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Reccomended outboard weight for a 3.8m inflatable, thinking of moving to 4 stroke?

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  • Reccomended outboard weight for a 3.8m inflatable, thinking of moving to 4 stroke?

    Hi, probably a much asked question but I've been using a 15hp 2 stroke on my 3.8m Bombard inflatable. Its Ok but I guess I would prefer something a bit less noisy /messy and that would be more likely tick over without stalling!
    Im looking at s/h 4 stroke motors but my current motor (Johnson 2 stroke) is around 35kg (77lbs) and the 4 stroke replacement would be aroung 52kg (114lbs). I cant get to try the 4 stroke on the water so I'm asking if its common to put a 15hp four stroke on a 3.8m inflatable without having to sit way forward for the best trim when in the inflatable alone.
    Also is 52kg an easy lift for one person? it seems heavy?
    Thanks

  • #2
    Keep you current 2 strokes motor, the weight difference between both motors is huge, sib will achieve an up bow attitude while floating and under way which will need to be correted shifting weight forward or going out with another boater up front.

    My 2 strokes Tohatsu weights 42 kilos and luckily lives sitting on transom perpetually, too darn heavy to put it on and remove it afterwords on constant basis.. Is the current motor having stalling issues ?

    Happy Boating



    Sea Rider 320, 380 Sibs, 450 Rib, 2 Strokes Tohatsu 5,18 & 30 HP Proud Smokers

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    • #3
      Thanks Sea Rider, its very tempting to go for another season with the 2 stroke. It usually gets used in the summer (brief in the UK!) for a bit of pottering and exploring around the coast.
      Each year I service it and clean the carb but last time out it was hard to get a stable idle speed. Required some synchronising of getting it started and pulling the anchor up if I'm fishing by myself. Earlier johnson carbs had a big air screw to adjust slow running but on this model it seems to be fixed according to the manual (IIRC). maybe its a sync. issue as well, or a combination of poor fuel or a bad mix. It needs some time to investigate.
      Given that its over 20 years old now I was looking for a change, so the kids could be more confident fuelling it and using it as well. It also seems to use a significant amout of fuel compared to the 4 strokes doing the same work. Four strokes are more abundant now on the s/h market than older 2 strokes but the 52kg hanging on the transom will no doubt need me to sit even further forward when 1 up, and lugging it on and off will be more difficult.
      The split in the tohatsu range means that the 9.8hp engine is only a bit over 38/40 kg, but I think that might be a bit underpowered when we're fully loaded /4 up. Ive been assured the 15 and 20 hp are the same except a carb change, but both are 52kg, similar to the Mariner range..
      I'll widen the search for a low hours 2 stroke to see what's around. Thanks.
      Last edited by Sailpower; January 14th, 2020, 10:49 AM.

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      • #4
        I believe you need to stop and look for the low speed fuel mixture screw.-------If you look you will find it.-----Or post some pictures of the carburetor.-----The 93 and newer 15 HP Johnson / Evinrudes run very nice at trolling speeds if in good condition.----Not sure why you would need to clean the carburetor every year.------Once every 10 years maybe !!

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        • #5
          A 2 strokes 9.8 HP short shaft Tohatsu is just 26 Kg, same 4 strokes is 37 Kg. but very underpowered for a 380 Sib along 4 boaters if like riding on plane at speed. Next are 9.9/15/18 HP with 41, the latter with 42 KG. Ideal for a 380 size Sib for family leisure would strongly recommend a 18 HP, has same powerhead as the 10 and 15 but with extra 3 HP and 50 CC. Check if can find one in excellent running condition, will leave any 9.9-15 HP behind by far...

          Click image for larger version  Name:	Sea Rider 380-18 HP.JPG Views:	1 Size:	211.5 KB ID:	10825506

          Personally would not dare to go for the 4 strokes motors, are quite heavy, will end injuring your back in the short run if no one is available to help you out.

          Happy Boating
          Last edited by Sea Rider; January 14th, 2020, 12:48 PM.


          Sea Rider 320, 380 Sibs, 450 Rib, 2 Strokes Tohatsu 5,18 & 30 HP Proud Smokers

          Comment


          • #6
            Here's a quick snap of the boat, also red..... The outboard has a 9.9 cowl on it, 1994 when I checked back. Not nearly as clean as yours!

            Currently its necessary to steer from one of the tubes - the fuel is under the seat and the anchor fwd of that in the bow. If oe person then an ext. handle and sitting well forward.

            Sea Rider - I agree about the weight, iirc the new Tohatsu 15hp EFi is around 43kg but out of my budget at over £2K The older carb models are over 50kg by the spec I saw.



            I'll look out for a better 2 stroke - being lighter good ones are hard to find. Its not so unusual for the wave height /chop to prevent comfortable planing I guess a 9.8 would get one person planing, maybe 2, but its reassuring to have some excess capability just in case.
            Thanks again
            Last edited by Sailpower; January 14th, 2020, 02:36 PM.

            Comment


            • #7
              Originally posted by racerone View Post
              I believe you need to stop and look for the low speed fuel mixture screw.-------If you look you will find it.-----Or post some pictures of the carburetor.-----The 93 and newer 15 HP Johnson / Evinrudes run very nice at trolling speeds if in good condition.----Not sure why you would need to clean the carburetor every year.------Once every 10 years maybe !!
              Hi Racerone, the low speed mix was pretty much all I could adjust, but it was a long way from the recommended start point in the manual but still not consistent. I agree about the yearly carb clean, but it's often 9 months from putting it away to using it again - I run it out of fuel to minimise any residue but for the time it takes its worth opening it up and checking there's no debris or sticky residue - I don't want to do it on the water later, but I take your point! I Should probably use it more!

              Basically it would hold an idle for a few seconds then die. Starting - It always seems to need full choke on the first pull - sometimes I can catch it but more often it needs to have the choke off and a second pull and then it's fine. Plug colour is good but usually we're in a restricted area when trying to do the adjustments so can't have it running WOT.

              Thanks.

              Comment


              • #8
                The low speed mixture is adjusted with the motor in gear , even tied to the dock !!-----If your 93 or newer Johnson motor does not idle / troll properly you should figure out what is wrong !!----I say that they run very well if in good condition.

                Comment


                • #9
                  Originally posted by racerone View Post
                  The low speed mixture is adjusted with the motor in gear , even tied to the dock !!-----If your 93 or newer Johnson motor does not idle / troll properly you should figure out what is wrong !!----I say that they run very well if in good condition.
                  Thanks, when the weather improves I'll have more time to investigate.

                  Comment


                  • #10
                    All those mentioned weights for the 2 strokes Tohatsu motors were excerpted from the Tohatsu web page, the only one that outweights the 50 kilo you mentioned is the Tohatsu 25-30 HP at 54 KG. Yes it's very uncomfy to have a boat at plane under chop and for others as well.

                    If wanting to keep the current motor you have 3 nice alternatives : Pass that 9.9 to a 15, it's just a carb swap. Which 9.9 model is it ? Used to have a Evinrude 1996- 9.9 HP motor, ended passing it to a 15 HP, a day/night water performance difference.

                    -Install a tiller extender, move yourself forward, sit inside middle deck on top of a proper cushion, combo will plane much faster when solo boating. If you are a heavy champ and like sitting on side tube will eventually unballance the boat and at speed could turn in a dangerous water behaviour....

                    -Remove and install the seat shown in picture more aft so to tiller drive sitting comfy on seat, will need to buy 2 seat holder patches and glue them well to said position.

                    Have tried through the boating years many driving positions, this is the one that worked the best on all water conditions...


                    Happy Boating


                    Sea Rider 320, 380 Sibs, 450 Rib, 2 Strokes Tohatsu 5,18 & 30 HP Proud Smokers

                    Comment


                    • #11
                      Originally posted by Sea Rider View Post
                      All those mentioned weights for the 2 strokes Tohatsu motors were excerpted from the Tohatsu web page, the only one that outweights the 50 kilo you mentioned is the Tohatsu 25-30 HP at 54 KG. Yes it's very uncomfy to have a boat at plane under chop and for others as well.

                      If wanting to keep the current motor you have 3 nice alternatives : Pass that 9.9 to a 15, it's just a carb swap. Which 9.9 model is it ? Used to have a Evinrude 1996- 9.9 HP motor, ended passing it to a 15 HP, a day/night water performance difference.

                      -Install a tiller extender, move yourself forward, sit inside middle deck on top of a proper cushion, combo will plane much faster when solo boating. If you are a heavy champ and like sitting on side tube will eventually unballance the boat and at speed could turn in a dangerous water behaviour....

                      -Remove and install the seat shown in picture more aft so to tiller drive sitting comfy on seat, will need to buy 2 seat holder patches and glue them well to said position.

                      Have tried through the boating years many driving positions, this is the one that worked the best on all water conditions...

                      [ATTACH=JSON]{"data-align":"none","data-size":"full","data-tempid":"temp_324147_1579095108594_624"**[/ATTACH][IMG2=JSON]{"data-align":"none","data-size":"full","src":"https:\/\/forums.iboats.com\/core\/image\/gif;base64,R0lGODlhAQABAPABAP\/\/\/wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw=="**[/IMG2]​
                      Happy Boating
                      Thanks. Ive already swapped the carb and agree about the performance increase. Its a 1994 9.9 with a carb from a later model - the part number links it to the 98,99,2000 models, all that was available. - The main jet / fast orifice is the same as the one for the 94 model based on the parts lists on line. It was also a long shaft but the extension section was easily removed and a shorter drive shaft fitted. So I'm happy enough that a 15hp would be the right amount of power (better with 18 when it fully laden).
                      I suspect its going to be a comprimise, finding somethong flexible enough for 1 person or 4 people is maybe a lot to ask.

                      One up I'm usually sitting on the floor leaning on the seat with a tiller extension clamped on to the tiller, or on the side tube for pottering - you're right about the ballance, at speed being in the middle /ballanced is essential. I had a 3.8 Honwave which wsa a great SIB, big tubes, except the floor made it very bulky to transport - the seats were much better positioned in that. With four people its finding somewhere to put everyones legs thats the problem especially with some gear to carry. WIll definitely look more at the seat position.

                      When i get back to working on this motor I'll maybe post on the other thread for Johnson /Evinrude, but the issue in the UK is parts for these older motors - I usually end up ordering from the US direct if theres a few things to get. Likewise finding anyone who wants to look at them to fine tune - seems most places prefer to sell new motors and don't have anyone with experience of old 2 strokes. So if you have an oldie you need to learn to look after it and hope it doesn't let you down as there's rarely anyone to help you out..Obviously we can't buy new 2 strokes here anymore and anyone with one, wants to keep it if its low hours.

                      I'm going to keep scanning the small ads for a good 2 stroke and maybe get lucky, or go for a 9.9 4 stroke and get used to displacement speeds while fully loaded.

                      Comment


                      • #12
                        The 1994 model 9.9 and 15 HP show a different high speed metering jet ( orifice ) on the carburetors.------Likewise the 2005 models have a different orifice between the 2 carburetors..------Check your information !

                        Comment


                        • #13
                          Originally posted by racerone View Post
                          The 1994 model 9.9 and 15 HP show a different high speed metering jet ( orifice ) on the carburetors.------Likewise the 2005 models have a different orifice between the 2 carburetors..------Check your information !
                          So, in the reply to Sea Rider I mentioned that I checked the differences between the carbs - 9.9hp versus 15 hp for 1994 models and those for the 1998/9 models I fitted - I probably didn't make that clear enough. The carb I fitted has a High Speed Orifice No.48, the same as would have been fitted to a 1994 15hp carb. The 9.9 had a No. 36 High Speed Orifice for 1994, and 1998/9.

                          I haven't fitted a 2005 carb but if I had planned to, I would have checked all of the orifice options before fitting it. Hopefully this agrees with the figures you have access to.

                          As previous post, when I get a chance to investigate the motor further I'll start a thread on the Johnson outboards section.
                          cheers.

                          Comment


                          • #14
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                            The 15 HP carb has wider fuel passages along high speed jets, you can install them on a 9.9 carb but the motor will run worse than a pig, that's from personal experience and frustration. To better the motor performance when carrying more boaters is going for a prop maximization, that's installing a les pitch prop to pull wot revs towards middle to max wot rpm range factory stated.

                            Will need to install an induction tach to check that out.. The only issue is that the Owner's Manual only shows 3 props to play with...

                            Happy Boating


                            Sea Rider 320, 380 Sibs, 450 Rib, 2 Strokes Tohatsu 5,18 & 30 HP Proud Smokers

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