Sail drive raw water intakes

[Captain Larry-DH]

I’ve pondered a bit on the subject, and come to the conclusion that saildrives are an excellent and stupid design. The way I see it, they evolved from inboard/outboard drives, which evolved from outboard motor drive units. The outboard drive that can be raised out of the water when the boat is at-rest is not so stupid.
Saildrives are stupid. They do eliminate shaft vibrations, and provide a drop-in solution for boat builders to reduce the labor and complexity of shaft seals and alignment. But the fully submerged aluminum drive passes a huge maintenance and corrosion nightmare to the boat owners to contend with, not to mention using a big rubber membrane seal that relies solely upon adhesive bonds to keep the boat from sinking.
Another issue with saildrives is that they take in raw water for engine cooling through a combination of slots and holes in the drive leg. The makers will boast that this is a feature - because it eliminates the need for another thru-hull. It also pulls water in down lower so there’s no aeration to interfere with water flow, nor is it likely to suck in surface weed. However if you suck in a fish, or weeds, or plastic bags, or anything else that obstructs the interior of the mysterious cavernous interior of the saildrive passageways, there is no easy way to clear the intake. You can’t poke a hanger or cable down the hose to clear it from inside the boat, and if you dive on it the complete pathway isn’t accessible from underneath in the water.
Regardless, I admit that I signed off on this stupidity and purchased a catamaran last year that has TWO of these design abominations installed …
So I realize I have to accept what it is, but I have done some proactive things to avoid emergency haul out from a blocked intake while cruising.
One thing I did was to drill four 1/16” holes around the main intake at the bottom of the drive, and threaded it with stainless seizing wire to block larger objects from getting sucked in. (Picture below). After I took the picture I coated it with spray-on aluminum primer and bottom paint.
The other thing I did was to add an extra thru-hull in the engine compartment that will not be used except in emergency, if one of my saildrives gets blocked. The intake hose would be diverted from the top of the saildrive to the spare thru-hull.
Of course, I posted this with the understanding that some of you will think this is stupid, that the seizing wire will induce galvanic issues and the extra through hulls create another potential risk. Have at it. I think those issues are smaller than the ones I’m trying to avoid.

 

[Ralph Johnstone]

What a $%#^@* nightmare. I hade no idea these things had so many problems looking for an inopportune moment to happen.

The other thing I did was to add an extra thru-hull in the engine compartment that will not be used except in emergency, if one of my saildrives gets blocked.

I'm just not comfortable with the SS wire through the aluminum body. Seeing as you now have a sensible, reliable thru-hull, why not use it full time and remove the SS wire and forget it. Another source of possible galvanic corrosion removed.

And for a plug-proof cooling water thru-hull :

 

[Mark Maulden]

In your third pic, what are the slots above the zinc for?

 

[Captain Larry-DH]

In your third pic, what are the slots above the zinc for?

Additional intake. I don’t know what the passages inside the drive look like.

 

[Captain Larry-DH]

Seeing as you now have a sensible, reliable thru-hull, why not use it full time

Because I don’t know (until I try it) if propwash, or large seas (if the sterns come far out of the water) will disrupt the intake flow. (There would have been major complications to installing thru hulls deeper and forward of the engines.).
I plan to try the new intakes as-installed. It’s not difficult to move the hose over when I want to . The new thru-hulls are pretty deep. Meanwhile they’re Plan B.

 

[Ralph Johnstone]

Just out of curiosity for all the enquiring minds out there, install a piece of clear, heavy (stands up to a small vacuum) vinyl hose temporarily in the cooling water line to the engine to see what the engine is sucking on when under power. Do it in various seas. Might be a foamy or could be a solid stream of water.

It'll either put your mind at ease OR give you something to lose sleep over night.

 

[Captain Larry-DH]

Just out of curiosity for all the enquiring minds out there, install a piece of clear, heavy (stands up to a small vacuum) vinyl hose temporarily in the cooling water line to the engine to see what the engine is sucking on when under power. Do it in various seas. Might be a foamy or could be a solid stream of water.

It'll either put your mind at ease OR give you something to lose sleep over night.

Will try them when I splash the boat in a week or two. I expect it will work just fine.
But I looked again at a galvanic scale table and decided to remove the wire intake “screens”. I’ll try using some heavy monofilament fishing line instead.

 

[Kings Gambit]

Whenever my saildrive sucks up kelp debris/seaweed I back flush it from the inside of the boat. I do this by connecting via a hose a manual pump to the water in-take hose leading to the water strainer. Strainer is mounted above the water line so I don’t get flooding when I remove the hose from the strainer to attach to the pump hose. Here’s the cool part. The engine access door giving me access to the strainer is in the aft head. It’s a short, quick hose connection to use the manual pump on the toilet to push water back out of (i.e., back flush) the saildrive. Of course, you have to stop the engine while engaged with it. It may or may not clear (back flush) a plastic bag

 

[Captain Larry-DH]

Of course, you have to stop the engine while engaged with it.

Good solution if it works. You need to stop anyway if the intake is blocked.

 

[Kings Gambit]

Good solution if it works. You need to stop anyway if the intake is blocked.

Well, actually. We’re sometimes able to crip along on slower speed if necessary. The blockage is not always total; some water getting through, etc. Judge by the amount of steam coming out the exhaust.

 

[Captain Larry-DH]

I just watched the latest video episode from Sailing Parlay Revival. If interested, watch this and in-particular the segment starting from 13:50. It illustrates most of what I wrote above about stupidly designed saildives, and shows how internal clogs can foul the internal passages for engine cooling water, and require an emergency haul-out and major disassembly to remedy.
I’ve had some doubts about whether I was fixing something that doesn’t need fixing. Any slight self-doubts have now dissipated about whether it was wise to add dedicated seacocks.

 

[Mark Maulden]

I’m curious..What Model saildrive is this?

 

[Captain Larry-DH]

It doesn’t matter- Yanmar and Volvo Penta - essentially all the same general design.

 

[Mark Maulden]

I think all made by ZF..I don’t think that hole in the bottom is a water intake..

 

[Captain Larry-DH]

I think all made by ZF..I don’t think that hole in the bottom is a water intake..

Not sure about OEM. There are differences between different models and makes but they’re all fundamentally similar. The hole in the bottom is definitely an intake.

 

[Captain Larry-DH]

Last week I was seeing white smoke/steam from one of my engines. Various spot checks on the engine block and exhaust riser with the IR thermometer showed it was operating within normal ranges, only a few degrees hotter than the engine on the other side, and the exhaust seemed to have good flow, yet the water coming from the exhaust was MUCH hotter than the other engine’s exhaust flow. I shut down that engine and didn’t use it again until a few days later when I needed it to maneuver into a marina service bay.
The mechanic I consulted ran various checks under load at the dock, and suggested I move the intake hose to the backup sea cock as a test. It resolved the issue.
With a partially obstructed saildive intake, having the ability to run the boat without it saved me an undetermined amount of yard labor to troubleshoot an engine cooling issue that wasn’t in the engine. I can only imagine the bills trying to find the blockage in the engine - and cooling system disassembly before the last-resort of hauling out.
I can correct the blockage at my home marina when I do scheduled haulout and other maintenance. No disrupted trip nor urgent haul-outs in unfamiliar places.
I highly recommend adding an intake at your next scheduled haul out.
PS- the mechanic said he’s never seen anyone do that before, but it’s a good idea. He said he’s going to start recommending it to saildrive and I/O powerboat owners

 

[Hello Below]

Last week I was seeing white smoke/steam from one of my engines. Various spot checks on the engine block and exhaust riser with the IR thermometer showed it was operating within normal ranges, only a few degrees hotter than the engine on the other side, and the exhaust seemed to have good flow, yet the water coming from the exhaust was MUCH hotter than the other engine’s exhaust flow.

I wonder if you had reduced the engine RPMs would the engine may have been happy with the reduced water flow? A moot point since you can now bypass the obstructed source.
An alternate raw water intake source has been on my mind since the first time I fished out a muscle shell wedged at the strainer entrance. I have been hesitant to add an additional thru hull/sea cock for something I may never use. However, I did recently purchased the pieces to extend a 3/4” hose from the toilet intake to the raw water strainer. One of these days I must trial it to see if it will actually meet the needs of the engine.

 

[Captain Larry-DH]

I wonder if you had reduced the engine RPMs would the engine may have been happy with the reduced water flow? A moot point since you can now bypass the obstructed source.

I tried reducing RPM and it did cool down the exhaust water somewhat but it still put out (less) steam. The other engine was my reference benchmark and the difference was very noticeable.