I'm looking at a new Tohatsu 4 stroke outboard. But I would like to know If any one knows the difference between the 4-5-and 6hp motors? The specifications page only shows tha the R.P.Ms make the difference in the HP. What am I missing? The bore
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Ray
Dave, The most important question is what size
and weight boat will you be pushing with this motor. By going with a 4 stroke or 4 cycle engine (both being the same)you will greatly reduce the amount of pollution you put in the air. A 4 cycle engine is also different in how it arrives at its horsepower ratings. While the engine weight, cubic inch, or cubic centimeter size is the same for all horsepower ratings, the differences are gained by changes in camshaft lift, duration, overlap and timing. Compression differences and carburation or fuel injection changes also have major input values. Horsepower ratings are most often gained at higher RPM ranges. Horsepower is the push that gives top speed. Torque is the value that can move a mass of weight off a dead resting position and into higher speeds. Torque originates at low RPM values. As you stated this same engine comes in ratings of 4,5 and 6 HP. The higher the HP, the greater the loss of comfort. Higher HP reduces smooth idle and fuel economy. It usually required more maintanance and always costs more. Sometimes fuel economy so greatly reduces range as to be unuseable. If you are pushing a light boat then higher HP engines are great. If you are planning to move heavy displacement boat then what is required is a high torque engine. Therefore to push your dingy, and since the motor weight is the same regardles of HP I would go for the larger HP engine. To push a heavy boat I would look for a motor with the higher torque value. Then try this motor and see how close it comes to pushing the boat at hull speed. Also study RPM ranges closely. If the engine pushes hard but the RPM range will only come to about 70% of its rated range then re-prop with a more powerful prop rather than higher speed prop. This should not only raise RPM but also speed. If the motor being used also generates power to recharge the batteries then you want the highest RPM while still running in a good torque range. Most engine dealers will explain these factors to you and also supply the props necessary to find the right range. So,I know I've confused you because I'm also lost. That's why "Speedy" has a pair of nice paddles, one for each of my wifes arms. Ray S/V Speedy
and weight boat will you be pushing with this motor. By going with a 4 stroke or 4 cycle engine (both being the same)you will greatly reduce the amount of pollution you put in the air. A 4 cycle engine is also different in how it arrives at its horsepower ratings. While the engine weight, cubic inch, or cubic centimeter size is the same for all horsepower ratings, the differences are gained by changes in camshaft lift, duration, overlap and timing. Compression differences and carburation or fuel injection changes also have major input values. Horsepower ratings are most often gained at higher RPM ranges. Horsepower is the push that gives top speed. Torque is the value that can move a mass of weight off a dead resting position and into higher speeds. Torque originates at low RPM values. As you stated this same engine comes in ratings of 4,5 and 6 HP. The higher the HP, the greater the loss of comfort. Higher HP reduces smooth idle and fuel economy. It usually required more maintanance and always costs more. Sometimes fuel economy so greatly reduces range as to be unuseable. If you are pushing a light boat then higher HP engines are great. If you are planning to move heavy displacement boat then what is required is a high torque engine. Therefore to push your dingy, and since the motor weight is the same regardles of HP I would go for the larger HP engine. To push a heavy boat I would look for a motor with the higher torque value. Then try this motor and see how close it comes to pushing the boat at hull speed. Also study RPM ranges closely. If the engine pushes hard but the RPM range will only come to about 70% of its rated range then re-prop with a more powerful prop rather than higher speed prop. This should not only raise RPM but also speed. If the motor being used also generates power to recharge the batteries then you want the highest RPM while still running in a good torque range. Most engine dealers will explain these factors to you and also supply the props necessary to find the right range. So,I know I've confused you because I'm also lost. That's why "Speedy" has a pair of nice paddles, one for each of my wifes arms. Ray S/V Speedy
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dave b.
speedy
speedy, WOW- I've been a machinist for 35 years, and I'v never had anyone explain it to me as well as you have!! Very nice. For this motor, I have heard that all 3 of them are the same, except that there is some kind of stop that's put on the carb to keep the R.P.M'sdown on the lower H.P> modles. But as you pointed out, there could be other things going on as well, timeing, carb jets, props. Thank you , I'll lok fwd. to hereing from you, or any one elence that can help me. Dave b. Nebr. ps- it for a 23.5
speedy, WOW- I've been a machinist for 35 years, and I'v never had anyone explain it to me as well as you have!! Very nice. For this motor, I have heard that all 3 of them are the same, except that there is some kind of stop that's put on the carb to keep the R.P.M'sdown on the lower H.P> modles. But as you pointed out, there could be other things going on as well, timeing, carb jets, props. Thank you , I'll lok fwd. to hereing from you, or any one elence that can help me. Dave b. Nebr. ps- it for a 23.5
Get the 6 hp if your wallet can stand it
A larger engine will get you up to hull speed at lower rpm than a less powerful engine. Fuel economy will be better and you'll have less noise and vibration. The first time the weather turns nasty and the nearest safe port is dead upwind, you will be glad you shelled out the extra bucks for more "grunt". Before you buy, make sure that the engine mount on the boat can handle the extra thrust and weight of a four-stroke. Most of them are designed for two-strokes. Happy sailing. Peter H23 "Raven"
A larger engine will get you up to hull speed at lower rpm than a less powerful engine. Fuel economy will be better and you'll have less noise and vibration. The first time the weather turns nasty and the nearest safe port is dead upwind, you will be glad you shelled out the extra bucks for more "grunt". Before you buy, make sure that the engine mount on the boat can handle the extra thrust and weight of a four-stroke. Most of them are designed for two-strokes. Happy sailing. Peter H23 "Raven"
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BART FORD
4 stroke motor
I have a 4 stroke Nissan 5hp. It is a great motor but I wish it had a charging loop and maybe electric start. It gets real tiring pulling the cord. I would opt for the biggest motor you can afford. If you ever go to the coast, the extra push would come in handy in a tidal situation. If you end up getting a 4 stroke, you want to buy the premium gas like Amoco. I had a bad carbon buildup one summer and the motor spent more time in the shop than on the boat. Also when you come in after sailing and tie up or put the boat back on the trailer, I have found it helpful to disconnect the gas line and run it until it stops. Makes it easier to start the next ime.
I have a 4 stroke Nissan 5hp. It is a great motor but I wish it had a charging loop and maybe electric start. It gets real tiring pulling the cord. I would opt for the biggest motor you can afford. If you ever go to the coast, the extra push would come in handy in a tidal situation. If you end up getting a 4 stroke, you want to buy the premium gas like Amoco. I had a bad carbon buildup one summer and the motor spent more time in the shop than on the boat. Also when you come in after sailing and tie up or put the boat back on the trailer, I have found it helpful to disconnect the gas line and run it until it stops. Makes it easier to start the next ime.
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