Hi,
Ok, I am talking from extensive experience sitting on a couch, but here is my thinking:
1) I first want to make sure that everything works properly under "normal" conditions; i.e. sailing around and not hitting anything. If I actually run into something big while going fast in the middle of the ocean, I kindof expect something bad to happen. I would like to minimize those consequences, but my primary goal is for this to work simply, reliably and without hassle for coastal cruising and island hopping (never more that 24 hours from land).
2) I am not looking for tilt up or swing up or anything in a collision. I expect the rudder to break and be pushed out the bottom of the boat and a new one inserted. More on ideas on rudder construction below.
3) I am thinking of this for a "small" harryproa, basically my version of sidecar. 30' lw hull, maybe 400 sq ft of sail max. I was thinking of a lw hull beam of 18", but I may have to go to closer to 24" if I do this.
Rudder construction:
I am under the impression that almost all the forces on a rudder are sideways forces. Since there is no rudder shaft, I would think that you could build a rudder that basically consists of thin carbon fiber plates, maybe 3" apart with foam in between. Then the rudder skin could be relatively thin fiberglass. This should give lots of strength sideways, but not that much along the chord. If it hits something, I would think it would pretty much fold up like dominos.
Drum:
Along with this, if the upper part of the rudder (the part that is never in the water) stays a rectangular shape, the drum insert can also be rectangular, and would be maybe 1.5" wide, which would allow the force of the collision to be spread over a bigger area than the trailing edge of the rudder.
In terms of marine growth, if the drum cassette can be removed from the top while the boat is in the water, the drum should be able to be cleaned out.
I figure the drum to be 12" diameter, and I could have a 9" blade with the rotation point pretty much in the proper place for balance. With a 4:1 aspect ratio, that would give me a 3' draft fully down, with a rudder area of 2.25 sq ft, or 4.5 sq ft for both together, which is only a bit over 1% of sail area, so it might be a bit small.
So, thoughts? It is possible to build a collapsable rudder? Can I realistically go higher than 4:1 on aspect ratio? Would I get enough lift/leeway resistance from rudders this small?
- Gardner
On Sun, Jun 13, 2010 at 8:57 PM, robert <cateran1949@yahoo.co.uk > wrote:
Friction in a bearing goes up significantly with diameter, requiring more sophisticated bearing such as ball bearing or pressure lubed, and also fine tolerances are more difficult with the larger diameters.. How exactly are you envisaging a kick up system if they go through the bottom of a hull. You either have to have sacrificial areas in the bottom of the hull, leading to difficulties in the structure, have blades that are designed to break off, or be prepared to lose the bottom of the boat
--- In harryproa@yahoogroups.com.au , Rob Denney <harryproa@...> wrote:
>
> Crud in the bearings, fouling, complexity, cost, damage potential, need for
> a flat bottom.
>> On Mon, Jun 14, 2010 at 9:43 AM, Gardner <gardner@...> wrote:
>
> >
> >
> > Hi,
> >
> > I am curious about your objections to drum rudders. The seem to be a proven
> > technology, since they are in so many production boats (Hunter, Menges,
> > Gunboat, maybe?).
> >
> > I understand the point about collisions, and it also seems like there might
> > be an issue with getting enough chord on the rudder, but those are the only
> > drawbacks that come to mind. What am I overlooking?
> >
> > - Gardner
> >
> > Sent from my iPad
> >> > On Jun 13, 2010, at 6:35 PM, Rob Denney <harryproa@..> > On Mon, Jun 14, 2010 at 5:17 AM, Gardner Pomper <gardner@...>wrote:.> wrote:
> >
> >
> >
> > G'day,
> >
> > I don't think much of in hull rudders, and even less of drum rudders, but
> > they are not too difficult to make from composites and acetal ball bearings
> > from McMaster Carr. You will need some turned alloy or steel (better) rings
> > to use as moulds. Let me know the width of the hull and I will do some
> > drawings.
> >
> > rob
> >> >> 2010/6/13 Helmut Müller <hmueller@...>
> >
> >>
> >>
> >> Ok, that is at least 10 times the price I am willing to pay. Anyone have
> >> any ideas on what it would take to make one (well, two, actually <grin>)? I
> >> would like to think that there would be an easy way to just build some extra
> >> rudders if you do hit something and pull out the old one (or push it down
> >> through) and replace it with a new one and (maybe) repair the old one at
> >> your leisure.
> >>
> >> Is this the system that Rapscallion was going to use? I remember his
> >> having in-hull retractable rudders, but I don't remember a mention of a
> >> drum, so it did not seem that they could steer while retracted, which turned
> >> me off.
> >>
> >> - Gardner
> >>
> >>
> >>>
> >>>
> >>>
> >>>
> >>> Gardener,
> >>>
> >>> I got an answer in 2008 from Paul Amon for the inboard rudder:
> >>>
> >>>
> >>>
> >>> Each unit as installed on a Melges 32 would be around 6900usd with hard
> >>> anodized bearing, rudder, tiller, housing.
> >>>
> >>>
> >>>
> >>> Quite expensive, no solution for us.
> >>>
> >>>
> >>>
> >>> Cheerio
> >>>
> >>>
> >>>
> >>>
> >>
> >
> >
>
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