Question on transmissions

675 Views 10 Replies 6 Participants Last post by cfm, Jun 23, 2008

Black Wishbone

Discussion Starter · Jun 22, 2008

Does anyone have a answer to why boats do not have multi gear transmissions. I would think just a two speed gear box would be good one for the hole shot and one once on plan.
Is it because of complication, durability, space, to much load on the shift? With how fast car transmission are shifting it would be usable in a boat.

I do know a few boats do have them be it is not main stream at all and that is were my question comes in.

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Fyhr Factor

· #2 · Jun 22, 2008 (Edited)

I would like to see a 2 speed out-drive too, like an over-drive for top speed at peak hp & torque, but it would definitely put more stain on the engine & gears, SO it would also need to have a Variable pitch prop to go with it, but can't see why those engineering problems couldn't be worked out..

They had variable pitch props for aircraft, so why not for a boat too?? then we wouldn't have to mess around finding the just right prop all the time..so if you increase the engines HP, just turn a dial on the dash and go.. :winker:

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RSCHAP1

· #3 · Jun 23, 2008

I had often wondered about the tranny myself. I have seen very few. Some BIG power BIG cat racers, and of course Vdrives, but why not family / pleasure boats too? Weight? cost?
Variable pitch props I have seen a few times. Never seem to catch on or work too well. Not sure why. I just saw a 211 Liberator Saturday that had an older "Power Pitch" variable blade prop on it. Maybe the springs and cams are too complicated for real world use??

I am curious about the multi gear transmissions though. Maybe with fuel costs they'll pop up now.

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ghart

· #4 · Jun 23, 2008

I've asked this question before and in my opinion never got an answer that made any sense from all the experts here.

I've been to hydroplane races and clearly heard a drop in RPMs after the initial start. I have to assume they are running some kind of gear reduction.

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axkiker

· #5 · Jun 23, 2008

I would think some sort of cvt might work well. Not sure if they have them yet that will handle the power...

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cfm

· #6 · Jun 23, 2008

98%-99.8% of all I/O boats plane well and reach top RPM/speed with one gear ratio and one prop size.

Many times when you modify an engine you can 'blow out the prop' anyway (so why more power added from gears ???)with too much power in low rpm while still having a prop that runs top rpm.

There really aren't too many applications that would need complexity of a multi gear transmission. Maybe a big single turbo set up that doesn't start to make big power until higher in rpm range ????????

BTW: if you start adding gears (underdrive) before the outdrive you will be multipling torque into outdrive. Don't think you want to do this to a Bravo or Alpha. They have a hard enough time staying together as it is.

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RSCHAP1

· #7 · Jun 23, 2008

That sounds like prop technology may need some advancment prior to the need for shifting...
It would seem like with enough gears a 10 hp Briggs and Stratton would be able to do the job.
??
I am sure that we/I ain't the first to ponder this though. Must be a reason there's no pistol grip 4 speed Checkmates...

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cfm

· #8 · Jun 23, 2008

RSCHAP1 said:
It would seem like with enough gears a 10 hp Briggs and Stratton would be able to do the job.
??

I know an extreme example - but no way !

You have to count in the prop pitch kind of like you do tire diameter.

Sure, you could throw a ton of gear at it, but then you'd need to adjust prop pitch accordingly and then you'd be back to pretty much square one.

Run huge gears but then you'd have to run huge tires ! Doesn't work too well, your back to same effective ratio but way heavier tires ! :laugher:

===========================

It's a great subject to think about, but you'll find as you go deeper into the thought that the multi-gear trans just isn't worth it for most pleasure I/O's .

Think out every little thing - don't just look at the surface.

Do not compare to other type boats - like some V-Drives that weigh under 2000lbs, use different prop styles, and have very little resistance to the water, oh, and usually big *** motors. They can also accelerate much faster without 'blowout' issues because of weight and design.

Many other performance boat propulsion systems don't lend themselves to multiple gears either.

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Black Wishbone

Discussion Starter · #9 · Jun 23, 2008

cfm thanks for you input it explains a lot. And I know boats ans cars are two completely different things. But I have to say I had a firebird/formula that in 3 gear I could go 0-100 faster then most car could do 0-60 at the time. And when I was lazy I did leave it in 3 but I still would not give up the 5 other gears.

Now on the boat I have a 22sx stingray. I have a 21p and 23. the 21p is the what the boat came with it has a great hole shot for skiing and has the best top end. But the 23p cruise fast and is for more stable at high speeds and I only lost a MPH or two. Now I don't think it would be worth installing a 2 speed in that boat but if it had one I think I would be very happy with the p23 I could get the hold shot I want and I would be happy with the stably at high speeds.

Like I said I know car and boat are completely different but if car are going up to seven speeds automatics from 3 speed from the 80's. It just make me think.

I would also think you can get a lower CI engine that produces high HP but low torque. And yes I know HP is a produce of torque vs rpm's. But a big block can produce a lot of low end torque vs a small block or even a 6 or 4 cylinder.

Like other people here it just something I think about.

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Black Wishbone

Discussion Starter · #10 · Jun 23, 2008

oh yea I for got my analogy of car vs boat as we all could agree the tranny would be the same the rear end = out drive and the wheels = prop. The only factor that is the problem and may be the biggest is wheel do not slip and props need to.

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cfm

· #11 · Jun 23, 2008

Thus different diameters of props.

A molecule of water only has so much resistance.

You need this resistance to propel forward.

If your prop does not have enough blade area to 'grip' the water than you'll blow out. However, too much blade area will be too much resistance and slow you down.

Take a V-Drive small 2 blade prop which is fastest on that boat aand put it on a 20'ft + I/O . It will not go anywhere - not enough surface to create enough resistance to move forward - blowout ! Prop will just cavitate !

Now, put on a too big diameter (too much surface area) with same pitch (same calculated forward movement with each prop rotation) and it will bog engine down because of too much resistance.

Remember - I really don't understand prop 'black magic' so take all this with a grain of salt.

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