3rd prop trial on 3.0 merc

[DXN]

I have an 1986 Glastron CX-19 with 3.0 Merc
I regularly carry 3 adults and 3 kids
want a good balance for watersports and I want to cruse around 40 - 35 when loaded
Have a hydrofoil

I started with a 21' stainless with a good amount of cupping. 45mph at 4700(with just me) I was very happy with this prop but its gone and I don't have the numbers.

It was a little bent and the guy said it was not worth fixing. too much prop and it was an old school design. He said Id be so much happier with a 19' (don't know the diameter but it just cleared the trim tab). It came out of the hole good but at 5 grand all I could get was 35mph.

Went to a 21' aluminum from merc again don' know the diameter I will get the #'s later but I took a flapwheel to the trim tab so it would clear. It would have cleared fine but I guess it has to much rake. It to got banged up. This Prop ran fare A bit slower out of the hole than the original, But I don't want to shell out the $ for a good stainless. 43 at 4500(3 adults $ 3 kids). May just replace this one but I'm worried It won't pull good

Now I'm thinking 4 blades are sounding good, Its the low to mid-range performance I'm after. Also hear it helps with bow-steer, and economy. Should I go with a 4-Blade 14 1/4 X 20' or 14 1/4 X 19'

Am I asking to much from this old 3.0. I'm not willing to drop my top cruse below 35 when fully loaded. but I don't want to hear it scream either

 

[steelespike]

Re: 3rd prop trial on 3.0 merc

I have a feeling your speedo is a bit optimistic. 45 is almost unheard of in a 3.0 with a 19 footer.
I'd like to see a gps speed if possible.Your numbers with the 21" ss prop are exceptional.
More to come.
It's hard to believe a ss prop wouldn't be worth fixing unless its just totally trashed.
According to your rpm the ss was a close match. at 4700 rpm.
and 40 something mph.
I think typical speeds are about 40-43 Your ratio may be something like 1.89 instead of the newer 2.00 The 1.89 would give a sliver more speed than the 2.00 but the 2.00 would have a sliver more hole shot.Your ratio may be on the outdrive somewhere.
I think the 19" 4 blade is a good bet.Typically a 4 blade will have good hole shot,handle better around the dock,stay on plane easier at reduced throttle and feel smoother.

 

[DXN]

Re: 3rd prop trial on 3.0 merc

I was actually debating with a friend whether it felt right, I wont be able to check it till I get another prop

What should I be expecting.

 

[DXN]

Re: 3rd prop trial on 3.0 merc

hey just found a very helpful thread
http://forums.iboats.com/boat-topic...e-topics/3-0-mercruiser-top-speed-411906.html

the spedo is probably a little off, looks like the most people are seeing is 43 with a similar setup and a 21'

 

[DXN]

Re: 3rd prop trial on 3.0 merc

I have a feeling your speedo is a bit optimistic. 45 is almost unheard of in a 3.0 with a 19 footer.
I'd like to see a gps speed if possible.Your numbers with the 21" ss prop are exceptional.
More to come.
It's hard to believe a ss prop wouldn't be worth fixing unless its just totally trashed.
According to your rpm the ss was a close match. at 4700 rpm.
and 40 something mph.
I think typical speeds are about 40-43 Your ratio may be something like 1.89 instead of the newer 2.00 The 1.89 would give a sliver more speed than the 2.00 but the 2.00 would have a sliver more hole shot.Your ratio may be on the outdrive somewhere.
I think the 19" 4 blade is a good bet.Typically a 4 blade will have good hole shot,handle better around the dock,stay on plane easier at reduced throttle and feel smoother.

I think my ratio is 1.94, not 100% sure though. I'll try the 19" 4 blade. calculated speed at 4500 would be 41.7 With all the advantages you listed I think I will be Very happy with it

 

[jestor68]

Re: 3rd prop trial on 3.0 merc

With your 1.98 gear ratio, if you're turning 4700 with a 3 blade 21, dropping to a 19 4 blade will surely produce excellent acceleration; but will likely result in over speed ( 4800 max).

A 20 pitch 4 blade should run about the same rpm as the the 3 blade 21, while still producing better hole shot.