Horsepower Rating Estimation. Am I overpowering?

JJ?

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Hey all,

I just purchased a new skiff and want to throw my 1983ish Merc 70hp on it.
However, there are a couple of problems that I've identified that I'd like to ask y'all about.

The hull is a 2009 and rated for up to 50hp.

A couple of justifications for thinking my 70 will be fine hanging on the back.
1. These early 80's Merc 70hp are very light (190lbs) so weight shouldn't be an issue, correct?
2. It can't be putting out anywhere near 70hp at the prop, right?
- I know Merc started rating hp at the prop in the early to mid 80's so I'm thinking it was probably rated at the block.
- The motor is 30yrs old and has had to have lost a step or two since it came off the production line.

Picture of it on the back of my old rig.



A big factor in this is that I will have to add a 5" riser plate to the transom to bring my cavitation plate flush with the bottom of my transom. I don't want to overpower and have the riser plate.

Anyway, what do you guys think? What is a more realistic valuation of the hp my outboard is actually putting out?
 

Frank Acampora

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Re: Horsepower Rating Estimation. Am I overpowering?

Oh, come on! You know what you are going to do and are just seeking justification here. Well, you won't get it from this forum's members. Iboats does not and never will recommend installing a higher than capacity engine. Watch the other replies that roll in.

Oh, By the way: My motto is "Too much horsepower is never enough."
 

matt167

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Re: Horsepower Rating Estimation. Am I overpowering?

legally, 50hp is 50hp no matter what age it is or how it was rated.. With that said, it should be fine. 2009 50hp will likely weigh over 200hp, especially a 4 stroke and most all outboards that small are 4 stroke ( Evinrude and Tohatsu are the exceptions ). The old 70 is an abomination split from the 65hp 3cyl which had a lot less bottom end than it's 4cyl predecessor.. You would probably get more 'snap' from a more modern 50hp 2 stroke engine. You have it, so it at least won't hurt to try it
 

kenmyfam

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Re: Horsepower Rating Estimation. Am I overpowering?

50hp max = 50hp max as far as insurance and legality is concerned.
What you actually do is your own decision.
 

nwcove

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Re: Horsepower Rating Estimation. Am I overpowering?

id guess your merc is probably making a healthy 60hp at the prop, so you would be over powered by 20%, then factor in the extra stress on the transom that would be added by the riser. play safe. jmo
 

JJ?

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Re: Horsepower Rating Estimation. Am I overpowering?

Frank - I am indeed looking for validation. But I also don't think it's realistically that much overpowering if any at all. For the record the skiff in that picture is a 14' Stumpknocker 2...THAT was overpowered.

Matt - I will probably end up giving it a shot and seeing what happens. I'm waiting on a call back from Brad at IPB to see what he says as well. I might be picking up a used 50hp yamaha 2 stroke short shaft soon so it might all not matter anyway.
 

JJ?

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Re: Horsepower Rating Estimation. Am I overpowering?

NW - I will definitely be waiting to hear from Brad at IPB before I rig it up. Is your 60hp guess a rough estimation? Or is there some math behind it?
 

nwcove

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Re: Horsepower Rating Estimation. Am I overpowering?

JJ™;4396925 said:
NW - I will definitely be waiting to hear from Brad at IPB before I rig it up. Is your 60hp guess a rough estimation? Or is there some math behind it?

no math involved , just an educated guess. if it were me, id be much more concerned with the extra torque on the transom with the addition of a riser, than the extra hp. jmo
 

Frank Acampora

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Re: Horsepower Rating Estimation. Am I overpowering?

104_7439.jpg104_7209.jpgworking copy.jpgresized55.jpg100_6190.jpg

Look: Mercs were always under-rated. Even though it is old, it was probably putting at least a good honest 70 at the prop. It has been said that Karl Keikshaefer considered full throttle for emergency use and the engines were rated at about 3/4. I don't know if this is true or an urban myth.

At any rate, here is a 10 foot hull originally rated for a 25 Merc for class racing. Look what it has on it now. Here is also a 14 foot flat bottom with a 125. Both are HIGHLY MODIFIED to accept the horsepower. HOWEVER I do not recommend this type of modification and would never sell the boat with this power on it.

So: Quit rationalizing your proposed actions, do whatever you decide t to do, but don't ask for permission from the forum.


Too much horsepower is never enough!
 
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JJ?

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Re: Horsepower Rating Estimation. Am I overpowering?

Thank you for the tough love. I guess I am just kind of looking for someone to say 'yeah that'll work fine and not cause damage to your transom'.

I guess I'll be ordering one of these and see how it goes. On another note, has anyone ever mounted these on to a cmc TnT? I'd imagine it would look kind of funny, but I'd like to keep mine so I don't have to do it manually every time.

Also, on my old skiff, I have an 1/8" or 3/16" diamond plate transom support.



Would it be worth it to have this cut down to fit my new transom to help distribute torque/weight?
 
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Re: Horsepower Rating Estimation. Am I overpowering?

......It has been said that Karl Keikshaefer considered full throttle for emergency use


An emergency such as "I FEEL THE NEED FOR SPEED"? :D

JJ, what's the deal with that diamond plate on your transom? I'd bet that's not original, am wondering if someone put that on there as a bandaid to hide a structural issue.
 

JJ?

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Re: Horsepower Rating Estimation. Am I overpowering?

It most likely is. I've never removed it actually.
It was my first boat purchase while I was in college and the price was VERY right, so I didn't expect perfection.
The transom has held up great for about 3 years now and have had more than a dozen "emergency situations" come up :D
Anyway, that's why I purchased the new skiff.

Anybody know anything about the extension plates being mounted to a cmc TnT?
 

Frank Acampora

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Re: Horsepower Rating Estimation. Am I overpowering?

Splashwell.jpg100_6068.jpg103_6241.jpg100_5969.jpg104_7210.jpg

As a reinforcement, diamond plate sucks. It is too soft and bendable. And a simple flat sheet will not provide the proper reinforcement.
Better to use .080 hard or aircraft grade aluminum. Yeah, more expensive but way better. I use salvaged street signs bought from recyclers. As single plate will not support the transom. You must plate it both inside and our and is possible gusset it at the corners like in the above photos.
 
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JJ?

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Re: Horsepower Rating Estimation. Am I overpowering?

WOW that is one beefy transom :eek:
Would the diamond provide any notable reinforcement?
What kind of recycler did you get the signs from?
 

Frank Acampora

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Re: Horsepower Rating Estimation. Am I overpowering?

exhaust.jpgCharger.jpg102_0245.jpg

I had brought a truckload of old midlegs and associated scrap outboard aluminum to a recycler. A municipal truck pulled up with a bunch of old signs. After they sold them I bought a few from the recycler for 5 bucks a piece. It was a good deal considering the aluminum would probably have cost me 16-20 bucks each.at a metal supply shop.

Diamond plate, especially thin plate inside the transom with nothing bent over to cap it and no other reinforcement will simply not be doing much at all. In order to have any benefit at all, it would need to be at least 1/4 inch thick. The only positive it will have is that it will keep the engine from falling off. That is, if the engine is through bolted.

Properly designed hard aluminum will do a much better job using less material. Notice that in addition to corner gussets and plates on the side of the splashwell, the aluminum is bent over the top of the transom providing resistance to bowing outward under the weight of the engine and thrust while running.

It is also possible to use 3/16 to 1/4 inch thick 3 X3 aluminum angle to cap the transom. You would need to cut one leg of the angle down to about 2 inches as a transom cap and the other leg runs down the inside of the splaskwell. Expect to spend about 40 bucks for the one piece of angle.

As a prophylactic measure, I reinforce all the transoms on my boats whether or not they need it.
 
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JJ?

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Re: Horsepower Rating Estimation. Am I overpowering?

I've been doing some research on torque and here are the numbers I've come up with. They're not exact but they get the idea across.

1983 Mercury 70hp (190lbs) -
(Assuming max rpm of 5,800 & 2:1 gear reduction ratio)
67 foot-pounds of engine torque
122 foot-pounds of propeller torque assuming a propeller RPM of 2,900
(If someone knows the actual gear reduction ratio that would be awesome)

Brand New Mercury 50hp 4cylinder (260lbs) -
43 foot-pounds of engine torque
98.9 foot-pounds of propeller torque with calculated propeller rpm of 2,575 (6,000rpm/2.33:1 gear reduction)

1989 Mariner 40hp Magnum 4cylinder (187lbs) - Might be trading my 70 for this.
38 foot-pounds of engine torque
76.6 foot-pounds of propeller torque

Is it a generally accepted principle that the propeller will produce torque approximately equal to twice the engine's horsepower?
Also, I have not yet found an equation that allows me to include the variable of increased engine height. After the height is added, how much effective torque will be added to the transom after factoring in the coupling of the hull and outboard?

Also, does the engine weight have any gravity beyond increased/decreased displacement?

After these calculations my concern has returned...damn...
 
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Re: Horsepower Rating Estimation. Am I overpowering?

JJ™;4402049 said:
After these calculations my concern has returned...damn...


You're way over-thinking this (that and you don't have all the pieces to the puzzle for what you're trying to do). You're attempting to calculate torque to the prop, where in reality what matters most to a transom would be thrust. And guess what - thrust is pretty much directly proportional to horsepower (assuming a correctly pitched prop). So just get a properly sized engine and be done with it.

(And no, I'm not gonna' go through the math to get from horsepower to thrust - it's been years since I've done that and then it was for airplane props).
 
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greenbush future

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Re: Horsepower Rating Estimation. Am I overpowering?

If the transom is rotted, I don't think that adding these items would be good. How would you tie it all the new metal to the hull of the boat? Not just the transom but the hull?
 

Home Cookin'

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Re: Horsepower Rating Estimation. Am I overpowering?

JJ™;4397224 said:
Thank you for the tough love. I guess I am just kind of looking for someone to say 'yeah that'll work fine and not cause damage to your transom'.

yeah that'll work fine and not cause damage to your transom.
There you go.
And since you got it from a stranger on the internet, you know it's true.

But I have a question about your photo--do you really have brick gunwales?
 
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