Subject: Re: [harryproa] Re: rudder anxiety
From: Gardner Pomper
Date: 2/19/2009, 9:46 PM
To: harryproa@yahoogroups.com.au
Reply-to:
harryproa@yahoogroups.com.au

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
>
>


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