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Old 02-19-2019, 10:07 AM   #1
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Prop Pockets/Tunnel Drives

These seem like a really good idea as you get shallower draft and reduced shaft angle so they're more efficient..... BUT.....since they aren't that common, there must be a downside. Can someone explain it to me ? Thanks in advance.
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Old 02-19-2019, 10:15 AM   #2
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IMO they reduce prop walk so that maneuvering is only done by spin not the side thrust possible with an open prop.
On faster boats they seem to increase wake probably because of reduced lift aft.
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Old 02-19-2019, 10:18 AM   #3
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We had them on 3 boats and there were pros and cons....
Good
- "V" struts very stable on ours
- depth clearance
- some prop protection
- better prop angle

Poor
- slightly less efficient
- inability to maneuver as well
- cannot walk the boats stern well at all

I would get one again but I would research the particular vessel and be aware of the limitations.
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Old 02-19-2019, 10:51 AM   #4
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I had them on two of my last boats: Mainship 34T and Pilot 34. The 34T had no negative issues that I can think of and all of the positives noted above. The Pilot 34 had prop clearance and/or cavitation issues but wasn't so bad.


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Old 02-19-2019, 10:55 AM   #5
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on the twins I have operated with them...you definitely needed wheel with split shifting to get a decent turn.


and the reduced dynamic lift on some ay affect top speeds but I had nothing to really compare them to.
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Old 02-19-2019, 11:17 AM   #6
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Greetings,
Mr. B. We have a 23' Penn Yan single with "Tunnel Drive". All of the above comments plus, you're limited to a maximum prop size for appropriate clearance between prop tips and hull. Shallow running is THE plus IMO.


On plane, I'm drawing about 1'.


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Old 02-19-2019, 01:15 PM   #7
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Thanks everyone!
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Old 02-19-2019, 01:20 PM   #8
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I built a tunnel into my single. No complaints but prop noise is pretty bad from 22kts up to top end of 29-30kts. I just cruise around 18-20kts and noise is not too bad.

Some tunnels are short and water has to make an abrupt change of direction, this increases noise and eats hp. Mine starts gently under the engine, and tunnel is over 10' long.
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Old 02-19-2019, 01:24 PM   #9
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I've never spent much time operating boats with tunnels/pockets. But most of the pros and cons mentioned above are consistent with what I've heard from owner/operators. I'll add one more, depending on construction method they can be more expensive due to increased man hours.
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Old 02-19-2019, 01:41 PM   #10
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An alternative to the tunnel is the Seabright or Higgins style hull. They used a box keel to both lower the engine for a straighter shaft run to the prop and to protect the prop. They are also considered a shoal draft hull. This type hull is not limited to small boats. Higgins used the design on landing craft and other large boats during WWII. Atkins has a lot of stock plans for boats that use this type hull.


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Old 02-19-2019, 02:42 PM   #11
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Greetings,
Mr. B. We have a 23' Penn Yan single with "Tunnel Drive". All of the above comments plus, you're limited to a maximum prop size for appropriate clearance between prop tips and hull. Shallow running is THE plus IMO.


On plane, I'm drawing about 1'.


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I once had a Penn Yan tunnel drive, on Keuka Lake no less.
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Old 02-19-2019, 04:48 PM   #12
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IMO they reduce prop walk so that maneuvering is only done by spin not the side thrust possible with an open prop.
I don't experience that same problem with my tunnels as the props are not mounted close to the top of the tunnel which reduces the constant pounding against the hull that water flying off the blades makes. Also, prop walk is good, for the same reason.

I had a 35' Tiara years ago that had tunnels and was a little noisey & needed some help when trying to walk the hull with the prop. I would still buy another "tunneled" hull but only after satisfying myself that the prop blades are sufficiently distant from the hull bottom.
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Old 02-19-2019, 04:54 PM   #13
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I don't experience that same problem with my tunnels as the props are not mounted close to the top of the tunnel which reduces the constant pounding against the hull that water flying off the blades makes. Also, prop walk is good, for the same reason.

I had a 35' Tiara years ago that had tunnels and was a little noisey & needed some help when trying to walk the hull with the prop. I would still buy another "tunneled" hull but only after satisfying myself that the prop blades are sufficiently distant from the hull bottom.
Yes - very shallow tunnels , barely there at all.
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Old 02-19-2019, 05:04 PM   #14
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My Phoenix 29 had tunnels which had a nasty habit of sucking air on a moderate beam sea. You had to slow down to about 11 knots from a normal 17 cruise in 3-5's. Other than that the plus side was the engines were in the cockpit, out of the cabin. The cabin was much larger than a comparable Bertram 28. Never had any trouble docking as long as both engines were running.
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Old 02-20-2019, 05:37 PM   #15
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Prop Pockets/Tunnel Drives

Starting in 2003 the Mainship 30 Pilot ll added a tunnel with a 5 blade prop. Itís nice to bring her in close to shore when anchoring at the beach. Having the tunnel means it has as a fairly short keel so the stern tends to walk a bit in rolling seas. If there is any extra noise due to the blade distance in the tunnel I donít know as I have nothing to compare it to though I have read the 5 blade minimizes noise and vibration. The original Pilot had a 4 blade prop with a full keel and no tunnel that was known to suffer from vibration due to the distance of the prop to the hull.
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Old 02-21-2019, 09:46 AM   #16
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Adding blades typically reduces vibration by increasing balance, but at the expense of efficiency and drag. The downside of tunnels, is limiting the size of the prop you can fit with an increased need to manage the flow over the prop since you have effects from the tunnel itself. Adding a blade increases surface area without having to further increase the diameter. You can then play with cupping and rake to get the tight flow around the prop until you kill any cavitation picked up along the way.
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Old 02-21-2019, 09:52 AM   #17
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I once had a Penn Yan tunnel drive, on Keuka Lake no less.


Greetings earthlings. I had a 23 once too. One of the deepest tunnels and I can confidently say that your 23 draws closer to 14 inches, having used that extra couple inches a time or two.

The Penn Yan was a much harder boat to get the prop right than most of the shallower tunnels out there. It does sort of sing when you get it right though.

I once went clean over a log hiding in the chop at around 24 kts without so much as a nick on the prop. Fun boat, but finicky.
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Old 02-21-2019, 09:58 AM   #18
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My 3870 has prop pockets...what I would consider "real" pockets. Sideways motion requires more throttle, but I also have a full length keel. For my size boat, and the horsepower, I think my props are too small. The efficiency (or is it the slip calc?) is something like 60%? But the tunnels don't allow anymore diameter than 20" which is a pretty tiny prop once you get it off the boat. Cummins repowers require something like a 26" pitch since the you can't bump the diameter.

Just my observation.
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Old 02-21-2019, 10:10 AM   #19
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My 3870 has prop pockets...what I would consider "real" pockets. Sideways motion requires more throttle, but I also have a full length keel. For my size boat, and the horsepower, I think my props are too small. The efficiency (or is it the slip calc?) is something like 60%? But the tunnels don't allow anymore diameter than 20" which is a pretty tiny prop once you get it off the boat. Cummins repowers require something like a 26" pitch since the you can't bump the diameter.

Just my observation.
A number of the 38's with 175 diesels had 21" diameter props.
But they really do not need to be more than 20" diameter - I would guess that your pitch is in the 21" range as well with 4 blade Nibral props.
Slip is not efficiency but I think your slip calculation is higher than it really is - take into account your actual prop pitch (scanned) your verified rpm and trans ratios at the highest rpm.
You will never get a good slip calculation when the boat is at or near transition speeds.
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Old 02-21-2019, 11:23 AM   #20
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A number of the 38's with 175 diesels had 21" diameter props.
But they really do not need to be more than 20" diameter - I would guess that your pitch is in the 21" range as well with 4 blade Nibral props.
Slip is not efficiency but I think your slip calculation is higher than it really is - take into account your actual prop pitch (scanned) your verified rpm and trans ratios at the highest rpm.
You will never get a good slip calculation when the boat is at or near transition speeds.
Ah, didn't know that! Yes, they were 20x22 nakashima that I had repitched to 20x20. She didn't like getting up on plane. I live aboard and have a lot of crap with me. I also anticipate getting heavier :/

I had my numbers backwards...calc'd slip is like 40%. That's 2800 rpm @ 18 mph. Assuming my tachs are good enough. Starboard engine needs a tune or adjustment. I'll be on the BOF for that though
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