The mystery of a proper prop and terrible performance.

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

  1. Ad Hoc
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    Ad Hoc Naval Architect

    There in lies the rub with 'sales figures.

    This guys 25 weighs nearly half that of MIA's...thus such performance specs are more likely than one which weights nearly twice as much.
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    I take that with a grain of salt, I read that his 135hp mercruisers burn 10 gals an hour each at cruise, that would be double the reality. The typical 25 Bertram Flybridge with twin sterndrives would be at least as heavy as this boat (7000 lbs) and would plane with a V8 sterndrive.
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    That's the whole point.
    It is all very subjective data out there - ostensibly 'sales' figures. And hard to get real hard data as a means of a metric other than empirical/subjective data.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    I checked out Bertram 25's for trailing weight a few years ago (daylight, permit) and it is over 4 tons easily. Built out here may have been heavier than USA build though.
     
  5. missinginaction
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    missinginaction Senior Member

    Thanks for the thought's, they're all good. As for the dead rise, I believe it's 20 degrees.
    254.JPG

    A lot of comments are about her displacement and you're right to question that. I have her original builders certificate but that doesn't show displacement. I looked at the Bertram 25 and other mid 20's boats trying to get a handle on this. One of the reasons I restored her is that her hull is solid fiberglass and quite thick. 3/4 inch under the engine tapering to about 1/4" forward. I had her gutted as I started the rebuild and added additional batteries and water tankage. I knew enough to position the heavy items amidships so as not to make her bow or stern heavy. She sits right on her original lines. If I really load her up for a trip she settles an inch or two but that's expected.

    I love those Berties. But this old Silverton is a completely different boat. Where are you going to find a 25' dual station flybridge cruiser today?

    I don't need or want her to be a fast boat. I just want to get her propped right (or as close as I can). I'm planning on taking her to Florida in the fall of 2021. I'd like to be able to carry a little speed going into some inlets and maybe running from weather. Other than that I'm fine with an 8 knot yacht.

    I'll contact a prop shop (maybe that guy in Tom's River). The nice thing about having a small boat is that you can ship your prop UPS.

    Thanks again for all of the replies....when I get a new prop on her and can run some tests I'll post the results. That'll be awhile though. It's a little icy up here.

    MIA
     
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  6. 7228sedan
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    7228sedan Senior Member

    Don't rule out adding trim tabs... your V is significantly deeper then my 28 Luhrs was. You'll need more lift to get over the hump. Prop work will definitely help there, but tabs also will make it a whole new picture especially if you have to run in any kind of weather. Having tabs heading into chop keeps the bow down allowing you to throttle back and remain on plane. Good luck! here's to spring.
     
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  7. Yellowjacket
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    Yellowjacket Senior Member

    7228 is correct, trim tabs will help reduce the power and thrust required to get you up on a plane significantly. As seen in Ad Hoc's pictures, your rig has a high hump to get over and that is part of the problem. The tabs will lower that hump and as I noted above that gets you moving faster and the prop works better the faster you go. This is one of those things where each thing may not make a huge difference but in sum the results are pretty dramatic. The first step is to get it on a plane and then you can look to getting it trimmed properly.

    Also, going into some smaller inlets with an 8 knot boat is a really good way to go swimming or not even go in if the tide is going the wrong way. Being able to speed up to keep your position relative to the waves is really important and if you're going to go in and out of inlets you need to be able to accelerate quickly to avoid broaching. In this case speed can be your friend.
     
  8. missinginaction
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    missinginaction Senior Member

    Yellowjacket, all of my experience is on the Erie Canal and Lake Ontario. I've never had to deal with tidal flows so I'm a little wary of the sea states in some of those inlets. I agree with you based on what I've read.

    Thanks also to 7228sedan, I know how hard you worked on the Luhrs!

    Perhaps the best solution is trim tabs and a 4 bladed prop with the most E.A.R. that I can find. Hung Shen makes a 16" with 40% more developed blade area than I currently have and a 17" with 57% more. Even with the 17" my blade loading would be significantly above the allowable limit. But the allowable limit is calculated at full throttle.

    With a set of trim tabs perhaps I can get "over the hump" as you say with 80% power. Now I'm applying about 167 HP to the prop instead of 209 horse power. The developed area of the 4 bladed 17 inch diameter prop is 154 inches. Plugging these figures into the allowable blade loading formula, adjusting for the decreased efficiency due to 4 blades and the lower speed of water at the prop (wake factor) gets me to a prop loading of 9.6 PSI. At 80 % HP (167 SHP) allowable blade loading works out to 9.0 PSI. That would be about as close as I could get unless I can find a prop with a little more developed blade area.

    The 17" prop would only leave me with 2 inches of clearance between the prop tip and the bottom of the boat. But the 4 blades would tend to reduce any vibration. If the vibration turned out to be a real big issue, I've noticed that I could move the entire drive system aft a couple of inches. To get the geometry to work this would involve lowering the engine about 3/4" so that the shaft would line up with the coupling to the V-drive. A bit of work but doable. I installed and aligned the engine back in 2014 so I know how this works.

    Based on the math this may be the best I can do. Looks like it's time to break out another thousand or so.

    Thanks very much to everyone who's commented. I know I say this all the time but your comments get me thinking. That's what it's all about I guess.

    Cheers,

    MIA
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    MIA says he hasn't pushed it beyond 3000 RPM. I don't think you can get a bead on the situation till it is floored.
     
  10. kapnD
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    kapnD Senior Member

    I agree with Mr Efficiency, why wouldn’t he push the throttle clear up?
    It may require that to get over the hump, and it is likely that once the boat planes, the throttle could be backed down considerably without falling off plane.
    I had some troubles once with a prop that lost its diffuser ring (proper terminology??), never did figure out why that mattered, but it did.
     

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  11. Barry
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    Barry Senior Member

    Might be worth joining the Silverton Owners Club for 25 bucks and asking the questions. Maybe someone has gone down the same path before
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    I would have to concur...as it is all bitty info which doesn't help in the analysis, other than just thoughts/suggestions etc and the biggest piece, the max rpm info..is missing!
    Full set of data aids considerably.
     
  13. Barry
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    Barry Senior Member

  14. missinginaction
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    missinginaction Senior Member

    We'll see come spring gentleman. When the engineering tells you that your prop loading is over 19 PSI and it should be no more than 9,7 PSI you don't need a full throttle test to tell you that there is a problem. The math doesn't lie. I understand your point(s) but in this case I'll respectfully have to disagree. If the prop loading was closer maybe. In this case though it's academic.
    Maybe I should have mentioned that there were two previous owners. I'm sure that they tried. When I bought the boat I was told "She won't plane." The math now tells me why.
    I'll let you know how this works out. Stay tuned in the spring/summer.
    As always, thanks for the replies....
    MIA
     

  15. Ad Hoc
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    Ad Hoc Naval Architect

    The lack of data is rather surprising for such a popular boat.
    Even their user manual offers little.
     
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