Troubleshooting Zen Basser performance(split off topic)

[Fritz]

Okay. I'm stumpped

Put a brand new 10 inch 17 pitch prop on today WOT is now just under 5000 and top speed with me alone and half tank fuel is 19 mph.

Now ,this motor did have a doelfin up until my second test today but I don't see any real difference with it on or off.

Thoughts?

Back to the motor too high theory --here's a better shot.

<img src ="modules/Upload/fritz/motor.jpg "> 

[DaveProka]

Hi Fritz,

I'm completely new to this motor stuff, but here area couple things I have discovered on my boat and through the literature from Nissan.

1. Make sure the anti-cavitation plate (that lowest horizontal fin -actually Nissan calls it something else, but I forget the name) is around 1" below the keel. If it's too high, you don't get "clean" water which lowers the performance.

2. Play with the trim, it will make a big difference in the way it gets on plane and how long it takes.

-Dave

[Fritz]

Yeah,Now I'm leaning toward the motor being to high(tip 'o the hat to bharbaugh)

The vertical distance from top of transom to bottom of keel is 21"

and now,with doelfin off, measuring at the keel (as in the photo) anti cavitation plate seems level with keel bottom.

I'm leaning towards cutting the transom an inch and remounting.

What do you think? Jacques? gurus?

[MikeS]

I think Yamaha is having a rebate sale on the 225 hp motors. Will that help?

MikeS

[JerryF]

I hate to see you start cutting chunks out of your new boat. However, I do agree there is something wrong with the performance. And that motor pic shows it to be mounted a wee bit too high to suit me. Do the deed. Only way to tell for sure.

Jerry

[Lucky_Louis]

Hi Fritz,

IMHO, engine height isn't the problem. I always start setting up an engine installation by placing the cav plate flush with my bottom and start lifting from there. It has been my experience that we raise the engine as high as we can go until it starts to cavitate in the turns - then lower one notch. That reduces the drag of the lower unit/leg to a minimum. With a 17" prop, at 5000 rpm you should moving faster than ~20 mph. Gut feeling tells me that your tach is wrong (2 cylinder tach in 3 cyl engine?), or your speed is wrong (units? knots maybe?) or you're cavitating.

The prop on it is a SS 13 pitch prop.

At 5500 RPM I was able to do 18 mph (GPS) with teenage son next to me in bench and 20 mph with him on back deck.

In going from a 13 pitch to a 17 pitch you only dropped by 500 rpm at WOT. I would begin to suspect that your engine isn't producing full power. Maybe take the 'ZenBasser' to you nearest OB dealer, and have him throw on a test prop. That will immediately tell you if you engine is performing up to snuff. Take it from there.[/u]

[Lucky_Louis]

http://www.rbbi.com/folders/prop/propcalc.htm

A great little calculator! Based on my quick research, I think your engine has a 1.83:1 gear ratio. Possibly a 1.87:1. Either way, not much difference. Using all your observations with propellor slip as the only unknown, you have ~50% slip. Not likely, because you would know something was up. Using a more 'normal' slip value of 15%, you should be doing around 40 mph. Even at 25% slip, you should be doing 33mph.

Hope this helps, I can feel your pain.

PS: has Dairy Queen paid for the damage they did to your lovely boat?

[Doug]

I agree with Lucky, the numbers do not compute. No way a 17 pitch prop is going to run 20 mph @ 5000rpm. It would be cavitating like crazy.
I don't think it is that high either. My Phantom is even with the keel with the jack plate all the way down but runs a few mph faster 2" to 3" up. So if the tach is wrong your engine is not making advertised HP.
Check the compression first, timing, advance, carbs, etc

[TonyGV-17]

Fritz,
The problem with my 40 was the tach I was using wasn't compatible with my motor. I was overspeeding when the tach said I was doing 4700 rpms. It took me about six months to figure this out. I tried different props and cleaned the carbs a gazillion times. If your motor is truely putting out 5500 rpms at WOT thats all its gonna do. Sounds like a tired motor. Have you checked the compression? I'm not sure about mercury/mariners because all I've ever used is OMC but compression being low isn't as important as all cylinders being equal or within 5-10 psi difference. My sister has a 50 Johnson on a 17' aluminum boat and it runs in the 35mph range. It might be worth a 100 bucks to have a good engine guy check it out.
Tony

[Fritz]

All, thanks, I agree, the numbers don't add up.

Tach: I will double check settings: It's a new teleflex tach with adjustable dial -- and settings are from teleflex for this motor. (BTW, 4 cylinder 50 Mariner--1983 probably built by yamaha)

Cavitation -- yes, it cavitates in the turns.

What about weight distribution? Is it possible there is too much weight forward -- or -- not enough aft?