Jurmo Trimaran 1970

Discussion in 'Boat Design' started by Tapani, Jul 29, 2022.

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

    Tapani Junior Member

    Greetings from Finland, recently registered, first post ;)

    Having lived for 60 years already I finally bought my first boat last summer. As mentioned in the title, it is a Finnish made Jurmo Trimaran from 1970, not much different from the cathedral hull boats of the era from the US. The boat was used only for a very short time in the early -70's and left in a shelter until 2018. It's never been in salt water and looks decent.

    I am somewhat aware of the quirks of the trihulls, but am committed to live with them, trying to iron out a few challenges the boat has and hopefully keep it for a long time. I run it only on a lake and in good weather at our summer home.

    The boat seems to be built structurally strong and stiff. The side spoons (?) and tunnels acts as stiffeners and the boat is an assembly of three separate parts, the hull, deck and a separate full size interior part ranging from the bow to the stern all bonded together.

    The first issue is a tendency to porpoise, which was quite bad when I bought the boat. With some mods I've been able to get it to a manageable level.

    I trust the motor which came with the boat is just too heavy, a Mercury EFI fourstroke 80hp from 2008 weighing 181kg. The boat was sold with a Chrysler 70hp two stroke when new, some 80kg lighter than the Merc. I have two plastic fuel tanks with app 50 liters capacity combined located aft almost under the motor well.

    What I've done is relocated the battery to the bow, lifted the motor 3" higher and swapped a four blade SS Trophy Plus 13 3/4 diam and 17" pitch. These made a big improvement.

    With an upgrade on the motor (ecu firmware & de-restrict) to 100hp the boat does 35 knots @6000rpm with just me onboard. The hole shot feels quite good, less than 6 seconds to 30MPH (Vessel view GPS).

    I think the mass / lift center balance is quite sensitive on this hull and I either need to trim the motor down or run fast(er than normal) to avoid the porpoising. At speeds over 30 knots the boat seems to "lighten up", raise higher, run lighter, much less steering effort needed and it tolerates more motor trim out. I've experimented with 4x20kg sand bags in the bow. These help, but surprisingly little. Increasing the mass is not a good direction, I think, the boat needs more stern lift at moderate speeds.

    I've had the anti ventilation plate about 1" above the keel line, but still not clearly visible. I now raised the motor to the final hole, but have not run it since (yet). Let's see how that affects it.

    I will experiment with trim plates before the fall comes just to provide a controllable extra lift to the stern.

    My aim is to be able to trim the motor to a better angle while keeping boat trim angle small enough to avoid porpoising at the most comfortable and efficient cruise @ 22-24 knots.

    Any comments to easing the porpoising further would be greatly appreciated.

    The other challenge is a soft transom, but more of that later.

    Kind regards,

    Tapani
     

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  2. bajansailor
    Joined: Oct 2007
    Posts: 2,917
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    Location: Barbados

    bajansailor Marine Surveyor

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

    Tapani Junior Member

    Hey Sailor,

    Thanks for the welcome !

    Well, yes, these are available, but I think trim plates would produce the lifting high pressure area under the aft of the hull, instead of the motor. Now I need the motor thrust to push the bow down / raise the stern to avoid porpoising and this seems like a waste in many ways. With automatic trim plates I would also benefit in keeping boat sideways level.... even if it is quite stable in this direction - relatively speaking, of course.

    Kind regards,

    Tapani
     
  4. Jimboat
    Joined: Feb 2002
    Posts: 261
    Likes: 18, Points: 18, Legacy Rep: 130
    Location: Canada

    Jimboat Senior Member

    Hi Tapani - it seems that your engine height is still too low. Raising height should reduce susceptibility to porpoising. This should also allow you to trim down more, helping porpoising.
    Porpoisng (PBR October 2020) https://aeromarineresearch.com/publications/PBR_October2020.html
     
  5. Tapani
    Joined: Jul 2022
    Posts: 5
    Likes: 1, Points: 3
    Location: Finland

    Tapani Junior Member

    Hi Jim,

    Thanks for your reply.

    I had the boat in water yesterday after raising the motor 3/4" further up (now full 3" higher than when I bought the boat). The AV-plate sits now roughly 1 3/4" above the keel line.

    Significant improvement

    Water pressure is still over 20 psi and solid. Hole shot good as ever, the bow trims down a lot sooner. Also much easier to trim (motor) up before porpoising starts, the boat seems to run higher in the water and somehow "lighter" at speed.

    I tried my new GoPro, one take here:

    I need alot more seat time to say much more.

    Any further comments are naturally welcome.

    Kindest regards,

    Tapani
     
    bajansailor likes this.

  6. Jimboat
    Joined: Feb 2002
    Posts: 261
    Likes: 18, Points: 18, Legacy Rep: 130
    Location: Canada

    Jimboat Senior Member

    Great that you're getting improved results! Engine height still sounds too low. if water pressure still ok, you might find more improvement by raising motor further. Best measure to reference is height of CL of lower unit torpedo relative to keel line.
     
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