When it comes to naval vessels, and in particular submarines, enormous importance–and secrecy–is placed on their prop designs. It is not an exaggeration to say that in some parts of the world, a photo like this, if based in reality, could lead to the severest of penalties.
But we are feeling mellow today, and have decided to share a few tidbits. In the spirit of modern statecraft, we will just say that it is up to the viewer to determine what is real and what is intended as disinformation. The blades above might be a Photoshop creation. Then again they could be based in reality.
Back in December we gave you a look at what was then described as a propeller for FPB 78-1. Prudence dictates that we not give away key engineering developments, so why publish such important data when would-be competitors might be lurking? The FPB on which these props may or may not be fitted perhaps does not exist, or could be real.
Stay tuned.
Posted by Steve Dashew (March 25, 2016)
March 25th, 2016 at 12:18 pm
Iceberg prop refit. Has been on Marine traffic clocked above 13 knots (top speed classified).
March 25th, 2016 at 2:57 pm
Hi Steve: While looking sternly at my network cable, is there any reason my F5-key gets nervous because of that rather large wheel at the right edge of the photo :)?
March 25th, 2016 at 9:44 pm
This is why you should always post first…THEN open the wine… =)
March 26th, 2016 at 1:24 pm
Steve – I take it you’ll have a clockwise- and counterclockwise turning prop as spares along as well?
March 28th, 2016 at 9:25 am
Hi Chris:
We always carry a spare set of props, but usually optimized for different conditions.
March 28th, 2016 at 6:48 pm
I wish I had activated my VPN before seeing the picture that I might or might not have seen!
Thanks!
March 29th, 2016 at 2:49 am
As the December post says, ” As long as we maintain our upwind ability, maybe we are better off giving away a little efficiency here, in return for a larger gain where we operate most of the time?”
That makes sense to me and it seems to me to be better to prop for the average conditions based on how a person actually uses the vessel taken from the logs, rather than how we think might.
As for military stuff, we used Agouti air in the 70s when I was in the RN to reduce noise from cavitation, which makes me wonder what we can learn from monitoring propeller noise.
One question. To date what do you think the maximum prop efficiency, at cruise spread, is that you have achieved. 70 to 75%? Props fascinate me in particular because my small sailing yacht is electric propulsion and the serial efficiency of the drive train and prop is key to range. My drivetrain is around 82% efficient and my prop around 60 to 65% it seems. The biggest gain is to be had from optimising the prop which must be the same for FPBs. On my little boat as the prime mover is wind my electric motor is a sailing auxiliary and my main aim is max efficiency at my cruise of 4 knots in calm conditions, as that is what it does most of the time. It’ll be interesting to know what the FPB criteria are.
March 30th, 2016 at 9:49 am
Hi John:
Can you give us more details on the “Agouti air” to which you refer? As to propulsion efficiency, we are no where near your numbers at cruise. By the time you deduct for the thrust bearings/CVs (if they are installed), packing glands, cutlass bearing(s), tranny, and then the prop itself, the very best a cruising yacht can expect is maybe 60%. And we don’t think we are at this point…yet.
April 1st, 2016 at 12:08 pm
Hi Steve,
It was more about noise from cavitation or at least that is what we were told back in the day. It is described here now declassified:
http://www.neely-chaulk.com/narciki/Agouti_Noise_Reduction
As for efficiency, I’m just a low power electric propulsion sail auxiliary and of course an ICE can’t get close to that but I’m assuming (like electric) the largest overall efficiency gain is to be had from the propeller design and in my fair weather coastal sailing I just want the best at cruise with a little in reserve.
John R.
March 29th, 2016 at 8:53 am
Steve, if you reduce the surface area of the props for best cruising efficiency at maybe 25% of the maximum engine power available, what effect does this lack of surface area have on top end speed with full throttle?
March 30th, 2016 at 9:44 am
Howdy Geoff:
The answer to that is subject to a lot of conjecture. We are in the process of various propeller experiments on different FPBs, and it will be at least six months before we have a real world take on the results.
March 31st, 2016 at 10:58 pm
We had the Prairie-Masker system on the Spruance class destroyer in the 80’s. A nice Wiki write up is available. I don’t know how effective it was, as my job was to shoot at stuff on land or in the air. In the propeller, it was described to me as filling and delaying the collapse of the bubble on the trailing edge of the blade. I wonder if varying the air pressure would effectively increase or decrease the chord length of the blade?
April 8th, 2016 at 6:56 am
Steve,
When do (builder and owner)sea trials begin for FPB 78 #01?
Lurkers and prospective owners are becoming scratchy.
Mark
April 13th, 2016 at 5:38 am
Howdy Mark:
Mid-June is when the fun begins.