Jeff wrote:
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.

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 like your logic Jeff. My opinion also. The danger of going too
small
is not a happy one, while going too big is not a terrible penalty.

Bart wrote:
....

I like your logic Jeff. My opinion also. The danger of going too
small
is not a happy one, while going too big is not a terrible penalty.

One thing I forgot to mention is that the number presumes a reasonable
prop - one that's about 55% efficient. If you use a 2 blade folder,
you might think you're underpowered. Mine work OK, but given my light
weight (10K lbs) and slippery hulls (11:1 length to beam) I really
expected to be able to power over 10 knots, but 8.5 is the practical
limit.

Also, while I think you have enough juice for your needs, I don't
think you will be overpowered. If you were using it as a motorsailer,
and expecting to exceed hull speed even when fighting a headwind,
you'd want 100 hp or more. And you'd probably be swinging a 4 blade
prop, although you can make a good case for a big variable pitch prop
on a motorsailer.

Bart wrote:
I like your logic Jeff. My opinion also. The danger of going too
small
is not a happy one, while going too big is not a terrible penalty.

Well, it costs more money, plus loss of space in the boat &
added weight. It's a case of "more is better" (up to a degree).


Jeff wrote:
One thing I forgot to mention is that the number presumes a reasonable
prop - one that's about 55% efficient. If you use a 2 blade folder, you
might think you're underpowered.

Yes but with a too-small or ineffective prop, more
horsepower isn't going to do much good, it's only going to
make bubbles & waste fuel.


.... Mine work OK, but given my light
weight (10K lbs) and slippery hulls (11:1 length to beam) I really
expected to be able to power over 10 knots, but 8.5 is the practical limit.


Wouldn't have expected that... is that full RPMs? Do you
think it's the props?


Also, while I think you have enough juice for your needs, I don't think
you will be overpowered. If you were using it as a motorsailer, and
expecting to exceed hull speed even when fighting a headwind, you'd want
100 hp or more. And you'd probably be swinging a 4 blade prop, although
you can make a good case for a big variable pitch prop on a motorsailer.

Yes, that would be the way to go. As for horsepower, we have
become lazy & spoiled... most boats could do quite well with
far less than the owner think acceptable... and I mean
sailboats, not the obscenely overpowered motor boats (for
example, my own motorboat is overpowered by about 30%, which
I would not call obscene but it's got one of the smallest
engines I've ever heard of in a boat of it's type & size).

Fresh Breezes- Doug King

DSK wrote:
Yes, that would be the way to go. As for horsepower, we have
become lazy & spoiled... most boats could do quite well with
far less than the owner think acceptable... and I mean
sailboats, not the obscenely overpowered motor boats (for
example, my own motorboat is overpowered by about 30%, which
I would not call obscene but it's got one of the smallest
engines I've ever heard of in a boat of it's type & size).

I don't know Doug. I think I'd call that obscene.

.... my own motorboat is overpowered by about 30%, which
I would not call obscene but it's got one of the smallest
engines I've ever heard of in a boat of it's type & size).



Bart wrote:
I don't know Doug. I think I'd call that obscene.


Well what about the same hull & same disp with twin 400s? Or
those 16' sliver shaped bass boats with twin 225 outboards?

I know of two sisterships of ours, one has the same engine &
the owner grips that he wants twins & more power, the other
has a 165 Perkins and the owner thinks the boat has "about"
the right amount of power. I think it would do just fine
with 90hp.

A fellow I used to work with tried to earnestly convince me
that his 17' runabout absolutely needed more than 150 HP in
order to "plane properly." Oh well.

DSK

I think you should take it out and put in a steam engine.

DSK wrote:
.... my own motorboat is overpowered by about 30%, which
I would not call obscene but it's got one of the smallest
engines I've ever heard of in a boat of it's type & size).



Bart wrote:
I don't know Doug. I think I'd call that obscene.


Well what about the same hull & same disp with twin 400s? Or
those 16' sliver shaped bass boats with twin 225 outboards?

I know of two sisterships of ours, one has the same engine &
the owner grips that he wants twins & more power, the other
has a 165 Perkins and the owner thinks the boat has "about"
the right amount of power. I think it would do just fine
with 90hp.

Bart wrote:
I think you should take it out and put in a steam engine.

DSK wrote:
.... my own motorboat is overpowered by about 30%, which
I would not call obscene but it's got one of the smallest
engines I've ever heard of in a boat of it's type & size).

Bart wrote:
I don't know Doug. I think I'd call that obscene.

Well what about the same hull & same disp with twin 400s? Or
those 16' sliver shaped bass boats with twin 225 outboards?

I know of two sisterships of ours, one has the same engine &
the owner grips that he wants twins & more power, the other
has a 165 Perkins and the owner thinks the boat has "about"
the right amount of power. I think it would do just fine
with 90hp.

There's not enough room in his engine room...and he would have to put a
paddle wheel on back and then he wouldn't fit in his slip....

Bart wrote:
I think you should take it out and put in a steam engine.


That would suit me just fine. I am comfortable with
technology on the level of shoveling coal.

DSK