello,

Out of interest I just had a short look at the class rules of the 12 foot dinghy and especially at the rules for the boom:

Boom: max length 3660 mm, max diameter 51 mm, max diameter front of the boom 41 mm, max diameter back of the boom 35 mm.

Diamters are approximately 10 mm greater in comparison to the standard GIS boom.

Regards,

Joost

How much sail area in that 12 footer? Is it an Uffa Fox design?

About the yard, I found that it took little downhaul tension to cause a lot of bend in the spar. How can you get the luff tension necessary if the yard keeps bending as the downhaul is hauled tight? This to me means that the Spruce I am using requires the yard to be fatter since I haven't heard others see this happen. I saw it immediately when I hoisted the tig in my driveway.

Cheers,
Clint

Hello Clint,

The sail area is very similar to the GIS: 9.5 m2 for the 12ft class versus 9.7 m2 for the GIS.

The 12 ft dinghy was designed by George Cockshott (an amateur boat designer from Southport, England, UK) in 1913 and introduced in the Netherlands as a racing class in 1914.

The yard does bend when tensioning the downhaul, but:
1. Bending in the aft part of the yard is beneficial in stronger winds
2. Bending in the front part of the yard doesn’t seem to make a difference for the required luff tension (in fact the additional bend in the yard keeps tension on the luff!)

The bend in the boom due to the tension in the downhaul flattens the sail in the bottom a lot because most of the draft is pulled out of the sail by the bend in the boom. Therefore it might be beneficiary to beef up the boom as Ralph has explained in his post but keep the yard quite flexible.

Ralph has almost finished the new boom and it will be tested on GISwerk soon.

Regards,

Joost

Thanks Joost, I am curious what the dim's are for the yard, then, on the 12 footer.

Clint,

The 12 foot dinghy's yard needs to have a bend in it of 45 mm - 65 mm's before rigging and putting tensions on the yard. The dimensions can therefore not be compared directly.

However: length 3660 mm, max diameter 58 mm, max diameter bottom end 41 mm, max diameter top end 35 mm.

Joost

Ok, so I was planning on a 50 yard which shouldn't be too stiff at all and a rectangular 50mm boom. Thanks for the info.

Cheers,
Clint