Way over propped?

[jcupo6]

Issue - Can't reach optimal RPM at WOT, VERY slow acceleration, struggles to plane. Current max RPM and speed is 2,800rpm at 24mph

Boat - 22 foot Starcraft holiday - about 2000 pounds with average load
Motor - 1978 120hp Mecruiser with 1 year old SEI 106 outdrive
Current prop - Aluminum Michigan Wheel #031027. Can't read the current pitch, but did some research and it looks like this model is 14-1/4 X 23.

Looking for a good overall prop. We mostly fish, will sometimes pull a tube. Thinking about going down to a 15-1/4 X 15 or 14-1/4 X 17, what do you guys, the experts, think! What do you think my top speed at WOT would be around with the smaller pitch, just a guess?

 

[jcupo6]

Re: Way over propped?

I apologize, for some reason I was thinking that you gain 100 rpm per 1? pitch decrease, but it?s actually 200 rpm. So I think the 15 pitch would be too much. I am thinking about just going with the 17. This should give me around 1,400 rpm?s putting me right in the 3,900-4300 rpm range for my motor. I am thinking about the Solas Amita here on iBoats, 15 ? x 17. Would this be a good choice, or do you think it would be too much of a decrease?

 

[steelespike]

Re: Way over propped?

While the 23 is too much prop you have more going on than prop selection.
At "about"200 rpm per inch you would have to go to a 15" prop to get to 4,400,
and I don't think you would make it.My guess is that it would normally have a 19 or 21" prop and run about 40 mph.A 17" would normally be used for skiing and such with a caution to watch the tach.Have you done a compression check on the motor?
Is it well tuned, timed right and timing advancing as it should?Throttle plates opening as they should?What is the outdrive ratio?

 

[Port-o-Call Curl]

Re: Way over propped?

First of all, I think you may be under-estimating the weigh on your boat, don't you fish with a couple of phat guys? Second, before you think about changing your prop, you may want to lose that ridiculous hat, I'm sure that is creating alot of air drag!

 

[glengold]

Re: Way over propped?

I recently purchased a 'new' used motor and had to add dolphin fins and move the motor up a hole, to get her to run in range. its a 55hp on a heavy fibergalss 16 ft center console

 

[jcupo6]

Re: Way over propped?

While the 23 is too much prop you have more going on than prop selection.
At "about"200 rpm per inch you would have to go to a 15" prop to get to 4,400,
and I don't think you would make it.My guess is that it would normally have a 19 or 21" prop and run about 40 mph.A 17" would normally be used for skiing and such with a caution to watch the tach.Have you done a compression check on the motor?
Is it well tuned, timed right and timing advancing as it should?Throttle plates opening as they should?What is the outdrive ratio?

Compression is 150 across the board, motor is tuned, timed and advancing correctly. New plugs and wires. New fuel lines filters and pump. Throttle plates do open all the way. Cleaned the carb and replaced the fuel accelerator. New Pertronix electronic ignition, new distributor and rotor. Gear ratio is 1.94.

 

[steelespike]

Re: Way over propped?

Boy youve got me stumped. You might try very carefully squirting gas into the carb at speed. Have you varified the gear ratio?Any chance there is an exhaust restriction somewhere?Maybe a rubber connector or elbow that collapses or is folded?
Maybe a bad plug/wire.3 cylinders would put you right about where you are.
Valve clearances wrong, maybe a bad valve spring letting a valve float.You might try pulling or shorting one plug at a time running on plane.Be careful of the electronic ignition.Is the polarity at the coil right?Have you checked the plugs? Any indication they aren't firing right.Any exceptionally clean?Run it in the dark see if there is any stray arcing at speed.

 

[jcupo6]

Re: Way over propped?

Boy youve got me stumped. You might try very carefully squirting gas into the carb at speed. Have you varified the gear ratio?Any chance there is an exhaust restriction somewhere?Maybe a rubber connector or elbow that collapses or is folded?
Maybe a bad plug/wire.3 cylinders would put you right about where you are.
Valve clearances wrong, maybe a bad valve spring letting a valve float.You might try pulling or shorting one plug at a time running on plane.Be careful of the electronic ignition.Is the polarity at the coil right?Have you checked the plugs? Any indication they aren't firing right.Any exceptionally clean?Run it in the dark see if there is any stray arcing at speed.

Thanks for your help, we're pretty stumped too! I am leaning against a motor issue, as this is a pretty fresh rebuild, but I will try your suggestions to rule everything out. I'm also going to try and get my hands on a 17" prop to see if that gets me closer to where I need to be.

 

[45Auto]

Re: Way over propped?

Did the boat ever perform any differently with this motor in it?

Or is it new to you and you have no basis for past performance?

 

[steelespike]

Re: Way over propped?

I don't know too much about I/Os but check the alignment from the motor to the outdrive.Or perhaps the cam timing is off. one or 2 teeth it would seem normal but be difficult to start in cold weather and lack top end.We had a 83 Honda that the dealer missed the cam timing during a head gasket repair.Car seemed comepletely normal until cold weather set in.Very difficult to start.It seemed to me a choke/carb issue and told dealer it seemed so.They were honest enough and smart enough to know they miss timed the cam and told us so.I'm sure if we had occasion to run the car at wot we would have had power issues.

 

[dozerII]

Re: Way over propped?

I am thinking it is a prop problem, my 18' Starcraft has the same I/O and I just put on a new Solas 15-3/10 X 21 from a an old chewed up Merc one and my wot is 3400 at 32mph it seems to run well but I think is over propped I feel a 19 would be perfect. We were able to tube with it quite well but it did lug a bit in the corners. If you go to a Solas make sure you order the offset trim anode which you will have to file it for clearance.

Glen

 

[steelespike]

Re: Way over propped?

Dozer, with a light load your motor should run about 4200 to 4600 with a 21 and approach 40 mph.I don't usually figure in diameter which usually just follows the design. I do feel that a A diameter in the 14" range may allow more rpm especially with the 3.0.you need 1200 more rpm to get to 4,600 thats normally about a 6" pitch change,putting you at a 15" pitch that is extremely low.On a 3.0 17" is usually suggested for skiing with the caution to watch the rpm with a light load.

 

[jcupo6]

Re: Way over propped?

Did the boat ever perform any differently with this motor in it?

Or is it new to you and you have no basis for past performance?

I did run the boat with the 140 with this same prop, acceleration did seem better, but I don't have any numbers. That motor caused nothing but issues, we never really had it out long enough to run WOT.

I am thinking it is a prop problem, my 18' Starcraft has the same I/O and I just put on a new Solas 15-3/10 X 21 from a an old chewed up Merc one and my wot is 3400 at 32mph it seems to run well but I think is over propped I feel a 19 would be perfect. We were able to tube with it quite well but it did lug a bit in the corners. If you go to a Solas make sure you order the offset trim anode which you will have to file it for clearance.

Glen

Okay, so you are running 32mph at 3400, did you ever run that boat with a different prop? My boat's heavier, so I guess we are running pretty close. I have called a few places and everyone is recommending a 17", so I'm going to go with that. Hopefulley I will get on the water this weekend, if so I will post my results. I would be very happy to be around 4200 rpm and 35+mph, but we will see!

Again, thanks for everyone's help!

 

[fishrdan]

Re: Way over propped?

For the guys running 15+" diameter props and having poor performance or thinking about buying a 15+" prop for a 120/140HP engine....

I'm dealing with prop issues on a 140HP Mercruiser right now and think my main problem is too much diameter. I'm running a 15.5 x 19P prop and it's fine except for low WOT RPM, 4200 (should be 4200-4600). I've also run a 15.75 x 17" prop at 5000' with poor performance. Mercury's prop selector suggests these pitches are fine, but they suggest a 14+" diameter prop. I don't think 1/4" diameter change within a specific pitch is bad, but under the impression that 1+" is bad.

I've come to the conclusion that 15+" diameter props (17P-19P-21P) are designed for high load conditions and not proper for normal use. They would be used on a heavy boat that requires a lot of blade area to push the boat, big cruiser with a V8, not a little runabout with a 4cyl. I'll be testing my theory next week with a used 14 x 19P prop and will let you know what I come up with.

BTW, my boat is a 18' Crestliner, 2500# loaded, 140HP Mercruiser, normal people & gear 300#, max people and gear 800#

 

[jcupo6]

Re: Way over propped?

Okay, took the boat out today with a brand new Quicksilver BlackDiamond 15x17" prop and my performance was GREATLY improved! WOT RPMs are now at 4100, up from 2800. Top speed is about 30mph. Hole shot is extremly quicker. She jumps up on plane much faster then I ever thought she would. I'm very happy with these results! Thanks to everyone for your help!

 

[thompy]

Re: Way over propped?

good to hear it worked out for you

 

[sethjon]

Re: Way over propped?

Personally, I'd go to 18. Why have something that will max you out on the top end. Better to err on the side of caution and an 18 should be a lot better.