what should a new 15hp 4 stroke do on a 1436 jon ?

[back4more]

just upgraded our 2013 6hp 4 stroke tohatsu to a 2014 tohatsu 15hp 4 stroke for our alumnacraft 1436. we took it out for the first time last weekend. after hitting the 2 hour mark on the motor I opened it up to WOT for maybe 35-40 seconds. using my lowrance mark 4 gps I found that the speed topped out a little over 17 mph. for a split second I thought it read 20 mph, but it was quick and I didnt see a 18 or 19 at all, but when it was reading 17 the reading was 17 constantly.

so we went from 13mph with 6hp to 17mph with 15hp (+ 135 pounds total weight too)

shouldnt I more of an increase with the 15hp 4 stroke?



1436 w/ 2013 tohatsu 6hp 4 stroke - 13+ mph (phone gps)
minimal chop, 545lbs in boat

Spoiler

- 480lbs 3 people (2 on mid bench, 1 on rear bench), 60lb motor, ~5lbs 3 life vests

1436 w/ 2014 tohatsu 15hp 4 stroke - 17+ mph (lowrance FF/gps)
very little chop, 680lbs in boat -

Spoiler

- 480lbs 3 people (2 on mid bench, 1 on rear bench), 115lb motor, 50lbs 3/4 sheet 3/4in plywood w/ carpet, 10lbs medium cooler w/ sm emergency paddle, PWC fire extinguisher, small battery for FF, 20lbs fuel tank/fuel, ~5lbs 3 life vests

 

[Boat2fast]

Seems slow...but, that's a lot of weight. Need to check the set-up on the prop. Find your operating range and check your RPM. If you are running too much RPM, you need more pitch; too little RPM needs less pitch.

 

[TNtroller]

Try making a test run with the 15 and the same weight as the test run with the 6hp for a better comparsion. Sounds like you need to move some weight up to the bow, maybe one of the smaller passengers if possible, or a cooler with some ice/water, or milk jugs with water, etc, to try some weight distribution and see if that helps.

 

[DrNip]

Looks like you asking a whole lot out of that 1436 and 15 HP with 3 people and all the excess weight. Was the 3 people just a test for the engine or are you actually out fishing with 3?

 

[bcbouy]

my 14' v hull does near 20 mph. with a 2011 15 2 stroke wot with nearly the same weight.id say its right where it should be.imho messing with the prop is wasting time and money .throttle back and enjoy the ride :LOL2:

 

[scoobeb]

That boat should fly even with that weight in it.I have a 1436 with a 20hp and it does near 30mph with me and near 28mph with 2 but with 3 it would probably be near 23-25mph.The 15merc/tohatsu(same motor) has a ton of power and you should do near 20-23mph with no problem.

 

[bcbouy]

scoobeb » Today, 13:37[/url]"]That boat should fly even with that weight in it.I have a 1436 with a 20hp and it does near 30mph with me and near 28mph with 2 but with 3 it would probably be near 23-25mph.The 15merc/tohatsu(same motor) has a ton of power and you should do near 20-23mph with no problem.

i respectfully gotta call bs on that. theres no way a 15 horse can get those speeds with #550 weight.especially a john.my 9.9 didn't get those speeds pushing a 120 pound 12 footer.

 

[scoobeb]

I have had a 15hp 2stroke merc on a 1436 that did near 28mph,you need to watch youtube my friend you will learn alot.I have seen a 15hp 4stroke nissan(same as the tohatsu and merc) with 2guys in a 1436 and they were flying.A 1436 is only 150 to 200lbs.If you can't get over 20mph with only 500lbs of weight in the boat then there is something wrong with that motor or it's propped wrong.Trust me on that,you can call b.s all you want but the facts don't lie.If you watch you tube there is hundreds if not thousands of videos with a 15hp moving well over 500lbs of weight well over 20mph.

A 15/20hp is a lot more powerful then people give them credit for.Like i said if anyone is having an issue getting over 20mph with only 500lbs of weight there is an issue with that motor.I weigh over 300lbs plus the motor i have is 106lbs and i had a 200lb other person on my boat,a 1436 crestliner which weighs all of 185lbs and top end was near 28mph with a 11pitch prop and it had an awesome out of the hole shot to.That is near 800lbs including the boat weight,just in the boat was near 600lbs and i hit near 28mph so numbers don't lie.I forgot to add it was a dead calm flat day,not a drop of wind so that can come into play,as with the tide to can also alter speed,if your going against the tide and the wind that can take some speed off what you could normally do.To get a true speed go on a flat calm day.A 15 would probably be a few hairs slower at say 24-26mph.They are the same outboard except for the carb.

 

[bcbouy]

so there you have it,back4more. our motors are broken and the numbers are wrong.

 

[scoobeb]

Go to youtube and you will learn alot my friend,the numbers don't lie.There is a guy with a 1236 and he has a 1976 9.9 2stroke johnson and he put a 15hp carb in it,with one person again he went close to 26mph and with 3 people and a dog he hit 21.1mph.He had the max of almost 550lbs in it.Look all over youtube there is hundreds if not thousands of videos that show the speed of a 15hp.

I had a 9.8hp 2stroke tohatsu on a 1236 which is like 20lbs lighter then a 1436,with two full adults,me at 250lbs at the time and my brother at 240lbs which is near 500lbs plus all the gear on the boat,tackle,bait bucket full of water,2 coolers,trolling motor,etc... we were at exactly 20mph so you can't tell me that it's not right.

I'm not going to debate this with you but 500lbs is not alot of weight in a boat and if a 15hp can't go past 17mph it's either propped wrong,there is way more then 500lbs in the boat,or the motor is not working right.There is no other reason,That is a 100%fact.

 

[scoobeb]

I found another one,15hp merc on a 14ft tracker,2 adults,2 children,battery,trolling motor,4 seats,all fishing gear,coolers,etc,they went almost 24mph gps,numbers don't lie again.

Hey Back4more if you want to learn alot go to youtube and find out what those guys are doing compared to what your doing,it may be as simple as a prop change,i go to youtube and ask questions all the time and many people have helped me out to get the most out of my engines.There is alot of smart people there as there are here,but you haven't really got enough info to fig out why it's only going 17mph because i'm telling you that is not right unless u have like 800-900lbs in the boat,or your way over or under propped.The bigger the prop the more speed you will get but the slower the whole shot along with as lower rpm,with a smaller prop the opposite happens,more get up and go and usually slower top end speed but your rpms will be higher.Remember your more then doubling your hp from a 6hp to a 15,a 4mph gain just doesn't sound right for the 9 more hp you gained.

You get to see live videos and they show the speeds in almost all the videos,plus you can see how the boats are loaded with gear and people.That is going to be your best bet.

 

[DrNip]

We get it already, the numbers don't lie and go to youtube, lol.

 

[scoobeb]

I'm just trying to help the guy,lol.

 

[back4more]

Thanks for all the input everyone. I understand that weight has a lot to do with the speed, and prop does too. I was just surprised to find that my 6hp pushed us at 13 mph but the 15hp will only do 17 mph with just 135 lbs more. Not what I had in mind when I sprung for the costly upgrade. I can understand not getting 28-30 mph with the weight I had in the boat, but a 4 mph increase isn't much considering the motor difference.

 

[turbotodd]

Friend of mine has a 20 hp Tohatsu (same motor but with 5 extra HP), power tilt even...on a 1448. With just me in it and nothing else (seats out, etc) with the stock prop, it runs 23 mph at 5700 RPM. 9" prop is what came on it.

I can't imagine a 15 on a 1436 being much slower unless the hull is severely hooked or damaged. Assuming the 15 came with the same 9" prop.

For the dude who had one running close to 30 mph, what prop is on it? Just curious.

 

[scoobeb]

11 pitch prop on my new 20hp suzuki gave me near 30mph,it was like 29.7mph for like 20 to 25 seconds in the calm part of the water,it was dead flat calm out of the wind,when i went back into the wind it dropped to 27.6mph for a constant average speed.The suzuki is one of the most powerful engines for it's size that is out there,plus it helps it's only 97lbs.

 

[scoobeb]

Remember the tohatsu can run near 6300 rpms,a 9pitch prop is way to small for that motor.I believe it's a 9.8pitch that comes on it,so it's near a 10pitch prop,go with a 11 and you should get over 25mph.There is a guy on youtube with a 1652 lowe jon boat at 450lbs and with 4 people on it he says it planes good with a new 20hp 4stroke merc.That to me is impressive.A 1448 jon boat is only like 260lbs unless it's all welded and a 20hp tohatsu should make that boat fly with one person.

There is a guy with an all welded monark 1448 with a 20hp 4stroke merc,same as the tohatsu and he's going 27mph across the water.So these speeds are very capable if you are propped right.

 

[flatboat]

Check your motor height. Cavitation pate should be flush with bottom . With most boats you need a small block under the motor to get it right . There is a lot of drag if it is to low . I've had 7 boats everyone has had to be raised to get it right . Use a tac to check rpm's Buy prop that runs at the top of the top of that range , the guys that build them tell you where they run the best . Every boat is different it takes time and effort to get things right on your boats setup , weight distribution is very important In small boats if it's wrong it's a barge , if it's right it's a rocket , speed is the indicator of correct set up . If you increase from 16 to 23 mph with proper set up think how you have increased you mpg . That's why you bought the new motor to go faster with better economy ....
As always just my opinion

 

[lovedr79]

my heavy as aoll get out grizlly 1448 SC with floor and a merc. 20 will only go around 23 with two people loaded. I have my plate about 1" or more above the bottom. I also switched to a 4 blade which made planning a lot easier. with the stock 10p nothing in the boat at all I got 27 once in dead flat still water.

 

[bcbouy]

fftopic: the op doesn't have a 20 hp.why do you keep comparing a 20 to a 15? 5 hp is 25% more power.put a 20 on an 18 ft. boat then compare.