The mystery of a proper prop and terrible performance.

Discussion in 'Props' started by missinginaction, Jan 25, 2020.

  1. DogCavalry
    Joined: Sep 2019
    Posts: 2,511
    Likes: 1,256, Points: 113
    Location: Vancouver bc

    DogCavalry Soy Soylent Green: I can't believe it's not people

    Would you consider an outboard as a supplemental source of power? Despite the insistence on a WOT test, once full cavitation has started, more throttle just means prop damage. So I agree with you there. Unless you can get a significantly bigger prop under there somehow, more power gives you nothing. The 302 already gives more than the prop can handle.
    Shaft angle definitely reduces available thrust. In fact, at 20°, the blades cyclically pass through a state of 20° too much AofA, 20° forward rake, 20° too little AofA, 20° aft rake. Imagine calculating that thrust.So an outboard, just to get you on the plane, and then only when on the ocean?
     
  2. missinginaction
    Joined: Aug 2007
    Posts: 1,094
    Likes: 248, Points: 63, Legacy Rep: 512
    Location: New York

    missinginaction Senior Member

    Hi Dog,

    Well 20 degree v-drives are typical in a lot of boats. I'm not sure what advantage there would be to having the prop perpendicular to the water.

    I just got back from my longest test ride yet, 4 hours. I was mainly just getting comfortable and doing some long running tests on the new cooling system.

    I use a Yamaha high thrust outboard as a trolling motor and as an autopilot. Even at a mere 9.9 horsepower the T9.9 will push my boat at 4 knots all by itself.

    I didn't want to push things too far as I was alone and couldn't run the boat and see what was going on back over the transom. But I did run the T9.9 up and then throttled up the main engine. Just to 2500 RPM. I wasn't out to plane her off, I just wanted to see what would happen. At 2500 on the main engine I was up to 10 knots. I know that the main will get to 3000 RPM on it's own, we did that last weekend. I'll wait until this weekend when I have a friend on board to keep an eye on things aft. Maybe, just maybe the T9.9 will provide just the thrust that the main is lacking.

    Like I keep saying.....We'll see. The good news is she's running great.

    MIA
     
  3. Mr Efficiency
    Joined: Oct 2010
    Posts: 10,401
    Likes: 1,035, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    You are a very patient man who is seemingly not expecting the world, but I think everyone wants to see this boat plane ! Looking at the picture you posted yesterday, she seem to be trimming level and high enough to dispel notions of being in the super heavy class. Trying to use the "kicker" simultaneously won't work, at the typical pitch of props used, at 10 knots you will be dragging the kicker and not having it pushing. I don't know about the 20 degrees, it sounds a lot of shaft angle, but I would defer to those with greater knowledge on that matter, are there greater losses through a 20 degree transmission ? Again, I don't know, or can see why there might be, others may know better. But when you consider you could have a prop working at 25 degrees to perpendicular to the direction of travel, that seems far from ideal to me.
     
  4. DogCavalry
    Joined: Sep 2019
    Posts: 2,511
    Likes: 1,256, Points: 113
    Location: Vancouver bc

    DogCavalry Soy Soylent Green: I can't believe it's not people

    I wasn't thinking a kicker, I was thinking a 75. Turbo boost. Once you're past the hump you wouldn't need it. But if you were close with 9.9, maybe 75 is overkill. A 25?

    The 20° thing is not unusual. The losses are simply accepted. I'll read through Gerr tonight. See what he says.
     
  5. Mr Efficiency
    Joined: Oct 2010
    Posts: 10,401
    Likes: 1,035, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    Beyond my ken, but I am wondering whether this exaggerated shaft angle is not causing cavitation, and that is why the revs don't seem to be translating into the expected forward speed, was the 3000 revs with full throttle ? It it wasn't, might be time to give it more throttle. Maybe the high shaft angle ( It would amount to 25% when the boat squats) requires a specialised prop. I saw one source that listed a 15 degree angle as losing 3.5% performance wise. 25 degrees is a lot more than that.
     
  6. DogCavalry
    Joined: Sep 2019
    Posts: 2,511
    Likes: 1,256, Points: 113
    Location: Vancouver bc

    DogCavalry Soy Soylent Green: I can't believe it's not people

    20° loss isn't transmission related, it's prop related. A prop shaft parallel to water flow gives 100% of thrust. A prop shaft perpendicular to water flow gives none at all. In between angles are somewhere between all and none.

    Edit: Gerr, pg 86. 15° should be taken as the upper limit. Above that, early cavitation.
     
    BlueBell likes this.
  7. BlueBell
    Joined: May 2017
    Posts: 2,120
    Likes: 654, Points: 113
    Location: Victoria BC Canada

    BlueBell . . . . .

    Really?
    Angle of attack variations through the path of rotation on each blade?
    Vibrations set up by blades pulsating through those pressure differentials each rotation?
    Thrust vectors alone are benefit enough to a perpendicular installation, no?
     
  8. Mr Efficiency
    Joined: Oct 2010
    Posts: 10,401
    Likes: 1,035, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    I think he has to give it full throttle if he hasn't already, if it is still only getting 3000 rpm, cavitation is not the issue. There is a good chance it is cavitating though, if he has just eased the throttle up to where it gets 3000rpm, and the boat can't get to 10 knots
     
  9. missinginaction
    Joined: Aug 2007
    Posts: 1,094
    Likes: 248, Points: 63, Legacy Rep: 512
    Location: New York

    missinginaction Senior Member

    OK. OK...maybe I could have expressed myself a little better.

    My mind was thinking in terms of inboard boats. How could I get a prop perpendicular? With a sterndrive sure but they have universal joints. I don't have that luxury.

    So it's a compromise. For the size boat that I have 20 degrees is standard. It has to be. If you tried to install say a 12 degree v-drive you'd never get enough prop under her. Unless of course you moved the engine into the main cabin and installed a longer shaft. But that would defeat the whole purpose of the v-drive. So it's a compromise.
     
    DogCavalry likes this.
  10. Mr Efficiency
    Joined: Oct 2010
    Posts: 10,401
    Likes: 1,035, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    So was 3000 RPM full throttle, MIA ?
     
  11. missinginaction
    Joined: Aug 2007
    Posts: 1,094
    Likes: 248, Points: 63, Legacy Rep: 512
    Location: New York

    missinginaction Senior Member

    Yes Mr. E, the other day 3000 was full throttle RPM. I even checked the linkage to make sure the throttle was opening fully.

    Today with the kicker giving her a little extra boost I was going just about exactly as fast at 2,500 RPM as I was last weekend with the main at 3,000RPM. I didn't want to push it any farther because being alone I didn't want to leave the helm and go back to the transom and look. The boat felt different, not squatting quite as much and it was considerably faster at 2,500. Once I can get an extra pair of eyes on her we'll do the experiment again.

    It's not cavatating. The new prop has a developed area ratio of 0.86. The calculated blade loading on this prop for my engine horsepower and shaft speed is under 10 PSI. Based on the math it is right on the money. I'll see if I can take a photo over the next few days when the boat is trying to plane. There is a LOT of water being pushed by that prop. Much more than the old prop (which was cavatating).

    That little 9.9 kicker is geared 3:1 and swings an 11" prop. It's not meant to push you fast but it does create a lot of thrust at low speeds. With the power trim I can tuck that engine under a bit so that the thrust will tend to lift the transom. It's not a lot of power I know but maybe, just maybe it might be enough. That's my hope for this weekend. If that works and she does actually get on plane, it would seem to me that a set of trim tabs would be an equivalent solution.

    We'll just have to wait and see. BTW, you're right when it comes to engineering problems I've learned to be very patient. I'm not so patient when I'm stuck in traffic though.

    MIA
     
  12. Mr Efficiency
    Joined: Oct 2010
    Posts: 10,401
    Likes: 1,035, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    OK, well I don't know where you go to with it next. Try the tabs, but don't expect much.
     
  13. Nidza
    Joined: Nov 2016
    Posts: 90
    Likes: 24, Points: 8, Legacy Rep: 10
    Location: Belgrade, Serbia

    Nidza Junior Member

    MIA, yes, in my case it is also a hobby boat, very similar story to yours, even the same expectations of boat, but fuel consumption somehow made a bit offset to running too often at high speed. Rather I use the semidisplacement potential and save time on river when going upstream the river at consumption of 6 liters/hour. At planing speed, I would save a lot of time on long upstream streches, but at 30 liters/hour it is far from linear correlation between fuel, time and distance. Even friends with planing boats for these holiday trips load the boats so much that they use them at displacement speeds in the end (no brute force power in engines).

    Anyway, use the expert knowledge of the members here, they helped me a lot a few times, I am an engineer, which helps, but in different domain. Only thing I noticed in the last posts is that you are not reaching the full engine speed at load and you said you checked the linkage, I guess with engine turned off. I would also suggest you to check if unloaded engine in neutral is reaching full rpm to see if you have some technical issue which could be easily fixed. It is unpleasant for your ears, but nothing that will damage the engine for a short time. As I understood you did changes on the engine, so it makes sense to check. If it does reach max rated RPM in neutral then you can continue exploring load, cavitation and other issues.

    Out of scope comment/question for members - was the “deformed” propeler for shafts which would compensate the shaft angle never built/designed because such a form is not possible, it would vibrate the shaft, forces towards shaft would be too high or due to technology limitation to huild such design?
     
    DogCavalry and missinginaction like this.
  14. 7228sedan
    Joined: Nov 2009
    Posts: 343
    Likes: 11, Points: 18, Legacy Rep: 60
    Location: New Jersey USA

    7228sedan Senior Member

    Glad to see you got away from the dock! My prop was a 16x16 3 blade with a medium cup. Tabs on that girl will make a tremendous difference. I used to plane at 2600 and hold 10 knots there. At 10 knots you're really close to planing speed for that hull. I know your goal is displacement speed cruising (I think). but being able to nail it and hit 15-16knots would be good. Especially true if it gets the motor up to recommended WOT RPM.
     

  15. missinginaction
    Joined: Aug 2007
    Posts: 1,094
    Likes: 248, Points: 63, Legacy Rep: 512
    Location: New York

    missinginaction Senior Member

    Good to hear from you 7728!

    I think you're right about the tabs. I don't expect much more than 20 knots and would be really happy with 23 as that's what the math says she should do. But that will have to wait for next year as I'm not going to get into all that until at least fall.

    I don't know if you read the recent posts but my buddy, who acted as my eyes at the transom (and knows the boat and the way it used to behave) made the comment "We're not going very fast but that prop is pushing a hell of a lot of water out back here." If I can get her moving just a little faster she feels like she'll come out of the hole.

    Trim tabs should help lift the stern and I think we just need a little, not a lot of help back there. Before the end of the season I'm going to run the fuel tanks down and the water tanks as well. If I remove some of my tools and supplies I'd say we can lighten her up by about 10% or about 700 lbs +/-. We'll try her again. I even have an idea to use the T9.9 kicker as a little booster. I can tuck the leg of the OB under a bit which should give us some lift. I don't want to use it all of the time but as an experiment it might help.

    There is no vibration in the drive line. She's quieter with the new v-drive and I think that even at displacement speeds she may be more efficient. I'm planning a short trip in September maybe 400-500 miles round trip. I'll get a better idea of her fuel usage by then.

    Regards,

    MIA
     
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.