New Boat motor HP vs. Older motor HP

[RedDB76]

Hi boys and girls,

I am considering getting a newer motor for my boat. I am currently running a 1988 90hp Merc on a 16' Lund that is rated for 90hp.

I have read that in the 90's the reference for a motor's horsepower was changed. Something like motor horsepower used to be measured at the powerhead, but is now measured at the prop? Therefore because of efficiency, an older 90 hp is not comparable to a newer 90hp, and the newer 90hp would have a lot more power than the old one.

I am ok with the speed on my current outfit, it is just a major gas hog, is cold-blooded, and I'd like something more dependable. So, I was thinking that maybe a newer 70hp (or maybe even 60hp) would be similar? Friends and relatives run similar (but newer) boats on 60 hp with the same speed as I can. I would guess their setups are lighter than mine. And - you don't see too many 16-footers rated for 90hp these days.

Thanks in advance,

[boatfixer]

In about 92, they started rating at the prop. You can figure approximately a 10% loss of HP going through the lower unit. Newer motors will tend to be more efficient and hopefully more reliable. IMO I wouldnt go less than 70hp. Even at that, I think you will see a drop in performance, maybe not much but it will drop. I dont know your budget, but take a look at a 80hp 4 stroke. More efficient, quieter, better low end performance, no 2 cycle exhaust and I doubt you would see a decrease in top speed when propped right.

[solbes]

Thanks for clarifying that for me a little. I couldn't figure out why on my new 16' Lund with a Yam F75 I can go faster than my older 16' deep-V with an older 2S 90. The old boat had a beam that was 8" narrower, which should make up for some of the differences in hull design (IPS vs non). The difference in horesepower measurement location makes sense and speaks to some of this difference.

As to your question on why there are very few 16' footers rated for 90. One of the differences has to be weight distribution. Those two strokers, while load and smokey, were 60-100 lbs lighter than their new 4 stroke EFI counterparts. Put a 370-400 lb motor on the back of these 16 footers and you notice a real difference.

That being said, horsepower does not necessarily correlate to weight. In Yamaha's case, the 75 weighs the same as a 90 because they are almost identical. Differences in the tuning and intake/exhaust are the only real differences I'm told.

[RedDB76]

Who makes an 80?

For weight, I was guessing my boat/motor - everything - weighed more than newer ones, and in combination with a less efficient hull design, I was losing some speed and fuel efficieny there as well.

I checked weights, and it looks like a newer 4-stroke 75-90 hp will be around 400#, or around 100# more than my existing motor. A 60hp 4 stroke is similar in weight to my existing motor.

I'm a little leary about adding an additional 100# back there -though, with a newer motor, I could probably get rid of my kicker.

My boat is a 1986, so I don't want to (nor can I afford to) put a brand new motor on it.

[solbes]

80hp would be a good compromise for weight/power. Yamaha used to make the F80 & F100 that were carbed, at least until 2000. Not sure when they changed to 75/90 carbed. Eventually they went EFI in 2005. Displacement for F80 was 97 cubes and weighed 356 lbs.

Suzuki has a 4S 80 that has 92 cubes and weighs 341 lbs.

I think Merc also used to make an 80, but this might have been the same thing as a Yamaha?