Relative merits of propellers

[Tmacmi]

I have two propellers. The one currently on the boat is three bladed and has the following stamps on it:

2 68
14RH10

It has been able to drive the boat at 5 knots (calculated hull speed is 6.7 knots)

I found a 2 blade propeller in the stuff left behind and it is stamped

9 79
13RH13

I am wondering about the relative merits of improving sailing speed vs. motoring speed?

We are not racers, but I would like to understand what I am gaining or losing in more detail than, more drag when sailing with three blade, less motoring speed with two blade.

Thank you

[bwindrope]

You are going to get a lot of folks asking for more information. I had this problem when I did sea trials on my Gulf 32. If your calculated hull speed is 6.7, and neither prop is getting you up to hull speed, you either have a weak diesel relative to your boat size, or both props are inappropriate.

You are right the trade off between 2 and 3 blades is driving power vs sailing speed. You should not assume either prop is the right size. Those numbers you give tell you how big the blades are, which way they spin, and what angle they are at. All critical data. You can get online and look up prop calculators for your boat size and such.

Bottom line: Whatever prop you choose should suit your local cruising conditions and should get your boat to hull speed. For me, I go three blade because here in the Salish Sea you spend a lot of time, and the most desperate time despite all your planning, pushing against either 5 knots of current or into 4-6 foot steep seas. Sometimes both. Like Cape Mudge. If you have a racing sled, naturally you don't want a 3 blade fixed, nor a 2 blade for that matter, but a folding prop.

See, lots of questions! Best wishes.

[Tmacmi]

Whoops fair point on additional information.

Its a 28 foot boat with 10' beam. Dry displacement 6,900 pounds. I guess I'd add another 1,000-1,200 with people water, fuel, etc.

It is powered by a Yanmar 2qm14 diesel.

We are in the Great Lakes....the gales of november come early, but no tides.

[tkeithlu]

To translate for you, 14" diameter with 10" pitch and right handed, built in Feb 1968 and 13" by 13" pitch, right handed and Sept 1979. You don't say what happens to your engine with each. If the engine over revs without getting you up to hull speed, you need more prop (greater diameter or greater pitch) and if it can't make it up to full revs then you need less pitch or less diameter. In that range, a prop with the same diameter and an inch less pitch might give you something like 300 more RPM.

[rbk]

Pitch and diameter aside to sum it up the three blade will be better balanced, provide better thrust both forward and back and unless your racing, only day sailing and/or locking your driveshaft you probably won’t notice any real difference when sailing. You could move to a fixed three blade Campbell sailor for some reduced drag and better performance for the price of a new fixed prop or for quit a bit more a folding prop but unless your existing three blade is bent, pink or otherwise no good I’d keep it on and the two blade as a back up.

[Tmacmi]

Originally Posted by tkeithlu

To translate for you, 14" diameter with 10" pitch and right handed, built in Feb 1968 and 13" by 13" pitch, right handed and Sept 1979. You don't say what happens to your engine with each. If the engine over revs without getting you up to hull speed, you need more prop (greater diameter or greater pitch) and if it can't make it up to full revs then you need less pitch or less diameter. In that range, a prop with the same diameter and an inch less pitch might give you something like 300 more RPM.

I bought the boat last year and it had the 3 blade on it, so I haven't tried the two blade which is why I am curious.

I also don't have a tachometer, so I'd have to gauge over rev by ear.

[MJH]

Originally Posted by Tmacmi

I have two propellers. The one currently on the boat is three bladed and has the following stamps on it:

2 68
14RH10

It has been able to drive the boat at 5 knots (calculated hull speed is 6.7 knots)

I found a 2 blade propeller in the stuff left behind and it is stamped

9 79
13RH13

I am wondering about the relative merits of improving sailing speed vs. motoring speed?

We are not racers, but I would like to understand what I am gaining or losing in more detail than, more drag when sailing with three blade, less motoring speed with two blade.

Thank you

Determining the best prop size and pitch for your boat is somewhat of a trial and error thing. A good place to start is by talking to others with the same make/model of boat, engine, and transmission ratio as your boat; all these items will affect your ultimate speed, compare apples to apples. I would also seek assistance from a prop manufacturer such as Michigan Propellers.

Note the maximum engine RPM and transmission ratio of your boat. With the prop you have on the boat and a clean bottom, find a calm day and drive the boat at full throttle noting the max RPM and speed in both directions. If you are maxing out the RPM of the engine WITHOUT any vibration or cavitation you are propped correctly. If you are getting vibration or cavitation you are over-propped and need to reduce diameter or pitch (Note that black exhaust smoke may also accompany an over-proped condition. If you are not reaching max RPM but there is no vibration or cavitation you are under-propped. With your test results consult your prop dealer or manufacturer if you are concerned.

Good Luck.

~ ~ _/) ~ ~ MJH

[Martkimwat]

You also need to know what gearbox forward ratio you have on your Yanmar, there are three to choose from, 2.21:1, 2.62:1, or 3.22:1 (reverse gear ratio is the same, 3.06:1 on all of them) You will find it on the makers plate on the aft facing part of the flywheel bell housing as I recall.

[stormalong]

I just looked up your Spirit 28 LWL - 22.64 ft. and I get a hull speed of 6.34, not 6.7.

Besides that, calculated hull speed is a theoretical number and actual hull speed is highly dependent on boat design and loading. Five knots under power is not an unreasonable speed for a 28 ft sailboat.

Can you ask the previous owner about the two props? My guess would be the one on the boat is the one he preferred and the other is the spare.

PS. If you want to sail faster you should look into a feathering or folding prop. A three blade is a lot of drag under sail.

[tkeithlu]

"I also don't have a tachometer, so I'd have to gauge over rev by ear."

Measuring over rev by sound really isn't going to help you find the right prop pitch, and might help you find a flying piston. Aside from this immediate task, the proper care and feeding of an engine really requires a tachometer. Better yet, a tach and engine hour gauge such as this one for under $100. Run it off the alternator.

https://www.summitracing.com/parts/s...SABEgIGk_D_BwE

For test purposes, here's a cheap optical tach.

https://www.harborfreight.com/search...l%20tachometer

[Tmacmi]

The tip for an aftermarket tach is helpful. Thank you.

For others:

My Spirit is the Fitch design. LWL of 23' , I was going from faulty memory. Its 6.4 knots hull speed.

The gear ratio of the transmission is 2.14

[roverhi]

I've got a 28' boat with 2GM20. Think it's got a couple more horsepower than 2 QM. Prop is a tired 13x13 2 blade folder. It will drive the boat at 5k at 2500rpm. Never tried to see what speed in calm flat conditions would be at redline which I think is 3600rpm. Personally have always felt 5knots is fast enough under power in a sailboat. The trade off of relative quiet at 5knot vice the engine hammering away to try and get hull speed just isn't worth it.

[angusbrodie28]

Hi Guys,

Any talk of propellers generates a whole spectrum of comment. A level of understanding is required. May I recommend "Propeller Handbook" by Dave Gerr. Easy to read, very comprehensive. All the best, Angus

[KadeyKrogen38]

Go to a good prop shop. They have software to determine the ideal diameter and pitch to meet you personal and engine/transmission requirements. They probably have or can obtain the torque curve for your model engine but what they need and do not have is what reduction ration is your transmission.

[Squanderbucks]

For diagnosing most any engine problem or prerformance questions most start with knowing what RPM the engine is running at at various throttle settings.
If you don’t feel the need for a permanently installed gauge how about a hand held digital unit. They are extremely accurate and require no installation.
Simply put a piece of reflective tape that comes with the Unit on the flywheel or harmonic balancer. Point the little “gun” at the tape while running the engine and you know exactly what it is doing. Cost only about $10-$12 on eBay or other online vendors.
Knowing your speed and Max RPM is the only way to know the best prop for your boat. It’s all trial and error after starting with what you have.
You don’t always have to buy a new prop if relatively small changes are needed. Just remove it and have it re-pitched or its diameter changed.
If your engine can not achieve the manufacturers rated max RPM ( over propped) you will greatly reduce the longevity of the engine. Unless you really only use the engine for short times such as docking and close maneuvering.
But if you cruise For hours to get places then you should really spend the time propping the boat/engine correctly.