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I have a 1981 15 sport that originally had a 70 merc, and am looking to replace it with a 40 or 50 4 stroke. My question is would a 40 four stroke be too small for this boat? I'm gonna be fishing on it and don't plan or want to go past 30 knots. Thanks!

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See the second boat on my personal page. I think I would have to say that a 50 is about as small as you'd want to go on a 15. With two adults it was fine. With four it was sluggish.

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Go for the 50. I converted my 40 to a 50. With just me in the boat I was seeing about 34-35 once optimized. Now with 50 hp I'm seeing 39-40.

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I had a 40HP 2 stroke on my 15 Sport. It was adequate for cruising with 2 adults and 2 small kids. Going with a 4 stroke, I would also recommend the 50HP.

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I have a F50 on my 15. More than enough for what I need.

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Ordered a new f50 Yamaha four stroke. I think I made the right call

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A 50hp should make that thing scoot!

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I wouldn't consider 50hp as under powering classic 15' whaler. I think under powering as using 20, 25 or 30hp on 15'.

A classic 15' with 50hp should be just as fast as max hp classic 13' with 40hp or the current 150 sport/Montauk with 60hp.

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I personally think with that F50......you are 100 hp short of fun!....just kidding, good luck with her.

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Agree with dgoodhue. A 50 is about the mid-point for powering a 15. It will deliver nice performance. Depends upon what you want. If you frequently run heavy loads, pull heavy skiers, or want higher top end go 60 or 70.

When my 40 was 40 it did very well but upgrading it to a 50 I realized a very nice performance increase.

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The 15 with a 50 is "acceptable". The 15 with a 70 is a speedboat.

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...and in 1976 the Sport 15 was rated for as high as a 75 hp. I can't imagine what that would be like. It was later de-rated to 70 max.

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I'm going to have to take you for a ride one day then, my little15 flies, with a fifty foot rooster tail, lol. And I'd still like to fit it with a V-6.

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Do you have any YouTube videos of this thing running? I don't mean from the boat itself, I mean from the shore....or another boat.

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Not yet, I've only run it fast at night, last August, I'm gonna try and hit the Homossassa meet this April. With two people on board one in front, one in back she hit 62 on gps with a new 28" cleaver. I'm running it this weekend I'll try so you guys can see how the hull rides nose down, flat and stable as a rock.

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I guess the rest of my comment above didn't post earlier, I wanted to mention there are a couple of easy mods you can make that will really improve the performance of your 15 classic hull no matter what motor you have astern. First you have to get weight forward in the hull by either or both adding ballast to the forward locker, how I tested it was with a thick garbage bag full of water, then weighed it. Around 50-80lbs does wonders. If you ever refurb or remove the wood seating, I moved my entire seating unit 9" forward of factory location. Second, if you ever flip the hull over for repairs, extend the strakes all the way to the transom. The all around benefit is dramatic, and it would be especially so for a boat with a smaller motor. See link below

http://i1320.photobucket.com/albums/u...3b07a9.jpg

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James, I didn't think y'all were doing the Homossassa run this year.

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Just being hopeful I guess...

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I have found that my boat prefers more weight in the rear. I have moved my gas tank around and people around. With two people on the rear seat my engine rpm is spins up to 6200rpm. If I put move the second person to the forward seat, I can only get just a hair over 6000rpm. It is about 1.5mph difference in top speed. The rear pad modification on your boat maybe affecting the trim of the boat WOT. You boat also has a lot more HP than mine does; it maybe closer to double. Their maybe also production variances in the hulls as well.

When I replace my motor and/or refinish my interior, I am definitely going to make the hull mods that you have. My boats (as it is set up) is just as fast with one person as two, because the out of control chine walking with one person. I probably won't over power my boat.

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I have a Honda 50 4 stroke on my 15. I feel it is about 25HP to low.
I can just hit 30 with just me on the boat (165 pounds)

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30 is slow for a 15' w/50hp. I would expect more than 35mph. Assuming the engine is running properly, it probably has too low of an engine mounting height or has the wrong pitch prop. The engine should be mount at least 2 holes up and the engine should be the manufacturer wot rpm range at wot with a light load.

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I agree - I used to get ~33mph with my 40 and me only in the boat once optimized. What is your WOT rpm? Does the engine over rev?