Fuel Flow and Economy Figures

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Running in new rig - Frewza F18 HT with Yamaha F115, 126km travelled, 63 Litres used. 3 POB, fishing gear, close to full tank of fuel 100L.

Was using about 2km/L @ 4400-4600rpm travelling at 35-40kmph was very swelly, boat went airborne a couple of times. 

Seems very underpowered under 4k rpm, iirc was going at 25-30kmph at around 3700rpm

Sounds about right or ?


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1st guess no number crunching..  115 underpowered for boat that weight.. under powered , not min powered  which would explain
Seems very underpowered under 4k rpm,
 you are at full power.. ok on flat water, get bit chop and no reserve power to maintain a constant speed when hit/ climb a wave.
 
That sound like what you have?
 Some further details Estimated total gross weight on the water.. ppl everything..?
And/ or  best seed and rpms at WoT at best trim on flat water?
 Which can calc gross weight on water reasonably close.

 
Also current prop pitch and diameter (stamped on the prop, usually centre  )
 Gear box ratio?  Think 4S 2:15  and 2S 2:1 check in manual.
 What rpms would you like to be able to cruise at and at what speed on flat water best trim?

 from the very basic data so far..
 would estimate boat weight.. min no gear 1 person, min fuel round the 1300 kg...say full tank gear 3 ppl ave 80kg, anchors etc.. 350/400kg(?) gross guestimate about 1650 kg (?)
 A 1200kg boat @ 115hp is min powered... and not to be able to take full advantage of the hull design in wakes, bars, and chops up.
 
Assuming 1600kg is close... to cruise around 30/32 mph, well powered (not max) you need around 95hp at the prop @ cruise rpms say 3900/ 4000 rpms and for best hull performance in chop another 15 to 20% on top... so need around the 115 hp @ 4000 rpms.. with a 19/21 pitch prop with enough grip  gives 12 to 15% slip.
For overall best economy, performance etc at full gross loaded weight on the water max WoT best trim rpms should be mid way in manufactures rpm range or slightly below(. think the yammy is 5000 to 6000)  so lets say 5500 rpms WoT
At ideal grip WoT  4 to 6% slip relates to about 45/ 47 mph WoT on a 19/21 " pitch prop. and around the 180 hp.
 Note HP at WoT is at max .. there is no reserve as in the lower rpms to manage chop etc...
 Anyway hope that helps to date.. give some understanding background and with supply of more accurate good data (questions above) can crunch numbers far better..

Bottom line thu.. ALL This done should have been done by the 'professional' sales person who sold a new engine .. repower.. or a boat / engine combo...
 
In the real world, they just want the sale commission, just plain bloody lazy, or havnt a bloody clue about this very basic stuff.
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Dream, Id expect a little better then that personally. 115hp seems on the lower end for an F18 in my eyes.

4600rpm in the 5.2m lazer with a 90hp suzuki is 28mph 3 POB.

Bigger boat with more hp I would expect more speed? Comes down to power to weight I guess? Any rough ideas how heavy she is?
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Says its 1150 kg trailerable weight, 13.25 x 17k on the prop.

I'd say about 250kg POB, 100L fuel, 27kg aux, 176kg motor, 50kg etc.

It's a 4 stroke Yamaha, i'll take pictures next time out on flat calm days for exact rpm, speed, fuel economy etc.

What should i be expecting anyways in terms of cruise revs/speed, fuel economy etc.
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Originally posted by Dream Dream wrote:

Running in new rig - Frewza F18 HT with Yamaha F115, 126km travelled, 63 Litres used. 3 POB, fishing gear, close to full tank of fuel 100L.

Was using about 2km/L @ 4400-4600rpm travelling at 35-40kmph was very swelly, boat went airborne a couple of times. 

Seems very underpowered under 4k rpm, iirc was going at 25-30kmph at around 3700rpm

Sounds about right or ?



4500rpm to get 20kts is pretty annoying - but then again theres a lot of metal in pontoon boats and 115hp is not a lot for any 6m hull

best to check your numbers properly - come back to us with exact numbers for maximum speed and rpms and we ll take it from there
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What should i be expecting anyways in terms of cruise revs/speed, fuel economy etc.
 
That is very much a personal preference....
 But bottom line what I put top of the list is the easy to maintain on the plain when it chops up, without having to work the throttle.....and still have the power to take advantage of hull design to do so in comfort at a reasonable speed..
 This is also a saftey consideration...if chop turns nasty, say tide change wind picks up in open water from barrier..lack in power means full on concentration for some hrs, lack in power that big wave, and easy to broach... same thing on a bar.
 I often wonder.. numbers never published, just how well powered the boats that crap out on bars really are.. hit that bigger wave, not enough power, broach... or just not having the power to keep in front of the following wave.. up on the back of the one in front.. swamped and/or broach....

General rule of thumb( from the data base  Im building up) boats around 5.2m thu around 7m 
 If ideal prop is 19" (subject to gear box ratio).. cruise 3800/ 4000 rpms @ around 30 to 34 mph will give the required performance when chops up.
 and have a WoT of around the 42/ 45 mph. flat water best trim

A BH says get back with good data..
ideally the data required...
 On flat water....put rpms around 3300.. trim up to just as bow starts to porpose and/ or the you hear change in tone prop ventalates... after 100m take the rpms and speed
 repeat, set to 3800, will find can trim up slightly more.. take rpms and speed
 repeat 4500  5000 and WoT.
 We take that many as often there is a error in there which can be picked up / discarded when graphed up.
 each rpm reading will be a little higher than set to...dont try and do it at that rpms...

 From the WoT number can work out gross weight on the water..

Says its 1150 kg trailerable weight, 13.25 x 17k on the prop.
I'd say about 250kg POB, 100L fuel, 27kg aux, 176kg motor, 50kg etc.

 At 1st glance, guess, would tend to say, pitch ok, but diameter way too small for that weight of boat... 14  14 1/4 more likely, subject to good data.

 I tend to lean away from 'official' weights...so often very inaccurate with 'justifiable' arguments for the purpose of making sales... thu some makes are spot on.

 PS also what is VERY important is the engine height is correct.. different makes have different heights in reationshio to the cavitaion plate.. eg mercs.
 Yammy, suzi , johsons/ evinrudes same.
 At best cruise, snap some pics...
 For general use boats (not pushing a tug, or a race boat.)
Top of the cavitation plate should have a few droplets of water running down the top like condensation on a window.. or very close to it

If under powered. min power, the best soln to lift the stern is a prop with good grip and much more rake....going to a 4 blade... the reversing issues of 4 blade , if, selected correct is a myth.. rather its far better improved, manoeuvrability at say the dock or on a big fish far better. and significantly improves chop performance.. improves not remedies.
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Under a heavy load the motor sucks in more air through the throttle body as you increase in WOT, at full WOT the inlet manifold vacuum decreases to the out side Barometric pressure . The Map sensor senses the decrease in inlet manifold pressure and relays a heavy load message via a voltage increase to the ECU , the ECU then enriches the fuel in relation to air flow through the throttle body. Under a light load less power is needed assuming you cruising the throttle body butterfly is not fully open or maybe only a quarter open the inlet manifold vacuum increases , the Map sensor senses an increase in manifold pressure and relays a light load message via a decrease in Map sensor voltage to the ECU the ECU then leans out the fuel and advances the timing at the same time.The Map sensor is a crucial part to the regulation of the fuel supply to your motor .                                                                       A dirty or clogged air filter can trick the Map sensor and relaying the wrong message to the ECU                                                         A slight leak in the vacuum system and the Map sensor will relay the wrong message to the ECU                                                        A dirty or faulty increase or decrease in voltage relayed back to the ECU will increase or decrease fuel supply                             If you have noticed a change in your fuel consumption i suggest you take it to your dealer and have your MAP SENSOR CHECKED.                  
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The biggest and most common issue be it ECU , carbed , injected.. since the introduction EGR in the late 60s, is carboned up inlet manifold..

 An essential part of service is cleaning the inlet with solvents.. under load.. be it in the water or with a prop loader  in the workshop.

The above is about maintenance... how does it explain not good idea to rev out/ free wheel an engine for longer periods..
 Be it ECU or a carb injection, variable mechanic dizzy.. the mechanics of the above remains the same thru jetting, idle circuits, power circuits, centrifugal weights ans springs, vacuum advance/ retard units etc.
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I put a new 200 hp Suzuki on he back of my seven metre boat a few months ago and have done 14 hours now, really pleased with it. keep forgetting to re-set the computer thingy but did today out raglan good conditions 3 up plenty of gear 54kms travelled for 32ltrs gas
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Did some more runs today

3800-3900 RPM
32-35 km/h
2.2-2.3km/L

so 16L per hour doing 18-19 knots @ 3800-3900 revs?

3 POB average 70-80kg per, fishing gear, spearfishing gear x 2.

Quite a nice calm day.


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If want to know how well powered/ prop correct need
detailed data. start at aprox 3500 rpms, trim to best trim travel 100m check trim, take rpms and speed.
 Repeat  set to 4000, re trim little more, take speed/ rpms.. continue at approx 500 rpm intervals to WoT.
 Best trim is just before ventilates and or just as bow starts to porpoise


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Went out for a fish here are some figures, nice flat day.

3700   33km/h   2.6km/L
4100   37 km/h  2.4km/L
4400   40 km/h  2.2km/L
4600   45km/h   2.2km/L
4900   50km/h   2.1km/L

will update more next trip, still running in motor.

Not very good at trimming boat but tried my best, video of boat on plane appreciate some feedback.


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Just been growled by Dream in PM Wink .. missed his post above with data etc
  the app I have made to make the calc easy rather than actual formula, ans also work in reverse engineering.. excel cant do that.. is in mph hence the conversions
 yammy 115hp
 2.15:1 gear box 
5000 to 6000 rpms
Prop pitch 17"   13 1/4" diam
3700   33km/h   20.5 mph 2.6km/L  26%
4100   37 km/h  22           2.4km/L    28.3
4400   40 km/h  25           2.2km/L    24
4600   45km/h   28            2.2km/L   18%
4900   50km/h   31            2.1km/L   15.5
 Total weight boat approx  calc to approx 1850 kg

Is 4900 WoT?  if so should be propped to around the 5500.. fully loaded With a slip 12 to 15% @ cruise and around 5% @ WoT
 If 4900 is WoT then need to go down to at least a 15" and either go to a 4 blade approx 13 1/2" or 3 blade 14" or 14 1/4"
 This will result in around 5500 WoT rpms, @ approx 34 mph Approx 5% slip and cruise @ 4000 rpms @ 22/ 23 mph
 unfortunately will increase fuel consumption maybe 10 or 12 %.. the problem being with the current prop have way too much slip, and the the under propped pitch is causing power head loads beyond what designed to do.

The trim video...dont confuse trim with engine height.. the  very brief glimpse of the cavitation plate indicated height ok.
 Trim is tilting the engine up while on the plane, till just before the bow starts to bounce (porpoising) and or the prop ventilates.. (sucks in air.)  faster the boat goes the more trim can get into it, the smoother the ride , the most economic point.




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Too much pitch is called over propping not under propping.... I'm sure you know this steps but others may be confused.
There are two types of people in the world: those that divide people into two types and those that don't.
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Just put a Merc 115 CT Pro XS on My Extreme 570 Game King, with a 16" Enertia prop. very pleased so far. Started with an 18" prop, but could only pull just on 6000 revs, versus max limit of 6300, so dropped the pitch by 2". Will try and 17P at some stage, but think the 16P is probably as good as it gets.

RPMs Knots L/H NM/L Slip
600
1000
1500
2000 6 5.5 1.090909 45.77%
2500 6.8 7.7 0.883117 50.83%
3000 8 10.5 0.761905 51.80%
3500 12 14.8 0.810811 38.02%
4000 18 16.8 1.071429 18.66%
4500 23 22 1.045455 7.61%
5000 26.5 26.1 1.015326 4.19%
5500 29.5 30.5 0.967213 3.04%
6000 32.4 40.7 0.796069 2.39%
6250 33.8 46.39 0.728605 2.24%
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Too much pitch is called over propping not under propping.... I'm sure you know this steps but others may be confused.

 Thanks...its one of those stupid regular errors one makes.. sitting down crunch numbers, writing hows whys up and get a mind set .. espec with dealing with underpowered boats

 BFB check your slip numbers... have re done them below
 Cruise around the 4000 rpms?

16" pitch 2.07:1 gearbox
 Gross weight on the water 1450/ 1500 kg

4000  18knt       20.7 mph   (18.7)   29.3% slip target 12-15%
 4500  23 knt      26.5 mph  (7.6)     19.5%
 5000  26.5 knt   33.5 mph (4.2)      16.7%
 5500  29.5 knt    34 mph   (3)        15.5%
6000  32.4          37.4       (2.4)       14.8%
6250  33.8         38.9       (2.2)         15%    target  4 to 7%

I would suggest that the grip (diameter/ cupping) is the issue rather than pitch
 WoT target fully loaded for most of the boats use, middle of rpms range.. which results in most cases , min load the owner would normally have would be close or on the max.
 Introducing a larger diamter prop would bring the slip numbers in range... but will also drop the WoT rpms...
 I would suggest  also adding 1" to the pitch.

 Of note  4s tend to have a narrow WoT range around 600 rpms  2S around 1000. 4S props in most manufacture ranges go in 1" increments.. and 2S in 2" increments
 
Ideally should bet the grip right then sort pitch...thu with a prop in ball park and good data, one can change both and still get real close.
 
 Ideally a well powered boat that weight /size to get best hull and sea performance and good reserve power at cruise rpms and less  for chop/ seas a 150hp would be far more appropriate....115 is at or just below min power.
 Well powered will also give a good 10 to 15% + better economy at the same speeds, and far better in chop/sea as the throttle doesnt need to be worked.
A 150  well proped would give a cruise about 30/32 mph...29 knt with around a 19" prop

 I really do hate having to make comments like , "under" or "min powered".. espec when someone has a new rig or repowered.

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Hi Steps, the gear box is 2.38:1 for the CT engine, the standard 4S is 2.07:1. Have checekd those slip numbers using the mercury online calculatorand my own calculations both align for 2.38:1.

Yes, Ideal cruise looks to be between 4000-5000 revs depending on conditions.

The Enertia Props are available in 1" increments, hence my comment that I will try the 17" and see what happens.
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That explains things
 hang on will just check a couple on the 2.38:1 ratio...
 yep close with in 0.05%...

looking at your correct numbers then

3000  8             9mph                   51
 3500 12             14mph                  38
4000  18knt       20.7 mph   18.7  target 12-15%
 4500  23 knt      26.5 mph  7.6    
 5000  26.5 knt   33.5 mph 4.2    
 5500  29.5 knt    34 mph   3       
6000  32.4          37.4       2.4      
6250  33.8         38.9       2.2            target  4 to 7%

 At 3500 you just coming over the bow wave.. not quite on the plane?...should at least be just on the plane... even at 3000/3200.
 3900/4000 should be nice on the plane and 13 to 16% slip cruise.
 If need to go to 4500 for cruise means requires more rpms to devalop that hp to get to cruise speed....which leaves very little, at least not 15 to 20% required as reserve power at cruise
  1st thing is to get that grip (slip right) or at least in ball park
 being under powered requires more rpms.
 So lets take 4500 as a cruise rpms... with slip of 14/15%
 on a 16" pitch gives a cruise 24/25 mph..
 And would give around 3 to 4% WoT. about 6000.

 Its a little strange that at your cruise 4500 have little too much slip yet WoT only 2%  bit tight... and a massive drop between 4000 and 4500

 Im questioning the data... espec the 3500 thru 5000.
 Accruate data is essentual

OK flat water get on the plane..fully loaded boat.. put rpms around (not spot on just below) to 3000, now trim up till either , just before the bow bounces (porpoise) and/ or just before the prop ventilates. Travel a good 50/ 100 m then take speed and rpms.
 Do not try to set at a given rpm or speed try to trim re set rpm , re trim etc.
 Now set rpms to around/ just below 3500, trim up as before  record the rpm and speed
 repeat at  approx 500 rpm increments to WoT
 you will end up with data like
 3150 rpms  x speed
 3600 rpm y speed etc
 the faster you go the more trim you can put in.

 
PS  you in the Waikato?  are we on salt water or fresh water?.. that too makes a significant difference






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Those trials were on Freshwater (Lake Karapiro, running up the lake), I trimmed it to get WOT top speed then dropped back to 6k then dropped 0.5k inrements without changing trim, that probably accounts for the increased slip at lower revs. 

The load was deliberately light, full tank of gas, and 2 POB.

What's the downside of having less than 4% slip?
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Do you normally use the boat on fresh salt water?
Fresh water has quite a bit more drag than salt water.


I trimmed it to get WOT top speed then dropped back to 6k then dropped 0.5k inrements without changing trim, that probably accounts for the increased slip at lower revs.

Which means from around 5000 down you are over trimed, which is not as significant as under trimed, till speed drops off around the 4000 mark.


The load was deliberately light, full tank of gas, and 2 POB

 When propping a general use planing hull light load.. basically as it comes off the dealers floor.. as they had it when propping, one aims for the high end of the WoT range.. the hopeful theory is  when loaded that will bring the WoT rpms down to around midway... In practice its far better to prop the actual boat to its normal fully loaded weight... espec on a well or over powered boat.. huge differenced in speed and economy

What's the downside of having less than 4% slip?

 
Well not a down side .. sorta...
you like detailed info then?Wink

Basic physics A given hp will move a given weight at a given speed.. therefore your boat car tractor aeroplane, will only travel at a given max speed. you are matching the target WoT rpms  to max speed for your boat weight..

 At WoT therefore the powerhead is at max hp at that rpms.. bearings, crank, 'rods, all loaded up, down thru the gearbox and to the prop at 100% .. no torque converter or clutch in between to take any change in weight friction etc. Say hit a wake or resistance the already fully loaded engine gets suddenly overloaded...eventually something breaks.
At the other end , holeshot  the engine cant produce enough power to take off, so slip is built into the poro,, and other stuff like holes in the side to allow exhaust gases around the blades, increasing slip further.. which means we can increase rpms.. and available power to get over the bow wave.
 The torque/ hp curves, gear box ratio are designed  (for general use boats to have reserve power at lower rpms and around 12 to 15% at cruise to around 5% woT.
 (Race boats area completely different setup.. heaps slip and pitch WoT.)

In particular if min or underpowered and slip is close to ) WoT then there will be less lip at cruise, right down to trolling..the power head doesnt have the 15 to 20% reserve power in those pms to handle chop at all, and powerhead is often fully loaded and overloaded when hitting a wave, momentum lost, drops off the plane, have to work the throttle all the , chew thru heaps fuel, loading and overloading crank, etc.. and warrantee issues.
 If  min/ underpowewred and in a boat 4.5 thru to 5.5/6m fully loaded and propped for around 5% slip WoT @ mid range, then at lower rpms you still have a little resrve poer and 12 to 15% slip at cruise and in chop...and a balancing act as to be able to maintain reasonable sped to stay on the plane without requiring 100% hp at any rpms .
the more the WoT slip gets to 0 the more overloaded and stress on the powerhead at cruise and lower rpms.
 A well powered boat.. aim for the same , thu often can afford to go to lower WoT slip if over powered.. basically because have a good 20% + reserve power at cruise and lower...and a reduculous WoT speed that would be an idiot to maintain.
eg 150hp on an old commander, fully loaded 3 PoB bimini etc and hitting around 50 mph.. starting to chine walk...and will if hit a big ripple.Like the wifes shopping basket car, it has a top speed you will never use, a side symptom so have reserve power to get up slopes fully loaded without always changing down a gear , to increase rpms to that extra power.
 @ 42/43 mph open up throttle slightly too fast the rear end kicks out like a V8 on a wet road hit the throttle rear end goes loose.
Bigger heavier boats have a little more slip at WoT 6m + getting out to around 8%

 less slip more speed per rpms
more pitch more speed per rpms
 Propping is a balance between slip and pitch without overloading the powerhead, for best performance and economy.
Rake effects the above also, but primary to make better use of the narrower thrust cone out the back to bring a high bow down...  and/or venation issues.. and improved propping performance.. due to smaller prop less hp loss, more for pushing.
 Im a great fan of 4 blades espec on min and under powed boats
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22 Mar 2024
Saltwater Fishing Reports
Bay of Islands Fishing Report - 22/03/24

Variety is the spice of life On one recent trip, the plan was to spend a... Read More >

22 Mar 2024
Saltwater Fishing Reports
Hauraki Gulf Fishing Report - 22/03/24

Fish where the fish are! Catching fish or just going fishing? I tackle this issue... Read More >

22 Mar 2024
Saltwater Fishing Reports
Inner Hauraki Gulf Fishing Report - 22/03/24

Thoughtful tactics required for better fish Over the course of each year the fishing varies,... Read More >

22 Mar 2024
Fishing bite times Fishing bite times

Major Bites

Minor Bites

Major Bites

Minor Bites