Prop selection for CD 33

[CPowell]

I am re-powering my 1980 CD 33 sailboat with a Yanmar 3YM30 motor. I am looking for recommendation for a propeller. The motor dealer recommended a 15X14 RH three blade. Any thoughts from those with experience is welcome.
Thanks,
Chris Powell
CD 33 - Spring Tides

[tartansailor]

Need to know the optimum engine rpm and the reduction gear ratio.

Dick

[Ron M.]

In addition to what Dick mentioned you'll need to know engine rotation an weight of the vessel, as well as your rudder aperture.
I had good luck with a new prop when I re-powered a few years ago.
Deep Blue Yacht Supply took all necessary criteria and and came up with the right propeller for my boat. Michigan 3 blade 13 X 13 L works nicely, reaches hull speed easily at under 2000 rpm's.
________
Volcano vaporizer

[Al Levesque]

Hi Chris, I received your PM but thought it best to reply here to encourage further input.

Our CD33 had a 14R9 prop with the former Universal engine. When we did a trial run with the new 3YM30 and the old 14R9 it was obvious that the pitch was too low. The diesel mechanic looked at the amount of overspeed and suggested that if I bought new I should try 14R12. I did that and found it almost exactly on target. I don't recall exactly but I think engine RPM maxed out under load about 50 RPM over the design max of 3600.

Both the old and new props are 3 bladed sailor type. The gear ratio is 2.62:1

I don't think the aperture is large enough for a 15 inch wheel. There may be space to rotate but I don't know what the tip effects would do to it.

Good luck with your new engine. Feed back your results on the prop.

[Maine Sail]

You might want to consider a Campbell Sailor prop. Norm will do the sizing for you. Excellent three blade fixed props and very, very smooth..

[CPowell]

To All:
Thank you for all your input! It is very helpful to receive advice from those who know! Keep the info coming and I will provide feedback to you when I sea trial the new engine and prop.
Chris

[CPowell]

To answer some of your questions about Spring Tides:
Boat weight is about 14,000 lbs
Max aperture is 17" high
Yanmar 3YM30 gear ratio is 2.62
Max RPM is 3600
Engine rotates clockwise viewed from stern
Recommended prop was 15X14 RH three blade
Chris

[Tod Mills]

With a 17" aperture, 80% would be 13.6", so a 14" would be pretty close (82%); a 15" is getting kind of big (88%).

The 80% figure is a rule-of-thumb that comes from books like Skene's as a safe figure to avoid vibration caused by change in flow as the blade passes the deadwood.

[fenixrises]

Hi Chris,

I pasted part of this from an earlier post:

Generally the prop on a displacement type boat should never turn faster than 1,000 RPM, because it will start to cavitate.

Unfortunately most modern sailboat engines have about a 2:1 reduction gear when they should have a 3:1 reduction gear.

The formula is relativly simple.

A 12" pitch means the prop wants to move forward 12" for every revolution. Of course water is a liquid and the prop "slips", normally for a sailboat with 2 blade prop the slip = about 50%. So in reality for each revolution the prop moves forward 6".

So 6" X 1,500 RPM(engine at 3,000RPM w/ 2:1 reduction) = 9,000"
9,000"/ 12" = 750' per minute
750' X 60 minutes = 45,000'
45,000' / 6,020'(nautical mile) = 7.47 knots

That speed, 7.47 knots should equal your hull speed or a bit less, say 7 knots, so you are not running the engine at full speed.

If this number is too high then you need to reduce the pitch, too low increase pitch.

The prop diameter vs. available engine HP is derived generally from a chart, Skene's has one.

For displacement hulls it is better to turn a large prop slowly than to turn a small prop fast(over 1,000 RPM). Less slip and cavitation that way and far more efficiency from the engine.

Large ocean going ships rarely turn their props over about 100 RPM.

To modify the above for your application is easy.

2.62 : 1 reduction @ 3,600rpm = 1,375 rpm

A three blade prop = about 40% slip so 14" pitch = 8.5". Plug those numbers into the simple formula.

8.5" X 1,375 RPM(engine at 3,600RPM w/ 2.62:1 reduction) = 11,690"
11,690"/ 12" = 974' per minute
974' X 60 minutes = 58,437'
58,437' / 6,020'(nautical mile) = 9.7 knots

So I would say too much pitch cause a 33 is never going to do 9.7 knots under power.

More likely about 11" pitch which by the above formula = 7.5 knots

Take care,
Fred

[darmoose]

Fred

I understand your formula (I think) and I was working out the numbers for my boat ,based on your formula, when a question occurred to me.

Doesn't the diameter of the prop have something to do with the result also? Particuarly for a given size of boat? Surely a 11 pitch is not the same for the 33 footer regardless of whether you use a 14" dia or a 10' dia, is it?

Darrell

[Oswego John]

and also the width across each blade's cup.

O J

[tartansailor]

I calculate the 14" pitch as being too high also.

Dick

[fenixrises]

Hi all,

The diameter of the prop is matched to the available HP of the engine. This can be seen with variable pitch props.

A smaller diameter = an engine with less HP.

Most small boats cannot use the available HP from their engine because the prop apeture is too small. Even boats with props on a strut rarely have the clearance needed for a larger diameter prop.

Case in point. Many owners of Westsail 32s opted for the larger engine, in this case a Perkins 4-108 with a 2:1 Borg Warner reduction gear. This is a 40~50 HP engine. But the prop apeture was too small to utilize the HP from the engine. So they used the biggest diameter that would fit with three blades. I would estimate that there was at least 20~25 extra HP with that setup. And the power could not be used at all. Most of these engines used one gal/hour at cruise speed, not very economical.

So the ideal engine for the WS 32 was actually the original stock engine. A 2 cyl Volvo that was about 25HP BUT the Volvo only had a 2:1 gear when it should have had a 3:1. This is just like the 13HP Volvo that many of you now have on your 28s and 30s. On Fenix the gear was even worse, 1.91:1. 13hp is plenty of power for a 28 or 30 and with a bigger prop and reduction gear it could be used effectively.

Magi, another boat that I built was 31' and about 13,500 lbs disp. The boat had a long fin and the prop was on a strut. Since I built the boat I made sure the strut allowed a large enough prop to be used.

The engine was a Yanmar 2GM 16, that's 16 HP with a 3.25:1 reduction gear. The reduction gear was a standard option from Yanmar. The Yanmar dealer down towards San Diego had never sold an engine with this combination. They didn't even have the reduction gear in stock. So they had to take the gear apart and change the gears. No cost to me you understand.

With this setup I was able to swing a 16"dia X 10" pitch, 2 blade prop. This resulted in a slow cruise speed of 4 knots @ 1,500 rpm. The engine used one qt/hr at this speed. On the other end the boat achieved hull speed @ about 2,800rpm, so it was slightly overpropped. But the best part was that the boat could do over 5 knts directly into 20 knt tradewinds and 6~8 ft seas.

The guy that bought the boat from me had the prop reduced to 14" dia. This allowed the engine to develop full speed, about 3,200 rpm as memory serves.

There is an old adage about sailboats and engines. One HP/ton of displacement for a true auxiliary, one HP/1,000lbs disp for a motor sailer. Back in the day small lightweight diesels were not available and the engines that were installed were usually heavy and slow turning, say 2,000rpm max, so a 2:1 gear was ideal.

These days with small lightweight engines it seems most manufacturers use an engine that = 500lbs disp/HP. In my opinion way too much HP. And the HP cannot be effectively used because the reduction gears do no reduce shaft speed enough. Prop apetures or tip clearance for the prop is not sufficient to allow the use of bigger props.

Generally diesel engines produce 22HP/gallon of fuel/hour. Some of the modern high tech truck engines can do better than this but not by much. So those 18 wheelers out there, when loaded, get about 5~6 miles per gallon. That's about 220~250HP at 1,500 rpm.

By the old adage 22HP = a 22 ton boat, pretty big vessel. Even 22HP = 22,000lbs isn't bad. A 3YM30 Yanmar is probably a 30 hp, 3 cylinder engine. For a 14,000lb boat that's about oneHP/500lbs, exactlly what the modern standard seems to be. But I would bet that that 30HP will never be used because the prop is too small to harness the power available and the reduction gear while better at 2.6:1, will spin the smaller prop too fast and thus cavitate.

Remember my old boat Magi cruised in a flat calm at 1,500 rpm. 1,500/3:25:1 gear = 460 rpm for the shaft. At that speed the engine was probably producing about 5HP. Remember 22HP/gallon/hr. Thus one quart/hour = about 5hp. 5HP!!! To increase speed to 6knts under the same conditions required about 2,400rpm and a doubling of fuel consumption.

It is easy enough to check the horsepower/torque/fuel consumtion curves on any new engine. The info can be found in the manufacturers brochures or online.

My new boat Sunshine has a Pathfinder(marinized VW Rabbit engine) diesel. It is 1.5 liter disp. about the same as a Perkins 4-108 in both disp and HP. This engine is rated at 50HP, way too much for my 36', 15,000lb boat.

If I had the money I change it to something similar to the Yanmar 3YM30. But even that would be 500lbs/HP. The reduction gear is a Hurth 2.5:1 so effectively I can run the engine at only 3,000rpm, it is rated at 4,000rpm, so 25% of the available power is wasted.

Since the prop is only 16" dia I could never use 50 HP, in fact my old 2GM16 with the big 3.25:1 gear would almost be ideal. Yanmar did increase the 2GM16 to a 2GM20 in later years, so slightly more HP at 20.

Think long and hard about repowering. Boat manufactures love the easy increase in profit by upsizing your engine, After all it costs very little to install a larger engine in the production line. But increasing prop size can almost never be done because it would require retooling the hull mold or a new strut. Even the shaft diameter might need to be increased, although 1" dia shafts are used extensivly in small high speed power boats with 300~400hp gas engines.

Most sailor prop blades are = to ~ 35% of the swept area. In other words if you take the diameter of the blade and convert that to a circle and compute the area of the circle each blade will be about 35% of the area of the circle. Thus a two blade prop would cover 70% of the area of the circle and a three blade 105%. That is why 3 bladers need less pitch, more area and less slip.

To address Darrell's question. You are right in that the diameter of the prop is also a factor. Otherwise we could all zoom about with 2" props. The engine prop combination must produce suffcient thrust to push the boat through the water. Hopefully this is done with as little wasted motion as possible.

Without digging out my Skene's I am recalling as I can from memory. Skene's has a number of nomographs for prop selection.
First thing is to determine the diameter. This amount is selected from the nomograph for displacement hulls. Next is the HP required at the expected shaft speed to turn a prop of that diameter. There is a nomograph for pitch selection as well but I find that following the simple formula is just as easy.

This is the method I used for determining the prop for Magi. It was pretty darned close to right on. In reality the prop I used was a used two blade Martec folding cruisers prop. This had far more area than a full on Martec racing prop and a bit more area than a fixed two blade sailor prop.

Prop selection is not an exact science. But you should be able to get quite close on the first try. While many people use three bladed motor boat props I do not think it is a good idea. A motor boat prop will have more blade area than a sailor type prop but it is not really needed while it has the disadvantage of being an even bigger drag. Remember that power boats do not drift along under sail. To move they are under power at all times.

Many of the new small diesels are used a stationary engines powering generators, a/c freezer units and water pumps. These engines often run at only 1,500~1,800 rpm and usually last 8,000~10,000 hours. They are designed as a unit.

Generators almost always run at 1,800 rpm thus producing 60 cycles per second, the usual for AC power in the US.

Perhaps that is why those used for marine power use reduction gears and props configured for less of the engines power. Considering the engines is under "full load" when ever it is running.

However most engine engineers say that a diesel engine in good condition and with proper maintenance can run constantly at 80% of its full rated power under load. I cite the 18 wheelers. Most of those engines are rated for a top speed of ~2,000rpm. Over the road they usually run at ~ 1,500rpm so 75% of full rated speed.

I think for the CD33 an engine of about 20HP and at least a 3:1 gear would be the right combination. With the 3:1 or if still available 3.25:1 gear would turn the same prop(15"X10~11") as the the 30HP engine and do so, more economically both at time of purchase and in the long run. It would save both weight and space as well.

Take care,
Fred

[CPowell]

WOW!!!! My head is spinning over the responses I have received! Thanks to all! BUT could someone sum this dissertation up and give me the best prop recommendation, size and pitch?
I have found all your replies very informative but how do I make a decision? Why did the engine supplier (Old Port Marine in Newport, RI) recommend a 15X14 three blade?
Chris

[tartansailor]

Assuming zero slip and 3200 rpm, you would in theory achieve
your 6.6 knot hull speed with a 6.57" pitch prop.

Now everyone has different concepts, and we respect all of them, but if it were my boat, I would like to cruise at 2800 rpm, ASSUME
20% slip, I would "call" in theory a 9.4" pitch.
So in the real world, we are looking at either a 9" or a 10"
I would buy the 10" stipulating that I could exchange it if
I found my hull wave punching ability different. +/-

IMHO

Dick