The mystery of a proper prop and terrible performance.

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

  1. Mr Efficiency
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    Mr Efficiency Senior Member

    The gearbox ratio is all wrong to be swinging bigger diameter props. If it is 1.5 to 1
     
  2. missinginaction
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    missinginaction Senior Member

    True. But I paid about $600 for the one that's in there (an Aquamet 22). I'll have to balance out the work involved with the cost of a new shaft. Good point about having a spare shaft.

    Fallguy: I have 10.5" from the shaft centerline to the hull. So yes, I'd have 1.5" clearance with an 18" prop. The issue is that the standard for clearance on a high speed prop is 20% of the prop diameter. This would work out to a little batter than 3 1/2 inches. A four bladed prop may mitigate this a bit but I'll have to do a little more research. I believe that 17 is as big as I'll be able to go. Yellowjacket nailed that one in an earlier post.

    Mr. E: As for the v-drive reduction.....Ideally a better (higher) ratio would be desired. I can compensate for the increased diameter with a corresponding decrease in pitch at the expense of some efficiency. It's not a perfect world and I need to play the cards I'm dealt.

    Remember, I'm happy at 8 knots or so most of the time and am just looking to have the option of some speed when it's necessary. Spending $1,000 or a little better isn't a problem (tabs/prop) but replacing a perfectly good v--drive? Not necessary in this case.

    Regards,

    MIA
    R
     
    Last edited: Jan 28, 2020
  3. gonzo
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    gonzo Senior Member

    How far forward from the transom is the end of the shaft? I am thinking that the shaft could have been shortened, which would decrease clearance.
     
  4. Barry
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    Barry Senior Member

    MIA
    I could not load this into the PM's
    But shows the pressure distribution on stepped and non stepped hulls
     

    Attached Files:

  5. missinginaction
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    missinginaction Senior Member

    Hi Gonz,

    I don't have any reason to believe that the shaft was shortened. There's a rudder behind it and the clearance between the prop and rudder is reasonable as it is. I'm the third owner of this boat. The original owner belonged to our boat club and owned it for about 25 years. There are some guys at the club that remember him (he passed away before I bought the boat). They tell me that he was a stickler for his maintenance but never made any major changes to the boat. The second owner worked for me back almost 20 years ago. He didn't use the boat much, it was just a cocktail barge to him for the most part. When I rebuilt the stringers I made them stronger but kept the engine beds in the same location (I made a jig to be sure, before I cut the original stringers out). Never touched the shaft log as it was OK..

    Barry,

    You gave me a lot to chew on.....I haven't started on the paper you PM'd me. It's looking like the case for the tabs is getting stronger. It may be a little while before I digest it all. I will get back to you.

    Thanks to you both,

    MIA
     
    Last edited: Feb 4, 2020
  6. missinginaction
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    missinginaction Senior Member

    Thought I'd post an update to this thread as so many were kind enough to weigh in. I've been doing more research, especially on the trim tab issue. Here's where I'm going with this.

    I spoke with the people at Bennett Marine and gave them the particulars about the boat. I have enough room along the bottom of the transom for trim tabs up to 42 inches wide. According to Bennett while I may have the room, 42" tabs would be far to aggressive for this size boat. They recommended 24" wide by 12" or possibly 30" wide by 9" tabs. Doing the math the 24x12 tabs give me a little less than 7% more surface area. Bennett said it wouldn't matter much one way or the other.

    I'm going to splurge a little on this set up and go with the 24X12 tabs with the Automatic Trim Pro option. There are a couple of reasons for this. I'll install the trim controls on the lower helm. I don't want to run a separate control on the flybridge. I do run the boat from up top a lot though. After talking with Bennett and reading the operators manual I realized that after I have the system set up, I'll be able to run the boat from the upper station and the trim tabs will automatically deploy as the system senses the boats acceleration and change in running angle off of displacement speed. The tabs will then automatically adjust as the boat planes. They'll adjust to running angle changes and as she heels to one side or the other, attempting to keep the boat at the programmed running angle. Tabs deploy when coming off plane (keeps the bow down) and retract at low speeds or when the engine is off. essentially once the system is "dialed in" I shouldn't have to go below to make any adjustments. I've read good and some bad about this system. I believe that the bad reviews may come from folks that did a poor installation. I installed a Garmin autopilot a few years ago. It had some bad reviews as well. I've never had a bit of trouble with it.

    As for the prop a 17" Hung Shen 4 bladed prop seems to be the best compromise. 10" or 11" pitch.

    Thanks again to everyone who took the time to write in. I would have discovered the prop but I don't think I'd have gone for the tabs without the advice that I picked up here.

    We're building replacement docks down at the boat club on the weekends now, won't be too long until spring.

    Cheers,

    MIA
     
    Last edited: Feb 19, 2020
  7. Barry
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    Barry Senior Member

    The narrower the transverse length of the tabs, the more that the tabs will have to be deployed to achieve the same lift. The higher the angle down is, the more drag the tabs will produce.
    The concept is to get as long as stagnation line as you can for optimum lift to drag ratio. If the cost is not too much, go with the 30. Trying to make the point that the
    area of the tab is not as important as the length of the stagnation line and the optimization of the lift to drag ratio.
     
  8. missinginaction
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    missinginaction Senior Member

    Hi Barry,

    I've read that but Bennett is telling me that it won't make a lot of difference. What you say makes sense though. Bennett's position was that either will work. I asked about the drag issue. They made a couple of observations. First, when installed as instructed, the hinge line on the tabs will be above the bottom edge of the transom. When the tabs retract they will be out of the water and not a drag issue. When deployed (again when installed as instructed) they only drop about 20 degrees from neutral. I can't recall the exact spec but the point was that any more and they would act as a brake. Bennett was insistent that either size would work. So why not go for the 30"?

    This is where the business part gets interesting. I can buy a 24" x 12" Auto Trim Pro kit for about $800.00. It has everything included. This is important as you need specific actuators (the hydraulic rams) that include a position sensor. The electronic control module needs to know the tabs position in order to function properly.

    They don't make a 30" kit. So in order to make my own 30" set-up I'd have to order a la cart, so to speak. You need to order the tab/hydraulic pump/actuators, then an adapter kit for the actuators (with the sensors) and then the Auto trim Pro Control Module and helm controller. A la carte this stuff works out to about $950.00 and I have to modify the actuators.

    My thinking is to trust Bennett in this case and just pick up the 24" x 12" kit. If, after installation I find that I want more side to side span I can pick up a set of 30" X 9" tabs for $300.00. They insisted that this would not be necessary.

    One last thing.... I've always been interested in aviation. Jets today use winglets on the wingtips to reduce drag and improve airflow. I was wondering if anyone has seen similar technology used on boat trim tabs. I'm thinking of a vertical piece of stainless steel blade riveted to the outer edge of a trim tab (or maybe both edges). Something like a 1" x 12" piece of stainless steel angle. Would this keep the water that's under a bit higher pressure under the trim tab from spilling out the side, especially as the boat accelerates from a resting state?

    MIA
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    I did that with some tabs once, it may have helped a little.
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    If you're going this route... then why not simple interceptors... much less intrusive and easier to install.
     
  11. missinginaction
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    missinginaction Senior Member

    I don't know what interceptors are Ad Hoc.
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    See here:
    Interceptors - Humphree https://humphree.com/interceptors/

    It is basically where a vertical plate is moved up and down from the transom and creates a differential in pressure distribution, in exactly the same manner as a 'normal' trim tab does, that provides the lift.
     
  13. 7228sedan
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    7228sedan Senior Member

    With the prop and the tabs, she's going to run like a new machine... can't wait to hear about the results!
     
  14. DCockey
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    DCockey Senior Member

    In the US market for smaller boats trim tabs for retrofit are very widely available and inexpensive. Interceptors are becoming move available but are generally a more premium product.
     

  15. Yellowjacket
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    Yellowjacket Senior Member

    Moreover, trim tabs create lift aft of the transom in addition to lift ahead of the transom. Interceptors create lift ahead of the transom. Wide trim tabs also create less drag for an equal amount of lift than interceptors. And as noted in the USA interceptors are more expensive than trim tabs. If you're trying to plane off, the trim tabs increase the area as well as create lift both under the hull and on the surface of the tab I'm not saying that they don't have a place, but if you're trying to get over the hump, a wide trim tab will do the job for sure. If you've got a trim issue and running nose high at moderate and lower speeds an interceptor will do just fine.
     
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