Why don't boats have gears?

[jakefrith]

I was thinking about using a CBR 600 motorcycle engine in a boat. This led me to look at the torque which is less than an outboard motor of similar power (100hp). Although I don't have figures to hand I seem to recall the CBR was 40 odd ft/ lbs @ 10,500 rpm ish and an outboard was more like 80-90 ft/lbs @ 6000 rpm ish.

However, the bike has the benefit of 6 gears and a clutch, which allow (in the lower load context of a motorcycle) furious acelleration from back wheel not moving at all to back wheel moving at 150 mph. It also would take it about an hour to use up a full tank of fuel (14 litres) flat out at 150mph if you could find a banked track to do it on.

Now, my last 2 stroke outboard engined 100hp boat flat out used a litre of fuel every 2 minutes or so. The above flat out motorcycle scenario equates to twice the fuel economy, a litre used about every 4 minutes. They are both at full throttle and full load. I am guessing the reason for this is that the marine engine is considerably larger than 599 ccs, (and that's also how it gets its extra torque).

To what extent would 6 gears (which can very rapidly and clutchlessly be changed up due to a sequential gearbox) mean that a motorcycle engined boat make up for its lack of torquey flexibility?

Would the motorcycle engine be more fuel efficient at different speed ranges, due to the ability of being able to keep the engine spinning at its most efficient rev range whatever speed the boat is going?

Have boats not got gears because the medium of water has traditionally allowed designers to get away without having them (you can still get going from standstill due to prop slipping, but prop of planing boat is designed to work most efficiently at point where engine is producing max power, so until it's got to max speed it's inefficient) Cars and motorcycles have always needed gears to pull away from standstill as they are pushing against an imovable object. Or could it be about cost of manufacture, ease of servicing, reliability in the marine environment when a boat engine will work after a fashion elbeit less economically with only one gear?

In short - Why don't boats have gears?
...And would a 100hp cbr 600 engine and 6 speed gearbox and sterndrive power a small 2 person 14 foot speedboat to a similar extent that a (more torqey but only one gear) 100 hp outboard motor would? (the two systems would be similar weight)
If it could power it as well, surely the fuel economy would be so much better?
My fag packet calculations are for flat out, but imagine the joy of redlining it in 3rd or 4th to get on the plane, then being able to shunt it down into 6th for economical lower revs cruising, still upon the plane. Surely the savings would be even greater in this type of scenario? (Cbr in bike form nipping around town and motorway faster than any car, but not completely murdering motor returns circa 60mpg)
Jake

[Ike]

boats do have gears. It's called a reduction gear. A prop does not turn at anywhere near the speed of the engine. If it did it would cavitate badly. It is simply not efficient to have gears like in a car.

[jakefrith]

Thanks for your reply. I know boats have a reduction gear. My question is why they don't have more of them like cars and motorcycles. A range of gears and the ability to select between them on the move so that a smaller engine can be used than would otherwise be necessary on a conventional marine instalation with only one forward gear.

[Fanie]

I think a gearbox where you can actually change to a different ratio to the prop could work very well. I have a 115 on the boat and there are many times I wished I had a faster prop on it.

To get on the plane you can use a low ratio where you have lots of power, then once on the plane, just get the prop to rotate faster and the revs could drop some.

Should work even better on a downhill :D

No but seriously, I think it could work quite well. You won't need a six speed box though. Just reverse, neutral and forward :D

OK OK, Serious seriously, good idea. I think 3 ratio's forward would be very nice.

[StianM]

I think the formula1 offshore boats have gears.

[Lt. Holden]

Jake,
I have always wondered the same thing! Take a lightweight higher revving motor like your Honda and use the transmission to control the motor rpm's while maintaining a relatively constant (for a given boat speed) prop rpm? What about in a jet boat application; where there is even less torque required?

Once you have the boat out of the hole and on plane, isn't the actual torque requirement less than that required to get on plane? So theoretically couldn't you go to an overdrive type gear to reduce motor rpm's and still maintain the impeller rpm to stay on plane?

What about with a lightweight higher revving diesel like a Kubota with a torque-multiplying transmission? It seems there would be some real benefits possible?

[Fanie]

60 % ratio, you have 40 % more torque for slow speeds like towing or using lots of power to get on a slow plane. Use when all twenty family members are on the boat.

1 : 1 for like your boats ratio is currently, nice middle of the road, use for conditions less favourable for high speed.

135% ratio, 35% more speed at the same revs. Conditions are favourable for speeding, the chicks are watching so let rip.

[daiquiri]

Boats can have transmissions with gears, usualy with 2 ratios, though some very high performance racers have more than 2. We are talking about boats like some Cigarette models, or the offsore racers which use 4 speed gears. You can find more here, for example:
http://www.weismann.net/offshore.html (4-speed gears)
http://boostpower.com (3-speed gears)

Then there's a Volvo Penta's Stepdrive (2 speeds) for pleasure boats:
http://www.stepdrive.co.uk/

It is not a widespread solution because of drawbacks like costs, complexity, increased probability of failures, weight etc. but technically it can and has been done.

[Stumble]

I am thinking that there are a few reasons why this wouldn't work as well on a boat as on a car.

1) the amount of torque required to keep a boat up and moving would prevent shifting to a much higher gear. Maybe you could drop a few hundred RPM from max, but not the 5k like in a car.

2) Props start to cavitate beyond a certain speed, so to use a higher gear to increase top end speed would require a variable pitch prop that could be changed while running.

3) The modest possible gains in performance would likely be offset by the cost and weight of the transmission that would have to be lugged around all the time.

At least these are the problems, but I wouldn't argue with someone if they can show me where I am missing someone.

[daiquiri]

One important consideration: a boat is not a car. :D
That phrase is not mine - it's been said by Antonio Soccol, a renowned italian boat designer and technical journalist. I like that phrase because it is so true and still too often ignored.

Cars operate in a highly variable range of situations. A car needs to continually change it's working regime due to it's surrounding environment - traffic lights, queues, crossings, speed limits, accellerations and decellerations, high speed at highways, low speed at city roads, climbing up the hills, descending down again, towing the trailer, etc. etc. It is a continuos change in RPMS, power and torque requirements, which would be impossible to obtain (and would soon destroy the engine) wihout a multi-gear transmission drives.

So first you have to ask yourself how do you intend to use your boat. If you will operate your boat in a way similar to the car in the example above (apart climbing up the hills :D ), then you might take into consideration a transmission with more than 1 gear ratio.

A 90% (or more) of boats are made to travel most of the time at a certain optimal speed (optimal in a sense of fuel consumption), will from time to time accellerate shortly to it's max speed and will troll slowly or go reverse when maneuvering in a port. All things which can be done pretty well without a complication of a multi-gear transmission.

Every transmission is a source of energy losses, so the less you install the less power you will steal from the prop. Commercial ships, bulk carriers and tankers, for example, which are designed to deliver their load with minor fuel consumption possible, have their props mounted directly to their slow engine's shaft - 1:1 gear ratio and very high efficiency.

A 2-speed drive can be useful for a planing boat during the transition from displacement to planing mode if you have props optimized for high speed, but its benefits have to be evaluated against the increase in weight and the need for an additional space in the engine room, besides the mechanical complexity.

By the way, remember that things you don't install will surely not break. ;)

High performance racing boats need multi-gear transmitions because they have a combination of super-cavitating props optimized for very high speeds (which perform poorly during the low speed, low rpm accelleration) and engines which deliver the power at high rpms. Since they need to maximize accellerations too (if they want to win the race :) ), the use of a multi-speed gearbox is obviously justified in their case.

[Mykul]

I have seen a few Nissan car commercials on TV demonstrating their variable ratio gear box. It sounds to me like a similar set up to what a snow mobile has. Wouldn’t that be more beneficial than a regular set of automotive gears?

[StianM]

I have seen a few Nissan car commercials on TV demonstrating their variable ratio gear box. It sounds to me like a similar set up to what a snow mobile has. Wouldn’t that be more beneficial than a regular set of automotive gears?

If I'm not mistaken the CVT is 5% less energy efficient than a normal gearbox.

[hmattos]

If you look at the Volvo Penta sterndrives made for larger boats in Europe, there is an option of having a two speed gearbox.
The main advantages are fuel efficiency and slow speed handling.
Without the 2 speed the creep speed is too high for docking and the engine revs at cruise are too high for economy.

We - explorermarine.co.uk - make high speed RIBs with outboards, so I have never used one myself.
HM

[Guest625101138]

I was thinking about using a CBR 600 motorcycle engine in a boat. This led me to look at the torque which is less than an outboard motor of similar power (100hp). Although I don't have figures to hand I seem to recall the CBR was 40 odd ft/ lbs @ 10,500 rpm ish and an outboard was more like 80-90 ft/lbs @ 6000 rpm ish.

However, the bike has the benefit of 6 gears and a clutch, which allow (in the lower load context of a motorcycle) furious acelleration from back wheel not moving at all to back wheel moving at 150 mph. It also would take it about an hour to use up a full tank of fuel (14 litres) flat out at 150mph if you could find a banked track to do it on.

Now, my last 2 stroke outboard engined 100hp boat flat out used a litre of fuel every 2 minutes or so. The above flat out motorcycle scenario equates to twice the fuel economy, a litre used about every 4 minutes. They are both at full throttle and full load. I am guessing the reason for this is that the marine engine is considerably larger than 599 ccs, (and that's also how it gets its extra torque).

To what extent would 6 gears (which can very rapidly and clutchlessly be changed up due to a sequential gearbox) mean that a motorcycle engined boat make up for its lack of torquey flexibility?

Would the motorcycle engine be more fuel efficient at different speed ranges, due to the ability of being able to keep the engine spinning at its most efficient rev range whatever speed the boat is going?

Have boats not got gears because the medium of water has traditionally allowed designers to get away without having them (you can still get going from standstill due to prop slipping, but prop of planing boat is designed to work most efficiently at point where engine is producing max power, so until it's got to max speed it's inefficient) Cars and motorcycles have always needed gears to pull away from standstill as they are pushing against an imovable object. Or could it be about cost of manufacture, ease of servicing, reliability in the marine environment when a boat engine will work after a fashion elbeit less economically with only one gear?

In short - Why don't boats have gears?
...And would a 100hp cbr 600 engine and 6 speed gearbox and sterndrive power a small 2 person 14 foot speedboat to a similar extent that a (more torqey but only one gear) 100 hp outboard motor would? (the two systems would be similar weight)
If it could power it as well, surely the fuel economy would be so much better?
My fag packet calculations are for flat out, but imagine the joy of redlining it in 3rd or 4th to get on the plane, then being able to shunt it down into 6th for economical lower revs cruising, still upon the plane. Surely the savings would be even greater in this type of scenario? (Cbr in bike form nipping around town and motorway faster than any car, but not completely murdering motor returns circa 60mpg)
Jake

Your question on the gears is a good one and there have been some fair answers so far. Fundamentally the water does not have traffic lights or big hills. There can be some benefit with gears to optimise the engine operation in adverse conditions such as strong winds and heavy seas but typically props can be well matched to the engine. Most boat owners have experimented with props to find the best compromise. If they had gears they would not have to fiddle with props so much.

I am doubtful about the motocycle and outboard comparison on fuel economy though. You need to take examples from current designs and style of engine. Also do not forget you some how have to cool the motorcycle engine. It will not be in a nice breeze as set up on bike - cooling pumps or fan will take power. The claimed fuel burn for the bike of roughly 160g/kWh is better than most diesels give.

I wonder whether your bike at 150mph really needs 100HP. Cyclists are now getting over 80mph and they generate about 1.2HP. Power goes up with cube of velocity so a well faired cycle could do 150mph with around 10HP. So what is happening with the other 90HP purportedly available from your engine.

You need to ensure you are comparing horses with horses and do not believe all the sales literature. Without looking at it in a bit more detail you could be very disappointed if you lined up your cycle engine powered boat against a 100HP outboard even if you had the slight advantage of gears.

Rick W.

[Mykul]

But to compare terrestrial vehicles to marine; wouldn't a marine gear box be similar to a differential, rather than a transmission. In which case the prop itself would be the transmission. An adjustable pitch prop would be a better solution than more gears, or am I wrong on that.

[MattZ]

The one litre every two minutes you mention for a 100HP two-stroke is very good. I would double check your calculations because the four-stroke would probably use almost the exact same amount of fuel. Your calculation for the motorcycle engine's fuel consumption is a far too optimistic.

Some boats do have two speed transmissions. Have a look at the product offerings from ZF Marine: http://www.zf-marine.com/ This is mainly done to allow higher engine speeds for generators, compressors, and hydraulic pumps while cruising at low speeds.

For low speed cruising the prop torque needed goes down very rapidly, so even the high speed cruising gear has more than enough torque. For optimum fuel economy you actually need a higher gear at low speeds. However, the increase in fuel economy is very low so the cost of an extra gear is not justified.

[FAST FRED]

The good folks at ZF would be delighted to sell you a 2 speed gear box. They're on the shelf , in a variety of ratios..

We have been looking for 2 years to find a good used one for the Box boat project.

The concept is to increase efficiency the boat would run at High Cruse at one ratio , and at max range cruise (1/3 slower) with less gear reduction. This would place a higher load on the engine at low speeds , increasing engine efficency.

"However, the increase in fuel economy is very low so the cost of an extra gear is not justified."

We have found sometimes a 300% difference between hard working engines and loafing engines per HP per gal of fuel consumed.

Since most folks usually choose to cruise at lower speeds a 3X increase in efficiency IS of interest , at least to us.

Look at the gen set articles by Nigel Calder in PBB , for some concept of how fuelish diesels are when unloaded.


It takes a certain amount of power to push any boat at a desired speed , how much you pay (in fuel or mechanical complexity ) is your choice.

FF

[tom28571]

I think we are missing a main ingredient here. Look at the propeller slip curves for a boat. High percentage of slip at low speed which reduces as speed increases to near zero at very high speed. This a clue that the prop in water is acting like a fluid coupling as some cars have had. The water is acting like an infinitely variable transmission of sorts. The drive wheels of a car do not normally allow for any slip so, in order to get the engine into its high torque range, it needs variable gearing. Some small advantage could probably be had with multiple gear ratios but not enough to offset the cost and complexity.

This is a separate argument from a selected gear ratio for cruising speed like Fred mentions. Most setups on outboards and inboards have this.

[CDK]

A couple of years ago I read on this forum that some people installed an automatic transmission in their houseboats to achieve better (=less worse) mileage and acceleration.
The ability to change gear does have advantages it seems and using an automatic gearbox straight from a car avoids the clutch problem. The propeller slip does make the power train much "softer" than in a road vehicle, but changing gear with a manual box without a clutch needs a very experienced helmsman.

[MattZ]

A couple of years ago I read on this forum that some people installed an automatic transmission in their houseboats to achieve better (=less worse) mileage and acceleration.
The ability to change gear does have advantages it seems and using an automatic gearbox straight from a car avoids the clutch problem. The propeller slip does make the power train much "softer" than in a road vehicle, but changing gear with a manual box without a clutch needs a very experienced helmsman.

It would not be too hard to rig up a way to operate a hydraulically operated clutch from the helm. Then you could avoid the loss of operating the hydraulic pump in an automatic transmission. (Not insignificant at high engine RPM`s).

[Marco1]

I was thinking that to compare shifting gears in a car to a boat it should compare to having a controlled pitch propeller...but then I thought that a CPP is like having a wheel that grows or decreases its diameter...a Controlled diameter wheel.

It is difficult to compare a car to a boat..,.however a 2 speed powerglide auto gearbox would probably do a good job for such test

[DianneB]

In aircraft, which also operate in "a fluid environment" and where the best combination of performance and efficiency are key, a fixed reduction ratio and variable pitch propeller seems to have been proven to be the best combination. I would imagine the same would also apply to watercraft.

[powerabout]

A couple of years ago I read on this forum that some people installed an automatic transmission in their houseboats to achieve better (=less worse) mileage and acceleration.
The ability to change gear does have advantages it seems and using an automatic gearbox straight from a car avoids the clutch problem. The propeller slip does make the power train much "softer" than in a road vehicle, but changing gear with a manual box without a clutch needs a very experienced helmsman.

Recreational ski boats/river cruisers in California have had 2 an 3 speeds trannys in them for over 30 years.
Some of their boats are 25' plus.
Torque converters are removed.

Mercury also made a 2 'speed' outboard but never went into production. It had a duo prop setup and used the props independently apparently?

and of course drag boats have used 2 speed Lenco's just like drag cars do.

The other use for them would be in any diesel powered boat that does 30+knots as with such a small rpm windows it means they do almost 10kts at idle.
Most large sport fishers have trolling valves, as in they let the trans slip so you can go slow.

[Zed]

If I'm not mistaken the CVT is 5% less energy efficient than a normal gearbox.

I think that is the other way they are slightly better than a manual in most cars I have looked at. The opposite to a traditional auto... FWIW.

[nukisen]

:eek:
It seems like many moore people then me does have this question in mind.

As I am planing about maybe build a bigger boat somewhere in the future I have also planned about engine and transmission alternatives.

My thought was to build it with a car diesel engine with an automatic gearing. But then I do think the output sped to the prop would be to much reduced. Then I was thinking about solve this problem by use an ordinary gearbox after the automatic but mount it backwards to gear out the rotation speed. Then it would be able to choose the output speed at the gearbox. Also it would be able to both forward and reversed.

Maybe this is an option for us redneckbuilders who have to build low budget.
:p

[CDK]

The thought about the diesel is the good part.
Automotive gearboxes are unsuitable, two gearboxes after each other do not change that.

What hp range are we talking about? Small marine gearboxes are not very expensive and save you a lot of fruitless experiments.

[nukisen]

The thought about the diesel is the good part.
Automotive gearboxes are unsuitable, two gearboxes after each other do not change that.

What hp range are we talking about? Small marine gearboxes are not very expensive and save you a lot of fruitless experiments.
:idea:
If you take an ordinary manual gearbox with a ratio at first gear 1:4 and second gear 1:2,25.

Turn it around and then you do have gear ratio 4:1 vice 2,25:1.

Then you have geared out again. ;)

Everything is possible, unless drunken sailor and small babies.

Cheers!

[FAST FRED]

In aircraft, which also operate in "a fluid environment" and where the best combination of performance and efficiency are key, a fixed reduction ratio and variable pitch propeller seems to have been proven to be the best combination.

This is because it has been many decades since aircraft used diesels , although this may change.

The usual gas aircraft engine MUST be at low power (compared to TO) for any cruise .The prop allows the load to be matched to the low (Say 2900 at TO and 1700 LRC) rpm needed to give some longevity.

FF

[dand0_4]

I am currently building a 16ft aluminum boat powered by a 350 chevy with a turbo 400 auto transmission. I have an 18 pitch four blade which should push me in 2nd gear, 1.48:1 ratio, 4000 rpm at 40mph with excellent accelleration. I can then shift into 3rd gear 1:1 ratio and cruise at about 2500 rpm at about 38 mph. with much better gas mileage than an outboard. Cooling is a combination of radiators and keel coolers.

[gonzo]

On modern engines, the same power will require about the same amount of fuel. The efficiency is very close.

[dand0_4]

A modern 4 stroke outboard at a cost of about 10 grand and maybe 70 hp will get comparable mileage, but I have a good running chevy that I paid $500 for, parts are cheap and it will push far heavier loads at far greater speeds if needed. My old 60 hp outboardmotor at 5000 rpm,30 mph got about 2 mpg. The chevy 350 at 2000 rpm and 30mph should do much better. Its not just a matter of power, the outboard has to turn much higher rpms for the same speed because it has only 1 gear.

[gonzo]

The Chevy has no gears unless you bolt them to it. You could bolt the gears to your old 60HP too.

[nukisen]

Dand0-4
Seems like a very interesting solution you have made there. :p

As a little bit of redneck solution guy of course I do like alternative thinking.
Specially when they seems to work well.

Well done!
:P

[dand0_4]

Gonzo, very true, but not very practical or easy. I am not disputing your point about power and efficiency, just relating a cheap and practical use for a very affordable, powerful and useful motor and auto trans combo. As nukisen has stated, I am a Texas redneck( I cannot deny it, although a fairly well educated one)We like big power and speed and it doesnt always make sense, but its fun!

[nukisen]

We like big power and speed and it doesnt always make sense, but its fun!

It doesn´t have to as long it is fun. hahaha

[gonzo]

I like Chevys. If the boat can take the weight it is OK. However, the thread is about why boats don't have gears. They usually have a single forward and reverse gear. Some applications do have a two speed gear to match the propeller curve to the HP curve at mid RPMs. In most applications it is not worth the expense.

[dand0_4]

If I get brave or crazy enough, a drag racer friend of mine will help me turn my 350 into a 383 stroker with aluminum heads and 450 reliable horsepower. This is what is in his everday drive to work car. Very dependable with thousands of miles and races under its belt. Should be good for 70mph or more!

[dand0_4]

Sorry if I am getting off track, Gonzo, you of course are correct in your assessment of the expense and complexity not being worth it in most applications. But the general idea of a cheap and plentiful auto transmission with gears for greater efficiency is a sound one.

[gonzo]

You have to add a thrust bearing to it but I've seen them installed. A marine transmission, with its cooler and already installed bearing is usually simpler. However, two speed marine transmissions are rare and really expensive. It starts becoming more attractive to modify something. 383s are the cheapest hotrod to build in the USA. Parts are all readily available. At 450HP, it will be revving pretty high and the overlap on the valve timing may make it ingest water at idle.

[will9000]

Most jet units dont require a gearbox because the engine is directly connected to the jet unit. They have gears in the sense, forward backwards. Hope this helps if your intrested in Jets.

[longtailboats]

I was thinking about using a CBR 600 motorcycle engine in a boat. This led me to look at the torque which is less than an outboard motor of similar power (100hp). Although I don't have figures to hand I seem to recall the CBR was 40 odd ft/ lbs @ 10,500 rpm ish and an outboard was more like 80-90 ft/lbs @ 6000 rpm ish.

However, the bike has the benefit of 6 gears and a clutch, which allow (in the lower load context of a motorcycle) furious acelleration from back wheel not moving at all to back wheel moving at 150 mph. It also would take it about an hour to use up a full tank of fuel (14 litres) flat out at 150mph if you could find a banked track to do it on.

Now, my last 2 stroke outboard engined 100hp boat flat out used a litre of fuel every 2 minutes or so. The above flat out motorcycle scenario equates to twice the fuel economy, a litre used about every 4 minutes. They are both at full throttle and full load. I am guessing the reason for this is that the marine engine is considerably larger than 599 ccs, (and that's also how it gets its extra torque).

To what extent would 6 gears (which can very rapidly and clutchlessly be changed up due to a sequential gearbox) mean that a motorcycle engined boat make up for its lack of torquey flexibility?

Would the motorcycle engine be more fuel efficient at different speed ranges, due to the ability of being able to keep the engine spinning at its most efficient rev range whatever speed the boat is going?

Have boats not got gears because the medium of water has traditionally allowed designers to get away without having them (you can still get going from standstill due to prop slipping, but prop of planing boat is designed to work most efficiently at point where engine is producing max power, so until it's got to max speed it's inefficient) Cars and motorcycles have always needed gears to pull away from standstill as they are pushing against an imovable object. Or could it be about cost of manufacture, ease of servicing, reliability in the marine environment when a boat engine will work after a fashion elbeit less economically with only one gear?

In short - Why don't boats have gears?
...And would a 100hp cbr 600 engine and 6 speed gearbox and sterndrive power a small 2 person 14 foot speedboat to a similar extent that a (more torqey but only one gear) 100 hp outboard motor would? (the two systems would be similar weight)
If it could power it as well, surely the fuel economy would be so much better?
My fag packet calculations are for flat out, but imagine the joy of redlining it in 3rd or 4th to get on the plane, then being able to shunt it down into 6th for economical lower revs cruising, still upon the plane. Surely the savings would be even greater in this type of scenario? (Cbr in bike form nipping around town and motorway faster than any car, but not completely murdering motor returns circa 60mpg)
Jake

Props are designed to operate at certian RPM. Exceede that given RPM they lose performandse and are inefficient. So if you use gears you have to change props. HI, Thai Sean

[michael pierzga]

Not sure if it was mentioned but the AUTOPROP....is designed to perform as an automatic transmission.

http://www.bruntons-propellers.com/Autoprop/Brochureandtestimonials/AutopropBrochure.pdf

[Brian@BNE]

In case anyone remains interested in transmission efficiencies then here is something of interest:
Manual - 97%
Auto - 86%
CVT belt - 88%
CVT toroidal - 93%

The article as it:
http://www.zeroshift.com/pdf/Seamless%20AMT%20Offers%20Efficient%20Alternative%20To%20CVT.pdf

Couple a toroidal tranny to a CPP and there would be a nice and flexible driveline. But it might also need a computer and diesel pyrometer to map the best combination of variables!

But with a drop in efficiency of 4% means that you would have to have a lot of different speed requirements (towing/trolling at different speeds/hi or lo speed cruise settings) to get enough fuel saving to be worthwhile. I'm guessing that most folk will cruise at just one speed, and have a trolling valve or kicker for periods of fishing.

[graftonian]

Back in the day when "Day Cruiser" meant a V8 in back, a V drive, and a minamalist cabin (wall to wall naugahide)..... a few people tried 2 speed auto transmissions. Worked great, especially when we took the boats out of the low water and went to Lake tahoe for the week-end.

[claydog]

In case anyone remains interested in transmission efficiencies then here is something of interest:
Manual - 97%
Auto - 86%
CVT belt - 88%
CVT toroidal - 93%

The article as it:
http://www.zeroshift.com/pdf/Seamless%20AMT%20Offers%20Efficient%20Alternative%20To%20CVT.pdf

Couple a toroidal tranny to a CPP and there would be a nice and flexible driveline. But it might also need a computer and diesel pyrometer to map the best combination of variables!

But with a drop in efficiency of 4% means that you would have to have a lot of different speed requirements (towing/trolling at different speeds/hi or lo speed cruise settings) to get enough fuel saving to be worthwhile. I'm guessing that most folk will cruise at just one speed, and have a trolling valve or kicker for periods of fishing.

Seeing as you brought up CVTs I might as well ask a question Ive had in the back of my mind for a while.

Would running a CVT that's very tuneable, like on a snowmobile, allow you to run higher pitch prop so that the boat gets on plane in low range then shifts out to operate at the props intended max RPM in high range?

[cbeckjr]

For months I have been wondering why don't I/Os have at least 2 forward speeds but had given up on the idea due to the complexity of it. But I wanted to ask 2 questions one of which is on the path of a torque multiplier and the other down a different path.

1. Use a lock-up torque converter (no tranny) between the engine and outdrive with a big pitch prop. Unlocked during acceleration to allow the engine to rev and get the boat on plane, then lock up the converter during cruising to drop the engine rpms down for fuel economy. Sort of a 2 speed tranny without all the gears and parasitic losses they incur.

2. Turbocharge your engine with a big pitch prop. Turbos really are mid range torque boosters so put on the big pitch prop and have an extra 100 lb/ft of torque to turn the big prop for similar low-end acceleration to that of a small pitch prop on a N/A motor but when you level out you would come off the boost and the motor would turn a much lower rpm.

Not looking to hijack just for some feedback on those 2 alternatives to the gear idea.

[whitepointer23]

I was thinking about using a CBR 600 motorcycle engine in a boat. This led me to look at the torque which is less than an outboard motor of similar power (100hp). Although I don't have figures to hand I seem to recall the CBR was 40 odd ft/ lbs @ 10,500 rpm ish and an outboard was more like 80-90 ft/lbs @ 6000 rpm ish.

However, the bike has the benefit of 6 gears and a clutch, which allow (in the lower load context of a motorcycle) furious acelleration from back wheel not moving at all to back wheel moving at 150 mph. It also would take it about an hour to use up a full tank of fuel (14 litres) flat out at 150mph if you could find a banked track to do it on.

Now, my last 2 stroke outboard engined 100hp boat flat out used a litre of fuel every 2 minutes or so. The above flat out motorcycle scenario equates to twice the fuel economy, a litre used about every 4 minutes. They are both at full throttle and full load. I am guessing the reason for this is that the marine engine is considerably larger than 599 ccs, (and that's also how it gets its extra torque).

To what extent would 6 gears (which can very rapidly and clutchlessly be changed up due to a sequential gearbox) mean that a motorcycle engined boat make up for its lack of torquey flexibility?

Would the motorcycle engine be more fuel efficient at different speed ranges, due to the ability of being able to keep the engine spinning at its most efficient rev range whatever speed the boat is going?

Have boats not got gears because the medium of water has traditionally allowed designers to get away without having them (you can still get going from standstill due to prop slipping, but prop of planing boat is designed to work most efficiently at point where engine is producing max power, so until it's got to max speed it's inefficient) Cars and motorcycles have always needed gears to pull away from standstill as they are pushing against an imovable object. Or could it be about cost of manufacture, ease of servicing, reliability in the marine environment when a boat engine will work after a fashion elbeit less economically with only one gear?

In short - Why don't boats have gears?
...And would a 100hp cbr 600 engine and 6 speed gearbox and sterndrive power a small 2 person 14 foot speedboat to a similar extent that a (more torqey but only one gear) 100 hp outboard motor would? (the two systems would be similar weight)
If it could power it as well, surely the fuel economy would be so much better?
My fag packet calculations are for flat out, but imagine the joy of redlining it in 3rd or 4th to get on the plane, then being able to shunt it down into 6th for economical lower revs cruising, still upon the plane. Surely the savings would be even greater in this type of scenario? (Cbr in bike form nipping around town and motorway faster than any car, but not completely murdering motor returns circa 60mpg)
Jake

gears work well, been there done that. i have had a boat with a 1400 datsun motor and 5 speed gearbox. another one with a 221 falcon with 3 speed box. one with a ford consul and 4 speed. a friend of mine builds a lot of boats with autos which work well. he fits props to suit 2nd gear because ratio is close to reverse, i have been in these boats heaps of times, they change gears in auto just like a car.

[CDK]

On a typical I/O the engine rests on the drive's inside bracket with the bell housing.
You would need a torque converter with the proper geometry, I don't think it exists. The prop in the water already acts as a torque converter, manufacturers are not interested in using a cascade because there is no need for it.

Your second idea implies that it would be advantageous to use the turbo during acceleration only. But that is not the case.
The drop in rpm is not large enough to return to normal aspiration and there is no need for that because a turbo charged engine is more efficient so why not make use of the investment.

The only short action acceleration aid I can think of is nitro injection. That you can use if the engine/prop match does not allow normal take off or if there is a high passenger load.

[Brian@BNE]

Gears for cars are typically necessary to provide enough power at low speed or for high loads such as towing or going up hills.

For boats the best way to achieve the analogous higher power or towing ability at low speed, and fuel economy at cruise speed, is with a CPP. It does the same job as a multispeed transmission does on a car.

[Squidly-Diddly]

They just aren't meant for hours at 90% of full power and neither are car motors.

Boat and airplane motors would be and need a whole other level of reliability for safety.

I've known of ski-boats with Ford 351 and 3sp AT(C-4 or C-6 I can't remember) with Vee-drive(motor bass-ackwards). I do remember the owner mentioning it had some marine 'hole shot' shift kit and the thing was setup for pulling skiiers without regard to much else.

Seemed much like a nice powerful smooth American v-8/at car.

[whitepointer23]

Gears for cars are typically necessary to provide enough power at low speed or for high loads such as towing or going up hills.

For boats the best way to achieve the analogous higher power or towing ability at low speed, and fuel economy at cruise speed, is with a CPP. It does the same job as a multispeed transmission does on a car.

show me a cpp for a planing hull under 30 ft and how much it cost. because i can't find any .

[Aharon]

Should work even better on a downhill :D

No but seriously, I think it could work quite well. You won't need a six speed box though. Just reverse, neutral and forward :D

OK OK, Serious seriously:p
very good, wholehearted laugh, 10x! That was the "1-2 humour punches"
ata boy!

[cbeckjr]

On a typical I/O the engine rests on the drive's inside bracket with the bell housing.
You would need a torque converter with the proper geometry, I don't think it exists. The prop in the water already acts as a torque converter, manufacturers are not interested in using a cascade because there is no need for it.

The only short action acceleration aid I can think of is nitro injection. That you can use if the engine/prop match does not allow normal take off or if there is a high passenger load.

I think i follow on your converter answer but i was thinkin a small pitch prop allows for a lot of slip out of the hole to let your motor rev more freely but then you have to rev 1000 rpm higher than you want to maintain speed. A big pitch prop turns lower rpm at speed but is a dog out of the hole. A device that would allow even more slip out of the hole but then tighten up to drop your rpms at speed seems ideal to me. 500-1000 rpm over what a big prop would let the motor rev out of the hole and then lock it up when on plane to sustain speed at a low rpm. I know the ratio is the same once the motor gets to that delta so it's not a 2 stage multiplier like a 2 speed tranny would be but it would put the motor in a higher powerband to take advantage of more horsepower. just thinkin' out loud. Props are always a trade-off, just wondered if a TC would equalize things a bit. Of course an affordable CPP would probably be better. More slip out of the hole then more bite at speed.

My Datsun with 305TPI and 3.23 gear turns low rpm on the highway but the stock torque converter (~1200rpm launch) was a dog out of the hole at the track. A 2500 stall converter (~2000rpm actual) would give much better 60ft times. Then I'd lock it up on the highway and still get great gas mileage.

On your nitro injection did you mean nitro methane or nitrous? It made me think of what I told my buddy a few weeks ago. I thought about building a motor with TPI induction. The long runners would have good torque at 2k rpm to maintain speed with a big pitch prop. And then just put on a 75 shot of nitrous to get the skiers up when needed. :D

[DCockey]

Boats have PROPELLERS.

Cars have TIRES.

That is a fundamental difference.

There is a direct, simple, constant ratio between the speed of the car and the rotational speed of its tires. This ratio is independent of the amount of torque being applied to the tires. (1) If the car isn't moving the tires are not rotating. In order to change the relationship between the speed of the car and the rotational speed of the engine a transmission/gearbox with multiple or variable ratios is needed.

The relationship between the speed of a boat and the rotational speed of the propeller(s) is NOT constant. Rather for a given propeller it depends on:
i) the torque applied to the propellor, which in turn depends on the engine characteristics, reduction gear ratio (if any) and throttle position.
ii) the boat speed vs resistance characteristics
It usually works out that a fixed propeller's speed vs load characteristics are a reasonably good match to an internal combustion engine's characteristics.

.................
(1) To be precise the ratio of the rotational speed of a tire to the speed of the vehicle changes slightly with the torque applied to the tire. This is a result of deformation of the tire though it is commonly refered to as "slip". Also, the effective tire diameter can increase very slightly with increasing speed due to centrifugal effects. These changes of the ratio for conventional road vehicles are on the order of a few percent or less.

[cbeckjr]

"Boats have PROPELLERS.

Cars have TIRES."

Well yea of course i get that. I'm not an engineer but not that stupid either. No need to yell lol. I'm askin' if allowing a motor to rev 1000 rpm higher than it normally would w/ a big pitch prop help 0-20 acceleration. Which you still haven't answered. While a prop is not constant like a tire, there is still a relationship between between the 2 isn't there? 19 pitch/200 ft/lbs vs 23 pitch/200 ft/lbs. Isn't that why you mix and match props on a boat according to desired outcome (top speed or holeshot)

"The relationship between the speed of a boat and the rotational speed of the propeller(s) is NOT constant" True, but 2300 rpm on a 23 pitch is higher mph than 2300 rpm on a 19 pitch. While not constant like a tire, there is a difference. And it's the same difference on Tuesday as it was last Thursday. That's what I mean by constant (maybe bad choice of words). It's not some random ever changing outcome. It is somewhat fixed and repeatable.

For the same weight, same pitch prop, same water density, blah blah blah, the acceleration is absolutely repeatable (maybe a better word than constant). But I'm asking if you were able to raise the engine's rpm with all else fixed (torque, pitch, etc) , would you see better 0-20 on a given prop or would you still need multiple ratios. Seemed like a yes or no kinda thing to me. :)

Funny thing is I'm not the only one who compares prop pitch to gear ratios. Pulled this from a boating magazine article.

"Think of a propeller as you would a car's axle ratio. The lower the ratio, the more pulling power from a standstill. The same is true with a prop. The lower the pitch, the better your hole-shot. However, this comes at a price: top speed.The lower pitch makes the engine reach maximum rpm at slower speeds.

Conversely, a higher pitch will deliver greater top speeds, but slower acceleration. Be aware that lower-horsepower engines can bog down if fitted with a propeller with too high a pitch and diameter, and that can wear heavily on internal engine parts."

Should I tell them they are nothing alike, or do you want to break it to him......

[DCockey]

cbeckjr, my posting above was not a response to you, even though it happened to follow you posting by 9 minutes. I started writing it before your post appeared.

[DCockey]

When I used "constant" in my note above, it was in the sense that in a car with a given tire size the ratio of tire rotational speed to road speed is the same at all speeds whether the car is accelerating, driving at constant speed, or decelerating. It's the same irregardless of the weight of the car, and whether the car is on level ground, going up a hill, down a hill, into a headwind or being helped along by a tailwind.

In a given boat with a given propeller the ratio of propeller rotational speed to water speed varies depending on:
- the speed of the boat throught the water
- displacement and trim
- accelerating, decelerating or constant speed
- ambient wind
- waves (if significant)

Changing the diameter and pitch of a propeller on a boat will change how fast the boat accelerates and the top speed of a boat. In that sense it is like changing the gear ratio of a car. As others have noted there can be advantages to installing a multible/variable ratio reduction gear/transmission converter in a boat. But the magnitude of the advantages are considerably different than in a car.

A car with a single ratio between engine speed and tire speed (no gear box/transmission or torque converter) would be very unpleasant to drive and either be very slow to accelerate and have limited ability to climb hills or have a low top speed.

In contrast most boats with a single ratio do quite well.

[Brian@BNE]

show me a cpp for a planing hull under 30 ft and how much it cost. because i can't find any .

Michael Kasten's article is a reasonable starting point for cost comparison and suppliers although its a couple of years old. http://www.kastenmarine.com/CPprops.htm

Richard's thread had some more suppliers, as well as some comments on cost also. http://www.boatdesign.net/forums/archive/t-30695.html If you are changing from petrol twins to single diesel then you'll need new shaft installation anyway, so don't just compare prop prices alone.

[whitepointer23]

Michael Kasten's article is a reasonable starting point for cost comparison and suppliers although its a couple of years old. http://www.kastenmarine.com/CPprops.htm

Richard's thread had some more suppliers, as well as some comments on cost also. http://www.boatdesign.net/forums/archive/t-30695.html If you are changing from petrol twins to single diesel then you'll need new shaft installation anyway, so don't just compare prop prices alone.

thanks for the links. interesting reading.

[nukisen]

So then I suggest we build a boat with a variator. Then you dont have to chose the gear as the torque handle this by itself. Then you have always have a transmition from the engine with perfekt balance all the way from displaced speed to full planing speed. And when top speed is reached we have a boat running in perfekt torque regarding highest effektive motor rounds.

[nukisen]

Sorry for the spelling and grammer.

[Aharon]

Nukisen, your ideas are way much important than spelling and grammar. Do not worry about them, just keep those ideas and suggestions coming.
I am a beginner myself, planning to build my first boat at age 56, and I never cease to marvel at the lenght people here will go to discuss any subject brought to light.
BDnet is like a "free university" where I can ask stupid questions and still be respected.
Way to go, you Swedish Seaman!
Aharon, from sunny Israel.

[nukisen]

:)!
A boatdesigner and builder is always respected! :)

[Lurvio]

I have been toyin with these ideas at some point myself. One thing you guys are forgetting (especially torque converters) are losses in the extra parts. With a slipping TC you probably won't get any more hp to the prop as the power is being churned in to heat in the converter.

What I've been thinking (for an inboard runabout type boat) is to use a basic car automatic trans without the torque converter. Stripping the TC you basically get a powershift trans with reverse, wet multidisc clutches on all gears. My thinking was that I might want to use the mean machine for trolling at some point and the engine idle might be too much speed for the poor little piece of balsawood and hooks at the end of the line.

Just my 2c worth. :)
Lurvio

[whitepointer23]

I have been toyin with these ideas at some point myself. One thing you guys are forgetting (especially torque converters) are losses in the extra parts. With a slipping TC you probably won't get any more hp to the prop as the power is being churned in to heat in the converter.

What I've been thinking (for an inboard runabout type boat) is to use a basic car automatic trans without the torque converter. Stripping the TC you basically get a powershift trans with reverse, wet multidisc clutches on all gears. My thinking was that I might want to use the mean machine for trolling at some point and the engine idle might be too much speed for the poor little piece of balsawood and hooks at the end of the line.

Just my 2c worth. :)
Lurvio

a company in melbourne used to sell auto's for marine use in the 80's. they used a 2 speed gm powerglide automatic with out the torque convertor and tailshaft housing. it was a neat looking unit. quite compact but a fairly complicated job to do your self unless you have access to machining equipment.

[powerabout]

I have been toyin with these ideas at some point myself. One thing you guys are forgetting (especially torque converters) are losses in the extra parts. With a slipping TC you probably won't get any more hp to the prop as the power is being churned in to heat in the converter.

What I've been thinking (for an inboard runabout type boat) is to use a basic car automatic trans without the torque converter. Stripping the TC you basically get a powershift trans with reverse, wet multidisc clutches on all gears. My thinking was that I might want to use the mean machine for trolling at some point and the engine idle might be too much speed for the poor little piece of balsawood and hooks at the end of the line.

Just my 2c worth. :)
Lurvio

Art Carr transmissions in the the US has been doing this for more than 30 years

[powerabout]

One problem you need to solve is to stop the prop from rotating when in neutral due to the drag inside the box.

[Lurvio]

Don't a car gearbox have a park gear, which locks the trans output? Maybe not all of them, but at least the ones I have seen used.

I have a couple of old Borg/Aisin Wagner autos lying around, maybe I should check what could be done with them. I have skill and some machinery which might come in handy in such conversion. :)

Lurvio

Edit: Borg Warner, Wagner is (among other things), a pig starring in a Finnish comic strip.

[nukisen]

Very easy to solve this!
exchange the convertor to a centrifugal clutch. Then the problem is no problem any more.
All you have to do is to put throttle to idle.
Even used in very modern cars nowadays. Also with cvh gears.
All parts already invented. just convert the output to prop axle instead of driveshaft.

[whitepointer23]

Don't a car gearbox have a park gear, which locks the trans output? Maybe not all of them, but at least the ones I have seen used.

I have a couple of old Borg/Aisin Wagner autos lying around, maybe I should check what could be done with them. I have skill and some machinery which might come in handy in such conversion. :)

Lurvio

Edit: Borg Warner, Wagner is (among other things), a pig starring in a Finnish comic strip.

yes you can use park , i have driven my friends boats with full auto trans and he uses the park function. you don't need an anchor , just put it in park.

[powerabout]

Very easy to solve this!
exchange the convertor to a centrifugal clutch. Then the problem is no problem any more.
All you have to do is to put throttle to idle.
Even used in very modern cars nowadays. Also with cvh gears.
All parts already invented. just convert the output to prop axle instead of driveshaft.
yes simple and you would put every marine transmission builder out of work
shock horror but nobody makes what you suggest, theres your answer

[whitepointer23]

glen l tornado with powerglide auto and cassale v drive.

[Lurvio]

A centrifugal clutch came to my mind also, but if the park does not lock input shaft as I don't think it does, there really is no need for it.

Centrifugal clutch is pretty easy to design and make, just need a couple of parts machined. Just made one about a year ago, works like a charm, has a brake drum from a Ford Transit truck (pair wheels in the back). My dad skimped on the springs strenght, so it connects under idle speed, but that works fine for the application it's in.

Lurvio

[whitepointer23]

A centrifugal clutch came to my mind also, but if the park does not lock input shaft as I don't think it does, there really is no need for it.

Centrifugal clutch is pretty easy to design and make, just need a couple of parts machined. Just made one about a year ago, works like a charm, has a brake drum from a Ford Transit truck (pair wheels in the back). My dad skimped on the springs strenght, so it connects under idle speed, but that works fine for the application it's in.

Lurvio

park only locks output shaft

[DCockey]

What would be gained by using a centrifugal clutch in a boat?

[nukisen]

For me who have a direct transmition from the engine to propaxle I have no prop with me when starting the engine. for those who uses a mechanical reverse gear will save the gears if they only change direction with idle speed. For hydraulic transmition it would be very unnesessary