new propeller installation and test resoult

hi guys; as a lot of you already know i have just rebuilt my boar and my motor;
motor mercruiser 4.3
stern alpha one gen II ratio 1: 1.65
boat dry weight 3300 lib so i would expect at least 4400 lib in running condition
first propeller tested a stainless still 14.75x 17 ( no enought rpm)
top rpm 4200 speed 31 knot fuel consuption 54 l per hour
at 3000 rpm 20 knot petrol 34 liters per hour
at 3200 rpm 22 knot petrol 35 lhr

then i tryed one 15.5 x 15 alluminium (18° blade angle) this is the propeller that old owner of my previous 4.3 efi with same stern gave me and i was using on the efi model
with this i had:
3000 rpm -- 16 knot 28 l/hr
3500 rpm -- 21,4 kn 36 lth
4700 rp ( max rpm) 28/29 kn--61 lhr

3rd tryal
alluminium 15x17

3000 rpm 16,7 kn 28,5 lhr
3500 rpm 22 kn 40 lhr
4400 rpm max speed 28 kn 57 lhr


4th tryal one alluminium stamped 15,5 x 17 ( shop that lant this to me told me that i should be 15 ( blade angle 10 °) but test seems confirm 17
3000 rpm 20 kn 28 lhr
3500 rpm 24 kn 35 lh
4200 rpm max 29 kn 54 lh

when i was in the shop, were they gave me last propeller and check angle of mine ecc they modify my 15,5x15 alluminium prop due to the fact that was damaged and noy it is 15x15.
with this new conf i got more or less same resoult ( just 0,5 knot less at same rpm, but i can reach 4800 rpm instead 4700 with same speed)

the alluminium 15,5x17 is the one that in cruise speed seems to be the best one, it use less petrol and more speed, and feeling is better while running in terms of confort, but unfortunatelly i can not reach more than 4200 ( temperature was stabile at 70 deg)

what is your suggestion? i'm in about to buy the final propeller, but in doubt what ( and especially if 3 or 4 blade) alluminium or stainles still.

thanks to who will reply

First of all, what is the WOT Rpm Range for that Motor?

jimmbo said:

First of all, what is the WOT Rpm Range for that Motor?

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on motor docs it is mention power at wot 4800 .... so i would say 4800
accordinglly to what i heard range should be between 4600-4800

So....which prop will give you 4800-4900 rpm at wot with a light load or you by yourself ? That is the prop you need. Aluminum or SS is your call, how often to you hit bottom and need a prop repair ?

airshot said:

So....which prop will give you 4800-4900 rpm at wot with a light load or you by yourself ? That is the prop you need. Aluminum or SS is your call, how often to you hit bottom and need a prop repair ?

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yes but i mean there are 3 blae 4 blade more angle less angle, i'm not sure what is better. accordinglly to the test obviously the 15x15 or 15.5 x 15 is the right one because it give me 4800 rpm but as said 17 in cruise speed run much better, but i don't know if i will stress too much the motor in that condition.

test was performed all just with me on the boat, 150/180 lt of petrol and 50 liters of water more or less

fortunatelly here we have sand so no so many risck for the propeller to be damaged in case of no deep water .

di you think that this pinch are ok for a 4,3 mercruiser in such boat? i mean, motor is new running very well but in searay website for mercruiser i think they suggest 15x17 (not sure wich ratio, but i think with mine) but i can not see any other reason that it could be powerless.

You Prop an Engine so it run at or Near the Top of the WOT Range. If you chose a prop because you like a certain Speed at a certain Rpm, and the Engine isn't getting to the WOT Range, you will be Torturing that Engine. But, hey, it is your Engine to wreck, and your Money to repair/replace it

jimmbo said:

You Prop an Engine so it run at or Near the Top of the WOT Range. If you chose a prop because you like a certain Speed at a certain Rpm, and the Engine isn't getting to the WOT Range, you will be Torturing that Engine. But, hey, it is your Engine to wreck, and your Money to repair/replace it

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This is why i'm asking en help here.
Motor is new i replaced everithing so i want to make it work as much nice as i can

All your testing at various speeds mean nothing except cruising information for yourself. The best prop for your engine and a long life expectancy is the one that gets your motor to max rpm with a lite load.
Whether you choose a 3 or a 4 blade depends on whether you want or need a better hole shot for skiing or pulling water toys. Folks that do a lot of pulling or carrying heavy loads will choose a prop with less pitch for the added weight. Many boaters carry multiple props, for heavy loads, light loads and just general cruising. If you only want to carry one prop, then get the one that fits the formula that is pretty much the standard for all speed boats.

airshot said:

All your testing at various speeds mean nothing except cruising information for yourself. The best prop for your engine and a long life expectancy is the one that gets your motor to max rpm with a lite load.
Whether you choose a 3 or a 4 blade depends on whether you want or need a better hole shot for skiing or pulling water toys. Folks that do a lot of pulling or carrying heavy loads will choose a prop with less pitch for the added weight. Many boaters carry multiple props, for heavy loads, light loads and just general cruising. If you only want to carry one prop, then get the one that fits the formula that is pretty much the standard for all speed boats.

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Thanks, i don't pully nothing (here in italy is not common) so actually i use boat for cruise or we ride.
99% of the time i run at 3500/3600 rpm never at full speed

ste80 said:

Thanks, i don't pully nothing (here in italy is not common) so actually i use boat for cruise or we ride.
99% of the time i run at 3500/3600 rpm never at full speed

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Typical running speed has no bearing on what prop you need. The props pitch needs to max the wot rpm of the motor, then it will be the correct match thru out your operating range

airshot said:

Typical running speed has no bearing on what prop you need. The props pitch needs to max the wot rpm of the motor, then it will be the correct match thru out your operating range

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Ok so choise has to be 15 pitch if i keep this ratio.
I just wander if 15 is normal for this weight and stern but i think so because it was on also on the efi that i bougt.

I just need to understand now if 3 or 4 blade.

In cade i will chose 4 blade pitch will remain 15? Or i need to modify?

ste80 said:

Ok so choise has to be 15 pitch if i keep this ratio.
I just wander if 15 is normal for this weight and stern but i think so because it was on also on the efi that i bougt.

I just need to understand now if 3 or 4 blade.

In cade i will chose 4 blade pitch will remain 15? Or i need to modify?

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Once again, does the prop allow your engine to reach max operating rpm with a light load ! Typically a 4 blade prop uses approx 2" less pitch to achieve the max rpm on the motor over a 3 blade prop. Gotta make up for the extra blade. Weight means little, max rpm under wot is the formula needed.

airshot said:

Once again, does the prop allow your engine to reach max operating rpm with a light load ! Typically a 4 blade prop uses approx 2" less pitch to achieve the max rpm on the motor over a 3 blade prop. Gotta make up for the extra blade. Weight means little, max rpm under wot is the formula needed.

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rpm under wot is the formula needed. yes at this point this is clear, and this is why i said i need the 15 pich . my last doubt is if people that has a mercruiser 4.3 lx with similar boat experiance find this number normal to understand if i can have any issue on the motor , but as said motor is running very well, also a mechanics that inspected my boiat told me same so i would not expect in motor, but just a check
thanks for supporting

Identical boats and motors could run different props, depending on how the boat is loaded, what the boat is used for. Other owners may do a lot of toy pulling or pack their boats with guests, so hard to compare the same prop.
Once a prop is chosen for general purpose use, props can be custom tuned to address certain issues a boat might have, like better holeshot, getting the bow lower or the transom higher depending on the type of activities the owner wants to do. Often folks carry multiple props to swap out depending on the days activities.