Suitable Sized Diesel

[CAVOK]

I’m looking at a 46’ Beneteau. It has the option of a normally aspirated 57HP Yanmar or a turbocharged 80hp Yanmar. Does anyone have any opinion on whether the 57 hp would be adequate for longer distance offshore cruising or is the extra price of the 80hp worth it. As well any thoughts on turbos versus normally aspirated diesels.
Are there any formulas for engine size for a given weight or length of boat.

[a64pilot]

Gut says 57 is OK, but having more power is nice.
A turbo does increase complexity and maintenance and therefore cost.
Depends on how important motoring is to you would be my answer.
Me personally, I think I would go with the turbo, but I’m a gear head and enjoy motors, but I also believe the 57 HP NA would be fine too, just not as fast in beating to heavy seas etc.
There are all kinds of formulas, what becomes clear to me with them is that the newer ones say you need bigger motors, and I believe that most new boat buyers when offered a bigger motor check that option box.
Was it needed? Old boats used to get by with much less power than is considered necessary in newer ones, I guess it depends on how big a hurry you are in?

[Dennis.G]

The 80 hp turbo is for the day sailors who want to get out of the harbor in a hurry on the weekends. Long distance traveling you will never use the extra power (big hit on fuel economy for very little increase in speed) of the turbo, and additionally will be glad to not have the added complication of a turbo engine. 56 hp is plenty for a 46' Beneteau. Of course there will be people with turbos who will insist they made the better choice, but now you have my opinion.

[Boston Steve]

Originally Posted by Dennis.G

The 80 hp turbo is for the day sailors who want to get out of the harbor in a hurry on the weekends. Long distance traveling you will never use the extra power (big hit on fuel economy for very little increase in speed) of the turbo, and additionally will be glad to not have the added complication of a turbo engine. 56 hp is plenty for a 46' Beneteau. Of course there will be people with turbos who will insist they made the better choice, but now you have my opinion.

Trust me you will be much happier with the turbo instant h.p. when u need it . Hearing that turbo wind up is like music to your ears. Turbos been around for years as long as you keep up with a maintenance regumate they last for years.

[Jim Cate]

FWIW: Our 46 foot yacht has a 43 hp Kubota and a three blade folding prop. After 16 years and ~62000 miles of full time cruising I have never felt the need for more power.

YMMV.

Jim

[Dennis.G]

The argument of wanting more power to handle heading into big seas will be heard often, but I say nonsense. Yes heading into seas under power will slow you down, but I find that I often need to reduce power and speed even further anyway to keep from pounding the boat apart. The ability to power into big seas at over 6 knots is something that does not really get used anyway unless you have a high pain tolerance (and assuming wife doesn't murder you).

[markpierce]

Originally Posted by Dennis.G

The argument of wanting more power to handle heading into big seas will be heard often, but I say nonsense. Yes heading into seas under power will slow you down, but I find that I often need to reduce power and speed even further anyway to keep from pounding the boat apart. The ability to power into big seas at over 6 knots is something that does not really get used anyway unless you have a high pain tolerance (and assuming wife doesn't murder you).

I totally agree. When heading into heavy seas, I reduce my 6.3-knot (1800 RPM, 43% power) "cruise" speed to something like a "slow" 5.5 knots (1400 RPM, 25% power) with a 80-HP John Deere naturally-aspirated, maximum-2400-RPM 4045 diesel in my fat, 14-ton trawler.

Also, the less-powerful engine makes best sense. You're a sailboater, not a motor-boat speedster. Besides, doubt the higher-horsepower engine will raise speed significantly but would probably consume lots more fuel if you tried.

[MarkSF]

If it's the Beneteau that weighs just under 24,000 lbs, isn't 57 hp ample, by most standards (including the various rules of thumb that I see)? It's not like the lower hp engine is underpowered.


You'll have slightly more hp per ton than I do, and I can push a big bow wave with the stern tucked almost under at full power. Definitely a bit overpowered, but hey.


In my boat, I have 24hp for 11,200 lbs which is 2.14 hp per 1,000 lbs. 57hp for 24,000 lbs is 2.375 hp per 1,000 lbs. That should be plenty.

[Diesel Bill]

I agree with with Dennis.G , You can only go so fast when slogging into weather and seas. I can do 8 knots under power in a calm but when going into head seas I'm lucky to do 5 knots while falling off a steep wave and getting pounded. I have twin Perkins 4-236 engines and only run them around 1450 RPM making about 20 H.P. each at that speed. No need for a turbo except for bragging rights.

[malbert73]

Furthermore there is now good technical material written on Morgan’s Cloud that shows that overpowering too much leads to a chronically under loaded diesel which causes glazing etc. with an 80 hp diesel you’d need to be burning a ton of fuel and digging a big hole in the water on a regular basis to adequately load it up

[RaymondR]

Originally Posted by Jim Cate

FWIW: Our 46 foot yacht has a 43 hp Kubota and a three blade folding prop. After 16 years and ~62000 miles of full time cruising I have never felt the need for more power.

YMMV.

Jim

Yep, 57 HP is more than enough for that boat and you don't want the added complexity of the turbo on a sail boat.

[CAVOK]

Hey thanks for all the well considered replies. I appreciate all your inputs. The complexity and maintenance of a turbo is something I don’t need, but so is having an underpowered boat. It sounds like the 57 hp will do the job from the general consensus.
The point of being slower in big seas makes absolute sense, as did the comment about running a big engine at lower power & having cylinder glazing problems.

[a64pilot]

A few myths.
First power production on a Diesel engine is almost flat fuel consumption wise, meaning to make x amount of power requires nearly identical fuel flow from either a large engine loaded or a small one running hard. There are efficiency sweet spots of course, but the efficiency difference is very small. So running the same speed in either a 57HP motor or an 80HP will burn the same fuel, excess fuel burn will only occur when you exceed 57 HP on the turbo motor, actually a turbo motor will burn less fuel as they are more efficient, they are more efficient as they harness what is normally waste heat, but the difference in fuel consumption is so small as to be difficult to measure.

It doesn’t take much to load an engine enough to keep it from glazing, on a boat that size a cruise speed in excess of 5 kts ought to be enough, glazing only really becomes an issue if your using the engine to charge batteries on a regular basis, often that isn’t enough load to fully heat it up to a temp that gives more complete combustion.

Nearly almost all larger boats are turbo charged, about the only way to avoid turbos is to avoid larger boats.
Decision turbo wise really ought to be do you want the increased complexity / possibilities of failure, is the extra power worth that, and is an NA motor available?
If the extra power isn’t worth the possible negatives that come along with it, then a turbo is not for you.

[GordMay]

Originally Posted by CAVOK

... Are there any formulas for engine size for a given weight or length of boat.

Yes.
An oft-quoted rough Rule of thumb for determining Horsepower Required for a Sailboat:
One horsepower per 500 pounds ( ± 50lbs ) loaded.
Westerbeke Corporation suggests 2 hp for every 1,000 lb of displacement for coastal cruising, and 2.5 hp per 1,000 lb of displacement for offshore.
In their book "The Gougeon Brothers on Boat Construction", the authors suggest a good rule-of-thumb is 1 hp for every 500 lb of displacement.
Some people suggest 1 hp for every ft. of boat waterline length.
Some people suggest 3 - 5 hp (continuous rating) per long ton (to calculate the boat's weight in long tons divide the displacement by 2,240).

[denverd0n]

57 hp is MORE than enough to drive that boat to hull speed, even into the wind. Trying to drive it beyond hull speed is almost entirely an exercise in wasting fuel and creating noise. Personally, I would not even consider the larger engine.