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Author Topic:   4-stroke & prop

Buffalo posted 08-18-2000 09:16 AM ET (US)
I'm repowering a 83 Mountauk with a 100 hp Yamaha. Does anybody know how this set up works out? What size prop? How will fuel economy be compared to my 91 Johnson 100 that is on the boat now?
Clark Roberts posted 08-18-2000 04:17 PM ET (US)
Tripper, the 100 Yamaha 4 stroke (aka 90 Merc 4 stroke) whighs about 100 lbs more than your 100hp V4 Johnson.. roughly 400lbs vs 300lbs... you may notice stern heavy handling and more "diggin in" on take off! Handling in general should not be upset. You may want to move some weight forward to compensate.. There's a slight difference in displacement 97cubic inches for the Yam and 99 for the OMC but the real difference you may feel is that the Yamaha may not have the hole shot that the old V4 two stroke has.. mid range accelleration will most likely be less "peaky" and a little slower with the 4 stroke but top end should be about the same.
The only way to get correct prop is to use a tach and experiment... you should select a prop that lets the engine reach manufacturers top rpm recommendation.. I usually prop for a couple hundred rpm more than recommended with boat lightly loaded.. then when she's loaded with fuel and gear and a passenger or two I don't loose any get up and go.... The 100 OMC is not famous for thrifty operation so don't be surprised if you get two to three times the fuel economy with the Yam/Merc. it will be quieter and smoother with no smoke.. Give us a report on how it works out... happy Whalin'.. Clark.. Spruce Creek Navy

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Author	Topic: 4-stroke & prop
Buffalo	posted 08-18-2000 09:16 AM ET (US) I'm repowering a 83 Mountauk with a 100 hp Yamaha. Does anybody know how this set up works out? What size prop? How will fuel economy be compared to my 91 Johnson 100 that is on the boat now?
Clark Roberts	posted 08-18-2000 04:17 PM ET (US) Tripper, the 100 Yamaha 4 stroke (aka 90 Merc 4 stroke) whighs about 100 lbs more than your 100hp V4 Johnson.. roughly 400lbs vs 300lbs... you may notice stern heavy handling and more "diggin in" on take off! Handling in general should not be upset. You may want to move some weight forward to compensate.. There's a slight difference in displacement 97cubic inches for the Yam and 99 for the OMC but the real difference you may feel is that the Yamaha may not have the hole shot that the old V4 two stroke has.. mid range accelleration will most likely be less "peaky" and a little slower with the 4 stroke but top end should be about the same. The only way to get correct prop is to use a tach and experiment... you should select a prop that lets the engine reach manufacturers top rpm recommendation.. I usually prop for a couple hundred rpm more than recommended with boat lightly loaded.. then when she's loaded with fuel and gear and a passenger or two I don't loose any get up and go.... The 100 OMC is not famous for thrifty operation so don't be surprised if you get two to three times the fuel economy with the Yam/Merc. it will be quieter and smoother with no smoke.. Give us a report on how it works out... happy Whalin'.. Clark.. Spruce Creek Navy