HP outputs of older models vs. newer models

[cassfam7]

I've been told that older model outboards hp do not match with the newer model outputs. What is the conversion. I have an older boat that specs an 80 hp for maximum outboard size. What would that be in modern hp?

 

[ZmOz]

Re: HP outputs of older models vs. newer models

Before the early 80s, HP was rated at the crank, after that, it's rated at the prop. The HP your boat is rated for is a completely worthless number...the formula they use to find it does not factor in new or old HP ratings.

 

[JB]

Re: HP outputs of older models vs. newer models

The changeover changed ratings about 5-7%, Cass.<br /><br />An engine rated at 80hp in 1980 would probably be rated at 75hp after that, but there were often other changes besides where they took the HP.<br /><br />The HP rating of your boat is calculated based on a number of factors and should be viewed as the maximum SAFE hp to put on that boat. It is NOT worthless.<br /><br />You would be fine with a modern 75hp and probably okay with an 80, as individual engines may produce real hp as much as 10% above or below the rating.<br /><br />Good luck.

 

[ZmOz]

Re: HP outputs of older models vs. newer models

Originally posted by JB:<br />The HP rating of your boat is calculated based on a number of factors and should be viewed as the maximum SAFE hp to put on that boat. It is NOT worthless.

As I pointed out in another thread, using the coast guard formula, a 16' flat bottom aluminum boat with a high performance prop rated 200lb, 120hp outboard would be just as safe as a heavy fiberglass deep V with a 300lb, 120hp crank rated outboard.<br /><br />How does that make ANY sense at all? It is not calculated based "on a number of factors". The only factors used are the length and width of the boat. The max HP number serves only as a very rough estimation of what you should be using on your boat. Hulls should be rated for a max speed and transom weight, not HP.

 

[JB]

Re: HP outputs of older models vs. newer models

Oh, excuse me, ZmOz.<br /><br />I didn't know you were smarter than the USCG and the engineers who built the boat, too.<br /><br />The USCG formula is designed for home builts to give the builder something to work with. It includes transom height.<br /><br />The ratings assigned to a boat are devised by the engineers and they take material, hull type, Hull rigidity, transom dimensions and other stuff into consideration. That is why two boats of similar dimensions have very different ratings. In many places it is illegal to use more than the rated HP.<br /><br />The bottoms of lakes are paved with know-it-alls who claim that the pros don't know what they are doing. The problem is that they don't know what the pros are doing.

 

[ZmOz]

Re: HP outputs of older models vs. newer models

Originally posted by JB:<br />The ratings assigned to a boat are devised by the engineers and they take material, hull type, Hull rigidity, transom dimensions and other stuff into consideration.

SOME boat manufacturers take it upon themselves to find a real world maximum based on actual tests...but many just use the formula LxWx2-90. My Glastron, for example, is rated at 120hp by Glastron. That's exactly the same number I come up with using that formula. It has a 150hp on it, and couldn't handle any better or safer while being "overpowered". Clearly no other factors were taken into consideration here, and that is true for many other manufacturers.<br /><br />You're right, some manufacturers do use other factors, but that's the exception, not the rule.

 

[cassfam7]

Re: HP outputs of older models vs. newer models

That was a great discussion guys. I appreciate all the information. As with most situations it will take some measure of judgment and common sense on the boater's part as well. It is a 70's boat rated for 80hp. I will likely look for a 50-70hp to replace it. That helps with cost as well. Thanks again gentlemen!

 

[rodbolt]

Re: HP outputs of older models vs. newer models

there is also the factor that at WOT the helm must be able to be turned 180* without ejecting the occupants. but no where in any of the abyc or USCG regs have I come acrossed hull strength or designs issues relating to HP. there are some flotation requirments as well. its very confusing to try to find the story but most of the regulations are from the 30's to the 50's. not much change has been done since then.<br /> not long ago I was going to repower a 26 jupiter from 200 HPDI to F225's. called jupiter and they said that while it would keep me inside the recomended max HP and the overall weight limits they did not reccomend it. in fact they sent the dealership a letter several days later stating the added weight would place stress on the transom that may exceed their engineers designs. that was strictly a CYA manuver on their part. dont blame them. we just replaced the powerheads on the HPDI's and kept rolling. but as far as the ABYC and the USCG the F225's would have fallen within the range. in my humble opinion I really think the numbers need to be recrunched and updated data used for max HP. props,engines and gearcase desighns along with power trim and trim tabs have come a long way since the 50's.

 

[ivar]

Re: HP outputs of older models vs. newer models

I think the ICOMIA 28 standard was approved in 81-82? In addition to what stated above , with HP measured on the prop shaft instead of the crank, the manufacturer would have to provide a recommended WOT rpm range, and then give a HP reading in the middle of this range. <br /><br />DNV, the Norvegian equivalent to Lloyds of TÜV, found that a 65HP SAE outboard would be like 52-53HP when rated after the ICOMIA standard.

 

[joblo33]

Re: HP outputs of older models vs. newer models

My 12' Sea Nymph (1979) is rated for 10hp by USCG and 15hp by candian coast guard. I think the USCG must leave a bit more of a buffer than canadian because I've run a 1991 15hp on there and it's fine.<br />Eric