90 Hp Merc, 17' Aluminum fishing boat with windsheild

[steam_mill]

Gents:

A friend bought a boat with an early 90's Merc 3 cylinder. I think the motor is propped wrong but it has exhibited something I've never seen before.

With a 19" Prop, motor turns about 4000 RPM at WOT (I think too much prop).

With a 17" Prop, motor will turn beyond 6000, I would have expected it to turn around 4400.

Another thing, with either prop, worse with the 19", the boat porpoises if you give it any trim. In other words, you cannot trim the motor out at all without the porpoising happening.

This leads me to believe the motor is mounted too hight on the transom, however, it cannot be lowered any further.

Comments?

 

[steelespike]

Re: 90 Hp Merc, 17' Aluminum fishing boat with windsheild

I'm pretty sure something more than a simple prop change is going on.
We need a gps speed to go along with the rpm.
Check the prop size numbers in the area under the prop nut.
Typically a 17 or 19 are pretty common for that motor.
Would help if you can pin down the year of the motor,
a brand and model of the boat would help as well?

 

[Texasmark]

Re: 90 Hp Merc, 17' Aluminum fishing boat with windsheild

That engine is running a 2.33:1 gearbox. I have an '02.

The boat is a 17' tin bass boat and it had a 17P Ballistic SS on it. Boat hull is stepped, but flat on the steps, weigh about 8-900, engine 300, 2 adults, 500, 3 batteries, gear, 10 gal fuel The boat was SEVERELY underpropped. Like you, the hole shot would pop your neck and the rpms would go out of sight if you tried WOT. I moved up to a 21 Ballistic and it did better but went on up to a 24 XL and that did the trick.....just under 50 at 56-5800 rpm, AV plate 1.5" above bottom measured vertically to hull.

I would expect your second prop to run that rpm as mine did. As far as the first one, the 19" I'd bet you are "plowing" and have too much boat in the water. Trimming out would raise the bow, increase the speed and the rpms.

Porposing is caused by a bad hull (hook) or trim/speed not in sync. The more trim you use (out) the faster you have to go to prevent porposing. You can be in a porposing situation and either tuck it in till it stops, or hit the throttle till it stops.

Trim can have a tremendous effect on boat speed/rpms with a given setup. Even on my little boat, I can tuck it in and easily loose 1500 rpms and 10-15 mph.

That engine is setup for 5000-5500 rpms. You need to run it near the top of that range. I am over it but not worried. Would like to see a picture of your boat, especially a side view of engine and transom and plate to hull position.

HTH,
Mark

 

[Silvertip]

Re: 90 Hp Merc, 17' Aluminum fishing boat with windsheild

It may be the engine is actually mounted too low which would provide a longer lever so to speak which causes excessive bow rise. The difference in prop pitches can indeed cause the significant rpm you saw, as does the prop design when coupled with the decrease in pitch. I've done a lot of testing on 40/50 HP johny/rudes and found that exact thing happening with a 13P stainless on a 16 foot V-hull. The boat was perfectly happy with a 15P cupped, rpm was at 5800 but the 13P would blow out. On that motor a 19P should be in the ball park and perhaps raising the engine would raise rpm and help with porpoising. But we don't know where the engine is mounted now so you need to give us a bit of help. Of course an engine that is down on power won't rev either but an engine down on power with too little load (as with the 17P) may spin freely but still not be providing full power.

 

[jimmbo]

Re: 90 Hp Merc, 17' Aluminum fishing boat with windsheild

The boat may have what is called a 'rocker'. The bottom of the boat, instead of the last several feet being straight is shaped like a rocker on a rocking chair. A deflection of 1/8" over a 6 or 7 foot lenth is more than enough to cause porposing. Regarding the propping issue. Are both props in good shape, no bent, broken, chewed up blades? Do the props show any signs of having been repaired? A 2000+ rpm change is excessive for a 2 inch pitch change. More info needed

 

[Texasmark]

Re: 90 Hp Merc, 17' Aluminum fishing boat with windsheild

The boat was perfectly happy with a 15P cupped, rpm was at 5800 but the 13P would blow out.

My opinion on that would be that the prop could spin up faster than the boat could accelerate. Since the distance to the surface was short and path of least resistance, the prop created a vacuum, sucked in the ambient air and you had your blow out.

Mark

 

[steam_mill]

Re: 90 Hp Merc, 17' Aluminum fishing boat with windsheild

Gents:

The 19" prop is brand new, outa the box.

The 17" prop is 9 years old. Is my personal "ski" prop used on my bowrider. Came with my brand new boat. no chips, nicks. As brand new.

Would motor too low cause the porpoising? I thought too high.

 

[Texasmark]

Re: 90 Hp Merc, 17' Aluminum fishing boat with windsheild

Gents:



Would motor too low cause the porpoising? I thought too high.

Don't know. Last engine that I had that could have been too low was an OMC 18 hp and it couldn't go fast enough to porpoise. OMC made their lower units a couple of inches longer back then I guess either because of their water pickup method, or transoms weren't standardized.

Mark