Rivet length question

[Greenheart]

I'm making a stainless plate box which I intend to locate in my dinghy's mast gate.

I've only riveted thin plates together - a combined maximum of perhaps 3mm. In this case it will be nearer 6mm...

...so how much length of rivet will I need, to be sure of there being enough deformed metal on the unseen side, to form a secure finished rivet?

Thanks.

Not strictly relevant, but here's the purpose...my mast foot is slightly too far aft to allow full raking of the rig, as I'm told I need to do for singlehanding. I'm reluctant to move the mast step because I've no reason to think it is wrong - the steering balance is perfect. But when the rig is deeply raked, the mast has nearly no support at the deck level...so I want to extend the 'gate' section of the deck a few inches aft, by creating a long steel box around the mast.

 

[coopec]

I'm making a stainless plate box which I intend to locate in my dinghy's mast gate.

I've only riveted thin plates together - a combined maximum of perhaps 3mm. In this case it will be nearer 6mm...

...so how much length of rivet will I need, to be sure of there being enough deformed metal on the unseen side, to form a secure finished rivet?

Thanks.

Not strictly relevant, but here's the purpose...my mast foot is slightly too far aft to allow full raking of the rig, as I'm told I need to do for singlehanding. I'm reluctant to move the mast step because I've no reason to think it is wrong - the steering balance is perfect. But when the rig is deeply raked, the mast has nearly no support at the deck level...so I want to extend the 'gate' section of the deck a few inches aft, by creating a long steel box around the mast.

I've always used the manufacturer's guides (particularly where strength is vital) Maybe this one?

https://www.arrowfastener.com/wp-content/uploads/2017/09/RT187M-Rivet-Guide.pdf

 

[dunedin]

.Not strictly relevant, but here's the purpose...my mast foot is slightly too far aft to allow full raking of the rig, as I'm told I need to do for singlehanding. I'm reluctant to move the mast step because I've no reason to think it is wrong - the steering balance is perfect. But when the rig is deeply raked, the mast has nearly no support at the deck level...so I want to extend the 'gate' section of the deck a few inches aft, by creating a long steel box around the mast.

This sounds very relevant. And perhaps what you are proposing is a bodge I the wrong place.
Many /most dinghies have an adjustable mast foot, something like this Adjustable Dinghy Mast Step 23x150mm

Not sure why need to rake aft, but only a small movement of 1cm at the mast foot will move the head 10cm or so - and the boom come lower at the same time, so watch your head
And wood or plastic would be much kinder on the mast at deck level than stainless steel

 

[Greenheart]

Thanks for that. I agree it sounds a curious solution for a problem that itself is a slightly grey area.

My mast step doesn't allow more than very slight movement of the mast foot. I realise the effect is exaggerated, 23ft above, but the mast still won't be within the support area at deck level. I don't want to attack the mast step to move it forward (partly because there isn't much space it can move into; partly because I don't want to cause serious damage to a critical component; and partly because except for my singlehanded purposes, the boat sails and balances perfectly already).

Strongly raking the Osprey's rig seems to be recommended for standard (two-man) racing crews when it's really blowing...

...and as I mostly singlehand, that point comes pretty early. There is an 'upper clew' which raises the boom when the rig is raked, helping to keep it clear of the water when heeled, and taking some bagginess from the foot of the sail to reduce heeling too...

...so I infer that raking is accepted procedure. Somewhere I had a note of the correct distances from masthead to transom, under different degrees of approved rake, and it struck me that my rig couldn't easily rake as far as is commonly considered correct...

...and if I do rake it that far, I'll definitely want more lateral restraint at deck level, than is possible at present.

I chose stainless plate in order that the box can be narrow enough to fit inside the original moulded 'gate' behind the foredeck, but also broad enough internally to hold the mast...and also, rigid enough to hold it securely. I can line it with plastic to soften contact.

 

[dunedin]

Raking aft in strong winds is often about opening the leech of the genoa, and widening the gap between genoa and mainsail. This is perhaps useful in very strong winds when two large race crew fully hiking and trapezing - but suspect not when solo helm getting overpowered in lighter winds, as much less wind to get through the gap. Raking aft will also increase weather helm and/or reduce lee helm. Unless you have lee helm, suspect you are chasing an unnecessary goal.

Also, if the mast foot is in the correct location, and the mast is at the very back of the mast slot at deck level, it is a bit of a hint that it was never designed to be raked further back than currently. Have a look at other Ospreys, as I suspect the adjustable mast foot will probably be pretty common (and the right way to do this). Otherwise forget it and just sail as is, unless big lee helm issue - in which case a smaller genoa might be a better solution (as I think some Ospreys had originally, back in the 1970s)

 

[Greenheart]

That's interesting, thank you. Especially about the slot between the genoa and main...because when I'm overpowered I'm likely to be under main alone already, which might be expected to make weather helm a bigger issue than depowering.

Surprisingly, I've rarely sensed a weather-helm issue under main alone, though I was caught out once in an increasing breeze, when she wouldn't tack. When eventually she did, I hurried home downwind. I reflected later that if I'd had the centreboard half-raised, the weather-helm would likely have been reduced; in that situation I wasn't keen to improve the steering by setting even more sail.

I'm glad you've made me think about whether to rake the rig at all. Nobody else seems to single-hand an Osprey, so nobody else's view is really relevant, and possibly the reason for raking isn't applicable.

I still plan to use the upper clew though, to reduce bagginess (and area) in the lower part of the sail, and to keep the boom up when I'm heeled hard over.

 

[dunedin]

That's interesting, thank you. Especially about the slot between the genoa and main...because when I'm overpowered I'm likely to be under main alone already, which might be expected to make weather helm a bigger issue than depowering.

Surprisingly, I've rarely sensed a weather-helm issue under main alone, though I was caught out once in an increasing breeze, when she wouldn't tack. When eventually she did, I hurried home downwind. I reflected later that if I'd had the centreboard half-raised, the weather-helm would likely have been reduced; in that situation I wasn't keen to improve the steering by setting even more sail.

I'm glad you've made me think about whether to rake the rig at all. Nobody else seems to single-hand an Osprey, so nobody else's view is really relevant, and possibly the reason for raking isn't applicable.

I still plan to use the upper clew though, to reduce bagginess (and area) in the lower part of the sail, and to keep the boom up when I'm heeled hard over.

If already sailing under main only, I would very definitely not be wanting to rake the rig backwards!

Also, are you using the existing controls - as wind gets up, going upwind max the Cunningham (ideally at least a 16:1 power ratio) and max the kicker (we used to have ridiculously powerful kickers back in the day - think in theory was 96:1 or so).
Does the boat have a mast ram - Rwo Mast Ram 51mm Width R4520 | Dinghy Sailing at sailboats.co.uk
The mast ram is on hard for a straight mast when underpowered, and eased to let the mast bend to depower

 

[Greenheart]

Mast ram? I wish! But the more you talk, the better I see my problem, thanks. The very low-tech system I inherited employs a mast-gate with space ahead of the mast, loosely containing wood chocks which prevent the mast bending into the void when it's all powered up. Only recently, I realised I mustn't leave those chocks in place in a breeze, otherwise the capacity to depower is reduced.

I suppose it's all insoluble as long as I singlehand a boat originally designed for a crew of three men; I'll never beat against more than about ten knots with full sail set, even trapezing...

...I have certainly tried to apply all the Cunningham and kicker I could, though I may have been too wary of busting some fitting or the deck/bulkhead/thwart to which it was attached. The problem may also be baggy old sails, which only money can solve...

...but I intend to add mast-track sliders to my old, possibly flatter, mainsail for easier hoisting/dropping and while I'm at it, I may sew reef-points in that won't halve the area like the huge reef I have in my regular mainsail. If I can set 75sq ft instead of 100, I can unroll some genoa too (or acquire a smaller one, like you mentioned), and hopefully maintain balance with a controllable area.

 

[dunedin]

Mast ram? I wish! But the more you talk, the better I see my problem, thanks. The very low-tech system I inherited employs a mast-gate with space ahead of the mast, loosely containing wood chocks which prevent the mast bending into the void when it's all powered up. Only recently, I realised I mustn't leave those chocks in place in a breeze, otherwise the capacity to depower is reduced.

Mast chocks work fine also, just harder to adjust - lots of race classes use chocks, often plastic like these but wood is fine Ronstan Mast Chock Set - 6 Piece | Force 4 Chandlery
Idea is lots of chocks so remove one or two at a time to increase bend

 

[Greenheart]

Yeah, I don't know why I didn't make the short mental step of realising what the chocks actually do, five years earlier. Each time I went out, I carefully packed the chocks into their space before tightening the forestay, effectively preventing mast-bend.

Can I assume that even in a force 3 on the nose, which has always proved nearly too much, I want all the bend I can get from the mast?

 

[dunedin]

Yeah, I don't know why I didn't make the short mental step of realising what the chocks actually do, five years earlier. Each time I went out, I carefully packed the chocks into their space before tightening the forestay, effectively preventing mast-bend.

Can I assume that even in a force 3 on the nose, which has always proved nearly too much, I want all the bend I can get from the mast?

Other on here are probably more expert than me regarding Osprey rig settings. Generally rather than a single chock there are 4-5 thinish chocks (often numbered and with a string threaded through an upper corner to keep on board when not in use). Hence it does not need to be all or nothing, can have them all in, or take a few or all out.
But yes, other than ghosting in a settled calm, having a bit of mast bend probably would help when got only half the designed crew weight on board. And unless helming from the trapeze, you probably have only a third of the designed crew leverage. Hence a bit of rig flex would help.
Note that generally need a good kicker control, led to both side decks where you can reach it, as kicker tension helps bend the mast and open the leech of the main to help depower upwind - but may need to ease slightly on a reach (not too much off on a run to avoid the head twisting the wrong way and the wet windward capsize!)

 

[Greenheart]

That's all really helpful, thank you. Your description of the chocks suggests you've seen mine, so I guess they're pretty standard equipment.

I'll enjoy testing the rig without the chocks in, to see the difference it makes.

Useful tip about leading the kicker control to the sidedecks too. I've been apt to 'set & forget'.

 

[dunedin]

Useful tip about leading the kicker control to the sidedecks too. I've been apt to 'set & forget'.

An Osprey is a lovely thoroughbred but POWERFUL sailing dinghy. Definitely not a boat to “set and forget“ any settings. Proper use of controls such as Cunningham and Kicker are essential both to “tame the beast” and to make safer / more enjoyable the sailing. Also fun to experiment and learn.
Very different boats, but when I was racing Enterprises (which have a biggish mainsail for size) many years ago when a skeleton shaped teenager, and a tiny girl crew, the boat was completely unmanageable above F2-3 unless seriously tackled the sail controls - Cunningham nailed and kicker on at limits enough to nearly snap the boom upwind (and played every 60 seconds or so in medium winds). Both dumped before bearing off onto a reach, kicker back on a bit before going further onto a run.
Also centreboard part way up helped when beating into proper strong wind - this would probably also help (done moderately) you if sailing upwind with main only, as will both improve balance and ice heeling moment.

 

[Greenheart]

Proper use of controls such as Cunningham and Kicker are essential...enough to nearly snap the boom upwind. Both dumped before bearing off, kicker back on a bit before a run. Also centreboard part way up when beating into strong wind - would probably also help if sailing upwind with main only, as will both improve balance and heeling moment.

I'll etch the main points on the wrists of my wetsuit. Thanks again.

And I won't worry about raking the rig, or riveting the mast gate.