Help with prop size and setback of motor

[slimdaddy45]

oiK I bought a Sea Nymph SS195 19.5' boat with a seized 88 hp Johnson which had a 13x 19 prop with t x t so Took a motor off my other boat which is a 1974 115 hp and put it on a CMC aftermarket T & T IT HAD A 13.75 X 15 PROP SO SHOULD I RUN THE 13 X 19 AND WITH THE SETBACK[ 5.5''], FROM THE CMC UNIT THE Cavitation plate is about 1'' above the bottom of the boat do I need to drop it a hole so its level Ive not had it in the water yet GUESS 2 ? IN ONE thanks

 

[jimmbo]

You can try the 19" with the 115 and see what it does but, I think it is too much prop for either engine on a 19 ft boat, and the 15 might be a better choice. Your 88 was prop rated while a 1974 115 will be crank rated, so the difference in power may be slight. Both engines are rated @5000 rpm, have a WOT rpm range of 4500 - 5500. They also share the same displacement of 99.6 cu. in. and a 2:1 gear ratio
Try to prop it so the engine will run in the 5300 - 5500 range. It will live a longer life. The 19 may have been what killed the 88hp, too much prop and the engine didn't get into the recommended range and was overloaded and overstressed.
When an engine is mounted with any set back, it can be raised higher. Again I would try it as it is mounted, if there is no ventilation upon acceleration, you can try raising it another hole. If you are running a higher performance design prop, you may be able to raise a couple of holes. Just be aware of water starvation becoming a possibility.

 

[WesNewell]

With a 5.5" jack plate you probably want to raise the engine more, not lower it for sure. That boat only weights 1125# according to iboats, so I'd start with the 19p prop for sure. Calculated prop pitch for a 115hp 2s merc 2.07gr with 500# people and gear is a 23p prop, so don't be surprised if you over rev with the 19p prop. Certainly don't think it's too much prop for that boat/engine combo.

 

[ahicks]

Why guess? Put it in the water and try it. Have both props, and the tools to change them, with you just in case.

 

[slimdaddy45]

You can try the 19" with the 115 and see what it does but, I think it is too much prop for either engine on a 19 ft boat, and the 15 might be a better choice. Your 88 was prop rated while a 1974 115 will be crank rated, so the difference in power may be slight. Both engines are rated @5000 rpm, have a WOT rpm range of 4500 - 5500. They also share the same displacement of 99.6 cu. in. and a 2:1 gear ratio
Try to prop it so the engine will run in the 5300 - 5500 range. It will live a longer life. The 19 may have been what killed the 88hp, too much prop and the engine didn't get into the recommended range and was overloaded and overstressed.
When an engine is mounted with any set back, it can be raised higher. Again I would try it as it is mounted, if there is no ventilation upon acceleration, you can try raising it another hole. If you are running a higher performance design prop, you may be able to raise a couple of holes. Just be aware of water starvation becoming a possibility.

The guy I bought it from said it got to hot and seized up Im going to rebuild it in the near future but gonna run my old 74 till I get it done

 

[slimdaddy45]

Why guess? Put it in the water and try it. Have both props, and the tools to change them, with you just in case.

I will just wanted to get some advise where to start

 

[slimdaddy45]

Finally got my boat on a lake I could open it up on since Ive only been on lakes with 10hp limit or idle speed on big motors so here's what I found out wot It runs 6000 -6100 rpms so the 13x 19 should put me to close to 5500 right I didn't get to try it but its pretty beat up that came on the 88 its on Cmc t&t unit with the plate about 1'' above the bottom of the boat it ran 28.6 mph gps on fishfinder

 

[Faztbullet]

Slap a new 19" on and go fish'n…….:fish2:

 

[slimdaddy45]

How much does the diameter of the prop makes a difference or is just pitch they make different dia of a 19 pitch for my motor

 

[Faztbullet]

Diameter can raise or lower RPM but usually only around 100 RPM ...I would see 300 RPM when going from a 14 to a 14.5 diameter with same pitch on my ole Bullet..You really don't want to drop below 13" diameter

 

[anzomcik]

Finally got my boat on a lake I could open it up on since Ive only been on lakes with 10hp limit or idle speed on big motors so here's what I found out wot It runs 6000 -6100 rpms so the 13x 19 should put me to close to 5500 right I didn't get to try it but its pretty beat up that came on the 88 its on Cmc t&t unit with the plate about 1'' above the bottom of the boat it ran 28.6 mph gps on fishfinder

Based off your speed your motor is turning closer to 5000 rpm, if you do the math. If you were spinning 6000 rpm you would be going closer to 35-37 mph

At least one of three things is happeneing
1. gps speed is wrong.
2. Tach reading is wrong
3. you mis typed a number

 

[slimdaddy45]

Based off your speed your motor is turning closer to 5000 rpm, if you do the math. If you were spinning 6000 rpm you would be going closer to 35-37 mph

At least one of three things is happeneing
1. gps speed is wrong.
2. Tach reading is wrong
3. you mis typed a number

this is with running a 13.75 x 15 prop so I need to go up to a 13x 19 or so prop to get the rpm's down to 5500 or a little below I don't wanna have to buy 2-3 props to get it right

 

[WesNewell]

If your numbers are correct, you've got a 35% slippage, which is pathetic. Either your boat is loaded down with a lot of extra weight and your hub is slipping or cavitating, or the numbers aren't right. As a refernce, with my 1000# boat and 60 hp merc 4s I get 5% slippage with a smooth ride and o% slippage when I trim it up with a wild unstable ride at 40 mph by the speedo, not gps.

 

[anzomcik]

Ill explain more, if your getting 6000 rpm with a 15 pitch prop you should be going at least 35mph, your going 28mph. Like Wes said if your truely getting 6k rpm and 28 mph your prop slip is 35%, thats horrible and means somthing is wrong.

So lets assume you have average slip numbers and your gps speed was correct, your RPM should be closer to 5000. You would be extremely over proping your boat by going to 19' of pitch. This would mean your tach is reading incorrectly.

Or you could have mis keyed a number when writing your post, easy to do and would make the math off.

Boat speed can be ball parked pretty close on a planing hull boat with simple math. Motor rpm / gear ratio x prop pitch / 12 x 60 / 5280 = therotical top speed. then multiply by .85 (prop slip)

So in your case
6000 motor rpm / 2 (gear ratio)= 3000 rpm of prop shaft
3000 rpm of prop shaft x 15(pitch of prop)= 45000 inches per minute
45000 IPM /12 (inches in a foot)= 3750 feet per minute of travel
3750 FPM x 60 (minutes in a hour)= 225000 feet per hour
225000 FPH / 5280 (feet in a mile)= 42.6 miles per hour
42.6 MPH x .85 (inefficiency of water propulsion)= 36.2 miles per hour after 15% slip.

 

[slimdaddy45]

Ill explain more, if your getting 6000 rpm with a 15 pitch prop you should be going at least 35mph, your going 28mph. Like Wes said if your truely getting 6k rpm and 28 mph your prop slip is 35%, thats horrible and means somthing is wrong.

So lets assume you have average slip numbers and your gps speed was correct, your RPM should be closer to 5000. You would be extremely over proping your boat by going to 19' of pitch. This would mean your tach is reading incorrectly.

Or you could have mis keyed a number when writing your post, easy to do and would make the math off.

Boat speed can be ball parked pretty close on a planing hull boat with simple math. Motor rpm / gear ratio x prop pitch / 12 x 60 / 5280 = therotical top speed. then multiply by .85 (prop slip)

So in your case
6000 motor rpm / 2 (gear ratio)= 3000 rpm of prop shaft
3000 rpm of prop shaft x 15(pitch of prop)= 45000 inches per minute
45000 IPM /12 (inches in a foot)= 3750 feet per minute of travel
3750 FPM x 60 (minutes in a hour)= 225000 feet per hour
225000 FPH / 5280 (feet in a mile)= 42.6 miles per hour
42.6 MPH x .85 (inefficiency of water propulsion)= 36.2 miles per hour after 15% slip.

Im gonna have to check it again to be sure whether it matters or not but the boat came with an 88hp with a 13x19 prop the motor I replaced it with is a 115HP WITH THE 13.75 X 15 BUT iLL GET IT ON THE WATER AGAIN to be sure what I thought it was tach may be wrong don't know but I just put a new rectifier on my motor

 

[WesNewell]

Just for reference:
[h=3]Your setup[/h]

• Boat type: Aluminum - Console
• Usage: Top Speed/0 ft
• Engine Manufacturer:
  Mercury
• Engine Type: Outboard
• Engine Year: 2017
• Number of Engines: One
• Engine Family: FourStroke
• Engine: 90 FourStroke 4cyl (2061cc)
• Drive / Gear Case: Mid Outboard 2.07 Gear Ratio

[h=3]Details[/h]

• Calculated Pitch: 17.18
• Calculated Weight with engine(s): 2,098.00 lbs
• Calculated Engine RPM: 5,750
• Calculated Boat Speed: 38.15 mph

 

[anzomcik]

whether it matters or not but the boat came with an 88hp with a 13x19 prop the motor I replaced it with is a 115HP WITH THE 13.75 X 15 BUT iLL GET IT ON THE WATER AGAIN to be sure what I thought it was tach may be wrong don't know but I just put a new rectifier on my motor

Did you drive the boat with the 88hp? how did it run? Speed? holeshot? RPM? Because if you didnt run it and get real numbers it doesnt matter. I seen many boats that have the wrong prop on them, and i know personally people swapping props on a boat prior to selling to keep the prop that performed well.

Some tachs are run from a rectifier, if yours was bad there is a chance your getting inaccurate reading.

My gut says getting 28mph is slow for a 115, but i dont know the condition of any of the equiptment. It isnt apples to apples but my 115 goes 44mph on a 17' glass walleye fishing boat, with a 17pitch

 

[b.gagnon]

With that setback you need to raise the motor more..The new motor is close to 100ish hp due to being measured at the powerhead and not the prop. I find it hard to believe you are hitting 6k check your tach.

 

[anzomcik]

while your taking the boat to the lake, bring your 19 pitch along and change it and run it after retesting the 15. If your only getting 28 mph out of the 15 pitch. i would be impressed if the 19 did anything significantly faster. I would also would not be shocked if it went slower, it may have trouble planing the boat.

please report back

 

[jimmbo]

Make sure the tach is set correctly.