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shawnfish

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was wondering why i never hear anything about changing props when doing mods to boats? i read in my manual that with added weight and in some cases reducing weight can drasticaly change motor performance because of size and pitch of your prop. sometimes the additional or reduction of weight causes the factory prop to either work to hard and not get the rpms for the motor to get maximum performance therefore causing problems with wear and part failure, or getting to many rpm's and causing problems. i noticed when i did my boat that at full throttle my motor didnt sound like it was running at full throttle like it did before and it took longer to get up on plane and when it did it wasnt the same as before. so does anybody have some input on wether added weight calls for more or less pitch or bigger or smaller blades to get the most performance from your outboard?
 
Yes, adding weight will reduce your wot rpms. If the rpms fall below mfg recommended, then you need to go down in prop pitch. Best to keep to the same style/diameter such that you are changing only one variable. Many people don't have tachs on small outboard. A small inductive tach might be worth getting.
 
FuzzyGrub said:
Yes, adding weight will reduce your wot rpms. If the rpms fall below mfg recommended, then you need to go down in prop pitch. Best to keep to the same style/diameter such that you are changing only one variable. Many people don't have tachs on small outboard. A small inductive tach might be worth getting.


where do you pick one up at? any idea on the cost? also i have a 3 blade 9 1/4 dia by 11'' pitch. so should i go with a 9 1/4 by 9'' pitch? and this is probally a dumb question, sst is stainless right?
 
Read this thread for the tach I use: https://www.tinboats.net/forum/viewtopic.php?f=4&t=21033

You need to know what your current wot rpm's are before calculating how much pitch to change.

I don't recommend using a SS prop on a small outboard. Lots of $, for little perf gain. Even more $ if you guess wrong on choice.
 
my wot operating range is 4500-5500 rpm's im just guessing that it close or below 4500 judging from the sound. doesent matter now anyhow, i ordered the tach in the link you posted, thanks. what would you say if i was only running at 4500, decrease in pitch wise? or is there some sort of general guide line for pitch and rpm's? thanks alot for the info, your a big help to me and others on here..
 
shawnfish said:
my wot operating range is 4500-5500 rpm's im just guessing that it close or below 4500 judging from the sound. doesent matter now anyhow, i ordered the tach in the link you posted, thanks. what would you say if i was only running at 4500, decrease in pitch wise? or is there some sort of general guide line for pitch and rpm's? thanks alot for the info, your a big help to me and others on here..

I like to be in the upper part part of the rpm range. If you follow the "rules" to try to make up a 1000 rpms, that would be a large pitch change. Small motors can act much differently than the "rules". Provide some aditional info, and let's see where that leads to.

What size/type boat?
What motor and hp?
What is the gear ratio?
GPS top speed?
How much weight did you add to the boat?
Do you have a "before" speed?
 
FuzzyGrub said:
shawnfish said:
my wot operating range is 4500-5500 rpm's im just guessing that it close or below 4500 judging from the sound. doesent matter now anyhow, i ordered the tach in the link you posted, thanks. what would you say if i was only running at 4500, decrease in pitch wise? or is there some sort of general guide line for pitch and rpm's? thanks alot for the info, your a big help to me and others on here..

I like to be in the upper part part of the rpm range. If you follow the "rules" to try to make up a 1000 rpms, that would be a large pitch change. Small motors can act much differently than the "rules". Provide some aditional info, and let's see where that leads to.

What size/type boat?
What motor and hp?
What is the gear ratio?
GPS top speed?
How much weight did you add to the boat?
Do you have a "before" speed?

its on a 16 ft loweline jumbo V. 1980 johnson 25hp(J25RCS) 12:21 gear ratio. no speeds but will check gps maybe this afternoon. the boat before i added decks,floors,etc empty weighs 360lbs (260 boat and 101 motor) id have to guess on added weight being around 200lbs give or take 20lbs
 
Once you have some speed numbers, can work through some rough numbers using some reasonable rpms.

Would you say adding your mods, decreased your performance about the same as when another adult was in the boat?
 
shawnfish said:
FuzzyGrub said:
shawnfish said:
my wot operating range is 4500-5500 rpm's im just guessing that it close or below 4500 judging from the sound. doesent matter now anyhow, i ordered the tach in the link you posted, thanks. what would you say if i was only running at 4500, decrease in pitch wise? or is there some sort of general guide line for pitch and rpm's? thanks alot for the info, your a big help to me and others on here..

I like to be in the upper part part of the rpm range. If you follow the "rules" to try to make up a 1000 rpms, that would be a large pitch change. Small motors can act much differently than the "rules". Provide some aditional info, and let's see where that leads to.

What size/type boat?
What motor and hp?
What is the gear ratio?
GPS top speed?
How much weight did you add to the boat?
Do you have a "before" speed?

its on a 16 ft loweline jumbo V. 1980 johnson 25hp(J25RCS) 12:21 gear ratio. no speeds but will check gps maybe this afternoon. the boat before i added decks,floors,etc empty weighs 360lbs (260 boat and 101 motor) id have to guess on added weight being around 200lbs give or take 20lbs

I have the same motor, a '81 25hp Johnson and similar size boat/weight. I do recall hitting around 4200 RPMs at WOT with a 9 1/4 x 12" prop. So this prop pitch was too big. I don't use this motor anymore, but I would like hear your results, i.e. what size prop gets in the proper RPM range. I would guess a 10 or 9 pitch as you stated.
 
FuzzyGrub said:
Once you have some speed numbers, can work through some rough numbers using some reasonable rpms.

Would you say adding your mods, decreased your performance about the same as when another adult was in the boat?

NOWHERE NEAR THE SAME. BEFORE WHEN I HAD SOMEONE(ADULT) UP FRONT ABOUT THE ONLY DIFFERENCE I NOTICED WAS THAT IT TOOK A FEW SECONDS MORE TO GET UP ON PLANE. THE SPEED WAS NOTICEABLE BUT PRETTY MINIMAL.

JUST GOT BACK FROM A TEST RUN, GPS SPEED WAS 14 MPH. IT WAS EASILY DOUBLE THAT BEFORE THE ADDED WEIGHT. ONE MORE QUESTION, IM KINDA IN THE PROCESS(IN MY HEAD) OF REDOING IT, I HAVE ALL MY ALUMINUM AND HARDWARE. IF YOU WORK OUT THROUGH YOUR NUMBERS AND COME UP WITH A GOOD ESTIMATE IS THERE A WAY TO FACTOR A DIFFERENCE IN YOUR NUMBERS IF ( IM GONNA) I WEIGHT EVERY THING THAT COMES OUT AGAINST THE ALUMINUM AND FACTOR THE DIFFERENCE WITH WHAT YOU FIND TO GET A MORE ACCURATE ANSWER?

AGAIN THANKS ALOT AGAIN GRUB!!
 
I ran some numbers through a prop slip calc. If you are even hitting 4Krpms you have excessive slip (42%). If you are at 4500, than slip is even worse. A V-hull boat should have a slip around 10-15%. Less is better.

If there isn't any issue with your engine or hull, your modifications and gear are probably allot higher in weight than your estimates. The weight adds up quickly, and it is very common to end up with significant performance problems on small boats/motors. Any chance of water logged foam? I'd try to put the boat on a diet, and retest with the tach.
 
FuzzyGrub said:
I ran some numbers through a prop slip calc. If you are even hitting 4Krpms you have excessive slip (42%). If you are at 4500, than slip is even worse. A V-hull boat should have a slip around 10-15%. Less is better.

If there isn't any issue with your engine or hull, your modifications and gear are probably allot higher in weight than your estimates. The weight adds up quickly, and it is very common to end up with significant performance problems on small boats/motors. Any chance of water logged foam? I'd try to put the boat on a diet, and retest with the tach.

there is not a issue with the motor or hull, and i removed the two foam boxes in stern(it was 100% dry) and the only foam left in the boat is in the back bench,front bench and the little bench way up in the bow. ive checked it and its dry as a bone. this is what i added weight wise to the boat.

1 7ftx4ft peice of 3/4 plywood (pine)
1 4ftx3ft peice of 1/2 plywood(pine)
1 2ftx2ft peice of 3/4 plywood(pine)
10ft of pine 2x4(pine)
battery(66lbs)
MK powerdrive bowmount(50lbs? my guess)

thats it, the batt. minnkota and 2ftx2ft peice of 3/4 is all on the bow. nobody sits up there but me so its empty when motoring(buddy sits right in front of me to the left a bit.

should have the tach by sat and will post my rpms for you, also leaving sunday for a cruise and plan to tear it all out and begin the mod with aluminum the following sunday when i get back and im gonna weigh what i tear out and the alum. going in. i really appreceiate your help! hopefully i can get the correct size prop so i dont have to buy 2. i'll keep ya posted
 
I have a 25 hp Evinrude on my 16ft Mirro craft with full decking, three battery (2 group 27 size and one group 24), to two to three TM plus seats, etc. Added a pretty good bit of weight.

Pre-decking i was running close to 25 MPH WOT and now around 23MPH

14 mph is really slow for that engine on your boat. I think even if you are not propped correctly you should still be getting a lot more top end speed

What prop is on the boat now?
 
Captain Ahab said:
I have a 25 hp Evinrude on my 16ft Mirro craft with full decking, three battery (2 group 27 size and one group 24), to two to three TM plus seats, etc. Added a pretty good bit of weight.

Pre-decking i was running close to 25 MPH WOT and now around 23MPH

14 mph is really slow for that engine on your boat. I think even if you are not propped correctly you should still be getting a lot more top end speed

What prop is on the boat now?

9 1/4 X 11 IT JUST WONT GET UP ON PLANE, IVE HAD MOTOR CHECKED OUT AND HAS NO ISSUES. IT USED TO SCOOT AT LEAST DOUBLE WHAT IT DOES NOW.....
 
FuzzyGrub said:
Any updates with wot rpm and gps speed?
14 mph on gps, 4100 rpm's at wot. i found a spare i had thats 9x10 but on the inside of the prop there are no teeth that slide on to the shaft splines? why is that?
 
That sounds like a prop for a pin drive or one that has a removeable hub and its missing.

At this point, I'd just drop down 2" of pitch, and see if that can get you over the hump and on plane. Once on plane, she may wind out. If not, there would appear to be something wrong with your motor.
 
UPDATE........

GOT BACK FROM VACATION AND FINALLY(AFTER A LONG DELAY) REMOVED ALL THE DECKING,FLOORING,TROLLING MOTOR,TROLLING MOTOR BATTERY ETC... AND WEIGHED IT ALL FOR
A TOTAL OF 280 LBS WICH GAVE ME A TOTAL WITH GEAR,BOAT,MOTOR AND GAS OF 715 LBS AND IF YOU ADD ME A GRAND TOTAL OF REAL CLOSE TO 875 LBS. THAT BEING SAID MY COAST GUARD RATING FOR BOAT, MOTOR,GEAR AND OCCUPANTS IS 800LBS. SO WITH THE BOAT IN FACTORY CONDITION MINUS THE MIDDLE BENCH AND WITH ME,GAS,STARTING BATTERY AND SOME TACKLE AND A FEW RODS AND OTHER GEAR FOR A TOTAL OF ABOUT 575 LBS, THE OLE SEA HORSE WAS RUNNING AT 5300 RPM'S AND A GPS SPEED OF 31 MPH WITH THE 9 1/4X11 PROP. NOW THE QUESTION IS(AND IVE PULLED MY HAIR OUT TRYING TO FIND THE WEIGHT CONVERSIONS) CAN ANYBODY TELL ME WHAT A 4X8 1/4 INCH SHEET OF MARINE GRADE ALUMINUM WEIGHS SO I CAN FIGURE THE DIFFERENCE IN WEIGHT FROM THE LUMBER THAT CAME OUT? AND ALTHOUGH IT MAY NOT MATTER BECAUSE I THINK GOD REALLY LOVES ME BECAUSE I RAN ACROSS A GUY FROM A FREIND OF A FREIND WHO SOLD ME A 1996 50HP HONDA 4 STROKE FOR 2OO BUCKS( AND YES IT RUNS, LIKE A CHAMP) AND EVEN DROVE(OR RAN IT) IT ON HIS 18 FT SPECTRUM BEFORE I BOUGHT IT.TURNS OUT THE GUY HAD A RENTER WHO LEFT IN THE MIDDLE OF THE NIGHT AND STILL OWED HIM 200 BUCKS AND THE NOT GREEDY GUY AT ALL THAT HE IS GAVE ME A DEAL OF A LIFETIME, OR SO I THINK SO. NOW I HAVE TO FIND SOMETHING TO HITCH IT ON UNLESS I USE IT ON MY BOAT WICH IS RATED FOR 40HP MAXIMUM. IF I DO IS THIS A BAD IDEA? AND CAN ANYONE GIVE ME ANY OPINIONS ON THIS MOTOR BECAUSE IVE HEARD GOOD AND BAD THINGS ABOUT 4 STROKES. I BELEIVE 2 STROKES ARE BETTER FOR MULTIPLE REASONS BUT ALSO BELEIVE 4 STROKES HAVE A FEW ADVANTAGES.. THANKS IN ADVANCE GUYS!
 
I guess that is one way to prove it was the extra weight. Your new numbers are good. Very little slip.

I found weigh for alum somewhere on this site. You will save slightly in weight, but main thing is never will rot.

Outside of liability (I always say that), the additional 10Hp shouldn't cause handling problems. The weight of a 50hp 4 stroke, will make it a challenge to stay near your capacity spec.

4 strokes are heavier, quieter, burn less gas, and powerband not the same 2stroke.
 
FuzzyGrub said:
I guess that is one way to prove it was the extra weight. Your new numbers are good. Very little slip.

I found weigh for alum somewhere on this site. You will save slightly in weight, but main thing is never will rot.

Outside of liability (I always say that), the additional 10Hp shouldn't cause handling problems. The weight of a 50hp 4 stroke, will make it a challenge to stay near your capacity spec.

4 strokes are heavier, quieter, burn less gas, and powerband not the same 2stroke.

THE QUIETER AND LESS GAS WERE THE ADVANTAGES I WAS THINKING OF, SEEING AS I FISH SMALLER LAKES AND LIKE THE SOUND OF A GOOD OLE 2 STROKE AND THE FACT THEY ARE A LOT MORE ME FREINDLY(EASIER TO WORK ON) I THINK IM GONNA SELL THE HONDA. PLUS IVE BEEN WANTING A 24V MINNKOTA WITH I-PILOT. YOU REALLY THINK I WILL ONLY HAVE A SLIGHT WEIGHT DIFFERENCE WITH ALUMINUM? IM THINKING IT WILL BE MORE. TIME TO USE THE SEARCH AND SEE. ANYONE HAVE AN IDEA WHAT THAT MOTOR MAY BE WORTH? COSMETICALY AND MECHANICALLY IT IS IN GREAT CONDITION. THANKS AGAIN GRUB FOR ALL THE INPUT!!! I REALLY APPRECEIATE IT!!!!!
 

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