Lower Unit Gears

[TooMuchFun]

I'm experimenting with replacing my 1:68 lower unit drive gear with a 1:59. Volvo has a nice setup where you simply replace the lower end of your drive unit.

Does anyone know which way the rpms should go as a function of change, and how much? In other words, when I drop the gear I am hoping to see additional speed (perhaps 4-5 mph) at WOT (due to prop turning faster at speed, but potentially the engine working harder to get there), and hope to not lose too much acceleration if I have enough hp to overcome the acceleration drop.

But I need to know what will happen to the rpms (i.e., gain or lose a couple hundred at top end?), so that I can prepare for a potential prop change to stay within the rev limiter and have all necessary props at the dock for quick changes.

All info. appreciated ...

TooMuchFun

[kevnmcd]

TMF - There are many factors that will effect what happens. Â*You can use this (http://www.go-fast.com/Prop_Slip_Calculator.htm) site to determine what the outcome will be but a lot has to do with the specs on your engine. Â*Where is the hp curve on your engine? Â*What rpm does your engine make max hp....that is where you want your WOT to be.


Why do you want to change your gears? Â*Can't you change your prop to accomplish the same results? Â*Just my .02

BTW - If you use the same prop and same max rpm and only change your gear ratio you will gain 4 mph.

[Essex502]

I think what you'll see is the "effective" torque go down thus you might lose MPH with the lower ratio in the drive unless you have big horsepower to compensate. If my memory serves...the effective torque to the prop should be reduced and the motor will have more load on it....or I could be full of $hit too. In drag racing we wanted to keep the rpm as high as possible and use the highest (i.e. numerically) gear ratio we could I used to use a 4.88 gears to keep my Trans Am in its preferred rpm range in the lights.

$0.02-1/2 worth http://www.lasvegashotboats.com/iB_html/non-cgi/emoticons/biggrin.gif

[TooMuchFun]

The challenge with a Volvo drive is that there are WAY fewer prop options (E1s through E4s) for the DPX drive I have, then what Mercury provides for. The up side is that the drive comes fully hydraulic, is built with a higher tensile steel (internally and externally) making it tougher to break, and has a greater sensitivity to steel compatibility when using in the salt than Mercury drives ... it for some reason is also much less prone to cavitation at high speeds than a compatible Mercury setup on the same boat.

I run a custom engine that is rated at handling 8900-9200 rpms, but is detuned for "recreational" use with a rev. limiter of 5250 and max propshaft hp of 650. I think I have enough hp to compensate, but don't know.

With my 1:68 gear ratio I am currently running the largest prop (E4s) that Volvo makes for the drive. At WOT I run about 100-150 rpms below the rev. limiter. If I drop to a lower prop I get a bit more crisp acceleration but lose about 5-6 mph at WOT and bump the rev. limiter right away.

The "hopeful" experiment is that with the lower gears I won't have to go down in props to increase rpms to the appropriate level, will not hurt acceleration, and will see increased top end.

To that end, what I need to know is which way will the rpms go as a function of changing gears? If it slows rpms a couple hundred, then I will have to go down in props and the experiment is not worth it. If it goes the other way or stays the same, then it is.

PS -- kevnmcd, the site is helpful http://www.lasvegashotboats.com/iB_html/non-cgi/emoticons/smile.gif.

TooMuchFun

[kevnmcd]

TMF - I believe your rpm's will go down with the same prop. The lower gears will be placing a bigger load on the engine turning the same size prop.

But it would give you the flexibility to work with a lower pitch prop to get you to your max rpm.

[kevnmcd]

Since your only a little below your rev limiter....have you thought about having your prop worked by a prop shop to get you those extra rpm's?

[TooMuchFun]

Thanks kevnmcd ... that's what I was thinking and if the case, then lower gear cost for change would be a wash and potentially not worth it for while I would gain 4-5 mph at WOT due to the gear change, I would lose 5-6 mph due to having to move to lower pitch props to bring the rpms back up.

Plus, bigger load on the engine and harder on the drive may make for faster wearing out of all and greater danger of breaking a drive?

I need to think this one over ...

TooMuchFun

[Essex502]

If you use the prop calculator then you will see that the RPM will go down if the speed remains the same. However, there are variables on whether the speed is the same or not.

Sounds like the Volvo drives are like Macintosh computers - better technology out of the box but inflexible as to expansion because of limited support by the manufacturer or third parties. Too bad.

[Essex502]

</span><table border="0" align="center" width="95%" cellpadding="3" cellspacing="1"><tr><td>Quote (TooMuchFun @ Sep. 29 2003, 10:59 am)</td></tr><tr><td id="QUOTE">I run a custom engine that is rated at handling 8900-9200 rpms, but is detuned for "recreational" use with a rev. limiter of 5250 and max propshaft hp of 650.[/QUOTE]<span id='postcolor'>
I just caught this...9200 RPM? Can't be a big block!

[TooMuchFun]

It's the GM Mark '6' Block ... 540ci and 8.8 liter. After the block it is all custom parts, self contained cooling system (similar to the new 496s), and built for two applications ... some Navy boats for extreme duty and harsh weather, and racing applications. On the latter, it is built to last 1-2 ocean races and to handle very high rpms but not for very many hours ... I don't have the spec lit. in front of me, and rating could be 78 or 8400 or something like that, but it is unusually high for a big block. For the recreational performance market they simply detune the engine for longevity.

You're right on the Macintosh analogy ... a prop switch would be the logical answer, but Volvo is incompatible with everyone else and their prop selection is very limited. My frustration is with the engine I have, I should see another 7-10 mph in comparison with similar boats and similar hp Teague setups for example, but with the drive and props I have I am already into slower acceleration than I would like to achieve at WOT speed, and dropping to the 1:59 gear will further slow acceleration numbers and force me into a smaller prop to regain acceleration (and rpms at top end), but will then cut into WOT speeds.

The folks at Commander put my engine, or more typically the smaller 525 hp into many of their tunnel hull boats and recommend them highly, but do not use the Volvo drives. They switch to one of the higher end Mercury or IMCO units and then experiment with props for max. performance acceleration and WOT speed wise. Hindsight is always 20-20, but I'm not ready to drop 6-8k on a new drive + additional monies on install labor to gain a bit more speed and acceleration at present. Maybe in a season or three ... we'll see.

Bottom line is that I sent the 1:59 gear back and will be content with what I have for another season. Besides, tomorrow is my court date for noise violations and I may be devoting monies to that challenge.

TooMuchFun

[Essex502]

Interesting data on the motor. Usually you don't want to spin big blocks with their long strokes at that high of an RPM due to the mass of the reciprocating components.

Oh well...hind sight is always 20/20 and foresight is sometimes blind!

The 1.59 gears might not be a bad idea at some date if for no other reason than to give you more options. Most of our Bravo drives are 1.5 so you're not that far off.

Have you looked into other third party prop manufacturers such as Hering?

Good luck in court and let us know what happens!

[TooMuchFun]

Have not heard of Hering or other custom prop folks. Do you have contact info.?

On the court thing, I showed up, the Henderson court is pretty low key, much different than Las Vegas, and when I got there the judge had already gone through all for the day and a stack of tickets remained where folks did not show up.

I got a continuance until 10/14 and was told by the desk folks not to worry just show up and show intent to comply and it will probably be forgiven.

Will see in two weeks.

TooMuchFun

[digginfordollars]

TMF, your ride is loud, sounds great to me but I think you are going to have problems if you don't muffle it http://www.lasvegashotboats.com/iB_html/non-cgi/emoticons/angry.gif I think we are all going to have to sooner than later. As far as working your props call Kevin at R&R Prop in Henderson. He is a member here (propman) and a all around good guy.

[Essex502]

</span><table border="0" align="center" width="95%" cellpadding="3" cellspacing="1"><tr><td>Quote (TooMuchFun @ Oct. 01 2003, 9:30 pm)</td></tr><tr><td id="QUOTE">Have not heard of Hering or other custom prop folks. Do you have contact info.?

On the court thing, I showed up, the Henderson court is pretty low key, much different than Las Vegas, and when I got there the judge had already gone through all for the day and a stack of tickets remained where folks did not show up.

I got a continuance until 10/14 and was told by the desk folks not to worry just show up and show intent to comply and it will probably be forgiven.

Will see in two weeks.

TooMuchFun[/QUOTE]<span id='postcolor'>
I don't want to lead you into believing it is a "custom" prop just a third party that might have some insight...

Anyway:

Hering Brochure (http://www.lipship.com/service/2001Brochure.pdf)

There was a gal on here a while back with prop shop, I believe, in Florida you might want to contact about alternate sources of props other than Volvo direct:

Julie Croce
Throttle Up Propellers
877-842-6855

(throttleup is her handle on this site.)

[TooMuchFun]

Thanks to both for the prop info. I will be exploring and let you know what I find ...

TooMuchFun

[Essex502]

Anytime amigo!