Prop questions

[arw2009]

Hi, Im replacing a bad prop and wondered if one of you may be able to give me some advice. I have a Thompson boat 21 foot with the mercruiser v6 4.3 . At top speed i was getting about 35 Mph, with a 14 1/2, 17 pitch prop. Im wating to get a little more mid range and top speed, better fuel Mpg. What would you recommend i was thinking a 19? the 17 had alot of low power but that was about it.. And what kind of top speed difference would a change make to around a 19 pitch?

thanks!

 

[sho3boater]

Re: Prop questions

It all depends on the rpm range of the motor. You need to be in that range, if the larger prop runs the engine too slow you go slower. You can go to the Mercury site (assuming that is what it is) and use the prop selector for it. If the old prop was for towing/ski and rpm were high at top speed, then you should be good with more pitch like you suggest. Any damage to your old prop will shave off speed, sometimes one little dent takes 5mph off. SS props are faster/more efficient usually, naturally more coin too. The last one I got was used from a local dealer and a great deal, so look around if your budget dictates.

You can also do the old tricks; taking any weight out of the boat, moving weight to the stern, synthetic lube, etc.

Find out your gear ratio and rpm range, then plug that into a prop calculator. You can change props and see what speed and rpm changes it will give you.

 

[hwsiii]

Re: Prop questions

Arw, 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 across 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
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

 

[kick bass]

Re: Prop questions

Look at my thread Prop selection (it's not majic)