props

wroys

Recruit
Joined
Apr 19, 2010
Messages
2
what is the best prop to use on my 1980 formula thunderbird 20 ft. with a merc 260
 

hwsiii

Commander
Joined
Jan 25, 2009
Messages
2,639
Re: props

Roy, if you will fill this form out to the best of your ability, I will help you as much as I can.

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
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 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
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 sticking out to the anti ventilation plate for a reference
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, number of blades 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 digital tachometer and hour meter here: http://cgi.ebay.com/ebaymotors/Tacho...Q5fAccessories $ 35
RPM _________ Speed ______ GPS ______ No. of people ______ Gal. 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 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
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
 

wroys

Recruit
Joined
Apr 19, 2010
Messages
2
Re: props

Hwsii:

1. Formula 1980 Thunderbird
2. 20 ft length, 4000 lbs, not sure of width as there is no decal
2a. no clue as to max rec. HP
3. 4 people, 30 gallons
4. Recreation
5. Deep Vee, I think it's 24 degrees
6. 1980 merc cruiser 260, model MCM260
7. 260 HP, gear ration of 1:1
8. 4800 - 5200 RPM
9. It is an I/O
10. It is not a bass boat and what is a pad bottom.
11. Hydrafoil (whaletail)
12. There are no markings, whatsoever, on the prop which is why I am looking for the best prop for the boat. It has three blades, it is aluminum and thin.
13. 5000 RPM, 4 people, no GPS, 40 MPH (question of accuracy of speedometer), 25 gal of gas.
14. Above sea level, no idea how many feet, but we are in Syracuse NY
15. Yes.
16. Do not know how long, as we bought the boat used. I think that I should be getting more speed out of the boat.
17. No damage, just thin.
18. More top end speed.
19. That's fine, but as I said, we are in Syracuse NY and it isn't boating season yet. Hopefully soon!

Thanks for your help.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: props

Welcome,I'm in Syracuse as well ;used to boat On the St.Lawrence.
I can't be very specific but::
I don't think your gear ratio is 1:1 Not on an I/O.
May be a label on the outdrive.Did you look under the prop nut for a prop number?
A pad is a section of the bottom that helps a fast boat get up free of the water.
Typically on a bass boat.
The whale tail will help get on plane but some sort of trim tabs are a better choice to improve hole shot and ability to stay on plane.The tail can eat up some speed as well.
Prop is thin? Designed thin or worn thin?
Just a seat of the pants opinion but I think your pretty close on speed if the speedo is close to accurate.
H can get the most out of your boat but at 4,000 lbs and a deep V it may not
get too much more.
 

hwsiii

Commander
Joined
Jan 25, 2009
Messages
2,639
Re: props

wroys, Spike is definitely right as always.

I find your recommended maximum WOT RPM as 4,200 to 4,600 RPM and your gear ratio as 1.32:1, please verify this with your local mercury dealer. Also pull the rear nut off your prop pull the prop and get the diameter and pitch for us as well.

We also need you to get us some new numbers for WOT RPM and speed with a GPS.



H
 
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