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Thread: Rating plate questions on HP

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    Default Rating plate questions on HP

    I bought a 2014 Lowe 1648MT last year new from a "sale". Thought it was a good deal so I bought it and figured I would power it later. It's now later and I'm looking for an outboard. I would prefer to put a jet on it, as I will be running the lil-su as well as other rivers. This has a 20" transom. It's rated for 1014lbs max weight and 555lbs persons. It also says it's rated for a 35 hp max. Dry weight on the boat is 275. These numbers are from AMDS where I purchased it.

    My questions are, is the 35 HP rating from the power head or the output. (can I put a 50/35 on it and still be in the rating max?) I don't know if there is really a 50/35, but that's my example. Is the rating there cause of thrust/torque on transom, or from weight of motor. 4 strokes being heavier than 2's, is that going to factor in my rating as well? Also, a 20" transom is a long shaft from what I have found online? Or do I need to measure every motor accordingly? Or are all brand outboards different as for as long/short/xlong? I ask this because I might buy a motor of CL and I don't want to waste my time if I'm looking at a shortshaft and needing a long. I don't see a lot of new, if any 35's out there. Is a 40 hp that much more to ruin my boat(riveted), transom and make it unsafe? I'm assuming the max weight is including the motor?

    I am new to the boat owner world as of this boat. It will mostly be the misses, the dog and I. But on occasion I would like to have 3-4 in this thing with maybe fishing gear. I want to be able to come back up any river I go down. Pretty much just a general use boat at this point...

    I probably missed a few things, but this is already longer than I intended. Thanks in advance for any advice you may have.

  2. #2
    Member Bullelkklr's Avatar
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    You will need a longshaft unless you put a jackplate on it. max weights include the motor and anything that you put in or on the boat. It is a transom structural strength rating, but I don't know how the manufacture's calculate it. Others will help on the motor sizes.

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    For a Prop driven Outboard, you figure HP at the Power head. converted to Jet, you figure HP at the output.. I've always been told you loose 1/3 HP or Torque Or what ever you want to call it.. so mathematically speaking a 50HP outboard, would actually be only a 33 (35) HP jet. (but I don't think it would be legal for the kenai, as i'm relatively certain they simply consider power head output...) You'd have to check with people more knowledgeable about todays outboards, but I'm of the opinion that most mid range outboards need to be short shafts for conversion to Jet, and with jets require a 25" transom.. So you'd need to talk to some of the more knowledgeable fabricaters (Gary at Greatland Welding on the Palmer/Wasilla hwy) or you'd need to buy and install a motor lift to adjust the correct placement of the Jet foot with the bottom of your boat for max performance.

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    Thank you for the feedback. I guess what I'm trying to ask, is a 50 HP with a jet on it, still within the limits of the plate. Or does the plate strictly refer to power output? Or is the plate just a "guidline"? I'm fully prepared to modify this thing as needed, but would rather just get the motor it's intended for. I'm familiar with boats, I just never had the option of putting together my own setup, and I'm coming to find out there are a lot of options leading me to more questions. o.O

    I believe that recently the HP limit increased to 50hp, and has to be 4 stroke or 2 stroke DI. I think possibly the hp limit might be not there if you have a 4 stroke. Not 100%, I would have to do a little looking, but I won't be going that far anyways I don't think. I'm in the Mat valley area. But I would def. like to keep in their regulations as I think sometime in the near future, this law will be statewide...

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    Supporting Member Old John's Avatar
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    Quote Originally Posted by millertime140 View Post
    Thank you for the feedback. I guess what I'm trying to ask, is a 50 HP with a jet on it, still within the limits of the plate. Or does the plate strictly refer to power output? Or is the plate just a "guidline"? I'm fully prepared to modify this thing as needed, but would rather just get the motor it's intended for. I'm familiar with boats, I just never had the option of putting together my own setup, and I'm coming to find out there are a lot of options leading me to more questions. o.O

    I believe that recently the HP limit increased to 50hp, and has to be 4 stroke or 2 stroke DI. I think possibly the hp limit might be not there if you have a 4 stroke. Not 100%, I would have to do a little looking, but I won't be going that far anyways I don't think. I'm in the Mat valley area. But I would def. like to keep in their regulations as I think sometime in the near future, this law will be statewide...
    the little Aluminum plate with max weight, HP, etc., is strictly recommended. There is a mathematical formula for figuring these things out but I don't have it at hand, and I'm too lazy to look for it.. The construction and strength of the Transom play a big part in suggesting safe HP for the boat. Yes, I believe you are correct, the Max HP figured at the power head, is 50 HP for the kenai, and with few exceptions has to be 4 stroke. A 16ft boat with a 50/35HP jet would be perfect for the Little Su... At least we enjoyed the one we had a lot....

  6. #6

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    Quote Originally Posted by millertime140 View Post
    I bought a 2014 Lowe 1648MT last year new from a "sale". Thought it was a good deal so I bought it and figured I would power it later. It's now later and I'm looking for an outboard. I would prefer to put a jet on it, as I will be running the lil-su as well as other rivers. This has a 20" transom. It's rated for 1014lbs max weight and 555lbs persons. It also says it's rated for a 35 hp max. Dry weight on the boat is 275. These numbers are from AMDS where I purchased it.

    My questions are, is the 35 HP rating from the power head or the output. (can I put a 50/35 on it and still be in the rating max?) I don't know if there is really a 50/35, but that's my example. Is the rating there cause of thrust/torque on transom, or from weight of motor. 4 strokes being heavier than 2's, is that going to factor in my rating as well? Also, a 20" transom is a long shaft from what I have found online? Or do I need to measure every motor accordingly? Or are all brand outboards different as for as long/short/xlong? I ask this because I might buy a motor of CL and I don't want to waste my time if I'm looking at a shortshaft and needing a long. I don't see a lot of new, if any 35's out there. Is a 40 hp that much more to ruin my boat(riveted), transom and make it unsafe? I'm assuming the max weight is including the motor?

    I am new to the boat owner world as of this boat. It will mostly be the misses, the dog and I. But on occasion I would like to have 3-4 in this thing with maybe fishing gear. I want to be able to come back up any river I go down. Pretty much just a general use boat at this point...

    I probably missed a few things, but this is already longer than I intended. Thanks in advance for any advice you may have.
    Max HP ratings are determined by a coast guard formula. It has nothing to do with the strength or construction of the boat. The basics are you take the length x the width at the top of the transom or splash well x 2 -90. So lets say 16 ft x 4 1/2 ft = 72 x 2 = 144 - 90 = 54. you can round 54 up to 55. Now the builder can rate the boat for 55 HP with remote steering ( less if it's a tiller ) if he puts enough flotation in the correct places ( level flotation ). He may chose to rate the boat for less power because he can use less foam or he just doesn't what that size motor on the boat. Glen

  7. #7
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    For the Kenai, it's strictly powerhead ratings. If you run a 60hp with a jet, which only gives you 40jet power, you'll get a ticket. Gar-ron-teed!!

    Those little metal boat templates are like yellow speed limit signs, they're just a safe recommendation, not the law, per se'...
    A boat dealer will (or can) only sell a combination boat/motor pckg under that template rating. If you wanna take that motor off later and power up, you can. A dealership will not sell it new out the door already "powered up". It's a liability/legality issue with them. What you do with it later, is on you....
    Years ago, I had bought a new boat motor combo (18' river boat with a Johnson 115jet). As per the dealer, that's all they could do at the time regarding hp.
    The following year, I sold the 115jet. I took the hull to the same dealership from where I got the package the previous year, they put a bigger motor (150hp VMAX jet) on for me, exceeding the template rating, and they did it with no problems.
    I did end up welding a few cracks in the transom a few yrs later, but that was from a combination of jumping down rapids, a few gravel bars, and an invisible log...
    Not from the newer engine weight.
    Unless somethings changed regarding the law and it's got more stringent, I say go for it.

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    You may also run into insurance issues if you go above the rated H.P. on the boat. I had to remind my insurance company that there isn't a weight and H.P. rating on vessels above 28' I think it is don't quote that though I can't remember exactly what the length is the cutoff.

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    Great info here. Thank you all! With what I've read I think I will look into a 60/40 or a 40/30 yamaha jet drive I have been eying, the difference being 38lbs. I was a little concerned about the weight of the power head but doesn't sound like that will be much of an issue. I figure I can always have a little extra throttle if/when I ever need it with the 60/40 and maybe save a little fuel by not being WOT. By all means let me know if I have this wrong tho. Thanks again for the advice.

  10. #10

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    Quote Originally Posted by dandmtritt View Post
    You may also run into insurance issues if you go above the rated H.P. on the boat. I had to remind my insurance company that there isn't a weight and H.P. rating on vessels above 28' I think it is don't quote that though I can't remember exactly what the length is the cutoff.
    There is no H.P, weight rating or flotation regulations over 20 ft. A builder may put a rating on a boat over 20 ft. but it won't be a black and yellow label like the ones on boats under 20 ft. Glen

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    There ya go from the person that definitely knows the regs.


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