Prop help please

[ezmac]

Need some help from the experts here at iboats. I own a 2004 17ft Century center console. The boat with about 15 gallons of gas,hull and motor is 3,350 lbs. this weight is without passengers which are usually 2 and gear. I use it for fishing only. Top speed is not a concern. The prop from the factory is an 18 1/4 X 17k. At WOT my 2004 90 TLRC Yahama will only turn 5000 rpm. I would like to get closer to 5500 RPM. Either stainless or aluminum, I have no preferance. I would like to know if it is a thru hub exhaust or not and how can you tell?

 

[steelespike]

I think you have the prop numbers wrong.Possibly 14 1/4 X 17?
We need your lightly loaded wot rpm and gps speed. While you don't care about top speed it is important in figuring how your setup works.
The value of your rpm depends on the accompanying gps speed.
My info indicates the hull is 1450 motor 189 and fuel about 90. I get 1729 for hull motor and fuel.
Prop size should appear near the base of the blades or under the prop nut.
Your's is a Thru hub exhaust. Note the body is hollow so the exhaust can pass through and around the hub and nut.

 

[ezmac]

Thanks for your answer. I will note that next time I'm out and post again. If I remember right it was around 30 mph. You are right the numbers were wrong. They are hard to read without some enhancement. The correct numbers are 13 1/4 X 17.

 

[steelespike]

Don't forget lightly loaded. Like you and the wife but not your 3 300 lb buddies and gps speed.

 

[ezmac]

Thnx steelespike

 

[ezmac]

Took the boat this morning to the lake to clock speed and engine rpm's. Could not trim the motor as the boat would porpoise. I mean that bad boy would have been airborne if I had held it there The motor has to be trimmed in and even then it has a tendency to do it. This boat is new to me and I am finding out more and more as I play with this thing. So the next project will be trim tabs before I will be able to solve my prop issues. There isn't any weight I can move forward. The trolling motor and engine batteries are in the center console. The access to the fuel tank is under the floor in the center console as well. I assume most of the tank is forward of that. . In the rear there is a small bait well on the port side and small storage box on the starboard. The only thing I keep in that are my dock lines. So if I want to go faster than idle I'm gonna try tabs and go from there. If I'm headed in the wrong direction perhaps someone can set me straight. Thnx all.

 

[Fly'n Family]

I can't imagine a 17' center console would need trim tabs, providing the motor itself has power trim. Are you on a lake or taking it out into the ocean where you have swells? As far as adjusting your WOT rpms, first make sure you know what the engine is rated for then adjust from there. Each inch of pitch is equal to about 200 rpms.

 

[ezmac]

It was on a lake. Smooth as glass. Motor needs to be close but not more than 5500 rpm. Completely trimmed down it will start especially if you cross another boats wake. Can't trim up at all. Don't even think about it. I had a Gheenoe do the same thing. it was less than 16ft. I would move a weight to the bow but there is nothing to move. The motor may be hung a little high but it is as low as it will go. Came from century that way. So, the only option I have is tabs unless someone has an alternative. Thnx for your interest.

 

[steelespike]

Is the motor trimming down all the way? Is it an al or ss prop?
Is the bottom clean and smooth no bulges or hollows?
Did someone do major work on thje hull.
Seems unlikely that a boat would go 11 years with a serious porpoising problem
Without some sort of attempt to solve it.

 

[ezmac]

No Title

Trimmed all the way down. Bottom is clean. aluminum prop. There is a boat like mine in for motor work at the marina that has tabs on it and a ss prop. Marina says that before he installed the tabs it was doing the same thing as mine. This boat was used very little for the first 5 years then put in dry storage. The previous owner added a water/fuel separator to the transom. I was poking around the other day and found under the floor a separator installed from the factory. What does that tell about the previous owner or the marina that put on the new one. The old one was on so tight I couldn't get it off. I had to bypass it. No hull work has been done.

 

[ezmac]

OK. Took it to the lake with just the two of us. I managed to move 140lbs of dead weight from the stern to the front seat even though she said she didn't like to ride up there. The GPS said the best I could do was 32.7mph @ 5200 rpm. I would like to get closer to 5500 which is max WOT. Gear ratio is 2.00. My dead weight caught a nice bass, I caught 0 but I digress. Anyhow, any ideas on which prop I need? Thnx for your help.

 

[Sea Rider]

Did moving weight up front including wife bettered the overall water performance while trimmed at plane at 0 deg (boat riding parallel to water level) ? If wanting to achieve top wot 5500 rpm need 1 less pitch, but that's not rocket science as you could achieve 200 or much more rpm. If passing 5500 rpm, max wot numbers will drop accordingly as soon you begin adding more load to boat. Will achieve better hole shot but will surely lose slight top end speed.

New prop must be tested on flat calm no wind water cond to get the max out of that horse.

Happy Boating

 

[steelespike]

Your numbers aren't quite right.5200 @ 32.7 nets 27% slip.Really high 12% is more typical.
If we use 12% slip and solve for rpm we get about 4300.
The 90 on similar boats is typically propped with a 12 or 13" prop at about 5500 rpm and speeds 29 - 32.
Maybe you could borrow a shop tach to verify your tach.With all these other boats running 12 and 13" it seems unlikely
yours is right with a 17".

 

[ezmac]

Sea Rider Yes it did help. I was able to hold wot long enough to get my numbers. The porpoising stopped and allowed me to trim up a bit but only a bit.
Steelspike I will try and get a tach and check my numbers again. If I can I will repost. At least now I know what neighborhood I need to be in.
Thank you both for your help.