Hello, I am a little confused over the relationship between engine HP, environmental conditions (wind, waves, current), and boat speed. If an engine of X HP will drive a boat at hull speed in calm conditions, how do other conditions, like high currents or rough surf change that? Why would a boat with 2X HP be better in that situation? Isn't the speed of the boat limited by the waterline of the hull? Why is an engine with more HP more desirable? How much does wind affect boat speed when motoring? Thanks, Barry Lenoble
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tom
Hull Speed is the ultimate limit
for displacement boats. But wind can stop you from reaching that speed. The same for waves. As an extreme example my brother and I were anchored out one night when the wind came up. The wind was coming straight down the cove and increased until the boat started sailing back and forth and being heeled as it turned. He had a 10 hp outboard that usually pushed the boat pretty well(28' boat). We decided to move to a safer location. But even with the motor at full trottle we couldn't move into the wind enough to pull in the anchor. Fortunately the anchor held. Next morning it was flat calm. Remember that the force of a given wind increases as the square. ie double the windspeed quadruple the force.
for displacement boats. But wind can stop you from reaching that speed. The same for waves. As an extreme example my brother and I were anchored out one night when the wind came up. The wind was coming straight down the cove and increased until the boat started sailing back and forth and being heeled as it turned. He had a 10 hp outboard that usually pushed the boat pretty well(28' boat). We decided to move to a safer location. But even with the motor at full trottle we couldn't move into the wind enough to pull in the anchor. Fortunately the anchor held. Next morning it was flat calm. Remember that the force of a given wind increases as the square. ie double the windspeed quadruple the force.
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Rick Webb
Think of it as your speed Through the Water
If your boat will do 6kts and the current against you is 4kts your boat will still be doing 6kts through the water but you will only see 2kts of forward speed. Kinda like walking up a down escalator. The wind slowing you down seems sort of like peddling your bike against the wind. The waves might be described as trying to go uphill and will slow you down that way. I always understood hull speed to a point on a very steep graph once you reach that point it is possible to go faster but the power needed increased to the point of even if possible not at all practical. So you are right a bigger motor than required is not going to do anything for you other than sink your transom further into the water a burn more gas. It may help some to drive through a wind a little in waves but none at all through currents. In the archives somewhere there is a formula to use as a guage to determine the size of motor required. It will probably need a smaller motor than you thought it did.
If your boat will do 6kts and the current against you is 4kts your boat will still be doing 6kts through the water but you will only see 2kts of forward speed. Kinda like walking up a down escalator. The wind slowing you down seems sort of like peddling your bike against the wind. The waves might be described as trying to go uphill and will slow you down that way. I always understood hull speed to a point on a very steep graph once you reach that point it is possible to go faster but the power needed increased to the point of even if possible not at all practical. So you are right a bigger motor than required is not going to do anything for you other than sink your transom further into the water a burn more gas. It may help some to drive through a wind a little in waves but none at all through currents. In the archives somewhere there is a formula to use as a guage to determine the size of motor required. It will probably need a smaller motor than you thought it did.
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tom
A dirty hull or towing a dink etc
will also slow you down. Within reason with excess HP you can overcome these extra drags. Also if you are aground the extra HP might let you free yourself. In calm conditions my honda 7.5 would push my old Mac 26 with a clean bottom at near 5.5 kts indicated at less than half throttle. At full throttle I might get another 1/2 knot. BUT I have been in conditions where full throttle only gave me about 3 kts due to a strong headwind. Also as the bottom got dirty it required more power to go 5.5 kts in calm conditions.
will also slow you down. Within reason with excess HP you can overcome these extra drags. Also if you are aground the extra HP might let you free yourself. In calm conditions my honda 7.5 would push my old Mac 26 with a clean bottom at near 5.5 kts indicated at less than half throttle. At full throttle I might get another 1/2 knot. BUT I have been in conditions where full throttle only gave me about 3 kts due to a strong headwind. Also as the bottom got dirty it required more power to go 5.5 kts in calm conditions.
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Bill O'Donovan
An analogy
This is more easily explained in the context of an outboard on a sailboat. A 25-foot boat will do okay with a 9.9hp and get a little more power with a 15hp, but anything beyond that is lost to the physics explained in the previous two posts. By the same token, a 5hp would be way underpowered and take a while to churn the boat through the water. So the ideal is the designated engine, either diesel or OB. By the same token, a three-bladed prop won't always help unless the boat is pushing 35 feet.
This is more easily explained in the context of an outboard on a sailboat. A 25-foot boat will do okay with a 9.9hp and get a little more power with a 15hp, but anything beyond that is lost to the physics explained in the previous two posts. By the same token, a 5hp would be way underpowered and take a while to churn the boat through the water. So the ideal is the designated engine, either diesel or OB. By the same token, a three-bladed prop won't always help unless the boat is pushing 35 feet.
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Rick Webb
prop thrust
Something else to throw into the equation is effective horsepower... actual thrust. Sailboats with outboards are moving away from using props designed for planing hulls. Think of it this way. Lets suppose a planing prop offers 50% efficiency but that a high thrust (larger diameter - less pitch) prop is 25% more efficient at the slow speeds of a sailboat. That would mean that a 10 hp would provide 5 effective horse power of thrust with a planing prop but an 8 with a high thrust prop would provide 6 effective horse power. However its looked at... a high thrust prop is the way to go on a sailboat. One problem is that most motors can't be reasonably converted to high thrust because the prop diameter is limited by the cavitation plate. If buying a new motor... look for a high thrust version, or one that will allow use of a larger diamter prop. Many high thrust motors will exceed the perfomrance of the next higher hp motor set up for a planing hull.
Something else to throw into the equation is effective horsepower... actual thrust. Sailboats with outboards are moving away from using props designed for planing hulls. Think of it this way. Lets suppose a planing prop offers 50% efficiency but that a high thrust (larger diameter - less pitch) prop is 25% more efficient at the slow speeds of a sailboat. That would mean that a 10 hp would provide 5 effective horse power of thrust with a planing prop but an 8 with a high thrust prop would provide 6 effective horse power. However its looked at... a high thrust prop is the way to go on a sailboat. One problem is that most motors can't be reasonably converted to high thrust because the prop diameter is limited by the cavitation plate. If buying a new motor... look for a high thrust version, or one that will allow use of a larger diamter prop. Many high thrust motors will exceed the perfomrance of the next higher hp motor set up for a planing hull.
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