Prop Question

[rme hunter]

I have a 17 ft Ranger Sport R70 Bass boat. The boat is 1995 model and I have a 125 horse Mariner Motor on the back. I can only run 40 miles per hour. It seems to me like I should be able to go a little faster than this. I currently have a 22 pitch SS prop on the boat. If anyone is familiar with this boat and knows how fast is capable of please let me know.

[Xi Bowhunter]

I have a 17 ft Ranger Sport R70 Bass boat. The boat is 1995 model and I have a 125 horse Mariner Motor on the back. I can only run 40 miles per hour. It seems to me like I should be able to go a little faster than this. I currently have a 22 pitch SS prop on the boat. If anyone is familiar with this boat and knows how fast is capable of please let me know.
Something doesn't sound right. I have an 1993 18ft stratos with a 120hp Johnson, and I can get a max speed of about 55mph. I also have a 22 pitch prop. It sounds like your timing might be off.

[rme hunter]

Well I thought about that but the boat runs great and starts very easy. It just doesn't go fast. I thought that maybe it was because it was so heavy. Do you know the dry weight of your boat?

[Marmot_Militia]

Setup has a lot to do with it. Do you have trim?

22" seems like a lot of pitch for a 120 hp motor, but I'm sure some will disagree.

Do you have a GPS to clock it?

Regards,
MM

[Xi Bowhunter]

Well I thought about that but the boat runs great and starts very easy. It just doesn't go fast. I thought that maybe it was because it was so heavy. Do you know the dry weight of your boat?
I do not know the dry weight, but it is HEAVY! I mentioned the timing because mine was doing the same thing, toping off at about 35mph, and really struggling. I found out it was the timing, tuned it, and it runs fine now.

[weedwalker]

Is your engine getting full RPMs? If not, your pitch is wrong. 2" more pitch will lower your engine about 400 RPM. 2" less pitch will raise it 400 RPM.

[rme hunter]

The MPH was gauged off of GPS. I will have the timing checked. I put a 21 pitch on the boat last night. I haven't had a chance to try it yet. I will tommorrow. I am only turning 5100 rpm's I think that is some of my problem.

[moosehead rollins]

My 17 ft Champion w/ 150 evinrude was turning 5100 rpms with a 22 raker prop. Light load GPS @ 68 mph. Motor height and prop diameter are pretty important

[daking]

5100 is within the acceptable range.

Motor height will make a difference. So will the way you use your power trim. How much boating experience do you have? Are you trimming your motor out when you get running. If you do so, you raise the nose of the boat, lessent he amount of wetted surface. Less drag means more speed. You should be able to see the difference where the bow wake rides on the side of the boat as you trim the motor out. The farther out, the farther back on the hull the bow wave will move. Gotta be careful though, too much trim is as bad or worse than too little. Play with it.

Boat load? If you're carrying a bunch of unnecessary junk on board, you know what to do.

One last thing. Most boat speedos work on what's called a bourdan tube and meter. The speedo head is hooked up to a tiny rubber hose that runs the length of the boat. This hose is connect to a pitot tube that hangs down below the bottom at the rear of the boat. The pressure generated on this tube is transmitted through a column of air to the speedo which is essentially a balance that translates the pressure into the speedo reading. In theory, more speed, more pressure, higher number on the speedo. In practice, this is a pretty delicate system for something that gets banged around as much as a boat on the water. It's probably lying to you.

My 16.5' Cajun with a 90 Yamaha will run a solid forty-seven with my fat kiester in the saddle. I suspect you'll get over fifty when you got yours just right.

[rme hunter]

I trim all the way out and have had several other people drive the boat to try and see what it will do. I have tracked the speed on a GPS and 40 is all that I can get right now. I think that once I figure it out I should be able to go at least 50MPH. I have a friend with an 18 ft tracker 90hp merc on that can out run me with ease. I must do something. I changed my prop yesterday and will try that tommorrow we'll see. I do appreciate all the advice and please keep sending it.

[daking]

Well, I don't think you have a sick motor, because if it can turn 5100 any minor timing problems or otherwise aren't the problem. It's spinning in the right RPM range.

How much stuff do you have on board? Batteries? Tackle? Left over Slim-Jims? Weight will make a difference. I'm sure you thought of this, but sometimes it pays to restate the obvious.

Does the boat sit in the water? Is the bottom clean? Again, you're probably way ahead of me, but reminder is good.

Going to a prop that will give you more rpms may help. If you went from a 22 to a 21, all things being equal, you should pick up 200 RPM. You could go to a 20 that would give you the magic 5500, but I doubt that 400 RPM is going to give you a whole lot of speed.

When you're running, how far back on the boat is the bow wake? I know on my boat, it seems that the bow wake at full throttle and at good trim is just behind the passenger seat. Your mileage may vary, but if your running on the bow wake somewhere under the casting deck, something is very wrong. You're keeping too much boat in the water.

One more thing. When you throttle up, do you accelerate strongly? There is a very slight outside chance that your prop hub is slipping a little and the hub is turning 5100 but the blades are turning less. THis is a very long shot, but at this point, it pays to consider everything.

I'm kind of stumped.

[C.L.Button]

Sounds to me like maybe the motor needs to be raised off the transom ? Motor bolted too low on the transom ?

When you watched someone go by, how much of the boat was in the water ? Terry's suggestion of the prop hub spinning is a possibility ?

The rest of your info sounds pretty correct to me ?

[rme hunter]

The motor is sit all the way down in the water. I mean as low as you can get it on the boat. Also will the plugs affect this at all?
It seems like when I am on plane the the wake is near the seats.
I will try the new prop soon and hope that it will help. I would like to go near 50.

[C.L.Button]

The motor is sit all the way down in the water. I mean as low as you can get it on the boat. Also will the plugs affect this at all?


Plugs can effect it but I doubt that is your problem ? Does it run better cold ,then seem to fall on it's face as it warms up ?

The motor being mounted all the way down on top of the transom does not sound correct ? With your boat on the trailer, Lower your motor down to a trimmed up running position and see how much of the lower unit is below the bottom of your boat. It should be from just above the water pick-up to the bottom of the skag. IF it is any more then that then I would say your motor needs to be raised.
Most newer boat mfg's have several sets of holes for mounting the motor to the transom. Take a look at raising the motor up to the next set or whatever will put the motor at the proper level ?

NOT being able to see this for myself I can only GUESS at what your problem is ? With that being said, make sure you measure twice, and move once ! :D

Good Luck.

[Marmot_Militia]

The motor is struggling for some reason.

Good points made before:
Clean the bottom of the boat
Lighten the load
New fuel
Use Carbon Cleaner (not carburetor cleaner) to clean pistons
Raise motor
Is the prop bent or nicked?
New fuel separator/filter
Correct oil mix
Timing/plugs/tune-up time
Lower unit lube/bearings
Less pitch

Let us know how it goes.

MM

[moosehead rollins]

Just from you saying " the motor is as far down as it can go" tells me that you need to raise that motor off the transom.

[C.L.Button]

rme hunter

What have you found out ? I'm curious where you are in your fact finding ?

[dead eye]

Without seeing the set up it's hard to tell but from what i've read on this problem it really seems the ur motor is too low on transom . I use to race outboars & the right set up would be with the motor in a staight vertical position the cavitation plate should be about parrelell with the bottom . that is looking from the rear of boat forward at the approxament same level as the bottom. Hope this makes sense to U. Then as U put ur motor jacked out as far as U can go without grabbing air .Using a cupped prop U shpuld be lifting bow up out of water so only about a foot or so of transom would be riding in water. Lift ther bow as far as U can without cavitating. If U do not have a cupped prop U will cavitate too soon & lose power & speed. Something is definately wrong cause U should be getting 50 to 60 out of that rig
This is lone winded But I'm tring.
Dead eye

[rme hunter]

I contacted Ranger boats and they said that the motor is in the correct place on the transom. They also said that I should be running a 24 pitch laser 2 prop. I am running a 22 pitch and still can't do anything. I did find a nick in one of my plug wires so I have ordered new ones. After that I am going to try a couple more props to see if I can do any better. If not I will go back to original setup and take it to a boat shop and see what they can do. The boat runs great and rides terrific but I just don't go very fast.