last years prop was spinning 4600rpm now 6000???? updated

[nailem]

i have a 86' sunrunner 220 cv. i bought it last and it came with a 21 pitch cant remember diameter prop. the boat could hardly get on out of its own way. i tried a 19p and 17p and ended up with a 14p 4 blade. with the 14p 4 blade i ran 4600rpm wot and about 38mph. so i was happy.

the boat had about 1/8th inch of hard bottom paint the looked like it was splashed on from 10 feet away. i have seen dirt roads that are smoother. so this spring i sanded it all down and painted it with ACT bottom paint.

i just put the boat in the water yesterday and it started and runs fine but now with the 14p 4 blade i turn 6000rpms.i back it down to about 5000rpm and run about 32 mph. i did not hit anything between storing the boat last year and launching this year so i can't believe the hub is spun???? or could it????

could the weight and added drag from the old bottom paint make this big of a difference? the only other thing i did was bellows and gimbal bearing. what are your thoughts?

 

[CATransplant]

Re: last years prop was spinning 4600rpm now 6000????

Re: last years prop was spinning 4600rpm now 6000????

Yes. Bad bottom paint can slow you down that much. It may be time to swap another prop back on there.

 

[hwsiii]

Re: last years prop was spinning 4600rpm now 6000????

Re: last years prop was spinning 4600rpm now 6000????

Nailem, you didn't tell us the horsepower and gear ratio of your outdrive, I need that information to help you with the correct prop.

H

 

[nailem]

Re: last years prop was spinning 4600rpm now 6000????

Re: last years prop was spinning 4600rpm now 6000????

boat has a 5.0l 230hp with alpha 1 gen one outdrive. i looked last year on the drive but could not find the gear ratio. i believe it has been painted over i tried scratching a bit were it should be stamped and came up with nothing. i looked it up and the original prob when i bought the boat was a 15.5x21
the spare was a 14.5x17 and the prop i settled with last year was 14 3/4x15 4 blade. like i said that spun about 4600rpm. it seemed to work good. but now that prop is spinning 6000.

all props have been aluminum

 

[hwsiii]

Re: last years prop was spinning 4600rpm now 6000????

Re: last years prop was spinning 4600rpm now 6000????

Nailem, my calculations show you should be using a 19" aluminum prop and do about 43 MPH. That bottom paint required a smaller pitch in order to negate the effects of the water disturbance on the bottom from the paint. You can try the 21" pitch you had on it before but I believe it will be better and have almost maximum RPM with the 19" pitch.
Here are my calculations if the gear ratio is 1.7 as I found.

Boat and Motor

Prop Picker

Prop Change

H

 

[nailem]

Re: last years prop was spinning 4600rpm now 6000????

Re: last years prop was spinning 4600rpm now 6000????

hwsiii, thanks where do i find that program? it is amazing that the bottom paint made that much of a difference. i knew when i was stripping the old stuff off it was going to help my top speed and fuel but did not think it was going to change the prop needed. the boat does sit at least 1.5" to 2" higher in the water. no kidding.

i sold the 21p that came with the boat but i have the 17p. it does not take long to change so next time im out i'll wet suit up and change it to the 17p just to try it. i'll start looking for a deal on a 19p. i bought the last one here at iboats. what diameter should i look for? i also liked the 4 blade, i do mostly family cruising and wakeboarding so top speed is not that important. should i look for a 19p 4 blade or a 17p 4 blade.

 

[Tacklewasher]

Re: last years prop was spinning 4600rpm now 6000????

Re: last years prop was spinning 4600rpm now 6000????

Hang on.

In the original post, he was running 4600 rpm and 38mph with a 14p. Or maybe a 15p. He says both in two different posts.

This year he is running 5000 rpm and 32 mph with the same prop.

nailem. Is this indeed what has happened?

if it is, this isn't a bottom paint issue.

 

[hwsiii]

Re: last years prop was spinning 4600rpm now 6000????

Re: last years prop was spinning 4600rpm now 6000????

Tacklewasher, he just forgot the real speed he was running last year with that prop. It is impossible for that combination at 4,600 RPM to reach 38 MPH at a 1.7 gear ratio, as that would be a -6% Prop Slip on a 14" Pitch and with a 15" Pitch it would be 1% prop slip and that doesn't happen either.
Nailem that is a custom program I wrote to find the correct props for boats, that I use. I don't know whether the prop you are running now is a 15" Pitch or a 14" Pitch but either way your Prop Slip is over 18%. My program is correct, with a 19" pitch 4 blade aluminum prop you should be able to reah about 41 MPH with a much lower slip ratio than you have been experiencing so far. Please get back with me after you change to the 17" prop and give me the WOT RPM and speed so we can run the numbers on it and do a verification of prop slip.

Prop Slip this year and last year

H

 

[TilliamWe]

Re: last years prop was spinning 4600rpm now 6000????

Re: last years prop was spinning 4600rpm now 6000????

nailem, confirm that your tachometer is reading correctly.

 

[nailem]

Re: last years prop was spinning 4600rpm now 6000????

Re: last years prop was spinning 4600rpm now 6000????

hwsiii, is correct in my first post i was going by memory on my current prop and tested props. i then looked it up and the correct current prop is a 4.75x15p 4 blade prop.

i will recheck my tach before i do anything but last season it was checked against a digital tack and was real close (maybe 25-50rpm off).

i found my post from last year and when i tested the 14.5 x 17 3 blade and WOT is running about 4200 RPM and 36mph per GPS. and the 14.75x15p 4 blade ran 35mph per factory speedo at 4600rpm. this was with 1/2 a tank of fuel and two men.

now with the same prop it planes out real nice but once on plane the rpms go way up. i know its not good to run it over 5200rpm so i let it get to 6000rpm twice only for a moment and backed it down to 5000rpm at that point the factory speedo read 32mph. i dont hold much weight in the factory speedo though.

it had so much bottom paint on it that when i was sanding it down i found body lines that did not show with all the paint on.

thanks for all the help. this is my first boat but i love to read and learn. i replaced the bellows, shift cable and gimbal bearing this spring and bottom paint, i made no other changes. i will be rechecking tach, and timing next outing. hopefully Wednesday.

 

[TilliamWe]

Re: last years prop was spinning 4600rpm now 6000????

Re: last years prop was spinning 4600rpm now 6000????

It certainly sounds like it had an excessive amount of bottom paint on it. And if that paint had absorbed water, it could have added a lot of weight. And could have created extra drag. I guess anything is possible.
The hub of the propeller isn't going bad and slipping is it?

 

[nailem]

Re: last years prop was spinning 4600rpm now 6000????

Re: last years prop was spinning 4600rpm now 6000????

im not sure, it was a brand new prop and worked fine till end of season. i will check but im not sure why it would be.

 

[Tacklewasher]

Re: last years prop was spinning 4600rpm now 6000????

Re: last years prop was spinning 4600rpm now 6000????

Don't trust the factory speedo.

Beg, borrow or buy (don't steal) a gps for speed.

To check if your prop is spun, make a chalk mark on the prop where the circle part of the cotter pin is. Go boating. When done, check if the chalk mark is still in the same place in relation to the cotter pin. If the mark has moved, you need your hub fixed.

 

[nailem]

Re: last years prop was spinning 4600rpm now 6000????

Re: last years prop was spinning 4600rpm now 6000????

took the boat out tonight and checked tach with reflective tape on harmonic balancer and digital tach. it read wot 4300rpm when the boat tach read 6000rpm at idle boat tach read 1200 digital read 800 so i guess i will move my question to the mercruiser forum but what would make my tach go off from last year? i will check the timing next time other than that im not sure.

also i guess im back to where i was last year. boat is all tuned up new oil, plugs and wires, cap and rotor, clean fuel filter, checked compression and although i dont remember my readings i know they were all within spec.

i am happy with the 14.75x15p 4 blade but why do i need such a small prop and still only hit 4300rpm? is my method of checking rpm flawed? it is a digital tach that is used to check fan and shiv speed for HVAC units. reflective tape and the unit shoots out a beam that is bounces off the tape. it is a good meter but maybe im using it wrong for motor rpm? i have never weighed the boat maybe water logged?

 

[nailem]

Re: last years prop was spinning 4600rpm now 6000???? updated

forgot to mention still reads 36mph, i need to get a gps!!!

 

[TilliamWe]

Re: last years prop was spinning 4600rpm now 6000????

Re: last years prop was spinning 4600rpm now 6000????

I don't hate to say it, but "I told you so".

On the back of your tach is a little turnable switch that will have markings like "4P" "6P" and something else. This allows the tach to be used in a boat with a 4 cyl, 6cyl, 8 cyl. engine. Trying turning that thing back and forth a few times. Or tap on the front of the guage, that also worked on mine!

 

[nailem]

Re: last years prop was spinning 4600rpm now 6000???? updated

will work on the tach but why do you think i need such a small prop when i should be able to run a 19?