Unable to motor at proper speed

[sagablu]

Hello all,

I have a 2018 Jeanneau Sub Odyssey 349, 34 ft (10.5m) and 12,000 lbs, (5390kg). The prop is a 15" Flexofold 3 blade that I assume was the factory prop. I am unsure of the pitch. The motor is a yanmar 3YM20, 21hp. The problem I am having is I am unable to attain the speed that I expect from the boat and from what I hear should be expected. I've been reading the Jeanneau 349 Forum, and I understand that seven knots at full throttle is to be expected. Under no circumstances can I get more than five no matter what I try, regardless of wind and tide. It appears that others have the same prop and I am at a loss. Reading the conversation someone else with the same boat had with Flexofold, The specification was a 16-in two blade, not
the 15 in three blade. The conversation went on to say that a 16-in three blade would provide more thrust but might not allow the motor to hit Max RPM. I can currently easily hit Max RPM.
Of course, the bottom is clean. I checked the prop in and out of the water, it seems to open and close without any restriction
The funny thing is a review I found did a test of folding props and Flexofold came out on top. Totally not understanding what's happening.

So my question is what do I do next. I'll buy another prop to get the proper motoring speed but I don't wish to buy multiples to try them out. This is a subject I do not know a lot about, so appreciate any suggestions and questions I should be asking

[Bowdrie]

Others will no doubt have more specific info, but if can "easily" hit max rpm that's an indication that the prop is not converting all the hp fed into it into useable thrust.
1, Not enough pitch, 2, not enough diameter, or 3, not enough blade area.
The "rule of thumb" is that the engine should just come-up to a little bit under max rpm when you have no more throttle left to give it, (understanding we don't really "throttle" a diesel, we "set the governor").
Although it's quite common to use a prop that will limit the engine rpm ~100>200 or so below its max.
I would add that nice clean props do make a boat go faster, props for our displacement speed boats have an airfoil cross section, the water flow is easily disturbed by surface irregularities.

[PerfectPirate]

When I first got my current boat, I was very disappointed in her performance under power. Despite having a theoretical hull speed of 7 knots, the best that I could get out of her in flat water was 5 knots. This occurred at 2100 RPM, which is the proper sustained cruising RPM for my Yanmar 2QM20 (ie. 80% of max RPM). When I applied more power, the RPM would easily climb up to maximum level, but the the propeller would only cavitate. My speed could even decrease.

So I started doing some research and bought a copy of Dave Gerr's "Propeller Handbook". Based on what I read there, I decided that I needed to try increasing the pitch of my propeller. Fortunately, my boat came to me with a 3-blade MaxProp (God bless the original owner!) and I could simply disassemble the propeller and change the pitch. So I increased the pitch by 2 degrees, which was one setting over on the propeller hub, and the difference was like night and day. Suddenly, 2100 RPM would get me just over 6 knots in flat water and full power RPM would push my boat to hull speed. There was no more cavitation or thrashing.

The original owner left a detailed written explanation of how he came to select the pitch. Because of hull clearance issues, the prop was a shade smaller in diameter than Yanmar's recommendation for that engine, so he had increased the pitch to 2 degrees (one inch of pitch) over Yanmar's recommendation, believing that he had pushed things far enough. He felt content with 5 knots under power and even took the boat from Lake Ontario down the ICW and to the Bahamas, and then back again. However, not wanting to experiment just a bit further meant that the first and second owners had to live for 45 years with inadequate performance which, in extreme conditions, might even have proved hazardous. Engine manufacturers' recommendations are only a starting point, arrived at by an educated estimation. However, each combination of hull design + engine + propeller is unique and some experimentation may be necessary to optimize performance. The real problem is that experimentation may incur some real expense and inconvenience. I was very lucky.

[Tin Tin]

Go onto Flexofold website and get a quote and see what propeller they come up with for you model boat. https://www.flexofold.com/propeller/...9-7d7b1a1ce213

[sagablu]

Originally Posted by Tin Tin

Go onto Flexofold website and get a quote and see what propeller they come up with for you model boat. https://www.flexofold.com/propeller/...9-7d7b1a1ce213

Another person had contacted Flexofold and asked the question. Attached is the conversation. Looks like the recommendation is 1 in larger than what I have, I am unsure about the pitch. Is the 1 in difference so substantial?

[hlev00]

Talk to flexfold.

[sagablu]

Originally Posted by hlev00

Talk to flexfold.

I certainly will, I was just trying to get an idea of what I need to know so I have a good path forward before I speak with the manufacturer. In other words I'd like to go in with an understanding of the relevant factors, so I can ask good questions and know that I'm receiving good information. Any insight anyone would have

[Adelie]

If you can adjust the pitch on you prop try doing that before buying a new prop.

1” greater diameter to match recommendations will probably help but try the cheap route first.

[Scrimshaw4]

Base on your pics it looks like you have a partially fouled prop. Before you get another prop I'd suggest you thourghly clean the prop down to bare metal. This will probably start another discussion but after cleaning I'd coat the prop with Prop Glide. Works as well as Prop Speed but at a lower price. JMHO

[sagablu]

Originally Posted by Scrimshaw4

Base on your pics it looks like you have a partially fouled prop. Before you get another prop I'd suggest you thourghly clean the prop down to bare metal. This will probably start another discussion but after cleaning I'd coat the prop with Prop Glide. Works as well as Prop Speed but at a lower price. JMHO

Most of those are marks from the barnacles. I purchased this boat this year, when it went in the water it was clean, same problem. Following, I was in the water for a while with my scuba gear cleaning the prop again. At that time I used the lanacote prop coding. Cleaning, seems to have made no difference. I wish it was that easy!

[skipperpete]

Before doing anything else, you need to confirm the gearbox reduction ratio, your engine came with 1 of 3 possible ratios, one of those is over 3:1. If the reduction ratio stamp is no longer legible on the gearbox simply select ahead, put a paint mark on the output coupling and manually rotate the engine and record/observe how many turns of the crankshaft it takes to achieve one full turn of the propeller shaft.
Hint, the prop shaft should rotate opposite to engine rotation.

[sagablu]

Originally Posted by skipperpete

Before doing anything else, you need to confirm the gearbox reduction ratio, your engine came with 1 of 3 possible ratios, one of those is over 3:1. If the reduction ratio stamp is no longer legible on the gearbox simply select ahead, put a paint mark on the output coupling and manually rotate the engine and record/observe how many turns of the crankshaft it takes to achieve one full turn of the propeller shaft.
Hint, the prop shaft should rotate opposite to engine rotation.

Thank you,

Good point, before I stop making phone calls I'll verify The transmission gearing. This is very helpful, appreciated

[NorthCoastJoe]

I would talk to a prop shop. When I got my Pearson 30 I put in a new Yanmar 3gm30f, I had a prop shop run the data and they gave me the ideal pitch and diameter prop for my boat, engine, and transmission. I love how it powers the boat, I have plenty of juice when I need it.

[davefromoregon]

Just for fun, have the bottom cleaned. You might be surprised at the results.

[sagablu]

Originally Posted by davefromoregon

Just for fun, have the bottom cleaned. You might be surprised at the results.

The bottom was clean when the boat went in the water, plus I cleaned it, and I had a professional diver clean at one time. Except for a barnacle on the strut and a few on the prop, the bottom was mostly clean. Regardless, the before and after results were the same. The problem lies elsewhere