What prop do I need....

[nitrosteve]

1. 1998 180 Monterey
2. 18ft,7ft 9 beam, dry weight 2100lbs
2a.190hp
3. 3 people, 20 gallons
4. Mainly cruising at 30 mph or faster.
5. I wouldnt consider it a deep-v but I don't know the dead rise
6. 1987 3.0 140hp Mercruiser I/O wrong year I know but it fit.....
7. 140hp Alpha one gen 2 2.0 ratio
8. 4400-4800
9. Anti-ventilation Plate height above keel of boat if it is an outboard in inches
10. ?
11. no
12. 13.75 x 19 Aluminum not sure the maker
13. 4950 rpm 42.1 mph GPS 3 people 10 gallons
14. Sea Level
15. Motor is in perfect working order.
16. Prop has been on boat for 6 months
17.would like higher cruising speed at less rpm. I would also like to lower the wot rpms into the correct range. It has very little damage mostly cosmetic.
18. See 17
19. Hmmm not sure I will get a chance to do this. I can tell you 30 mph with 3 in the boat 10 gallons of fuel is about 3700-3800 rpm. 35 is right at 4200rpm.

On a side note I have a 14x17 vengence SS prop that seems to take alot more positive trim and raise the bow. My current prop planes out well but you cannot trim it up near as much without it washing out. Not sure why this is. Thanks for your help..
__________________

 

[Tail_Gunner]

Re: What prop do I need....

1. 1998 180 Monterey
2. 18ft,7ft 9 beam, dry weight 2100lbs
2a.190hp
3. 3 people, 20 gallons
4. Mainly cruising at 30 mph or faster.
5. I wouldnt consider it a deep-v but I don't know the dead rise
6. 1987 3.0 140hp Mercruiser I/O wrong year I know but it fit.....
7. 140hp Alpha one gen 2 2.0 ratio
8. 4400-4800
9. Anti-ventilation Plate height above keel of boat if it is an outboard in inches
10. ?
11. no
12. 13.75 x 19 Aluminum not sure the maker
13. 4950 rpm 42.1 mph GPS 3 people 10 gallons
14. Sea Level
15. Motor is in perfect working order.
16. Prop has been on boat for 6 months
17.would like higher cruising speed at less rpm. I would also like to lower the wot rpms into the correct range. It has very little damage mostly cosmetic.
18. See 17
19. Hmmm not sure I will get a chance to do this. I can tell you 30 mph with 3 in the boat 10 gallons of fuel is about 3700-3800 rpm. 35 is right at 4200rpm.

On a side note I have a 14x17 vengence SS prop that seems to take alot more positive trim and raise the bow. My current prop planes out well but you cannot trim it up near as much without it washing out. Not sure why this is. Thanks for your help..
__________________

3 people, 20 gallons
13.75 x 19 Aluminum
4950 rpm 42.1 mph GPS 3 people 10 gallons
140hp Alpha one gen 2 2.0 ratio

Very good numbers....

On a side note I have a 14x17 vengence SS prop that seems to take alot more positive trim and raise the bow. My current prop planes out well but you cannot trim it up near as much without it washing out. Not sure why this is. Thanks for your help..

That makes perfect sense the SS material allows for complex curves or shapes that alum cannot duplicate..Hence more grip or bit..Like a very sharp It cuts better than a dull one..In this case a ss sharpens far better than alum

a 18 or maybe a 19p 4 blade might be the answer..On a 3.0 4950 is just a tad high rpm level..But thats a personal opinion 4800 would be a more comfortable spot...the 19 should win the day. Solas is a great brand of prop

The case for a 18 would be a improved hole shot..Your really splitting hairs here your current numbers are very good. But the four blades will lower plaining speeds.

 

[nitrosteve]

Re: What prop do I need....

Yeah the current prop is a real screamer. But I just don't like that 4950rpm.

So you wouldn't recommend going to a 21 3 blade and maybe squeezing a few more mph out of it?

What diam. on the 19 4 blade?

Would the 4 blade remove some of the torque steer it has at WOT. It seems like it's pulling one direction until I get it fully trim/planned out then it seems neutral.

Make sense about the ss prop it is a lot more complex looking.

Thanks for your help.

 

[extreme76]

Re: What prop do I need....

If your goal is lower overall rpms, and higher cruising speed at moderate rpms then I agree with Tailgunner on a 19 pitch 4 blade. You could accomplish it also with a 3 blade 21 but I think you will like the 4 blade better. The 3 blade will likely be faster at WOT, but I bet it isn't at cruising rpms. The reason for this is the 4 blade will have more stern lift (particularly at lower rpms), thus reducing the surface of your hull cutting through the water which makes up for any additional drag of the 4th blade. Everything is an educated guess, but the 4 blade will have a better hole shot and will be smoother throughout the range and I would try it first.

 

[NHGuy]

Re: What prop do I need....

"would like higher cruising speed at less rpm. I would also like to lower the wot rpms into the correct range."
All you will get with a 4 blade 19 is better bite for takeoffs and heavily loaded cruising. It will only drop your rpms a touch, and will cruise at very slightly lower rpms. If you want to turn fewer rpms you need a taller pitch prop. Using the 200 rpms per inch formula a 20" prop should theoretically bring your WOT rpms down to 4750 and a 21 will bring them to approximately 4550. You should see a little more top speed too.
The other factor is you will lose some holeshot with the taller prop so keep your 19 and have it fixed for watersports or full boat days.
I'm not a big proponent of 4 bladed props on light hulls such as yours. The main benefit for you is stern lift and lower speed planing, but as I understand it 19" is 19" of pitch.
Who can tell us what is the slip difference between equivalent props of 3 and 4 blades?

 

[nitrosteve]

Re: What prop do I need....

Good point does the more blades equal less slip?
Also does more pitch equal less slip?

 

[Tail_Gunner]

Re: What prop do I need....

A 4blade simply due to the greater surface area will be more mech effecient..with one side note. There will be a point where the increased suface area will start creating more drag than it pulls..Well i have never measured this the general speed is thought to be about 30 mph.

Hence 4 blades for hole shot and midrange
3 blades for top end speed.

For myself i call performance characteristics....guess where 90% of your boating occurs..

 

[nitrosteve]

Re: What prop do I need....

Lets hope its not at top speed..