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need for speed


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sorry this is in the wrong room..i couldnt get the other one to work. but anyways i have a lund pro angler (i think) its a 17' boat with a walk through windshield and a 90 hp johnson (rated for a max of 115hp). Im just wondering if anyone has any ideas to make it a little faster?? thanks for any tips.

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To get it up on plane faster, you could try one of the delta wing style products.

Next winter, if your mech. inclined, pop off the heads and take them in for a revamp, bigger valves, port and polish. An example would be like the Johnson 40 to 50 horse, the displacement is the same just better breathing for the 50 horse.

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Kyle, why don't you kill 2 birds with one stone and put a 115 hp Yamaha four-stroke on your boat? Your boat will then be at maximum horsepower, that 115 Yamaha will make that 90 Johnson look silly, and your fuel consumption will drop significantly. (I guess that's 3 birds with one stone...even better).

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i wish i could just get a new motor but i dont have the cash. what would a 115 yammy or honda cost?? and what would the trade in value be on a 90 hp johnson that is 4 years old?? any other tips??

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Kyle: Hold on a minute!! These suggestions are all drastic (except for D-Man's). The SS prop may help (more later), of course a repower to a 115 Hp would do the trick, a hydrofoil won't increase top end speed and porting/polishing heads would help. The hydrofoil will help your holeshot but decrease your top end speed because of drag. I don't think porting/polishing is a good solution because this is big bucks and you would need to find someone who knows what they're doing (retune carbs...).

I suggest before you replace your motor or tweek your engine, you do a little analysis. Your original question was "how can I improve top end speed?" Assuming your engine runs well (clean gas, clean filter, no hesitation...), then your next step is to look at your propellor. If you have a great holeshot, then it's quite possible you could increase the pitch of the prop to give you better top end. To do this analysis, you'll need to jot a few things down:

1) Look at the engine manual and write down the Max operating RPM. Also write down the gear reduction, although I suspect it is 2:1.
2) Look at the propellor and write down the pitch (usually this is stamped along with the model number: 123456789 x 21). Usually the "x21" number is the pitch. Call a dealer if you can't find the pitch.
3) Take your boat out to a lake (calm day, minimum weight) and run it up to top speed (fully trimmed, WOT). Write down your max RPM.

Why all this data? Think of your propellor as a big screw - As the prop rotates, it advances forward. The pitch number is the theoretical distance the prop would move forward in 1 revolution. For example, a 21 pitch prop should ideally move the boat forward 21 inches in 1 revolution. In the real world, this doesn't happen because of slippage (or loss of efficiency). It's not necessary to go into this discussion. Suffice to say, a decent prop will have 10-20% slippage or 80-90% efficiency. With your aluminum prop, we'll assume it's a 70% efficient prop. Now that you know you're recorded your top speed RPM, you can plug this into a calculation of theoretical speed:

Theo. Speed = Efficiency x [RPM x Pitch] / [Gear Ratio x 1056] (the 1056 number is a factor to convert units to MPH)
For example,
Theo Speed = 0.70 x [5300 x 21] / [2 x 1056]
Theo Speed = 37 mph

To continue the example further, you switch to a SS prop and increase the pitch from a 21" to 23". You would lose a couple hundred RPM's however the SS props are much more efficient because the blades don't flex like the aluminum ones do. Take a look at the theoretical numbers again,

Theo Speed = 0.85 x [5000 x 23] / [2 x 1056]
Theo Speed = 46 mph

So, it is possible to gain up to 10 mph just by changing your prop to a higher pitch, SS prop. Lastly, you want to balance your prop selection with your motor's RPM range. At full throttle, you want to stay within the max operating RPM range stated in the manual (typically 4500-5500 for 90 Johnson). Keep one other thought in mind, if you go for top end, you'll loose some of your holeshot. In other words, you can't have your cake and eat it too (unless you get a variable pitch prop for $1000+). If you want to do water skiing as well, consider having 2 props - high speed/cruising and power prop for watersports.

Call around to some boat dealers to find one who has loaner props for test drives. If they know you are looking to buy, they should be quite helpful.

Good luck and I hope this helps.


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