Prop Recommendation

[blorr]

Hey, sorry for the double post. I would love your thoughts on what prop to get. I got my motor problems fixed so sorry about the earlier post.

Here is all of the correct info on my boat.

1993 Dynasty
18 Ft.
4.3 Mercruiser
175 HP
Alpha 1 outdrive
RPM WOT range is 4400 - 4800
1775 Dry weight

Had a 15X19 Solas Stainless steel prop.
Would only get 4100 at WOT
Top speed was only 37 mph with one person in boat.

My primary purpose is slalom skiing. I would like to get decent hole shot with 3 others in the boat and be able to pull a skier at 35mph.

Thanks for any help!
Brent

 

[blorr]

Re: Prop Recommendation

Here are my more detailed answers

1. Year, make and model of boat
1993 Dynasty 18ft.

2. Length, width and base weight of boat, look for boat decal on back of boat
18 ft long
7? 1? wide
1775 base weight

3. Number of people and gallons of gas normally on boat
4 people
14 gallons

4. What do you use the boat for
Slalom Skiing

5. Is it a Deep Vee and if so how many degrees of deadrise
I don?t think so

6. Year, make. manufacturer and model of motor
1993 Mercruiser
4.3 Liter V6

7. HP and gear ratio of motor IMPORTANT
175 HP

8. Manufacturer?s recommended Wide Open Throttle (WOT) range
4400 ? 4800

10. Is it a bass boat or does it have a pad bottom
NO

11. Does it have a hydrafoil, dolefin or trim tabs
Has hydrafoil, but I can take it off it that would be better.

12. Make, model, diameter, pitch, number of blades and whether SS or aluminum prop
15X19 Solas Stainless 3 blade

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.
WOT Rpm 4150
37mph by GPS
1 person on the boat with 20 gallons of gas.

14. Are you at sea level or a higher elevation, give us the elevation in feet
1,000 ft.

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
Has not been tuned up lately

16. How long has this prop been on the boat and why, at this time, do you think it is the wrong prop
Been on one week because my other one was busted up.

17. Does the prop show any damage that you can see
No, brand new

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
Overall it feels as if it underperforms on hole shot and top end speed.

 

[Bondo]

Re: Prop Recommendation

Hey, sorry for the double post.

Ayuh,... Hey, I posted on your Other thread...

 

[hwsiii]

Re: Prop Recommendation

Blorr, after running my numbers I find that your boat should be able to do about 46 MPH by two different speed analysis I ran, and sine you have an I/O we can't change the height of the outdrive. I found your motor's reommended max RPM to be 4,800 RPM and your gear ratio to be 1.84:1, please verify this.
You are only at 1,000' elevation so we don't have much loss of HP from that, the hydrafoil could very easily be holding your speed down, so lets take that off until we have the motor working right and see what happens. I only show a 9% Prop Slip at 37 MPH, which is good with a SS prop. It appears that the motor is NOT producing the HP it should and that is why we are not attaining the WOT speed we should. Please check the following:

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 and timing advance, as all of the foregoing could be the reason your prop is not attaining full RPM

Boat and Motor

Prop Picker

Prop Slip

H

 

[blorr]

Re: Prop Recommendation

H,
Thanks so much for your help!!!

Can you run that same analysis with a 15 3/4 X 15 SS prop?

Thanks,
Brent

 

[hwsiii]

Re: Prop Recommendation

Blorr, in my opinion you are making an error in not fixing the problem and instead just changing prop pitch, as this problem might get very expensive in the future if you don't do anything now.
Having said that, if you just want to change prop pitch to get your RPM close to where they ought to be, I would recommend a 16" or 17" pitch prop in the same make and model of prop you have now. If you use a 15" pitch prop your RPM will increase to about 5,200 RPM. and that is way too high.


Prop Pitch change

H