I've noticed that when when I am at a dock and there is a moderately strong aft wind and I need to back out of the dock, the outboard (1980 7.5 hp Johnson SeaHorse) doesn't make much progress. It seems to have plenty of power going forward but in reverse in an aft wind it just seems to cavitate and sometimes pop out of the water with the waves. If I need to go slow backwards, without fighting wind and waves, it works fine enough. Is this a problem with the motor, prop, trim, user error, or the fact that the boat is so blunt at the back? How well does your boat/motor combo work in reverse? I don't have it in the budget to buy another motor now so I am hoping for an easy solution.... Thanks, sPk
Reverse on O'Day 25?
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Reverse
I have a similar problem on my 15hp Johnson. Great forward and bad reverse. I think my problem is that the po installed a high pitch prop - I think around 12 degrees. I have bought a 5 deg prop but have not installed it. I think the prop will fix it. Prop cost around $100 and looks like an easy install. Scott
I have a similar problem on my 15hp Johnson. Great forward and bad reverse. I think my problem is that the po installed a high pitch prop - I think around 12 degrees. I have bought a 5 deg prop but have not installed it. I think the prop will fix it. Prop cost around $100 and looks like an easy install. Scott
Prop blade shape is the culprit
Outboard propellors are made with a concave surface (facing aft) for maximum efficiency in going forwards, but they have convex surfaces on the rear side of the blade. That means that they are about 70% more efficient in forward than in reverse and,in reverse, they cavitate more easily. Result is that the reverse mode is much less effective than is the forward mode. Its nice to keep that in mind when you want to check way while approaching a dock!
Outboard propellors are made with a concave surface (facing aft) for maximum efficiency in going forwards, but they have convex surfaces on the rear side of the blade. That means that they are about 70% more efficient in forward than in reverse and,in reverse, they cavitate more easily. Result is that the reverse mode is much less effective than is the forward mode. Its nice to keep that in mind when you want to check way while approaching a dock!
Cavitation is the culprit!
The problem may be that you have a standard outboard and not a "sail drive!" Most outboards now have the exhaust port through the center of the props, or through a port just above the prop but below the cavitation plate. When the motor is put in reverse, a "cushion" of exhaust bubbles forms around the blades and the blades no longer can "bite" into the water. You basically have no way of stopping or going backwards at higher speeds. With a power boat this is not much of a problem because when they come off plane, they settle into the water and stop. With a displacement hull, by design, never comes off the plane, the forward progress continues long after the motor is shut off. What you need to do is to drill a number of holes (I drilled 7, 1/4" diameter holes on each side) above the cavitation plate into the exhaust shaft. Just take the lower unit off and check where the exhaust shaft is, usually towards the back, but some new "off shore" motors have it in the middle of the shaft. Get a piece of stainless or aluminum sheet about 1/8" thick, cut to fit at the bottom of exhaust shaft in the lower unit and "glue" it into the lower unit exhaust shaft on the inside. You may have to drill 3 very small holes with self tapping screws to hold this in place. Use high temperature silicone gasket-in-a-tube to seal and "glue" in place. This should do the trick. Of course, you can always take it to your local marina and have them make the "sail-drive" conversion for about $300.00. Your choice. Best of luck!
The problem may be that you have a standard outboard and not a "sail drive!" Most outboards now have the exhaust port through the center of the props, or through a port just above the prop but below the cavitation plate. When the motor is put in reverse, a "cushion" of exhaust bubbles forms around the blades and the blades no longer can "bite" into the water. You basically have no way of stopping or going backwards at higher speeds. With a power boat this is not much of a problem because when they come off plane, they settle into the water and stop. With a displacement hull, by design, never comes off the plane, the forward progress continues long after the motor is shut off. What you need to do is to drill a number of holes (I drilled 7, 1/4" diameter holes on each side) above the cavitation plate into the exhaust shaft. Just take the lower unit off and check where the exhaust shaft is, usually towards the back, but some new "off shore" motors have it in the middle of the shaft. Get a piece of stainless or aluminum sheet about 1/8" thick, cut to fit at the bottom of exhaust shaft in the lower unit and "glue" it into the lower unit exhaust shaft on the inside. You may have to drill 3 very small holes with self tapping screws to hold this in place. Use high temperature silicone gasket-in-a-tube to seal and "glue" in place. This should do the trick. Of course, you can always take it to your local marina and have them make the "sail-drive" conversion for about $300.00. Your choice. Best of luck!
Motor mod?
I took a look at my old Johnson... And I found out that it does indeed have the exhaust below the cavitation plate toward the back of the lower unit. This is very interesting. F.A.A.F.H, did your 'sail drive' modification make a significant difference for reverse performance? thanks for all your ideas and input! sPk
I took a look at my old Johnson... And I found out that it does indeed have the exhaust below the cavitation plate toward the back of the lower unit. This is very interesting. F.A.A.F.H, did your 'sail drive' modification make a significant difference for reverse performance? thanks for all your ideas and input! sPk
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