Now that the sailing season is over for me and the boat is on the hard I am thinking of replacing my o/b motor with a new one. Currently have an 8 hp Tohatsu. I would like to replace with something with alittle more hp. Was thinking of moving up to 9.9 hp. My boat is a Hunter 27' 1992. The owner of the marina where I winter the boat thinks I should move up to 15 hp given that the difference in weight is minimal but the added hp would be a plus. My questions are; how much weight is the factory installed motor mount rated for and is there really value to that much more hp? P.S. This is my 1st year with a sail boat (motor boated for many). Although I have not posted any questions prior to this I have found this site extremely helpful and have learned much just reading the posts at this site. Thank you in advance for any help with my questions.
how much hp?
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Power question
The engine you have is more than enough. Unless there is something wrong with it. Going to a bigger engine is pretty much a waist of time and money. Your boat is going to only go so fast. Its better to run a small engine at full power than to run a large engine at 1/2 power.
The engine you have is more than enough. Unless there is something wrong with it. Going to a bigger engine is pretty much a waist of time and money. Your boat is going to only go so fast. Its better to run a small engine at full power than to run a large engine at 1/2 power.
Don, that rule sure works for a diesel,
but he's talking gas. My Yamaha manual says that running full throttle for extended operation is abuse. The warranty won't cover abuse. I would go for the fifteen. Most boats around here with OBs use that size. But try to get one with a exhaust diverter to aid with reverse.
but he's talking gas. My Yamaha manual says that running full throttle for extended operation is abuse. The warranty won't cover abuse. I would go for the fifteen. Most boats around here with OBs use that size. But try to get one with a exhaust diverter to aid with reverse.
I think where you would notice the extra HP
is when conditions are really nasty (lots of chop & swell). Many of the 27's out here repower with the Honda 9.9 and it serves them well in fair weather. My 270 has an 18hp diesel and I've needed every bit of it in small craft warning conditions. I'll vote for the 15 if extra weight is not an issue.
is when conditions are really nasty (lots of chop & swell). Many of the 27's out here repower with the Honda 9.9 and it serves them well in fair weather. My 270 has an 18hp diesel and I've needed every bit of it in small craft warning conditions. I'll vote for the 15 if extra weight is not an issue.
The mathematics of hull speed
Skip, the question of how much horsepower is needed as an auxiliary is tied to the characteristics of the sailboat hull. Keelboats are "displacement" hulls, that is, they push aside water rather than plane on top of it. There's a mathematical formula which I won't go into here that expresses what the most economical horsepower would be for displacement hulls of certain weights and dimension; that number is called the "hull speed". It's not an absolute limit on how fast the hull can go, but on how fast it can go with maximum efficiency. My 28-foot, 7400-lb., 10 1/2 foot wide sloop was equipped by the factory with a mere 13.5 hp diesel because any more horsepower than that is beyond its' theoretical hull speed of 8 knots. The hull speed of my Catalina 25 was 6 1/2 knots, so your hull speed is somewhere around 7 knots. Having stated the theory, I'll point out that the trend today is to put much more powerful engines in new boats and I think there's some merit in the idea when you consider that some conditions, such as strong tides or winds, require a little extra oomph to keep hull speed. If that 15hp really weighs about the same as your 8hp, then it wouldn't be a bad idea to go for a little extra power. Make sure your bracket and transom can take it!...
Skip, the question of how much horsepower is needed as an auxiliary is tied to the characteristics of the sailboat hull. Keelboats are "displacement" hulls, that is, they push aside water rather than plane on top of it. There's a mathematical formula which I won't go into here that expresses what the most economical horsepower would be for displacement hulls of certain weights and dimension; that number is called the "hull speed". It's not an absolute limit on how fast the hull can go, but on how fast it can go with maximum efficiency. My 28-foot, 7400-lb., 10 1/2 foot wide sloop was equipped by the factory with a mere 13.5 hp diesel because any more horsepower than that is beyond its' theoretical hull speed of 8 knots. The hull speed of my Catalina 25 was 6 1/2 knots, so your hull speed is somewhere around 7 knots. Having stated the theory, I'll point out that the trend today is to put much more powerful engines in new boats and I think there's some merit in the idea when you consider that some conditions, such as strong tides or winds, require a little extra oomph to keep hull speed. If that 15hp really weighs about the same as your 8hp, then it wouldn't be a bad idea to go for a little extra power. Make sure your bracket and transom can take it!...
Forget weight, consider power torque
The weight difference is much less important than the forces on the motor mount caused by the power of the engine. Let's face it. You are going to crank up the throttle hard a few times, even if you normally run it at half power. The 15hp engine will just about double the stresses on the motor mount even though it only weighs a tenth to a fifth more. I'm not opposed to the larger engine. I went with a 15hp because it had electric start and a generator to recharge the battery on my 27' ODay. But you should find out what your mount is rated and perhaps buy a stronger motor mount. It would be a shame to come in to the dock too fast and gun it into reverse, just to have the engine and mount fall off the boat (quickly followed by running into the peer and damaging the bow [immediately followed by lots of swearing and yelling]). Sincerely, Capt Jim
The weight difference is much less important than the forces on the motor mount caused by the power of the engine. Let's face it. You are going to crank up the throttle hard a few times, even if you normally run it at half power. The 15hp engine will just about double the stresses on the motor mount even though it only weighs a tenth to a fifth more. I'm not opposed to the larger engine. I went with a 15hp because it had electric start and a generator to recharge the battery on my 27' ODay. But you should find out what your mount is rated and perhaps buy a stronger motor mount. It would be a shame to come in to the dock too fast and gun it into reverse, just to have the engine and mount fall off the boat (quickly followed by running into the peer and damaging the bow [immediately followed by lots of swearing and yelling]). Sincerely, Capt Jim
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