I/O Sterndrive Performance (Seacraft 23') Mercruiser 5.7

[Hooligan]

Gentlemen: I am looking for some numbers for the performance of my I/O with 5.7 Mercruiser sterndrive. Motor is Carbed Mercarb 2 barrel. Factory target WOT is 4200-4600.

Currently I am overpropped (new one on way) and am only turning 4000 RPM at WOT turning a 15 1/4x17 Black max prop. Leg is Alpha one gen II 1.47. New Prop is 15x15. Mercury claims 200 RPM drop per inch of pitch.

Boat now does 34.5 knots at WOT. Boat is light, has no top and no real gear onboard except safety gear. What kind of numbers should I be seing out of this setup? Top speed of your setup and what motor/ drive/ efi or carbed (2 or 4 barrel)? Do you know what your prop dimensions are?

I am cruising at 3000 RPM and moving at about 28 knots which seems a good cruise speed though im sure the new lesser pitched prop will change things.

Please advise, I did a search and turned up some miscellaneous info on props but none right on target in terms of I/O performance on multiple boats.

Thanks in advance!

Brendan.

[bigeasy1]

Brendan,this may be of some help to you.I have a 1974 23 footer(Tsunami/Sceptre).I also have the same engine as you only with a four barrel carb,but the hp is pretty close.I'm running an Alpha one drive with 1.65 gears.
My boat is pretty heavy fully loaded with fuel,crew,tackle,arch,top downriggers, etc.It weighs in the vinity of 5600 lbs.
I got a lot of help on prop selection from Ken at Prop Gods,he was able to set me up with the right size.
Also extremely helpful was Fr.Frank from this site,and I pasted his prop size vs gear ratio explanation to the end of this reply,it should be required reading,and is great info.

Here's where I started
*23' Seacraft Tsunami
*prop size------14.5x17 alum 3 blade
*engine--------350 5.7 mercruiser 4bbl carb 260-270hp
*outdrive-------Mercruiser Alpha One
*top speed-----34 mph at 4600 rpms
*cruise speed---24 mph at 3500 rpms

Here's where I am now after help from Fr.Frank and prop gods,recommended going to a larger diameter and pitch prop. I went to a 15-1/2 x 17 Mirage stainless prop.

*New prop------15-1/2x17 Mirage ss
*top speed------39 mph at wot 4500 rpms
*cruise speed----27 mph at 3200 rpms

So, I gained 5 mph more top end with only a minor drop in rpms,and I gained 3 mph at cruise at 300 less rpms which is great.

This is as close as I'm ever going to get to the perfect prop.I don't know if this helps you any,but Prop Gods and Fr.Frank are worth talking to if you need prop help.


The following is a cut and paste from an old question on prop vs gears answered expertly by Fr.Frank of this forum


--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------your gear ratio is fine. Your prop needs changing.
Allow me to use a pickup truck as an analogy. The differential gearing is analogous to the lower unit gearing, and the tire diameter is analogous to the propeller pitch.

If I want to apply torque to the pavement more quickly I can change out the gears in my differential from the current 3.73:1 to 4.10:1 gears. If I do this, I will have better acceleration under load, higher rpms at cruise, lower axle rotation for a given rpm, and better responsiveness to power inputs.

OR, I can change my tires on the rear to a smaller diameter tire, which changes my applied drive ratio just like changing the diff gearing. Dropping from my current 265x75-16 tires to 245x65-16 tires gives me almost the exact same effective results as changing to 4.10 gears.

On the other hand, if I want to have lower rpms at cruise and a higher top speed, I can change the diff gearing to the optional 3.45:1 gears. This will reduce my acceleration, but give me lower rpms at a given speed, and probably increase my top speed.

OR, I can change my tires on the rear to a larger diameter tire, which changes my applied drive ratio just like changing the diff gearing. Increasing from my current 265x75-16 tires to 285x70-16 tires gives me almost the exact same effective results as changing to 3.45 gears.

In both cases, changing the tire size when it comes time to replace the tires costs a lot less, and produces the same results.

If your current gearset is undamaged, you can more economically change your effective power to thrust ratio by just changing the prop size.
__________________
Common Sense is learning from your mistakes. Wisdom is learning from the other guy's mistakes.

Fr. Frank says:
Jesus liked fishing, too. He even walked on water to get to the boat!

[oldfielder]

I am seeing almost identical numbers in terms of speed with the same engine drive combo. My top end is right around 40mph and my cruise is slightly slower than yours at the same rpm. Sounds like I am a bit heavier in terms of hull weight with gear.
Are you sure the max rpm range isn't 4400-4800?
I will check my prop size when I get home later.

[Hooligan]

Great info BigEasy- nice to hear some theory and also what your engine/drive/ prop setup is. I am sure that the tach is reading around 4000 at wide open so Ill focus on the prop. Seems strange as the motor doesnt have much trouble getting to its 4K. I have a 15x15 prop that just arrived UPS that I need to try. Will post results.

I guess somewhere around 40 mph is right with this combo, thank you for sharing your performance info OldFielder. If you could share your prop size Id appreciate it.