installed new prop,same size and lost speed?

[stylesabu]

I damaged my prop so I replaced with another 13 1/2 x 15 whuch is stock for my motor.i was at 6000 rpm and only doing about 28 mpg. with old damaged prop i had it to 34mph( i didn't have a tach at that time). the new prop has a coupler insert with 4 grooves in it and 4 springs with nylon inserts, all spaced 90 degrees apart. the old prop was solid rubber coupler. I think prop is spinning on shaft? I had also added a whale tail to lower unit which i was told would increase speed and power. but on my test run i was only one in boat previous have 200 pounds more wieght. it looks like i'm going to have to replace prop again, i would like a prop that could handle more wieght, I'm willing to sacirfice speed. its a sylvan 16ft deep v 1982, with suzuki dt 75hp, stock prop is aluminum 13 1/2 X 15, I'll be carring about 30 gallons of gas, 400 pounds in passengers,my self and another. and about another 300 pounds of gear. I have made this trip several times trwice in rental boats,with 50 hp motors 4 stroke,which never plane out until unloaded and in a buddy boat 16ft with 90hp 2 stoke,which is able to plane out. he does lose speed, but its 3 times faster than the rentals, hence i have purchased a boat. let me know what other info you need. thanks

 

[hwsiii]

Re: installed new prop,same size and lost speed?

Styles, welcome to Iboats prop forum and we will try to help you as much as we can but without more information no one can help you, and I will tell you I need more information than anyone else. But if you would like my help fill this form out and I will help you as much as I can, if you would prefer to not go to this much trouble there are other people who will gladly help you.

Iboats Boat and Motor Info

1. Year, make and model of boat
2. Length, width and base weight of boat, look for boat decal on back of boat
2a.What is the maximum recommended HP for your boat
3. Number of people and gallons of gas normally on boat
4. What do you use the boat for
5. Is it a Deep Vee and if so how many degrees of deadrise
6. Year, make. manufacturer and model of motor
7. HP and gear ratio of motor IMPORTANT
If you don?t know the ratio, you need to pull the plugs out and put a piece of tape a c ross the prop and the lower unit and then cut it between the prop and the housing and turn the motor until the tape lines up with each other, it is easier if two people do this, so one can watch the prop while the other counts the revolutions of the motor
8. Manufacturer?s recommended Wide Open Throttle (WOT) range
9. Anti-ventilation Plate height above keel of boat if it is an outboard in inches, use a board under the keel and stiking out to the anti ventilation plate for a referene
10. Is it a bass boat or does it have a pad bottom
11. Does it have a hydrafoil, dolefin or trim tabs
12. Make, model, diameter, pitch and whether SS or aluminum prop
13. WOT RPM and speed from your current prop and how much gas and how many people were in the boat for the test data and is the speed by GPS. Make sure you trim the prop up until it starts ventilating and then just trim in until it quits ventilating. If you do not have a tach you can buy a Tiny Tach for $ 50
RPM ___________ Speed ___________ No. of people ____________ Gas ________
14. Are you at sea level or a higher elevation, give us the elevation in feet
15. Has your motor been tuned up lately and have you checked that the carburetor butterfly is opening all the way, checked compression, and looked at the plugs and checked spark, all of the foregoing could be the reason your prop is not attaining full RPM
16. How long has this prop been on the boat and why, at this time, do you think it is the wrong prop
17. Does the prop show any damage that you can see
18. What problems are you trying to cure or what are you looking for the boat to do that it is not doing the way you think it should or to your expectations
19. If you are trying to attain a better cruising speed and fuel savings or trying to attain a faster speed I will want you to take your boat and run it with 1 or 2 people and give me the RPM and speed readings starting at 3,000 RPM in 500 RPM increments all the way to WOT.

REMEMBER, The numbers I give you will be NO better than the information you give me

The only thing I ask of you is to come back and give me a report of WOT RPM and speed for my database.


H

 

[stylesabu]

Re: installed new prop,same size and lost speed?

1) boat sylvan 16' 1892,model sea monster 2) 6ft wide 16 feet long,its a center console, 1 seat so i think it weights less than the ave 16ftit does have wooden floor with thin marine carpet 2a) max wieght is 1350 pounds w/70 hp or 1200pounds 90 hp motor 3)2 people, and 30 gallons going in 4)fishing when fishing 2 guys and a cooler 550 pounds 5) its a deep v but i don't no degrees. 6) 1986 suzuki 75dt 3 cylinder 2 stroke,75 hp model number 07501-607015 7) 75 horse 27/13 or 2.08 gear ratio stock prop is 3X13 1/2x 15 aluminum 8)i can't find WOT but manual states 4800 - 5500 rpm 9) no anti ventalation plate 10) no and no 11)hydro foil just installed, 12)prop aluminum, i bought it from iboats #41102317 13) no tach at time of test run,wot, 3 people 550 pound people wieght, 6 gallons of gas.gps speed 37 tops at wot 14) chicago i think at 1000 ft 15) engine runs strong plugs and filter new 16) just bought boat prop was slightly damaged, i was ok with the way it performed just thought a prop with no nicks would work better.17) no, but i thgink it may have spun on shaft see previous story 18) we carry quite a bit of gear into canada were we fish, it a three1/2 hour trip if boats a re at 20- 22 miles per hour, has taken 6 1/2 is past with rentals. just would like to know what is best prop for my application about 1300 pounds gas ,gear people. 19) i will get reading on exact, mph people, rpm next week, and if you recommend different prop i will also test run that with specs. my tach may be off but i have a fluke 88 meter with rpm, so i can calibrate mine and double check rpm. this is as much as i can tell you. thanks again. i do also have a 13 1/4 X 17 3 blade prop that i will check performance on off buddy 90 hp boat, same size, probably 300 pounds heavier, more seat and cabinets

 

[hwsiii]

Re: installed new prop,same size and lost speed?

Style, all outboards have an anti ventilation plate from the fatory, it is the one right above the prop. Please measure the distane from the antivent plate to the keel, and if it is higher or lower.

H

 

[Silvertip]

Re: installed new prop,same size and lost speed?

Unless the new prop is exactly the same as the old one (meaning that it is from the same manufacturer, same model, and same design) you just learned that all 13-1/2 x 15 props are not created equal. "Design" is the critical word here. Different manufacturers have different design factors for their props so it is common to have different performance characteristics when changing props between manufacturers. One may have better hole shot and less top speed. Your loss of speed could also be due to the hydrofoil you added. When making prop comparisons you need to make them under identical conditions.

 

[stylesabu]

Re: installed new prop,same size and lost speed?

its even with bottom of the boat, if any thing it maybe 1/2" higher. i appreciate the help.

 

[hwsiii]

Re: installed new prop,same size and lost speed?

You are very welcome Styles, and Silvertip is right, blade geometry is everything in a prop and 90% of the general public has NO idea how important it is. Diameter and Pitch matter, but blade geometry can matter even more. We want to move your motor up one hole higher on the transom than it is now. When you installed the new hydrafoil it could be dragging through the water causing drag because the motor is too low in the water column, that is why I asked about the anti vent plate. Then I want you to run the boat again and see if the hydrafoil is completely out of the water when you are planing, that is what is required. And get me the new WOT RPM and speed, and do you have power trim and tilt on your motor.


H

 

[stylesabu]

Re: installed new prop,same size and lost speed?

I DO HAVE POWER TRIM ON BOAT. and i am familar with adjust trim, to max rpm's without porpusing

 

[stylesabu]

Re: installed new prop,same size and lost speed?

good call. as this boat is an 82 and motor is 86. i boughtfrom original owner and is possible that motor was incorect from inital set up. I should be able to get boat out this weekend and have results. Thanks again

 

[hwsiii]

Re: installed new prop,same size and lost speed?

That sounds very good Styles.


H

 

[stylesabu]

Re: installed new prop,same size and lost speed?

alright got boat out yesterday. I raise motor on transom 1 bolt hole, i put my original prop back on 13 1/2 x 15, i filled in nick edges with alumilloy,which works well.i had top speed of 36 mph wot per gps,tach was at 6000 which i think is off(sorry) i had my fluke meter to compare rpm reading and it was flucuating i did see it read 4800 at one point wide open throttle.i than install a 13 1/4 by 17 prop speed dropped to 33 and rpm dropped 500rpm. so i think tach work i just need to recalibrate. I wish I could have true rpm but closest safe water is a hour away. and my engine doesn't like running on earmuffs, still gets hot in a couple of minutes

 

[stylesabu]

Re: installed new prop,same size and lost speed?

HWSIII, any idea if I'm running right prop? do you still need more info?

 

[hwsiii]

Re: installed new prop,same size and lost speed?

Styles, if I knew what RPM you were turning I could tell you, but I don't know if you are turning 4,800 or 6,000 RPM. So I don't know.


H

 

[stylesabu]

Re: installed new prop,same size and lost speed?

i'm going fishing in 2 weeks, and i don't think I'll get a chance to put it in the water before then. I installed another tach and it seems to be right on.Ill have the numbers in three weeks. one other thing, I have read that i should be able to see anti cavitation plate while cruising, i have a hydro foil installed. does it make a difference as to how fast im going as to weather I'll be able to see hyrofoil? I couldn't see it on Fridays test drive,it was under the water,but 37 mph is the fastest I've been able to get,I'm not that concerned about speed, i just want enough power to haul fully loaded for 3 hour trip into canada. Thanks again

 

[hwsiii]

Re: installed new prop,same size and lost speed?

Your motor needs to be raised higher then, the hydrafoil should be OUT of the water. it has something to do with speed, but it is performance that matters.


H

 

[stylesabu]

Re: installed new prop,same size and lost speed?

so when i'm on plane, at whatever speed i should be able to see hydrofoil?
Thanks again

 

[hwsiii]

Re: installed new prop,same size and lost speed?

Anytime you are at cruising speed you should be seeing the hydrafoil OUT of the water, not just see it. Anything in the upper 1/3 of your RPM range.


H

 

[stylesabu]

Re: installed new prop,same size and lost speed?

Im going try to get it out again, after i raise the motor. thanks again for your help

 

[stylesabu]

Re: installed new prop,same size and lost speed?

ok went up 1 more hole on transome, have working/accurate tach. here are the #'s 13 1/4 X 17 prop, 1 bad in preety bad shape 4900 rpm at wide open,34 mph top speed pretty choppy water, switch to 13 1/2 X15" prop wot 37 mph 5400 rpm which i think is about max for my motor.what prop size would give me best economy taking in 1400 #'S and still get up on plane say may 24 26 mph? I don't mind carrying 2 props plus a spare. Thanks again hwsiii

 

[stylesabu]

Re: installed new prop,same size and lost speed?

also I was carry 4 people 500 pounds w/17" prop and only 250 pounds on the 15"