Dejay,
Well said. Thanks. If you want more details on harryproas, join the chat group at
HarryProa@groups.io | Topics https://groups.io/g/HarryProa/topics and read Rick Willoughby's posts. Rick is probably the most knowledgable aero/hydro dynamicist on BD.net, has designed world record human powered boats and analysed, optimised and sailed on the Melbourne 18m/60' Harryproa which is currently for sale.
Hi Rob, They are what would be traditionally called overbalanced. Spade rudders have up to around 15-20% leading edge because when you go too far above that, any over correction self reinforces. When they self-reinforce, they add considerable stress and risk losing control. Reversible rudders either have to be 50/50 or completely rotatable (which adds complexity). One of them is then placed ahead of the CLR where any tendency to lose control will be amplified.
Harryproa rudders are in oversize cases so they can be raked forwards (one way rudders which rotate 240 degrees around the leading edge) or aft (2 way rudders which only rotate 60, enough to steer). Rake happens automatically when you shunt, can be set for particular balance requirements and is easily altered.
This can be reduced with a coupler, but again at the cost of simplicity.
Couplers are ok on small boats in smooth water, not so good in waves. They also limit your steering options. Being able to use either or both rudders, together or in opposition, at any depth the boat will float in, is a handy thing.
The contact point with the boat is considerably smaller, which results in greater localized stresses (and decreased tolerances for deflection if coupled). I acknowledge that by making them pivot up, the overall peak stress in a collision that you have to design around is reduced though.
The contact point with the boat is via a large diameter drum supported by multiple bulkheads and shelves. Far more surface area than a daggerboard case and above the waterline so no fouling or leaks.
Specific to Harrproas - they are also asked to bear almost all of the lateral resistance. I have a hard time with the engineering that deliberately increases loads on one of the most complex/vulnerable parts of the boat for simplicity. An analogy would be a common rail diesel being superior in energy conversion and easier to use/maintain for the uninformed (you don't have to worry about underloading or running them up periodically), but being anything but simple - and you won't ever find one on my boat. If Harrys had a leeboard in the windward hull, I think that could accept most of the other design choices.
It's a lousy analogy. Making the rudders and their attachments strong enough is the same as making the masts, beams, daggerboards, bunk bases and galley shelves strong enough: Analyse the loads, decide on a safety factor (much easier for rudders if they kick up), build and test them. Pultruded carbon strips and simple cnc cut nose and tail moulds in the Harryproa plans make rudder building an easy job. Making the mounts simple is a challenge, but far less of one than making daggercases and in hulls rudders collision proof.
Shunting works best with large rudders and no fixed foils to stall, which may be why there are so few videos of this type shunting. If the fixed foils are on the windward hull, the difficulties they cause are magnified.
I have tried several leeboards, they tend to misbehave at speed, but on the lee hull they are the best option if you want to start with small rudders. My advice would be not to make them too pretty or permanent as they will be ditched pretty quickly. Unless you have a death wish, they and the rudders should be mounted on the hull, not in it.
A lot is asked of these rudders:. 1) To control yaw as their primary function 2) to limit leeway 3) to control the CLR, 4) to pivot up in collision and continue to function while pivoted 5) to remain in sync with their partner.
Correct. Not easy, but well worth it once you make it happen.
1,2,3, and 5 are all discrete functions in trim tab rudders with a separate leeboard and are generally fault tolerant as a result - which is why they fit with my desired simple. They have their issues, most surrounding fault mode #4 - but everyone involved with them acknowledges that and are ready to jump on to a solution / alternative. I would too, and likely will use something other than initially proposed. But I'm not looking for a Swiss army knife rudder.
Without the first part of #4 you have an unsafe boat. If this is ok, and you don't mind all the hassle and limitations of the trim tabs in cases, go for it. For me, if it is not as safe as I can make it, I am not interested in designing, building or sailing it.
The second part of #4 is not a requirement, but would be nice. #5 is nothing to do with the rudders, their structure or attachment.
No one who has used them on a proa would call trim tab rudders "fault tolerant". Quite the opposite. Make a mistake in the raising/lowering/sheeting on routine and something will break.
I do respect your experience, as well as others who have chimed in here, but I often feel (wrongly or rightly) that you are generalizing from the context of the boats you know, when some of the compromise that I'm making (in theory at this point) are specifically to address those factors and that is what I'm looking for feedback on.
I would be remiss if I generalized from boats I have no experience of. ;-) Ask specific questions, I will answer them, with the reasons why. You might like to note that my generalisations are based on probably building and sailing more different proas and arrangements than everybody else on this forum combined. That includes pretty much every variation you have suggested.
Can I ask if your opinion of rocker would be the same when specifically used in the context of the windward/lifting hull exclusively, as I described?
You can ask and I will answer, but I don't know what you are describing. Please reference or repeat it.
(And I get that if you depend on the float to counteract bow down attitude, then you need to provide more buoyancy and account for the torsional loads). Would you think that the kayak rudders would have more success on the hull that is not being depressed, therefore no likely to be submerged? Etc. (I know it is hard to convey tone - these are meant as legitimate questions, not rhetorical).
I consider them legitimate, but was confused/cranky when you wanted a boat which answered none of your requirements.
I think the kayak rudders will be more effort than they are worth, except on a kayak sized boat where performance is not critical. They will be less drag (slow and causing the nose to depress) if they kick up on top of the hull, but there are collision issues with this. Build a model, you will see what I mean.
Re: rocker= asymmetric hull on edge, there are vastly different mechanics involved in lateral vs. vertical effects when you have a denser, incompressible fluid on the bottom of the boundary, so your comparison is not really a proof, but I will do some reading on the matter.
I didn't say it was a proof, just seems logical, and another reason not to have rocker. To save yourself some reading time, hold the back of a spoon under a tap and see whether it is sucked towards the denser, incompressible fluid or pushed away from it.
Can you explain how Harrys are distinct from ndrua to me? The reason I equate them is because they are both shunting boats with the longer hull to leeward. They both have their rigs, lateral resistance, and steerage in the lee hull. Compared with PP, they have more weight to windward, necessitating longer hulls and allowing for narrower beams. To me, the mechanics determine the type, not where you live on the boat etc.
If you can see similarities, no problem. But it is the same as saying modern Pacific proas are the same as traditional ones. They aren't, in almost every relevant way. Materials, righting moment, performance, hull shapes, steering, leeway resistance, rigs, comfort, seaworthiness, watertightness, platform stiffness, skill required, build time, value, weight, payload, etc.
Proportionally how much weight goes to windward and the length of the hulls is a spectrum which can be illustrated by the 80' cargo Harry approaching Russ's boat to my guesstimate.
I design to requirements, not formulae. Bucket List is/was even more extreme. There is nothing wrong with the weight distribution in modern pacific proas if you can accept the lower righting moment or have an active crew. My problem with them is sitting in the shower to leeward and pumping water/moving ballast to stop them falling over.
This is already too long, and discussing CLR with rocker will be even longer as it is complex, so I'll have to generalize. Yes, rocker adds to hobby-horsing which may briefly change CLR dynamically, and sailing bow down, which statically changes CLR, and allows for weight trim of CLR. But by having the bulk of the CLR concentrated in the middle, variations from waves can have less of an effect dynamically. CLR and centre of buoyancy are correlated, but can be varied from one another and a lot of the effects are from the centre of buoyancy, not CLR.
I suggest you build 2 models and compare this. If that doesn't convince you, build a one or 2 person version of what you are planning. You will find your "yes" points outweigh the rest of it. You will probably also find that it does not make a whole lot of difference to the sailing unless you optimise everything else. However, you will also find that it is harder to build, heavier and more expensive which, added to the performance and comfort loss is why i don't use it.
Ultimately, if the rudder issue can be solved simply, then a relatively narrow main/ww hull with a pivoting (fore/aft) centerboard [edit: I meant lee board] would gain the upper hand over a rockered assymetric. (everything is connected :/ )
True, but toss the leeboard and make the rudders bigger and it will be even upperer. Move the rig and the rudders to the leeward hull so you have some living space, safety when (not if) you are caught aback and don't get a sore neck trimming and you will be closer to the 'ultimate'.
The rudder attachment on the cargo proa is simpler than on any multi I know of apart from the canoes steered with paddles. They also add #6 to your list: vertical lift. If they work. Stay tuned.
Cheers
. I would not be unhappy with a Harry either, despite the different set of compromises.
