7500lb boat (loaded 2556), 5.7omc.... prop suggestions?

[Centerline2]

I would like to hear from others who have a similar weight boat, about what prop seems to work the best for them.

there is a LOT of information and questions regarding prop issues on EVERY boating site... and we all know that there are those who stretch the truth a bit for no other reason than to lead others think they have a more powerful or better tuned boat... and some of them actually may have!

but we hardly ever read about how the boat is loaded, or its displacement weight.

a boat that is without any supplies, tools, or gear and with a half tank of fuel, no water or waste on board, will have much better performance than the exact same model of boat/power package that is fully loaded and stocked for a 2 week cruise.... so it is not really a fair comparison when talking, power, tuning or prop selection.

My boat, a 2556 (which, the model shouldnt matter as much as the weight of the vessle) is always loaded for cruising, and the displacement weight of the boat only is right at 7500lbs.

the engine, a 5.7OMC package (of which the top cruise rpm should be 4600), and a 15dx13p prop pushes it at WOT to 21kts/24mph, at a max rpm of 4300.... and it sounds as if the engine is laboring, with the secondaries open and sucking the fuel at a rate of 27-28gph...

I have a new spare prop that came with the boat, and it is a 15x15. I tried it with slightly worse results. (Im willing to trade it for a usable prop with a lower pitch... or sell it)

using an online "prop selector" calculator, it recommends that I switch to a 15dx11p prop...

contrary to what some other boaters want from their craft, Im not after speed so much as I am an engine that runs easy, carrying the load that im asking it to carry.

this boat (with its larger trimtabs) will easily stay on plane at least down to 6.5kts, and I would like to have the ability to have the tach run up to 4700, just so when im running at 45-4600, I know the engine is not laboring so much... and NO, I dont want to run it at 4700, or even 4600 for long periods, but I want the engine to have the reserve power to insure its not laboring all the time when I am running near the upper end of its operating range ...

I know the reserve power will come at the cost of speed, and thats fine, as there is a balance somewhere in there that allows for better fuel economy than what im getting at this time.

what do you other "heavy" cruisers (or fishers) with omc packages run for a prop? does a 15dx11p seem unreasonable? Thanks

[builderdude]

Can't really contribute on this one as I'm running a Bravo but wondering if you could clarify a couple things:

What's your final drive ratio?

How'd you determine your overall loaded weight, you had it lifted?

[builderdude]

Can't really contribute on this one as I'm running a Bravo but wondering if you could clarify a couple things:

What's your final drive ratio?

The 7500 lbs weight you listed, is that your overall loaded weight? How was that determined? Guessing you had it weighed when lifted?

[desuch]

I own A 1988 2556 bayliner also and have been trying to figure this prop. issue out myself i have the 5.7 lit. engine with the omc cobra outdrive am now running a 15.5x13 prop. and getting about 4100 rpm i have contacted bayliner on this issue there answer was the prop with this package came with the 15.5 x 15 it didn't do well ,just purchased a 15.5 x 11 prop. have not tried it out yet ,i have been told my engine needs a tune up but its a new engine i have about 50 hrs. on it and runs like a top ,i also would like to get the 4600 rpm thats its suppose to get if you get any answers please let me know

I have heard that whole problem might be the way the outdrive is geared ,don't know

[Centerline2]

"builderdude" post=827511 wrote:
Can't really contribute on this one as I'm running a Bravo but wondering if you could clarify a couple things:

What's your final drive ratio?

The 7500 lbs weight you listed, is that your overall loaded weight? How was that determined? Guessing you had it weighed when lifted?

I havent actually checked the overall final ratio, but according to spec its supposed to be 1.43-1.... I have thought about double checking it with my photo tach to see what the actual ratio is, but havent gotten around to it yet.

the weight... full of fuel and water, empty waste tank, with all of our normal gear onboard, setting on its galvanized tandem trailer, on a certified scale with the trailer disconnected from the tow vehicle, it weighs 9550lbs.... I am figuring the trailer weight at close to 2000lbs.... = boat weight of 7500.

[Centerline2]

"desuch" post=827519 wrote:
I own A 1988 2556 bayliner also and have been trying to figure this prop. issue out myself i have the 5.7 lit. engine with the omc cobra outdrive am now running a 15.5x13 prop. and getting about 4100 rpm i have contacted bayliner on this issue there answer was the prop with this package came with the 15.5 x 15 it didn't do well ,just purchased a 15.5 x 11 prop. have not tried it out yet ,i have been told my engine needs a tune up but its a new engine i have about 50 hrs. on it and runs like a top ,i also would like to get the 4600 rpm thats its suppose to get if you get any answers please let me know

I have heard that whole problem might be the way the outdrive is geared ,don't know

what is the weight of your boat?. with the numbers you describe it sounds like we are both heavy and have the same gearing. but if you are unladen, then somewhere you are losing power.

i would be very interested to know how the 11 pitch prop works. as with yours, my engine has just been freshened and a carb rebuild, and all settings are right where the manuals say it is supposed to be.... so im waiting for an answer from YOU about that 11p prop ;-)

[MadWill]

For what it is worth as a reference point:

Had a 250HP mercruiser with 1.62 ratio drive. It spun a 13 x 17.

Purchased after owner upgraded engine to 300HP MPI, they left original drive and had a 14 x 19P.

1.62 self destructed and was replaced with a 1.47 ratio

Prop on that is a 15 x 15P

The boat loaded is approx 5500lbs, I can spin 4850 RPM (4800 to 5200 recomended) with 300HP and 1.47 ratio. Boat is a 2352 Trophy.

Cheerd Adam

[desuch]

since now I have the engine and drive unit winterized it won't be till spring now to try the 15.5x11 but i will post it when i have test ran, this prop issue has driven me crazy i now have 2 other props to try and sell because they didn't work out ,living in alaska I have to buy a prop to try

[Centerline2]

"MadWill" post=827540 wrote:
For what it is worth as a reference point:

Had a 250HP mercruiser with 1.62 ratio drive. It spun a 13 x 17.

Purchased after owner upgraded engine to 300HP MPI, they left original drive and had a 14 x 19P.

1.62 self destructed and was replaced with a 1.47 ratio

Prop on that is a 15 x 15P

The boat loaded is approx 5500lbs, I can spin 4850 RPM (4800 to 5200 recomended) with 300HP and 1.47 ratio. Boat is a 2352 Trophy.

Cheerd Adam

all things considered, this leads me to believe I should have the 11p prop... your mercruiser drives delivers a higher percentage of the engines hp to the prop, and with 300 engine hp and a slightly higher ratio you are still only turning 4850 with the 15x15... I have less hp, less ratio, and less pitch and getting 4300rpm..

in your case, like mine, I believe there is a benefit to be had by dropping a an inch or two in pitch, as the loss in speed will easily be made back (and possibly a slight gain) by the higher rpms of the engine/prop, and at the same time, allow the engine to run without laboring quite so much.... which means the engine rpm would be higher at maximum power, and there would be more reserve power left when the boat is running at the 4850 rpm... which almost always equates to better fuel economy.

[MadWill]

I have been thinking about the enertia 15.2 x 14P, the current prop is the enertia 15 x15P. The extra diameter will give a little extra thrust and 14P some more RPM's. I still havent had the ideal day for testing, so I may still get 100 RPM more if not in 15knot winds and dont have to use my trim tabs.

In Aus, Im looking at $1000 per prop. If I get 4900 to 4950 RPM's I will be happy.

Test day 1, I only got to 4760 RPM and 70klm / hour downwind in 20 knots. Had tabs deployed as chop was pretty rough and couldnt trim

Test day 2, got to 4850 and 65klm / hour into 15 knots. Got lots more trim up, but still some tabs.

Fuel economy though was really good compared to a 15 3/4 x 15P mirage plus and 14 × 17 elite spitfire 4 blade (cant get smaller) both of which stopped at 4600 revs and 60 klm / hour.

I reckon I will get close. Both tests 2 pob + 400 litres fuel. I usually have 4 - 6 pob. So 14p is still in consideration.

Cheers Adam

[Koppers_Kove]

Good rule for prop selection is 1" pitch = 200 rpm. If you drop from 13" to 11" you should pick up the 400 rpm you are looking for. By doing this your boat will cruise at a slower speed at same rpm than your current setup.

[Centerline2]

"Koppers Kove" post=827656 wrote:
Good rule for prop selection is 1" pitch = 200 rpm. If you drop from 13" to 11" you should pick up the 400 rpm you are looking for. By doing this your boat will cruise at a slower speed at same rpm than your current setup.

yes, this is about as close as I can figure it with the information I have to work from... I was hoping to hear some confirmation from other omc owners who are running an 11 or 12p prop, or that are struggling with the same issues I am..... its difficult to spend the money on a prop when you arent sure its actually the prop or a hp problem with the engine. it sounds as if the engine is delivering when comparing to what I have been reading from others...

[desuch]

I know there is someone out there that had the same problem but i can't find them there was a lot of these boat made with 5.7 lit. cobra package bayliner doesn't have an answer i contacted them already

[dmcb]

You cannot use weight for prop size. A Volvo, Merc, or OMC cannot be compared for prop size because the gears may not be the same.

You really answered your question on your first post.

You should prop your boat so it reaches the rpms suggested by the engine mfg.

You are turning 4300 rpms and want 4600 rpms.

Each pitch change should result in 200 rpms change for each inch of pitch. Or each inch of dia.

If your prop has not had a pitch change they can be changed 2 inches either way.

What I would do is take your present prop to a prop shop and have them lower your pitch by 2 inches.

You should be about right doing that.

Doug

[desuch]

thanks for the info its not cheap trying to figure this out ,i have repitched 2 and purchased 2 so now i am at 500 bucks just trying to get the rated 4600 rpm ,haven't tried my last prop yet now its in winter storage