Hi,
Been lurking for a bit..
I don't do this stuff for a living, so read with scepticism ;)
I had a look at Gardner's pdf of the layout, and one thing that struck me quite strongly is that the rudder position just felt wrong.
I set out to think about how the rudders might work and in the process came to some interesting conclusions.
The thinking is convoluted - that is me I am sorry.
So thinking on where the vectors for drag and drive line up on the generalised harry proa...
The Harry has the sail-plan drive all the way to lee-ward. And some of the drag is moved out to windward.
And the sail-drive points forwards and leewards [as for all sailing boats]
First some simplifying assumptions to disprove:
1* that the drive is centred in the leeward hull.
2* that the lee ward and wind ward hulls are symmetric around the sail drive
3* that the hulls are rigidly aligned (the lovverly skiming dish idea is different, much more like a wind-surfer :).
If the drive points forwards and leewards from the centre of the lee hull- then the windward hull is relatively forwards of the drive vector and the harry would round up into the wind, pivoting around the centre of drag.
Which no-one complains about :).
Neatly disproving one or more of the simplistic assumptions above :).
Possibilities:
1) The rig is forwards in the lee-hull so that it balances the (relative to the drive vector and lee hull) forwards windward hull :).
Hard to see how this happens with a Rig mounted on a a central mast with the main sail behind the mast, and the head sail is smaller - limited by the unstayed mast..
2) lifting the bow rudder changes the drag the right way.. as the stern rudder balances the forwards wind-ward hull :).
3) no, the hulls are all rigidly aligned.
So we have a winner - the asymmetrical rudders.. :)
This is supported [a bit] by the suggestions from lots of people to sail based on the rear rudder... [this could be to make the boat handle more like all the other boats - as opposed to a forklift :) ]
Now I have one reason not to like the rudders moved towards the centre of the boat.. there is less correction for the offset drag of the windward hull by a corresponding rudder offset..
This could be addressed by making the rudder larger, or adding a moveable centreboard, ...
But there is another reason not to like the rudders in the (relative) centre of the boat.
The moment arm from any rudder to the centre of drag for a proa is a variable distance. Much like it is in a surf-cat.. where the bows can dig in, or a hull lift, or the crew sqat far out over the stern- the hull shape in the water is variable - so the centre of drag can move rear and foreward, wind and lee-wards.
If the centre of drag moves 1 metre and the moment arm avgs 4 metres, that is a moment arm change from 3.5 to 4.5 metres - ie 28% change in the steering effect.
If the centre of drag moves 1m and the steering moment is 2m, then that is 1.5m to 2.5m or 66% effect.
As the centre of drag is moved more than 1m the impact on the steering is increased.
This becomes cataclysmic when the centre of drag meets the rudder location - and you have no steerage at all.
So moving the rudders towards the centre of the boat and onto the beams risks them being very close to the effective centre of drag of the boat for some sailing conditions - where they would need to be much larger to have the same control effect.
The centre of drag of the boat would change depending on a lot of factors - where the crew moved on a small boat [reminding me of sailing over-crewed surfcats], amount of stores in each hull, heeling moment [ie wind strength, sail carried, gust or calm], wave shape.
Then there is the impact of a breaking wave - changing the speed of the rudder through the water...
So even if the rudders where huge, the moment arm could change by a large factor due effectively random factors - leading to steering that is difficult to predict.
Imagine if the steering effect changed by 200% when a wave passed underneath and the rudder lost power in the wave - round-up and capsize sound familiar?
When you want the boat to be predictable it is not..
So for my cruising boat, I rather like the idea of large rudders safely distant from the centre of drag.
Dave
wishing he was at lake or even at sea.
--- In harryproa@yahoogrou
>
> Hi,
>
> A new job and a continuing desire to test out my feelings towards a harryproa have prompted me to try again with a layout for a harry just big enough for me to do extended cruising, with enough bunk space for my wife to accompany me if she likes. There are a couple different ideas in this design that I wanted to pass by the group:
>
> 1) The galley is laid out perpendicular to the ww hull. There is basically just a 2'x2' area to stand on. The double bunk is behind you and the galley is in front, and is the same width as the double bunk (4.5').
>
> 2) I moved the head out of the ww hull to the cockpit. Before the outrage starts, let me just say that with a hardtop, I felt like it would be feasable to drop a privacy curtain down around the head when it is in use, and it avoid putting a potentially "aromatic" head right under either the galley or bunk in a boat this small.
>
> 3) There is a removeable 7'5 wide hard deck which doubles as a dinghy ramp. Having a good dinghy with a powerful (15+hp) engine greatly expands your range when cruising. The proa has no engine; I figure I can either tow it from the dinghy, or perhaps lower the dinghy ramp and just use the engine if I need it. I would like feedback on this idea.
>
> 4) No piece is wider than 7.5' when dismounted, as I am still on my "fit it in a shipping container" kick.
>
> 5) I have drawn the hulls the minimum length I think they can be. Since the actual hulls have no walking space in them, it would be trivial to just extend them. My back of the envelope calculations imply that it would add about 10 sq ft of panelling to extend the lw hull by 1 foot, and about 15 sq ft to extend the ww hull by 1 ft. So, to extend the lw hull to 38' and ww to 24' would add less than 500 sq ft.
>
> 6) My very rough preliminary calcs show about 1100 sq ft of panel as drawn. I am guessing about about 1 lb/sq ft, plus beams and rig, to come in about 1600 lbs. Sailing weight probably about 2000 lbs with a displacement about 3500 lbs for a 1' draft on each hull.
>
> I would like feedback. I have a potential lead on a relatively local builder who might be interested in working with me, so once I have panel drawings done I was going to approach him to see what sort of pricing we can work out.
>
> I am interested on innovative ideas on how to do a build contract on something like this. A builder won't give me a reasonable fixed price, despite the simplicity of because it is so new to them and they can't risk the loss if things don't work out. I don't want to just pay by the hour to have them play around and figure it out. Anybody have any ideas on a compromise? I have been leaning towards trying to get a fixed price on just the lw hull, since it has the fewest pieces and use that as a basis for estimating the rest of the boat.
>
> Thanks all,
>
> - Gardner
>
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