Capt. Rob wrote:
The engine is predicted to need 65 HP to make hull speed.
I have 10 extra HP.

Bart, I'm no expert on engines, but that seems underpowered.

No, it seems just about right, unless the priority is to go at hull
speed regardless of the conditions or fuel cost.

I thought
hull speed should occur within 60-70% of rated horsepower in calm
conditions.

No. First of all, you don't really want to go a hull speed at all on
most displacement sailboats because fuel usage will go up dramatically
for the last 10% of speed.

And while you don't don't to run a light diesel at 100% for long
periods, 80-85% should work fine. At 60% you might start to worry
that you're running too slow.

For example....and my math is admittedly fuzzy here....the
35s5 makes hull speed with 16.6 HP on her 28 HP Volvo diesel.

The "rule of thumb" is that 1 HP per 500 pounds displacement gets you
just shy of hull speed (S/L ratio of about 1.3). So you would need
about 23 HP, Bart about 63.

With your
vessel's weight and wetted surface ratio, 10 extra HP seems way to low.
A 46 foot J Boat carries a 76 HP engine, but doesn't it weigh a lot
less than your boat?

The J46 is about 6000 lbs lighter, so by my reckoning it could use a
little less power. However, if you look at the Yanmar lineup, there
is nothing at 64 hp; the next size down could certainly be viewed as
too small. Also, the hull speed for the J is probably higher, both
because of a longer waterline and because the achievable S/L ratio is
probably somewhat higher than 1.34.

I thought I saw an Ercison 46 for sale some years
back and it had more like 105 HP.

It could be an old gas engine - I'm not sure how they actually get
rated but I don't think they are generally run at 80% for extended
periods. Certainly tons of 30 footers were overpowered with Atomic
4's. Also, lots of boats get re-powered with whatever happens to be
sitting in someone's warehouse.