Prop selection Sundowner 215

[concentric]

Hi,
Starting my second season with my 1989 Fourwinns Sundowner 215, 5.7l 260hp Cobra. I am a little confused with my top speed.
I reach 4000 rpm, and WOT is 4200-4600, top speed (GPS) is 35 knots.
Running hours is 175.
Prop is Michigan Wheel 14 1/2 * 19. Something is wrong!

Does anybody know what prop the boat was delvered with, and what top speed was expected with the new boat?

I bellive the prop should be 21 or 23 and top speed well above 40 knots
, but with bigger prop the revs wil die.

The smaller 205 hp is by Fourwinns said to do 36 knots with a 19 prop (with a 24% higher prop revs due to other gearing and WOT)

Please... any thougts on this

Best regards/
Mats, Sweden

 

[steelespike]

Re: Prop selection Sundowner 215

Yes I would say your numbers are way off probably would run a 21-23" prop and reach the rated rpm.Something is way off you should be overeving in my opinion.
I would first ask is the motor in excellent tune?Does the throttle open all the way? Is the prop in excellent condition.Is the bottom clean?What elevation do you run at?Do you trim out and does the trim work right?If this all checks out do a compressioin check.Let us know what you find.

 

[hwsiii]

Re: Prop selection Sundowner 215

Concentric, it appears at first glance that Spike is right as always. If you will fill out my form I well help you as much as possible.

You might try reading this for a better understanding of changing props for better performance, and for the major differences between aluminum props and stainless steel.

http://forums.iboats.com/showthread.php?t=369057

If you would prefer to not go to this much trouble there are many other people who will come along and gladly help you.

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 and what do you use the boat for
4. Year, make. manufacturer and model of motor and recommended Wide Open Throttle (WOT) range
5. 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 across the prop and the lower unit and then cut it between the prop and the housing then do the same thing on the flywheel and turn the motor until the tape lines up with each other on the prop, it is easier if two people do this, so one can watch the prop while the other counts the revolutions of the motor
6. Anti-ventilation Plate height above keel of boat if it is an outboard in inches, use a straight edge under the keel and sticking out to the anti ventilation plate for a reference
Motor Transom Height

7. 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.
RPM _________ Speed ______ GPS ______ No. of people ______ Gal. Gas ________
8. Make, model, diameter, pitch, number of blades and whether SS or aluminum prop and does it have a hydrafoil, dolefin or trim tabs
9. Has your motor been tuned up lately and have you checked that the carburetor butterfly is opening all the way by just pushing the throttle at the helm and not at the carburetor, checked compression, and looked at the plugs and checked spark as well as timing and advance, all of the foregoing could be the reason your prop is not attaining full RPM
10. 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 and does the prop show any damage that you can see

REMEMBER, The numbers I give you will be NO better than the information you give me. I use a computer program I designed to find the correct pitch and then knowledge to pick a Better prop design for your boat.

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

 

[concentric]

Re: Prop selection Sundowner 215

OK hwsiii here comes some more details, very thankfull for your help

1. Four Winns Sundowner 215, 1989
2. LOA 21? Beam 8?2280 lbs without engine, 3191 lbs with engine
2a.King Cobra can take 300 hp I think
3. 2 adult, 1 children, 20 gallon
4. Motor is OMC CKing Cobra 1989 5.7 l, WOT 4200-4600
5. 260 Hp, ratio 1.41
7. Current condion is 41 mph (GPS) at WOT 4000 rpm, 2 people. Tach is the originally mounted one.
8. Michigan Wheel 14 ? x 19 3-blade aluminium, slightly cupped. In good condition
9. The previous owner had a ?rebuild? of the quadrajet carb, don?t know how much he did. Fiddled with the Butterflie valves yesterday, but are not sure how they should feel, should it be dead easy to move them by hand, by grabbing the valve and turn it open?
Seems to be a platic cam with a notch on, not sure of function.
The The boat is still on land (late spring) so timing and other checks will be done in a couple of weeks.

10. My aim is to get the boat in a good running order. For now I think some power is missing. It will be used for the family to go out to a vacation island, and some watersports.

-I?m running at low altitudes
-The throttle opens up all the way, the vacuum butterflies moves at higher revs, but is still not checked if they open all the way (90?) at full speed.
-The bottom is clean, but painted with the white self cleaning bottom paint, maybe a little bit of drag there?
-To reach maximum speed, I trim out to ventilating, then back a bit, and there is the maximum speed.

 

[hwsiii]

Re: Prop selection Sundowner 215

Concentric, you seem to be as anal as I am about wanting to do everything there is in a very systematic way. LOL

Let me say that I find your boat should be able to attain 46 MPH at WOT. I also show at this time you have a prop slip of right at 20%, which is high in my opinion.

Prop Slip

My personal opinion is that your motor is NOT producing the full 260 HP that it should be, my calculations using Crouch show your engine is only producing about 210 HP.

HP Produced

With only gaining another 400 RPM you can get to 46 MPH. And from everything I read it seems like everything else is fine on the boat.




H

 

[concentric]

Re: Prop selection Sundowner 215

Thanks hwsiii for the info, and as you say i have to find more power in my motor (it should be more).

Is it possible to say anything about if i have the right prop for now or the "efficency" of my boat hull from the slip precentage.

The calculation software you are using is it a spreadsheet made by yourself?
Really impressive in that case, have you bulit up a database with all sorts of boats/hulls/motors?

Found another one (propcalc.excel) for dowloading, but dont really get the rigtht figures calculating a real life scenario. maybe missunderstood the input...

Anyway, in a couple of weeks my boat sits in water, and furter tetsing will be done.

 

[hwsiii]

Re: Prop selection Sundowner 215

Concentric, it is not verified yet, but it is not as efficient as you need, because it is not able to reach your maximum RPM, and without being able to do that I do not know what the true prop slip is because as most props turn more RPM the less prop slip they have.

I use a total of 8 different spreadsheets to find the correct pitch as well as prop slip, effective prop pitch and other analysis that I perform for any particular hull form and Horse Power, although I have a total of 12 spreadsheets for much more detailed analysis and 19 charts as well, thank god they are able to feed each other information to check and recheck my analysis. And I will tell you, I am very proud of my program, and it appears to work very well from my results on this forum. And I do have a database built up from all of the boats that I have propped on Iboats, and it is very helpful at times. It is almost 8 MB in excel.

I will just say that Propcalc is alright for giving you a range of pitches and speeds, but it definitely does not narrow it down. They actually use Crouch to try and determine the variables, and I use Crouch the way it should be used in my opinion, for weight and HP size changes.



H

 

[concentric]

Re: Prop selection Sundowner 215

hwsii, really amazing job you do. I can see my self getting absorbed in spreadsheets, but for now I listen to you, that worked it out so fine.

One more question: Is 46 mph my estimated topspeed with 19 inch prop and same slip as today, or can i dream of 50+ mph with a correct sized prop (21?) and a better slip. Of course with all 260 horses healthy.

/Mats

 

[hwsiii]

Re: Prop selection Sundowner 215

To the best of my knowledge NOBODY can or has designed a program to get the numbers just right, and that includes commercial programs on the market today that cost thousands of dollars.

I have seen many many times that just by changing props people have gained as much as 5 MPH, but I will have to say it is not the norm, and I will also say that my program is really for finding a starting point for a prop and then seeing where the prop is deficient in prop slip and effective prop pitch and other considerations, BUT the most IMPORTANT part is picking the right prop design and blade geometry.

But personally I do not believe you will ever reach 50, sorry.



H

 

[concentric]

Re: Prop selection Sundowner 215

hwsii, now I have some more correct data.
My starting point is (with correct tach reading):
14.5x19 prop Michigan whhel, 40.8 mph, 3850 rpm, 17% slip
after that i changed to 14.25x19 turningpoint hustler and get:
43 mph at 4150 rpm, slip increases to 19 %.

Good thing that i nearly reach recomended wot rpm, and gained some speed.
But slip is also increasing.

So is it possible that the heavy deep V-hull of my boat never will get a slip lower than this? or is more to gain with the "optimal" prop? And if so could there be a suggestion out there for the optimal prop, how would a fourblade or stainless prop affect the slip and speed.

PS i?m still working with my motor to get out some more ponnies but that will hardly do any major improvements on slip.