| Author |
Message |
    
Brian Burke
Member
Username: burkstar
Post Number: 14
Registered: 05-2009
| | Posted on Tuesday, October 20, 2009 - 03:55 am: | 
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My outdrive has had its day on my MR470 but I have a replacement. Only problem is its a 1:98R and the original is a 1:84.
What does the number relate to: is that the amount of revs per prop twist or visa versa and if I was to fit the 1:98 what would happen and can I compensate for it by fitting a different pitched prop? |
makomark
Senior Member
Username: makomark
Post Number: 1715
Registered: 09-2008
| | Posted on Tuesday, October 20, 2009 - 10:12 am: |
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The ration is input rotations to output rotations.
Going "up" in ratio will unload the motor a bit. We did a similar 'swap' with an old 165 and were very satisfied with the end result. Hole shot improved, sensitivity to load in the boat was reduced and didn't care about the slight loss of top end speed. |
Brian Brackett
Advanced Member
Username: kainon
Post Number: 998
Registered: 03-2009
| | Posted on Tuesday, October 20, 2009 - 11:15 am: |
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I went from a 1.65r to a 1.98 and went from a 19p to a 23p prop to compensate.
1.98r means the engine makes .14 more of a complete turn before the prop turns 1, so a 1.98r drive requires the engine to almost make a full 2 revolutions to 1 revolution of the prop. If your engine is a little old and weak this will help it, especially with a heavier load. ( but never overload your boat beyond the rating ) |
Ricardo E.
Advanced Member
Username: ricardomarine_vp
Post Number: 697
Registered: 03-2009
| | Posted on Tuesday, October 20, 2009 - 01:02 pm: |
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Ultimately, there is no substitute for a correct over-all gear ratio. M/C designed this ratio for the 470.
Compensating with a different pitch prop may get you by.... but that is all it will do!
I will say that the size of the hull can make a difference whereby it may work OK.
. |
PugetSounder
Advanced Member
Username: pugetsounder
Post Number: 502
Registered: 04-2007
| | Posted on Saturday, October 24, 2009 - 03:01 pm: |
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Currently I have a 1.98:1 leg with a 21P prop on my 19' with a 470. It's not the original leg to the boat. It's not a huge problem for runnability but here are the issues.
At max RPM (4200) my top speed is 32 MPH which is not fast. Cruising speed is 22 - 24. I can run the engine to a lot higher RPMs if I wish. The boat can still do 40+ but at the risk of blowing up the motor. The fuel useage is quite good at 3.5 GPH.
I bought a good used upper unit with the 1.84:1 ratio out of a 4.3L boat, that I plan on swapping sometime this winter. My thought is I should be able to do 40+ but at the proper RPM range and cruising speed should be faster as well. I may have to experiment with diffrent props but we will see next spring. |
