Best prop for me?

[Ats666]

Hi, I've got an old 18 foot long pontoon with an early 80s 18hp Mercury (serial 7198298).
The motor will only rev up to 3260ish rpm in forward (verified with Tach/hour meter) .
I did not check gps for speed.
Compression in the two cylinders is 155psi and 160psi.
Idles perfect and does not stumble to its maximum rpm.
It's got the factory prop on it (48-43586-13).
When I put it in reverse I can get the revs up for maybe 2 seconds but then it slows down again.
​​​​​​
No water in toons. Doesn't mater if it's 2 adults or 5 adults with fishing gear on board, max rpm doesn't really change.
​​​
This is my second season with the boat. I don't think the carb has been gone through in 10 years.

​​Linkages appear to be moving correctly.
​
Do I need to go through anything in the motor or carb?

These are the lowest pitch props I could find. 9s

#1
Solas Amita
11x9 rh
1211-110-09
No hub kit required

#2
Michigan Vortex
10.5 rh
992501
Hub kit: xhs-120

#3
10.5 rh
322800
Hub kit: 314899


​​Will a 9 pitched prop be enough and if not who should I contact/deal with to get me a better
one?

Thanks
Allan

 

[Scott Danforth]

with 'toons, proper height is critical. if your too deep, you will also lug the motor. make sure your motor AV plate is about 2" below the water when running

then from there, you should be in an 8 or 9p with only 18 hp.

 

[ahicks]

What is your target rpm? In a perfect world, how many rpm would you like this new prop to turn up?

 

[Ats666]

The motors max is 5500 rpm so I thought it should be somewhere around there at wot.

​​​​I troll with it 95% of the time, the other 5% is wot.

Need some more top end out of it for the ride home.

Any reason to no spin it up that high?

 

[WesNewell]

You should find the lowest pitch prop you can. You are lugging your engine way down and losing power with a 13p prop. A 9p would probably bring your rpm's up to ~4100. Still not ideal, but a lot better than 3200. Ideally, you want something closer to 5000 rpm.

 

[ahicks]

The rpms you are getting with the present prop are so far off what you're after, they're almost of no use at all. Another vote for a 9" pitch prop here. A cheap one. Usually the kind that don't need an adapter are less expensive. Better yet though, would be a used one. A few calls to local prop repair facilities could pay off.

 

[Sea Rider]

Would check firstly if current prop, tach, engine are doing their homework right before playing with less pitch props. An 18 is a very underpowered engine for a toon use which has more hull drag to overcome fast compared to a same size standard mono hull boat with same engine.

One thing is propping right any engine to rev say middle to max wot rpm range factory stated with a fixed, near fixed load, other story while constantly varying load as in your case. No matter which prop you choose will only attain slow displacement speed provided that deck weight is evenly distributed, engine is well height transom seated and trimmed.

Engine being that old, available prop selection could be scarse.

Happy Boating

 

[Ats666]

Installed a rebuilt 9.5 pitch 10.38 diameter prop.
Getting it up to 4500rpm now.

Also raised the motor up 1 inch while I was at it

Thanks to everyone for the advice

 

[Sea Rider]

Engine is still revving on the lugging side as currently loaded which is no bueno, need a much less pitch prop, say a 8 if wanting the engine to rev at least middle to max wot rpm range and head to terra firme much faster and efficiently than with current 9.5 prop.

Raising/lowering any engine on transom is no indication that prop will achieve its best thrust as you don't have the slightest idea about what's going on at back of engine. Ideal transom/engine match is for water flow to skim at fast displacement speed right under upper small plate while engine is trimmed to 90°. Visually check if you have dialed that height position, if not sit engine back till that cond is met.

Any toon/boat with a optimized tarnsom height along a prop maximization is a day/night boating difference..

Happy Boating

 

[Ats666]

No one makes a prop with that low of a pitch though.

I've got the motor height so the cavitation plate is 1-2 inches below the water at speed with 2 passengers/gear and weight spread out.
​​​​​​My motor is as close to 90 degrees as I could get it using the provided holes.

 

[Ats666]

I called the prop shop I used is going to see if they can repitch another 9.5 they had on the shelf to 7 for me.
​​​​​I'll post back when they back to me

 

[WesNewell]

Try the 7p prop. That should bring your rpm within the recommended range.

 

[Sea Rider]

I called the prop shop I used is going to see if they can repitch another 9.5 they had on the shelf to 7 for me.
​​​​​I'll post back when they back to me

That I know the max recommended size for a good repitch is 2 sizes down. Shelf one will probably be max repitched to to 7.5. ? Will be interesting to see how much more rpm is achieved per one pitch drop. Anyway, toon should perform much better. When testing combo on flat calm water cond with engine set at 90º pull head back and report at which lower leg height is water flow passing by once toon it's on its way at full wot.

A prop maximization along an engine/transom height optimization works very well for under powered engines pushing heavier boats or ones with more hull drag than traditional mono hull ones.

Happy Boating

 

[Ats666]

With the repitched prop I'm up to 4700rpm with a light load.
Looks like it's the best I can do

 

[Sea Rider]

With the repitched prop with current ? pitch achieved just 4700 wot rpm while lightly loaded. Still far from max 5500 wot rpm range, worse if were boating as usually loaded. Need to drop many pitch sizes or will continue to lug the engine.

Happy Boating

 

[Ats666]

Moving the tilt/trim pin one more notch and removing almost 80 liters of water from 2 sections of the toons I'm up to 5160rpms.
Also upon winterizing the motor I found a loose ground for the CDI box.
​​​​​No more fun this year.
Thanks for the help everyone