OB replacement suggestions - SailNet Community

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Go Back   SailNet Community > On Board > Gear & Maintenance
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  #1  
Old 03-13-2006
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OB replacement suggestions

I currently do quite a bit of sailing in the chesapeake bay and surrounding areas in my c&c 24. My current outboard is a Johnson SailMaster 6.0hp. I have found in heavy currents and tides that the 6 is sometimes just not enough. The first thought that came to mind is going up to a 9.9hp. Would it be outrageous to go to a 15hp? I have read that physically the 9.9 and 15 are identical, just changes internally.

Thanks
Andrew
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Old 03-13-2006
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If price is not a concern, buy the 15 h.p. motor. Better to have the power and not need it than to not have the power you need.
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Old 03-13-2006
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You should get a motor that is matched to the task. A motor that is too powerful will cost you more when you buy it, and it'll use more fuel throughout it's lifetime. A small motor will push a displacement boat up to hull speed fairly easily, but it takes an inordinate amount of power and fuel to make it go any faster than hull speed. Your boat's hull speed is a little over 6 kts., and the 15 hp. motor won't push it any faster than the 9.9. The extra 5 hp won't help you.

I had a 7.5 hp motor on a 25 ft boat that I'm sure is heavier than your C&C, and it pushed the boat to hull speed (slightly over 6 kts.) with power to spare. The 9.9 would be more than enough. I suggest you get the 9.9 with an extra long shaft.

I wouldn't be surprised if you would occasionally encounter 6 kt currents in your area, and in that much current, the 15 wouldn't be any more likely to get you through than the 9.9. I tried to pass through a bascule bridge in Florida once, and the 38 ft. boat's 55 hp diesel was barely able to push the boat through, against about a 7 kt. current. If you steer the boat out of the deeper water, and into as shallow water as you dare, the current will be weaker, and you might be able to get through.
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Old 03-13-2006
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I've got an ageing 9.8HP Mercury two-stroke on Sanctification, and I know that I'll be replacing it in the next year or so. It is the original motor, over 23 years old, and is starting to make an ominious ticking sound at high RPM.

Any thoughts on two-stroke vs. four-stroke? I really like how quiet the four-stroke engines are, but don't know squat about reliability or longevity or DIY-repairability.
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Old 03-13-2006
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Quote:
Originally Posted by Sailormon6
A small motor will push a displacement boat up to hull speed fairly easily, but it takes an inordinate amount of power and fuel to make it go any faster than hull speed. Your boat's hull speed is a little over 6 kts., and the 15 hp. motor won't push it any faster than the 9.9. The extra 5 hp won't help you.
I disagree. While the smaller motor may be able drive the boat to hull speed, I doubt it would do so against the strong currents the boat is frequently used in. Further, the larger displacement (edit: actually same displacement for both motors, but you know what I mean) motor would not need to be run as hard as the smaller one to reach a given speed, regardless of the conditions the boat is used in. And I suggest that the difference in fuel consumption between a 9.9 and a 15 h.p. engine is pretty small. Certainly not enough to be a deal breaker, IMHO.

Last edited by Fstbttms; 03-13-2006 at 08:44 PM.
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Old 03-13-2006
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"While the smaller motor may be able drive the boat to hull speed, I doubt it would do so against the strong currents the boat is frequently used in..."
Hull speed is hull speed, it doesn't change in a current, only the speed over ground changes, and HP won't affect that. I'd say that the 9 is all the engine a 24' needs, whether the prop stays in the water is probably more important than useable horsepower. Lugging a more powerful engine isn't necessarily better than running a smaller one at an optimum speed - the cruising RPM mode.

But hey, these are just the voices of experience speaking..
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Old 03-13-2006
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A C&C 24 has a displacement hull, as opposed to a planing hull. As a displacement boat moves through the water, it creates a bow wave and a stern wave. The boat moves in the trough between the two waves, and she's a prisoner of those waves. The boat's length determines how fast the bow wave can travel, and the boat cannot climb up over it's own bow wave and travel faster than it's bow wave, unless you can get the boat up on a plane. Five horsepower is nowhere near enough power to drive your boat over it's own bow wave.

This principle works the same regardless of whether the boat is moving through the water at 6 kts., or the water is flowing under the boat at 6 kts.

That's why you almost never see a 23-25' sailboat with more than a 10 hp. outboard. If you could make a displacement sailboat get up on a plane simply by putting a 15 hp. engine on it, you'd see a lot more sailboats with 15 hp. engines. The only person I know who put a 15 hp. motor on his sailboat did it because he got a better price on a 15 hp. motor than on a 10.

Last edited by Sailormon6; 03-13-2006 at 09:00 PM.
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Old 03-13-2006
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I am glad I asked! Most of the people (friends) I talked to are power boaters and just dont understand. One of them said get a 25. I think power boaters are a couple of clicks away from crazy, we have all seen them on the water. I am just thinking of a time that I was approched by a squall and had to hot tail it to shore, this is the reason for thinking of the 15. Money is a player in my purchase, but so is my safety in the bay. If there was 7kts of current plus a against tide and wind of 15+ in the teeth I am going nowhere fast. Then again no engine would help along the way in this situation. I plan to try out a 9.9 and a 15 from some buddys and make a list of pros and cons. Please feel free to post any info or situations with your OB's.
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Old 03-13-2006
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I understand the principle of hull speed, don't need it explained to me. I'm not suggesting that he try to drive the boat faster than that. I'm suggesting that the more powerful engine would be better suited and more able to provide good handling under power in the conditions lightyear22 would be using the boat in.

That being said, I just looked at maybe a dozen C&C 24 ads online and not one of them was being sold with anything bigger than a 9.9 h.p. outboard. So I defer to the "voices of experience."
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Old 03-14-2006
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In his books on boats, Dave Gerr suggests that propellers make a bigger difference in performance than people realize. Most small outboards are used to push light boats up onto a plane. The diameter and pitch of their propellers are set up to push a little water at a time, but really fast. On a displacement hull, as stated above, more power isn't going to get you anywhere any faster. What's needed is perhaps propeller with a bigger blade area (three or four blades instead of two?) and a deeper pitch, so that it will push more water a little slower, in keeping with the hull form. Maybe yours has this already, but it's more likely to have a more "standard issue one-size fits all" type. Perhaps you could look for a more efficient prop (or at least check to see what yours is) before spending on more horsepower.

Another way to think of the situation is like the difference between a garden hose and a fire hose. You can turn the garden hose up high and get a fine spray of water shooting from it that the kids can play in. Or you could have a fire hose turned low and use the same volume of water to irrigate the garden. Your boat is like the garden- you want the bigger, slower hose.
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