how fast would a bigger motor make it go!

[redlinj]

I have an '88 Sylvan super sportster 21'. The boat came with an 88 hp johnson. The boat is rated for 200 hp. The 88hp pushes the boat (aluminum) 34 mph on the GPS. Is there a formula to determine how much faster a larger motor could push this boat? I'm pretty sure that if I doubled the hp I wouldn't come close to doubling the top speed but is there any way to scientifically figure this out without actually putting a bigger ob on?

 

[SigSaurP229]

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

Pics of said boat would be nice not because I have anything of any help here but am seriously interested in seeing it.

 

[roscoe]

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

Your Sylvan is essentially a 21' Starcraft SS (SuperSport).
Same factory, same hull, different interior trim.

The hull itself has some limiting factors to top speed.

I'm guessing you might get low 50's out of her.

Best place to find the answer is in the Starcraft forum.

Starcraft Boats

Very active forum, and someone will be able to give you the number.

 

[superbenk]

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

I thought I read somewhere that every 10HP gets you roughly another 1MPH above a certain base and below a certain top speed. That could be completely wrong though. I know it's surprising how much HP you need to add to see any benefit. Maybe it's totally different with OBs and aluminum boats.

 

[Frank Acampora]

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

The general rule of thumb is that it takes four times the horsepower to double the speed. However, there are a lot of variables. If you roughly double your horsepower to the 200, I suspect that yes, you would get in the mid 50s

 

[Chris1956]

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

I have a 21' SeaSwirl with a 150HPV6 Johnny. It weighs in at 2700#, and will max out at 41MPH, downhill, with the wind. I expect your boat will be lighter and with more HP, your performance will certainly be better than that. You should approach 50MPH, but unless the hull is of a fast design (like a bass boat), I would not expect too much more.

 

[V153]

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

I replaced a prop shaft rated 90 with a crank rated 140 and went from 54 to 63. Boat is a 15' Baja.

In my particular case: Roughly 50 additional hp gained roughly 10 mph. So that's what, 10 hp per 2 mph?

I doubt you'll find any formula, way too many variables.

 

[metriccrescentwrench]

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

I doubt you'll find any formula, way too many variables.

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!!!!

 

[V153]

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

I doubt my 'formula' would benefit the OP. Totally different hulls.

Personally I'd like to see him hang a 250 HO ETEC on that tinny. Just keep an eye out for loose rivets ...

 

[Lager]

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

Not only will a larger engine give you more MPH, it will also allow you to travel at your max speed right now with a lot less throttle imput.This will decrease your fuel bill and eventually save you money because any engine run at full throttle uses a lot of gas.If you can kick it back a notch and achieve the same MPH, the fuel efficiency increases dramatically.
Just a thought if you need to convince a significant other as to why you need a larger engine.

 

[UncleWillie]

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

The general rule of thumb is that it takes four times the horsepower to double the speed. However, there are a lot of variables. If you roughly double your horsepower to the 200, I suspect that yes, you would get in the mid 50s

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:

 

[UncleWillie]

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

Not only will a larger engine give you more MPH, it will also allow you to travel at your max speed right now with a lot less throttle input.This will decrease your fuel bill and eventually save you money because any engine run at full throttle uses a lot of gas.If you can kick it back a notch and achieve the same MPH, the fuel efficiency increases dramatically.
Just a thought if you need to convince a significant other as to why you need a larger engine.

That is like saying that if you only intend to only drive at 60mph on the freeway; get the largest V8 engine you can fit in the car. You will get a lot better gas mileage compared to a small 4 cylinder engine because the larger engine will be barely doing any work. :facepalm:

The larger engine will always have more moving parts and larger parts, creating more internal losses compared to the smaller engine. Engineering physics will show that to get the best efficiency; run the smallest motor that will do the job at 100% power.
The efficiency may be at maximum; however the life of the engine may very well be considerable shorter. :grumpy:

Your speed will be proportional to the amount of Power being delivered.
Each gallon per hour is going to produce ~20hp. (Some estimate as little as 10hp.)
Regardless of the size of the engine; any specific power output is going to use approximately the same amount of fuel.
There is No Free Lunch!

 

[Mischief Managed]

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

Drag increases with the square of speed. If you go 34 MPH with 88 HP, and all conditions remain the same (weight, lower unit/prop drag, prop efficiency), 200 HP will give you 51.25 MPH.

The math: Top speed with 200 HP = the square root of: ((34*34)/88)*200.

Weight and drag will likely increase with more HP, so the top speed will likely be a little lower than 51.25 MPH. That said, a high performance SS prop can easily add 2-4 MPH, especially with higher HP, so you could see 55.

 

[Chris1956]

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

Gee, A lot of answers, some based upon mathematics, some based upon experience and extrapolation. The answers, even the mathematical ones are divergent from one another. I would love to see the actual results.

 

[redlinj]

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

Yes, actual results would be GREAT!!!!!! However, they will have to wait til Spring. In the meantime, a Sylvan ss 21 according to the manufacturer weighs in at 1100lbs dry. To me, it looks sleek and fast. Pics of said boat would also have to wait til I get it uncovered. The idea of this thread for me was to determine what HP would be the best "bang for the buck". I have a older Johnson 115 in the barn, but by Mischief Managed formula ( which I preferred because of the results and I could understand the formula a bit easier!) the 115 could push the boat about 39 mph. Whereas, a 140 by the same formula could do about 43 mph. This info is much more than I had before the thread and pretty much falls in line with what I was thinking. I guess one must determine if an added investment is worth the extra cost. However that being said, maybe the 115 would make the boat run more efficiently at WOT and cruising speed.

 

[Mischief Managed]

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

I replaced a prop shaft rated 90 with a crank rated 140 and went from 54 to 63. Boat is a 15' Baja.

In my particular case: Roughly 50 additional hp gained roughly 10 mph. So that's what, 10 hp per 2 mph?

I doubt you'll find any formula, way too many variables.

My formula says you were making 122 prop shaft HP to go 63, if the weight, drag and prop efficiency remained the same. Since a 140 likely weighs quite a bit more than a 90, I would guess the prop shaft HP is higher than 122 and some of the excess HP is absorbed by the extra weight. I'm not sure how much HP is lost by a 90 degree change in rotation, but I'd guess 5%. That would mean 133 Prop shaft HP for 140 crankshaft HP (assuming it's an outboard. Another thought to consider is that HP ratings are typically publisjhed, plus or minus 10%.

 

[snowman48047]

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

My 20' Alaskan had a 50hp originally. Top speed about 21-22, 50hp replaced with a 115, new top speed about 40. There is a link somewhere here to a chart you can input specs to. These numbers match up to the chart.

 

[dan02gt]

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

My formula says you were making 122 prop shaft HP to go 63, if the weight, drag and prop efficiency remained the same. Since a 140 likely weighs quite a bit more than a 90, I would guess the prop shaft HP is higher than 122 and some of the excess HP is absorbed by the extra weight. I'm not sure how much HP is lost by a 90 degree change in rotation, but I'd guess 5%. That would mean 133 Prop shaft HP for 140 crankshaft HP (assuming it's an outboard. Another thought to consider is that HP ratings are typically publisjhed, plus or minus 10%.

Also they are just peak HP numbers taken from one tiny place in the power curve. The entire curve is what's really important. For instance a Mercury 200HP 2.5L EFI has a really narrow and crappy power curve compared to a Mercury 200HP 3.0L Optimax. Even though they are both rated at 200HP the EFI is gutless compared to the Optimax.

 

[redlinj]

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

OK Snowman, nothin' like teasing me! So, any idea where the link to this chart might be hiding?????????

 

[UncleWillie]

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

Drag increases with the square of speed. If you go 34 MPH with 88 HP, and all conditions remain the same (weight, lower unit/prop drag, prop efficiency), 200 HP will give you 51.25 MPH.

The math: Top speed with 200 HP = the square root of: ((34*34)/88)*200.

Weight and drag will likely increase with more HP, so the top speed will likely be a little lower than 51.25 MPH. That said, a high performance SS prop can easily add 2-4 MPH, especially with higher HP, so you could see 55.

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:

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.

... maybe the 115 would make the boat run more efficiently at WOT and cruising speed.

There is a lot of truth in that statement.
"The Physics will show that to get the best Efficiency; run the smallest motor that will do the job at 100% power."

BoatTest reported that my boat went 40.4mph with a 135hp 4 cylinder engine.
My 220hp V6 gets about 50mph.
The math shows a speed increase of 24% with a power increase of 63%.

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!
Your results WILL vary. :joyous: