Prop Selection Help

[Mr. M]

Hi! I was wondering if I could get some recommendations for determining a pitch\prop for my boat. I am leaning towards 11x9 but still undecided.

Boat: Tracker Sportsman 16 (Mod v Jon)
Engine: 1996 J25REDA Johnson 25hp 2cyl
Current RPM: 4780-4800
Current MPH: ~21-22
Current Prop: 10x13 (Damaged)

Notes:

Heavy load
2x batts
275lbs driver

 

[rallyart]

You'll get more response in the Prop Questions forum. Perhaps one of the Mods will move this there.
I think a 9" pitch will be to low for your use. Maybe a 10.5X11 would work as you are not at the max operating RPM yet. If your current one is damaged, you might even keep the top speed you have now and just run higher in RPMs Going from a 13 to a 9 is a 25% change in gearing. That's huge.

 

[Mr. M]

Thanks for the reply. I saw that section after I posted... Mods please move.

 

[Sea Rider]

What's the WOT range of that OB ? is it around 5000-6000 / 5500-6000 wot revs ? Is it possible to repair that current prop and tach the combo at wot as you did on last run ? then will know by how much less rpm OB runs compared to factory WOT rpm range, can fo from there on for a prop maximization to suit your personal boating requeriments. This procedure will be better than going blindy buying other prop alternatives.

Happy Boating

 

[Mr. M]

Gear Ratio: 2.15
RPM Range: 4500-5500

The main reason for looking for a new prop is the boat is very stern heavy and tends to drag causing spray from the front. I have thought about getting the prop repaired but for the cost a new prop could be had.

 

[steelespike]

Is your speed by gps?
I think a 10 or 11" 4 blade prop Should help get the stern up and the rpm.
The rule opt thumb 200 rpm per inch doesn't work as well on these reactively little props.
I lean to the 10" because Its better to be too small because it will give more power and you can use it
by keeping an eye on the rpm.If you were to get too much prop you shouldn't use it for an extended time.

 

[Sea Rider]

You are underpropped by 700 wot rpm, a Johnson 10 x 11 prop should rev near max wot revs. Ain't 25 HP underpowered for that size boat, what's the max HP transom allowed ? Is OB well transom height seated, trimmed, are you achieving over or backward transom/OB splashes ?

Happy Boating

 

[Mr. M]

Yes speed is from a lowrance fishfinder gps. I was looking into 4 blades but it seems there where not to many options for the small prop. Not sure max HP but yes seems like a bit under powered. Outboard is trimmed and appears seated well. The boat is taking spray over the gunwale but partially due to a offset mount fish finder. OB is splashing backwards.

 

[Sea Rider]

I usually work with OM props by the pitch, assume there isn't too many pitch sizes to choose from. Is there a 10 X 10-11 pitch available ? Can you post a nice pic of lower leg, want to check the distance from small upper water deflector plate to lower AV plate. If OB is splashing backwards you're loosing bit of top speed, hole shot, achieving more water drag on lower tail. Your combo will definitely benefit going for a prop maximization along sitting OB at the sweet transom height.

Happy Boating

 

[Mr. M]

There are 10.5x10 and 10.5x11 available. When I went to a local shop out here he said the motor never shipped with a 13p. Book showed a range of 9p-11p\12p. I will try and snag some pics when I can

 

[rallyart]

4750 rpm X 13" per revolution gives 61750 inches in a minute. I'm skipping the gear ratio as it won't change and the slip between the props won't be hugely different. If your's is damaged it might have more slip than a new one so your rpm might be higher than normal.
61750 divided by 11" would give 5600 rpm. This is not deadly accurate as there are many factors but it will give you a close estimate. The 200 rpm per inch of pitch won't be that accurate at low pitch rates. Going lower than 11 would be a risk. We don't know if the rpm is accurate, or if the pitch is accurate, or how much your current prop is damaged, or what the technology is of the different blade shapes, or lots of other things. But, the math is fairly simple to give a good starting place.

Many prop shops will let you trade in a new prop that is undamaged to try another. Generally, they want you to be a happy customer. Try one then another, if you can.

 

[steelespike]

I think a 4 blade 11" will get up quicker and increase rpm.

 

[Mr. M]

I still owe pictures but I went will a 4 blade 11p. Speed stayed the same 21-23 mph while RPM increased to 5200-5300. Boat is a lot better on the water and it no longer drags the stern causing spray.

 

[steelespike]

Nice to see an improvement when we have a success.