prop shaft leaks under tow - SailNet Community
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post #1 of 17 Old 07-19-2007 Thread Starter
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prop shaft leaks under tow

My volvo md6a on my Islander 30 overheated and terminaly failed during a recent trip to the San Juan islands. I had to call vessel assist for a tow of about 15 miles. The tow boats speed was about 8 knots over the ground. After about 3 hours I noticed water over the floorboards in the main cabin. The engine compartment does not have a pump. All in all I got about thirty to fifty gallons of water aboard. As soon as the tow ended so did the leak. Now it does not leak a drop. The prop shaft is the only place it could have gotten in.
What is my problem and how do I fix it ????
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post #2 of 17 Old 07-19-2007
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Jbrock-

Eight knots is probably above the hull speed of your boat, and as such could have caused some problems.

Generally, if a keelboat is being towed, the towing vessel should take care not to exceed the hull speed limit of the keelboat, or damage can result.

You may have a thru-hull near the waterline or a vent near the waterline that could have become immersed during the tow. I wouldn't assume that the water came in the propshaft. I would recommend you check the thru-hull and vent fittings to see if you possibly lost a hose during the tow or something similar.

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post #3 of 17 Old 07-19-2007
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Why is there no pump in the engine compartment? Not that it was the cause but did you have you tranny in gear?
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post #4 of 17 Old 07-19-2007
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Actually I am going to suggest just the opposite. Your stern tube bearing and seal are designed to load and seal under forward thrust from the prop. They are not designed for a heavy drag on the prop. It shouldn't leak at only eight knots but....

The Dog's points are quite valid as well. Towing at that speed would cause more squat at the stern than normal, and immerse discharges normally above the waterline. I'd start there with a garden hose and a spotter in the engine room.

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post #5 of 17 Old 07-19-2007 Thread Starter
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No leaking thru hull no loose hose the prop shaft is the source of the leak but why wold it leak so bad under tow and none when stopped ?
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post #6 of 17 Old 07-20-2007
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Once more (patiently),

You need to examine those, if any, thru hulls above the normal waterline by use of a garden hose, pressurizing them to check for water ingress. Also, the thru hulls you have aft do not, with the boat at rest, have the same head pressure against them as they would under tow. Examine discharges in particular. As to the prop shaft, re-read my above post.
Good luck.

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post #7 of 17 Old 07-20-2007
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That was a very nice reply...
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post #8 of 17 Old 07-20-2007
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It is known as "Strange Nautical Phenomena" check all the thru-hulls just to be sure and don't be towed that fast again. It also puts too much strain on your deck fittings.
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post #9 of 17 Old 07-20-2007 Thread Starter
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Thanks for the suggestions. The drag on the prop is what I believe caused the seal to leak under tow. However, I get up to 7 to 7 1/2 knots under sail am I not still getting the same drag on the prop under sail ? It so far does not leak under sail but I am concerned about it. The tow operator wanted the engine transmission in neutral so I imagine the prop would be spinning faster than normal. Yes I do believe he was towing too fast for my boat. How do I go about inspecting the seal to be sure this does not happen again ? I don't believe it should have leaked at 8 knots.
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post #10 of 17 Old 07-20-2007
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"However, I get up to 7 to 7 1/2 knots under sail am I not still getting the same drag on the prop under sail ?"

The attitude of the boat (bow up/down...not whether it was having a bad day) may have been significantly different when being towed as opposed to under sail.

Just a suggestion as I don't know - I wasn't there.
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