HP rating

[puddle jumper]

I was told today that back before 1990 the hp rating was taken off the fly wheel and modern engines are taken off the prop. What I was also told was my boat hp rating may be over rated for modern engines. Do I have a concern with my pre 1990 boat with the newer outboards and need to look to a lower hp engine than the boat tags says I can have.

Thanks

 

[Willyclay]

Re: HP rating

The effective year for the prop shaft rating of outboard horsepower that I have seen discussed most often is 1985. How much pre-1990 is your boat?

 

[JB]

Re: HP rating

I would not worry about the difference in HP of modern vs. pre-1985 engines. If the rated HP of the boat matches or exceeds the rated HP of the outboard you should be okay.

In an older boat I would only be concerned with the condition of the boat, mainly the transom.

 

[seahorse5]

Re: HP rating

The ratings for max. hp have nothing to do with construction strength of a boat and have not changed for many years. The ratings are determined by a mathmatical formula that is a function of transom width (not boat width) and length and whether it is tiller or remote steered and if it has a hard chine flat bottom or not.

 

[JB]

Re: HP rating

You left out transom height, SeaHorse. That does not speak to the OP question, though.

I would still be concerned with rot in any pre-1990 boat, regardless of math gymnastics.

 

[nwcove]

Re: HP rating

i agree with JB, mathematical formulas dont add a weak /rotted transom into the mix. the relatively small difference from crank hp....which would be slightly higher than at the prop shaft.....to prop shaft hp....which would be slightly lower than at the crank, is quite small.

 

[puddle jumper]

Re: HP rating

My boat is a 89 but is all aluminum construction. So no need to worry about rot and there is no cracks.

I didn't think that there would be any problem but you never know on till you ask. The way I was told there would be like a 20hp drop in the rating.

thanks again.

 

[nwcove]

Re: HP rating

the only drop in hp from a crank rated motor to a prop shaft rated motor would be the "parasitic drag" from spinning the impeller,and overcoming the friction from the gear case. i.e...a motor rated at 90 hp at the crank, might only be putting 85 hp to the prop.....if the water pump and gearcase are using 5hp to spin......probably more like 1.5 to 2 hp at best.

 

[HT32BSX115]

Re: HP rating

the only drop in hp from a crank rated motor to a prop shaft rated motor would be the "parasitic drag" from spinning the impeller,and overcoming the friction from the gear case.

And anything else the engine is spinning like an alternator, circulating pump and power steering pump etc(if equipped)..............

My current 7.4L 454 engine is rated at 330 crankshaft HP. (although I think Mercury optimistically did it excluding the other stuff the engine is spinning.......they are trying to sell engines!!!) ..............With a drive on the back it's rated at 300 Propshaft HP.

I don't think you can automatically assume a 10% drop from crankshaft to prop but the analogy is the same....and it would probably be somewhat less in a small outboard...

The difference is also probably not discernible by most of us...........

 

[JB]

Re: HP rating

My boat is a 89 but is all aluminum construction. So no need to worry about rot and there is no cracks.

I think you will find that your transom is wood inside an aluminum skin, PJ. If that wood rots you risk dropping your outboard to the bottom of the lake.

 

[puddle jumper]

Re: HP rating

I think you will find that your transom is wood inside an aluminum skin, PJ. If that wood rots you risk dropping your outboard to the bottom of the lake.

I will have to take a closer look.