Pros and Cons of jackshaft power.

[Capt Sport]

I posted this in the boat design forum and only got a few responses. Maybe I'll get more response over here on the metal boat forum. I understand the jackshaft set-up, what I'm looking for is more information on how the boat performs.

I'm interested in jackshaft power for my next boat. And, was wondering if some of the more experienced designers on here could tell me the pros and cons of powering this way.

It seems to me that a boat with jackshafted power would be much better balanced, ride better and be more efficient. But I'm sure I'm missing something, or more boats would be built this way. It can't just be that outboard boats are cheaper to build.

Just to give a little background on the boat I have in my head:

Aluminum
Center pilot house (full walk around)
24-28' lenght overall
8'6"-10' beam (dependant on length)
diesel power

What I'm looking for is a rough water boat, with extended range. That is as cheap to run as possible. Think of it as some of the Pacific north west boat designs without the great big full width house and cabin. I'm not looking for the fastest boat on the water either, cruse at 20 knots top out at about 25 knots.

Thanks in advance for you help and input,

jr

[Frosty]

Capt -- weve all seen your thread, --and if like me,dont understand your question. A jack shaft is a method of driving a particular drive by an engine.


What do you think a jack shaft is?


All threads get posted on the "same" main notice board.

[Capt Sport]

Jack Frost,

Thanks for the reply, I understand what a jackshaft is and how it installed. But what I'm really interested in is how do boats of similar design handle given jackshaft power vs I/O power vs outboard power. I would assume that all things being equal (deadrise etc) each boat will handle the same conditions differently. And, the differences in ride characteristics, efficiency etc are what I'm after. For example is one design better in rough water than another, is one more effiecient then another design and why.

Thanks again for your reply,

jr

[alan white]

It's better to ask what fore/aft engine placement does to seakeeping characteristics.
Generally, an engine down low and centered fore and aft puts more mass at the center of motion, meaning the boat yields more easily to any forces applied to its ends, lifting or yawing more quickly, "feeling" its way through more efficiently.
Normal shaft drives work better and better the further forward the engine is located due to the ever-lessening angle of each parallel part of the system.
A jackshaft would place a third parallel shaft into play for reasons not associated with boats, perhaps to power hydraulics, winch, etc..
It wouldn't be needed to place an engine in a particular location, however, since there are gear drives for doing that, usually V drives or simple parallel setups (chain, belt, or gears).
Again, a jackshaft is parallel to, but not axial to either engine or prop shaft, and it might also be termed an intermediate or transmission shaft, and hence it would serve no typical location function. In other applications it might assist in further speed reduction or allow going around corners, something most all boats don't need.

Alan

[Bob S.]

I believe the OP is asking about the characteristics of a boat equipped with engine amidships driving a transom mounted I/O unit through the use of a lengthy horizontal shaft. I believe a few production glass recreational boats are so equipped.

In my opinion, if you really buy into the I/O type of drive then the jackshaft is the way to go if for no other reason than to rid the cockpit of the lousy big engine box that all conventional I/O boats come with.

[alan white]

And this is called a jack shaft? I see. Sorry jr.----- I stand corrected.

A.

[Willallison]

There are a host of pro's and cons for any drive system, a jackshaft being no exception. But there are two main reasons why you would emply a jackshaft. The 1st, as has already mentioned, is to free up cockpit space (possibly at the expense of space further fwd...) The second is to ensure that the boats centre of gravity winds up where you want it. Depending on the boats intended service and speed, this could be anywhere from between about 50 and 65% of the waterline length (aft of the bow) for a planing hull.

[Frosty]

A jackshaft can enable you to move the engine relative to your chosen drive by lengthening or shortening it ,--therefore a jack shaft does nothing for handling more than re-distribution of weight by being able to drive from the engine from vaious distances.

Calling a boat 'jack shaft' powered is extremely confusing.

[alan white]

The gear reduction appears to be in the I/O drive. The shaft must spin at engine speed... balancing that shaft must be important---- it would be going 2-3 times the speed of a regular prop shaft, right?

A.

[Frosty]

Yes,-- Engine speed-- hence lengthy shafts are not good,---well to be avoided if possible. But at least dont need to have splines unless very long.

[Capt Sport]

My apologies for the confusion, Bob S has it correct in that I'm interested in an I/O connected to a midship mounted motor as in the photo he's posted.

The shaft does spin at engine speed just like a drive shaft for an automobile. They can be made safer by passing the jack/drive shaft thru a tube designed to keep it from flailing about in the unlikely event a u-joint fails.

Thanks for the replys,

jr

[Rusty Bucket]

I know the term "jackshaft" and I thought it already had a home. If that's a jack shaft then what's a drive shaft? I think the terminology is confusing at best. regards rusty

[TerryKing]

Originally Posted by Rusty Bucket

I know the term "jackshaft" and I thought it already had a home. If that's a jack shaft then what's a drive shaft? I think the terminology is confusing at best. regards rusty

I learned from my Grandfather, who was the "belt wizard" at the old New Departure (GM) ball bearing plant in Connecticut, that the "Jack Shaft" was the main shaft that ran down the overhead of the factory and drove all the machinery by belts that came down to each machine.

This seems to be consistent with the definition from www.thefreedictionary.com:
(Mach.) the first intermediate shaft, in a factory or mill, which receives power, through belts or gearing, from a prime mover, and transmits it, by the same means, to other intermediate shafts or to a line shaft.

So, the shaft that receives power from the Prime Mover (Engine) and transmits it to (load or loads) sounds like a reasonable definition. "Drive Shaft" is, I think, more general...

Many years ago, when I was about 12, The Old Man (AKA A.C. Gilbert Sr. of Erector Set Fame) gave an old Wisconsin-powered garden tractor to me and my friend Mark. We eventually got it to run. A.C. and Mark's father conspired to give us absolutely no help, while keeping an eye on us. It had "The Jack Shaft" which was chain driven by the engine, had a 'twin-disc' clutch, and drove the chain to the drive axle.

So the term was pretty familiar. Of course, that was about 1950

[rayk]

Drive shaft for propulsion, jack shaft is a PTO.
Jack is an assistant, for helping out around the place.
Plenty of jacks around a boat.

There is a meaning behind jack shit.

[alan white]

Yer an ancient tractor mariner, Terry. We used jack shafts on mini-bikes in the sixties to center the engine and to play with ratios (the rear sprocket was expensive and the front sprocket was part of the centrifugal clutch).
I can understand the definition of the jackshaft in a factory. It wasn't axial to either prime mover or machinary. I've got the feeling the name caught on with boats as slang, which is right and proper.
I worked out a V drive that used no gears (1:1 ratio) a few years ago. It used (on paper) a universal joint, so all power was transmitted through needle bearings. Would save, I assume, 5-10% power usually lost through gear contact. I just bought a digital camera (a first for me) and I'll have to upload a drawing of that design.

Alan