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The Right Size Outboard Gas Engine for a 18-ft Boat?
I would like to know the proper size outboard gas engine that I should
put on a 18-ft aluminum semi-V/deep-V boat. According to Tracker web site, they stated that the max HP for their 16-ft deep-V aluminum boat is 40HP. I would assume that the max HP of a similar 18-ft boat "may" be around 50HP (?) Obviously, this 50HP figure is just the maximum limit. And I may want something smaller than 50HP to save gas, to reduce noise, to reduce weight that an electric motor has to pull. But if I go to the other extreme by getting a very small engine, I may spend too much time "getting there" especially against the current. Somewhere between the maximum and the minimum, there may be a horsepower rating that is appropriate for my need. Let explain what my needs a - Put the gas engine in a 18-ft alumimum boat. - Carry 4 adult males (let's say 750-lb total). - Use it strictly for fishing, not for skiing. - I like to drive my car at or below speed limit, not over it. Likewise, I operate a boat at moderate speed only. - Want a quiet engine (I guess I need a 4-stroke engine). - Want to save gas money. What size gas engine should I get? A range of horsepower should be fine. Thanks for any info in advance. Jay Chan |
"Misifus" wrote in message news:yChWe.20499$dm.12431@lakeread03... wrote: I would like to know the proper size outboard gas engine that I should put on a 18-ft aluminum semi-V/deep-V boat. According to Tracker web site, they stated that the max HP for their 16-ft deep-V aluminum boat is 40HP. I would assume that the max HP of a similar 18-ft boat "may" be around 50HP (?) Obviously, this 50HP figure is just the maximum limit. And I may want something smaller than 50HP to save gas, to reduce noise, to reduce weight that an electric motor has to pull. But if I go to the other extreme by getting a very small engine, I may spend too much time "getting there" especially against the current. Somewhere between the maximum and the minimum, there may be a horsepower rating that is appropriate for my need. Let explain what my needs a - Put the gas engine in a 18-ft alumimum boat. - Carry 4 adult males (let's say 750-lb total). - Use it strictly for fishing, not for skiing. - I like to drive my car at or below speed limit, not over it. Likewise, I operate a boat at moderate speed only. - Want a quiet engine (I guess I need a 4-stroke engine). - Want to save gas money. What size gas engine should I get? A range of horsepower should be fine. Thanks for any info in advance. Jay Chan There should be a plate on the boat giving the maximum horsepower for that hull and the maximum number of people or weight allowed. -Raf -- Misifus- Rafael Seibert http://www.ralphandsue.com Bigger engine may not use more gas. Unless you run it wide open. The smaller engines can use more fuel, as they have to be run in the upper range of the throttle to get the performance required. |
Generally, less than 20 percent of the rated max hp will significantly
degrade performance. As for the difference in noise between a 40 and 50 hp engine, it's insignificant. Besides current engines are much quieter than previous two-strokes. ed marine engine digest |
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On Thu, 15 Sep 2005 12:47:50 -0700, barry wrote:
wrote: : I would like to know the proper size outboard gas engine that I should : put on a 18-ft aluminum semi-V/deep-V boat. : According to Tracker web site, they stated that the max HP for their : 16-ft deep-V aluminum boat is 40HP. I would assume that the max HP of a : similar 18-ft boat "may" be around 50HP (?) 50 HP seems very low for an 18 Ft. boat. I'd think something closer to 75 to 100 HP would be more reasonable. An 18-ft Aluminum is a LONG way from a Bayliner! And some engines (not car engines, but inboard diesels and I THINK outboards) are MOST efficient at or near WOT. 40-50hp sounds fine for his boat and his needs. I'd say go 4-stroke for quietness, although you might want to check the specs of your boat to see if there's an engine weight restriction. And considering your other post about taking it on and off, you MIGHT get by with a 25hp 2-stroke, depending on how slow you're willing to go. Lloyd Sumpter "The Tin Boat" Mirrocraft 12 |
Generally, less than 20 percent of the rated max hp will significantly
degrade performance. I assume you mean an outboard engine will reduce its performance significantly if we operate it below 80% of its max hoursepower rating. Do I understand this correctly? In other words, I need to run an outboard engine at or near its peak horsepower rating. This also means that I need to buy an outboard engine whose _max_ horsepower rating matches the cruising speed of the 18-ft boat that I intend to operate at. I probably want to cruise at a leisure speed of 10 to 15-miles-per-hour (here the miles are not nautical miles). What kind of horsepower an outboard engine should have in order to push a 18-ft boat with 4 adults males on it at the cruising speed of 10 to 15 miles-per-hour? Any idea? Thanks. I appreciate your help. As for the difference in noise between a 40 and 50 hp engine, it's insignificant. Besides current engines are much quieter than previous two-strokes. Good to know this. This means I should concentrate on getting the proper horsepower to push the boat instead of being distracted by other factors. Jay Chan |
... And some engines (not car engines, but inboard diesels
and I THINK outboards) are MOST efficient at or near WOT. I assume WOT means "wide open throttle". This means I need to get an outboard engine that will run at or near its rated max horsepower when it can push the 18-ft boat at the cruising speed that I intend to operate the boat at. (The speed that I intend to run the 18-ft boat is like 10 to 15 land-miles per hour). This sounds good to me because this means I can buy a low cost small engine and run it near top speed instead of buying an expensive big engine and run it at low speed. This works out great for my money. Thanks for the good news. 40-50hp sounds fine for his boat and his needs. I'd say go 4-stroke for quietness, although you might want to check the specs of your boat to see if there's an engine weight restriction. Thanks for confirming that 4-stroke engine is quieter than 2-stroke. I am not sure if I really need a 40-50hp engine because my intended cruising speed is only 10 to 15 land-miles-per-hour. Actually, I don't know because I really don't know how small an engine can push the 18-ft boat at 10 to 15 miles-per-hour cruising speed. And considering your other post about taking it on and off, you MIGHT get by with a 25hp 2-stroke, depending on how slow you're willing to go. Actually, I am thinking of 9.8hp because many lakes in New Jersey (where I live) have restriction to limit the engine to no more than 10-hp. And a 9.8hp 4-stroke engine is around 90-lb; two persons with a carrier should be able to hoist it. I am currently leaning toward getting a 9.8hp as long as it can push the 18-ft boat with 4 adult males on it at 10 to 15 land-miles per hour. What do you think? Thanks for any info in advance. Jay Chan |
Actually, I am thinking of 9.8hp because many lakes in New Jersey
(where I live) have restriction to limit the engine to no more than 10-hp. And a 9.8hp 4-stroke engine is around 90-lb; two persons with a carrier should be able to hoist it. I am currently leaning toward getting a 9.8hp as long as it can push the 18-ft boat with 4 adult males on it at 10 to 15 land-miles per hour. What do you think? You are going to need at least a 15 to do what you intend to do, but I believe that a 25 is going to be much more appropriate. This is kind of disapponting to know this; I am hoping that a 9.8hp will be good enough. Seem like I will need a 25hp outboard gas engine. In this case, I need to get a very powerful electric trolling motor for lakes that are electric only or limit to 10hp-or-less. Luckily, those lakes that have restriction tend to be small lakes (with some exceptions). Oh well... Thanks anyway. Jay Chan |
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Boy I hope you don't ever intend to go into rivers and that the lakes
have no current. A 9.8 with that load might not even hold its own in a stiff current. I remember watching my brother in a 15ft aluminum boat with an older 25 Hp on it sitting in the Little Kanawa river near Parkersburg, WV. The river was high and flowing fast, but not yet at flood stage. He was sitting still with the engine WOT. Could not make any headway at all and barely got it back to shore where we could tie it back up to the dock. At normal current it would do OK and get on plane with him in it. I couldn't see it doing much with 4 full sized men in it though. Yes, you are probably right. Some old post said that a 9.8hp would push a 12-ft boat only at 12 mph with 2 adult males and other gears. I would guess that the same engine will only push a 18-ft boat at 6 mph or less with 4 adult males and other gears. This would be too slow even for me. On the other hand, someone else in another post said that his 9.9hp new engine with a new prop could push a 18-ft boat at 20 mph (probably with only himself in the boat). I guess this would mean around 10 mph if there are 4 adult males on the boat instead of just one. Seem like the best course of action a - Get a powerful trolling motor and use it in electric-only lakes. Electric-only lakes tend to be very small anyway. - Get a 9.9hp outboard engine and use it in 10-hp-or-less-only lakes (there are many lakes like this around here (New Jersey)). And play around with it with various number of passengers to see how slow it can get, and whether the speed is acceptable. I would think that this should be OK because those lakes don't have any current to speak of. At that time, I will get enough information about the boat/engine combo, and I will be able to decide whether I need a bigger horsepower engine to run the boat in sal****er. - If I discover that 9.9hp is only good enough for freshwater lakes and is not appropriate for sal****er use, I will get a larger engine (like 40hp) and use the 9.9hp engine for trolling. You see, I need the 9.9hp engine to run the boat in 10-hp-or-less lakes anyway. This means it will not be wasted. Jay Chan |
On Fri, 16 Sep 2005 09:08:28 -0700, jaykchan wrote:
Actually, I am thinking of 9.8hp because many lakes in New Jersey (where I live) have restriction to limit the engine to no more than 10-hp. And a 9.8hp 4-stroke engine is around 90-lb; two persons with a carrier should be able to hoist it. I am currently leaning toward getting a 9.8hp as long as it can push the 18-ft boat with 4 adult males on it at 10 to 15 land-miles per hour. What do you think? I think a 9.8 might be a bit "lean" for that, although I Hear You about the size. Here, "under 10hp" means you don't have to license it. Then again, there's lots of 15hp engines around with 9.9 "tags" on them... As for speed, there's this problem called "hull speed". In most boats, either you're below hull speed ("displacement mode"), or well above it (planing). So either you're going about 5-6mph, or 15-20mph. 10 mph is in that "in-between" area where you're not really doing either. So either you get an engine big enough to plane the boat (probably at LEAST 25hp for your boat and 4 adults), or stay in displacement mode at 5-6mph, and you could do that with a 7.5. In other words, a 9.9 probably won't go much faster than a 7.5, but a 25 would to MUCH faster. Make sense? Lloyd |
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I think a 9.8 might be a bit "lean" for that, although I Hear You about
the size. Here, "under 10hp" means you don't have to license it. Then again, there's lots of 15hp engines around with 9.9 "tags" on them... As for speed, there's this problem called "hull speed". In most boats, either you're below hull speed ("displacement mode"), or well above it (planing). So either you're going about 5-6mph, or 15-20mph. 10 mph is in that "in-between" area where you're not really doing either. So either you get an engine big enough to plane the boat (probably at LEAST 25hp for your boat and 4 adults), or stay in displacement mode at 5-6mph, and you could do that with a 7.5. In other words, a 9.9 probably won't go much faster than a 7.5, but a 25 would to MUCH faster. Make sense? Thanks and Thanks Again! This sounds very good to my plan. When I operate a rental boat, I never put it in plane mode. Therefore, I probably was running at 5-6 mph instead of 10 mph that I original thought. Because I don't intend to operate the boat in plane mode, I can use a low cost small engine, such as a 7.5hp that you have recommended. Honestly, the price difference between a 8hp and a 9.8hp engine is quite small comparing to the total cost of an engine. Therefore, I will probably choose the engine depending on which one is available on the used boat. If the used boat comes with a 7.5hp engine, I will use it. If the used boat comes with a 9.8hp engine, I will use it too. I am not picky as long as the engine is within the 10hp limit that many lakes in NJ state parks only allow. I will think about the 25hp engine later (may be for sal****er rse). This should work out great for my plan! Jay Chan |
I think a 9.8 might be a bit "lean" for that, although I Hear You about
the size. Here, "under 10hp" means you don't have to license it. Then again, there's lots of 15hp engines around with 9.9 "tags" on them... I forgot to respond to this in my last post. The 10hp limit has nothing to do with license. Here in New Jersey, many lakes in state parks only allow a boat that is electric only or has a small gas engine that doesn't go beyond 10hp. If the gas engine in a boat is over 10hp, we cannot run the boat in that lake. This significantly limits the number of freshwater lakes that I can run the boat on (something like from 10 available lakes reduced to only 2). And two of the lakes that I really want to fish have that 10-hp limit. If I want to use a large engine, I will have to use it in sal****er or those two freshwater lakes mentioned above. Currently, my primary plan is to run the boat on freshwater lakes. Running the boat in sal****er is only an afterthought ("that would be nice" kind of thing). Therefore, outfitting the boat with a gas engine that can run in most of the freshwater lakes is the first priority. And I am glad to hear that a 7.5hp gas engine should be fine for a 18-ft boat with 4 adult males on it as long as I am OK with running the boat in displacement speed. Two years from now, I may have explored enough of the freshwater lakes, and I may start thinking of running the boat in sal****er. At that time, I may decide to get a 25-hp or 40-hp outboard motor, and use the 7.5hp engine for trolling. You have helped me to make the right selection. Thanks. Jay Chan |
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