Starting point for correct prop for cabin cruiser

[256barry]

Recently purchased a 96 Rinker 266 fiesta Vee! Dry weight 5760, average cruising weight 8000lbs! Boat takes very long time to plane out and top speed is 27! After seeing videos, I feel the prop is incorrect!
New engine 5.7 L (300) WOT 4200-4600 rpm, Bravo 2 outdrive 1.50 gear ratio, current prop 15 3/4x 25x3

 

[paulswagelock]

What max rpm do you see as currently set up and what is the gps speed (not speedometer)?

 

[256barry]

Current Max rpm 4000 , have not gps speed!

 

[Texasmark]

Got a speedometer? Need these parameters to find out how you are doing:
Max engine rated RPMs....from the engine mfgr. data
Outdrive gear ratio
Current prop pitch
Actual engine operating RPMs when running at normal load, full throttle trim set to "sweet spot"....max velocity.
Boat actual speed...Big guns on here mention that speedometers are usually in error by 5-10% even if working properly......but any number is better than none at all.

Expect that the result you get is a non-scientific, WAG! Only way you really know how to max out, is to guess and run it!

You have conflicting data in expecting improvement. What you would do with prop pitch to get a better "hole shot" will cut into your top speed.....unless engine torque curve "increases" as RPMs increase (not likely) or hull to water resistance is reduced (some how) due to any increase in thrust at the higher RPMs you'd be getting with your lowering the pitch for the hole shot improvement. You can't have your cake and eat it unless either the engine HP increases, or the load it is expected to push is reduced....load being defined as anything that causes resistance to moving the hull through the water.

There is a possibility that hydraulic trim tabs mounted to the transom could help your hole shot while having no effect on top end, whereby (more guessing) you could up your pitch for more MPH yet have a decent hole shot.

 

[paulswagelock]

Well, simply based on the max rpm of 4000 you could drop to a 21 or 22 pitch prop, have a significantly better hole shot, and be at least the same top speed, assuming you engine is in good running condition. Running the calcs you should me in the low 40 mph range at wot with a 22 pitch.

 

[256barry]

Got a speedometer? Need these parameters to find out how you are doing:
Max engine rated RPMs....from the engine mfgr. data
Outdrive gear ratio
Current prop pitch
Actual engine operating RPMs when running at normal load, full throttle trim set to "sweet spot"....max velocity.
Boat actual speed...Big guns on here mention that speedometers are usually in error by 5-10% even if working properly......but any number is better than none at all.

Expect that the result you get is a non-scientific, WAG! Only way you really know how to max out, is to guess and run it!

You have conflicting data in expecting improvement. What you would do with prop pitch to get a better "hole shot" will cut into your top speed.....unless engine torque curve "increases" as RPMs increase (not likely) or hull to water resistance is reduced (some how) due to any increase in thrust at the higher RPMs you'd be getting with your lowering the pitch for the hole shot improvement. You can't have your cake and eat it unless either the engine HP increases, or the load it is expected to push is reduced....load being defined as anything that causes resistance to moving the hull through the water.

There is a possibility that hydraulic trim tabs mounted to the transom could help your hole shot while having no effect on top end, whereby (more guessing) you could up your pitch for more MPH yet have a decent hole shot.

Thank you

 

[256barry]

Well, simply based on the max rpm of 4000 you could drop to a 21 or 22 pitch prop, have a significantly better hole shot, and be at least the same top speed, assuming you engine is in good running condition.

Thank you

 

[paulswagelock]

Thank you

If your motor is healthy, bottom is clean, and you get a decent prop, I calculate you could run around 40 mph with a 22p

 

[QBhoy]

Without even hearing anything about how she performs as she is. I can almost certainly tell you just from when I got to the bit about 1.5 ratio and 25” pitch...jeez. The set up you describe...I’m amazed it’s even getting to 4000 rpm.
just another wee thing. Is that rpm range Defo correct ? You’d think a modern 5.7 recent engine would be a vortec type thing and likely the range would be 4400-4800 or 4600-5000 or similar ? Yours may differ, of course. Anyway. If someone asked me off the top of my head what a good starter prop for getting correctly propped on this was...I’d naturally think it would be around the 17-19”. I’d be amazed if it could turn anything onto the 20’s of pitch with that ratio...on that boat and that engine.
Just thinking about the rpm range and 300hp you mention. I honestly don’t think that engine can make 300 hp unless it had the usual higher rpm range I mentioned earlier. Anyway. All the best.

 

[256barry]

Without even hearing anything about how she performs as she is. I can almost certainly tell you just from when I got to the bit about 1.5 ratio and 25” pitch...jeez. The set up you describe...I’m amazed it’s even getting to 4000 rpm.
just another wee thing. Is that rpm range Defo correct ? You’d think a modern 5.7 recent engine would be a vortec type thing and likely the range would be 4400-4800 or 4600-5000 or similar ? Yours may differ, of course. Anyway. If someone asked me off the top of my head what a good starter prop for getting correctly propped on this was...I’d naturally think it would be around the 17-19”. I’d be amazed if it could turn anything onto the 20’s of pitch with that ratio...on that boat and that engine.
Just thinking about the rpm range and 300hp you mention. I honestly don’t think that engine can make 300 hp unless it had the usual higher rpm range I mentioned earlier. Anyway. All the best.

Thank you

 

[Sea Rider]

If the wot rpm range is 4200-4600 rpm factory stated for said motor and powered with a 25 pitch prop achieving just 4000 wot rpm, the motor is only 600 wot revs from max range, going down 3 pitches max shoud put that motor revving at max 4600 rpm that's if the +200 wot revs increment per one pitch down works ? So a 22 pitch if available should do the trick to better the slow hole shot as currently loaded. If a 22 pitch is non existence get a 21 pitch prop.​

Happy Boating

 

[QBhoy]

If the wot rpm range is 4200-4600 rpm factory stated for said motor and powered with a 25 pitch prop achieving just 4000 wot rpm, the motor is only 600 wot revs from max range, going down 3 pitches max shoud put that motor revving at max 4600 rpm that's if the +200 wot revs increment per one pitch down works ? So a 22 pitch if available should do the trick to better the slow hole shot as currently loaded. If a 22 pitch is non existence get a 21 pitch prop.​

Happy Boating

Honestly. There is just no possible way a 22” is right for this boat. Not a chance.
To put some perspective on this.
I have quite a serious performance boat for its type. It’s 18ft. Weighs about 1400kgs. Has about 300hp merc mpi. Has a similar ratio of 1.47. At 67 mph it will run a 21” aired out on its pad at about 5100/5150 rpm. A 23” is running late 4000’s if all is perfect. But not as well as a 21”. That’s on a stepped hull of only 18ft and literally almost only the outdrive in the water.
There is no way in gods green earth a 26ft cruiser with the same power and ratio is running anything like into the 20’s in inches of pitch.
Something just isn’t right here. As mentioned before...how can this engine produce its highest form of possible horsepower ever seen in its marine standard form...unless it’s rated up at the 5000 rpm or a little more, peak of its rpm range ?
I haven’t ran the numbers...but they don’t make sense at a glance. To me at least.

 

[256barry]

Honestly. There is just no possible way a 22” is right for this boat. Not a chance.
To put some perspective on this.
I have quite a serious performance boat for its type. It’s 18ft. Weighs about 1400kgs. Has about 300hp merc mpi. Has a similar ratio of 1.47. At 67 mph it will run a 21” aired out on its pad at about 5100/5150 rpm. A 23” is running late 4000’s if all is perfect. But not as well as a 21”. That’s on a stepped hull of only 18ft and literally almost only the outdrive in the water.
There is no way in gods green earth a 26ft cruiser with the same power and ratio is running anything like into the 20’s in inches of pitch.
Something just isn’t right here. As mentioned before...how can this engine produce its highest form of possible horsepower ever seen in its marine standard form...unless it’s rated up at the 5000 rpm or a little more, peak of its rpm range ?
I haven’t ran the numbers...but they don’t make sense at a glance. To me at least.

So 17 to 19 pitch would be a better starting point!

 

[256barry]

So 17 to 19 pitch would be a better starting point!

Also double checking my connections to make sure RPM is accurate

 

[QBhoy]

So 17 to 19 pitch would be a better starting point!

I think so Barry. Unless I’m being completely daft and missing something.
Just about the rpm range Barry. Where did those figures come from ? And am I right assuming when you said “300” you mean 300hp ?
Im not the man to ask, but loads of guys on here will be able to tell you exactly what specs your engine are with a serial number or more details of it. I honestly think most repower engine units like yours will have an rpm range of about 4600-5000 rpm in that form and vortec heads. Once you get the details confirmed (unless you’re sure already) then you can get her propped well. But my gut honestly tells me you almost certainly can’t run a prop anything like what you have now.
I even know of a scarab excel 21 with a 7.4 330hp. A bravo 1 with 1.5 ratio. That only just runs a 23” prop to about 4500 approx. But the 7.4 has a lower rpm range than your usual 5.7 like yours. Needless to say, the scarab is quite a quick thing too. Just to give you some perspective of what I’m getting at. So I’d think you’d certainly be much less pitch than that for sure ? Hope that helps.

 

[256barry]

I think so Barry. Unless I’m being completely daft and missing something.
Just about the rpm range Barry. Where did those figures come from ? And am I right assuming when you said “300” you mean 300hp ?
Im not the man to ask, but loads of guys on here will be able to tell you exactly what specs your engine are with a serial number or more details of it. I honestly think most repower engine units like yours will have an rpm range of about 4600-5000 rpm in that form and vortec heads. Once you get the details confirmed (unless you’re sure already) then you can get her propped well. But my gut honestly tells me you almost certainly can’t run a prop anything like what you have now.
I even know of a scarab excel 21 with a 7.4 330hp. A bravo 1 with 1.5 ratio. That only just runs a 23” prop to about 4500 approx. But the 7.4 has a lower rpm range than your usual 5.7 like yours. Needless to say, the scarab is quite a quick thing too. Just to give you some perspective of what I’m getting at. So I’d think you’d certainly be much less pitch than that for sure ? Hope that helps.

I will do some re-checking on my engine for serial numbers! I was going by the template stamp to the top of the motor!

 

[Faztbullet]

Somethings wrong with motor or ratio...