Prop replacement

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KenKrawford
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Re: Prop replacement

Post by KenKrawford » Tue Nov 26, 2019 7:09 am

I replaced my OEM with the new prop recommended by Catalina about 9 or 10 years ago. There was no "deal" at that time regarding the price. I believe I paid slightly over $400 back then. Since I sail on a rather small lake my performance changes with the new prop are pretty meaningless to most people. I only had minor intermittent episodes of cavitation with the OEM prop but none at all with the new one.
Ken Krawford
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C350 # 351
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TBOT422
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Re: Prop replacement

Post by TBOT422 » Tue Nov 26, 2019 5:20 pm

Cuthbert,
What's your hull number? You seemed to have shot holes in my theory that Catalina changed the transmissions with Hull No. 400 when they changed the standard prop to the Michigan M Series 14x10. I figured they increased the gear reduction to lower the prop RPM and help with the cavitation problem as well as the prop diameter change. No I have no idea how they determined which transmission to install.

On a side note. Flexofold has offered to let me try one of their props for a week and return it if it does not perform as I hope. I am thinking about buying the 15X10 with an additional set of 15x11 blades, then run a performance test of my existing 14x10 and both of the the Flexofold blades, then select the one that seems best. Unless there is significant performance improvement under power, I will not likely spend $2K on the folding prop. If there is an improvement that I feel is worth the money, I will have a 14x10 prop available for you if you want it. Presently talking to my bottom cleaner to see if he is willing to assist with the R&R tasks. If I decide to go this route, it will be late Dec, or early Jan.
Gary & Janet
The Best of Times (Hull #422)

cuthbert
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Re: Prop replacement

Post by cuthbert » Wed Nov 27, 2019 11:28 am

I'm hull #69 and the transmission as per my earlier post led to the 15 x11 fexofold recommendation. I am also a little confused as to their sizing recommendation, I didn't read it as being consistent across all 350's transmissions, but will go back and re read all the posts to try and make sense of it. What does seem straightforward is that of all the folding options flexofold is the optimal for boats that are not hauled out every season as they require no greasing/servicing. Reading on some other forums where one of the SF bay area divers posts prolifically, it would appear that any good bottom diver should be able to change the prop in the water.

They offered me a buy (direct from Europe) and return in a short window if it was not right, but the window was so short as to be impractical.

I chased down Catalina and they quoted $725 for the 14x10 M series but 6 week lead on an order. I went direct to Michigan wheel and they quoted similar. Although a little bit of shopping around distributors yields some stock around $500 price point.

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TBOT422
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Re: Prop replacement

Post by TBOT422 » Wed Nov 27, 2019 1:11 pm

My diver said he has done it but does not do it anymore. Too many small pieces to lose. I'm still researching.
Gary & Janet
The Best of Times (Hull #422)

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Re: Prop replacement

Post by TBOT422 » Fri Nov 29, 2019 12:46 pm

I am still confused by propeller 'optimization'. I tried thinking of a semi-trailer to help me understand it. In the case of a semi, it seems to me that the RPM and speed are directly related. Suppose you have an unloaded semi running at 3000 RPM and a speed of 60 mph. If somehow a helicopter could add a cargo load to the trailer, and no throttle adjustment was made, the RPM would decrease as it is harder to pull the heavier load. Since there is no 'slippage' in the drive train or wheel connection to the ground, the speed would drop proportionately. Increasing the throttle to the previous RPM would return the speed back to 60 mph as long as the load was below the engine's power curve capacity.

However, with a boat propeller, it seems to me that boat speed and RPM are not directly related. In the case of a boat, the propeller seems to be the 'load'. It is harder to turn a bigger prop or a higher pitch, thus more 'load'. If you have a boat with small propeller running at 2500 RPM and 4 knots, at a specific throttle setting, then could somehow replace the prop with a larger prop, the engine RPM would drop as with the increased load on the semi. However, increasing the throttle back to the previous RPM would INCREASE speed since the larger prop creates more thrust which results in more boat speed. Again, as long as the engine has the power capacity to support the load of the larger prop.

It seems that propeller manufacturers 'size' their prop for a specific boat by selecting the largest prop they can that falls below the engines power curve at rated RPM. This should ensure the engine has enough power to turn the prop at any RPM without exceeding the power curve. However, in the real world the 'load' from this 'optimal' prop may exceed the engine's power capacity due to marine growth on either the prop or the boat bottom., or any other factor that creates more 'drag'.

Most manufacture's imply a change in diameter of 1" increases or decreases RPM by around 250 RPM (assuming the same pitch). If that is true, does it imply that a 14x10 prop produces the same (or at least very similar) thrust (and thus speed) as a 15x10 prop if the engine was running 250 RPM higher with the 14x10 prop? i.e. Suppose a manufacturer sized an optimal prop as 15x10 for a given boat. It is sized based on the boats rated Hp at 3000 RPM. Let's assume the boat speed with the 15x10 prop at 2250 RPM is 5 knots at a specific throttle setting. If we could magically replace the prop with the 14x10 at the same throttle setting, the engine RPM would increase to around 2500 RPM since there is less load from the prop. Would the thrust and thus the boat speed still remain somewhere near 5 knots?

Where this is all going - If most of the above is true, and our boat presently is under-propped with the M Series 14x10, I am beginning to see little cruising speed advantage of increasing to the Flexofold 15x10 prop under power. True, it apparently would increase boat speed at 2500 RPM since it is bigger and creates more thrust. But wouldn't the 14x10 create a nearly similar thrust around 2750 RPM? Even though 2750 RPM is outside the Catalina recommended 'cruising RPM' of 2000 - 2500, would it be any more harmful to the engine since the 'load' is roughly the same? (i.e. higher RPM, but further from the maximum engine power curve). One advantage of the bigger Flexofold prop might be in higher top end power. That power could be useful when battling tidal current, wind, waves, etc. for short periods of time, but it appears to me that at normal cruising speeds there may not be any significant advantage. Then of course, there is the advantage of the 'folding' blades, that could reduce boat drag by about 30% when sailing.

Any comments regarding where I am wrong would be appreciated.
Gary & Janet
The Best of Times (Hull #422)

tedseward
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Re: Prop replacement

Post by tedseward » Sat Nov 30, 2019 11:32 am

Science and theory and engineering discussions aside I can only provide my experience...for what it is worth. I have hull 375 from 2006 and it came with the original prop. Best cruise speed I could get was 5.9 knots. Based on all the PR at the time I switch to the "M". Cruise speed became a blazing 5.95 knots. Went to boat show and bought a Flexifold that they recommended for Catalina 350. I seem to remember it as a 15 x 9 although it may be a 15 x 10. Cruise speed now up where it should be...about 6.4 knots. No black smoke, no cavitation, no oil burning. Added benefit of a little better speed under sail with prop folded. Life is good - I hope :)

Ted Seward
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Re: Prop replacement

Post by TBOT422 » Mon Dec 09, 2019 3:48 pm

Sargentef,

What engine & transmission do you have?

You commented that fuel consumption increased with the Gori propeller. What was your previous propeller, and how much did the fuel consumption change?

Thanks
Gary & Janet
The Best of Times (Hull #422)

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TBOT422
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Re: Prop replacement

Post by TBOT422 » Sun Dec 15, 2019 11:41 am

I believe I have settled on going with the Gori prop. Its Overdrive (OD) mode supports my logic that being slightly over-propped is not a bad thing as long as you operate at reduced RPMs while in OD. Basically, the Gori prop with OD almost acts like a second transmission. By allowing the blades to be positioned in different pitches dynamically the propeller essentially has a higher gear ratio (OD with more pitch), and a lower gear ratio (regular with standard pitch). The regular pitch allows the engine to achieve full rated power when needed (fighting current at a bridge opening). The OD pitch allows the boat to 'cruise' or motor-sail and achieve the same boat speed at less RPM than the regular pitch. Brunton's AutoProp does something similar, although it is a feathering prop rather than a folding prop.

From Geoff Prior at AB Marine:
A propeller is ‘sized’ to absorb / transmit the full engine power at a specific engine speed. If you are looking for maximum power output from an engine, then this will always be at the maximum rated speed. In some cases where customers wish to limit engine speed and noise – propellers are sometimes sized to the normal engine cruising speeds, meaning that can be slightly ‘over-propped’ and the engines will never achieve their maximum rpm in gear. This is what the OD does.

At say 2000 rpm in OD you will get the equivalent thrust of approx. 2600rpm in std (or a propeller sized to reach the full rpm of 3000), which = more boat speed. In your case it is approx. 0.95kt more & you will be doing approx. full boat speed.

You will get more speed at the same rpm or the same speed at approx. 600rpm less = a fuel saving / extended cruising range / less engine noise & wear & tear.
I could note that Yanmar's Service Advisory YASA2011- 021 – M, also notes pretty much the same thing. They state that when using OD, the maximum cruising engine RPM should be 500 RPM less than the maximum loaded engine speed at WOT measured with the propeller in OD.

AB Marine also provided some sea trial results on a C350 (Hull #207) which showed an average (with/against wind & current) speed of 6.45 knots at 2900 RPM (standard pitch 15x11) and 2450 RPM (in OD mode). The average speed at 2100 RPM was 5.95 knots in OD. I presently have to run around 2750 RPM to see 6 knots with my 14x10 fixed blade. Additionally the folding prop should give me better sailing performance, and may be a little less likely to get entangled with the numerous crab traps along the FL west coast.

It will probably be early February before I splash the new prop, but I plan to do some actual performance tests with my existing prop and the new Gori. I will report what I find.
Gary & Janet
The Best of Times (Hull #422)

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