1436 with 15hp Johnson speed questions

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aarolar

aarolar

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I am struggling to get what I think is appropriate speed from my 1436 Lake Lowe with a 1992 Johnson 15hp on it. With just two adults (200lbs each) fuel and starting battery I am only able to run about 15mph. Previous owner of this motor had it on a similar boat and claimed it was running nearly 28mph. So far I've moved the motor up so the cavitation plate is above the bottom by 3/4" and got zero gains. It's running a 9-1/2 x 10 prop.
 

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My initial gut reason was that 28 mph would be a tall order. However, I note several claims on the internet of similar set ups in the low to mid 20's. Hopefully someone with a similar set up will chime in.
 
LDUBS said:
My initial gut reason was that 28 mph would be a tall order. However, I note several claims on the internet of similar set ups in the low to mid 20's. Hopefully someone with a similar set up will chime in.
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My previous experience says I think it is possible lightly loaded it should definitely be doing better than 15
 
NOT 28 mph unless they are guessing !! I have had a couple 14' V bottoms that won't go 28 with minimal equipment. I would venture someone had a 15 hp hood on a 20 hp motor to get 28 mph. The flat bottom has more friction drag on the water than a V bottom on plane. From personal experience ( I have had a bunch of small boats in my 60 plus years of boating) if that 15 hp is running perfect, you might see 22/24 with one person, tops 20 with two adults on a flat bottom. You might gain another 2-3 mph on a V bottom. I base that on accurate GPS readings not a pressure speedo or a poor quality phone app. Accurate speed readings need 5 satellites for real accuracy, only three sats can drop accuracy by 20-25% which is what phones typically use.
 
aarolar said:
I am struggling to get what I think is appropriate speed from my 1436 Lake Lowe with a 1992 Johnson 15hp on it. With just two adults (200lbs each) fuel and starting battery I am only able to run about 15mph. Previous owner of this motor had it on a similar boat and claimed it was running nearly 28mph. So far I've moved the motor up so the cavitation plate is above the bottom by 3/4" and got zero gains. It's running a 9-1/2 x 10 prop.
Click to expand...
I think it might have to do with total weight in the boat. I wonder how much weight the previous owner had in it. 🤔
 
I get 9mph out of my 1436 with 5hp 2 stroke. 8mph with 2 200lb adults and a 110lb kid. I got 23 on my 1648 with a 20hp 4 stroke. I'd pick up a cheap tachometer and check rpm. Prop hub could be slipping or maybe the prop is just too big and the motor is struggling to turn it.
 
Like mentioned, it's all guesswork until you get rpm reading. But my years on the water are telling me that 15 pushing two solid men on a narrow boat ain't horrible. Put a 150lb guy and a kid up front and you'll get into the 20's.
 
Weldorthemagnificent said:
Like mentioned, it's all guesswork until you get rpm reading. But my years on the water are telling me that 15 pushing two solid men on a narrow boat ain't horrible. Put a 150lb guy and a kid up front and you'll get into the 20's.
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Even with just me in the boat I only hit 17mph matter of fact with me and my father (200 and 250) and a half a 48 quart cooler full of water it ran 15mph.
 
My 1436 started out with a 6 hp. It would barely do 11 mph. A tach, two props, and some rearranging and it would do 14. You want to be on plane at the upper end of the rpm range of the engine. It could be that you have one prop for loaded down, and a different prop for running light. Tilt pin also needs to be at the best setting.
 
Best/easiest/cheapest way to tell how fast you're going in a small tinny? I'm curious, but not wanting to spend a fortune to find out.
 
Have a tach hooked up headed to the river this afternoon see what happens
 
LaqueRatt said:
Best/easiest/cheapest way to tell how fast you're going in a small tinny? I'm curious, but not wanting to spend a fortune to find out.
Click to expand...
Phone app if you have a smart phone or a handheld gps. There some cheap ones out there now....I have a couple old Garmin 12 units I let folks use to get speeds, even taped one onto my R/C car ro get speed readings. GPS is going to give you your most accurate speed...
 
I had a 13 ft v hull aluminum with a 15 hp 4 stroke Yamaha tiller that would hit 28 + mph. My Tohatsu 9.8 would do almost as well………It’s all about having the right prop, rpm, and engine height…..Not to mention that each boat and how you load/balance it is different. If you want to dial it in for your specific scenario, it’s going to be a little work…….
 
Small boat/motor packages are finicky when it comes to weight, hull condition, prop selection, prop condition, how well the motor runs, current etc. anything above 20 isn't bad, above 25 is real good. Under light loads, you lose MPH as weight goes up.
 
aarolar said:
I am struggling to get what I think is appropriate speed from my 1436 Lake Lowe with a 1992 Johnson 15hp on it. With just two adults (200lbs each) fuel and starting battery I am only able to run about 15mph. Previous owner of this motor had it on a similar boat and claimed it was running nearly 28mph. So far I've moved the motor up so the cavitation plate is above the bottom by 3/4" and got zero gains. It's running a 9-1/2 x 10 prop.
Click to expand...
I h
aarolar said:
I am struggling to get what I think is appropriate speed from my 1436 Lake Lowe with a 1992 Johnson 15hp on it. With just two adults (200lbs each) fuel and starting battery I am only able to run about 15mph. Previous owner of this motor had it on a similar boat and claimed it was running nearly 28mph. So far I've moved the motor up so the cavitation plate is above the bottom by 3/4" and got zero gains. It's running a 9-1/2 x 10 prop.
Click to expand...
I have exactly the same setup, plus my prop isn't in the best shape. I get 20-21 consistently down river with around 600lbs total minus hull weight and 17-18 up river. With just me and my son (180+50) plus 6 gallons of fuel, and maybe 75-100lbs of misc I get about 20 up river and 22 down river. All that said my cavitation plate is 1.5-2" too low. I temporarily picked it up (didn't have enough transom to leave it that way, scared it would fall off) and regretfully I didn't clock it, however I believe I was doing 28 easily, honestly feel like it was closer to 30 down river. Significant speed increase. This test run was bare bones, ~375lbs including motor minus hull. Haven't gotten around to building my riser bracket so can't clock it to prove it but fully intend to when I get time
 
rebeldrummer96 said:
I h

I have exactly the same setup, plus my prop isn't in the best shape. I get 20-21 consistently down river with around 600lbs total minus hull weight and 17-18 up river. With just me and my son (180+50) plus 6 gallons of fuel, and maybe 75-100lbs of misc I get about 20 up river and 22 down river. All that said my cavitation plate is 1.5-2" too low. I temporarily picked it up (didn't have enough transom to leave it that way, scared it would fall off) and regretfully I didn't clock it, however I believe I was doing 28 easily, honestly feel like it was closer to 30 down river. Significant speed increase. This test run was bare bones, ~375lbs including motor minus hull. Haven't gotten around to building my riser bracket so can't clock it to prove it but fully intend to when I get time
Click to expand...
Mine is a 1985 15hp with ALOT of hours on it.
 
I had a 14ft Grumman with a 48" wide flat bottom Mod V hull back in the 80's, the motor was a 20hp with 9.9hp decals, with two guys in it two batteries, a trolling motor on the bow, and stick steering up front on slight chop with the right prop I got 23mph on the fish finder, which used one of those wheels to read speed. It turned out those were not accurate because when GPS came out, that boat never topped 19.3 mph. even with just me in it.
 

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