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Spacing twins on the transom

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  • Spacing twins on the transom

    Okay, never heard this topic addressed on the forums before and I've done a search or two without finding it. So the question is...when you mount twin outboards on the transom, is there a rule of thumb for spacing, ie. distance apart? Obviously they have to be separated enough so each has clearance throughout its turning radius. But once that is accomplished there's the question of...what is better, closest as clearance allows? or further apart, and how far is too far apart?
    Reason I ask is I'm restoring an early 60's Texas Maid 15 ft. aluminum V-hull runabout. I've got two Johnson RD's (35 hp) in the shop that may become one reliable outboard...if the outboard gods keep smiling on me. That's one power option. Power option two is mounting the two Johnson FD-11's (18 hp) that I have...both are sweet runners and are sitting idle, so I thought "why not try the pair?" Either power option will be steered with a refitted cable-over-pulley system that I put back in place since I had all the necessary items lying around.
    Don't expect to get 36 hp out of the 18 hp twins, but I'm curious to see what the pair will do on the water.
    Anyone out there with experience fitting twins on relatively small craft who can address the spacing questions?

  • #2
    Re: Spacing twins on the transom

    I've recently been looking into the same thing (one 25 hp or twin 15 hp's). I'm interested to hear some of the responses. There is an active topic on the AOMCI website about connecting twins as we speak. I have no real world knowledge at this point, but I have read a lot of pros and cons.

    1. Looks awesome especially with a pair of vintage outboards like ours.
    2. Back up motor if there are problems.
    3. Better handling, but only if you can get the screws far apart. Might not be noticeable on a small boat.

    1. Higher fuel consumption.
    2. Less top end than a single of the same hp.
    3. Twice the maintenance.
    4. Both have to be running in sync. You don't want one thats stronger than the other.

    Here is a website I found very useful. It goes much deeper into the setup on a moderate v-hull Boston Whaler.



    • #3
      Re: Spacing twins on the transom

      I'd say, look at your interconnect steering link. If not adjustable, there's your answer. Sometimes the boat hull configuration dictates it. Like I saw a pair of white (older) Mercs on a catamaran. obviously each engine was behind a sponson which really put them apart.

      Otherwise I'd say that you want to keep the props in the water so you want to locate them where you will always have a good undisturbed flow (from parts of the hull preceeding the prop). So that would dictate as close to the centerline as is reasonable.

      Prop to prop interference could be a problem too, but I'd bet that by the time you make enough room for them to rotate side to side without touching, you have them far enough apart to prevent interaction.

      My 2c.

      If you are new to boating or have a new boat, a knowledgeable friend could show you how to operate your boat and save you a lot of grief, maybe some money, and maybe your life.


      • #4
        Re: Spacing twins on the transom

        Thanks Wayne and Mark. I fitted both the FD-11's on the boat yesterday and have them spaced about a foot apart right now. As Wayne mentioned in the Pros, it does look real sharp and will provide back up power if one quits on the water...I guess trial and error will eventually tell me how it performs with the outboards at different spacings. As for the Cons, don't mind running two tanks as I don't get on the water as much as I'd like to (few lakes, far from home), I've outgrown the need for speed, and the FD-11's are light on maintenance needs, very reliable and stout running. The Whaler article is interesting. Next step is to fab up a steering link between the two outboards using a turnbuckle and diagram provided by the fine guys on the AOMCI board. Then I have to install the dual outboard control and cables before I can run it down to the local municipal lake...maybe by early to mid March I'll be able to get it wet for a test run. Also wonder how the spacing will affect the steering. When I had one outboard installed and centered, I had 1.5 turns of the wheel in either direction to full stop...or three turns stop-to-stop. I think that will decrease as the cables will wind up being shorter with two outboards...but that's just a guess as this is uncharted waters for me. Keep the input coming.


        • #5
          Re: Spacing twins on the transom

          I recently worked on a Whaler with 2 Rude 70s. It had two single control boxes that bolted to each other so you should easily find controls.

          And that twin engine sound is Sweeeet!


          • #6
            Re: Spacing twins on the transom

            hhmm not too sure about smaller Engines like you have but as a general rule, they need to be spaced so the cav plate is in the correct running position, and need to be as close together as possible so when you do a turn and the Boat leans one Motor does not end up out of the water, so there is no hard and fst set spacing, but rather it is dictataed by the Boat design and shaft length, and take into account turning as well. of course a Cat does not use the same considerations, as it is in fact two boats joined together (sort of)


            • #7
              Re: Spacing twins on the transom

              Need Dhadley in on this...I believe there is a specified amount of tow-in/out for positive steering.


              • #8
                Re: Spacing twins on the transom

                I have an old Merc manual that says toe-in should be such that the prop wash converges about 100-200 feet behind the boat. Guess it works out to be a couple of degrees.


                • #9
                  Re: Spacing twins on the transom

                  Good info...thanks Frank.


                  • #10
                    Re: Spacing twins on the transom

                    Okay, looks like I'll be calculating deadrise on this v-hull and measuring for toe-in this afternoon, according to the whaler article posted by Wayne. Interesting. I'm hoping that if I managed to get the motors mounted correctly, ie. exactly perpindicular to the transom, I'll have that 1/2 to 3/4 inch of toe-in mentioned in the article. Can't wait to get the dual control and cables later this week. This is getting interesting. Thanks for the all the posts, I hope Dhadley wonders by with his .02 cents worth.


                    • #11
                      Re: Spacing twins on the transom

                      I'm late but here....Noelm is right - hull configuration dictates where the motors should go. Don't be surprised if you measure a given distance from the center out and find out the AV plates are at different heights. If I were doing it I'd place the motors so the AV plate is even with the hull. If the hull has lifting strakes you'll want to center the motors on them unless they are 10-12" away. As Mark mentioned, the tie bar will have some bearing on it too.

                      1/2 to 3/4" of toe in is a lot but it may not make that much difference on small motors. We usually look for 1/8 to 3/8".

                      As for steering you should still have 3 turns (or whatever you have with a single) lock to lock unless the boat is so narrow you run out of cable travel. If it's that narrow it could influance where the motors go too.


                      • #12
                        Re: Spacing twins on the transom

                        Okay, measured for toe-in on the outboards today and checked deadrise on the hull. Center-to-center measure on the prop shafts is 19 and one quarter inches. Center-to-center on the nose of the gearcases is 19 and one eighth inches. Looks like I have 1/8 inch toe-in. As for deadrise, it's nonexistent...the boat tapers from a deep V at the bow to virtually flat at the stern. The anticavitation plates on both FD-11's sit about 3/4 inch higher than the bottom of the hull, with no way to adjust them further downward. I've attached a brochure that shows how the Texas Maid Impala looks, courtesy of the Fiberglassics site. My main concern now is that I may be barking up a dead tree...brochure puts boat weight at 420 lbs. Put me behind the wheel (250 lbs.), add the two FD-11's (75-80 lbs. each) and two gas tanks and the total load is pushing 860-900 lbs. depending on you figure the weight of tanks and gas. So I'm wondering if the old girl is going to top out at a whopping 10 mph at WOT?
                        Attached Files
                        Last edited by tmcalavy; February 4th, 2008, 06:27 PM. Reason: text error


                        • #13
                          Re: Spacing twins on the transom

                          Photo didn't work.

                          I think the 18's should be ok. What is the max hp rating and load?


                          • #14
                            Re: Spacing twins on the transom

                            You have 1/8" toe out, not in.


                            • #15
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                              Re: Spacing twins on the transom

                              Wayne, photo loads for me. Max hp rating is 60 hp, no load rating on the brochure or anywhere on the boat/plate. It came with an Evinrude Lark IV, which I believe is a 40 hp...that beast weighed well over 150 at least.
                              Dhadley...right you are, 1/8th inch toe out...is that okay?