how fast would a bigger motor make it go!

Frank Acampora

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Re: how fast would a bigger motor make it go!

100_5980.jpgresized55.jpgworking copy.jpg

You can not use the cube formula because we are dealing with a planing hull. The four times formula is only an estimation to show that, as most people would assume, twice the horsepower would yield twice the speed is not valid.

The variables will certainly be weight and load in addition to wetted area of the hull. Over about 40 MPH, air resistance will also come into play. Given the same load or total weight--plus or minus some for the weight of the larger engine-- The higher horsepower engine will lift the hull higher for less wetted area, thus drag will not increase as much as expected. Disclaimer: Every hull is different and will respond differently to increased horsepower however, it is a given that significantly more horsepower will increase speed within certain parameters.

NOW: This hull was at 55 MPH with a 90 and when repowered with a 125, speed jumped to 60--that's 5 MPH for a 35 Horsepower increase. HOWEVER: Since the hull was already riding very high (about the last 18 inches) the larger engine did not lift it higher so there was an increase in drag (both water and air) due to speed. And, the faster I get this hull to go, the more horsepower it will take per MPH increase. Thus another 5 MPH may require 100 horsepower increase.
 

Mischief Managed

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Re: how fast would a bigger motor make it go!

Actually, you are on the right track. :joyous:

The Force(Drag) is proportional the Square of the Velocity.
Power(hp) is the Force time the Velocity.
Twice the Velocity pushing 4 times the Force requires 8 times the Power.
The Power is proportional to the Cube of the Velocity.

There are a lot of other variables that make and accurate answer problematic.
But, assuming laboratory conditions and a simplified textbook answer....

If 88hp produces 34mph.
200hp is 2.27 time the power.
The cube root of 2.27 is 1.31.
1.31 time 34mph yields 44mph with 200hp.
The extra 112hp will theoretically buy you 10mph. Your results will vary.
This all assumes that the larger hp engine and added fuel does NOT add any extra weight.

55mph /34mph = 1.62, Cubed = 4.23, * 88hp = 373hp to theoretically go 55mph.
Realistically, well over 400hp may be required.

This is a classic case of diminishing returns. Sorry! :blue:

You really think it will take 400 HP to push a 21 foot aluminum boat at 55 MPH? My 25 foot, 6000 lb boat goes 53 MPH (GPS) with only 310 HP.
 

UncleWillie

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Re: how fast would a bigger motor make it go!

You really think it will take 400 HP to push a 21 foot aluminum boat at 55 MPH? My 25 foot, 6000 lb boat goes 53 MPH (GPS) with only 310 HP.

I am just running the numbers, the reality of the situation is going to be quite different.
The differences between hulls and how they are loaded makes a huge difference also.

Given the limited information that we have, we can only extrapolate using the information that we have, plug it into the standard formulas and realize that it may be extremely far from reality.

The other option is just to make a wild guess and apply an optimistic or pessimistic correction based on gut feelings.
The "Your Results Will Vary" line is not just a cute catch phrase, it is the general Disclaimer.

Having said all that; I am surprised that the 88hp engine is only moving the boat at 34mph.
The trim may be very far from optimum.
If it is overly heavy in the stern, it may be plowing way too much.
 

redlinj

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Re: how fast would a bigger motor make it go!

Actually Uncle Willie, considering that the OBC of the boat is 200 hp, I was surprised the boat went that fast with only 88 hp. The boat doesn't have trim taps, but the 88 does have trim/tilt and I've experimented up/down and in/out and every which way in between. So what top speed do you think it should be getting. Motor appears to be in good tune, and, has even compression. I wish I knew if that motor came with the boat when new. 1988 Sylvan, 1988, 88 hp spl. A coincidence? I don't know!!!!
 
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Re: how fast would a bigger motor make it go!

Power is Force(Drag) time Velocity(Speed).
Twice the speed is 4 time the Drag(Force).

But it is 4 times the force at twice the speed, so 8 times the power.
Power is to the CUBE of the Velocity.


Power
The power required to overcome the drag is given by:
e31430f0898268091f410282a89503b1.png

Note that the power needed to push an object through a fluid increases as the cube of the velocity.
A car cruising on a highway at 50 mph (80 km/h) may require only 10 horsepower (7.5 kW) to overcome air drag, but that same car at 100 mph (160 km/h) requires 80 hp (60 kW). With a doubling of speed the drag (force) quadruples per the formula. Exerting four times the force over a fixed distance produces four times as much work. At twice the speed the work (resulting in displacement over a fixed distance) is done twice as fast. Since power is the rate of doing work, four times the work done in half the time requires eight times the power.




Dang Willie, it's been a while since I took fluid dynamics (plus a few brain cells have been sacrificed in the interim). Some of the cobwebs are starting to shake loose now though.







Using the Cube Formula, calculates an expected speed increase to 47.5mph.
Using the Square Formula, calculates an expected speed increase to 51.6mph.
So a real life example falls somewhere in between!





OK, here's where the variation from the cube law comes from. In boats, as in airplanes, there are two types of drag (I'm using the aeronautical terms 'cause that's where I learned it) - parasitic drag (friction) and induced drag (in a boat that's from the hull redirecting water downward to produce enough lift to stay on plane). Parasitic drag (friction) does increase as the cube of speed. Induced drag actually decreases as speed goes up - an airplane wing or a boat will be at a lower angle of attack as it goes faster, thus less drag. At very low speeds (like when a boat isn't fully on plane) induced drag is the major contributor of total drag. Induced drag it drops off quickly at first as the boat goes faster, then gradually falls off slower with and more speed. So the TOTAL drag goes up at a bit less than the cube of speed. It's going to be different for various situations (that's why there's drag-polar charts which give the Cd at varying angle of attacks for any particular airplane wing design).

I looked at some of the charts for boats that let you figure out speed at different weights or power levels and it appears that they assume a coefficient of 2.7 or thereabouts (like you said, somewhere between 2 and 3). Of course that's only a close estimate.
 
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UncleWillie

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Re: how fast would a bigger motor make it go!

Actually Uncle Willie... I was surprised the boat went that fast with only 88 hp. The boat doesn't have trim tabs, but the 88 does have trim/tilt and I've experimented up/down and in/out and every which way in between.
So what top speed do you think it should be getting....

I do not have a clue! :joyous:

I am not familiar with the "Sullivan Super Sportster 21".
I just did a search to see what one looked like and found an '89 and '90 but but they were equipped with I/O drives. They appeared to be well made aluminum bow riders.
So I guess yours would be about the same except with an OB motor attached.
I was picturing a large, Tiller Steered, very light, no frills, open, Aluminum boat.

Based on your report of 34mph at 88hp;
Your speed with the 200 will be somewhere between 45 and 55 according to the math.
I will also guess that your fuel efficiency is going to be less. :D
 

Mischief Managed

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Re: how fast would a bigger motor make it go!

This reminds me of a conversations I had with Bell Labs Phd I used to work with when I was measuring signal to noise ratio with an optical sprectrum analyzer. My technique was really fast, easy and accurate, his was absurdly complicated. His response to my work was roughly: "sure it works in practice, but does it work in theory?".

I'm tellin' ya, just use the speed squared method and the results will be very accurate on a planing hull. Also, the torque and HP curve will make absolutely no difference in top speed, as long as the delivered HP and drag are the same, the speed will be the same. Acceleration my be different with a peaky torque curve, but not top speed.
 

redlinj

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Re: how fast would a bigger motor make it go!



You can get pretty darned close going with Frank said - required power is roughly proportional to the square of speed, or inversely, speed is proportional to the square root of power.

sqrt(200/88) * 34 = 51.

So, put a 200 on that thing and let us know how good our predictions are!!!!

All right gentlemen, after a Spring, Summer and Fall of wheelin and dealin and swapin motors I have some results. Came across a pair of 1976 Evinrude 200's. Was able to make one decent one out of the two. Unfortunately, could only get it up to 4900 rpm's. (5500 optimum at wot). Compression even at 135. Top speed was 42 mph on gps. I should have rebuilt the 3 carbs. I really believe they were holding me back! Now the real results. Picked up an '82 Evinrude 150 hp which ran a whole lot better all the way around. Hit 44+ mph on gps. So, going with the above formula sqrt(150/88) * 34, we get 44.4 mph on gps. I'd say pretty darn close. I'd say if the 200 evinrude was running properly, we would have hit or come close to the 51 mph mark. The boat is out of the water now and I have pics but don't seem to have much luck posting them. I'll keep trying. Thanks to all the engineers, mathematicians and boat people who found this topic of interest. At this point my curiosity has been satisfied.
 

Illinoid

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More importantly...The faster you go the slower you age...so you are younger longer. Einstein was a genius and would still be alive...if he had had a fast enough boat;-)
 

roscoe

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More importantly...The faster you go the slower you age...so you are younger longer. Einstein was a genius and would still be alive...if he had had a fast enough boat;-)

True, but only if you drive in the right direction. :)
 

roffey

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Nov 22, 2012
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Here's my question, I thought top speed of the boat was in direct proportion to the RPM, prop pitch and the amount of prop slip. The higher the pitch the more horse power you need to hot max RPM, right? The hull design will create or lessen the prop slip. A 21 inch prop will go 21 inches in one turn, now how fast can you make the prop turn one revolution and how much prop slip is created.

Just my thoughts, LOL. Feel free to correct me as this really is a question and my understanding.
 
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Maclin

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May 27, 2007
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Roffey, the drive ratio is part of that equation. RPM is how fast the engine spins ahead of the gears, then the final drive ratio affects the prop's speed.

If two engines have the same drive ratio then any RPM comparisons work. If each has a different drive ratio then that must be part of the equation.
 

undone

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Interesting. I also made the switch with almost the exact same motors 90hp Evinrude (same as the 88), to a 1976 200HP Evinrude on a 19' center console that weighs 1,200 lbs.

My speed jumped from 35 to 45. Here's the deal with the 1976 200's, they were considered a very weak 200, many say even when new it's closer 160hp, plus it's crank rated.

I had the same problem of getting the RPM's high enough, carbs clean, compression great, starts and runs great, just hard to get up into the correct RPM range. Had to drop down to a 17" prop to get up to around 5200 RPM's. I did a great deal of setup work, changing props, raising and lower the motor, etc. just to get to that point. I never did go to a SS prop because I just don't use the boat that much, I don't think it's been in the water for at least five years now.

There was another person on here with the same motor and the same problem, just had issues getting the RPM's high enough. When I say high enough, I mean you expect a 200hp motor to use more than a 17" pitch prop and only get to 45MPH on a 19' boat. My hull was more of a flats/bay design and had a great deal of wetted surface when on plane, it was never designed for speed. But Yamaha testing (their website) showed a top speed of 54 with one of their 150 2 strokes on this hull.

This was all done back in 2005 on this site.
 
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