New Prop

[Deleted member 117556]

Just had a new fixed 3 blade 15 x 10 prop installed replacing the original 13 x 9 prop that came on my boat. Three different prop suppliers recommended the 15 x 10 prop for the boat including Catalina where I purchased it. My assumption is that most or all 310's have 15" props? My question is because the diver who installed it said there was 1/8" to 1/4" clearance between the new prop and the hull. Wondering if others have 15" props with minimal clearance like that?

Took the boat for a spin at slack tide and early returns were promising. Did not lose any RPMs at full throttle (still 3,500) and boat speed increased by anywhere from 0.5 - 1.0 knots. Hard to tell exactly because many variables but we are going on a 2 week trip next week and will have a better idea after that. Backing seemed to be much better too.

 

[Justin_NSA]

That's very close, But as they say, " miss is as good as a mile".

 

[Stu Jackson]

Had you asked HERE before you did that?

 

[KZW]

If you are going to the trouble to replace a prop, why not get a folding or feathering prop???

 

[Stu Jackson]

If you are going to the trouble to replace a prop, why not get a folding or feathering prop???

Perhaps because some people don't like extra moving parts that require maintenance under their boats.

Besides, that's not the question he asked.

For #1: Our C34s use 15x10s. Something sounds odd. Have you checked with Smitty's website? He may have discussed it, he did everything else with his boat.

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[Stu Jackson]

Just for grins & giggles, I did a forum search on "C310 prop", not "+c3120+prop" or anything else fancy.

I found this discussion about folding/feathering props, but it included this:

I have a '77 C30 which I have repowered with a Yanmar 2GM (13HP)and the prop that was recommended was a 13x9 which I put on. Performance was to say the least poor with 4 knots cruise at 2900 RPM and at max 3600 I was getting about 4 3/4 knots. Poor performance in reverse. I had them change the pitch to 11 inch and recup the edge a bit. WOW! Total different boat.

Forget the pitch for a moment, the DIAMETER is what I think you may have a problem with.

 

[jssailem]

the diver who installed it said there was 1/8" to 1/4" clearance between the new prop and the hull.

All I know is what I have read in the books and experience...

When reading your statement "there was 1/8" to 1/4" clearance" that appears to be a concern. The material I have read suggests at least 15% of the prop diameter should be the clearance between the Prop tip and the hull bottom.

What is the minimum allowed clearance between the propeller blade tips and hull bottom? How much space should exist between the propeller and strut or keel?​

The clearance between the propeller blade tips and the hull bottom should be at least 15% of the propeller diameter and ideally, 20% or more. So for example, a 20” diameter propeller would have 3” minimum clearance and better still 4” or more. A common guideline for recommended propeller to strut or keel clearance is 20% of propeller diameter measured between the propeller blade edge and strut leg or keel. This is often measured at a point on the propeller blade edge about 70% of the distance from the shaft centerline to the blade tip.​

I believe this is to reduce the potential for prop cavitation between the blade tip and the hull. The 15% rule would indicate on a 15" prop that there should be approximately 4 inches clearance. Perhaps the reason the other boats have 13" props it to get that 2" gap for efficient performance.

When I reported my boat the prop size was an important element of the design decision.

You might want to re check the issue with your prop provider before heading out on a 2 week voyage.

Inadequate space could be a recipe for disaster. Hit something that makes the prop jump and you have holed your boat. Likely would go unnoticed until the water is above the floor boards. That would be my concern.

Good luck. There is nothing better than a prop that enhances your boat performance.

 

[Deleted member 117556]

Had you asked HERE before you did that?

Yes I did on March 20th and got minimal feedback although Leo said he has a 15" prop. Catalina indicated that is the recommended prop diameter for the 310. Gori & Flexofold also recommended 15" props for my engine and the 310 although of course they recommend folding props

 

[jssailem]

Yes I did on March 20th and got minimal feedback

Selecting the correct propeller size: How to choose the right propeller for your boat - Practical Boat Owner

It is a challenge to deal with these issues when they feel foreign. Looking back on your March 20 query, @sail sfbay provided an excellent primer for the boat owner with limited knowledge about choosing the right prop for a boat. It specifically addressed prop size. (reprinted below for your support).

It is up to the boat owner to check his boat and do the measurements to validate that his boat can accept the prop that is being offered. Only you can use the tape measure to judge the distance from your prop shaft to your hull bottom. With the correct information the prop folk can provide competent answers as to how big a prop you should have.

Clearance is a critical issue. Your statement that only 1/8'th to 1/4 inch is the gap indicates the prop diameter is too large for your boat.

It is your boat and you can choose to address it or ignore it.

Once this resistance is known, engine and gearbox specifications can be combined with maximum propeller diameter (clearance beneath the boat is typically 12% of the prop’s own diameter) to determine the prop’s minimum requirements. The vessel’s ‘duty cycle’, which is simply a judgement as to whether a boat is to be used for heavy or light commercial or pleasure purposes, then also influences a designer’s final specification.​

​

Propeller diameter is first decided by the type of boat. A large-diameter slow-revving propeller is the most efficient type but this won’t provide the necessary acceleration to get a sports cruiser onto the plane. Acceleration is achieved by increasing the rotation speed, which usually means a smaller diameter propeller. The fact that many modern sports cruisers have limited propeller clearance space is another reason for fitting smaller diameter propellers.​

​

To accommodate the loss of thrust from the smaller diameter, the pitch is increased to give more thrust from the propeller’s movement through the water rather than from the blade area. It is in fact a combination of the three elements: diameter, pitch and rpm that provides the thrust at the given speed of the boat. The higher the speed of the boat, the higher the pitch should be. Also the higher the pitch, the greater the efficiency of the propeller. Higher pitch can be used by selecting higher gear ratios, but this is not really viable for most outdrive powered craft due to the small choice of ratios available and in this case it is usually necessary to work with the supplied ratio.​