I am a bit puzzled about a circa 4mm wide radial gap around the back corner of the prop hub which seems an obvious path for a lot of the exhaust gas to bypass the normal through-hub route. I know that my outboard set-up is totally standard with the 12.25X15" OMC prop supplied new with the outboard (normal mid-range 3.5" gearcase type). I have also checked the thrust bearing and this is the correct OMC part.
Is there somebody out there with a similar age johnson 50 who can confirm what I am seeing. All other outboards that I have seen have a relatively small radial clearance (and hence very limited gas path) in this area with the stepped prop hub entering for a fair distance inside the gearcase housing. On my engine, the prop hub does enter the housing a little but is alongside a chamfered collar which is a mile away i.e. approx 4mm. Is the exhaust gas intended to partially vent across the prop blades on this particular outboard perhaps ? I have seen some references on the site to "prop porting" to allow some gas over the prop but these are relatively small holes, not this large radial gap?
This is a new project and I am basically not yet getting on the plane and this is just one factor that puzzles me.
By sports boat and ski boat standards the set-up has a very low power to weight ratio but there are a number of shetland 536 cabin boats like mine with 50hp on the water here in the UK successfully planing and reaching moderate speed on the plane which is all I aiming for, perhaps 22mph. So far I have yet to get above 11 knots on the GPS
With the the 15" pitch OMC prop speed gradually gets up to around 7 mph at 4100 rpm, then ramps up quickly to around 5000 (feels like slip/vent/cavitation) obviously not getting over the hump /tried a much larger pitch i.e. a standard 19" pitch OMC prop, only because it was available - did not expect much of a result which proved the case although the 4100rpm speed increase was not there. I am going to test the 15" pitch prop again this coming weekend (have marked it across hub) to see if there is hub slipping. Also I now have a new hustler 13" pitch prop to try if the 15" OMC one is slip free. I am not really concerned about top speed and fancy the 13" pitch might do the job for me as long as over-revving is not an issue (WOT=4500-5500) because I do like to carry a bit of load on the boat (three people and fuel etc could be as high as 2250lbs displacement).
I have other concerns as well regarding trimming, which I have tried in various positions including fully in.
Is the engine supposed to be vertical to the water surface when at rest and fully trimmed in ? (I would say mine is - the power trim fully in rests the leg on the limit pin which is positioned in hole closest to transom. I thought that when fully trimmed in the leg would "tuck under" the transom very slightly to help the take-off ?
Any ideas please. I will post again once I have done the trials above. Possibly I will end up trying smart tabs but I want to eliminate all other possibilities first.
Incidentally this my first post, it is interesting to see the different USA terms, for example the term " kicker " (just called an auxillary in UK ) is new to me. I am not sure but I think a hole shot is what I am looking for right now.
Regards Gemini
Is there somebody out there with a similar age johnson 50 who can confirm what I am seeing. All other outboards that I have seen have a relatively small radial clearance (and hence very limited gas path) in this area with the stepped prop hub entering for a fair distance inside the gearcase housing. On my engine, the prop hub does enter the housing a little but is alongside a chamfered collar which is a mile away i.e. approx 4mm. Is the exhaust gas intended to partially vent across the prop blades on this particular outboard perhaps ? I have seen some references on the site to "prop porting" to allow some gas over the prop but these are relatively small holes, not this large radial gap?
This is a new project and I am basically not yet getting on the plane and this is just one factor that puzzles me.
By sports boat and ski boat standards the set-up has a very low power to weight ratio but there are a number of shetland 536 cabin boats like mine with 50hp on the water here in the UK successfully planing and reaching moderate speed on the plane which is all I aiming for, perhaps 22mph. So far I have yet to get above 11 knots on the GPS
With the the 15" pitch OMC prop speed gradually gets up to around 7 mph at 4100 rpm, then ramps up quickly to around 5000 (feels like slip/vent/cavitation) obviously not getting over the hump /tried a much larger pitch i.e. a standard 19" pitch OMC prop, only because it was available - did not expect much of a result which proved the case although the 4100rpm speed increase was not there. I am going to test the 15" pitch prop again this coming weekend (have marked it across hub) to see if there is hub slipping. Also I now have a new hustler 13" pitch prop to try if the 15" OMC one is slip free. I am not really concerned about top speed and fancy the 13" pitch might do the job for me as long as over-revving is not an issue (WOT=4500-5500) because I do like to carry a bit of load on the boat (three people and fuel etc could be as high as 2250lbs displacement).
I have other concerns as well regarding trimming, which I have tried in various positions including fully in.
Is the engine supposed to be vertical to the water surface when at rest and fully trimmed in ? (I would say mine is - the power trim fully in rests the leg on the limit pin which is positioned in hole closest to transom. I thought that when fully trimmed in the leg would "tuck under" the transom very slightly to help the take-off ?
Any ideas please. I will post again once I have done the trials above. Possibly I will end up trying smart tabs but I want to eliminate all other possibilities first.
Incidentally this my first post, it is interesting to see the different USA terms, for example the term " kicker " (just called an auxillary in UK ) is new to me. I am not sure but I think a hole shot is what I am looking for right now.
Regards Gemini