G'day,

What you say is mostly correct. I am addressing these problems as follows.
1) The rudders can be supported down to very near the water surface,
as long as they are well faired. The shaft above the water can be
much wider than the shaft inside the rudder wide as it sees water less
often. this can be improved even further on boats that don't fly a
hull by moving them to windward. Even if they were unfaired, the
drag is still less than that of two daggerboard slots in a cat.
2) The support for beam mounted rudders is very strong. The problem
with encased rudders is that between the fuse breaking and the rudder
blade actually floating free there is half a second or so of very high
loads which destroy the structure. With beam mounted rudders they can
be fully supported from down to kicked up. Because the fuse for beam
kick up rudders only handles the fore and aft loads, it is quite small
and easily tuned.
3) Beam mounted rudders allow simple variable draft. The 15m under
construction has max draft of 1.6m, min steering draft of 800mm, min
draft 200mm.

The 15m will be sailing in 3-6 months, so we will have some answers then.

Could we see the calculations for the 6" rudder stock, please?

Keep up the good work on sketch up. The boat looks good. I have
stayed out of the discussion, but if you have any specific questions,
please let me know.

The bows and underwater area from the beams forward are solid foam
with glass over them and bulkheads/floors between them and the rest of
the hull. I think this is a better option than steeply sloped
(necessary for anything other than trees) very strongly built bows.

regards,

Rob

On Fri, Feb 20, 2009 at 11:46 AM, Gardner Pomper <gardner@networknow.org> wrote:
> Hi,
>
> Since I seem to be the one voicing the most concern, I think I should
> clarify the points that bother me.
> 1) rudders need to be effective at low speeds for shunting - this makes them
> larger than usual
> 2) rudders are the primary leeway control - also makes them larger
> 3) harryproas are fast - increasing the force on the rudder
> 4) long lever arm - most underhung rudders are supported within an inch or
> so of the top of the rudder. For harries, we are talking about 2-3 feet.
> This increases the level arm for the center of force from about 1' to about
> 4', quadrupling the force on the rudder. From crudely applying the info from
> Dave Gerr's "Nature of Boats", that means the rudder stock has to increase
> by a bit more than 50% in thickness.
> My very rough calculations for a 5.5 sq ft rudder at 25 knots, would require
> a solid stainless steel shaft 6" in diameter.
> 5) rudders have no protection - underhung rudders often have skegs, plus
> mini keels, giving obstacles 2 other, fairly solid things to hit before they
> hit the rudder. Even if we went with underhung rudders on a harry, the
> bidirectional nature of a proa makes these bumper guards impractical.
> So, to summarize my concerns, the rudder supports (shaft, bearings, etc) are
> subject to a MINIMUM of 4 times the load on a comparably sized catamaran,
> probably considerably more. Plus, they really need to kick up, since there
> is no protection. Since they are new, an even bigger safetly margin should
> be built in for the unexpected (like Doug's mishap). You don't want the
> rudder to let go the first time you are in a gale, surfing down breaking
> waves.
> Now, on the other hand, I desperately want them to work. The combination of
> kickup rudders, light weight and long hulls makes the draft of a harryproa
> typically about a foot or so. This makes even other catamarans seem like
> deep draft boats. I have actually been in many places in the Bahamas and
> Florida keys, Cheasapeake Bay, where even a 2' draft would have allowed me
> to really have a lot of fun.
> In addition, I really do like the idea of nothing tearing a hole in the
> boat. I wonder if it isn't worthwhile to slope the underwater profile of the
> bows so that if you did hit a submerged object, like a log or a shipping
> container, that the hull would just ride right up over it, instead of a
> sudden impact.
> Anyway, I think I am really going to have to see some numbers, and
> especially some real on the water experience before I trust the rudder
> systems. I expect I will have the rest of my design all ready to go by the
> time that happens.
> - Gardner Pomper
> York, PA
>
> On Thu, Feb 19, 2009 at 9:02 PM, Robert <cateran1949@yahoo.co.uk> wrote:
>>
>> -The sailing loads are less on a Harry so the loads on the rudders
>> shouldn't be that much more than on other multihulls. It is simply a
>> case of beefing them up enough. The enough bit is what we are
>> learning. The Vis rudders work but twist a bit under high loads and
>> can be a bit of a handful at high loads while partially lifted. Bain
>> has had no troubles with his. I think the difficulties in the present
>> Vis system can be overcome with more tow to stop any twist and a cuved
>> blade rather than a straight one to keep the rudder closer to balanced
>> at all depths of immersion.
>> The beam hung rudders work but Doug hadn't designed for loading from
>> a different side while surfing. and didn't put the reinforcing to take
>> tension. This is simply a bit of extra glass.
>> Going to Harriette type rudders, if you don't have the kick up system,
>> then it is no different in principle to spade rudders, only having a
>> 30-50% longer lever arm. The kick up system adds complexity but
>> doesn't seem insurmountable. The point is that the boats are trying to
>> get away from holes in the hull that can shatter the boat if you hit
>> anything hard at speed. You can have crash boxes or kick up systems or
>> tear on the dotted line type systems, but something has to give. It
>> would be quite easy to put holes in the hull like most of the faster
>> boats with their centre boards and spade rudders, but Rob likes to
>> eliminate what he sees as a potential weakness. There have been quite
>> a few boats that have been sunk or immobilised from hitting things
>> with their rudders or centreboards.
>>
>> I reckon the beam hung rudders will get sorted out pretty well as
>> there is nothing intrinsically difficult about them, just making sure
>> there is enough carbon in the right places. My only objection is for
>> telescoping crossbeams. Farrier had to recently organise a patch up
>> job on his rudders due to unexpected loadings. Rudders are a concern
>> for most boats.
>> My preference is either modified Vis types on outriggers or Harriette
>> type in a flare that gives plenty of bury. I am still working on the
>> kick up system to my satisfaction but I think I have a system that
>> would work and I am tempted to have a disposable, crushable bottom
>> third of the blade.
>>
>> -
>> >
>> > I am glad to hear about someone actually sailing these things. We
>> need more
>> > real life harry stories. The rudders are always a concern. It sounds
>> like
>> > you really stressed them out. Other than the loose gudgeon, can you
>> think of
>> > other stuff to make them stronger. I don't accept operator error,
>> because
>> > that is exactly what I am likely to do. I need a boat that can deal
>> with me
>> > being wrong.
>> >
>> > Good luck on your trip.
>> >
>> > - Gardner
>> >
>> >
>>
>
>