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Old 02-08-2009, 21:52   #1
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Losing Power Intermittently When Planing

I have a Nissan 8hp 4-stroke engine on my inflatable dinghy. When the boat has many people in it (and doesn't plane), and I run the engine at full speed, it works fine. When the water is calm and I run it at full speed and plane, it runs well. When the water is rough and the dingy is going up and down waves, then the engine tends to lose power momentarily. Usually, when this happens, I feel the boat lose thrust for a second and I hear the engine rev higher. I don't think the propeller is coming out of the water though it's hard to check when you're in the boat. I've already rebuilt the carburetor, and also changed the fuel pump, due to other problems. What else should I check? The engine has had this problem every since I bought it new four years ago.
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Old 02-08-2009, 21:55   #2
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Sure sounds like cavitation... is the prop low enough to receive a clean flow of water?
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Old 02-08-2009, 22:05   #3
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Well, there are about five holes I can choose from to adjust the angle of the shaft as it enters the water. I think I have it on the middle one now. I think I've tried the lowest one as well but I can try again.
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Old 02-08-2009, 22:06   #4
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More importantly, how far below the keel is the prop? When the boat is moving slow there is plenty of water for it to grab, but when planning it may be cavitating due to turbulence caused by the keel...its a very real problem. I have seen stainless steel props on powerboats turned BLUE by overheating due to cavitation.
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Old 03-08-2009, 00:58   #5
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The problem could be with your fuel float in the carb. The bouncing up may be elevating the float and it may stick just enough to shut off your fuel to the bowl. Just a guess.

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Old 03-08-2009, 01:41   #6
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Could it be a lose ignition or spark plug lead?
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Old 03-08-2009, 04:42   #7
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If the revs go up when this happens, it's not a lack of fuel or spark. While there could be a mechanical slip somewhere this is almost certainly cavitation or the prop coming out of the water.

The holes to adjust the engine angle for the planing angle (bow up or down more). Most dinghies I have used do fine with the engine in the full down (first hole) position.

As the boat gets up on a plane, it rises in the water and there is less boat below the water line and thus less prop depth. If you have trouble getting up on a plane in this position consider adding a planing fin to the lower unit.

The problem you are having is not uncommon.

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Old 03-08-2009, 06:16   #8
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Christian, if you think the problem is the keel interupting the flow of water, are you saying I should deflate the keel and not use it? Other than changing engine angle, is there anything else I can do? I've already replaced the float in the carburetor and that didn't seem to help. I'll check the spark plugs.

I will put the engine back in the lowest position. It does plane that way, but it also causes the bow to lift more when trying to get up on plane initially, which I guess is ok.
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Old 03-08-2009, 08:25   #9
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If you deflate the keel, you will have less directional stability and I doubt that will solve the problem. However, each combination of dink and motor is unique so don't be afraid to try it.

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Old 03-08-2009, 09:17   #10
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I suspect two things cavitation or aeration or both.

Aeration is when air gets to the blades. Aeration can be prevented by getting the propeller lower in the water and/or trimmed properly. It can also be reduced by running the engine easier in chop. Aeration you feel and hear with a quick reduction in power and sometimes an increase in RPM when the propeller suddenly has air around the blades.

Cavitation is when a very temporary vacuum is produced. When the vacuum collapses the energy is transmitted back to the blade which you feel as vibration and sometimes hear. Cavitation will eventually damage a propeller. Cavitation can be stopped by loading the propeller less or by increasing the surface area of the propeller or by reducing the pitch of the propeller....or a combination of the three factors.

Its easy to get these confused since the symptoms are similar.
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Old 08-08-2009, 07:34   #11
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Motor Mounting Height is an important factor in achieving optimal performance. The motor should be as high as possible without ventilating or loss of water pressure. This minimizes the effect of hydrodynamic drag while underway, allowing for greater speed. Generally, the antiventilation plate should be about the same height as, or up to two inches higher than, the keel.

The ideal trim angle is the one in which the boat rides level, with most of the hull on the surface instead of plowing through the water.

If the motor is trimmed out too far, the bow will ride too high in the water. With too little trim, the bow rides too low.

The optimal trim setting will vary depending on many factors including speed, hull design, weight and balance, and conditions on the water (wind and waves).

Ventilation is a phenomenon that occurs when surface air or exhaust gas (in the case of motors equipped with through-hub exhaust) is drawn into the spinning propeller blades. With the propeller pushing mostly air instead of water, the load on the engine is greatly reduced, causing the engine to race and the prop to spin fast enough to result in cavitation.

Cavitation is defined as the phenomenon of formation of vapour bubbles of a flowing liquid in a region where the pressure of the liquid falls below its vapour pressure, at which point no thrust is generated at all. The condition continues until the prop slows enough for the air bubbles to rise to the surface. The primary causes of ventilation are: motor mounted too high, motor trimmed out excessively, damage to the antiventilation plate, damage to propeller, foreign object lodged in the diffuser ring, vegetation caught on the lower unit, or anything that interrupts the flow of water into the propeller blades.

Tilt and Trim for Small Engines
Small Outboard Engine Tilt and Trim: Theory and Practice
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Old 08-08-2009, 08:39   #12
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Possibilities include cavitation or prop slippage. Does the prop connect to the shaft with a sacrificial pin or with a rubber clutch fitting. Wear of the rubber part in side the prop will sometimes cause the prop to slip on the shaft, loosing thrust and causing the engine to race. This more commonly would occur with a higher boat load. In your case it is more likely air cavitation as it occurs with one person in the boat at high speed. Are there other props (different size or pitch) available for the motor? Sometimes changing to a different prop will eliminate the problem. Ask the engine manufacturer.
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