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Discussion Starter #1
I done a little research and discovered that I can convert my 2001 Mercury 40hp to a 50hp by changing the carburetors, reeds, and removing the reed stop. It is my understanding that the only difference in the 50hp and the 40hp in that year were those 3 parts. The 60hp is the same also but the exhaust porting is different on it making the swap much more difficult and expensive. I found a guy on a high performance tiller motor page "Ooser Outlaws" that has the three carbs and reed block for sale for $150 plus shipping. He said that he has done the sap before on a 1948 and went from 31mph to 41mph. My boat currently is an 1860 flatbottom riveted hull thats rated for a 75hp max. It runs 24mph with the 40hp (no til/trim) 10.5P prop loaded to fish. Do you guys think that doing the swap would be very beneficial on a boat like mine with all the weight of a bowfishing setup? I know his boat was completely stripped out with nothing but hull, motor, gas, and driver. Mine has generators, lights, fish barrels, 4 guys, 2 champion 2000 generators, 8 250w hps ballast, and other random items. I figure if I could gain 5 mph it would probably be worth it. If I gained 10 then I would be extremely pleased.
 

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I really don't think it would be enough of a difference to make it worth it, I went from a Johnson 40 to a 50 on a 1648 and I couldn't tell any difference at all. if you really want more speed just sell your 40 and find one on craigslist to replace it with a little more hp. you would notice a big improvement moving up to a 75 at the least, ive got a buddy with a 150 on his 1860
 

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I really don't think it would be enough of a difference to make it worth it, I went from a Johnson 40 to a 50 on a 1648 and I couldn't tell any difference at all. if you really want more speed just sell your 40 and find one on craigslist to replace it with a little more hp. you would notice a big improvement moving up to a 75 at the least, ive got a buddy with a 150 on his 1860
I considered doing that but I cant find any newer 90hp motors or anything around that size for a decent deal. I wont buy an older motor and put on the boat because I dont know its history and I wont sacrifice 100% reliability for speed.
 

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I say it's only $150 and a couple hrs labor so why not
Good point. If I only gain 2mph its not a big deal. I can probably sell my 40hp carbs on ebay individually and make more than my $150 back.
 

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Ive been thinking about doing this to my 90 and make it a 115 with a ecu swap. I just havent found enough feedback from people who have done it to do it.
 

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150$ for 10 horses, that's like 15 doll hairs for one horse and horses are expensive around here. All jokes aside I think it would be worth a shot may not help a lot then again it may be just what your looking for. Hell I would pay 150 just to not have to worry about taking the motor off the boat and fool around with people trying to sell it.
 

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If a 25% increase in horsepower isnt noticable something is very wrong. Im not saying you are gonna think its a different motor, but dollar for dollar dime for dime if you cant feel that sort of increase there wasnt one.

You already did the research and sourced the parts. Why not do it for that dollar amount and see if it works. If I could list all the stupid crap I've spent 150 bucks on pointlessly Id have to invent some new sort of 150 dollar pen to write it down with.
 

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Also if you gain a full 10 hp, then you will need a different prop to utilize the power without over revving your engine, so that's another 100 bucks, just something to consider
 

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You might want to double check that info , I do a lot of swaps like this and that 50 hp engine still has the reed retainers in it. Find a parts website that has a schematic in it and cross check all the part numbers, if the bore and stroke are the same, check the carb #'s and the timing module. If you go to Boyesen reeds or carbon fiber it's ok to remove the reed stops but if you suck a stainless reed through the block by overflexing it its game over.
 

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I'd check the swap more. I'd look at the carb specs. If the throttle bores are the same a jet change maybe all you need to do. The timing module with the carb mods and you maybe golden.
 

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Discussion Starter #13
thanks for all the info guys. I actually found a 2004 mercury 115hp 2 stroke last night that I am thinking about pickup up instead. I can sell my 40 for relatively close to what I am going to have to pay for the 115hp. Boat is only rated for a 75, however, that is because the rating factors in if it has a console or not. I ran the calculations of my hull dimensions with a console added and it would be rated for 165HP. So I think that it will be ok. I still plan on adding some transom bracing.
 

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I wouldn't recommend putting a 115 on that 1860, especially with it being riveted. That motor weighs roughly 350 lbs if its a 2 stroke and more if its a 4 stroke. My boat is a 1872 Diamondback and its only rated for a 90 and I have a 115 on it. If for some reason you are dead set on getting it you will need to brace the transom. Is the transom all aluminum or does it have some wood in it?
 

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I wouldn't recommend putting a 115 on that 1860, especially with it being riveted. That motor weighs roughly 350 lbs if its a 2 stroke and more if its a 4 stroke. My boat is a 1872 Diamondback and its only rated for a 90 and I have a 115 on it. If for some reason you are dead set on getting it you will need to brace the transom. Is the transom all aluminum or does it have some wood in it?
transom has 2.5" thick plywood in it and its in good condition. As far as weight goes the boat is a 2001 and I checked on a 2001 75hp Mercury and it was 385lb and this 115hp is 345 lb so its actually lighter than a possible max hp engine from 2001. The weight hanging off the back isnt as big of a concern to me as all of the extra thrust. I'm not dead set on this and could be talked out of it if anyone has had bad experiences. A lot of people will say not to do it but few people have had anything happen to justify it. One of my friends in our NC bowfishing club has a Grizzly tracker that is rated for a 90hp with a Mercury 200 Black Max on it. He has had that motor on it for 5 years now.
 

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The motor ratings are a liability concern in most cases. If they tell you it will hold a 200 and you bolt it on and it rips the transom off theyve got issues. If it can really handle a 150 and they tell you 75 in theory it should eliminate or reduce the issues.

Ive seen a great many boats that are running double or more the tag rated hp. Many of them are 20+ year old boats that have had that motor for 20 years. Im not saying be careless about it, but If your boat is in good condition and is properly braced I dont see it being an issue. I plan to strap a 115 to my 1760, and couldnt care less what the tag says. Mine is an allweld with a full aluminum transom, so its a little different, but the theory remains the same.
 

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I'd slap that 115 on the back and let her eat!!! I have a 90 optimax on my 1860 and want more power. The motor weighs 375 dry and the boat still only drafts in about 8". :D
 

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The motor ratings are a liability concern in most cases. If they tell you it will hold a 200 and you bolt it on and it rips the transom off theyve got issues. If it can really handle a 150 and they tell you 75 in theory it should eliminate or reduce the issues.

Ive seen a great many boats that are running double or more the tag rated hp. Many of them are 20+ year old boats that have had that motor for 20 years. Im not saying be careless about it, but If your boat is in good condition and is properly braced I dont see it being an issue. I plan to strap a 115 to my 1760, and couldnt care less what the tag says. Mine is an allweld with a full aluminum transom, so its a little different, but the theory remains the same.
Sounds like pretty sound advice to me.



When I looked up how the boats hp rating is obtained I was extremely surprised. Being an engineer myself I thought that each individual boat was rated based on model specific structural calculations. When I looked up the formula its actually just a ratio of the length and width multiplied by one of three factors then rounded to the nearest multiple of 5

For my boat with a tiller it is
X=18' length X 85"/12 (Beam) = 127.5

for tiller steer the factor is (X(0.8))-25=HP rating
so (127.5(0.8))-25=77 rounded to the nearest multiple of 5 is 75 HP which is what my boat is rated for.

For remote steer it is (x(2))-90=HP rating
so (127.5(2))-90= 165 HP rating


I cannot believe how simplified the rating method is......Its kind of scary.
 

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Sounds like pretty sound advice to me.



When I looked up how the boats hp rating is obtained I was extremely surprised. Being an engineer myself I thought that each individual boat was rated based on model specific structural calculations. When I looked up the formula its actually just a ratio of the length and width multiplied by one of three factors then rounded to the nearest multiple of 5

For my boat with a tiller it is
X=18' length X 85"/12 (Beam) = 127.5

for tiller steer the factor is (X(0.8))-25=HP rating
so (127.5(0.8))-25=77 rounded to the nearest multiple of 5 is 75 HP which is what my boat is rated for.

For remote steer it is (x(2))-90=HP rating
so (127.5(2))-90= 165 HP rating


I cannot believe how simplified the rating method is......Its kind of scary.

Didn't know that. Thats pretty crazy. However, I also dont see lots of talk of ripped off or severely damaged transoms on new boats. So maybe the math that supports it is setup so "light" on HP that its just on the extremely safe side.

I, like you, assumed that there was some sort of test or system the manufacturers followed to arrive at said number. Sorta like a crash test rating. They hook some device to the hull and try to rip it off and calculate the force necessary or something.

Alas, im not all that suprised the dont want to detonate a hull everytime they make a new model.
 

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Didn't know that. Thats pretty crazy. However, I also dont see lots of talk of ripped off or severely damaged transoms on new boats. So maybe the math that supports it is setup so "light" on HP that its just on the extremely safe side.

I, like you, assumed that there was some sort of test or system the manufacturers followed to arrive at said number. Sorta like a crash test rating. They hook some device to the hull and try to rip it off and calculate the force necessary or something.

Alas, im not all that suprised the dont want to detonate a hull everytime they make a new model.
Destruction at this point, given known materials is all done in software usually (from my experience getting my engineering degree and working doing destructive testing). Basically the goal is always a simple formula so that you can give a rule of thumb to people, especially for custom built stuff or if they make modifications, or to give a starting point.

There's what's referred to as a factor of safety, used for most engineering fields. Generally speaking, automobile industry uses a factor of 3, and with marine applications it's usually considered best to use a slightly higher factor, to take into account harsher environment, lower quality control, and material degradation. I've generally seen a factor of safety of 3.5 used for stresses and force, for commercially produced items in the marine industry. Note that this doesn't mean it'll hold 3.5x as much, as there may be engineered points of failure. A motor snapping the transom and sinking into the water is bad, it creating too much force for a hull would be worse (ignoring the implication of being in the ocean stranded without an engine).

My personal take is that swap isn't exceeding the original rating a crazy amount, I'd consider making sure the area has good wood/metal and considering additional bracing, and just not try to push it as hard as possible in the roughest conditions. And I'll get back to day dreaming of owning a boat (which requires too much money and a vehicle swap to do currently...) I'll have to stick to my paddleboard or the shore, and my super under-powered human limbs.
 
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