Need prop guru for prop size/pitch for '95 Merc 25hp.

[Kymasabe]

I'll try to get all the info in here so I don't waste anyones time. I'm trying to correct two problems: 1. I need more top end speed, 2. I thought I had a fuel delivery problem or coil problem but after having my boat in the shop, they tell me it's rev-limiter.
Boat: 1995 Starcraft Seafarer 14 DLX, 14.5 feet aluminum, aprox 450 lbs.
Motor: 1995 Mercury 25hp two-stroke, remote steer/throttle/shift.

My boat was running around 20/21 mph says my GPS. I called Mercury and they tell me that motor with that size boat should run in the 25-30 mph range and that I've got too much lower end in the water. So, added a Bob's Machine Shop Mini Manual jackplate, raised the motor almost three inches.
Removed my stock 10&3/8th's X 13 Mercury prop and installed a Turning Point Rascal 10&1/8th X 13 aluminum prop. With both changes, I'm now running at around 24 mph. But, at WOT, I'm getting a bucking/hesitation/hiccup/cutting out thing going on. Was starting before the changes, is worse now after the changes. Local marine dealer that does my service told me today the at WOT, the boats running at 7000 RPM and the hiccup I feel is the rev limiter kicking in.
So, I'm going to take one of my old, Mercury props and have it adjusted to a 15 pitch at a local prop repair place but the question is...
...what is the right prop for this boat/motor.
Oh, forgot some info, LOAD: Half the time I'm alone, other have I have one passenger. I'm around 270lbs and my passengers are usually my teen daughters, biggest one is around 150 lbs. Usually carry 12 gallons of fuel, small cooler, tackle box, 2 rods, and small baitwell with a 10 gallons of water and 2 dozen shrimp, as well as bow-mounted trolling motor and it's battery.
Above speeds and RPM were alone, no bait or baitwell water.

I look forward to hearing the prop advice.

 

[steelespike]

Re: Need prop guru for prop size/pitch for '95 Merc 25hp.

I suspect It isn't a prop problem.
I'm surprised the turning point allows it to rev so high.It is a very agressive prop.
It must get the boat up so well it can rev easier.
Why not have the turning point repitched.What is the max rpm rating on the 25?
Is the motor as high as it can run? the Rascal should allow maximum height.
You should raise it to a point just before venting(cavitation) becomes a problem.
Did they do an actual rpm check or are they assuming because the limiter cuts in at 7,000?.Seems odd you were getting bucking before the changes. The limiter
may be failing. A 13" prop seems about right to me do you know the gear ratio.
In my opinion at 7,000 you should be pushing 30 mph.Of course that might depend on your ratio.

 

[Kymasabe]

Re: Need prop guru for prop size/pitch for '95 Merc 25hp.

To answer some of your questions, yes the motor is just about as high as it will go. I was starting to get some blowout on turns with the Mercury prop and does not occur with the Turning Point prop. Max RPM range of the 25hp, per Mercury, is 5000-6000 RPM's. Mercury told me today that optimum operating range is 5500-5800 RPM's. The boat was tested today with the mechanics tach mounted on the boat, boat in the water, running down the channel that I usually run, with my usual full load of fuel and everything, less the tiny cooler, tackle box, and couple of rods (and of course no bait or water in the baitwell.) Yes, was getting a little bucking before the change but intermittant, not as often. I did change fuels recently though, went from premium fuel from the local gas station to 90rec, 90 octane non-ethanol marine fuel from a local marina, with a splash of Mercury Quick Clean in the fuel (and Sta-Bil). So maybe the new fuel is allowing it run a pinch faster as well. I called the mechanic to make sure his tach was set to the right settings for my motor, bad idea, insulted the guy, but he said it was and that the RPM reading was accurate. I don't want to repitch the Turning Point prop yet because I may find I need more than 15 pitch and all I can bend the TP to is a 15. If I leave it as-is, I can return it to my local prop shop for credit towards a different prop. Solas makes a 15 pitch prop for my boat and I can have that repitched, up to a 16.5 or 17 if I need to go that high.

 

[hwsiii]

Re: Need prop guru for prop size/pitch for '95 Merc 25hp.

You neglected to mention how many RPM you were turning, and I found your gear ratio to be 2.25:1. please verify this.




H

 

[Kymasabe]

Re: Need prop guru for prop size/pitch for '95 Merc 25hp.

Hwsiii, the marina tech and tach determined today that the motor is running at 7000 rpm's at WOT.
I'm second owner, don't have a manual, not sure how to verify the gear ratio without calling Mercury in the morning.

 

[hwsiii]

Re: Need prop guru for prop size/pitch for '95 Merc 25hp.

You might try reading this for a better understanding of changing props for better performance, and for the major differences between aluminum props and stainless steel.

http://forums.iboats.com/showthread.php?t=369057

7. HP and gear ratio of outdrive IMPORTANT
If you don?t know the gear ratio, you can pull the plugs out of the motor and put a piece of tape across the prop and the lower unit and then cut it between the prop and the housing then do the same thing on the flywheel and turn the motor until the tape lines up with each other on the prop, it is easier if two people do this, so one can watch the prop while the other counts the revolutions of the motor.

9. Anti-ventilation Plate height above the bottom of the transom of boat if it is an outboard in inches, use a straight edge or a board under the keel and sticking out to the anti ventilation plate for a reference, and take about 4 pictures for us to see.

Prop Slip

Something is wrong with the prop slip, there is no way that you should have a prop slip of 37%, and that is using 7,000 RPM with a gear ratio of 2.25:1 gear ratio, please check your gear ratio the way I show you how in the above paragraph.

Take about 4 pictures of the antivent plate as well, and I also want to be able to see where your pin is set for the angle of your motor while you are running.



H

 

[Kymasabe]

Re: Need prop guru for prop size/pitch for '95 Merc 25hp.

OK, will check on that tomorrow. Boat is still at the marina, will do my best to get the measurements you need. Thank you for your patience and for your help with this.

 

[steelespike]

Re: Need prop guru for prop size/pitch for '95 Merc 25hp.

If anyone can get you straightened out its H.
As far as venting I don't think you would be happy with the merc prop repitched
unless they add some cup.

 

[Kymasabe]

Re: Need prop guru for prop size/pitch for '95 Merc 25hp.

I agree with you there steelspike as the old Merc prop was starting to blow out a little and why I replaced it with a Turning Point Rascal. I did ask the local prop shop to add some cup to it when they repitch it to a 15.
And even then, I know that 15 pitch probably won't be enough, I may need more, I may need a 4-blade prop, but at least it's a cheap modification and though not 100%, will at least tell me if I'm heading in the right direction. The mechanic is going to leave his tach on the boat for me so I can sea trail with the repitched prop and get an accurate read on the new RPM's.

One more thing that occured to me this morning, one more reason it's now acting up more than it was, I trimmed the motor one pin hole higher, got a little bit of the boat out of the water and picked up a little bit more speed, was probably one more minor adjustment that brought the RPM's up just a hair and put me over the rev limiter.
Consistantly running over 6000 RPM's, I'm surprised I didn't blow up the little Merc.

 

[Kymasabe]

Re: Need prop guru for prop size/pitch for '95 Merc 25hp.

I stopped by the boat today and took some pics. Looks like cav plate is about 1/2" or so above the bottom of the boat. I should have taken a better picture of the trim pin position, I have the pin in the second-from-the-top hole. I did contact Mercury today and confirmed it is a 2.25x1 gear ratio. I had forgotten that previous owner beat up the bottom of the boat a little and has some kind of rough, black patch on it. Is it possible that it's causing turbulence/aeration and causing some prop slippage? Anyway, here are some pics, I hope they help.

 

[Kymasabe]

Re: Need prop guru for prop size/pitch for '95 Merc 25hp.

WOW, my pics worked. Here are more. My cav. plate had a hydrofoil on it at one time, still has holes in the plate. Could they be causing problems too?

 

[Kymasabe]

Re: Need prop guru for prop size/pitch for '95 Merc 25hp.

My local prop guy is recommending a PowerTech SRA3 10x15 stainless prop but I'm not doing a damn thing until I get the word from hwsiii !!

 

[hwsiii]

Re: Need prop guru for prop size/pitch for '95 Merc 25hp.

Kymasabe, you took superb pictures and all the ones we really needed, and Spike is definitely right as always, that it is not a prop problem.

I stopped by the boat today and took some pics. Looks like cav plate is about 1/2" or so above the bottom of the boat. I should have taken a better picture of the trim pin position, I have the pin in the second-from-the-top hole. I did contact Mercury today and confirmed it is a 2.25x1 gear ratio. I had forgotten that previous owner beat up the bottom of the boat a little and has some kind of rough, black patch on it. Is it possible that it's causing turbulence/aeration and causing some prop slippage? Anyway, here are some pics, I hope they help.

The REAL problem you have is all that junk he put on the boat to patch the hole in the boat, and it is right at the centerline of the prop stream and couldn't be in a worse place unless it was at the transom itself. It is creating tremendous amounts of air influx into the prop stream and thus making your prop slip supremely high, just like you mentioned. With the setup you have and the right prop, with the bottom clean, you should have a prop slip no higher than about 12% and yet yours is at 37%, if your RPMs are right. When that is fixed right you will find that your prop slip will drop tremendously and since the prop will have much more bite your RPM will go down as well.

Do not have the prop shop raise the pith on your prop and you are definitely right, do NOT buy another prop yet.

Bottom Path

Lets raise the motor another 1" first and see how much that helps, we could have a problem because of the areation, but I believe we will gain speed and RPM. The rev limiter will stop it from getting too high, so it will be alright.

Hopefully, whatever he put on there you will be able to sand off, but first start with a bondo cheese grater and then go to about a 50 grit sandpaper and get it as smooth as you can, you don't really need to go finer than about 100 grit paper for the final finish. But be sure the finish is smooth and does not have ragged edges.



H

 

[Kymasabe]

Re: Need prop guru for prop size/pitch for '95 Merc 25hp.

OK, so here's the plan. I'm having the marina guys install a tach as it seems likes it's a must-have piece of hardware. My old QuickSilver prop is already at the prop shop getting repitched to a 15...just in case...and I plan on spending a good part of the weekend scraping that crap off the bottom of my boat. Then...baby steps. Sea trial after minor adjustments. If I do too much at once, I won't know what's working and what's not. So, I'll report next week when I have some results.
Thank you all for your help.

 

[hwsiii]

Re: Need prop guru for prop size/pitch for '95 Merc 25hp.

I will be waiting Kymasabe.



H

 

[steelespike]

Re: Need prop guru for prop size/pitch for '95 Merc 25hp.

OK, so here's the plan. I'm having the marina guys install a tach as it seems likes it's a must-have piece of hardware. My old QuickSilver prop is already at the prop shop getting repitched to a 15...just in case...and I plan on spending a good part of the weekend scraping that crap off the bottom of my boat. Then...baby steps. Sea trial after minor adjustments. If I do too much at once, I won't know what's working and what's not. So, I'll report next week when I have some results.
Thank you all for your help.

Good plan; we allways really imtend to try one thing at a time so you can tell what works.
To many things at once one might cancel another or introduce other complications.