Engine height and Props testing

[lateralinfo]

so i ran the tests mentioned in moonlight's sticky topic. here are the results and i would appreciate your expert opinion on interpreting the data:

The Boat and the Conditions:
Boat is 40 foot seamaster half cabin used for a diving school, weighs 4 tons and fitted with twin DF3000AP mod 2017 with stainless steel suzuki 3x16x20 props.
Fuel tank had 1,000L and fresh water tank had 300L: I usually operate at half those tanks capacities, but also should account for extra divers weight with their gear (on average 2 crew and 5 divers with their gear, but sometimes up to 14 divers with their gear).
90% of the trips are less than 4 miles out with rare occasional trips up to 20 miles out. Sea conditions are usually good: no diving in rough seas and no long off shore trips.
during the following tests, the sea conditions were good, slightly windy but not too much. Also should be noted that the boat has minor fouling formations on it, but this is how it will be 90% of the time.

Engines Height:
I put the boat on plane with engines trimmed out to adequate level. I experimented with my hydraulic jackplates up and down, in combination with trimming in and out the engines.
I was able to figure out the best height for my engines: ventilation plate skimming the surface with most efficient revs/speed/consumption ratios.
All the following tests were conducted at this same engine height.

Initial Data Table:
Engines trimmed in (all the way down).

RPM (revs)	Speed (km/h)	Fuel Efficiency (km/L)	Fuel Consumption (L/min)
1,000	9	1.1	9
1,500	14	1.0	14
2,000	18	0.8	22
2,500	23	0.6	36
3,000	30	0.7	48
3,500	38	0.6	62
4,000	48	0.5	94
4,500	55	0.5	112

Max Revs at WOT:
After full throttle, engines trimmed out (up) up until the props started slipping then backed in (down) a notch to grip again.
For some moments, it would go up to 54,000 but doesn’t hold for long. Important to note that the wind was in our back and we had following seas during this part of testing.

RPM (revs)	Speed (km/h)	Fuel Efficiency (km/L)	Fuel Consumption (L/min)
5,100	70	0.4	175

Hole Shot test:
It takes 25 second to reach max revs when throttle is hit hard from idle speed.

Optimum Cruise Speed:
Boat put on plane, engines trimmed out (up) up until the boat starts to porpoise and/or props started slipping then backed in (down) a notch.

RPM (revs)	Speed (km/h)	Fuel Efficiency (km/L)	Fuel Consumption (L/min)
4,000	47	0.6	75

Expert Opinion:
It is important to note that a Suzuki head engineer is suggesting to switch to Suzuki stainless steel 3x16x17 propellers (Suzuki doesn't manufacture 4 blade props).
another expert prop shop is recommending 4x15.25.16 because they would give more grip and torque due to heavy load (losing on top speed isn't a big deal for my operation).

any thoughts on the right prop for my boat?
thanks a lot.

[Scarab]

Sorry-can't help much with the prop question, but I hope your engines arent really revving to 51000! lol

[lateralinfo]

oops...yeah there was a 0 that shouldn't be there lol

[tsturm]

Didn't we already do this?????

[Moonlighter]

There are apparently conflicting statements about the data presented in the table.

Your post says:

”I was able to figure out the best height for my engines: ventilation plate skimming the surface with most efficient revs/speed/consumption ratios.
All the following tests were conducted at this same engine height.

Initial Data Table:
Engines trimmed in (all the way down).”

Now, it cant be set at the optimum height as per the first sentence, and then also be set with the engines trimmed all the way in, at the same time, as per the header above the table!

Not possible.

So what was it?

Common sense says that the tests as per the sticky should be done with the engines operated AS THEY SHOULD BE by an experienced skipper. Eg trimmed in to push the boat up over the hump onto the plane, then trimmed out and up to optimal as revs rise. If you want to put it another way, trim the engines to their best position at each point of the rev range when you do this test.

If the data presented is a result of a test run where the engines were kept trimmed all the way in and all the way down, it is basically useless data because it doesnt represent the reality of how the boat should be operated under most “normal” conditions.

If it is the alternative, and I can only presume that the engines were trimmed in to get onto the plane and then trimmed up and out to optimal for the rest of the rev range, then, the data shows that the engines are way over propped.

5100 is far too low at WOT. By a long shot. Your target is to gain 900rpm.

As the sticky says, you need to prop these engines so they hit 6000rpm, or very close to that, in order to get optimum performance and economy.

Whoever told you that Suzuki doesnt make 4 blade props is wrong. See (approx) page 41 of this Suzuki 2020 catalogue:

http://www.suzukimarine.com/~/media/...%2019%20KB.pdf

Anyway if you stick with a 16” prop, then the 3 x 16 x 17 is the lowest pitch available in that dia from Suzuki, and it seems very doubtful that 3” extra pitch is going to get you the extra 8-900 rpm needed.

By all means test those props that are being suggested, but make sure you agree with the suppliers that they are being tested and will be returned for full refund if they dont perform to requirements.

To be honest, I am really starting to think that the pre is a more fundamental problem at play hear that is being danced around ...... the boat may be underpowered - too heavy for 300’s.

[lateralinfo]

Didn't we already do this?????

This is a follow up to my other thread but this time i made the tests mentioned by moonlight in his sticky topic. I thought maybe those findings would help further

[Moonlighter]

There are apparently conflicting statements about the data presented in the table.

Your post says:

”I was able to figure out the best height for my engines: ventilation plate skimming the surface with most efficient revs/speed/consumption ratios.
All the following tests were conducted at this same engine height.

Initial Data Table:
Engines trimmed in (all the way down).”

Now, it cant be set at the optimum height as per the first sentence, and then also be set with the engines trimmed all the way in, at the same time, as per the header above the table!

Not possible.

So what was it?

Common sense says that the tests as per the sticky should be done with the engines operated AS THEY SHOULD BE by an experienced skipper. Eg trimmed in to push the boat up over the hump onto the plane, then trimmed out and up to optimal as revs rise. If you want to put it another way, trim the engines to their best position at each point of the rev range when you do this test.

If the data presented is a result of a test run where the engines were kept trimmed all the way in and all the way down, it is basically useless data because it doesnt represent the reality of how the boat should be operated under most “normal” conditions.

If it is the alternative, and I can only presume that the engines were trimmed in to get onto the plane and then trimmed up and out to optimal for the rest of the rev range, then, the data shows that the engines are way over propped.

5100 is far too low at WOT. By a long shot. Your target is to gain 900rpm.

As the sticky says, you need to prop these engines so they hit 6000rpm, or very close to that, in order to get optimum performance and economy.

Whoever told you that Suzuki doesnt make 4 blade props is wrong. See (approx) page 41 of this Suzuki 2020 catalogue:

http://www.suzukimarine.com/~/media/...%2019%20KB.pdf

Anyway if you stick with a 16” prop, then the 3 x 16 x 17 is the lowest pitch available in that dia from Suzuki, and it seems very doubtful that 3” extra pitch is going to get you the extra 8-900 rpm needed.

By all means test those props that are being suggested, but make sure you agree with the suppliers that they are being tested and will be returned for full refund if they dont perform to requirements.

To be honest, I am really starting to think that there is a more fundamental problem at play here that is being danced around ...... the boat may be underpowered - possibly too heavy for 300’s.

That is all I can offer. Done.

[lateralinfo]

*** what an awesome detailed reply! Thanks a lot moonlighter, you never disappoint
to clarify: engine height was set on the jackplate (at level 3) and all the testing was done at that same level and jackplates didn’t move throughout.
what i meant was that the engine were trimmed in during the testing: maybe i misunderstood the instructions, i should have trimmed them up accordingly with every 500 rpm? Is it worth it to redo the trating you think?
And yes, i keep hearing that the boat might be underpowered i am trying to make the best with what i have. Starting to lean towards 4 blades honestly...

[Moonlighter]

I understand now. You had the jack plate set at optimum, but then kept the motors fully trimmed in. EXCEPT Until WOT, where you trimmed all the way out till they lost grip then came in a bit. Im not sure I understand the rationale for that approach, but it is, what it is.

That will definitely affect the numbers (except WOT trimmed out) when doing the rpm test. I suspect this would be most noticeable in the 3500+ rpm range and up to WOT.

So yes you could re-run the test with more appropriate engine trim settings across the rev range, and I would expect to see better numbers in the mid rev range, but if you did the WOT with both jack plate and engine trimmed to optimum, your WOT FIGURES WONT CHANGE.

So the story still points to significant over propping and/or underpowered.

All you can do now is to test other props based on local recommendations and see what you find.

At least now (and if you redo the tests with engines trimmed out to optimum throughout the range) then you have a solid baseline to compare the new props being tested with.

From what I have seen over the years, prop selection is part science, and part art. 4 blades add another dimension again. So I would encourage you to run ANY new props you try thru the full rpm tests as per the sticky so that you then get good, solid, and complete data to review and compare. Then add in the “seat of the pants” factor.

good luck.

[lateralinfo]

Tx a lot...very helpful

[Vronsky]

5100 RPM at WOT is indeed really low...
WOT on mine is approx 6200, but the 'rev limit' message appears when exceeding 6100

[D'oh999]

I will be a 3rd, 5100rpm is Way Over Propped. That being said, the Prop People suggestion of 15 1/4x16 seems more logical than Suzuki's 16x17, but either will be much closer to where you need to be.