Hull/motors/weather/sea state

[Knucklehead]

Horse Power vs fuel consumption.
Assume for the moment, the boat hull is the same, weather is the same and the sea state is the same. Additionally, the cruising speed remains the same, xx knots. Beginning with 300 HP and ending with 660 HP. Again, they are diesel and with the same number of hours.
Which would provide, low or high, the least/best use of Diesel fuel.
I am considering the purchase of a MY and would much prefer to put funds into the boat instead of it's exhaust. Bigger motor lumbering along or smaller at a higher RPM. Similar to a Chevy 283 or 427.
I've looked into Chapman with no real definitive answer.
Any old sea dogs out there care venture a response?
Karl

[valhalla360]

Assuming the smaller engine isn't overloaded, it would be the more efficient.

BUT!!!! not by a large amount. The bigger savings would be in terms of up front costs generally being lower for the smaller engine.

If the 600hp engine is drastically heavier and it's a planing boat, the extra weight might have more of an effect.

[Knucklehead]

Originally Posted by valhalla360

Assuming the smaller engine isn't overloaded, it would be the more efficient.

BUT!!!! not by a large amount. The bigger savings would be in terms of up front costs generally being lower for the smaller engine.

If the 600hp engine is drastically heavier and it's a planing boat, the extra weight might have more of an effect.

I am going to make an assumption; on the ICW, planing is not the norm with the large numbers of "no wake" zones. Especially with a 44 - 48 foot MY (Navigator, Ocean Alexander, Viking/Princess). Yes, they are not designed for the ICW, but, the Admiral has an important position with where and when.
I had not considered cost's of maintainance. Good point.
So, low to mid HP engines would be best overall????
Thanks,
Karl

[Cheechako]

Get plenty of HP vs marginal. The engine will last longer. Also, watch out for HP ratings. Some makers rate their engines at high rpm and call them light weight... when really they are just smaller engines if you can see thru the hocus pocus.

[Scot McPherson]

my father bought an egg harbor 37 with twin cat 3208s...it was too heavy to plane...major and expensive disappointment

[a64pilot]

Displacement boat, or planing vessel?
It matters, greatly.
For a displacement boat in my opinion I'd want to be able to push hull speed at 75% power, any bigger engine is a waste.
For a planning vessel I'd like to be able to get it on plane just barely at 50% power, or maybe said better, once on plane, just hold it it at 50% power.

However I would never cruise a planing vessel, ever. I have to far to go, and don't have the funds to spend on fuel.

[a64pilot]

The brake specific fuel consumption of a small Diesel engine is pretty flat, meaning just about any of our motors will burn close to 1 gl an hour to produce 20 HP. A 600 HP motor will burn slightly more and a 30 HP one slightly less, but its not much different. The difference is the frictional losses of the bigger motor, but it's turning a low RPM, so the friction is down.
However, it's parts and services expenses will be much greater, and you will need to periodically "blow it out" and that does burn fuel

[Suijin]

If the majority of your time is going to be spent in displacement mode and a lot of time in the ICW I would be looking exclusively at trawlers and other displacement boats. Lower fuel consumption, better use of space and generally a more comfortable boat.

A64's recommendation is a good one; enough HP to drive the boat at hull speed plus a reserve for when you need it (wind, sea state, etc. )

[barnakiel]

Many engines provide best fuel efficiency at about 50% to 75% of their rated rpm.

So a small engine run very hard and a big engine run at very low revs will be less efficient than when each is run at about 50% of their rated revs.

Otherwise the consumption is only up to hp used and this will be nearly the same for similar but bigger / smaller engines.

200 grams per hour per hp used.

I have seen low figures of 185 and higher at 220 grams. If you missed the right engine your consumption will be sub optimum by about 10%.

b.

[Knucklehead]

Thanks guys.
This info will used to assist me in selecting the type of MY/trawler in my search/purchase. I knew the 75 percent of motor range, but, forgot.
Again, thanks,
Karl

[valhalla360]

A follow up:
- 300 vs 600 hp is a drastic difference for the same boat. The designer should have provided recommended hp. If you deviate drastically, it is likely to be a problem. (high or low)
- Unless you are overloading the engine, don't expect much difference in longevity. Unless you abuse the engine, expect the life of the engine to be measured in decades.
- If you can stomach the fuel bills, a significant percentage of the ICW can be done at planing speeds.
- If it's a displacement boat, even 300hp is likely overkill. 600hp is just silly.
- Not ideal but if you throttle back a planing boat will get close to the same efficiency as a displacement design. Just make sure to open her up from time to time so the engine gets up to full operating temp. So yes, it is viable to cruise in a planing boat.

[ranger58sb]

Originally Posted by Knucklehead

I am going to make an assumption; on the ICW, planing is not the norm with the large numbers of "no wake" zones. Especially with a 44 - 48 foot MY (Navigator, Ocean Alexander, Viking/Princess). Yes, they are not designed for the ICW, but, the Admiral has an important position with where and when.
I had not considered cost's of maintainance. Good point.
So, low to mid HP engines would be best overall????
Thanks,
Karl

There are many places on the ICW where planing would be possible, and actually normal. And in some cases, planing could be more comfortable for certain sea states... rather than slogging through it at trawler speeds. Look at the Albemarle Sound, just for one example. Any of those boats could be a good fit, if they float your boat.

As to the horsepower question... I'd suggest you ignore it at first. Find the boat where all the other features fit your requirements. Maybe at that point you can compare models of that boat with various engine packages, but mostly you'll find builders will have installed viable engines for various anticipated use patterns.

Then once you buy your boat... ease back on the throttle whenever you've got the time or inclination (or when sea states permit). Going SLOW -- with whatever engines you end up with -- will save you more money than any difference in overall installed horsepower. Going slow with smaller engines will save you a whole dollar and a half compared to going slow with larger engines. (SLOW means below theoretical hull speed.)

Many will point out diesels like to be loaded and run at the correct operating temps. Yep. But you can do that, even going slow. Minor detail. The difference in longevity will be measured in decades, not minutes.

-Chris