HP ratings? Old - vs- Current ??

Kymasabe

Petty Officer 2nd Class
Joined
Jul 15, 2007
Messages
179
I have a 1970 DuraNautic 12 foot aluminum boat (for now) that has an '82 Johnson 9.9 on it that runs great. However, with full load of fuel, kid, tackle, rods, small cooler, can't get boat on plane. The plate on the transom says the boat will take up to 12 hp outboard and honestly, I can't remember ever seeing a 12. So, was thinking of throwing on a 15hp outboard.

The question is...was 10 or 12 hp back in 1970 rated differently than in the 80's or 90's, or even in the 2000's with 4-strokes? Was a 10-12hp back in 1970 stronger or weaker than my '82 9.9hp or a 9.9 form the 90's.
Can I put a newer 15hp on the boat without tearing it apart or should I reinforce the transom somewhere?
I just saw an ad for a 1974 18hp Evinrude at a great price...but think it might be too powerful and too heavy so am still thiking 15hp instead. Many of the Johnson/Evinrudes of the 80's...the 9.9 and 15's were almost identical with minor differences like carbs throat sizes and exhaust ports.
 

nobleprime

Cadet
Joined
Jul 17, 2008
Messages
20
Re: HP ratings? Old - vs- Current ??

Hello,
I am not sure where you are from but in the US is it illegal to put an engine on a boat that is higher than the max hp rating. That being said I don't see why your boat wouldn't be able to handle the extra 3 hp.

As for the new versus older debate, the older outboards are rated from the power head while the new ones (I think after the mid-80's) are rated at the prop. That being said the newer ones are slightly more powerful because, with the old ones, there would be a loss of power in the transition from power head to prop. I think it boils down to around 5% difference. Hope that helps
 

Kymasabe

Petty Officer 2nd Class
Joined
Jul 15, 2007
Messages
179
Re: HP ratings? Old - vs- Current ??

Well, the label on the transom is 38 years old and barely legible. A little elbow grease and a stiff breeze and it'll be a fond memory. I can't see how anyone could enforce overpowering a small boat if the label has worn off.
But thank you for the "rated at powerhead - vs - rated at prop" information. That gives me a little insight into the old HP rating system.
 

dajohnson53

Lieutenant Commander
Joined
Apr 28, 2004
Messages
1,627
Re: HP ratings? Old - vs- Current ??

Well, the label on the transom is 38 years old and barely legible. A little elbow grease and a stiff breeze and it'll be a fond memory. I can't see how anyone could enforce overpowering a small boat if the label has worn off.
But thank you for the "rated at powerhead - vs - rated at prop" information. That gives me a little insight into the old HP rating system.

Just in the FWIW dep't. When I was a kid in the 60s we had a 12 foot Lund with a ~67-68 Merc 9.8. That thing went like a bat out of hell, and I have no doubt that it would get on plane with an adult, kid, and the rest of the stuff. Maybe your 12 foot is stouter and heavier, but that Lund was pretty heavy too. Is your engine running really well - full RPMs, good compression, good prop?

As for the rated HP: there is also a US Coast Guard formula for rating HP for a given hull. It is based on length, width, transom height and whether the boat is tiller or remote steering (If I recall correctly). You might want to check that out - you can find it by searching these forums and/or do a yahoo search on the US coast guard site. That would be the default rating for your hull by your insurance company and/or water law enforcement in your area if your boat's rating label is missing or illegible. Although you and I don't think there would be a problem with running the boat with a 15, your insurance company might not cover you if it's overpowered. You might want to check that out to see if it's to your advantage or disadvantage should the label become unreadable or lost.

Just as another thought - a post-80s motor of the same hp would be a little more powerful than the 70s version as others have said. Maybe you just need a newer, better running ~10 hp?
 

Kymasabe

Petty Officer 2nd Class
Joined
Jul 15, 2007
Messages
179
Re: HP ratings? Old - vs- Current ??

Good question. IS it running to full RPM ?? I don't really know. It may still be a weight distribution problem too but I'm not sure. Still working on it.
 

wbeaton

Commander
Joined
Jul 30, 2006
Messages
2,332
Re: HP ratings? Old - vs- Current ??

Your 1982 9.9hp is powerhead rated. Motors after 1987 or so had to be prop rated. So your motor is less powerful than a newer motor. Some outboard makers used to make 12 hp motors. 18 hp is too powerful. The Merc 9.8 hp of the 1960's was conservatively rated and produced more hp than the OMC 9.9hp.

It is unlawful to operate a boat without a capacity label where I live. Its not enforced much, but a friend of mine spent a few weeks with his boat impounded because he was suspected of overpowering and didn't have a capacity plate to prove otherwise. His was an issue of too much weight on the transom not hp, but it comes down to the same thing.

Look at your boat setup. Try to weight the load forward. Try adjusting the trim pin location. You can also try adjusting the vertical height of the motor on the transom. Do these things before you start looking for a new motor. a 9.9 hp should be fine. My 12' boat max rated at 12.5 hp with a 1985 9.9 hp (or various 1950's 10 hp motors) gets two adults and gear on plane with no problem. Slower to plane, but fine just the same.
 

SnappingTurtle

Lieutenant
Joined
May 4, 2008
Messages
1,251
Re: HP ratings? Old - vs- Current ??

I can just say that my 1981 6HP motor is now on a list in Germany (I just found out) of being down rated to a 4HP by the German Water Police. There is a law here that requires a professional license for all boats with a 5HP or above, this means that the illegal (on paper for use by non-licensed boaters) motor, is really legal by the newer HP standards.

According to them I could also actually run a older 18-20HP (powerhead rated) motor on a newer Inflatable that is only rated for a 15HP (prop rated).

I don't know what formula they used to come up with this number, and don't really care, but they said I would be in the clear legally, and not to worry if I did it.

Personally, I think a newer 15HP would be enough, and I really don't like the idea of having to lug the extra weight of a larger motor around, whether on the boat, or carrying it to the boat. But it does open up the choices when looking for older, more powerful used motors for the boat.

Different countries, like different States, have different laws. Don't even want to get into the legal, smart, court, insurance, and liability debate, so I won't.

That said, there are a lot of Americans here in Germany that think all American traffic laws apply here, while having no idea of the real German traffic laws, and their argument is that if it is illegal where I am from, it must be every where else. It is always funny to hear them screaming about all the supposed dangerous speed violations they have witnessed, that aren't violations at all, only to watch them break every rule on the books while driving here.

This is not a another “yea, go ahead and overpower” post. Check your local laws to see what applies in your area, where ever you are, and follow these rules.

On paper many older boats are not safe, even know they are “horsepower legal” on paper, because they are no longer in the condition they were when they were rated. Those power ratings on your boat, are for a boat that is in good condition, not for one that has a half rotted transom.

None of my boats have a plate with horsepower ratings. One is newly registered as a homebuilt and the HP rating given by the authorities is way over what I would consider to be appropriate. The other is rated for maximum weight, but not horsepower. I do have a data sheet I found in the internet, for this boat with a "recommended" HP rating, but not a maximum. It is just not on the boat plate. The third boat is a Kayak, and the HP supplied by me is way underpowered. ;-)
 

Chinewalker

Fleet Admiral
Joined
Aug 19, 2001
Messages
8,902
Re: HP ratings? Old - vs- Current ??

I have an identical 12-foot Duranautic and I can say that it flat out flies with a 9.9, even with a couple adults on board. It'll actually plane off with a 5.5 with 2 adults and my stepson. How much gear are you loading? Might also want to look at a lower pitch prop for more low-end grunt.
- Scott
 

Kymasabe

Petty Officer 2nd Class
Joined
Jul 15, 2007
Messages
179
Hey Chinewalker !!

Hey Chinewalker !!

Sent you an IM with my load and set-up.
 
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