Over-propped?

[No Worries]

New to me 1985 250 sundancer with alpha 1 and 260 Mercruiser. She doesnt give me more than 3800 rpm's and currently has a 4 blade prop which o suspect is not stock. At about 3000 (where i like to run her) i can trim up the drive and trim down the tabs and get about 27 mph out of her. Once i decided to run wot (strictly out of curiousity) i was surprised to see her only hit 3800 rpm's. Thank you all for any input on this one. Anchors Aweigh!!

 

[kenny nunez]

You need to get the RPM s to the rated max which is around 4400. The worse thing you can do is have it over propped. It will get better fuel mileage with correct propping.

 

[No Worries]

You need to get the RPM s to the rated max which is around 4400. The worse thing you can do is have it over propped. It will get better fuel mileage with correct propping.

I'm only assuming that it is over propped. Opinion?

 

[stonyloam]

Well, can't tell if you are overpropped unless we know what you have on there now. The number of blades is not important, the prop diameter and pitch is. Should be stamped on the prop hub. There will be two numbers, diameter and pitch. Is that a gps speed? Once we have a accurate tach and speed reading and the prop diameter and pitch, should be able to tell the direction to go.

 

[QBhoy]

If the tacho and speed are correct and accurate, well over propped.
I'd imagine on that set up, if you are achieving 27mph at 3000rpm, it is. Must be hard to get her to plane assuming all data is correct ?

 

[No Worries]

Well, im just trying to validate my suspicion for the time being. She'll be pulled out at the end of October and I'll check the prop at that time. It is a gps (garmin) speed reading...i guess its a dead issue until i see the prop.

 

[No Worries]

If the tacho and speed are correct and accurate, well over propped.
I'd imagine on that set up, if you are achieving 27mph at 3000rpm, it is. Must be hard to get her to plane assuming all data is correct ?

I think shes getting on plane just fine but maybe i can't tell? On my 19 footer i could tell the instant she planed. This one, not so much but i think she is. Strong holeshot and seems to get up just fine. Again, new to me and maybe i just cant tell. For what its worth, i dont think my bow is up in the air.

 

[Scott Danforth]

thats a big boat for a 260hp/alpha combo. assuming a healthy boat my guess 38-40mph max

85 was the change over year for the flywheel vs prop rating

have you weighed the boat? being 33 years old, you could be toting around 1000# of water which would make it a bit sluggish

need to post the prop numbers, verify the tach reading and the speed per GPS.

 

[No Worries]

thats a big boat for a 260hp/alpha combo. assuming a healthy boat my guess 38-40mph max

85 was the change over year for the flywheel vs prop rating

have you weighed the boat? being 33 years old, you could be toting around 1000# of water which would make it a bit sluggish

need to post the prop numbers, verify the tach reading and the speed per GPS.

If i follow you correctly, im comparing your response to flywheel/rearwheel hp in a car...that makes sense to me. As far as being waterlogged...the surveyor didn't didn't feel that was the case. That being said...i haven't had it weighed. I'll verify the prop in the fall but the rpm #'s are from the boats tach and the speed #'s are from my garmin gps.

 

[Scott Danforth]

ok, so your speed is correct. now go to the auto parts store, get a loaner shop tach and verify the RPM. while there, get a timing light and dwell meter and verify your points dwell and timing.

 

[No Worries]

ok, so your speed is correct. now go to the auto parts store, get a loaner shop tach and verify the RPM. while there, get a timing light and dwell meter and verify your points dwell and timing.

Will do and thank you. On a side note, she has electronic ignition (thunderbolt) but ill check the timing and get a shop tach.

 

[boatman37]

I have the same boat but a 1986 and I have all the factory manuals. Top speed according to the manual should be about 32 MPH. I have a 14.5x17 3 blade aluminum and can hit about 33-34 MPH (GPS) but only max at about 4000 RPM. According to my manual mine came with a 16x16 3 blade aluminum. I was thinking of trying a 15x15 on mine to get the RPM up. My manual says 4200-4600 max. I haven't tried a shop tach yet. The water has been polluted this year so I haven't even tried full throttle yet this year. I have a volt meter with a tach setting so I am going to hook it up to verify readings. My sweet spot seems to be about 3300-3500 RPM at about 27-28 MPH. This is going by sound and feel so not scientific at all. Just feels the smoothest in that range. I can stay on plane at about 24 MPH but any lower and it falls off.

 

[stonyloam]

Well since you can't look, can take a guess. Your numbers suggest that you are running a 17" prop. If you go to a 15 inch you would probably pick up about 400 rpm, and give up about 1 mph in top end speed, but it should get up on plane a little quicker.

 

[Rick Stephens]

I would be concerned that if overpropped and used frequently, you are putting a lot of wear and tear on your motor. Like drag racing, run it overpropped enough weekends and you'll need a rebuild before you know it.

 

[No Worries]

I have the same boat but a 1986 and I have all the factory manuals. Top speed according to the manual should be about 32 MPH. I have a 14.5x17 3 blade aluminum and can hit about 33-34 MPH (GPS) but only max at about 4000 RPM. According to my manual mine came with a 16x16 3 blade aluminum. I was thinking of trying a 15x15 on mine to get the RPM up. My manual says 4200-4600 max. I haven't tried a shop tach yet. The water has been polluted this year so I haven't even tried full throttle yet this year. I have a volt meter with a tach setting so I am going to hook it up to verify readings. My sweet spot seems to be about 3300-3500 RPM at about 27-28 MPH. This is going by sound and feel so not scientific at all. Just feels the smoothest in that range. I can stay on plane at about 24 MPH but any lower and it falls off.

Your numbers are on point with my limited time on this boat! I've been running at 3400 and 28 mph or so when i play with the trim. It also sounds and feels good to me there...interesting. The lack of max rpm still concerns me, though. I gotta have a look at that prop.

 

[boatman37]

if the water is clear enough hit the trailer button and have a couple bigger guys sit up on the bow. you may be able to read the numbers on your prop. worst case it is easy to pull in the water. just be sure to not drop anything

 

[HT32BSX115]

If you go to a 15 inch you would probably pick up about 400 rpm, and give up about 1 mph in top end speed, but it should get up on plane a little quicker.

It might not lose any speed at all. the engine will be developing more HP turning a higher RPM.

Realistically, that engine should turn 4600 RPM [WOT range for a 1985 260 is 4200-4600 RPM ]

If the engine is otherwise running properly, capable of achieving rated HP and has the correct drive ratio (1.50:1 for that engine if I am not mistaken) that package should be propped to make 4600 RPM when the boat is loaded with full fuel, gear and whatever number of people you'll typically run in it.

If the boat should make do 38-40 MPH at WOT(with that HP engine) , then according to the prop-slip calculator (with 15% slip) a 17p would yield 42MPH and a 15p prop would net you 37mph.

15% slip may be optimistic. It may be more like 18% for a boat that heavy. If you have a 4blade prop, your slip might be slightly less but you'll have more drag (more blade area in the water causes more drag) with resultant slightly less top RPM.

Other things to check if you're not sure.....look at the following thread: List of possible causes of low WOT

Cheers,

Rick

 

[No Worries]

It might not lose any speed at all. the engine will be developing more HP turning a higher RPM.

Realistically, that engine should turn 4600 RPM [WOT range for a 1985 260 is 4200-4600 RPM ]

If the engine is otherwise running properly, capable of achieving rated HP and has the correct drive ratio (1.50:1 for that engine if I am not mistaken) that package should be propped to make 4600 RPM when the boat is loaded with full fuel, gear and whatever number of people you'll typically run in it.

If the boat should make do 38-40 MPH at WOT(with that HP engine) , then according to the prop-slip calculator (with 15% slip) a 17p would yield 42MPH and a 15p prop would net you 37mph.

15% slip may be optimistic. It may be more like 18% for a boat that heavy. If you have a 4blade prop, your slip might be slightly less but you'll have more drag (more blade area in the water causes more drag) with resultant slightly less top RPM.

Other things to check if you're not sure.....look at the following thread: [url]http://forums.iboats.com/forum...6-list-of-possible-causes-of-low-wot?t=295070List of possible causes of low WOT[/URL]

Cheers,

Rick

Great response! Thank You!

 

[No Worries]

Update...it looks like im running a 14.5x18 4 blade. A couple of weeks ago on Flat Water I hit 36 mph on the GPS at around 4200-4400 RPMs. That's where she maxed out.

 

[boatman37]

good numbers. i am no prop expert but if a 16x16 should get you 4600 at 32mph (both max as per my manual) what should the 14.5x18 do in comparison?