Herbie the Volkscruiser Proa
...the worst case scenario for the rudder is when hard pressed downwind (down by the bow) and decelerating into the back of a wave.
Sure, but this is true of all shallow draft boats is it not? Nothing special about this one.
A schooner rig will give you the ability to balance the boat, but if that is your only means of steering then the rig will be operating inefficiently because the aft sail will always be under-sheeted.
We discussed this under the Simple Proa thread. Again nothing new. There is a price to be paid for simplicity; in this case the schooner rig is slightly under sheeted for optimal efficiency. We aren’t racing, are we? It’s OK if you want to optimize the performance, but the brief in this particular design was simplicity, was it not?
Even taking into account the leeboard in the ama, the CLR will still be forward of midships, plus you have drag of the ama causing a turning moment to windward as well. To get the best out of the schooner rig, you still need some method of shifting the CLR aft.
Again this is well-plowed ground. ALL narrow hulls have their CLR forward of where one imagines it, and ALL daggerboards/leeboards help to bring it back—as does rudder input. This is basic design stuff. We’re looking for bang-for-buck simplicity, right? Yes, added complexity often will incrementally increase performance. Question is, how complex do you want to get? Raising and lowering rudders, dealing with exposed rudder collisions, moving leeboards fore and at; each of these needs to be weighed, not against perfection, but against the design brief, no?
The Newick style rudders are probably the most efficient. However, I like side hung linked rudders because you don’t have to raise and lower them on each shunt (good for tight situations) and because there are no penetrations through the hull taking up space. OTOH the linked rudders are a bit like the schooner rig in that one of the rudders (the forward rudder) will usually be operating at low lift, so you are paying a penalty in unnecessary wetted surface.
This is a plywood boat with hard chines and un-battened lugsails. Are we really concerned about half a percent increase in wetted surface? 😉
For asymmetric (cambered) sections, 30-35% may be right, but for symmetrical sections, 25-30% is more the norm and in engineering you should consider the worst case. Similarly, the worst case scenario for the rudder is when hard pressed downwind (down by the bow) and decelerating into the back of a wave.
A schooner rig will give you the ability to balance the boat, but if that is your only means of steering then the rig will be operating inefficiently because the aft sail will always be under-sheeted. Even taking into account the leeboard in the ama, the CLR will still be forward of midships, plus you have drag of the ama causing a turning moment to windward as well. To get the best out of the schooner rig, you still need some method of shifting the CLR aft.
The Newick style rudders are probably the most efficient. However, I like side hung linked rudders because you don’t have to raise and lower them on each shunt (good for tight situations) and because there are no penetrations through the hull taking up space. OTOH the linked rudders are a bit like the schooner rig in that one of the rudders (the forward rudder) will usually be operating at low lift, so you are paying a penalty in unnecessary wetted surface.
Also, the sharpie chine will contribute leeway resistance, (dare I mention vortex chine-runners?) that will also bring the overall CLR forward. I am imagining that on longer passages the way to go would be to put in a reef in the aft sail (or even raising a small jib!) thus turning the boat into a proper ketch. The fun of a schooner is that there are so many interesting sail combinations.
This is a plywood boat with hard chines and un-battened lugsails. Are we really concerned about half a percent increase in wetted surface?
That’s the thing. It’s not intended to be a racer, and I’m sure Jzerro would clean Herbie’s clock in a race. It’s intended to be a relatively inexpensive, simple, safe and “fast” multihull, if that is even possible. As is usual for this sort of thing, I would guess an R/C model would help clarify some details.
This design is very close to my own thoughts (and budget) at this point. There is a lot to like in it’s pretty careful adherence to the design brief. Keeping build complexity low while also keeping costs under control is not easy if you are still intending to have it perform reasonably well.
I wrote up a long post last night about my thoughts on this design, but alas was too slow and my posting timed-out and Proafile determined I was a bot. I was just slow, and I lost a fair bit of commentary and was too frustrated to restart.
I like the scow-ish ends more than I thought I would - because they allow offset mast(s) with similarly offset sheeting - eliminating fussy lateral sheeting spars a la Brown/Beiker. I’m not so sold on the balanced lug schooner rig as I’d rather optimize flow attachment and fiddle with underwater foils for balance.
I’ve been making cardboard models for a while now trying to get things looking right (I’m not talking only cosmetically).
Please keep this design on the front burner - it has already caused me to question some of my assumptions - and that is a good thing!
—
Bill in Ottawa
I wrote up a long post last night about my thoughts on this design, but alas was too slow and my posting timed-out and Proafile determined I was a bot. I was just slow, and I lost a fair bit of commentary and was too frustrated to restart.
A bit off topic, but I get that a lot. Now I always copy the text of the message to the clipboard before I post it, so that I can re-post it if it goes awry.
You just take the lower gudgeon point and move it well “aft” of the skeg—the hinge line is not parallel to the skeg, rather cants forward (at the top of the rudder). There’s been a lot of talk over on mhml about doing just this—including Bill Gibbs’ very powerful 51’ cat Afterburner.
Dave, I mulled this over and tried to make a sketch. Is this what you are talking about? The dashed line represents the line of rotation of the rudder, giving some balance.
I’m sorry about the post cut-off time - Miss Moneypenny being a little overly efficient. I’ll see if there is a control in the back end that will extend the time limit. That said, I’ve lost too many long posts on various forums, and frequent copying to the clipboard is always good practice.
Below is an idea for a CLR shift style steering system. I’ve had this idea for a while but thought it looked a bit ugly, and it’s not very efficient (due to the very low aspect ratio foils, more a case of just shoving some lateral area down under the water). However, it works quite well with Michael’s scow bow style proa because it could be largely hidden under the overhang. Basically it’s just a double ended sweep pivoted at midships. It could be operated simply with a line from either end of the sweep. It’s simple enough but in reality it would be quite a big piece of equipment.
Steering using the front rudder will only work if you can get the CLR far enough aft, which means you would need to lower a fixed board somewhere way aft on each shunt, so it adds more parts complication. If you don’t shift the CLR, which would naturally be only about 25% aft of the bow, then there is not enough leverage for the bow rudder to work.
When I made the suggestion about the front rudder steering, it was with the assumption that the adjustable leeboard would still be part of Herbie’s makeup, so that should attend to CLR issues.
With the suggestions about the pair of side-hung rudders, that seems to fit in quite well with Herbie’s design brief, not to mention the benefits that come with having externally mounted rudders whose workings can be tewwithout causing a great deal of grief. There’s a certain symmetry in a proa design that features tandem “foils” both above and below the waterline.
Dave, I mulled this over and tried to make a sketch. Is this what you are talking about? The dashed line represents the line of rotation of the rudder, giving some balance.
Precisely. You’ll need to “thin” the edge of the skeg, plus use Tom Speer’s proa daggerboard section for the rudder—both in order to get flow past the nose of the rudder in order to create the balancing force, but that’s not a big deal. You can see that, as you extend that lower gudgeon further and further out, the rudder can be made to balance even on the opposite shunt. So figure out a clever hinge/latch that switches the pivot point from length A to length B when you shunt. Make it automatic if you would, please?
Below is an idea for a CLR shift style steering system ... Basically it’s just a double ended sweep pivoted at midships. It could be operated simply with a line from either end of the sweep. It’s simple enough but in reality it would be quite a big piece of equipment.
I quite like this, Mal. Would it be improved by putting it to leeward of the main hull, though? That way it can bear on the hull and the pivot needn’t be as massive. I first heard this concept on AYRS (natch). Henry Gilfillan did half a dozen riffs on it; most included “proper” daggerboards in trunks with the connecting piece across the tops of both boards. Strongest and simplest had this piece on a fixed pivot as you show it but well above decks, however that would only let you bring a long daggerboard halfway up when grounding. A bit more trouble was to leave the pivot off and arrange a bit more friction in the daggerboard trunks. Now you can raise or lower the whole thing, as well as pivot it.
Other AYRS concepts included various schemes for withdrawing the rudder when centered (thus unloaded) and progressively immersing it as you increase the angle of attack. In this way, when the helm is balanced, there is no drag from the rudder at all; it’s only in the water when needed—and all automatically. Takes a clever bit of linkage to make it so, and you know how I feel about complexity… 😉
I have another favorite geometry I’ll drag out to air; let me do a little trig first. Look for “Simplest Proa Rudder” coming up!
Dave, I mulled this over and tried to make a sketch. Is this what you are talking about? The dashed line represents the line of rotation of the rudder, giving some balance.
Michael,
Either way, the effective in water rudder area (shaded blue) is hopelessly small and the proportionally much larger above water rudder area will get smacked about badly by waves. Changing the pivot axis as you have will make this worse and also leave a gap when the rudder is at an angle for weed, debris (and fingers) to get jammed in…....
Overlaid on your sketch is a suggestion for you…...You can tune the rudder area, profile and counterbalance to suit.
Obviously you would also reduce the gap between the rudder and the bow to a minimum.
Rob
Would it make sense to put the sails on different sides of the mast, fore and aft? If this was arranged so that the forward sail had a higher efficiency than the aft sail then they would have the same arrangement after a shunt.
Overlaid on your sketch is a suggestion for you…...You can tune the rudder area, profile and counterbalance to suit.
Obviously you would also reduce the gap between the rudder and the bow to a minimum.
Rob
Hi Rob,
I think Michael was trying to keep the rudders no deeper than the hull this time, so they can’t hit the bottom. Your way will work as well, but now the deeper rudder is unprotected—it’s the first thing to hit in a grounding.
As to problems with the angled pivot line, I would say that you are correct—however, a number of multihulls are already doing this, and aren’t experiencing the problems you point out. So, just guessing here, but perhaps the advantages simply outweigh the disadvantages, so are more enthusiastically reported? There’s been a fair amount of traffic about this over in the [MHML] multihulls forum. Their archives are here: http://steamradio.com/pipermail/multihulls/
Dave
I think Michael was trying to keep the rudders no deeper than the hull this time, so they can’t hit the bottom. Your way will work as well, but now the deeper rudder is unprotected—it’s the first thing to hit in a grounding.
Dave,
Which is why I indicated Dmax on the sketch…..
I don’t know how much rocker Michael has on Herbie, and he could get more by lifting the nose up to the waterplane…...Even that in itself should more than double the current effective rudder area.
And with more rocker (ie less forefoot) would not that help minimise (slightly) CLR migration?
Rob
Which is why I indicated Dmax on the sketch…..
Doh! Sorry.
I don’t know how much rocker Michael has on Herbie, and he could get more by lifting the nose up to the waterplane…...Even that in itself should more than double the current effective rudder area.
Yes it would. I’d be a bit happier if the hull had a bit of “keel” as it were, so that rudder had something to hide behind. Then again, that’d recreate the “finger guillotine” you objected to earlier, wouldn’t it?
And with more rocker (ie less forefoot) would not that help minimise (slightly) CLR migration?
Why yes, I believe it would. 😊
Dave