Prop size, again....

I'm about to give up.

You need to have an experienced person who will step on your boat, make a thorough visual inspection, possibly measure her weight and lcg position and perform a range of test runs to fully undestand where is the problem.

I (or anyone else here) could suggest you a new prop to install but it would be a wrong thing to do, because at this point it would be based on input data of unknown validity. First you need a reliable assessment of the situation.

That's the best suggestion I can give you at this point, while sitting at the other part of the globe.

Cheers mate, and let me know what did you discover. I got really curious about this.

 

Well thank you. What kind of tests are you thinking? I'm pulling the boat next week, and wanted to order props over the winter. So, i'll have to act fast.

 

Propcalc uses "3 blade std. propeller" what ever it means. More information on their site:

http://www.castlemarine.co.uk/

 

Lets put the high drag aside and assume it is the hull/keel shape. The 18X18 props will consume about 85HP each at 1578rpm doing 17kts. THe efficiency will be about 73%.

If you can fit 20 X 18 then prop rpm will reduce to 1510rpm and prop efficiency will lift to 76%. A 20 X 19 prop will drop rpm to 1450 and the efficiency stays at 76%.

If you have room to fit larger diameter then this will improve prop efficiency by reducing slip so the rpm drops a little as well. Obviously increasing the pitch will reduce engine speed. An 18X20 prop will give also give 17kts at 1450rpm but the prop efficiency remains at 73%.

The motors will certainly have the torque to run the higher pitch around the 17kt speed and you should get better engine efficiency. The downside is that you may reduce top speed because the engines will not rev out.

If the slip has a weight scale it would pay to check the weight.

I would like to see a photo of the back of the keel and the bottom of the hull around the props.

One test to do now is to do a steady run at your desired speed, say 18kts, and get an accurate fuel burn. At least this will provide a baseline to compare any changes.

I would expect that working the engine harder at lower rpm will improve motor efficiency.

Rick W

 

I'll get the pics for you in the a.m. (i'm at work,pics are home). I can tell you that the keel is about 5 inches wide and is square at the edges. The bottom is smooth and without obstructions.
My only fear of increasing diameter is I need to be able to troll on one engine around 3-3.5 mph. Mark

 

Rick, we do agree on the figures about his cruise speed. It will be around 17-18 kts, be it one prop or another (reasonably chosen, of course).
The problem, as I see it, is this boat's top speed. The other guy's boat runs at 30 kts while this one does 22 and will make it up to max. 24-25 kts with new props.

So I believe that, while it's ok to advice firepiper about the best prop for his current boat configuration, there remains the fact that this boat is for some reason hydronamicaly inefficient. In a sense that it has a high resistance compared to the other similar boat - and that will spoil the overall propulsive efficiency whatever prop you choose to install.
That issue needs to be resolved first - if possible.
If, after all have been done to improve the boats' characteristics, firepiper sees that it is related to some non-modifiable feature like engines weight and position, interior structures, ifurnitures or whatever - than he accepts the things as they are and choose the best prop for the boat he has.
IMHO, as always.

Btw, your prop efficiency estimates are still so optimistic... I mean, 73-76%...

 

The bigger diameter improves grip. I cannot give exact numbers but the relative figures something like 800rpm on engine give 3kts with 18X18 then the bigger prop will only need 660rpm to do 3kts with 20X18. So you are right to be concerned.

So something else to check is the current rpm at slowest required trolling speed. You could need something like 20% less with the bigger diameter prop.

Rick

 

By the way - firepiper, are you sure your friend is not giving you mph numbers instead of kts??? That's very important and is a common mistake. 30 mph would be only 26 kts - in that case you just put the right prop and your boat is ok! :!:

 

I'm starting to wonder. But, even then, i'm not quite at 26 kt (23kt), and would still like to drop some cruise rpm's. (<3000) He's not a friend, just a guy that has a similar setup (8.1's w/1.5's) on the same boat. Now i'm more confused!!

 

I get the impression that the hull is not designed for high speed. From what I can gather it has a deep entry and large keel that is chopped off so a lot of extra drag compared with a more usual planing hull. The cabin is typical and no flybridge so no more windage than usual.

The prop efficiencies given are always the best case. Irrespective the relativity is the important consideration. Going up in diameter will improve efficiency and drop engine revs for any given speed.

Rick W

 

Any idea on the props being used on this boat?

Rick W.

 

I believe that, since we are dealing the ordinary-real-world boats, it would be more correct to give efficiencies as close to reality as possible. It is useless to tell the person that his prop will have 74% efficiency, knowing that you still have to detract a lots of things from that number.
I recognize that even 63-64% which comes out of my spreadsheet is optimistic for the comercial props of this size, but at least is trying to get closer to the reality.

Don't misunderstand me, Rick - I'm in no way questioning the corectness of your methods (which are proven, as we know), but I'm trying to put myself in the skin of persons like firepiper who are not prop experts yet need to have some real numbers that apply as close as possible to his boat.

Now, about this case. I think pretty much the same thing - that this boat is not designed for high speeds. and I believe that the other guy is really planing at 26 kts (30 mph). And that is the speed that can be approached.

This is my final result, I've put the corrected boat's weight in the spreadsheet:
3-bladed, 0.61 E.A.R. Michigan Wheel type prop, 18"x15" should give 25 kts top speed at 4550 engine rpm.
Cruise speed at 75% throttle will be 19 kts at 3700 rpm.

Both max and cruise engine rpms would be correct for this engine, for the reasons I have explained in this thread (that guy had the same engine):
http://www.boatdesign.net/forums/showpost.php?p=231072&postcount=15

And the last phrase in that post also remains valid.

 

Do not disagree with the result but it does not answer the original question:

"I would like to cruise more like 26-2800 rpm @ around 18-20 kt. "

I am suggesting a 20X18 would be a good choice to achieve this.

Are you recommending the only option is to fit an 18X15 and run the engine at 3700rpm??

Rick W.

 

I've had to work till very late last night to respect a deadline of a project I'm doing and it was very late when I wrote the last post.
So I'm here right now principaly to give you my public apologies for having criticized your results in public rather than discussing those questions in private with you. It is not my style and just didn't think about how would I feel if someone did it to me. Your competence is proven, I respect your work and I believe that Firepiper can trust your advices.
So, once again, sorry for my lack of style.

I agree with you that a 20x18 prop will give about 18 kts@2900 rpm, with some 90% throttle, I guess. Depends principaly on how you model the boat's resistance.

For my calculus, I have taken into a consideration the engine data too. JSH gave us the curves for Crusader 454 engine here:
http://www.boatdesign.net/forums/showpost.php?p=230858&postcount=13
For the reasons I have quoted in my previous post, the correct RPM range for this engine should be between 3500 and 4600.
So, although it is true that Firepiper wanted to run at 2900 rpm, I think he should be adviced for higher regimes, because at 2900 this engine's torque is in the instable regime, i.e. it needs to be compensated manualy for the increase/decrease in resistance (due to wave activity or wind, for example).

Once I've adviced him on that, I know I did my best and it becomes a matter of Firepiper's choice.

Now let me explain how did I choose the prop.
A 3x18"x15" E.A.R. 0.61 propeller allows the engine to work in it's stable regime of torque - at 3700 RPM at cruise and 4500 RPM max speed.
Diameter 18" is identical to the prop already installed, so I have taken it as upper limit for the diameter because it already fits the available space under the hull and probably allows for correct clearances between the prop and hull.
I have choosen the pitch 15" considering the above reasoning about the RPMs.
The number of blades and E.A.R. are chosen taking care of cavitation issues.
Finaly, the prop needs to be readily available in boat shops, so I have considered the commercial series only.

So, I absolutely don't say it is the only prop possible. and it is not the most efficient one for the cruise. This one simply appears to be the which satisfies the above design considerations.

That's all. I repeat, from the purely hydrodynamic point of view regarding the props characteristics I agree completely with your results - it's just that you can't consider this separated from the engine characteristics, imho.

Best regards

 

For the load calculation at 17kts I determined what power the 18X18 prop would absorb at 1510rpm. This was 85HP at the prop so say 100HP allowing for auxiliaries and shaft/box losses. This is well within the engines power capability. Will deliver 230HP at 3000rpm. Fuel burn per engine from engine data is estimated at 43lb/hr.

Going to the 20X18 prop the power required to do 17kts drops to 82HP or say 97HP allowing for auxiliaries and other losses. Fuel rate drops to 41lb/hr.

The engine data only gives full throttle power and best efficiency is around 3600rpm. Would be reluctant to assume best efficiency at lower throttle is at the same rpm.

The 18X15 prop gives 17kts at 3400rpm and prop power is the same as the 18X18. There may be a slight advantage as the engine could be in a more efficient range but you would want to see fuel rate at lower throttle settings to confirm this.

The 43lb/hr on each engine should be equivalent 12.4 US gallons per hour for both.

Rick W.