21 foot very slow hole shot, prop to big?

[soaringhiggy]

Ok, first time here, and I am sure this has been done a million times, but I could use some help.

New to me 87 sea ray sorrento 21 foot s21
5.7 liter 260 hp
alpha one with a 1.5:1 ratio
al prop 21 pitch 4878122 A40L 21P (dia 13.75")
centerline 20'8"
beam 96"
dry wt 2820
gal full 40
WOT is listed as 4200 to 4800 RPM

Other info:
I live at 4300 feet
2 adults (well by years anyway) 2 kids

Tech info as I know it:
given approx 3% hp loss per 1000 feet 12% loss (approx 31 hp) 260-31=229 hp

As far as I know the carb has not been rejetted

I am getting a WOT of 3000 rpm, engine is running well.

Once the hour that seems to be needed is over and the boat is on plane pulling a tube with 2 people I can throttle back to 2500 and run there with no problem.

Prop is to large.

ok if my thoughs are correct each pitch give 300 to 400, (lets call it 350 for ease) rpm reduction or increase, depending on which way we go, therefore if I go to a 19 pich prop it should give approximate 1050 rpm which would give a wot of 4050, still a bit low but may be livable.

Should I go as small as an 18? seems to me that top end would be much reduced at that point.

back calculating from here with the reduction ratio if 1.5:1 my prop is turning somewhere between 800 and 1100 rpm less than it is designed to at WOT, no wonder it wont come out of the hole.

Suggestions??

What would the advantave of a stainless vs al prop be?

3 blade vs 4?

If I am off the mark here tell me so I can learn!

Thanks

 

[junior1113]

Re: 21 foot very slow hole shot, prop to big?

has this been run at or close to sea level? 4i would look into rejetting, going to a 19 will only gain like 300-350 and 17 will gain about 700 a 15 would get about 1000rpms. 4 blades are good but all you need is to be hitting rec limits and hole shot will follow. dont really have any expierence with altitudes here in florida good luck

 

[TilliamWe]

Re: 21 foot very slow hole shot, prop to big?

Confirm your tachometer. I find it hard to believe it can get on plane while only turning 3000rpm.

And you need to be propped to run at your WOT RPM range, or you will damage the intake and exhaust valve, due to "lugging" the engine.

 

[soaringhiggy]

Re: 21 foot very slow hole shot, prop to big?

tach is accurate, confirmed with an optical tach.

when out today while getting on plane only 3K, once on plane I was able to get 33-3400.

compression are all right on, and engine runs very smooth.

The answer to the above post is that this boat was set up for lake washington in the Sattle area and then lived in Couer D'Alene Id which is about 2000 to 2500 feet elevation.

I can not here any pre ignition, but that does not mean that it is not occuring.

 

[junior1113]

Re: 21 foot very slow hole shot, prop to big?

i'd just buy a 15 pitch and get your 800-1000 rpms and see what happens? extra props are always a good thing.

 

[soaringhiggy]

Re: 21 foot very slow hole shot, prop to big?

put a 15" dia 17 pitch on yesterday, I will try it out this afternoon and post the results, I hope it does what I need.

I understand there is quite a difference in the prop cord ratio when droping to the 15 and don't want to go there if I do not need to.

If this does not do the trick I will put in a high altitude gear set in the leg.

 

[junior1113]

Re: 21 foot very slow hole shot, prop to big?

i would'nt run that motor with any thing less than 4200rpms. or rebuild is in near future$$$$$$$$$$$$$. good luck.

 

[soaringhiggy]

Re: 21 foot very slow hole shot, prop to big?

I have not run the boat yet, can not tell you what the rpm will be, but all of the calcualtions show that it should be above 4200 rmp.

 

[soaringhiggy]

Re: 21 foot very slow hole shot, prop to big?

Well the results are in!

Ran the boat today,

15" dia 17 pitch prop.

on plane in 3 to 4 seconds and 50 or so feet.

Top speed 44 miles per hour
5000 RPM.

Most likely could have done a 19 prop but the hole shot would not have been as good. Pulls like a train.

Now all I have to do is put on the Bennet trim tabs.

Will order a stainless blade 17 for skiing this winter, the 3 blade al prop will be fine for this season as the kids are only tubing at this point.

YES! Nailed it.

 

[NoKlu]

Re: 21 foot very slow hole shot, prop to big?

If the wot is between 42-4800 rpm and you are turning 5000 it's still not right. 5000 is over revving your motor. Yea it sounds picky but props are cheaper than motors.

 

[soaringhiggy]

Re: 21 foot very slow hole shot, prop to big?

that is true, however, I do not have to run it that hard. To get to 5k, I had to trim finely, on most hole shots and running up to WOT I never exceeded 4800 which is with in tollerances. 200 rpm is with in 4% of the expected range of 4200 to 4800 rpm, a very small precentage. I will give you that it is over, but not by much.

I also can say that coming out of the hole the rpm is about 4400 and builds to the final high rpm, very easy to control the over rev situation.

As far as jetting, this carb is listed as good to 6k feet in elevation, if the carb needs to be jetted there are several indicators listed by merc, I have not observed any of the symptoms that tell me the carb needs to be rejetted for this valley. I am going to Red Fish lake above Sun Valley this weekend, evevation is 6400 feet. Will let you know how it runs there.

At the end of the day you are correct, it is over the specified range.

 

[NoKlu]

Re: 21 foot very slow hole shot, prop to big?

I only stated that to be on the safe side. Running on the top end will shorten the life of the motor just as lugging will. Picking a rpm in the middle of the 42-48 range will get you better engine life and you have a little room on top for things like tach accuracy, sharp corners, and another person driving your boat that will not be watching for revs. Glad to hear it made the boat jump out and pull hard. Have fun with it.

 

[junior1113]

Re: 21 foot very slow hole shot, prop to big?

a 21p ran 3000 rpm and a 17 ran 5000 that's crazy. 2000 rpm difference? never heard of that, 800 would be alot. thinking motor is runing better now for some reason. try the 21 again for the hell of it just to see and post back really interested thanks

 

[soaringhiggy]

Re: 21 foot very slow hole shot, prop to big?

No, It went from 32 or 33 WOT on plane to 5k when finely plained.

Once the engine hit the torque range it really made a difference. The hull was able to get fully on step. If the rule of thumb of 300 per inch is used 1200 rpm is expected, and I got a bit more than that, but I think that it was just the ability to get fully on step. If I could have been fully on step with the 21 it may have gone higher the first time.

I will try it for you.

 

[junior1113]

Re: 21 foot very slow hole shot, prop to big?

you got it wrong i've had 12 boats. 2" in pitch change will result in 300-350 rpm change so 17 to 21 is about 600-700. not 2000 rpms some things up.
if a 17 spins you 5000 a 21 should have spun you 4200 4300 not 3000. just want you to under stand why this sounds weird to me. maybee tack just started working correctly, or a bad cylinder came out to play. i have never heard of 2000 rpm change with 4" pitch change.for your info a 1" pitch change only results in 150-175 wot rpm change. dont know who told you 1" pitch would change 350 rpm but that is wrong and anyone here will agree.
honestly i think tach was stuck and would'nt go over 3000. i would strongly recommend a new tach or a tiny tach to conform readings. just trying to help.

17p@5000=19p@4650=21@4300 these are realistic #'s
17p@5000=21p@3000 this is a joke, just cant be, ill bet my life

if 5000rpm is real than a 17 pitch 4 blade would be most beneficial to put you at about 4600 and still have good hole shots

 

[dazedandconfused]

Re: 21 foot very slow hole shot, prop to big?

This was an ineresteing read. Learned a few things in this one.

 

[soaringhiggy]

Re: 21 foot very slow hole shot, prop to big?

Junior,

I have though about how to respond to your post, I hope that our conversation continues, as it is interesting and informational.

I have only been on this site for a week and I do not know people here yet. I would like to maintain open discourse on this and other topics with you and all others.

I do lack in experience in selecting propellers for such small boat, that is a given, and I admit it freely, that is why I am here.

You state "2' in pitch change will result in 300-350 rpm change so 17 to 21 is about 600-700." and "for your info a 1" pitch change only results in 150-175 wot rpm change. don't know who told you 1' pitch would change 350 rpm but that is wrong and anyone here will agree"

I got the pitch change information directly out of the Mercruiser manual; Alpha One Power Package 200.230.260 publication 90-42234--6 586 Page 32 "If engine speed is too low, replace with a lower pitch propeller. Normally, a 300 to 400 RPM change exists between propeller pitches.?

You also state "17p@5000=21p@3000 this is a joke, just cant be, ill bet my life"

I recommend that you don't make this bet. As I started with 33 or 3400 rpm to 5000 is a difference of 1600 to 1700 rpm. Using Mercs instrutions 400 rpm with 4 pitch changes (17 to 21) we can expect the following: 1200 rpm change @ 300/pitch, and, as much as 1600 rpm change @ 400 per pitch with an average change of 1400 rpm.

The change that I saw we 1600 rmp, almost exactly what Merc had prodicted. I freely admit the 5000 rpm was after much trimming and slight chop on the water and was perfict for max rpm. I could get 4800 repeatedly. I expected the pitch change to give me 4600 (the middle of the expected pitch changes with a 1400 rpm increase)and was pleasently suprised when I got 4800.

I do not know where your suggestion for a 14" prop came from, was it a program, calculation or other method?

I spoke with engineers from both Mercruiser and Michigan Wheel and they agreed with my approach and calculations, so I tried it. I posted the results that I obtained from my small sea trial.

If you do not think that the results are correct I will invite you into my home, we will eat some ribs, have a soda and go to the lake and I can recreate the conditions with both propellers and you can verify if I am correct or not.

As I stated before I calibrated the tach with an optical tach and verified the accuracy of the instrument.

You also state"you got it wrong I've had 12 boats", I am glad that you are willing to share and I am sure that you know much about boats and running them as well, I also have a small amount of maritime and other mechanical experience.

I do not intend this post to be confrontational, and I look forward to talking with you in the future.

SH

 

[junior1113]

Re: 21 foot very slow hole shot, prop to big?

merc is refering to 1 size pitch change meaning 21-19 = 400 or 19 to 17 =400 not 21-20 =400 and 20-19 =400 now i see where you put 3300- to 3400 with the 21. i was going by original post where you stated 3000 rpm wide open. so you got 1600 rpms by changing from 21 to 17. all im saying is these are what the results would be of 21 to 13 pitch. also cant see you backing off to 2500 while on plane with the 21. all im saying is, tack must have been sticking or motor came to life with the 17. i'm not telling you your wrong i beleive everything your saying. but i believe you were running like 4000 with the 21 but tach was messing up. glad you got it right though thats all that matters. just dont want you to believe 21-17 = 1600 rpms. about those ribs where do you live? kiding lol

 

[hwsiii]

Re: 21 foot very slow hole shot, prop to big?

Gentleman, there is a mathematical formula for pitch change within any given model of prop, and that is OLD RPM x (Old Pitch/New Pitch) = NEW RPM. So that means a 21" Pitch prop at 3,400 RPM x (21/17)1.235 = 4,199 New RPM and that is a change of 800 RPM for 4 inches of pitch change, which is 200 RPM per inch of pitch change. So Junior is right Higgy, you misunderstood what the book meant when it stated that there is about a 350 to 400 RPM in each pitch change what they meant was that their normal pitch change is 2" of pitch between each prop NOT 1", as 2" is the normal change of pitches between props.
When you get into the lower pitches like a change between a 10" prop to a 6" prop there is a very dramatic difference. 3,400 RPM x (10/6)1.67 = 5,666 New RPM, that is a change 1,133 RPM per every 2" of pitch. So the old tale of about 350 to 400 RPM change for every 2" of pitch change only works on props in the area of 20" pitch.

Prop Pitch Change 21"

Prop Pitch Change 10"

Higgy, I can't explain why you had such a dramatic difference between the two props unless you have a pad on the bottom of that boat or the blade geometry between the two props is dramatically different. I have a printout of a friend of mines trial of three props a 17" a 16" and an 18" pitch. The 17" is a 13" x17" and the 16" is a 13 1/4 x 16" pitch, the 17" is able to turn 5,750 RPM and the 16" is only able to turn 5,050 RPM. The difference is in the blade geometry not the diameter, and they are both stainless steel.

Larry's Prop Test

H

 

[junior1113]

Re: 21 foot very slow hole shot, prop to big?

thanks for the formula h learn something new every day. did'nt realize the diifernce in those small pitches. everything i deal with is in the 17-23 range .

ss props can change rpm that dramatic i know from expierence from standard to aggresive double cuped in same pitch can result in 300+m diffe

ive seen 1000 rpm change when going 21 ss with cup to 17 alluminum

but these are both allunimum props so geometry is out of the question.

1600 is impossible