Boat, Motor or Prop top speed limit?

[RobsTV]

Have a '73 Wellcraft V20 Center Console, used with 2 divers and third person to remain in boat, plus dive gear. Here in Tampa area, need to go out 20 to 30 miles for desired water depth. So max MPH is most important thing. Storm kicks up, we need to get back in. Other disasters, we need to get back in.

Was running '75 Evinrude 135HP with 13-1/4x17 Alum Prop.
Not set up correctly, as top speed of around 42MPH (GPS) at Max RPM of 4500RPM.
(speeds were with 36 gal fuel, two people, no other gear during tests).

Just swapped to a '83 Evinrude 175HP that was originally on a bass boat that the seller said ran 65 MPH with it's 14-1/4x21 SST prop. Of course we knew we would not get 65 MPH out of our V20 with that motor. Best we could do so far with that motor/prop combo was the same 42MPH at 4500RPM. But, the motor sure ran much better than the 135HP at that speed.

Just ordered a 15x17 SST II prop, which should hlep get RPM's up.

Using prop slip calculator, it seems HP has nothing to do with speed?
With set RPM, Pitch and gear ratio, it should run a certain speed.
Huh?????????

Looks like a 100HP will run the same top speed as a 200HP based on the calculator?

 

[QC]

Re: Boat, Motor or Prop top speed limit?

it seems HP has nothing to do with speed?
. . .
Looks like a 100HP will run the same top speed as a 200HP based on the calculator?

All other things being equal, HP has EVERYTHING to do with speed. I question:

a) the calculator you're using
b) Your tach
c) Your speed

Need to know where you are getting a, b and c . . .

 

[RobsTV]

Re: Boat, Motor or Prop top speed limit?

Using calculator from:

http://www.go-fast.com/Prop_Slip_Calculator.htm

Using the numbers from above, slip looks to be correct, so I am guessing the rest is correct, and HP is not involved.?.?

B: Old OEM OMC tach, and confirmed with new OMC tach.
C: Garmin GPS

Why I am confused is according to calculator:
If you have gear ratio of 1.86, and pitch of 21, at 4500 RPM you should run 48MPH if you had 0% slip. As shown, HP is not one of the variables. A 100HP 1.86/21/4500 and a 200HP 1.86/21/4500 would both run 48MPH if ideal.?.?

In my example of actual 42 MPH, that's slip of 13%.
After changing prop to the 15x17 I would need to run at 5500 RPM to get to same end results speed limit of around 42 MPH. (but looks like according to posts going from 21 to 17 "might" only get me up to 5300 RPM's or 46 MPH best and 40 actual if slip stays at 13%).

 

[QuadManiac]

Re: Boat, Motor or Prop top speed limit?

The calculators only tell you what theoretical speed a prop can push with a particular RPM, drive ratio, pitch and slip. If you are underpowered, you will never be able to reach that RPM with a higher pitch prop, pushing as much water as your boat displaces out of the way. Can you go higher than 4500 RPM with the new motor? What's its optimal WOT RPM? If you are capable or revving higher than the optimum, go more prop, if you can't reach it, go less prop.

 

[RobsTV]

Re: Boat, Motor or Prop top speed limit?

Thanks.
Of course I know more HP equals more speed, but the calculator had me baffled.

Also, along with this topic, and aiding to confusion, it seems that I am getting the same top speed with the 135HP as I do with the 175HP, or about 42MPH. Please explain why that is if you can. It is like there is a unmovable speed limit present.

To get more than 4500RPM's, both the 135hp and the 175hp would need lower pitch, which I thought would mean lower top end speed as well. So max speed has been obtained?

 

[Silvertip]

Re: Boat, Motor or Prop top speed limit?

That's the thing about prop calculators. People think they can slap a 21P prop on a 35 HP motor and get the same speed as that same prop on a 175 HP engine. Fact is, IF the 35 can spin that prop at the same RPM as the the 175 that would be true. The "real" fact is a 35 would not be able to get away from the dock with that prop. So the factor you are missing is HP. The calculator assumes you would not think about over propping that much. Besides, that 175 on a barge vs a 175 on a Ranger would still not perform the same -- they wouldn't even be close and that engine on a barge would self destruct in a matter of hours with that prop. Maximum "observed" engine rpm on any given boat tells you whether you have too little pitch, too much pitch, or the exact pitch. Speed is what you get when the engine runs at the manufacturers recommended wide open throttle RPM.

 

[RobsTV]

Re: Boat, Motor or Prop top speed limit?

So it seems the boat used determines the speed limit mainly?
That 175HP on a bassboat doing 65MPH, and doing 45MPH on the V20.
Still confused by 135hp Vs 175hp on same boat doing same max speed.

On another subject.
If pitch is dropped to obtain (increase) a desired RPM of say 5500 RPM, and nothing else is changed, it would seem fairly correct or dialed in. If you then raise motor, or add trim, then RPM increases to say 6200 RPM, is this still fine as long as you don't push it to that higher RPM? Is this still a much better scenario than running a motor rated at 5500RPM at max of 4500RPM due to too much pitch?

As you might notice, I like to know how and why I am making changes instead of blindly following the cattle.

Thanks in advance for the info!

 

[QC]

Re: Boat, Motor or Prop top speed limit?

Still confused by 135hp Vs 175hp on same boat doing same max speed.

If they are the same gear ratio, one of them is sick or extra super duper healthy . . . Just because it says 175 on the cowl does not mean it is making 175 bhp today.

You should select the prop that gets you into the WOT range when running a typical load and trimmed/set-up to go as fast as possible. If you want better hole shot select a propeller that gets towards the top of the range, maybe slightly better economy, prop towards the bottom. The most important part is using a typical load.

 

[QuadManiac]

Re: Boat, Motor or Prop top speed limit?

Bravo for not blindly following. I think you are overproped for your 20 with the new motor. I suspect when you drop pitch a bit you will find RPM picking up significantly. Remember that power is related to torque times RPM, so you gain in a squared relationsip by both moving up the torque curve and increasing RPMs. The ideal WOT spec for a motor is usually where it has reached it's torque peak. With the current motor/prop, you have not reached that point. When you do, i think you will see less hull wetted area and a good increase in velocity. Physics just doesn't like pushing all that water around, you know?

Good Luck!

 

[QC]

Re: Boat, Motor or Prop top speed limit?

The ideal WOT spec for a motor is usually where it has reached it's torque peak.

In the interest of education (not blindly following), that there is 100% false . . . Peak torque is usually 1000 RPM or more below Peak power. Also, it is more important to understand prop load curves as that is what determines how much torque is required at a particular RPM. The only time a Marine engine takes advantage of "peak" torque, is jumping up on plane. The only place that it is at full horsepower is at WOT RPM, with the engine at WOT (with a low pitch prop you can be at WOT RPM without being at WOT).

Also, it is torque times RPM, but then there is this divide by 5252 thing that eliminates the squaring deal. Yes, if torque remains the same and you increase RPM, then horsepower increases linearly, but not in a "squared relationship". I am not trying to be argumentative QuadManiac, just want the right info to get out.

This chart illustrates prop demand curve. Yes it is a diesel, but the concept is the same for gasoline engines, they just run at higher RPM and less "peak" torque. Doesn't matter the prop load curve still falls below the max torque line except at WOT. Remember torque has dropped at max RPM (the prop demand "curve" is the straighter line on the bottom of each chart. Power, then torque, then fuel)

Edit: BTW, the gap between the lines is reserve power, torque, and the fuel burned if you use the "reserve". This reserve is used for hard acceleration and climbing hills . . . yup no hills except swells in the water, so 99% of the time marine engines are operating on the prop curve, not in the reserve area. Exception would be examples like a ski boat used as a ski boat . . .

 

[RobsTV]

Re: Boat, Motor or Prop top speed limit?

O.K., motor wear (and lack of) makes sense too.

The '75 135hp had 150psi to 155psi in all 4, so it was tight. (stored 15+ years).
Also fresh carb rebuilds, tuned and decarbed.
Motor was at proper height.
Small hole in water passage to exhaust forced scrapping the 135hp
http://forums.iboats.com/showthread.php?t=197178

The '83 175hp was just installed on it's "see if the $400 motor works" trip. (huge YES).
Raised it two holes compared to the 135hp. Might need to go up another.
Compression was only around 80psi on all 6 cylinders
(I know, different beast, and 80psi is acceptable).
Still need to decarb top end.

Will decarb and install 15x17 SS prop, then test and report back in a couple weeks with max speed results.

Thanks for helping me think!

EDIT: Uh, looks like they need a "smack the poster button" here. I goofed on numbers from 135hp. Been months since it was in the water, and I am getting older... Looking at my post from last year at wellcraftv20.com, and original 13x19 hitting 4800RPM max and 38MPH (GPS). Went down to a 17 pitch, but it didn't make any noticable difference. Was thinking I should have dropped to a 15 pitch with the 135HP. Also noticed in v20 forums that 40 to 45MPH is about it for that boat, and plenty of 225HP V20's only run around 42MPH max. Sorry for the confusion.

 

[QuadManiac]

Re: Boat, Motor or Prop top speed limit?

QC, you are correct, and I am humbled and embarassed for speaking with so little care. Of course it's not a squared term, i was using that term somewhat blithely to imply that you get a 'bump' from both. And, assuming torque doesn't fall off very quickly, peak power will occur somewhat higher in RPM relative to peak torque. Mea culpa. I'll be more careful next time!

Thank you for the detailed explanation.

 

[QuadManiac]

Re: Boat, Motor or Prop top speed limit?

QC, you are correct, and I am humbled and embarassed for speaking with so little care. Of course it's not a squared term, i was using that term somewhat blithely to imply that you get a 'bump' from both, and if normalized torque and RPM both change linearly, their product is, effectively, a second order term divided by a constant.

And yes, assuming torque doesn't fall off very quickly, peak power will occur somewhat higher in RPM relative to peak torque. Mea culpa. I'll be more careful next time!

Thank you for the detailed explanation. Question for you, please. All 3 plots contain 2 curves each, but both Y axes are labeled the same function, just in different units. (for instance, gallons/hour and liters/hour.) Why, since all units are linear, do the two curves differ so much? I know I'm missing something, but what? Are the upper curves the 'prop demand curves', where after the 'bump', the boat is on plane... and if so, what are the units?

 

[QC]

Re: Boat, Motor or Prop top speed limit?

Yeah it's the no hill thing. Engines don't haved to run at full load. Think about a propeller like a water pump, an almost perfect load that rises with speed. The faster you run it, the higher the load. That's why they use water for dynos etc. pumping is easy to measure. The hull is what gets in the way of a load like that, you need to get up and over. If you are not patient, you need to venture into the gap. Getting more temporarily out of the propeller than a nice straight line. It requires more torque to make more horspower at a given speed, which takes more fuel . . . Once the load stabilises, it's on the line. The torque the power and the fuel rate all rise and fall with the RPM equally with a fixed load, because they are all directly related.

bhp = torque x RPM / 5252

 

[RobsTV]

Re: Boat, Motor or Prop top speed limit?

Updating after new prop.

Installed 15x17 SST II OMC used $75 prop from ebay.
Decarbed and installed thermostats.
Now turning 5400-5500 rpm at 42-43mph (GPS) trimmed out, 2 people, 28 gallons fuel, smooth salt water, which was as good as I had hoped. 32-33mph cruising speed at around 4100rpm.

Thanks for the input on how props work!

 

[walleyehed]

Re: Boat, Motor or Prop top speed limit?

well, I'd say most of your questions were answered above quite well...
If I may add to this, you went from a 2:1 Ratio 135, to a 1.86:1 Ratio 175....the end result will be near the same pitch prop.
There is also a way to gain speed/economy without loosing the RPM you have now (which is the bare minumum for that engine).
6" of jackplate would help considerably due to the length of the boat and it's design. I would guess with the same prop and the engine on a plate and up some, you "could" gain another 150-200RPM...you can do the math on the speed.. Your slip will be lower as well due to having less gearcase in the water.
If you WERE to go with a jackplate and got the RPM high enough, installing a Turbo1 in a 17 would bring the R's down slightly but the cruise and top-end speed will increase enough that the cost of the prop and plate would be paid for in short order by off-setting the cost due to much better fuel economy.
It all depends on "how" important the speed and economy actually are to you. The Turbo1 will put you in a whole different class than the SST is in.