Maximun Speed

Bouba said:
I probably didn’t make myself clear. If it’s was my boat I want those 400 revs.
I’ve only owned 90 hp and 150 hp outboard boats (not a monster like you have) but 400 revs at the top end can easily add a few knots.
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How will finding an extra 400 ( to 6000 ) rpm effect the real world cruise speed at typically say 3800/4200 ?
Which the OP reports is below 20 knots and hints he’s disappointed .
Surely he needs to cruise at a higher speed knots wise with a reasonable eco rpm ideally sub 4000 rpm in the interests of useable range .
Petrols don,t suffer overload excessive EGT issues as acutely as diesels so reaching at predescribed “ book “ rpm is not important here imho .
He needs a prop to suit the torque curve ,to suit mid twenty to high twenty knots cruise below or around what ever the eco ( ish ? ) rpm he is happy to fund fuel wise .

18/20 knots is poor .
 
Portofino said:
How will finding an extra 400 ( to 6000 ) rpm effect the real world cruise speed at typically say 3800/4200 ?
Which the OP reports is below 20 knots and hints he’s disappointed .
Surely he needs to cruise at a higher speed knots wise with a reasonable eco rpm ideally sub 4000 rpm in the interests of useable range .
Petrols don,t suffer overload excessive EGT issues as acutely as diesels so reaching at predescribed “ book “ rpm is not important here imho .
He needs a prop to suit the torque curve ,to suit mid twenty to high twenty knots cruise below or around what ever the eco ( ish ? ) rpm he is happy to fund fuel wise .

18/20 knots is poor .
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I would guess that the extra 400 would translate all the way down the curve as far as speed is concerned however acceleration (holeshot) might change but , personally, that wouldn’t worry me
 
I'm surprised nobody asked what the sea state was. A small chop could also have a significant impact on speed.
 
I think there is some misunderstanding around propeller properties and dynamics here. With reference to my previous posts, I’ll add...
To suggest getting another 400rpm would mean more speed, is almost certainly not true. All figures and details being accurate (which I think unlikely), in order to gain 400 rpm the prop pitch would need to be lowered by usually about 2”. This would not only make the actual prop pitch even more unlikely, but also undoubtedly cause a reduction in speed.
As long as an engine is within the recommended rpm range, it should be at its peak power. The only occasion a drop in pitch and a raise in rpm would mean an increase in speed would be if the engine was not finding its way into the recommended rpm range with the larger pitch.
Anyway, I still find the 14” pitch prop unusual for this engine and gearcase. As mentioned, usually it would be an odd number...outwith a specialist prop.
Loads of other variables like tankage, weather, engine trim and ofcourse real gps speed vs dreamometer speed.
All the best and suggest another run with all figures recorded from the smart craft this engine will most likely have. Check the prop too
 
Hi peter

So it’s a 14” right enough. To try and claw back some dignity, I’ll add that it’s not a standard prop and indeed a specialised performance prop. I might even add that it looks very like just the example i gave of such a prop. The excellent Enertia. Perhaps it is. These are one of only a couple I know of that come in one inch increments for this gearcase size. Excellent props.
Regardless, I would hazard a guess that all is not as it seems. With that low a pitch and that boat..it should likely be pulling more rpm trimmed up and running ideally. I’d have though that it would be best suited using a prop an inch or two higher in pitch.
One thing that might not help...is if it is indeed an enertia, it may not be the blistering performance orientated enertia, but the less capable enertia eco version. With the massive diameter stated, it just might be. These are aimed at lightening acceleration and low planing speeds, but perhaps not favoured for faster cruising speeds or top end. Might explain it. As an example, a typical 17” or 19” prop for this engine would be around 14” diameter.
 
Hi QBhoy,
Selecting a propeller is just like playing puzzle game with a blindfold on. We pick up a piece from the box but just feel something wrong although it is recommended by someone else.

I just use the "propeller selector" in Mercury website and there come 4 suggestions:

Enertia 15 x 15, Enertia 14.7 x 16, Mirage Plus 15.75 x 15, and Vengeance 14.5 x 15

The "calculated" top speed of these 4 propellers is 42.89mph which is even faster. I am not pursuing the top speed, but I guess
higher top speed equals to higher cruise speed. (Am I correct?)
 
Exactly as you say. It’s a black art.
But one thing for sure is you need to gather exact and certain details to enter into the mercury prop calculator. One which I used myself. I’ve found it to be fairly accurate over the last few years. If you put crap in, you get crap out though, haha.
As long as you are sure of the exact details, it should be able to advise well.

Drive ratio

Max rpm achieved during optimum normal use conditions with engine trimmed well

Max speed achieved in gps

Current prop pitch

Are all required for any sound propeller estimate.

If one of these is wrong...it’s no use at all, as you will know.
 
I went out for testing again yesterday. Only me on the boat, 275 L of fuel, 100 L of fresh water.

This is what I got:


6100 rpm (WOT) , 30 ~ 31 knot.
5100 rpm, 21 ~ 22 knot.
4800 rpm, 18 ~ 19 knot.

wind speed: less than 2 knot from my starboard.
swell: 0.3 ~ 0.4 M (every 8 ~ 10 second) from 10 ~30 degree
Temperature: 33 C.

Engine was auto-trimmed by Mercury ActiveTrim.

What do you think, captains?
 
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I don't know how well auto trim works,but can you turn it off and try different trim angles manually? this way you can eliminate trim angle as a cause of being slower than expected
 
Now you are over reving. The engine is outside its operating range and might be close to automatically cutting out. This could be as Julians maybe suggesting that it’s trimmed far too high. Personally I would not push it past 6000 (if that is the operating range).
If your dealer (engine or boat) is a good person they may have a range of other props for you to experiment with.
 
Bouba said:
Now you are over reving. The engine is outside its operating range and might be close to automatically cutting out. This could be as Julians maybe suggesting that it’s trimmed far too high. Personally I would not push it past 6000 (if that is the operating range).
If your dealer (engine or boat) is a good person they may have a range of other props for you to experiment with.
Click to expand...

Interestingly, the figures in that linked doc show 37.2kts at 6300rpm!
Converting 37.2mph to kts gives 32kts - I can't help wondering if there has been a unit mix-up?
 
The operation manual says the rev range at WOT is 5800 ~ 6400 rpm. So I must have pushed
to limit. On the other hand, unit mix-up and trim position are possible factors behind.

I think I will use this propeller current, the engine is just 9 hours old. I will accumulate some
more hours, at least after the first maintenance, then perform the experiment again.
 
peterjaw said:
The operation manual says the rev range at WOT is 5800 ~ 6400 rpm. So I must have pushed
to limit. On the other hand, unit mix-up and trim position are possible factors behind.

I think I will use this propeller current, the engine is just 9 hours old. I will accumulate some
more hours, at least after the first maintenance, then perform the experiment again.
Click to expand...
That 6400 changes everything, and already you’ve increased the rpm a lot .
I agree with FlowerPower that the Internet figures are not quite right. Here’s my reasoning; for the boat you have two styles weekend and activ, then you have a choice of petrol or diesel sterndrives , then you have outboards. Single upto 400hp and doubles upto a pair of 200hp. That gives a mathematical certainty that whoever does the tests and writes the reviews, did not have the same engine/hull combination that you have. And that’s without taking into account propeller choices.
The only thing that limits the almost countless permutations is the fact that it only comes with Mercury engines (except a VW inboard, I think). Otherwise it would set some kind of boating record for choice (and confusion).
So your plan to sit and wait, and see how she performs over time is spot on. And I’ll repeat, 30knots is fast for a three ton single engine boat. Time to enjoy your boating!
 
Hi Peter,

I think your performance figures are very poor for a 8mtr boat & 300hp Verado when you see my ones in post No.9, our boats are very much the same.

Is the Enertia prop is a 3 Blade propeller ?

If I was you I would be getting my hands on a Vensura 4 blade 17”. My 750cc with the 300hp is running a 19” the same boat with 200hp Verado fitted is running the 17” Vensura 4 blade. With the 17” you have the option to modify it down 2” if required but I think your boat will be fine with the 17”

I would have liked to have my full open throttle up to 6200rpm (6400max ) so ordered an 18” MMP Apollo – 993204 £309.28
Material: Stainless Steel
• Diameter: 14.375
• Rotation: RH
• Pitch: 18
• Blades: 4

It was no way as good as the Vensura. We were down in top end speed & fuel burn was way up even at 4500rpm.

All the best CS
 
Agree mostly with the above. I think part of the problem is that it’s an enertia eco from what I can tell. The extraordinary large diameter goes some way to offset the normal pitch dynamics and calculations. An engine with that power could easily get away with a 17” or more traditional diameter performance prop like the standard enertia, laser or similar.
 
Thank you very much for all your inputs, captains.

I am not satisfy with the performance though I know 30 knots is fast enough. Sine I have
spend the money on big (not the biggest though) engine, I guess it should provide me
something more.

From the weather forecast, my cruising area will be rough and windy for the next 2 weeks.
I won't have chance to go out anyway.
 
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