Prop shopping, confused and in need of help

[KRS]

The numbers on my prop are 4878122-21, and I need to find a replacement. The iboats prop sale looks like a good time, but it appears they offer many different pitches, etc. and it is a little confusing.<br /><br />Once I know what I have, I can get ideas on a replacement.<br /><br />1987 16' bowrider, 2.5Liter Mercruiser stern drive alpha one, rated at 120 HP.<br /><br />Iboats offers everything from 11" pitch to 27" pitch. Then when I click at pitch (such as 21" as I believe that the 21 in the above number means 21" pitch) there are many choices and some have different diameters.<br /><br />I would like more top end speed, but who wouldn't I get 30 mph as an absolute top speed, trimmed and running well at 4,400 WOT (manual rates that engine at 4,200-4,600 I believe).<br /><br />I don't know how this speed compares, I'm open for thoughts.<br /><br />I posted this here first, by accident.

 

[vipzach]

Re: Prop shopping, confused and in need of help

KRS something doesn't seem to add up. Assuming that your motor is running right, with a drive ratio of 1.98.......you should be able to hit around 44 MPH! <br />Either the drive ratio is wrong, the motor isn't quite running correct or something else is wrong. <br />Has the boat always performed this way or is it new to you?

 

[walleyehed]

Re: Prop shopping, confused and in need of help

I show 35-37mph with a 21 at 4400RPM<br />Are you using a dash-mount speedo (pitot tube) or GPS for speed???

 

[KRS]

Re: Prop shopping, confused and in need of help

GPS for speed, the boat was new to me 2 years ago, and it has always (2 years) been like that for me.<br /><br />44 MPH or even 35 MPH! Wow, I would like that, but not getting that kind of performance. The motor must be running right, if it's hitting 4,400 RPM, so the only other variables are 1) GPS is faulty (but I've checked it while driving my jeep) or 2) tach is wrong (but it's wrapped up and sounds like 4,400? or 3) gears ratio has been changed.<br /><br />As for the pitot tube, the pitot is broken, and I haven't layed out the $15 for a new plastic one.<br /><br />Now I'm even more confused.

 

[ron7000]

Re: Prop shopping, confused and in need of help

hey, i was going to say that i have a 19' bayliner, 2002 though, with the 3L which is basically the same engine, an inline 4. Mine has a 2:1 drive ratio and a 14x21 prop (14" diameter, 21" pitch). Mine tops out at 41-42 mph at 4600-4800 rpm.<br /><br />Now your boat is 16', I can't imagine it weighing more than mine. There's a post here somewhere of prop pitch and drive ratios versus speed. Fact is, if your engine is turning 4400 rpms and your only doing 30-32mph, then either you have a lot of growth on the hull and/or drive causing massive drag. Or your prop pitch is way lower than 21". If you can post a decent pic of your prop, we can tell if it's close to a 21 pitch or not. Other possibility is a spun hub in the prop. or your tachometer is off and your really not doing 4400 rpm.

 

[KRS]

Re: Prop shopping, confused and in need of help

Just re-checked the part # stamped on the prop. Number located just around the prop nut washer, stamped (actually casted) in a half-moon around what would be the center of the shaft.

 

[KRS]

Re: Prop shopping, confused and in need of help

I looked at the repair order from the mechanic who worked on my boat in '04, the gearset he installed cost $239.10 and the part # is 43-55778A3.

 

[ron7000]

Re: Prop shopping, confused and in need of help

in this chart efficiency is set to 1 which means there's no prop slip which is not realistic. So this chart represents the max theoretical speed your boat could go provided you know your outdrive or transmission gear ratio, and the propeller pitch.<br />Figure a realistic prop slip or efficiency value, which will vary as speed varies, to be 0.8 to 0.9. So just multiply the speed value at 4500 rpm by 0.8 or 0.9 to get a estimate of what top speed would be. If you've had a gearset change, I don't think it'll go much higher than 2:1 for a alpha outdrive. Going lower will increase prop rpm which means more speed, which isn't the case.<br /><br />Thought regarding drag, do you have doel-fin or sting-ray fins on your outdrive cavitation plate? I tried a set on mine this past season, found out the outdrive on my boat sits lower in the water than most so the fins caused massive amounts of drag one on plane doing 30mph or better and dropped my top speed to somewhere between 30-35 mph. Remove them if you got them.<br />Since someone had the boat before you, it's possible that prop was sent away to be repitched lower so it will say 21 on it but it really isn't. Compare it to a new one at a boat store, see if it looks close. One band-aid solution to a motor not reaching full rpm (because it's tired or not tuned) is repitching the prop lower.<br /><br />hope this helps.<br /><br />

 

[ron7000]

Re: Prop shopping, confused and in need of help

http://boatmotorrecyclers.com/RRCatItems.asp?MFG=MC&CAT=GS <br /><br />part number you listed for your gearset is 1.98 to 1, which is what it should be.

 

[KRS]

Re: Prop shopping, confused and in need of help

I do have a doel-fin, I've been thinking about removing it to see what it's like without it.

 

[KRS]

Re: Prop shopping, confused and in need of help

Okay, pix here: Look very closely at the prop fin edges, you'll see why I'm re-propping.<br /><br />

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[walleyehed]

Re: Prop shopping, confused and in need of help

your prop by # is a 13-3/4X21C.<br />the replacement is a 48-832832A45, 14-1/4X21.<br />If we have a known GPS reading, and if the gear ratio is factory correct, we have a tach problem....the boat would not plane out if we had a slipped hub...tachs DO go bad. Any GOOD shop can verify the tach for you.<br />If you still have issues, I'd look into a motor tune-up and verify with another GPS...My Dad's 120 on an 18ft swinging a 17P will do 33-34 at 4000...different gear ratio, but that's for a reason...no reason your smaller boat with a bigger engine should be slower.

 

[KRS]

Re: Prop shopping, confused and in need of help

Kenny, <br /><br />I believe the gps to be good.<br /><br />No local shops, what else can I do to check the tach? Is there a cheap aftermarket one available that I can hookup on the water for a test?<br /><br />You are right about the planing thing, I do get on plane just fine, and the MPH is a steady 29.5-30.5mph, so I don't believe it's a spun hub either.<br /><br />Several have mentioned a tune-up, however my question is... if I'm reaching the posted WOT RPM, what would a tune-up do, it won't spin any faster than the WOT RPM?

 

[walleyehed]

Re: Prop shopping, confused and in need of help

The tune-up may let it go on up to 4600-4700, who knows...<br />But, something is off, and I believe it's the tach...Has it "always" turned the same rpm by the tach??? (at wot)<br />Another item we don't know is, Has the prop ever been repaired before???

 

[KRS]

Re: Prop shopping, confused and in need of help

The prop is in terrible shape, as shown in the pic. If it's been repaired, it would have been prior to me owning it, so who knows.<br /><br />The tach hasn't always read the same, my WOT used to be in the 3,000's (I don't recall exactly what), and on my last trip, I looked down and it was 4,400. I didn't question it because 4,400 is what is should be.

 

[vipzach]

Re: Prop shopping, confused and in need of help

KRS, you can go to a local auto store and by you a cheap tach and hook that up to do your testing. If your WOT's used to be in the 3000's that would be a little closer to the speed you are going, but is way under what the motor should be turning. Your next step definately needs to be getting that tach checked.

 

[walleyehed]

Re: Prop shopping, confused and in need of help

Ditto...need to get the tach figured out.

 

[KRS]

Re: Prop shopping, confused and in need of help

roger that, will do.... follow up to come.

 

[whywhyzed]

Re: Prop shopping, confused and in need of help

Most tachs have a 4/6/8 cyl dipswitch or individual coil wire terminals for different engines, you can do a visual on that without getting another tach.<br />...if the tach is right... time to pull up the floorboards and see how many tons of water you are hauling with you. <br />Although I can see the tires of your trailer in 1 <br />pic- they're not squatting...fingers crossed for the tach.<br /><br />Could the coil, or points not cause the tach to read funny too?

 

[walleyehed]

Re: Prop shopping, confused and in need of help

That's why I would recommend a tune-up...especially if it hasn't been done in the last 2-3 years...<br />Has the Idle RPM changed any???