Prop size for heavy hauling and power

S

SEA3PO

New member
I have a Tug Boat sort of question.....I have a working tug much like the Ranger 21 and I have a Yanmar QM30 (30hp diesel) normally asperated...normal rpm is about 2000rpm-2200rpm.

I would like to increase my pulling power...not necessarily my speed....(roughly 10 knots wide open) I tow things like large boat docks, log booms and blocks of Cement weighing just over a ton....

My question....

For a new propeller do I want pitch or blade size for power ?

How close to the bottom of the boat should the propeller blade come ? I have a 16" rh 12pitch on there now. I measured from the center of the shaft to the glass bottom and it is 10" so could I use a 18" prop? (that would be 9" with an inch of clearance ?

Thanks in advance

Joel

SEA3PO 22 C-Dory Cruiser
Kea 22' S&S working tug
 
Of course, the lower the pitch number, the more power that your motor puts to the water, and the bigger diameter, the more water is being pushed on. The key is to understand the WOT specifications for your Diesel drive train. If you can't get WOT in the right RPM range, then you've got too large of a diameter of prop, or too high a number pitch prop or both. The WOT spec drives all prop driven craft I believe. I know prop aircraft operate under the same rules.

It sounds like you must be a professional captain to be doing all of that tugging around, so the above is probably a naively simple answer to what must be a more complex problem that I don't understand.
 
Joel, I get confused on this stuff. Check an authoritative source, or consult the maker of the boat. In any case, you want the desired rpm to be reached at the max speed you plan. My gut feeling is less pitch, not more, if you are more heavily loaded and moving through the water more slowly than with the boat not towing.
 
Ha! You are correct, I do use that WOT ratio with the C-Dory and twin 40 gas Yamaha's But that Yanmar is a different beast...amazing diesel, it starts easily anytime...never complains and I have never been able to drag it down...I have pulled 1000' of 3/8" galvanized chain off the beach and it just keeps pulling... or a string of logs..no problem with the diesel....(a bit of a problem when load way exceeds the weight of the tug) even with that it just keeps -pulling.. slight question about who is pulling or pushing..
 
Joel,
Lots of variables. As Dave says the prop clearance is 20% generally accepted, but…some go as low as 10% in a slow speed prop. 15% to 17% is generally accepted as minimal. Another but--and that is with the Kortz Nozzel--which many tugs use for max efficiency. There can be less than 15% there--and the nozzle is foil shaped. (One of my neighbors has been experimenting for several years, with making his own Kortz nozzles for an electric drive, solar powered cat--and come up with some very efficient designs, using low wattage, at about 4 knots).

For the YM30 (and the Q should be similar) [29.1 mhp@3600 rpm]
Maximum output at crankshaft * 21.3 kW@3600 rpm But the QM is an older engine--and I probably would not run it any where near the new ones top RPM.

According to the current (YM) series, the best RPM for torque (52 ft lbs) and HP (23) is about 2500 RPM--just a little over where you are running the engine.

You want to go to the max blade size, with 15% blade tip clearance, and pitch it to get 2500 RPM, where you are at your max load towing. This may not be quite as good for max speed, but you are not interested in that--and the displacement hull design will be the limiting factor in any case.

I think you would be OK with an 18" prop, and 10 inch pitch…However, I would join "Boatdiesel.com" and specifically the Yammar QM forum, which has 76 pages on the QM series.

Do you want to go faster with the load you are now pulling? It is really hard to know if you will gain with the new prop. What is the max RPM the diesel will reach currently with the loads? (Of course at a certain point the slip starts increasing rapidly--and you may be near that point already--the new prop may raise the speed slightly.)
 
The weight and displacement of the boat, the hull design, and water line length of the boat, the engine type, the horsepower, the transmission design, the balance between power and getting to theoretical top speed at displacement and numerous other factors play on prop size and pitch. If you boat is a factory built well designed, the factory should be able to give you some curves that would allow you to pick the sweet spot for what you are trying to do. Otherwise, it is a very expensive try this combo and that combo process to find the sweet spot yourself.

From the sounds of what you describe things are working fine, good cruise and plenty of power so, why mess with it.

I'm a firm believer that in most cases boats that have proven history and reputable designers have the best compromises already on your boat at delivery. Not always the case, but for 99 % of the buyers it is.

I get a kick out of guys who "chip" the engines in their trucks. Invariably, they find that performance gains in one place results in crappy performance in the other and go back to what the factory put in.

Anyway, wish I had a tug to go along with my C-Dory. It would be fun to go around Lake Powell and just pull out tamarisk plants.
 
OK, I think I understand.... plus the main thing was how much clearance between the blade and the hull I need to maintain.

The S&S company only built 9 of these boats...mine originally went to a Yacht Club as the Officials boat....then sold thru a broker to some guy in Ventura who did some cheesy wood work (inside) and put it out for sale... I have owned it over 20 years... working it most of those... it originally had a 2 cylinder Yanmar which would easily make hull speed of 10 knots...but I needed POWER so I got the Yanmar QM30 (cast iron) from a large Catalina sail boat that went aground in Santa Barbara.... I rebuilt that engine and it has been amazing, what a sweet heart.

I have no clue what it originally had for a prop.... I took it off and used it as a partial trade-in on a 16 RH 12 pitch... that prop worked really well until I ran it to far up a ramp while towing a disabled boat... gave the blades a nice curl.

Thru the magic of EBAY I installed a 13 RH 16 ($40) so I could finish the season....the tug responded quickly, seemed to run a bit faster and as expected did not have the GRUNT I needed... worked fine anyhow...

During the winter I watched EBAY for a larger propeller and purchased a 15 RH 17.... it is not exactly what I wanted but it is working in that direction... (also about $40)

I think what would be ideal would be something like 18 RH 12

It has been entertaining watching EBAY sales... some folks ask $1000 and more, then there are numerous $650 props.... I kinda like those
$40 props .....easier to replace than repair.. somehow just not the exact size I need.....

As I always say: People in hell need ice water

Joel

SEA3PO
Kea the only heavy tug in the county
 
Good point.... is having a fourth blade better for torque? I have seen them on EBAY but never thought about the additional blade surface area giving me more thrust.....I don't care about speed as the hull limits me to about 10 knots....after that it probably would take 200hp to make 11 knots.

Joel
 
to me, the fourth blade is like adding four wheel drive ... and you might not need it in your case

I would email Ken at Prop Gods for his 2cw

best
 
Generally speaking you need more blade surface for pulling. Larger dia. and/or more blades. The amount and sector that the trailing edge is cupped will also factor in. Pitch may need to be lowered. Drive gear ratio is also an important consideration. Bigfoot outboards for example use larger props but the gearcase is beefier and the ratio is lower.

Get the advise of a prop tech experienced in diesel engines. Trial and error can get very expensive!

Regards, Rob
 
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