Building Paterson 8 in Dubai

[john_hamilton]

progress pics?

[doug1720]

Will do.  Since moving I have not been able to find my charger but I will use my Blackberry if still cannot be found.

Had to delay my flight back from Riyadh and so will be a later start tomorrow!

Doug

[Stuberry]

I don't understand why you would need a compression strut going forwards to the stem. Surely the solid lowers will be just as happy working in tension and compression.

[Will_Lee]

Hi Doug,

A low boom has got to be better aerodynamically, but we felt ergonomics trumped that concern. After we had Aquamarina, with a book 1.1m from the deck, we started to really value a high boom.

Atum has 1m from the deck to the bottom of the gooseneck fitting, which is a bit above the bottom of the boom. Antidote is 0.95m.

Atum has tension struts as solid lowers, with a very visible backward kink at gooseneck level. We did not have the problems of the jib catching etc, but it is easy to understand how this could have occurred, esp if we has increased the rake any more.

A low gooseneck and heavily raked boom for ergonomics is something we considered, but felt it was too radical. It would allow for a low stump with substantial lowers though.

[doug1720]

I don't understand why you would need a compression strut going forwards to the stem. Surely the solid lowers will be just as happy working in tension and compression.

Thanks.  Agreed.  I had seen this solution on other types of boats though.

[doug1720]

Hi Doug,

A low boom has got to be better aerodynamically, but we felt ergonomics trumped that concern. After we had Aquamarina, with a book 1.1m from the deck, we started to really value a high boom.

Atum has 1m from the deck to the bottom of the gooseneck fitting, which is a bit above the bottom of the boom. Antidote is 0.95m.

Atum has tension struts as solid lowers, with a very visible backward kink at gooseneck level. We did not have the problems of the jib catching etc, but it is easy to understand how this could have occurred, esp if we has increased the rake any more.

A low gooseneck and heavily raked boom for ergonomics is something we considered, but felt it was too radical. It would allow for a low stump with substantial lowers though.

Thanks for the detail, very helpful and I know I am asking some basic questions...would be a lot easier if I could see some boats!

I think I am going for a long stump, in terms of rake what should I be looking for?

Doug

[Phil Alderson]

I don't understand why you would need a compression strut going forwards to the stem. Surely the solid lowers will be just as happy working in tension and compression.

Thanks.  Agreed.  I had seen this solution on other types of boats though.

The reason that some boats put solid lowers and a fwd strut is to get a fully triangulated stump-strouds-forestay combo. As there is a lot of depth to the triangles the structure becomes very stiff so you can build the bits lighter.

Without a forward strut the stump needs to be angled back further than the mast so that the rig tension is pressing it down pulls the top of the stump back approx 80 degres from the horizontal. If you are going to have a standard kicker (and it is worth building the structure for one as a backup for when the gnav breaks) then the fwd load on the gooseneck from full kicker needs to be taken into account.

If you have no stump you have to build your hull-wing structure strong and stiff enough to take all the rig loads. If you build a stump with solid lowers that due to the geometry are sometimes in compression and sometimes in tension, then you still need to build enough strength/stiffess into you hull structure to take all the rig loads.

The question to ask yourself is: Will the stump structure take enough of the rig loads to let you make the shroud bulkhead and wings and bridge deck or foredeck lighter than they would be with no stump and is the difference more than the additional weight of the stump structure?

Other questions to ponder are;
What is the windage of solid lowers, compared to wire?
How much does 1m of mast weigh and how big a difference will this make to the all up sailng weight?
Do I ever want to double stack the boat or to ship it to a different country in a container? Will a stump cause issues with doing this?

[Stuberry]

Some things I partially understand and some things I don't:

1. Why would a mast stump with solid lowers need to be any more raked than a mast with wire?

I'm imagining a free force diagram with rig tension loads coming down the mast and acting vertically and a forwards horizontal load on the gooseneck.

Diagram attached.

Where there is both tension and compression forces on one solid lower I don't know which one will be bigger and how this balances out.

Carbon is stronger in tension than in compression, right? So as long as they are strong enough to take the compression loads they will easily be strong enough to handle the tension.

No engineering from me, just logic. Are they mutually exclusive?

[Phil Alderson]

Stu your diagram and logic look about right, logic and engineering are not mutually exclusive, you just need to be careful when you simplify things to keep the important things and get rid of the unimportant.(hopefully I am doing this but I could be wrong)

To simplify the analysis I think it is best think about the joints between sections of the stump as pinned joints i.e. they will flex also think about the hull being very flexible.

The simplest is a boat with no stump and a perfectly straight mast , when you put the rig tension on the sides of the boat will flex up and in slightly.  As this happens the distance from the shroud points to the gooseneck will reduce. If you had solid lowers, then the gooseneck would be pushed forward.

With this setup the only additional stiffness that the stump provides for the boat comes from the joints between the lowers and the stump (very little) and the stump to hull and spine connection. If the stump is about 1m from the deck connection to the gooseneck, then it either needs to be a large diameter, or very heavy to give you enough stiffness to make a difference.

Now think about having the stump angled back a long way, the rig tension puts the solid lowers in compression, and they press down at the shroud points forcing the sides of the boat down and out, this balances the up and in from the shrouds, and the boat is stiffer. The kicker loads reduce the effectiveness of this which is why the stump needs to be angled back further than you might think, and calculating the best angle would be extremely complex.

If you are using the stump to make the boat stiffer, then for it to be a gain you need to get back the of the extra weight of the stump and lowers back by  making other bits lighter.

To summarise
No Stump ==> Bulkheads need to resist all rig loads
Free standing stump ==> Bulkheads need to resist all rig loads minus stiffness gained from stump
Stump with solid lowers that go into tension ==>  Bulkheads need to resist all rig loads minus stiffness gained from stump and tiny bit from joints.
Stump with solid lowers that are always in compression ==>Bulkheads need to resist all rig loads minus stiffness gained from stump and minus compression of lowers acting at shroud points and a tiny bit from the joints.

It is worth noting that when you use a stump you get stiffness from the stump to hull joint, however you lose stiffness from the mast below gooseneck to mast above gooseneck joint.

Carbon is stronger in tension than compression but the stump to lowers joint, and lowers to hull joint is complex, can you be sure to get the full benefit of carbon's strength if the fibres are not lined up properly through the joint. There have been a lot of broken gantries because joining small tubes together at acute angles is difficult to do.

[JimC]

A low boom has got to be better aerodynamically,

I don't believe that any practical boom height closes the gap sufficiently that it has an end plate effect (need to be inches at best I reckon), so I think you cn ignore that particular factor.
Higher is more heeling moment, but also more wind because of wind sheer, goodness knows what the best tradeoff is.
Therefeore *I* (for what little my opinion is worth) think that main/jib interaction is the main argument for a lower boom, but there is more than one way to skin that cat. I find Bethwaite's arguments for the mast cuff thing on the 9ers sufficiently compelling that I had one made for the Canoe, included in the meaured area, but must confess that I very rarely get round to rigging it. Its a suprisingy difficult thing to get right and convenient to rig. I bet the Bethwaites went through a few prototypes, and whilst their solution is a good compromise for a one design I'm sure it could be done better. If I had sailmaking gear/skills its something I think/hope I would play with a lot to get right.

To my unskilled eyes a number of the 2005 rules rigs look like too much rag crammed onto too little spar: it wouldn't suprise me if the last few percents of sail area are of minimal benefit on some boats, but I lack expertise... Its also suprising how little difference sail area makes to boat speed if you look at relative handicaps of otherwse similar boats...

Another reason for over-raking the stump is the practical one of making it easier to get the kicker anchor pivot point in the right plane. Assuming a tension kicker it also means that the mast compression is opposing the kicker push: if both mast compression and kicker loads are pushing the top of the stump forward...

[dwlee]

Just a quick question.   Does anyone use a stump and not have the goose-neck on the stump?

I think the jib/main interaction is more important than it is usual given in comparison to mast height, particularly in lower wind speeds and high aspect ratio sail plans.   One Javelin has its boom around 250mm lower than any other and due to the rules the mast height is also lower by 250mm and it was particularly fast in lower wind speeds compared to other hulls of the same design. 

Doug, have you meet Rob Fordyce who is up in Dubai?

[Banshee Ambulance]

Just a quick question.   Does anyone use a stump and not have the goose-neck on the stump?

The E5s use a stump of sorts with the gooseneck on the mast but it is pretty low (almost deck level) so more of a mast step. As a consiquence they still have wire lowers.

[Will_Lee]

The stumped boats I can think of:

Atum Bom
Mango Jam
Suicide Blonde
Halo Jones
Loco Perro

all have the gooseneck on the stump.

Dangerous Beans has a highish complete foredeck and has a low gooseneck only about 300mm above that.

What are you thinking?

[MK]

Nautilus Pompilus has a stump, and i think the gooseneck is on the mast just above where the stump ends, but i will check on saturday, this is on a 97 rig

[phil_kirk]

Am i right in saying that Loco has no lowers. 


Phil A is correct in saying that solid lowers in compression reduces the loads on the bulkheads and hull structure however I am not sure that you can save much weight from the main bulkheads under the mast/shrouds.  E-numbers bulkheads were all 8m foam with 1 layer of weave on one side and a layer of biax on the other.

There is a saving to the boat's all up weight because the solid lowers and stump of a stumped boat can be counted as a part of the hull weight where as the lower m of the mast and the wire lowers of a non stumped boat can't.

We didn't go for the stump because of the practical problems of getting two boats into a single garage.