I have thought abut a hinged drum, but the mechanics of having a large enough space above to allow the partly raised blade to kick forward leaves a large area without the reinforcing bulkhead needed to support the strains on the rudders. There is also a matter of bury to reduce the need for excessive bulking up of the bearings. If someone can overcome these problems I would seriously consider them. An anti cavitation plate or simply kicking the rudders forward reduces the ventilation problems.
Rudders have always been a weak point of boats. They need to be strong enough to withstand the considreable sailing loads, not so fat that they cause too much drag, have the ability to survive extraordinary loads without crippling the boat, avoid ventilation, bea able to have some steerage in reverse. Add reversibility and balance to a Harry design and it is a wonder the original Vis design worked as well as they did.
I am trying to reduce the loads on mine by having a leeboard. This also brings the COdrag more to lw. For sailing, I would prefer the blades more to lw but I don't see any way to do that that doesn't leave them too vulnerable. There is also the option of two easily retractable leeboards to adjust the balance and just use the rudders for steering.
--- In harryproa@yahoogroups.com.au, Mike Crawford <jmichael@...> wrote:
>
>
> Personally, I'd say it all comes down to what you want to do with the
> boat.
>
> I'm not sure anyone can argue against the superior hydrodynamics of
> rudders that emerge from the hull bottom, either on a fixed stock or as
> retractable rudders in drums. You can also get away with less strength
> in the stock/blade this way.
>
> One thing that would worry me would be the drum bearings. They're
> do-able, but there's less expertise, and fewer manufacturers in this
> area, when compared to a rudder stock. And these rudders will be
> enduring more sailing forces than a normal monohull or multihull drum
> rudder.
>
> But that's a minor point. Half of these boats qualifies as
> experimental as each person makes changes to the design. To worry that
> something hasn't be done before is to eliminate the Harryproa.
>
> The collision issue would be the major one for me. Let's say you hit
> a whale, submerged shipping container or log, or uncharted sandbar,
> while doing ten+ knots.
>
> a) With the external rudders, the mechanism will break away. The
> best case is that you simply re-attach the rudder with a new "fuse", and
> then go on sailing. The worst case is that you can't use the rudder,
> and will instead have to steer with a backup mechanism. Neither case
> involves structural damage, a weakening of the boat, or a hole in the hull.
>
> b) With internal rudders, the following might happen:
>
> - The rudder mechanism breaks the hull, water enters, and the boat
> no longer works.
>
> - The crash box functions, so there's no sinking, but there's still
> some structural damage, and there's still water in part of the hull.
>
> - The breakaway rudder fails properly, but now can no longer be
> raised or lowered due to its post-crash shape, and there's still a
> likelihood of damage to the bearings.
>
> In all these cases, there's a good chance of structural damage, water
> in the hull, damage to the bearings, and an inability to repair the
> damage while at sea and return to sailing normally.
>
> ---
>
> I grounded out two years ago and ruined my daggerboard supports when I
> let someone else helm for an hour. While I was able to get them
> repaired, but the truth is that I was lucky. And had we been doing ten
> knots, my deck would have buckled under the stress. That would have
> been a very unfortunate day on the water that could have involved injury
> and great expense.
>
> So the idea of having every foil below the waterline kick up is more
> important to me than ever. That means either a Harryproa, a Dragonfly
> tri, or a Seacart tri (I think Seacarts can do this, but I'm not sure).
> Given the huge difference in price between the three, the Harry is the
> easy winner.
>
> This is not an argument to use external rudders. I'd like to see
> people develop the drum rudders because they fascinate me, and also
> because I'd consider drum rudders myself (albeit with the drum mounted
> on a pivot that can allow the entire mechanism to kick up).
>
> And if you want to go fast, avoid steering balance issues, have an
> easy up/down system, and stay away from experimental designs, the drum
> is a good choice. Especially if you're not going to make an ocean passage.
>
> But I'm a fan of being comfortable with the worst-case scenario. It
> might not be likely, but it does happen. I'll personally go with an
> external option.
>
> - Mike
>
>
> On 6/13/2010 7:43 PM, 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@...
> > <mailto:harryproa@...>> 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
> >>
> >> On Mon, Jun 14, 2010 at 5:17 AM, Gardner Pomper
> >> <gardner@... <mailto:gardner@...>> wrote:
> >>
> >>
> >>
> >> 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
> >>
> >> 2010/6/13 Helmut Müller <hmueller@...
> >> <mailto:hmueller@...>>
> >>
> >>
> >>
> >>
> >>
> >> 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
> >>
> >>
> >>
> >>
> >>
> >
>