Critique this ob mounting

Discussion in 'Outboards' started by fallguy, Sep 17, 2020.

  1. fallguy
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    fallguy Senior Member

    side view
    0579AFA2-F5CF-46F7-9E32-803F02A27F0C.jpeg
    Port side stern
    B06576B2-2B1A-4CE9-B59F-3F1A9510F8E4.jpeg
    Star side stern
    81CA81A2-EB2B-44B4-86B5-A4E43EB13D33.jpeg
    Straight edge off hull bottom
    1CA0AEB5-169E-4DCB-9AEB-D428A7D7F8AD.jpeg
    Engine on jackplate from port
    10450BB0-4F90-4350-ACD7-8C1EB350F448.jpeg B1DCFE72-C39E-4669-9084-F6D1940AE4C1.jpeg F6ACBFCE-4ACE-4B6E-ABF5-5C75DA831CDE.jpeg 6823E0C7-1DB4-45E1-AF19-331F5A77D9F4.jpeg Just looking for criticisms.

    90hp vmax xl mounted on a 5.5" jackplate

    boat is semi-displacement

    speed hope is 25 knots

    laser for expected waterline is 5.5" above cav plate leading edge and 11" above trailing edge...

    mostly wondering if I should mount higher

    jack plate is all the way up

    I am in the higher holes on the plate. You can see by the lower slotted I can probably go up, but then engine bracket holes would be above jack plate which is a bit foreign to me
     
  2. fallguy
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    fallguy Senior Member

    Transom was built at 14 degrees as I had some concerns about the bow running a bit high. We can always trim up the engines a bit, but you have to remount and wedge to go the other way.
     
  3. bajansailor
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    bajansailor Marine Surveyor

    Your Skoota certainly does look very impressive, with excellent quality workmanship from what we can see in the photos.

    Re the depth of the transoms, did you know that you would be fitting these O/B motors when you started building the boat?
     
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  4. fallguy
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    fallguy Senior Member

    Ah, yes, you are so wise.

    What happened is this.. Richard Wood's wisely specified Yamaha F70s for this build. So, we built the transom for the F70 which is a 20" shaft.

    I hired an electrical consultant and he was worried about alternator output from the F70s and said it would not support an active autopilot in much wind. He suggested we get double the alternator output. Around that same time, a fellow from Alaska I met said he had twin F70s on a workboat of his and he said that Alaskan currents and winds can be vicious and the F70s had to be swapped off his boat. The boat is designed to trip from Seattle to Alaska. So, I discussed with Richard and decided to bump to F90s. We also want 25mph top speed. Well, it turns out those F90s are available in xtra long, so for a jackplate expense, I would be able to raise powerheads up 5" and reduce drowning engine risk.

    And that is the story. The jack plate also allows some vertical adjustment for any error in final transom height.

    Great question.

    Any opinion on the cav plate height vs bottom/waterline?

    Does that massive engine look silly on that little transom?

    I am still worried I am a bit low.

    I should mention we never discussed wanting autopilot with Richard early on, so he never had a chance to discuss the issue the consultant had prior to hull build. Autopilot heavy wind capable was not in the sor.
     
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  5. bajansailor
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    bajansailor Marine Surveyor

    It does look like a bit of an afterthought, re how the engine is much higher than the transom.
    But if the transom is strong enough as it is, to support the extra loads from the bigger engine and being higher up, then that is the main thing.
    Have you fitted both motors to the transoms, or is this just a 'trial fit' on the starboard side?

    I think it would look better if the transom height was raised up a bit, but I appreciate that this would be a LOT of work to do now, especially re the filling, fairing and re-painting on both sides.

    We had a similar situation with the 49' power cat in my avatar - she originally had a pair of 70 hp short shaft outboards, and there were local cut-outs in the transoms - the cut-outs were filled in when these were subsequently replaced by a pair of 115 hp O/B's, followed some years later with a pair of 150 hp O/B's (she is now on her second set of these - Suzuki this time - the previous ones were Yamaha).
    It was fairly easy with an ally cat to just weld in some extra structure to fill the cut-outs - an extra length of square hollow section was also later welded across the top of the transom as well when the longer shaft 150 hp engines were fitted. But they are still standing proud of the transom by about 4".
    Here is a photo from a few years ago when she still had the Yamahas -

    P3060345.JPG

    Edit - a question - @fallguy, re the laser line in your last photo (I presume that it is a laser?) - is this where your waterline will be painted?
     
    Last edited: Sep 17, 2020
  6. kapnD
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    kapnD Senior Member

    I’d definitely raise the transom, possibly just add a new one over the existing?
    I know that pretty much nullifies the usefulness of the transom steps, so perhaps the motor could be moved to the extreme inside edge to allow access?
    The jack plate will be partially submerged at rest, and maybe the motor, inviting corrosion problems.
    The designer should be able to tell you the best height for the motor, and maybe eliminate the jackplate complication.
     
  7. fallguy
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    fallguy Senior Member

    C068830C-1B3B-4C4E-898A-4558D6B4305E.jpeg
    I am not modifying the transom. It is not even a discussion. I asked for apples; not oranges! I only want to know how high to set the cav plate up for the rockered bottom because it is different from the vee hu ll I am used to.. the angles are different.

    Yes, the laser is expected static waterline at rest. So the bottom of the jackplate will be in water. The engines will tilt out.

    Today I asked a rigger and he said some pontoons he has rigged can easily run an extra long on a 20" transom with little effect on performance. He told me mount it up in the fourth hole and lower with the manual jack if needed. He also said it'd be fine. The transom has 8 layers of db1700 on the top. It doesn't budge with the 360 plus 220# more on the cav plate (me).

    It also looks huge outta water, but here is another view. Remember, I wanted 5" higher for keeping engines out of any weather
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    Certainly the jacking plate gives you a lot of leeway for adjusting motor height, as does, I assume, the outboard bracket itself, with its different holes. Just going on eye, and if what the pictures shows is fully "up", I'm inclined to think I'd like to be able to raise it higher. You absolutely do not want want water hitting the leg above the splash plate, the one above the cav plate, that will cost speed, fuel, and create a fountain. I am a little surprised by the amount of up-sweep aft, in the hull, it seems more than the pictures I have seen, but could be an optical illusion.
     
  9. Ad Hoc
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    Ad Hoc Naval Architect

    My first impressions are:

    Visually, the transom appears 'dwarfed' by the OB, it just looks weak.. a small bracket holding a large engine, which can expect to be subjected to 2-3g in a big sea. I hope you've done your calc's ok??
    The load going into the transom can be larger than expected.
    Looks a tad high too... the engine.

    Have you considered adding a small "Trim Tab" extension, a permanent part of the hull bracket . It is not a trim tab per se, in the true sense. But this'll help aid flow better to the prop:

    upload_2020-9-18_11-37-20.png

    It will also aid in providing a little bit of a trimming moment to level her out easier.
     
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  10. fallguy
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    fallguy Senior Member

    thanks

    I am a wee bit nervous now about my transom. Can't lie. Motor is big! Last thing we want is motor to break off and sink in 150 or 800 fow.

    It is glass on 1.5" 26# composite. Transom is 28" wide inside and 36" outside or so. Want to review the laminate privately?
     
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  11. Ad Hoc
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    Ad Hoc Naval Architect

    Sure, happy to help out.
     
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  12. Mr Efficiency
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    Mr Efficiency Senior Member

    A bloke once told me that the Hughes "Spruce Goose" was never going to fly properly, because on visuals alone, the engines were "too small". Appearances are not always the best guide.
     
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  13. Ad Hoc
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    Ad Hoc Naval Architect

    Just like your intuition o_O
     
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  14. Mr Efficiency
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    Mr Efficiency Senior Member

    One thing there is never any doubt about, your legendary "charm" ! :)
     
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  15. philSweet
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    philSweet Senior Member

    The risk of drowning the engine typically comes with the risk of ventilating the prop. At displacement speeds, which is where you will be in a big chop, you need the thing as low as you can stand it. Jack plate and long shaft were both good ideas. The cowl can handle the occasional dunking, and it's worth it to keep the prop in the water. I don't know if anyone makes a shaft extension kit for the Yamaha 90, but they were cheap and handy back in the day. You added a 5" spacer where the foot attaches, and swapped out the shaft and shifter with new ones. I ran an extended 30" Merc 15hp on a 28' boat and the cowl sat 1" out of the water at rest - but the damn prop stayed in the water!
     
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