15 or 17 pitch 4 blade (with different diameters) - help please

[Toronto boater]

I posted my stats a few months ago and got some helpful advice. Have a follow up question.

I have a 19.5 foot Sunbird Corsair Cuddy with an OMC 4.3L Cobra I/O engine with gear ratio of 1.68:1

With two passengers, and mimimal load, I get the following stats from the 14.8 x 17 aluminum Solas, 3 blade propeller:

wide open throttle (trimmed up): 4500 RPMs at 41 mph
3000 RPMs (trimmed down): 26 mph
coming off plane at about 21-22 mph

With a couple more passengers on board it is really slow out of the hole and doesn't hold plane well at lower speeds. I want to increase the performance to get better hole shot and handling at lower speeds. I am willing to sacrifice a couple of mph at the top end. I am an avid slalom waterskiier.

The helpful advice I received previously was to try a 4 blade 15 or 16 inch pitch prop, (dropping 1 to 2 inches in pitch in order to compensate for the 4 blade)

I asked my mechanic to order a 16 inch 4 blade. My mechanic just called Solas, which recommended that I put on a 4 blade, 14.5 x 17 prop (aluminum). The thinking was that the drop in diameter to 14.5 (from 14.8 which I currently have on the 17 pitch 3 blade) should compensate so that my RPMs remain in the proper range (the proper range for my engine is 4200 to 4600) and therefore I need not drop all the way to a 15 pitch.

The other possibility was to use a 14.8 x 15 inch, but I would prefer not to drop down that much in pitch so as to not lose that much top end speed.

Any thoughts?

Thanks.

 

[cpubud]

Re: 15 or 17 pitch 4 blade (with different diameters) - help please

the thing with props is you have to find a happy median, you cant have top speed and best hole shot with the same prop. you need to find the right prop that gives you the best of both yet still the right wot rpm for the motor you are running, with the same load on the boat [weight wise]. if the weight changes on the boat so does the prop performance. it takes a lot of props and a lot of note taking to find the proper prop.

 

[steelespike]

Re: 15 or 17 pitch 4 blade (with different diameters) - help please

Take a look at the Turning Point Hustler props.15"pitch.Close to ss performance at al prices.There are a couple of Turning Point reports here.
They are said to improve overall performance.40% off.

 

[Tail_Gunner]

Re: 15 or 17 pitch 4 blade (with different diameters) - help please

Take a look at the Turning Point Hustler props.15"pitch.Close to ss performance at al prices.There are a couple of Turning Point reports here.
They are said to improve overall performance.40% off.

Spike is probably right...some things come into play actually .

1. What is the mfg top rpm rating...4600??? that's your target.

2. How to balance the boat on plane aka stern lift or bow lift..you have a cuddy which would probably benfit from bow lift...

A three blade 15 Hustler would give you a good hole shot and if it is cupped it would probably act like the a 16p ..The 16p 4 blade would be my choice however..Smoother and more midrange and if your motor is strong the 4 blade should give you enough lift...less wetted surface to make up for some top end loss..

You wont know until you put on the props and take it for a run.

 

[hwsiii]

Re: 15 or 17 pitch 4 blade (with different diameters) - help please

Toronto, you are going to gain right at 400 RPM by changing 1" in pitch and .3" in diameter. I don't know what the change to the four blade will do, as well as the change in the elements of prop geometry between the two props, but it sounds like it ought to work well to me.

H

 

[Tail_Gunner]

Re: 15 or 17 pitch 4 blade (with different diameters) - help please

Toronto, you are going to gain right at 400 RPM by changing 1" in pitch and .3" in diameter. I don't know what the change to the four blade will do, as well as the change in the elements of prop geometry between the two props, but it sounds like it ought to work well to me.

H

Respectfully anyone short of Walleye of Dhadley givng such advise i would say BS on diameter. I noticed you disregared a post to DH advise on a O/B rpm rating.

Ken is that you..if so why have you changed up so much on dia and max rpm level's...

 

[hwsiii]

Re: 15 or 17 pitch 4 blade (with different diameters) - help please

Tailgunner, my name is H. W Slater and I live in Pascagoula, MS. I do not know who the Ken is you are referring to in this post. But I do have considerable knowledge about propellers and their effects when diameter, pitch and geometry are changed, if you notice in my posts I talk about the mathematical changes to be expected when people change diameter and pitch in props in the same model of prop. These formulas I have designed myself, from experience and physics. I ask for manufacturers WOT RPM settings and gear ratios, as they are required for these formulas to work correctly.
I am not here to denigrate or put anyone down, I am just trying to help the people coming here asking for advice. As far as disregarding Dhadley's advice on RPM for a particular engine I guess this is what you are talking about.
Dhadley, "Typically when folks refer to an 150 Oceanrunner they are refering to a 60* motor. Those can be safely propped out at 5800-6000."
hwsiii, "It sounds like you have a prop with a decent amount of rake in it, which is lifting your bow high. I don't know what the real RPM or lower gear ratio is, but what I found in my database says it should turn 4,500 to 5,500 RPM and it shows the gear ratio as 1.86 to 1 with the boat weighing in at 2,350 pounds empty. if it is a 1998 too. If i use these numbers asd a starting point, and add the motor, 40 gallons of fuel, a couple of people and sundry items I come up with a total weight of about 3,700 pounds. If I also use this weight I find that if you change to a 17" prop in the same model of prop you have now, your RPM will rise to about 5,200 to 5,300 RPM with a speed of about 40 some miles per hour. But if you change models of prop to something with less rake it is hard to tell, because I don't know what other geometry of the prop changed as well. Using the 17" prop you will get on plane much quicker."
I did not disregard dhadley's post, I just try to use the manufacturer's recommendations as a baseline, and I think that is reasonable. Most of the people coming to this forum asking for advice have no idea about the different elements in a propeller and the diffferent geometry of props and how they can help add speed to different hull forms or actually slow them down and make them very difficult to handle if they pick the wrong elements in a prop for their boat. A lot of them believe that if they pick a stainless steel prop with the same diameter and pitch as the aluminum one they are using now they should be able to run their boat faster, and we both know that may or may not be true. I am only trying to help people find the correct propeller for their boat. I have amassed a large database on propellers and what happens to a particular hull form when you change diameter or pitch within the same model of propeller. My formulas have been designed using this database as the model for my answers, so I do very much believe that what I say is very correct.
I am sorry if my answers don't agree with other peoples on this forum, but that doesn't change the fact that I do believe they are correct.

H

 

[Tail_Gunner]

Re: 15 or 17 pitch 4 blade (with different diameters) - help please

Calm down a bit.. i need to calm down plz explain your gearing down to a smaller dia hub and getting a increase in revolution's..This sounds like creating tourqe with some kind of chaos theroy..

Or are you redefining mass..kinda like pulling touqre out of nowhere..in the end the prop is still a given pitch with a given bite and dia..I cannot see how dia would change a thing.

 

[hwsiii]

Re: 15 or 17 pitch 4 blade (with different diameters) - help please

Tailgunner, sorry but I don't quite understand what you are asking in your question. Maybe I am just not clear headed this time of the morning. It appears you are asking me about when I discuss changing Diameter of the prop that I am talking about changing the diameter of the hub? Is that correct?

H

 

[hwsiii]

Re: 15 or 17 pitch 4 blade (with different diameters) - help please

Tailgunner, anytime you decrease the overall diameter of a particular propeller without changing the basic geometry of the prop you decrease the surface area of that propeller and anytime you decrease surface area you decrease thrust and there is less mass (water) being moved by that propeller. Any time there is less water being moved it requires less torque and horsepower to move it so, there is more horsepower and torque available for higher RPM from that prop.
If that is your definition of Chaos Theory, then so be it.
Just because you don't know me doesn't mean I am NOT right. If you care to we can go into this much deeper. I can't imagine that you choose to be so contentious about something as small as this. There appears to be a problem here that I can't quite understand. You got upset when you felt that I discussed my opinions with the person posting and asking for help with his prop choice without agreeing with Dhadley. I made it very clear in my post that I didn't know the TRUE RPM or Gear ratio of his motor, but I stated my database showed the information I stated for my calculations. If you believe I do not have the right to share my knowledge with other people on this forum, then I will ask you who died and made you King. You called me Ken and acted like I was your friend and I had turned on you and attacked your basic premises of boats, motors and props. If you truly believe that call Ken and talk to Ken NOT me. I have never said anything rude or drogatory to anyone on this forum, so why did you pick me to be so rude to?
Toronto I am sorry for interrupting your search for a more suitable propeller for your boat, motor and uses.

H

 

[freddyray21]

Re: 15 or 17 pitch 4 blade (with different diameters) - help please

does your boat have trim tabs? If not then you might look at smart tabs as a solution. You tach numbers for that prop are about right so I don't know if a prop is your problem. I put smart tabs on my Searay and it made a big difference in hole shot, top end and planing at lower speeds.

 

[Tail_Gunner]

Re: 15 or 17 pitch 4 blade (with different diameters) - help please

Dropping 400 rpm with 1" of pitch and a smaller dia....Well ok if you say so...Not gonna go there...Playing god not.. props are quite expesnsive there we can agree...as they say carry on.

Peace out.

PS: as to Ken or Dhadley....they are not involved in this childish game of thoughts....In any manner or any form..

 

[hwsiii]

Re: 15 or 17 pitch 4 blade (with different diameters) - help please

Tailgunner, who was the person that stated "I cannot see how dia would change a thing." and "Playing god not". You give advice on props on a regular basis, are you playing God? Just because you don't agree with my statements doesn't mean you are Right and I am Wrong. Who is the Child in this diatribe? I don't try to make someone appear to not understand the basic laws of physics, in fact my formulas are totally based on physics and mathematics, what are your thoughts based on.

H

 

[Toronto boater]

Re: 15 or 17 pitch 4 blade (with different diameters) - help please

Thanks to all for the suggestions. I think my post started a dialogue that may have been better left for private posts... hope all is ok, on a good natured forum which is very helpful to the newbies like me.

I will try a 17" inch 4 blade for now, and my mechanic suggested I could switch it to a 15 inch pitch if necessary and if I couldn't get my RPMs to the right target.

Will let you know how it is in a few weeks once I am back out on the water again.

Thanks again to all for their time and advice.

 

[MikDee]

Re: 15 or 17 pitch 4 blade (with different diameters) - help please

If you have a 17" pitch 3 blade prop, and you add another blade, you're going to lose rpm, regardless of the fact that it's .30 of an inch less diam., that's not enough drop in diam. to make a difference when adding another blade, you really need to drop to a 15" pitch 4 blade prop.

That's my inexpert opinion