PROP size yamaha 250 2 stroke

[highlander]

I recently purchased 2twin yami 250 to replace twin yami 225 2 stroke on 2000 grady 272. Engines run great but increased about 40% fuel consumption which really destroys my range. I normally run up 3800 rpms and have consistently run this way with the 250's. My guages don't show any higher fuel consumption but it obviously consumes more fuel due to 50hp increase. I want to know if prop size makes a difference. These motors were on a different make of boat and maybe I need to change pitch for 272 grady?? thanks

 

[DennisG01]

What props are on there and what is the Max RPM you can get (trimmed out fully till ventilation or no more speed increase) with a light to normal load in relatively calm conditions?

40% is a lot. A LOT. Depending on the answers above, it's possible that something else is going on.

 

[Sardinia306Canyon]

Your boat should be much more fuel efficient with 4 strokes and definitively not burn 40% more.

Did you noticed a noticeable lower stern due the higher weight of the 4 strokes?
Are you sure new engines sit on same height and not too low; anti ventilation plate skimming or just below waterflow?

However, what Dennis1 already wrote, without more detailed informations about rm and fuel burn it's not possible to give a useful answer.

If you have time the do a test ride and write down:
rpm - speed and fuel flow in 500 rpm steps from idle to wot and copy them here.

Chris

 

[Ekea]

so, you went from 225 two strokes to 250 two strokes? unless that is v4 to v6 or they are completely different in some other way, they should be nearly identical.

what props were on the 225s?
what props are on the 250s?
what was/is max rpm with both of the above set ups?
are you sure the 250s are in good shape?

 

[seasick]

How can you have 40% more fuel consumption but the gauges don't show that? What gauges?

What did you do wit the old props?

 

[georgemjr]

"My gauges don't show any higher fuel consumption" - then you don't have any higher fuel consumption.

"it obviously consumes more fuel due to 50hp increase." - that isn't "obvious" it isn't even indicated on the gauges nor is it necessarily true.

" increased about 40% fuel consumption" - where are you gathering this 40% greater consumption information from, if not the gauges?

Listen to those that responded before me and figure out max rpm to determine correct prop set up, only other variable is engine height. F250s are NOT thirstier than F225s.

 

[highlander]

Well boat on blocks at marina so cant run now...original props went with old motors and last trip was to hotdog ,which is 45 miles.....trolled all day and the 150 gal tank went dry 10 miles on return trip....i normally burn 100 gals going to hotdog and back....made that trip dozens of times and fuel burn always the same. Last two times (using 250 2 strokes) I had the same experiance with 150 tank empty before reaching the barn. I dont run them any harder than the 225 2 strokes.....so thats 50 more gals to go the same distance.
Tanks are always topped b4 the trip and always just 2 people onboard. I run on flat days always and 3700 rpm burn and I troll 7-9 knots. The only thing I can think is these are not the original props for my Grady. Percentage is just a guesstimate but gals per hour on my guages show same amount of burn. So how are they burning 50+ gals more?

 

[seasick]

Well boat on blocks at marina so cant run now...original props went with old motors and last trip was to hotdog ,which is 45 miles.....trolled all day and the 150 gal tank went dry 10 miles on return trip....i normally burn 100 gals going to hotdog and back....made that trip dozens of times and fuel burn always the same. Last two times (using 250 2 strokes) I had the same experiance with 150 tank empty before reaching the barn. I dont run them any harder than the 225 2 strokes.....so thats 50 more gals to go the same distance.
Tanks are always topped b4 the trip and always just 2 people onboard. I run on flat days always and 3700 rpm burn and I troll 7-9 knots. The only thing I can think is these are not the original props for my Grady. Percentage is just a guesstimate but gals per hour on my guages show same amount of burn. So how are they burning 50+ gals more?

They are probably not burning more gas. Assuming you did use more gas and I am not convinced your method of estimating consumption is accurate, the gas must be leaking out somewhere. There could be a leak under cowling in which case the gas will end up flowing out the prop and may not notice ot. That is an extremely dangerous situation.
You could be leaking gas into the bilge another serious situation.
I suggest that you check for leaks under the cowling and you don't find any and I don't thing you will, you redo your tests
The WOT speed and actual revs will help determine if the prop is close. You need the specs for the prop, pitch and diameter. Thay may be stamped on the outside of the prop or on the inner bushing casting. To see that spec if inside the hub requires removing the prop.

 

[DennisG01]

Chris - good point about engine height. I think it would have to be quite far off to experience that consumption difference... but it's VERY worthwhile to check and this can also be done on the hard - whether by lowering the engine if there's ground clearance or just comparing holes to where the 2-strokes where.

What are the specs on the current props compared to what your old props were?

Seasick brings up some excellent points, too.

I still think this all comes back to "something else is going on".

 

[Sardinia306Canyon]

Chris - good point about engine height. I think it would have be quite far off to experience that consumption difference...

Yes, thats what i believe too but maybe the combination of too deep and other factors, however i don't believe that the new 4 strokes burn 40% or the 50 gallons OP wrote more, thats virtually impossible and gauges show correct fuel burn. Not sure what kind of gauges OP has, but all newer outboards calculate fuel flow by counting injector cycles and that is very accurate, very! If Op has the old impeller driven Yamaha fuel Flow Sensors or a 3rd party fuel flow metering device the problem could be there, or OP has by mistake set the fuel gauge calibration to a totally wrong value.

Honestly i believe that there is a error in OP's GuessTimating about fuel consumption, either gasoline was stolen or a trip forgotten.
I would exclude a fuel leak as the smell would be very noticeable and where should go 50 gallons of fuel?

One thing what could be is that the new propellers are way to small in pitch and/or diameter or damaged and OP has to run 6000 rpm for same speed he could reach with the old engines before, that would explain burning 40% more fuel

His boat is out of the water, so nothing really useful (particular rpm-speed-fuel flow) could be done to verify a hypothetic 40% higher fuel flow, what would be helpful is

• model and year of Yamaha 250HP
• kind of gauges are installed
• kind of gauge showing fuel flow
• verifying if fuel flow calibration is set correctly
• pulling all spark plugs and check for heavy fouling
• a photo from side and height as AV Plates side using a ruler or similar attached to hull to show most exactly where AV plates are
• double check if new engines are same length as the previous ones, same hole on 5" longer model would make LU sitting ways too deep
• checking for gas smell in bilge
• thinking again and once more if there is a error in OP's fuel burn calculation, possibility of fuel theft or incorrect fuel pump

Chris

 

[highlander]

to clear things up...i purchased 250 2 strokes to replace the 225 2 strokes as i was told installing 4 strokes requires new transom wholes....installer concerned boat always slipped and i might have to deal with transom rot and it would be costly to replace the transom....there has been no smell of fuel and no sign of fuel leaking at the cowling as the paint would show erosion.....aft bilge is clean and forward bilge has been checked with no sign of fuel....i guess i will have to wait til spring when i get back down to check prop size

 

[DennisG01]

Drill new holes? No.

Paint showing errosion from gas? No.

Wait till Spring? No.

Take a look back at the advice given - there are a number of things you can do now.

There should be zero, if any difference, in the fuel usage between those two engines. It's quite possible that there would be a slight increase in fuel economy.


Chris... I was wondering about the leg length as well...


Seasick... did you notice that Chris used the term "anti ventilation" plate? (for most reading this, that's an inside joke)

 

[Punch53]

Well boat on blocks at marina so cant run now...original props went with old motors and last trip was to hotdog ,which is 45 miles.....trolled all day and the 150 gal tank went dry 10 miles on return trip....i normally burn 100 gals going to hotdog and back....made that trip dozens of times and fuel burn always the same. Last two times (using 250 2 strokes) I had the same experiance with 150 tank empty before reaching the barn. I dont run them any harder than the 225 2 strokes.....so thats 50 more gals to go the same distance.
Tanks are always topped b4 the trip and always just 2 people onboard. I run on flat days always and 3700 rpm burn and I troll 7-9 knots. The only thing I can think is these are not the original props for my Grady. Percentage is just a guesstimate but gals per hour on my guages show same amount of burn. So how are they burning 50+ gals more?

9 knot troll? Blue Marlin?

 

[Sardinia306Canyon]

Drill new holes? No.

I was wondering about the same, we did many repower from 2 strokes to 4 strokes and i can't remember that the the hole pattern was different.
And that includes my own BW 23 Outrage where i installed first twin Yamaha 150 with carburetors, but changed because of absurd fuel consumption to twin 150 HDPI and after one exploded to twin F150 and we did not have to drill new holes.

Regarding the Anti-Ventilation or Anti-Cavitation plate
There is some inconsistency between the naming and the function of the plate sitting above the propeller.
Most outboard manufacturers call this plate Anti-Cavitation plate, including the Italian rigging instructions of the brand new Honda BF350 and the unknown brand rigging instructions here: https://www.bandofboaters.com/forum/main-forum/boaters-forum/49502-anti-cavitation-plate-question
However, other call it Anti-Ventilation plate.
The difference between cavitation and ventilation is described here https://chieftain.training/propeller-cavitation-propeller-ventilation-explained/
Another good read is on ContinousWave (yes Boston Whaler) here: https://continuouswave.com/forum/viewtopic.php?t=5509


Chris

 

[DennisG01]

AV... yes, in some cases it's wrongly referred to as an AC plate - even by the manufacturers. But as you (and other know), cavitation and ventilation are entirely different things.

I've always been a prononent of "nicely" correcting people in posts so we use the correct terms. Seasick tends to point this out to me when I do it - in a fun/sarcastic way

 

[Ekea]

do you know that the 250s are healthy? do you have compression test numbers? do these have a vst? my vst was constantly overflowing, leading to fuel to continuously leak

even if you dont have WOT numbers, was your cruse mph at 3700 rpm the same with both motors?

have you run the engines on muffs and looked around for leaking fuel, rich exhaust smell...

when you re fuel after the recent trips, do you put in the same amount of gallons as the capacity of your fuel tank? another issue could be a vent or pick up tube issue

 

[highlander]

OK again, I have made the trip to the hotdog out of indian river dozens of times over many years and it has always taken about 100 gals of fuel....sometimes a little more or less depending on sea conditions. I made the two last trips with 2 stroke 250 ox66's and the fuel burn both times emptied my 150 gal tank and the fill at the gas dock proved that the tank was empty plus or minus a couple gals. I have not checked compression or looked at the prop size since on blocks ,as i live in Pa. I did have a fuel line rupture 2 years ago under the cowling that was not discovered while trolling until i got back to the slip. The fuel removed all of The paint under the outside of the cowling where it sprayed. Visual inspection of the 250s during fall winterizing shows no paint damage. I have never found fuel in forward or aft bilge nor have i smelled during these last two trips. I am planning in spring to compression test the cylinders and possibly replace props at that time. These 2 stroke 250's sit no lower or higher on the transom than the 225's. The 40% is a guess, as it now takes 150 gals to go the same distance as the 225's consumed 50 gallons less. Don't know how else to explain as the fuel guages do not work correctly. When i have the 150 gal tank filled the guage shows 2 bars and when empty 1 flashing bar. I wish I had another way to guage the fuel consumption but the boat is 24 years old and has a few glitches. I appreciate your responses as it gives me clues as what to look for this spring,

 

[highlander]

9 knot troll? Blue Marlin?

great white

 

[DennisG01]

Highlander, to be clear about what I said about fuel vs paint... I meant that it is not "guaranteed" to remove paint. In other words, having intact paint is not a definitive answer to whether or not fuel is leaking. It certainly COULD remove paint - but I've seen plenty of cases where it hasn't. All I mean is to not discount fuel leakage without physically checking for it.

Regarding engine height... from what you just wrote it sounds like the power head is physically the same height when looking at it. But as noted above... what about shaft length? If the 250's have longer shafts than your old engines, then the props and gearcases would be 5" lower in the water creating a lot more drag.

 

[Ekea]

were the 225s HPDI engines? maybe the HPDI vs OX66 is the fuel culprit. maybe someone that knows more about 2s motors can chime in