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Discussion Starter #1
Hi everybody;

I'm boat shopping and was looking at a 2001 jeanneau so 37 that said the shaft is direct drive.
I'm wondering if there are transmissions or drives that I should be careful with?
Does direct drive mean the prop turns at the same speed as the engine?
Thanks for any info.

Jim.
 

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Tartan 27' owner
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Does direct drive mean the prop turns at the same speed as the engine?
Yes. 1:1.

On larger, heavier boats it is more common to find a reduction gear of, say 2.x:1. With the reduction gear a larger prop can be spun giving more thrust.

That said, there is nothing wrong with direct drive transmissions if the prop can be sized to efficiently move the boat with some power left over for when needed. You would need to do a sea trial under power to asses how the drive train performs.
 

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Courtney the Dancer
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I'd be surprised on that new of a boat that it is truly a direct drive (on older boats direct drive meant no transmission at all, you shut the engine down, pulled a lever that moved the camshaft and then started the engine again, rotating the opposite direction). I would guess they are just meaning there is no V-drive, but you never know.
 

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Dirt Free
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I'd be surprised on that new of a boat that it is truly a direct drive (on older boats direct drive meant no transmission at all, you shut the engine down, pulled a lever that moved the camshaft and then started the engine again, rotating the opposite direction). I would guess they are just meaning there is no V-drive, but you never know.
jrd22 has it right and that model does not have a direct drive. You can't always trust that the broker who wrote the ad' actually knows what he is talking about.
 

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jrd22 has it right and that model does not have a direct drive. You can't always trust that the broker who wrote the ad' actually knows what he is talking about.
Sorry to disagree with you ( again :) ) but in this case I think the broker who wrote the ad' actually does know what he is talking about.

I was pretty sure but I checked, just in case....



 

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Dirt Free
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Sorry to disagree with you ( again :) ) but in this case I think the broker who wrote the ad' actually does know what he is talking about.

Sorry, I don't understand what you mean. Your diagram clearly shows a gear reduction unit and all your photo shows is a shaft drive which does not mean it is a direct drive.

"direct drive" is a very specific type of drive without a gear reduction unit. No production pleasure craft is built that way. For more information on what is is, try here Tugboat Society of America
 

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Sorry, I don't understand what you mean. Your diagram clearly shows a gear reduction unit and all your photo shows is a shaft drive which does not mean it is a direct drive.
Thanks all, I think I get it. So how about more info on saildrives and v-drives.

Jim
 

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V-Drives often make it hard to service the engine as the service points - waterpump, alternator, etc - are hard to reach/

Saildrives are basically like the lower drive unit of an outboard coupled to an engine mounted inside the boat. Lots of aluminum in the water and if strict maintenance is not adhered to there can be serious corrosion problems. Easy for the builder though.

Conventional shaft drive like the diagram above is the simplest most reliable system.
 

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Here's a V drive transmission:



Ignore the arrow in the right hand image, intended for another discussion.

The engine sits 'backwards' in the boat, the gearbox at the forward end, and the shaft exits the V drive under the engine.. Many such installs put the stuffing box and its gland/seal directly under the oil pan in an almost impossible-to-access place. A most compelling reason to install a dripless seal, as a spraying leaky gland can do nothing good for the pan.
 
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Discussion Starter #11
So v-drives are similar to a transfer case in a 4wheel drive truck? I wouldn't think that's efficient for long periods or high rpm.
Thanks again.

Jim.

GO HAWKS.
 

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Discussion Starter #12
Here's a V drive transmission:



Ignore the arrow in the right hand image, intended for another discussion.

The engine sits 'backwards' in the boat, the gearbox at the forward end, and the shaft exits the V drive under the engine.. Many such installs put the stuffing box and its gland/seal directly under the oil pan in an almost impossible-to-access place. A most compelling reason to install a dripless seal, as a spraying leaky gland can do nothing good for the pan.
So 180 from the point of the arrow is the gearbox output?
 

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So v-drives are similar to a transfer case in a 4wheel drive truck? I wouldn't think that's efficient for long periods or high rpm.
Thanks again.

So 180 from the point of the arrow is the gearbox output?
Yes... the coupling you see in the photo is under the arrow in the diagram, the shaft is shown exiting at an appropriate angle for the hull, carries on through the shaft log and strut to the prop.

It's really no 'less efficient', though there's an 'extra' gear or two involved. It's not an unusual practice. It gets the engine more aft vis a vis the prop location, often getting it further out of the accommodation space and opening up other options there.

With today's proliferation of aft cabins, though, I think they are used less often now as the engine can't sit so far under the cockpit anymore. From a service/convenience point of view a straight drive (better term than 'direct) is probably better all round.
 
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Many different configurations. I think the terminology should help communication. Firstly ,straight drive (the usual) V drive (engine in backwards) and Z drive (like sail drive and azimuth) The reduction ratio (engine RpM compared to prop RPM) is a function of the transmission . No trannie? direct reversing engine or fully adjustable prop or no reverse .Each system has pros and cons in different applications Improvement in that paddle wheels are not common any more. I just noticed Faster beat me to straight/ direct bit.
 

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Sorry to disagree with you ( again :) ) but in this case I think the broker who wrote the ad' actually does know what he is talking about.

Sorry, I don't understand what you mean. Your diagram clearly shows a gear reduction unit and all your photo shows is a shaft drive which does not mean it is a direct drive.

"direct drive" is a very specific type of drive without a gear reduction unit. No production pleasure craft is built that way. For more information on what is is, try here Tugboat Society of America
Sorry, I didn't intend to confuse the issue. jrd22 had said, "I would guess they are just meaning there is no V-drive", which is what I thought you were referring to. I have often seen "direct drive" in broker listings - I think it a check box on their automated system and as indicated, is commonly used to differentiate conventional from V or saildrive.

I completely understand what a traditional direct drive is. I was on a canal boat earlier this year and we double-locked with a boat that had a gorgeous open-case engine with direct drive. It used a sliding cone arrangement that engaged/disengage the shaft for forward drive and engaged a set of side gears (probable not the correct terminology) to reverse the shaft direction to go astern. I don't recall who made it but it was from an old fishing boat and apparently it was not uncommon for the cone to jam after several days fishing and a little "persuasion" might be required to disengage the drive.
 

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"direct drive" is a very specific type of drive without a gear reduction unit. No production pleasure craft is built that way. For more information on what is is, try here Tugboat Society of America


not any more... sabb had/has a nice direct drive box with pitchable props. I like them better than a transmission
 

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.... I was on a canal boat earlier this year and we double-locked with a boat that had a gorgeous open-case engine with direct drive. It used a sliding cone arrangement that engaged/disengage the shaft for forward drive and engaged a set of side gears (probable not the correct terminology) to reverse the shaft direction to go astern.....
One of the places where I used to work had no road access and we commuted to work on a small ferry.. we walked on but it was big enough for a couple of semitrailer trucks.

When it went in for refit and inspection we used an old harbour passenger ferry with diesel direct drive. NO gearbox of any kind.. they reversed engine rotation for reverse. I believe many deep sea freighters still do this...
 

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I believe many deep sea freighters still do this...
Correct. The most common propulsion plant for large ocean going ships is the slow speed diesel. Engine is coupled directly to the propulsion shaft and propeller turns at the engine RPM. Reversing is done by stopping the engine, shifting the camshaft, and restarting in reverse. Some of the largest are inline 12-cylinder engines with a bore of around 1 meter and run below 100 RPM.
 

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Back in the day, the working schooners had direct reversing engines coupled " straight" to the prop shaft. When unloading dockside, engineer would keep an eye on the pilings and stop/restart and idle the other way until the vessel began moving again. Usually make and break before semidiesel and full diesels became popular and affordable. Usually air start but with good timing you can bounce on compassion to go backup. Pro- mechanically simple ,con -really hard on generator brushes .So the larger vessels had designated gen sets. As engines began operating at higher RPM ,reducing transmissions became necessary with the bonus of a neutral/ reverse, usually using a planetary (bands and disks)
 
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