Prop concepts

My boat runs 19x19 heavy cup three blade round eared low rake props and cruises at 30 mph at 3600 rpm. Best WOT I can get is about 4100 rpm. I'd like to see 4800 rpm at WOT.

If I go to 6 blade rollas that are 17.5x22.5 what will happen? More blades but less diameter?

If I go 17x22 4 blade cleavers what will happen?

Thanks for guesses - expain the dynamics if you will.

 

without knowing much here, I'd go with the 4-blade, as long as it is applicable, I would think it at the very least less risky. I have other reasons too, but I cant back them up really.

 

1: heavy cup says that you don't have the right prop.
2: you have have provided nowhere near the correct info to select a prop.
3: at minimum you need to provide is Blade Area Ratio (BAR) and shaft horsepower (SHP) for any of the data you have given so far to make sense.
4: To get the best prop performance under modern theory, you need infinitily thin, infinite number of blades corrected for the Goldstien factors to get optimum performance.

 

Go take a look at this link on props. http://continuouswave.com/cgi-bin/propcalc.pl

 

I was really looking to understand how the number of blades effects the pitch required. If for a given application 19x19 works well with three blade props, what if I went to 4 or 6 blades? Would I need to decrease the pitch of the prop to keep the same speeds and gain the same rpm?

 

Pitch is effectively independent of # of blades in the selection process. Pitch is selected as a function of boat speed, desired shaft rpm, and propeller diameter traded off against avaliable torque and cavitation regimen. # of blades is selected based upon required thrust and is a function of individual Blade Area Ratios and blade section loading coefficients.

From the info you have given it shows that the prop you have is asorbing all the HP-torque the engine can put out. The question is wether or not you are at the proper operating point, and there is not enough info provided to determine that, though I will say that with 50% slip, it points to an overloaded prop so going down in diameter is not the answer. Because of the interference effects, increasing the number of blades decreases efficency. It is almost always better to go with a larger diameter and less pitch to get the blade area down and disk area up.

 

First off - I am very impressed by your answers. Let me provide all the information I can at this time:

Vessel is 42 feet long, 9.6 wide, 21 degree deep vee and without any steps.
Weighs approximately 15,000 lbs

Weight is centralized - motors are mounted 6 feet forward of transom. 4 fuel tanks forward of those and genny and water forward still. It runs flat no matter what I do.

Arneson surface drives capable of handling 1200 lbs torque and any prop diameter required.

Motors: 425 HP 502s. WOT is rated at 5000 rpm
Borg Warner 1.5:1 reduction
Results today with 19x19 three blade round eared low rake props:
30 mph by gps at 3600 rpm
40 mph by gps at 4100 rpm
Can't get higher rpms than 4100 rpm.

Does this help?

 

I ran a couple of quick numbers...and you know....the prop you have doesn't seem to be doing that bad...from the props point of view.

Given that the engines can't make WOT and the estimated hull ehp jives with the estimated prop absorbed hp for those speeds and rpms....I would start looking on the other end of the powertrain for the problem.

But this is just quick back of the envelope stuff using some Series 62/65 data and some generic high cavitation regimen prop data. Now we need to start sharpening the pencil. We also need to start looking at the engines (temperature, fuel flow, air mass flow, etc) and the gearboxes-shafting and make sure that is all correct because it looks to me that you are getting less than 1/3 of the possible installed hp delivered.

 

the fuel lines are half inch from pickups all the way to the fuel pumps. My flow meters show .9 gph at 3600 rpm total. about 20 gph per motor. Motor temps are normal - 170-220 oil and water around 160 - raw water cooled.

I am going to try some smaller props next week and see what happens.

again- these are surface drives

 

I know, that's what's bothering me. You should have enough HP to be getting to the other side of the efficency point, but you're not. Even with a 10% increase in pitch due to the cupping.

How close are the two tips BTW?

How much transom submergence at speed?

Edit:

Go reat this thread, especally Ranchi Otto's graph, your application is in that ballpark.

http://boatdesign.net/forums/showthread.php?t=13646

 

when on plane the props are at surface since it is a surface drive. The deep vee is probably 2.5 feet to the bottom from the surface at rest. Props are around 4 feet apart.

 

I believe that a "Rule of Thumb" for fuel consumption/horsepower is that a gas engine will generate approximately 10 HP for each gallon (US) of fuel used. Using this formula, you are only generating 200 HP at 3600 RPM. Believe jehardiman may be correct -- look to the engines.

 

What happens if you trim the drives up or lower them? Does the RPM change If it does'nt you have a RPM limit problem on the engines.

 

To get to 4100 I have to trim the props up rather high. I get a rooster tail and no additional forward speed.

I elieve that I am not getting to the "meat" of the HP curve as well but I thought it was either because:

1) Props too large
2) Transmission reduction ratios are not what the cases are labelled
3) The boat is heavier than I think.

1) I will try two different sets of props this weekend and post results
2) I have no idea how to check the trans. Is it possible that they are somehow reversed? I have one rotating clockwise and the other in the opposite direction (counter rotating motor). Can the reduction ratio work in both directions? If they didnt have reduction at all and were 1:1, would my prop/speed results make sense?

3) Many others have told me their gas powered Harleys are around 15,000 lbs so that is what I am working with.

Always open to ideas - thanks!

 

You need to make a phone call to Arneson --they will be very helpfull.