Barrel length & diffuser

Discussion in 'Props' started by Tapani, Aug 13, 2022.

  1. Tapani
    Joined: Jul 2022
    Posts: 5
    Likes: 1, Points: 3
    Location: Finland

    Tapani Junior Member

    Hi all,

    I am looking for more stern lift under cruise speeds of 18-25 knots. My boat is the Jurmo Trimaran introduced in the "design" section.

    I have a 100hp Mercury Fourstroke and run a 4 bladed Trophy plus 13 3/4 diam and 17" pitch. 35 knots GPS is easily reached keeping tje revs at 6000, a bit more maybe with a different prop, but not really needed. I run into the limiter WOT.

    The barrel of is relatively long but lacks a diffuser.

    Being an absolute beginner, I've not been able to find info on wether an increase of the barrel length and adding a diffuser generate stern lift at all speeds or just during hole shot. Would a diffuser help me at the cruise speed range mentioned above?

    I don't mind loosing a few knots or lowering the rpms a bit.

    Any suggestions on a "better" prop for me?

    Thanks for any insight,

    /Tapani
     
  2. baeckmo
    Joined: Jun 2009
    Posts: 1,654
    Likes: 670, Points: 113, Legacy Rep: 1165
    Location: Sweden

    baeckmo Hydrodynamics

    Sorry Tapani, but what are you referring to with the terms "barrel" and "diffuser"? I suspect you are talking about the propeller hub, or? Before you change anything, I suggest that you run a series of tests, where you measure trim angle at different speeds. Use the keel as referrence line.

    With constant throttle, vary the engine tilt angle and note the resulting speed, boat trim and rpm at one or two speeds in the cruising range to find "the sweet spot", then we can discuss if this spot can be achieved by other means f.i. trim tabs or interceptors.

    Edit: your phone has a set of physical instruments that are useful for this kind of experimenting. Download the app "phyphox" to get access to the lot; there is an inclinometer that is quite useful and precise.
     
  3. Tapani
    Joined: Jul 2022
    Posts: 5
    Likes: 1, Points: 3
    Location: Finland

    Tapani Junior Member

    baeckmo, thanks for your reply !

    Yes, I'm talking about the prop hub. Tube and possible diffuser at the end.

    My apologies, there's another post of my challenge under the "Design" forum. See Jurmo Trimaran 1970 https://www.boatdesign.net/threads/jurmo-trimaran-1970.67208/

    As explained, my problem is - has been since I bought the boat - porpoising. Especially at the most efficient cruise speeds. One reason may be my heavy engine, the Merc weighs 181 kg dry. The boat was sold with a 70hp three cylinder Chrysler in 1970 - probably quite a bit (80kg?) lighter.

    I've made progress as explained on "the other side", but, I would still like to improve the 18 - 25 knot cruise range.

    If the reference for all trim values is the keel line I can trim the engine up from that reference a few degrees running at 35 knots and find the sweet spot. Small angles are difficult to measure and quantify with the meter on the outboard, but let's say it's a few degrees on the positive side. I think the boat runs well at that speed and that the boat trim angle feels small, the bow goes down and the stern seems to lift. This makes me believe that the prop shaft is parallel to velocity or very close.

    Within the cruise window I can trim the engine quite a bit out from the max negative, but feathering towards "0-trim" (AV-plate parallel to keel line) will eventually induce porpoising, and I need to trim down and then come up a bit, but not to "0". Boat trim angle is noticeable larger than when running at 35 knots, i.e. bow higher. All this meaning, that the prop shaft line is "far" from optimal (horizontal) and I need to use the thrust to prevent porpoising. Adding a two people to the aft makes things quite a bit worse.

    As explained on the "other side" raising the motor quite a bit, moving the battery to the bow and swapping props has helped tremendously.

    This morning I added 40kg sand bags as ballast to the bow and it helped the situation further.

    I add a link to this morning's GoPro material, I know it's quite long (6 minutes), but feel free to check it out if interested. I ran practically at two different rev levels, 4000 and 6000rpm, you will notice the difference. These are roughly 22 and 35 knots respectively (forgot my GPS to the shore, unfortunately). At 22 the motor trim is slightly negative from the keel line and at 35 knots it's positive. I also deliberately induce porpoising when doing 22 knots.



    So, my thought is, that I need other tools to produce more stern lift across the cruise speed range to relieve the prop thrust for forward movement.

    I would like to understand the potential in props better and, if possible, try something before adding trim plates or interceptors.

    Kind regards,

    /Tapani

    PS Playing with the GoPro is actually quite fun. Apologies for the sound, I need to find how to turn the sensitivity down - it saturates real easy. Same thing for the digital stabilizer.... it seems to stick with the shore lines not the boat :), especially in turns.
     
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