correct prop size

[ednhr12]

I have a 1984 trophy 1910 CC with a same year 125 force outboard, the prop that's on it is a 13-1/4 x 17 Michigan SS prop, (has Coligan stamped on the side) the boat struggles to get on plane (best result is moving passengers to the bow, but still doesn't feel like getting full power) and the engine is only getting to about 3200-3500 rpms at WOT, Speedometer is reading about 26-32 mph.
Engine tag has 125hp @ 5500 rpms and operating range between 4500 to 5500. Got 150 psi on all 4 cylinders, engine runs really good, starts right up and idles at around 7-800 rpms.
So, I'm not sure if the prop that's on it is the original factory? or previous owner changed it out? I believe the boat is suppose to weigh around 2100 lbs empty, it has a 40 gal. gas tank and the sticker says it's a 5 passenger max boat capacity. as you can see there's a lot of question marks ......help!!

 

[Scott Danforth]

welcome aboard

what does the boat actually weigh - weigh it
factory props were cheap aluminum props
you state the boat motor doesnt feel like full power, get that fixed before you worry about a prop

 

[Sea Rider]

Check if prop specially if being old is in the process of becoming fully spun, need to make a hub slip test. Engine must be 100% healthy before going for it. If bad replace with a new one, the issue is : exactly same one or one with less pitch ?

Happy Boating

 

[Texasmark]

Something is lying to you if I plugged in your gear ratio correctly at 2:1 which I think is accurate for 4 or 6 cyl. Mercs in that hp....I had a I6 tower and it was 2:1. Go-fast.com BAM prop slip calculator says that your actual average speed and rpms (3500 and 28 mph) are accurate per your inputs but your prop slip is zero %.

That just doesn't happen....a screw in wood yes, but not a screw in water. Lots of variables but on a boat like that loaded as you say, 15% is not unreasonable. Gotta get accurate info to know where to look for your problem. Idle rpms on that engine per the book are 675 +/- 50 so you are right on with rpms. Is your speed via speedometer or GPS? Speedos lie.

Years ago I ran a 17P on an 18 ½' Caravelle tri hull with a Johnson 125 which was engine rated so prop wise less hp than you are running (prop rated 125 hp).....about 85% lower on mine due to brake vs prop hp and I had no problem running in recommended rpms with wife and 4 kids and 18 gal of gas and full of gear. Water skied me on slalom or 2 kids at a time on 2 skis, plenty of whoopie on hole shot. I do recall that with me alone I could run 39. I remember that number as I kept wanting to hit the magic 40 and never could get there with that prop.....not bragging here, just giving you a comparison as to what to expect.

Assuming your rpm were at your max of 5500 and you were running your 28 mph your zero slip speed would be 44 and slip would look like 38% which is abnormally high.

While you are thinking about what is telling you the truth and all, how about shoot us some clear pictures of things like hull condition and engine mounting height in the vertical to the horizontal hull (90 degrees...just a few degrees of trim out).

 

[jimmbo]

I believe the gear ratio for that engine in 15:26. I doubt the engine would be rated @5500 and have a WOT range of 5000-5500. Either it is 125 @ 5250 and WOT 5000 - 5500, 125@5000 WOT 4750 - 5250, or possibly but unlikely, 125@5500 WOT 5250-5750

 

[89retta]

Check and make sure the butterflies are opening all the way in the carbs.