Beneteau 361 _ Is my current propeller efficient?

[Phil_Ben361]

I often wonder if my 2 blade fixed propeller is adequate for the boat (and if it is the correct size) and what more a 3 blade propeller would offer besides more drag if it is a fixed one or more costs if it is a folding. The boat is a Beneteau Oceanis 361, engine is Volvo Penta 29hp, gear ratio is 2.35:1, the 2 blade propeller size is 16X11 and I can get the following figures when motoring AGAINST 13-14 knots of true wind (17-19 apparent depending on speed) and a little bit of chop:
At 2000rpm speed of 4,3 knots.
At 2500rpm speed of 5,2-5,4 knots
At 2600rpm speed of 5,9-6,1 knots
At 2800rpm speed of 6,5-6,6 knots
At 3000rpm speed of 6,8-7 knots
Maximum rpm is 3400 and speed 7,6-7,8.
“Cruising rpm” I like to use is around 2600. If I have to motor against heavy seas I would not like to exceed 2800rmp. Above this there is a lot of noise and vibration plus increase of consumption, not the best if I have to make a long trip since my fuel tank capacity is only 75 litres. Of course speed drops a lot in heavy seas but I haven’t made a similar test to check it.
What do you think of these figures?

 

[petersea]

Looks Good

You rpm/speed figures look pretty good to me. It would appear you are prop-ed correctly. A three blade fixed can be around $500 vs. a three blade folding/feathering costing around $3000. Don't expect you boat efficiencies to change much if you go with a three blade prop. you will "smooth" out the drive train with a three blade it will reduce vibration but will not increase speed or lower rpm if you get the correct size three blade prop. Let me caution that the correct size is critical as over propping or under propping will have long term devastating effect on the engine. Fixed folding and drag are a can of worms I won't go near but you are best suited to answer that based on are you willing to spend the money for the extra 1/4 knot of boat speed. Pretty much how do you use your boat and your type personality. Go see a good prop shop for sizing a three blade prop.

 

[Phil_Ben361]

If change to 3-blades....

If I was going to change to 3-blades I would definately go for 16X10 which is the size recommended by Beneteau (fixed or folding is a different issue).
I am mainly concerned if I get the best out of the whole current combination. I must say it's suprising (though nice!) you mention that there would be not much difference if change to 3-blades in terms of performance. I though 3-blades would have a big difference. But I am not an expert AT ALL.
Thanks

 

[Ken13559]

Re: Looks Good

I changed from 2RH 15"x10" stock prop to 3RH 14"x6" (with 2" cup) Campbell Sailor Prop.
Love it. Gives good bite, reduced prop-walk and alot more smoother ride and quieter prop/shaft noise (no thumping). Sailing isn't affected much by the 3 bladed Campbell Sailor 'cos the blades are narrower to most fixed prop.

 

[Mike B]

B361 Prop

The B361 came with a 3 blade prop. We had an Autoprop 3 blade feathering prop installed at delivery. We have a Westerbeke 27 hp engine and get 6 kts at 2000 RPMs, 6.5 to 7 kts at 2800 rpm. Speeds vary baded on sea conditions and amount of growth on the hull. With a properly pitched prop you should be able to see the engine reach max designed RPMS at full throttle. From your specs it looks like you're right where you want to be.
Mike

 

[pturje]

Kiwi Prop

The performance you are experienceing with your current prop seems appropriate, so my guess is that you are close to an optimum pitch for the diameter. I changed from a 3 bladed Michigan style to a thin bladed Campbell prop when I re-engined my O'Day 37 this year and the decrease in drag at low speeds is noticable though not dramatic, but that could be due to the boat having conservative performance to start with. I would be tempted to try a Kiwi Prop http://www.kiwiprops.co.nz/. These auto feathering props are increasingly used in the Vancouver area with good reports. Look for distributors and retailers in your local area.

 

[petersea]

it is a correct size thing

Phil, it is not the number of blades so much as the size of them and the pitch that are the issue when changing number of propeller blades. The three blade will be smoother then a two and a four will be smoother than a three etc etc. In theory you could put a five blade prop on and IF it was sized correctly it should not change the engine speed / boat speed relationship (maybe slightly) .Your engine needs to reach full mfg rpm's at full throttle. Cruise speed should be 75 to 80 % of mfg rated rpm's for long term health of engine. Overproping will "load" the engine causing dramatic lose of engine life and some other problems along the way to a full replacement. I will tell you I changed out my two blade for a three blade and it was a remarkable difference in "quality" as far as less vibration and smoothness. You may notice some more "punch" when in rough seas or cross current. You may even notice a difference in prop "pull" in reverse. Trust me they will not be that noticeable. fixed vs. folding/feathering is a cost and performance issue you will need to decide for yourself. Slightly more drag with a fixed prop vs. less drag but more complicate mechanical folding or feathering and a large cost difference. A lot of boat makers are putting on three blades (or offering them as a option) on boats your size. I know I was very please when I changed mine and strongly recommend it . For what it is worth I went with a three blade fixed, but I'm not concerned about the drag issue. Good Luck with that issue ! !:stirthepot:

 

[Steve Dion]

Re: it is a correct size thing

Phil:

Whatever you decide, please be sure that you have the prop shop determine your prop sizing. These guys have computers that will size the it to your specific engine/trans/boat.

 

[Paul Ouellette]

Phil :

We went through the same experience when we swapped the 3 blade fixed prop that came with the boat to a two blade folding prop.

Our 2 blade Flex-a-Fold prop sho'o'o'o'o'ok the boat (at certain RPMs...) & so we changed to the 3 blade Flex-a-Fold & that corrected the problem.

My advice is to put the two blade prop aside for a spare, just in case the 3 blade (that you'll likely be getting) should ever be lost.

You won't regret the decision.

Cheers, P.
www.winpipe.ca