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Old 20-09-2006, 11:38 PM   #1
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gear ratio's

If you have an engine producing its best power at say 5000 RPM, and you have a selction of drives, is it better to go for (say) a 1:1 gear ratio or a 2:1 ratio ( obviously changing the prop pitch to suit the setting )
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Old 21-09-2006, 12:29 AM   #2
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I believe, turning the prop more slowly, and with the appropriate higher pitch, using a numerically higher ratio, is supposed to be more efficient, resulting in it being faster, but the increase in torque reaction is obviously detremental to the handling.

We ran the lowest ratio we could (1.36:1 at the time) so we could run the lowest pitch possible, in order to reduce the torque reaction, paddle wheel effect and other associated problems. (we were of course running a single prop, though I don't think the duo-prop system completely eliminates torque reaction, rather it reduces it a bit.)

Having said all that, quite a few 2litre drivers choose to run with 2:1 ration, instead of 1.87:1, so they're going for more pitch, lower prop rpm. Be interesting to hear their reasons for choosing that setup.

I would however, value the opinion of some more qualified members, such as Graham Stevens, Adam Younger, or John Guille, to verify, or poo poo my theory, and would of course love Lorne Campbell, Steve Baker, and a few others to join boatmad and share their knowledge, but somehow think that's unlikely.
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Old 21-09-2006, 01:33 AM   #3
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Does this effect the possible risk of damage to the gearbox? Say running a lower ratio could survive an offshoure 'air time' better than a high pitch highratio box?
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Old 21-09-2006, 08:59 AM   #4
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I guess the lower unit has to be experiencing far higher torque peaks if power has been through more of a reduction in the upper gear set, but I don't know if the lower unit of the Volvo is a weak link.
I do know that there used to be a deliberate weak link drive collar between the upper & lower units on the first DPX drives, this was there as a way of protecting the mechanicals, but used to regularly fail under heavy racing, admitedly, it was generally on the big block Volvos, of around 400hp and up.
I believe they changed that 'weak link' to be 'not so weak'.

With the Mercruiser drive (Bravo) the lower unit is the stronger part, and failures in it are rare. It's almost always the upper that fails with them, but that info's not much use to you.
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Old 21-09-2006, 09:06 AM   #5
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thanks.
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Old 21-09-2006, 09:16 AM   #6
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One will be more driveable than the other to that particular individual it's whats suits YOU & the boat best. Testing , Testing , testing
A slower turning higer pitch is more efficient and in THEORY quicker but that might not be what is required.

Example being boat at the time was quicker with a old 175 on a 2:1 Than the XR2 at 1.87. I found dropping from a 28" to a 24" made driving much easier but top end loss was signifficant
Dunno if it helps or not just gotta try it

Edit: was spinng the fek out of it on the little one
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Old 21-09-2006, 09:34 AM   #7
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Quote:
Originally posted by Jon Fuller
I believe, turning the prop more slowly, and with the appropriate higher pitch, using a numerically higher ratio, is supposed to be more efficient, resulting in it being faster, but the increase in torque reaction is obviously detremental to the handling.

This is generally accepted as say a base theory - but from there I think a number of practilacalities take over - hull type, power to weight ratio, avaialble props, prop diameter / prop type, prop height. etc.

So you end up with an equation with many variables all effecting efficiency in way or another - whilst having an effect on handling as well.

Back to original question - if Iwas going for top speed in a straight line and all other factors are equal - yes lower gb ratio and higher pitch prop - spinning slower thus keeping blade speed down.
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Old 21-09-2006, 10:26 AM   #8
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My feeling is the same a Jon's. Since I am using a DPX out of it's intended range, I did a bit of research. As Adam just said, for out 'n out speed the high pitch has it but at lower engine speeds the high pitch prop will not work as well as it could and acceleration and low end performance will suffer.

For the DPX, since E2's are the finest pitch, but still quite high, the gear ratio I needed was not offered for my engine but was available for the petrol option. I was concerned about the strength of the drive if I used a higher ratio especially since I am using a diesel. I spoke to Volvo tech and their opinion was that the DPX was more than capable of handling the torque but that ratio was not offered because they didn't expect it to be needed.

The DPX still uses a section which will sheer but my understandng is that it's intended for protection on impact. It's simply the joining sleeve between the lower and upper unit and it has a groove machined around it to form a stress raiser. I carry a spare. The Volvo man said I would never need it.

For general boating, I wish there were lower pitch props available and I could use the original lower unit.

In use, I'm not aware of the torque reaction and if I open the throttle wide without holding the steering wheel, the boat takes off in a straight line. That doesn't mean there isn't any though.
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Old 21-09-2006, 10:36 AM   #9
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To be fair, I think thre torque reaction monster, is most noticable when travelling pretty fast, with very little hull other than a couple of feet wide along the centreline in the water to fight the torque. Kinda like ballancing on a water ski, whilst absorbing 300hp's worth of 'twist' at 2700rpm (in the case of my engines/drives)
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Old 21-09-2006, 10:38 AM   #10
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To optimise the speed, I have 'fiddled' with the blades of the prop, changing the pitch on the blades. With my so-called record set, I feel a tug to the right when I power up. This is not noticable on the race-set.
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Old 21-09-2006, 10:57 AM   #11
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Steer left

I'm an save you posting it mate
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Old 21-09-2006, 11:19 AM   #12
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(empty post as requested)
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Old 21-09-2006, 04:02 PM   #13
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Quote:
Originally posted by verytricky
To optimise the speed, I have 'fiddled' with the blades of the prop, changing the pitch on the blades. With my so-called record set, I feel a tug to the right when I power up. This is not noticable on the race-set.
Marc, maybe you've 'fiddled' more with one of the props than the other.
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Old 22-09-2006, 12:04 AM   #14
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Quote:
Originally posted by jw.
Marc, maybe you've 'fiddled' more with one of the props than the other.
Yes. I have.
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Old 22-09-2006, 12:51 PM   #15
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Old 22-09-2006, 05:56 PM   #16
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Quote:
Originally posted by Jon Fuller

I do know that there used to be a deliberate weak link drive collar between the upper & lower units on the first DPX drives, this was there as a way of protecting the mechanicals, but used to regularly fail under heavy racing, admitedly, it was generally on the big block Volvos, of around 400hp and up.
I believe they changed that 'weak link' to be 'not so weak'.

We had problems with both DPX drives on our 96 32 Velocity. We also had the 435 hp big blocks.

Volvo was consulted numerous times and finally the problem mainly went away. Perhaps they finally provided us with the "not so weak links".
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