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Author Topic:   Johnson 35 to Johnson SPL48

funhog posted 02-18-2013 11:31 PM ET (US)
I swapped engines out on my 13whaler(82yr) over the holidays. Top speed wwith the 35 was about 25. With the biger 48 it tops out at 31(two handheld garmin gps units used for accurracy. Got a little better performance raising the the engine one hole up off the transom. The odd thing is: With the smaller engine I would lose a little speed with mor passangers. with the 48 , it doesn't matter if its just me in the boat or three gorown men at 630 pounds, it tops out at 30-31mph.I also have a stand-up ctr console in it that weighs about 100 lbs.I was expecting a little more speed than this; am I not being realistic here. Alsoaof note;at top speed and just before full throttle the engine starts to get this strange sound Wa-Wa-Wa-Wa sound that dissappears when I back off the throttle a bit. almost like something is out of round , but there's no vibration or roughness to it Any thoughts would be appreciated.(It's not prop cavitation)
The engine is an 89 2cyl in great shape with 148 compression in both cylinders. the engine has significant holeshot over the 35 and runs very smooth untill you get to the very top end.Back it off a bit and the noise goes away and it doesn't lose bit 1-2mph. The prop is in great condition and is a 11 3/4" X 17". I'm not sure its hitting 5k rpm Im about to buy an inexpensive tachometer when I find one. Maybe I have to much weight of this size prop?Or, maybe my old ears cant hear the engine screaming and I need a bigger prop. Sucked-up a lot of valves back in the 60s racing VW's
An engine manual is on order, and a tach when I locate one on the internet.
Thanks for any an all comments

Tom W Clark posted 02-19-2013 12:01 AM ET (US)
I find it hard to believe you can get 35 MPH with a 25 HP outboard.
The 50 HP (call it 48 if you like) should be good for close to 40 MPH.
The motor is going "Wa-Wa-Wa-Wa" because it is grossly underpropped and over revving.
The 48 SPL has a 4500-5500 RPM WOT range and a 2.42:1 gear ratio. You should be running a 19" to 21" pitch prop.
The motor could be raised on the transom as well.
There is far more to having a two stroke outboard in-tune than just good compression numbers.
funhog posted 02-19-2013 12:56 AM ET (US)
Thanks for the reply Tom. I re-read my post and I believe its worded correctly. "I had a 35hp Johnson and swapped it out with a 48SPL". The Johnson 35 gave me an avg. top speed of 25mph over the yrs depending on the ocean surface conditions. I believe you have your numbers backwords.
Either way I greatly respect your input on raising the engine one more hole higher and also checking the "tune" of the engine. I think I found a shop who will possibly let me renting/demo with the intent to purchase the prop size you recommended.
Thanks!
dg22 posted 02-19-2013 09:37 AM ET (US)
funhog, My 1973 50Hp Evinrude performs about the same on my 13-footer. I may get 33 mph with me alone. The gear ratio on mine is 2.66:1 and I currently have a 19 pitch prop. I'm reving around 5200 RPM WOT. I'm actually looking for a 17 pitch prop but the dia. is more around 13" with my prop. I need a little extra power to get my daughter out on 1 ski. I agree, get a tach before investing into a new prop.
BQUICK posted 02-19-2013 11:19 AM ET (US)
Boat waterlogged? A 1968 33 Evinrude I had ran 34 mph. A 50 should rin easy 38-42. A 50 Merc even more.....
Tom W Clark posted 02-19-2013 11:24 AM ET (US)
Ah, 25 MPH with the Johnson 35 HP outboard motor. That is slow. A Sport 13 with a Johnson 35 should be good for close to 35 MPH, lightly loaded. I know, I used to own one.
This makes me wonder if your boat is unusually heavy, and thus slower.
dgoodhue posted 02-19-2013 02:34 PM ET (US)
Do you have power trim on either motor? And are you trimming the motor out? Trimming the motor out is worth about 4 mph on my 15' whaler. Even without trimming the motor out both numbers seems low. Your 35 should have more top end than your 48 currently has if set up with the proper prop and engine height.
dg22 posted 02-19-2013 02:46 PM ET (US)
I thought the same thing but if you plug into Jim's calculator my numbers you'll get 33 mph with 5% slip.
I think with the correct SS prop and max out my RPM's which is 6000 for my motor I'd be in the high 30's.
Don't forget guys this is a boston whaler with hook in the hull and back in the day you measured speed with a boat speedo not a GPS. From what I have read hear at CW, most people that are getting high 30's or low 40's are running power trim.
funhog posted 02-19-2013 03:06 PM ET (US)
Thanks to ALL for your reply’s. There’s a consensus that: the boat should be a little faster at top end, I need a bigger prop, and the info I need to select one is attainable with a tachometer, and good performance marine shop. I just stopped by a well known shop here in So. CA. and they concurred. Apparently there’s a Rev. Limiter on the engine – it’s maxing out and that would probably account for the peculiar sound when the engine nears its top end( over revving). $180 for a Tach and harness and another $130 for a prop and I will most likely be golden.
All of you mentioned expected speeds with a light load. I stated in my original post that I usually have about 630 pounds. 2-adults, bait well , stand-up consol , 12 gal fuel , and an engine that’s about 60 pounds heavier the previous 35hp. I would think with this weight load the boat is on the heavy side but very manageable. Don’t have power trim but I’m not opposed to the minor trim positions I have to make for the different sea conditions. The hull is bone dry and solid. I just hope I can bring it up into the low to mid 30mph without much trouble. Once in a great while the Southern Calif. Coast is has no wind and no swell. We call it Lake Pacific and it’s fun to go faster than the normal seas will let you go normal . Thanks again for all you input and comments you have me going in the right direction.
Regards,
Peter posted 02-20-2013 07:38 AM ET (US)
I has a Super Sport 15 with a Johnson 48 SPL. The motor turned an 11 3/4 x 17 aluminum propeller. It had no problem pushing the boat to 36 MPH with a light load. On the 13, I think you need more pitch, either a 19 or 21 inch as Tom suggests.
funhog posted 02-20-2013 09:46 PM ET (US)
Thanks for the info,Peter. I purchased the Tack & Harness along with a service manual earlier today. Dug out an old timing light from my past and plan to have a productive weekend. Also purchaed a new water pump impeller and was given a heads-up that its a little tricky with a shift linkage disconect just under the carburators. My SELOC manual doesn't mention anything about this little step.Hopefully I"ll get it back in the water on Sunday and have enough info. to figure out which prop to get. It might have been cheaper to just buy the 2 props and try them both out and pick one?
dg22 posted 02-21-2013 09:13 AM ET (US)
funhog, Once you report back your RPM WOT, I'm sure Tom or other's can suggest a good SS prop for your boat. Prop design is a big factor too.
You would think a prop that would give some bow lift would be idea with the 13-footer due to the slight hook in the hull design but I could be wrong.
Binkster posted 02-22-2013 04:45 PM ET (US)
Actually the hook at the stern on a 13 footer is about 3/4". that's not a SLIGHT HOOK. They were not designed for speed, the hook made the boat more controllable at the 30mph top speed that was available by 1958 vintage outboards.
rich
dg22 posted 02-23-2013 04:07 PM ET (US)
Binkster, that would explain why more HP does not increase the speed significantly with the 13-footer unless you have power trim.

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Author	Topic: Johnson 35 to Johnson SPL48
funhog	posted 02-18-2013 11:31 PM ET (US) I swapped engines out on my 13whaler(82yr) over the holidays. Top speed wwith the 35 was about 25. With the biger 48 it tops out at 31(two handheld garmin gps units used for accurracy. Got a little better performance raising the the engine one hole up off the transom. The odd thing is: With the smaller engine I would lose a little speed with mor passangers. with the 48 , it doesn't matter if its just me in the boat or three gorown men at 630 pounds, it tops out at 30-31mph.I also have a stand-up ctr console in it that weighs about 100 lbs.I was expecting a little more speed than this; am I not being realistic here. Alsoaof note;at top speed and just before full throttle the engine starts to get this strange sound Wa-Wa-Wa-Wa sound that dissappears when I back off the throttle a bit. almost like something is out of round , but there's no vibration or roughness to it Any thoughts would be appreciated.(It's not prop cavitation) The engine is an 89 2cyl in great shape with 148 compression in both cylinders. the engine has significant holeshot over the 35 and runs very smooth untill you get to the very top end.Back it off a bit and the noise goes away and it doesn't lose bit 1-2mph. The prop is in great condition and is a 11 3/4" X 17". I'm not sure its hitting 5k rpm Im about to buy an inexpensive tachometer when I find one. Maybe I have to much weight of this size prop?Or, maybe my old ears cant hear the engine screaming and I need a bigger prop. Sucked-up a lot of valves back in the 60s racing VW's An engine manual is on order, and a tach when I locate one on the internet. Thanks for any an all comments
Tom W Clark	posted 02-19-2013 12:01 AM ET (US) I find it hard to believe you can get 35 MPH with a 25 HP outboard. The 50 HP (call it 48 if you like) should be good for close to 40 MPH. The motor is going "Wa-Wa-Wa-Wa" because it is grossly underpropped and over revving. The 48 SPL has a 4500-5500 RPM WOT range and a 2.42:1 gear ratio. You should be running a 19" to 21" pitch prop. The motor could be raised on the transom as well. There is far more to having a two stroke outboard in-tune than just good compression numbers.
funhog	posted 02-19-2013 12:56 AM ET (US) Thanks for the reply Tom. I re-read my post and I believe its worded correctly. "I had a 35hp Johnson and swapped it out with a 48SPL". The Johnson 35 gave me an avg. top speed of 25mph over the yrs depending on the ocean surface conditions. I believe you have your numbers backwords. Either way I greatly respect your input on raising the engine one more hole higher and also checking the "tune" of the engine. I think I found a shop who will possibly let me renting/demo with the intent to purchase the prop size you recommended. Thanks!
dg22	posted 02-19-2013 09:37 AM ET (US) funhog, My 1973 50Hp Evinrude performs about the same on my 13-footer. I may get 33 mph with me alone. The gear ratio on mine is 2.66:1 and I currently have a 19 pitch prop. I'm reving around 5200 RPM WOT. I'm actually looking for a 17 pitch prop but the dia. is more around 13" with my prop. I need a little extra power to get my daughter out on 1 ski. I agree, get a tach before investing into a new prop.
BQUICK	posted 02-19-2013 11:19 AM ET (US) Boat waterlogged? A 1968 33 Evinrude I had ran 34 mph. A 50 should rin easy 38-42. A 50 Merc even more.....
Tom W Clark	posted 02-19-2013 11:24 AM ET (US) Ah, 25 MPH with the Johnson 35 HP outboard motor. That is slow. A Sport 13 with a Johnson 35 should be good for close to 35 MPH, lightly loaded. I know, I used to own one. This makes me wonder if your boat is unusually heavy, and thus slower.
dgoodhue	posted 02-19-2013 02:34 PM ET (US) Do you have power trim on either motor? And are you trimming the motor out? Trimming the motor out is worth about 4 mph on my 15' whaler. Even without trimming the motor out both numbers seems low. Your 35 should have more top end than your 48 currently has if set up with the proper prop and engine height.
dg22	posted 02-19-2013 02:46 PM ET (US) I thought the same thing but if you plug into Jim's calculator my numbers you'll get 33 mph with 5% slip. I think with the correct SS prop and max out my RPM's which is 6000 for my motor I'd be in the high 30's. Don't forget guys this is a boston whaler with hook in the hull and back in the day you measured speed with a boat speedo not a GPS. From what I have read hear at CW, most people that are getting high 30's or low 40's are running power trim.
funhog	posted 02-19-2013 03:06 PM ET (US) Thanks to ALL for your reply’s. There’s a consensus that: the boat should be a little faster at top end, I need a bigger prop, and the info I need to select one is attainable with a tachometer, and good performance marine shop. I just stopped by a well known shop here in So. CA. and they concurred. Apparently there’s a Rev. Limiter on the engine – it’s maxing out and that would probably account for the peculiar sound when the engine nears its top end( over revving). $180 for a Tach and harness and another $130 for a prop and I will most likely be golden. All of you mentioned expected speeds with a light load. I stated in my original post that I usually have about 630 pounds. 2-adults, bait well , stand-up consol , 12 gal fuel , and an engine that’s about 60 pounds heavier the previous 35hp. I would think with this weight load the boat is on the heavy side but very manageable. Don’t have power trim but I’m not opposed to the minor trim positions I have to make for the different sea conditions. The hull is bone dry and solid. I just hope I can bring it up into the low to mid 30mph without much trouble. Once in a great while the Southern Calif. Coast is has no wind and no swell. We call it Lake Pacific and it’s fun to go faster than the normal seas will let you go normal . Thanks again for all you input and comments you have me going in the right direction. Regards,
Peter	posted 02-20-2013 07:38 AM ET (US) I has a Super Sport 15 with a Johnson 48 SPL. The motor turned an 11 3/4 x 17 aluminum propeller. It had no problem pushing the boat to 36 MPH with a light load. On the 13, I think you need more pitch, either a 19 or 21 inch as Tom suggests.
funhog	posted 02-20-2013 09:46 PM ET (US) Thanks for the info,Peter. I purchased the Tack & Harness along with a service manual earlier today. Dug out an old timing light from my past and plan to have a productive weekend. Also purchaed a new water pump impeller and was given a heads-up that its a little tricky with a shift linkage disconect just under the carburators. My SELOC manual doesn't mention anything about this little step.Hopefully I"ll get it back in the water on Sunday and have enough info. to figure out which prop to get. It might have been cheaper to just buy the 2 props and try them both out and pick one?
dg22	posted 02-21-2013 09:13 AM ET (US) funhog, Once you report back your RPM WOT, I'm sure Tom or other's can suggest a good SS prop for your boat. Prop design is a big factor too. You would think a prop that would give some bow lift would be idea with the 13-footer due to the slight hook in the hull design but I could be wrong.
Binkster	posted 02-22-2013 04:45 PM ET (US) Actually the hook at the stern on a 13 footer is about 3/4". that's not a SLIGHT HOOK. They were not designed for speed, the hook made the boat more controllable at the 30mph top speed that was available by 1958 vintage outboards. rich
dg22	posted 02-23-2013 04:07 PM ET (US) Binkster, that would explain why more HP does not increase the speed significantly with the 13-footer unless you have power trim.