can 50hp engine push 16ft. bass boat?

[schang]

Hey iboats,

I'm keeping it plain and easy, can a 50hp mercury 4-stroke be able to replace an older model(1985) bassboat 16 ft. that originally had a 90hp johnson? Can it still push the boat around without damaging the motor? Will the overall proformance be that much of a difference? I don't really care that much about speed, just need a good motor...

Thanks for any info,
Chang

 

[kenmyfam]

Re: can 50hp engine push 16ft. bass boat?

Hey iboats,

I'm keeping it plain and easy, can a 50hp mercury 4-stroke be able to replace an older model(1985) bassboat 16 ft. that originally had a 90hp johnson? Can it still push the boat around without damaging the motor? Will the overall proformance be that much of a difference? I don't really care that much about speed, just need a good motor...

Thanks for any info,
Chang

need some weights of the boat, fuel and gear to give a reasonable answer. A 6 hp will move the boat but not on plane and very slowly. What kind of performance are you looking for ????

 

[jevery]

Re: can 50hp engine push 16ft. bass boat?

I doubt you'd be happy with the results. If the boat is rated for 90HP, a 50HP will be 55% of rated power. You'd likely have to prop it with around 9 - 11" of pitch to get it up on plane and top speed wouldn't be much above 20 MPH. Weight distribution and load would become very critical. A 75HP would be a much better choice for a 90HP rated boat, or if you have a 50HP motor, look for a boat rated for no more than 75HP.

 

[ddrieck]

Re: can 50hp engine push 16ft. bass boat?

My old 78 16' Starcraft Tinny had a 50hp Merc 2stroke on it when I bought it. I jumped on plane, and pushed along just fine. However, it was no speed demon and the fastest it would was 25mph measured with GPS

 

[jdsgrog]

Re: can 50hp engine push 16ft. bass boat?

what is the year and make of the boat? like everyone else said, depends on the weight and what you are looking to do. Also, if you plan to fish with a buddy, fill the livewells, etc., my guess is that you would be very disappointed with the fifty.

 

[PaulsMarineSalvage]

Re: can 50hp engine push 16ft. bass boat?

It will move the boat but you'll be unhappy with the performance. My father did just what your saying. 50hp on a 16ft boat that should have a 90.

It does 20 mph with 2 people and gear, more people and it is a real slug. Takes forever to plain off and drives like a plow......

 

[PatDaddy67]

Re: can 50hp engine push 16ft. bass boat?

The pump laws state:

1. Fluid velocity (V) goes directly as motor speed (N).

2. Pump differential pressure (Dp) goes directly as motor speed squared (N^2).

3. Pump power (P) goes directly as motor speed cubed (N^3).

Since a prop can be modeled as a type of pump we can use these laws.

The assumptions made that make the above relationships true include things like an ideal pump (100% efficient), no tip losses, etc?

So, for your question the following applies:

P = C * N^3 where C is a constant of proportionality in units of power over speed cubed.

Let?s calculate an example: If your boat with the 90Hp motor tops out at 40mph we can calculate C at 40mph.

(1) C = 90Hp / (40mph)^3 = 1.4 *10^-3 Hp/mph^3 or .0014 Hp/mph^3

You may be thinking at this point that this obviously is not a linear relationship? Well, it?s not, but for our purposes it is a useful tool for estimating speeds over a range of powers.

So, now for the new 50Hp motor:

Nnew = the cube root of Pnew / C or (50Hp / .0014 Hp/Mph^3)^(1/3) which equals:

32.88 mph

Again, this estimate assumes 100% efficiency in the system. The good news is that the prop efficiency of your new motor and old motor should be similar in magnitude when operating at the same engine RPM. There are other factors that will affect this estimated speed. If at 33 mph your boat is coming off plane and slightly plowing, the drag would be significantly higher than this estimate would account for, thus making your actual speed much lower. Is your new motor significantly lighter than the old motor? Does it require less fuel to be hauled for a day?s run?

I hope this helps?

PatDaddy67

p.s. If you send me the exact operating characteristics of your current boat, I will generate an operating curve for you.

 

[izoomie]

Re: can 50hp engine push 16ft. bass boat?

My first boat was a 16' fiberglass bowrider with a 50hp johnson, both 1980. The boat with a family of four did not have any hole shot but would plane nicely and pull kids on tubes all day.

I hit a rock in georgian bay to end that fun time but that's another story.

 

[JustJason]

Re: can 50hp engine push 16ft. bass boat?

I have a 16ft fiberglass runabout that I think is slow with a merc 120 IO.

 

[Moody Blue]

Re: can 50hp engine push 16ft. bass boat?

I have a 16ft fiberglass runabout that I think is slow with a merc 120 IO.

Yeh, its all a matter of perspective

No doubt the 50 will push the boat, but performance will be substantially compromised compared to what you are used to. Just how badly is impossible to tell because of so many variables. Like dropping a 4.3L V6 into a Corvette. Sure it will still move but you would be sadly disappointed in the performance.

 

[PatDaddy67]

Re: can 50hp engine push 16ft. bass boat?

Something else to look at is thrust. The acceleration of your boat is directly related to thrust which is the differential pressure that your prop generates. When the differential pressure of the prop is equal in magnitude to the combined water drag and air drag, your boat can go no faster for a given power output. Since from before we know that Dp goes directly as N^2 we can form the following relationship:

Dp2 / Dp1 = (N2 / N1)^2

Thus if we use our answers from before we find that Dp2 / Dp1 = .68

What this means that the thrust of the 50Hp motor is roughly 70% of the 90Hp motor. If the mass and drag of your boat remains constant from before to after, your acceleration will be 70% of what it was before. If you are trying to pull up a skier you can see how this might be more difficult with the new motor. If you?ve been boating for a while you know that there is a point of high drag just before the boat lifts to planning mode. If your available thrust is not more than this maximum drag you will not be able to plane. This will be a problem unless you are doing nothing but trolling all day long.

Enjoy!

PatDaddy67

 

[Moody Blue]

Re: can 50hp engine push 16ft. bass boat?

Pat, if the rated HP is only 55% of the original motor, how can the thrust be 70%? So a 35hp motor will produce 1/2 the thrust of a 90hp?

Not arguing, just curious as to the relationship between HP and thrust.

 

[steelespike]

Re: can 50hp engine push 16ft. bass boat?

Another point A 50 will be busting a gut at 20 to 25 a 75 or a 90 would be relaxing
and using about the same fuel at the same speed.

 

[PatDaddy67]

Re: can 50hp engine push 16ft. bass boat?

Well? It?s like this:

Power is a function of thrust, velocity and time. The pump laws relate thrust (pressure head of the prop) as the first integral of power with respect to time and speed is the second integral. The same way as torque in an engine is the first integral of power with respect to time and rpm is the second.

Think about it? Say your skiff will go 20mph with a 25Hp motor. Given that the drag curve for the hull is a fixed property of the hull for a given displacement; on a whim you decide you want to make your skiff go 40mph. Guess what? If the efficiencies are the same and the displacement of the boat is constant (drag 1 = drag 2), then you will need a motor with (N2/N1)^3, or a 200Hp motor to achieve you goal. That?s why fast boats need so much more power to go only 20 or 30 mph faster. My boat will go 30mph with 750Hp (34? Carver Montego) while my friend requires 2200Hp to go 70mph (35? Baja). Our displacement is different but by far the biggest factor is the relationship of power required being proportional to the speed cubed for a given configuration.

Looking at it from the other end:

Distance is a zeroth order function. It is simply the displacement from your origin.

Velocity (speed in a given direction) is the first derivative of displacement with respect to time.

Acceleration (change in velocity per unit time) is the second derivative of distance and the first derivative of velocity.

Acceleration can be related to a force by the relationship F = m * a where m = mass of the object, and F = the force applied. Differential pressure (Dp) of a prop or other pump element can be related to force by the relationship Dp = F/A where F = the applied force (thrust) and A = the area of the active element (effective prop area). So, by doing some substitution and a bit of math, we can say that Dp is by function the second derivative of distance and the first derivative of velocity.

Energy is defined as moving something through a distance using an applied force yielding units of length*force like ft-lbf.

Power is the first derivative of energy with respect to time, in other words, how much energy can be produced per unit time.

Since we already related thrust (force) and pump speed as well as force and energy we can now relate power to speed and thrust to speed by the relationships:

P ~ N^3

And

Dp ~ N^2

Where ~ means ?is proportional to?

What this all boils down to is:

To double the boat speed if the prop efficiency is constant you need to double engine speed. In doing this you are generating four times the differential pressure across the prop thus requiring eight times the amount of power to do so. There is the relationship in a nutshell.

I hope this helps?

Enjoy!

PatDaddy67

Here is a link to some useful info: http://www.engineersedge.com/fluid_flow/pump_laws.htm

 

[schang]

Re: can 50hp engine push 16ft. bass boat?

Thanks everyone,

Well, judging by all the different feedbacks it would not be such a good idea to down-grade on hp for the boat. By knowing this we will most likely not purchase the 50hp yamaha. Thanks again everyone for some very good information, we will look for a 75-90hp engine. BTW, the boat is a 1985 monarck 16ft. bassboat that weighs.....900-1000lbs? I am guestamating.

Thanks again,
Chang

 

[kenmyfam]

Re: can 50hp engine push 16ft. bass boat?

The pump laws state:

1. Fluid velocity (V) goes directly as motor speed (N).

2. Pump differential pressure (Dp) goes directly as motor speed squared (N^2).

3. Pump power (P) goes directly as motor speed cubed (N^3).

Since a prop can be modeled as a type of pump we can use these laws.

The assumptions made that make the above relationships true include things like an ideal pump (100% efficient), no tip losses, etc?

So, for your question the following applies:

P = C * N^3 where C is a constant of proportionality in units of power over speed cubed.

Let?s calculate an example: If your boat with the 90Hp motor tops out at 40mph we can calculate C at 40mph.

(1) C = 90Hp / (40mph)^3 = 1.4 *10^-3 Hp/mph^3 or .0014 Hp/mph^3

You may be thinking at this point that this obviously is not a linear relationship? Well, it?s not, but for our purposes it is a useful tool for estimating speeds over a range of powers.

So, now for the new 50Hp motor:

Nnew = the cube root of Pnew / C or (50Hp / .0014 Hp/Mph^3)^(1/3) which equals:

32.88 mph

Again, this estimate assumes 100% efficiency in the system. The good news is that the prop efficiency of your new motor and old motor should be similar in magnitude when operating at the same engine RPM. There are other factors that will affect this estimated speed. If at 33 mph your boat is coming off plane and slightly plowing, the drag would be significantly higher than this estimate would account for, thus making your actual speed much lower. Is your new motor significantly lighter than the old motor? Does it require less fuel to be hauled for a day?s run?

I hope this helps?

PatDaddy67

p.s. If you send me the exact operating characteristics of your current boat, I will generate an operating curve for you.

Ah Yes !!!!
Just what I was thinking
Pass me another beer

 

[CaptainHook]

Re: can 50hp engine push 16ft. bass boat?

Thanks everyone,

Well, judging by all the different feedbacks it would not be such a good idea to down-grade on hp for the boat. By knowing this we will most likely not purchase the 50hp yamaha. Thanks again everyone for some very good information, we will look for a 75-90hp engine. BTW, the boat is a 1985 monarck 16ft. bassboat that weighs.....900-1000lbs? I am guestamating.

Thanks again,
Chang

Sounds like the best Idea. And that is really funny. The boat that I spoke of previously that is my Dad's is a 16ft Monark. 50hp just doesn't cut it!!

 

[trendsetter240]

Re: can 50hp engine push 16ft. bass boat?

Well, some great technical info here...I plan on reading it again when I have less beer and more coffee in me

I had a 50hp engine on my 17ft runabout boat last summer. Worked fine with just myself, acceptable with 2 people and just plowed water with any more.

If it's just you and your kid,wife or extra light friend using the boat to go fishing than it will be fine with the right prop. If you plan on loading it up with people and or weight you will not be a happy boater. Also, as Steelespike mentioned, you will not be getting great fuel economy from that 50 because you will need to run it at WOT to get anywhere.


Cheers~!

 

[95cajun]

Re: can 50hp engine push 16ft. bass boat?

something els to consider is the weight and the depth that the motor sits in the water. it may cavitate because the balance of the boat will be way off. what to you have the boat or motor? boat = buy a bigger motor at least 1/2 to 3/4 of what the coast guard rated the hull at. Motor = buy a lighter hull with a rating of 75hp to 100hp. thats the designed power rating for you hull the rule is 49 hp over the rating max no minimum I'm aware of cause nobody underpowers a boat...LOL

 

[4JawChuck]

Re: can 50hp engine push 16ft. bass boat?

My 16' Tuffy Sportsman came with a 50 Yamaha originally, it would do 34 mph unloaded with just me and 10 gallons of fuel. Add three more people and it dropped to 29 mph. I ran a 13" pitch prop on that motor and the hull is a modified vee or cathedral hull.

With a Merc 90HP motor the boat does 51-53 mph with a 22" pitch prop and moves out really well, but not what I am used too. The boat is rated for a 80Hp motor from Tuffy. Strangely enough I get about the same mileage with the larger motor I just get there twice as fast.

This summer I have a 200 Merc V6 going onto the boat after some hull mods and transom reinforcement and underdeck fuel tank etc., I am hoping for about 70+ mph with a 26" pitch prop. My goal is to cruise at 50 mph at half throttle, most motors seem to get the best mileage around that throttle position.

I was not happy with the 50Hp motor at all, so slow to plane and adding passengers made things worse. If you were the only one riding in it then it might be OK but the noise of running at WOT all the time would get annoying to me, something to consider. Don't discount the time it takes to get to the fishing spot either, I find the larger motor gives me another 1.5 hours of fishing time each day if you have to travel a long way on the water...not to mention time getting off a big lake when a storm is brewing.

One last thing to consider is most bass boats have a pad on the hull which makes the boat faster with a smaller motor compared to a true Vee hull, the planing performance will still be slow but once its up on the pad it may have a decent top speed once its propped right. Weight is sensitive on pad hulls, run light they are OK with lower HP motors.