Twin Engine and Potential Prop Damage

Ok, going to survey Friday on a sundeck trawler with twins. My question is, with exposed props and shafts, how vulnerable are they to damage? I travel at 6-7 knots. At that speed how much damage would a soft grounding cause in mud or sand. What about a floating or semi-submerged object such a log? Am I to treat them (the props) as if they were eggs or are they more durable then that? Thanks!

Depends on the keel or lack of a keel. Our twin prop sundeck has a keel that extends about 5” below the props. We were coming up the Hudson in May two years ago and there was a lot of visible and also submerged debris. We hit things that we never saw 15 times or more just on the Hudson. I assumed that our props would be toast. However when we pulled the boat for the winter there was absolutely no damage. Have not grounded the boat yet so I cannot speak to that question. Good luck with the survey.

The boat is a Heritage East Nova 42. She does have a keel, but not sure how deep it is in relation to the props. Will find out on the survey haul out. As a side note, it'll probably be raining during survey so I'll get a great opportunity to check for the dreaded Taiwanese leaks....

Yes there are leaks, but not just on Taiwan boats. My boats worst leaks were from President (also made in Taiwan) trying to put a straight aluminum window on a curved cabin side. I took the 4 long narrow almost useless windows out and fiberglassed the openings closed. I added 3 10 X 10 vent hatches to make up for the lost light and get a lot more ventilation. Anyway good luck, hope the survey goes well.

Greetings,
Mr. S. Well, over the last 30+ years I've hard grounded, soft grounded and hit "stuff"?? and only had to have props attended to on one occasion and that was in a single engine Marine Trader.



Just know where you're going or want to go, be watchful of your surroundings, the bottom (if it is visible), your charts and your depth sounder. One more thing...don't be hesitant to contact locals (towing companies, marinas etc.) to ask for directions.

At 6 knots, you've got a bit of "forgiveness" when it comes to hitting submerged debris. A grounding in soft mud often does little damage if you are moving slowly.

A hard grounding or touching rocks is another matter.

We carry a spare set of props with us. In 30 years of boating with planing hull boats, I've yet to damage any props. But the Chesapeake is very forgiving with mostly muddy bottoms. I've churned up some mud on occasion, and one time (in a previous boat) plowed through some mud when I felt the props "thump/thump/thump repeatedly as the blades churned through the mud. But I was at idle and no damage was done.

Fortunately, I'm single-engined with a protective keel and haven't had a problem. My two-prop friends have different stories, however.

Each grounding is different. I have a friend with an Island Gypsy 32. Full keel and sand shoe protecting the prop and rudder.

Last year, he had a soft grounding on a sandbar. He was able to back off on his own. When he hauled in the fall, he found damage to the keel. because the boat sat in the water for months after the grounding, water wicked into the keel. The repair involved a substantial removal of keel material and re-glassing.

Whenever there is a grounding, it is worth it to consider a short haul for inspection. Fully protected gear doesn't protect you from all possible conditions.

Hit something hard with a starboard prop at idling speeds, it rolled the edge of a Michigan Bronze 22 by 20 3 blade prop. And I have a 2 foot keel back there.
We got into shallow water very quick.

I straightened edge out on a haul, but something is wrong still, have a vibration now when engine is revved up. So it will have to be looked at on next haul, either the 1 3/8 shaft is bent after the cutlass or the prop is bent or might be a worn cutlass bearing.

I have an even .004 or less all the way around and while turning the prop, coupler to trans clearance. So it seems to me the shaft is ok, but if it was bent after the cutlass, that would not be apparent checking the clearances.

If you treat them like glass you have a better shot at a long and happy life.

High Wire said:

If you treat them like glass you have a better shot at a long and happy life.

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That's kinda how I've treated my boats over the years. Never really pushed them or took chances. Our last boat was a Lagoon 410 catamaran in which we lived on for 5 years. In over 12,000 miles on the ICW, did we ever find the bottom? Of course we did. But I knew the trouble spots and approached with caution. Of the times we have grounded, it was soft and we backed off. So far in 40 years of boating I've never damaged a boat. Knock on wood. That's reason for the question. Just how conservative I would have to be with exposed props. Sounds like if I used the same diligence we should be fine. Hope this survey goes well. The last one went all of 1-1/2 hours before I called it off and rejected the boat. I guess at this point I should introduce myself and will do so at the Welcome Mat. Thanks for all the responses and reassurances!

I've had twins most of my life and never damaged anything. I've run up on sand/mud banks, etc., but not at speed. My first big boat was in 1961. Plus running boats in the military, ships, etc.

If you're exploring an unknown area with poor charts, you go slow. I do it in Alaska where some surveys are 100 years old. But carefully.

We have twins, no prop damage. Yet, I know people who seem to get repairs every week they boat.

Every boat that goes cruising will probably run aground in time.

The single engine keel protected hull offers the best chance of not being damaged by the grounding.

IF the boats engine power requirement is such that twins are needed there is no choice.

Few displacement boats will require twins to go cruising.

No props are immune from damage however options

Some designs are more vulnerable that others. The Great Harbours are a twin engine trawlers that do not have exposed shafts and props. The N-37 is a twin engine trawler with skeg protected props and rudders. It can sit on the bottom without damaging the props/short shaft or rudders. No keel as designed similar to tows. 40,000+ miles no prop damage.

Sidclark said:

The boat is a Heritage East Nova 42. She does have a keel, but not sure how deep it is in relation to the props. Will find out on the survey haul out. As a side note, it'll probably be raining during survey so I'll get a great opportunity to check for the dreaded Taiwanese leaks....

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Sid,
I have the same boat vintage 1984. PM me if you want to talk.
We actually bumped a few times on a sand bar Saturday in Lake Michigan that we were at last week with room to spare.
I dove the running gear and no issues. The keel is very solid.



Some of the other areas are more of an issue.

The port lights in the aft cabin leak like a sieve. That's my next project.


Inspect the waste system thoroughly (don't ask why. LOL)


I love mine, but I have a ton of time invested into her.

We just replaced 11 portholes in our boat. It was a lot of work but they are beautiful.

Comodave said:

We just replaced 11 portholes in our boat. It was a lot of work but they are beautiful.

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I do not want to side track this thread, but where did you source the replacements?

The original portholes were PYHI. Beckson bought PYHI in the 90s I think. They closed PYHI up and there are no parts for them. Even if I could have gotten parts I would have replaced them since the boat is 31 years old and the plastic frames were at the end of life. I went with New Found Metals portholes. I had to cut out the holes and make them a bit larger and fill the old drain cutouts with thickened epoxy. Overall it was a challenging project but not difficult. It just seemed neverending because there were 11 portholes to do. Packing the buytl tape was the worst part.

On a twin engine boat, the greatest likelihood of damaging a prop is not from a "standard" grounding (getting into water that it too shallow); it is from the prop hitting the side rocks/rip-rap of a narrow stretch when having to get out of the narrow channel when passing another boat. The Rock Pile on the ICW is a great example. Rocks abound just outside of the channel, and your props are hanging out there just waiting to hit something - there's no protection from the keel.

I've always done more damage by hitting objects floating under it near the surface.. Palm fronds here in South Florida are the biggest culprits. They lay flat on the surface and are hard to see. Those and the other assorted flotsum and jetsum floating ijn big city waterways!

There are bazillions of twin screw boats out there, seldom with prop damage. It could just be a math issue...

-Chris

Beings an assistance tow guy and living in a busy working marina....just about every day a twin screw boat gets hauled and blocked for prop, rudder, shaft or strut damage.

Just hauled a commercial tug with a strut ripped off from picking up old line laying on the bottom (not uncommon).

Few keel boats with protected props and shafts though....so the basics of design match the basics of realities in my world.

But, like all damage and emergencies, apply them to a specific crew and the numbers game becomes less accurate.

ranger42c said:

There are bazillions of twin screw boats out there, seldom with prop damage. It could just be a math issue...

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OK. There are bazillions of twin screw boats out there, many without prop damage. It could just be a math issue...



-Chris

I agree that you will see many more twins in for damage because of sheer numbers, but the singles are almost non existent.

You would think you would see more.