Wrong Prop...24mph @ 3500 rpm

[andrewterri]

89 mercruiser 3.0 alpha 1 with SN OC547466. I have a 14 1/2" diameter x 17" pitch. The boat is running way under what I think it should. With two adults and a 1 1/2 year old in the boat with about 70 pound of gear. All in everything weighs about four hundred pounds and that includes the extra layer of ply on the decking. The boat runs 24 at 3500RPM and will only get to 4k before it starts bouncing like the engine is hitting the rev limiter. With only myself on the boat the WOT rpm will reach 4200 and run about 26mph which I know the rpm is too low but the speed seems very low as well. What do you guys think? Is the prop wrong for this boat. It is currently a 4 blade and want to get a 3 blade because we cruise to the beach more than we tube. I do not know what I should be looking for, I thought I did and apparently I was wrong.

 

[Scott Danforth]

check your tach you may have the tach on the wrong setting

you may want to weigh your 29 year old boat too. if the flotation foam is soaked, you could be carrying 800# of water with you.

by reading the cryptic message, you currently have a 17p 4-blade?

you could also have a fuel delivery issue.

your motor should be able to hit top RPM. in a 1700# 17' boat I could hit 41mph at 4800 RPM with a 3.0

what boat do you have the 3.0 in?

 

[Bayou Dave]

Extra layer of plywood on the deck? Why?

 

[andrewterri]

the boat is a searay 160. the previous owner had put it in plywood because of wood rot that has taken over the boat. The boat was one of those finds that were to cheap for a good wrenching toy :lol:​ I paid 800 for the boat because it did not run. Brought it home to the neighbors dismay, and got it running after a short adjustment to the shift cables, improper setup causing interrupter to be engauged. Boat had great compression at 145 each so I kept working on it over the years. It started as a toy years ago because we (girlfriend now wife) have seadoos but since now married with kids and the boat has become a major part of our lives in the summer as that is the main thing I have in common with the older step kids and its a great way for us to bond together and do something we enjoy. I do not know anything about the PO but do know it was sitting outside for years before I got it so wet foam is possible. I am going to take the cooler, anchor, sand toys, hand tools, and all the other BS that is carried on that boat and do a run by myself. Like I previously said I could get to 4200 by myself but that is a long way from where it should be even with 20 gallons of gas. We are hoping to get through the summer with a new prop, perhaps a 14 3/4 x 15 to get the rpm high enough. We looked at new boats yesterday and plan on getting a chaparral at the end of the summer or during the Oklahoma boat show in January. Any advice on a new prop sure would help as from my understanding I will be able to us it on the new alpha one with the next boat.

 

[andrewterri]

The boat is a 1989. The factory dry weight is 1900, length 16'9, beam 85", fuel capacity 20 gal., based on the verified tach numbers checked by multimeter on coil and gps speed I am showing to have a 17% prop slip. That seems slightly high to me. What do you guys think? This is what lead me towards wanting a 14 3/4 x 15 three blade prop. We don't tube with the boat now because an unknown knocking in the engine so we just cruise around then go to a beach for the baby to play. That's why I was thinking a three blade. less drag in the water and smaller pitch to increase rpm. By my numbers it should increase 2-400rpm over the 17 thereby increase wot to 42-4400 with typical load. By myself it theoretically take rpm to 4400-4600. It is also very important for me to note (and I forgot) the boat looks like the previous owner let the boat break loose from its mooring and get into rocks. The hull looks like buckshot hit it and it was patched up but is rough and not smooth. The damage is mostly on the starboard side of hull. I know this will drastically affect the top speed but I am more concerned about the engine RPM as not to be lugging the engine.

 

[Rick Stephens]

Might be worth a quick trip to a scales and see what it weighs now. And I can't but recommend you take a screwdriver or a drill and poke around to make sure it has a transom and stringers enough to be safe for the family. I understand the last season thing before you get the family a new barge. But safety is everything and knowing for sure what you have is minimum towards keeping the little ones, wife and self safe.

I wanted to get a better slalom ski pull so I dropped a 17 inch to a 15 when I had a 3.0L in my boat. Gained only a couple hundred RPM in my case, since I went from a 3 blade 17 to a 4 blade 15. Surely gained the jump in hole shot I was looking for though I had to watch top end not to over rev. SO yeah, you can get a fairly run of the mill 15 inch 3 blade like a Solas from iBoats and it will speed up your motor, for whatever reason it is lugging.

 

[nola mike]

Compression test, then weigh the boat. Doubt the wrong prop is your issue

 

[andrewterri]

compression is 143, 135, 143, 141. I do not like to see #2 a little lower but still within spec. The boat is rotted everywhere!! :facepalm:​ The transom with the tap test sounds solid and a moisture sensor is just to expensive. There is no flex when I push on the outdrive. The stringer the front motor mounts to is rotted pretty bad as well as the sides of the bildge area. This is what lead us to deciding to buy a NEW boat. People just do not know how to take care of them, do proper maintenance, or fixing things CORRECTLY even if it takes more time. Since I can not justify fixing the stringers correctly with such a small boat to began with we are just going to buy a new one so I will know it is cared for correctly just as soon as I can pay off the big green machine...JD is very damn proud of their tractors. We should have that done this year which is why I am trying to just make it through the summer without causing any more damage to the engine for the next owner as I will sell the boat as a project. This boat turned out to be very good for our family when I was in college and could not afford anything else. I want to pass it on to another family who can continue to do some work and have a good boat for them. While the motor mount is rotted it is still strong enough to hold good alignment as I check it every 2-3 weeks since the bell housing gasket kit is only 10 dollars. I did find the fuel mechanical filter to be the original ceramic. The spring under it was broken and the gasket with the four slots were so small I am surprised the boat even ran. Someone put the notoriously unsafe inline filter on it and I think that saved it as there was a lot of rust in it from the gas tank. Without that inline it would have made it to the carb.