CONTINUOUSWAVE

Q1: if a NEWPORT 17 engine is changed to 70-HP from 100-HP, but the new engine weighs 100-lbs less, what change will occur in performance?

BACKSTORY. I have a 17' Newport. In 2000, I powered it with a Yamaha F100 four-stroke-power-cycle engine that weighs 355-lbs. The Yamaha F100 has been a great engine, and I like it very much. However, I've always felt it looks huge and out of place on the stern of that little NEWPORT 17. Previously the engine was an Evinrude 100-HP two-stroke-power-cycle engine that was smaller, lighter, and looked like it belonged on the boat

I'm guessing the NEWPORT 17 wasn't designed to have a 355-lbs engine on it's transom.

If I switched to a modern Yamaha F70 engine, I think I would be pleased with the look, but I may constantly feel the boat is underpowered and sorry I made the switch. Often there are several people on the NEWPORT 17, while at the same time pulling a double-inner-tube or a skier. I only use the NEWPORT 17 for recreational boating, such as cruising with friends and pulling little kids water skiing. The 100-HP does a great job of handling a full load of passengers and still pulling-up skiers. However, the bow sure gets high sometimes with the heavy 100-HP on the transom, and I often have to have folks sit up in the bow to counter balance.

Q2: is even thinking of re-power with 70-Hp stupid?

Without knowing any information about the present performance of the boat with the 100-HP engine, to give you an accurate estimate of how the performance will change (if you reduce to 70-HP and reduce weight by 100-lbs) is going to require some guessing about the total boat weight.

The general behavior of a moderate speed planing hull is to achieve boat speed in proportion to the power-to-weight ratio to the 0.5-exponent multiplied by a coefficient called the hull factor. With the data you provide only the change in power is known. The change in total boat weight is not known accurately. To make a guess, the total boat weight will have to be estimated by an informed guess, based on having knowledge of the appropriate hull factor. I demonstrate below:

Assume the NEWPORT 17 with 100-HP is able to accelerate to 40-MPH. Assume the hull coefficient is 180. We can deduce the weight using the Crouch Speed Prediction formula, which I have implemented in a calculator. (See https://continuouswave.com/calculators/crouchCalc.php )

INPUT DATA
HORSEPOWER =100
HULL FACTOR = 180
BOAT SPEED = 40
The calculator predicts a total hull weight
POUNDS = 2025

To predict top speed with 70-HP and a reduced weight of 100-lbs, the calculator is given

INPUT DATA
HORSEPOWER = 70
POUNDS = 1925
HULL FACTOR = 180
The calculator predicts a boat speed
BOAT SPEED = 34.3

If you have accurate data about the top boat speed with the 100-HP engine, a better estimate of the boat total weight can be made, and then a better estimate of the new boat speed with 70-HP will be possible.

Note: the hull factor of 180 is a conservative value for use with moderate speed Boston Whaler classic hull designs. Read the notes in the Calculator page for more details.