Longtail Evolution

See the video above.

 

On the subject of stainless steel longtail propellers, of the 3 pictured here, 2 are American longtail props and are not surface piercing. The Thai prop is but it is a tapered key design so not compatible with the most common American longtail shaft which is 3/4"-10 unc. I'm being a bit selfish here because I use Thai aluminum propellers and thread them myself 3/4". This brings up the debate of whether to use an aluminum prop as the sacrificial element of the drive line or not. Broken propshafts are not out of the ordinary and can strand you miles from home. There is something to be said about the longevity of a stainless prop though. I'm on the fence.

 

Any propeller works in partially submerged surface piercing mode, maybe just not as efficiently, ha. The larger surface piercing props for some American outboards don't necessarily look like any of these, probably because they are not particularly meant for shallow water.

Lots of ways to reduce the possibility of a broken driveline, and also reducing damage to the prop. For the shaft itself, it can be made from a stronger or more corrosion resistant steel alloy; or it can be increased in diameter, instead of cutting corners. Using an adjustable slip clutch, whether mechanical or magnetic, can instantaneously reduce the huge shock load and minimize damage, from collisions of the propeller. Aluminum props are softer/ more sacrificial and much cheaper compared to stainless; probably less likely to result in stranding, which I think is way more important than prop longevity. Of course that's assuming the long tail can be swung around or there's some other way for someone to easily replace a broken aluminum propeller with a spare- in situ.

I have successfully been using some of these ideas for many years in my battery powered Airline travel and go anywhere one man, pack in boats. My RC props are way more fragile than aluminum, but an adjustable mechanical clutch and and unsheathed spring Steel long tail means I've never broken a prop or shaft. I also use collapsible carbon fiber drive shafts when minimum weight or compactness require it.

Hope this helps.

 

Here's a video of above pictured longtail. The cavitation plate is called the "surface tracer". I emailed the manufacturer inquiring about his stainless prop and his reply was "it's not meant to be run on a surface drive". You can see in the video the prop is run almost always totally submerged. So you are right they can be briefly run surface piercing but they are inefficient at doing so. This type of cupped prop is run on all American longtails to the best of my knowledge.

 

Some short tails have five and six blade configurations and very different shapes as this link shows,




Hope the link works. Arneson and the military may also have their own versions of surface piercing props. Of course, these are not meant for shallow water as I had indicated previously.

Ps. Some designs of mud motors cannot be run continuously in surface piercing mode, probably because it over revs and destroys the motor..

 

Ps. Some designs of mud motors cannot be run continuously in surface piercing mode, probably because it over revs and destroys the motor..[/QUOTE]

Only 2600 hp? That's a long way from a jon boat with a Briggs & Stratton. My surface drive runs at 4000 rpm all day long throwing a rooster tail.

 

Response was my guess to post on the specific Mud Motor above, "it's not meant to be run on a surface drive".

Governor can keep the motor below red line, or watching the tachometer to adjust throttle was the old school method I knew.. But things may be way better /different with built-in protections now, I haven't had dealings with newer, small ICs -for at least a decade. So I'm basically on the same page, admiring your rig, and learning from your postings.

 

Just a good longtail video today, oughta wrap this up.

 

My favorite part of the video in post #35, is when he casually mentions the multi speed transmissions.

 

Jeff that "modified surf board" in your photo, no thanks! There are 600 easy ways to die driving that boat and all of them very painful, I take my hat off to the skippers.

 

That is a anti-ventilation plate. Ventilation and cavitation are two completely different phenomenons. Surface piercieng (ventilated) propellers do not need a plate since the upper half is out of the water. The claim of one outperforming the other is not valid unless you can prove that it is true in every operating conditions. I believe the shorter tail will outperform a longer tail in tight areas like zigzagging between trees.

 

The picture of Shorter's drive shows two cables that feed through fairleads on the transom. They appear to be capable of swinging the shaft horizontally. The float behind the prop is painted to indicate the waterline and appears to be the vertical control. The two cables did not look to me like they were angled for raising the prop, although they almost certainly could be used that way. In the Wikipedia article about Shorter, this is part of the description of the drive: "The drive shaft was guided horizontally above the water line through the ship's stern." Sounds like steering to me.


I thought that David Taylor would be pretty well known in this community. The model test basin at NSWC Carderock should be familiar. My career took an odd turn or two, so what seems familiar to me may be far from the beaten track. Never worked on anything going higher than the moon. OK, the moon was a summer help job cleaning the floor in a machine shop, but the parts did go there. I wish I had been there (at a safe distance) when one of the machinists dumped a 20 gallon trash can full of magnesium turnings into a bonfire on Revere Beach. Sunburned his back as he ran for his life. He wasn't one to toss in a handful to see what he was up against. You don't have to be smart to work for a rocket scientist.