why don't boats have transmissions?

[BigB9000]

I never could understand it. I'm not talking a T56 6 speed manual trans here, I was thinking something like a 2-speed powerglide or something.

just something to drop the RPMs down to save on fuel. why not?

 

[redfury]

Re: why don't boats have transmissions?

Boat motors are designed to run at or near max RPM's so they don't carbon up. Also makes them more complicated, plus...how are you going to change gears without adding a bunch of extra weight.

I think it comes down to simplicity and keeping a design that works, plus interchangeability/parts availability. Ever notice that car manufacturers use the same basic components from year to year, just making minor changes? The Chevy 350 is still around...how long have they used that basic design?

Of course, nothing says you can't break the mold, and with the 4 strokes becoming the norm, it would be more feasible. I'd like it just so that I could troll at 1.5 mph with a 200hp motor if I felt like it.

 

[thefairlaneman]

Re: why don't boats have transmissions?

Transmissions are designed with gears that alow you to coast. This is what makes them efficient when you have gravity to help. On water you cant coast so theres the answer.The part about carbon build up is true too

 

[mattttt25]

Re: why don't boats have transmissions?

2 speed transmissions are available with diesel packages. You'll find them on some 32'+ sportfish.

 

[ovrrdrive]

Re: why don't boats have transmissions?

I've wondered the same thing myself many times. The answer that boat motors have to run at a certain rpm goes out the window when you're talking about I/O or inboard motors because they are just car motors with a bunch of anti-spark equipment installed.

It seems like a high revving 3.0 could benefit from at least a 2 speed tranny to drop the rpm a little. A simple transmission would weigh in less than a typical passenger.

Seems like it could make sense to me...

 

[F_R]

Re: why don't boats have transmissions?

Boats "shift gears" by changing the propeller. In an ideal world, you would select a propeller that is in "high gear" all the time. Since you want to compare with a car, you can leave your car in high gear all the time too, and rev it up and slip the clutch to get it rolling. With the boat, there is built-in "clutch slippage" as the prop slips in the water because there is no solid contact as there is with tires on the road. Once you get it up on plane the prop grabs the water more efficiently.

Yes, a boat runs less efficiently at wide open throttle than at mid speed. So does a car. Remember 55?

 

[haskindm]

Re: why don't boats have transmissions?

It takes very little power to keep a car moving at a steady speed, once you have reached that speed. Rolling resistance is very low, so the only real obstacle is wind resistance which is comparatively low. Do an experiment. On a clear, flat road, run your car up to 25 or 30 mph and then shift into neutral or push in the clutch - see how long it takes the car to come to a stop. Now run your boat up to 25 or 30 mph, brace yourself, and quickly shift into neutral. You will find that the boat stops very quickly. The difference is the resistance of the water. Someone once told me that a "boat is always climbing a mountain", and that is a pretty good analogy. There have been some propellers that change pitch at higher speeds, and some transmissions used in larger boats, but the amount of "gear change" that you could use and still maintain speed, versus the benefit that you would receive just does not work out. If gas prices continue to increase, some things that aren't feasible now, may become feasible. We will have to wait and see.

 

[EricR]

Re: why don't boats have transmissions?

Those two speed transmissions have not worked really well in planing hull applications from what I was told by a tech with ZF Marine. ZF and Twin Disc have about 95% of the market in inboard diesel marine reduction gears. They said the units have a very high failure rate.

Now I am not talking stern drives here, or the new aziopod units, just conventional shaft/strut/prop/rudder arrangements.

 

[koberlee]

Re: why don't boats have transmissions?

Your outdrive is a one speed trans with forward and reverse gearing and you prop is your rear end gear.
A trans that would change gears on a boat would attempt to lower the motor speed while maintaning the higher rpm at the prop then you would drop out of your motors rpm band and you would have the rpms at the prop but you would not have the power from the motor to keep the boat on plane and cutting through the water.
the only way it could work is to maybe do a .8 to 1 or so ratio and I would assume that this type of change is not worth the added weight and complexity.

 

[oops!]

Re: why don't boats have transmissions?

good thread......more input !


don...bond-o....super nova....45......wca....hbxt02385...

care to comment?

 

[oops!]

Re: why don't boats have transmissions?

(add rating)

 

[Frank Acampora]

Re: why don't boats have transmissions?

The propellor and water form a simple automatic transmission torque converter.

An automobile needs gears because the engine develops power in a relatively narrow rpm range. It also runs nowhere near its maximum power at any legal cruising speed. This concept is important as I will explain. Therefore, because it has a built in automatic transmission, it is not really necessary to have gears to keep the boat engine in its power band. It is also assumed that outboard engines will run at or near their power band most of the time.

Now, regarding overdrive: Let us say that it takes 100 horsepower to cruise a boat at 30 MPH. Let us also say that this is not at wide open throttle. If you had gears, no matter what gear you were in it would still take 100 HP to cruise at 30. Since the prop loads the engine and since the load is much greater than the load to roll an auto, if you use an overdrive gear, the prop will try to spin faster at a given engine rpm and load the engine more. Thus the engine will spin slower. Since it will still take 100 horsepower to go 30 MPH you will need to open the throttle more to try to generate the 100 HP at the lower RPM. This will give lower manifold vacuum and low manifold vacuum equates to higher fuel usage. Thus the engine will use more fuel cancelling out the overdrive. If it takes 10 gallons an hour to cruise at 30 MPH it will take 10 gallons no matter what gearing you use.

Conditions are quite different in an auto cruising along the highway in overdrive--- It is not anywhere near using its maximum power so the excess displacement waste gas. The small amount of extra throttle opening to generate the horsepower in overdrive is well offset by the savings in rpm at these low horsepower settings, manifold vacuum will stay high unless climbing a steep hill, thus slowing down the rpm will save fuel under most conditions.

There were props manufactured that would shift pitch (some are still around). However the object there was not fuel savings but performance: The lower pitch accelerated the boat quicker and got the hull on plane faster while the higher pitch was set for the engine's power range at WOT.

 

[QC]

Re: why don't boats have transmissions?

Well I didn't make Oooops' list but I am gonna chime in anyway . . .

For fuel economy alone a trans would make some sense. I actually think the best would be a Continuously Variable Transmission (CVT) . . . however, using current marine engines would be a problem as they are not designed for a deep lug, like some state of the art automotive engines are. Also, the weight comments, and especially . . . COST . . .

There are no marine manufacturers that want to increase the cost of their products for limited perceived benefit. Yes, there could be better hole shot, better economy and even better top speed as you could use the same prop for all loads and speeds with a CVT, annnnnd get the exact same engine RPM (peak power) regardless of output shaft speed. But . . . it would cost more for an engine that could handle that and of course for the trans and all for maybe 10% better economy. Like I usually say, $5 a day for the average pleasure boater. All sounds good until you have to build and sell one . . . .

 

[wca_tim]

Re: why don't boats have transmissions?

Frank and others are right on point. I look at it this way, if you were to run a two speed transmission for optimum performance, you would set the 2nd gear ratio to give you the prop pitch range where the best performing props are available, and then a lower gear (more gear reduction) to give you more torque to accellerate out of the hole quicker.

once a boat is propped right for a given gear ratio, max speed is not going to be very different for two gear ratios (sure some difference for the extra drag for the higher gear ratio / lower pitch prop combo since the blade speed will be higher) max speed is limited not by the gear ratio or prop pitch, but by the horsepower of the engine.

so the only potential advantage of having a second gear once we get our max speed dialed in is low end accelleration. Turns out that by dropping pitch just a little, we get a lot more bottom end without losing a ton of top end. We can also make up the hole shot by inducing cavitation with vents, etc...

Also consider that the transmission for the majority of i/o pleasure boat applications we're dealing with, is in the lower gearcase. Why? because if you put the transmission in the boat, the engine has to move forward eating up valuable cockpit space. Add that we can't put it in the lower gear case without making it a lot bigger (slowing the boat down) and more complex (you think your alpha drive comes apart now, ad a bunch more parts...)

What we know for cars and motorcycles doesn't necessarily apply until you get going a good bit faster than we typically do. Think about it almost any car will hit over 100 at wot... Case in point, my lasy hot rodded zx-11 would run a bit faaster in 5th gear than in 6th because the way I had it geared it reached the speed where it couldn't overcome air resistance before it hit max horsepower. Drop a gear and it would pick up 5 or 10 mph. (You're a better man than I if you can read the spedo more accurately than that goin that fast).

the long and short of it is that its expensive and there really isn't that much to gain in the vast majority of applications. The crux of the issue is the the fact that a boat has to displace water (and air) while a car has the much easier job of displacing air...

 

[45Auto]

Re: why don't boats have transmissions?

As many of the posters above have stated, the propeller slipping in the water acts like a transmission in a boat. No need for any additional weight or mechanical complexity to replace something that's free!

This chart is from my boat, but it will be very similar for any planing hull.

As you can see, up until about 30 MPH the slip is acting similar to a CVT (continuosly variable-ratio transmission). It's allowing the motor to rev up to reach the point where it can supply the power needed to drive the boat at that speed. If the prop WASN'T slipping, it would be like trying to drive your standard transmission car locked in 4th gear with the clutch permanantly engaged. If the prop couldn't slip, you would need a transmission in the boat just like you do in a car.

To make the power required to drive a typical boat, the engine is big enough (displacement) to have a broad enough torque curve that a transmission is unnecessary when combined with the prop slip.

Some specialized boats do use transmissions. I believe the 100+ MPH ski-racing boats do. This is because the engines are tuned to deliver lots of horsepower within a narrow RPM range (peaky torque curve). So the transmission allows the engine operate within that RPM range.

If your boat is propped right, the engine RPM will be so close to where you want it to be when cruising (for efficiency and power purposes) that a transmission is unnecessary. For example, at 60 MPH my Suburban is turning about 2200 RPM towing my 5000 lb boat. An additional gear wouldn't do me any good because the engine couldn't pull it. My boat turns 4600 RPM at WOT (4400-4800 recommended), and is turning 2200 RPM at 28 MPH, which is it's maximum efficiency cruise speed. A transmission which would allow me to lower the RPM would be equivelent to an additional gear on the loaded Suburban. All it would do is lug the engine.

Be kind of funny to put a small displacement, peaky (narrow RPM range where it makes HP) motor in a boat. You'd need a transmission in that case to allow the engine to operate at the RPM where it made the required power. Kind of like riding an old (1970's) CR125 motocrosser ...... talk about "rowing the gears"........

Kind of a rambling post, hope it makes sense!!

 

[Mischief Managed]

Re: why don't boats have transmissions?

Mercruiser offered (or planned to) a two speed transmission between the drive and the engine for a very short time in the mid 90s, if I recall. I am 99% sure I read about it in a 1997 Popular Mechanics magazine.

The problems with this arrangement are:

You are increasing the torque to the drive. This is the kind of option only a performance boater would typically want and pay for. When coupled to a high performance engine, the extra torque can kill the drive.

The transmission added more than a foot of length to the engine package which was not something most existing designs could swallow without major modifications.

I think a two speed would be awesome coupled to a 350 MAG and a Bravo 3 in a pocket cruiser.

 

[Bondo]

Re: why don't boats have transmissions?

Mercruiser offered (or planned to) a two speed transmission between the drive and the engine for a very short time in the mid 90s, if I recall. I am 99% sure I read about it in a 1997 Popular Mechanics magazine.

Ayuh,....

It was considerabily Earlier than that,... The old TRS,+ TR had a 2 speed box....

As noted above,... It just Ain't practicable....

But,....
I've got the next Best thing,....
My L&S Torque-shift prop starts out at 11" of pitch,+ shifts up to 26" of pitch.....
I can,+ nearly have ripped the arms off people foolish enough to want to waterski behind my boat.....

 

[oops!]

Re: why don't boats have transmissions?

Well I didn't make Oooops' list but I am gonna chime in anyway . . .

please forgive me your guru-ship......my most humble and sincer apoligies....and to any of our other super pros not mentioned :redface:

i just love these kind of posts where all the people who really know what there talking about....."chew over a concept" makes for a fantastic learning opertunity


and btw kevin......what happened when you made supreme? did bruno make you give back your keys to the admrials lounge? or did you get to keep the key forever?......im closing in on it....and once i have it....dont wanna loose it

lol

cheers
oops

 

[wca_tim]

Re: why don't boats have transmissions?

I've got the next Best thing,....
My L&S Torque-shift prop starts out at 11" of pitch,+ shifts up to 26" of pitch.....
I can,+ nearly have ripped the arms off people foolish enough to want to waterski behind my boat.....

that's funny... and quite frankly sitting here thinking about it torque shift props really probably are the best way to go from a cost / performance benefit standpoint...

I spent some "haze grey and underway" time on a spruance class destroyer. Powered by 4 LM2500 gass turbines turning two screws with controllable reversible pitch props... once in a while they would wind it up all the way. on the "holeshot" was like a wall of water coming out from under the stern. If I remember right, it could do from full forward to stop in one length and accellerate from 0 to full speed of "greater than 30 knots" (a good bit greater when they really wanted to...) in something like 3 lengths. 75,000+ shaft horsepower.


btw oops... "superpro" I'm flattered, but to set the record straight, I'm a backyard hack... especially compared so some of the folks on here...

 

[45Auto]

Re: why don't boats have transmissions?

Here's a link to one of the ski-racing boats. 1500 HP through a 2 speed Turbo 400. Got to be crazy to ski at 120 MPH!!

http://www.clubmarine.com.au/internet/clubmarine.nsf/docs/MG23-1+Club+Marine+Southern+80