Talador,
You've hit on a chief element of compromise that trips up a lot of folks when looking at the harryproa.
What's better: a) The most-efficient design? Or, b) A slightly less-efficient design that avoids likely failures or massive slowdowns inherent in the more efficient design?
This is a subjective question. But unless you're an out-and-out racer competing in ideal conditions, where option A would rule, there are often some pretty clear reasons for choosing option B.
For the propeller shaft, I'd go with your position that directing it through the keel, with a faired housing where exposed, would produce less drag.
But I'd quickly drop that efficiency for the ability to see the prop and shaft, as well as the ability to quickly raise them and drop them to clear off weeds. /Any/ weed on that system is going to sap a lot more energy than is lost to the side-mounted shaft-prop combination.
For the proa, two under-hull spade rudders would create less drag than the equivalent surface-piercing rudders, which are going to need more wetted surface area, and more rudder angle, to do the same work.
For the proa, two under-hull spade rudders would create less drag than the equivalent surface-piercing rudders, which are going to need more wetted surface area, and more rudder angle, to do the same work.
But it's really easy to get seaweed or plastic bags hung up on near-vertical foils with such a shallow-draft vessel. I can't tell you how often I lose several knots in my catamaran when seaweed hangs up on the daggerboard and rudders when I'm within a mile or two of shore.
The kick-up rudders are not so bad because I can glance at them to see how much weed they're pulling, and then kick them up and back down with the tillers if needed. They are easy enough to clear that I'll do it regularly on those low wind days when the surface weed hasn't been blown to shore. The daggerboard is a bit of a pain, though, so I normally it until I just can't take the drag any more (since I can't see the weed, I often can't tell it's there until it's pretty obvious).
So, if I were to race, I'd want the surface-piercing rudder/foils instead of the spade rudders, if only to be able to see when they need to be cleared. Being able to clear them would be the second bonus.
Add to that the ability to lift the rudders to account for water depth or trim, and to have them kick up instead of break when they hit a shoal or log, and there's even more of an argument to go with the less-efficient design. Part of that is for performance reasons, and part to avoid showstoppers that result in a DNF.
Cruising-wise, it would also be nice to be able to lift the foils completely clear of the water in a big storm, and also to beach the boat without breaking the hull or the steering system.
So, what's better? While everyone's answer is different, there's a clear argument to go with the design that's less efficient in ideal conditions and remarkably more efficient in non-ideal conditions. Particularly given the likelihood of encountering weed in an endurance human-powered boat race or in a near-shore sailing race.
- Mike
On Mar 28, 2015, at 12:03 PM, taladorwood@yahoo.com.au [harryproa] <harryproa@yahoogroups.com.au> wrote:
Rick, You were moving right along in that video : )
The drag on the shaft, or close enough that it doesn't matter, is the same whether it is 100 mm vertically or angled over a meter to get to that 100 mm depth.The primary problem though is the penetration through the surface of the water. The drag there is much worse than it looks like. There is a very good reason a lot of work and effort is put into attaching rudders, props, keels etc to the hull underneath the boat. Wing root drag is minimal compared to the surface interference drag and ventilation. It is much cheaper, easier, and less maintenance to simply stick the appendages through the water, the only reason for going to all the trouble of mounting the stuff on the hull under the water line is to decrease drag. Eliminating that minuscule surface drag must be important to at least a few boat manufacturers, so at least a few people agree with me.About the weeds, a fouled prop is horrible. Rain cuts some airplane propellors efficiency by 15 to 30%. The more critical the surface the worse the effect. I don't have a good solution for the weeds. The prop itself creates negative pressure that sucks the weeds and lines in.If the weeds are primarily floating on the surface though, eliminating the two surface penetrations might help a little, the shaft might be steering weeds straight to the prop. Putting the shaft in an aerodynamic keel might help.Talador
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Posted by: Mike Crawford <mcrawf@nuomo.com>
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