3.0L prop help... Again

[kart74]

I have a 1983 Sea Sprite 780 XL 17' boat with a 3.0L mercruiser and a Alpha 1 Gen 1 drive. When I got the boat it had a damaged but useable 15x15 3 blade prop on it. Since it was damaged I came on here looking for advise on what prop to get. Unfortunately the tach was off when I was giving my initial numbers. I have got through the engine to make sure everything was ok (carb, compression, ignition) and got everything to spec, or replaced. I ended up getting a 14.75x17 4 blade solas prop from iboats. I also got a new tach. With 2 adults and 2 small kids in the boat pulling nothing I was getting 4200-4300 rpm and 35mph. The take off seemed a little slow but not too bad. Well over the last week we went on vacation and used the boat a lot. Tubing wasn't an issue, but pulling up a skier was. Pulling up a skier made the take off very slow, and if the skier was inexperienced or not very strong they couldn't get up. I skied behind it and got up just fine, but it didn't just pull me up. The last day pulling a tube I had to slow down fast and unfortunately we were over a sand bar. Well under the sand was something hard which messed up the new prop some. 2 of the blades have the tips bent over a bit. I think I can get it straightened out to use it still, but I am wondering if this still isn't the correct prop since the low end take offs were so bad while pulling. Any thoughts or help would be great. Thanks

 

[MikDee]

IMO, A prop bigger then 14" is a bit much for a 17' boat, especially for a 4 blade. a 14"x 17" 3 blade, or 14"x 16" 4blade should do the trick?

 

[steelespike]

Very typically a 3.0 in a 17 ft boat would use a 17" pitch for water sports with a boat load of people and have to watch the rpm lightly loaded.
A 21" pitch would be for average cruising and produce close to 40 mph. but be sensitive to added weight and a struggle for water sports.
If you are sure the motor is healthy and tuned right,timing advancing as it should you need to see what the gear ratio is
and see if there is water in the foam under the floor.My info indicates 1.98 ratio.

 

[Starcraft5834]

I run a 14.75 × 15 Solas in my 3.0. Its a 20ft cuddy and heavy. The 15 gives me punch i need.. i get 4500 rpms 31mph... consider 15. Should help get your rpms in right range and lots of torque

 

[Starcraft5834]

Forgot to say its a 4 blade too.

 

[jestor68]

Has your 1983 motor been rebuilt? If not it has likely lost some of it's get up and go.

The motor is talking to you and telling you what it needs.

It needs to go up to 4600 rpm with a 15 pitch. If you intend to do water sports, you need to prop it to get right to the top of the rpm range when not pulling something.

4300 rpm isn't enough, since that's right at the bottom of the rpm range, with nothing left.

 

[kart74]

that makes sense. The motor has not been fully rebuilt. The head was gone through and I have 135-145 on all cylinders for compression, but I am sure it is a little tired. So it sounds like a 15 pitch is the right direction. Do I go with a 3 or 4 blade? What about diameter? I see there is a 15" and a 15 4/5" in the 3 blades and the 4 blade is 14 3/4"

 

[steelespike]

A 15" 4 blade should help hole shot and handling.

 

[jestor68]

It sounds like the 15 X 15 three blade prop was the correct one to begin with.

 

[kart74]

Well since the last time I posted I changed over from points to an electronic ignition, and it made a huge difference. Now I am over reving the 15 pitch 4 blade (5100rpm) and with the 17 pitch 4 blade I am getting 4600 rpm and 36.5mph! unfortunately the 17 pitch hit a log or something and got banged up. I straightened it out and it works fine (produced the numbers above damaged), but think I should get another new one and use this as a spare. So my question is if there is much difference between brands or models of props? Both of these are solas amitas4 props. Would it make any difference to go to a rubex or a mercury spitfire? I am looking to stay with aluminum and don't want to spend a ton, but if there is a better performing prop for a little more I would do that. Thanks!

 

[jestor68]

Another Solas would be the most cost effective.

The Amita and Rubex are the same prop, except for the hub type.

 

[jestor68]

And, your numbers are still off.

Before the ignition upgrade, your 4300 rpm @ 35 mph equaled no slip.

Your present 4600 rpm @ 36.5 mph equals only 1% slip.

A more likely 10% slip would yield 4920 rpm @ 36.5 mph.

 

[jimbo_jwc]

Check on my last post for prop sizes this is for 1987 3.0 that had 40hours on motor when I bought ,it close to new engine performance

 

[Chris1956]

Kart, I find it hard to believe that a change over to electronic ignition would make that large of a difference. Maybe the new ignition gives a false reading on the tach? Tachs on I/O motors are driven off the ignition system.

 

[jimbo_jwc]

Concure with Chris on tach as my boat was doing 50 mph till I found out pick up hose broke stiff at bend and would comeoff at full speed and not measure thru metered orfice . I still haven't tried 2 of newly aquired props with GPS and 1 person w/light load less fuel but 42mph at 4600rpm is fast to me . No one has ever said anything about overreving a 3.0 and dammaging it though .

 

[steelespike]

If it pulls a skier now Therte isn't an awful lot more to be had with al props.
I've seen a couple of independent Spitfire tests that looked good. You might try a search and see what you think.
The spitfire might let you go up an inch in pitch and maintain 4600, or another way go the same pitch and it might give you 100 or 200 rpm.
Anyway you look at it there isn't a lot there.
The Rubex is the the same as Amita but with a removable hub.

 

[kart74]

The speeds were taken from GPS on my phone. The tach is brand new as well so it seems like that shouldn't have an issue. The 36.5mph run was with a good wind to our back and perfect flat water. It only happened once and after I was 34-35 was all she would do.

 

[steelespike]

I wonder if the motor was on 3 cylinders and the electronic ignition got all 4 going.
Motor is still suspect or perhaps the hull has issues.
A lightly loaded 17 footer with a 3.0 and a 21" prop should run around 4800 and close to 40 mph.
Probably be sensitive to added weight and be sluggish in water sports.
If a 17 gets it done for you just be aware there is still some sort of underlying problem.