Engine Size

[StuM]

Originally Posted by StuM

So;
10 HP SAAB about 1.11 gallons per nm.
Perkins 4 -108 about 1.18 gallons per nm.
Not exactly "three times higher"!

Doh!

That should obviously be 0.111 and 0.118 gal/nm

[tkeithlu]

Guys, hull speed has gotten too little attention and power to weight too much in this discussion. Reaching hull speed takes surprisingly little HP in even a very heavy boat. Exceeding it takes bunches. You're talking aout 40 to 45 foot boats. A 35' waterline has a hull speed of 7.9 knots, 40' waterline 8.5 knots, and a 45' 9.0. So, my 43' overall 30 ton boat does 6.5 knots at 1600 RPM, 7.5 at 2100, and 8.5 at screaming speed on a 140 HP engine. If you are staying at those gentle speeds, those of you with the seemingly small engines are doing just fine; those that have to fight a 7 knot current or want to beat a bridge closing are going to need a whopper of an engine.

[MooGroc]

One factor that I don't think I've seen mentioned is engine room space. One reason I'm a fan of smaller engines is that they give you a lot more room to work. My boat originally came with a 29hp. The PO replaced it with a 39hp. It's really shoehorned in there and is a bitch to work on.

I never feel the need for the extra HP and would much prefer something smaller and easier to work on. Cheaper and more fuel efficient would be nice bonuses.

[Dockhead]

Originally Posted by a64pilot

Two motors, both rated to run a max continuous RPM of 3600, both engines propped to just barely reach 3600 at full throttle.
One run 99% of the time underway between 2,000 and 2,200 RPM, (58%) the other run 99% between 2,800 and 3,000 (80%)

The one run at the lower RPM will outlast the one run harder. That is my experience, engines run between 50% to 75% seem to do the best, Ones run much lower do well also assuming they are run up at higher power ever so often.
Example a Sport Fisherman may troll at not much above idle all day and night, but almost always will run them up on the way out and on the way home.

But it's load more than RPM, but we are assuming properly propped and if so load and RPM are pretty consistent.

This is absolutely correct. I also agree with Valhalla's comments above.

It is a myth that diesel engines need to be run at 80% of full power, all the time. The less load and heat, the better, as long as there is enough combustion pressure to keep the rings seated, and enough heat to prevent fuel condensation and coking.

And as someone wrote -- fuel consumption of a diesel engine is an almost direct function of power produced. A smaller engine run at a higher percentage of its max power will be slightly more efficient, but hardly enough to notice. This is very different from the case with gasoline/petrol engines, spark ignition engines, which rely on throttling to modulate power -- such engines are much less efficient when used at a small percentage of max power, but diesels are different.


So I'd rather have a somewhat oversized, rather than somewhat undersized engine, personally. Although I am a keen sailor, and make many hundreds of miles upwind every year (unlike most cruisers who don't really sail to windward much), motoring ability is still extremely important. Practically all cruising boats these days are in reality motor sailors which make usually more than 50% of their miles motoring or motor-sailing -- that's just reality, and nothing to be ashamed of.

You may rarely need the extra power, but when you do -- oh boy, you really need it -- like trying to bash upwind against a gale with tide running against it -- something which might happen to you, as it has happened to me a couple of times (most memorably in Borkum Riff two years ago in a NW gale).

So my next boat will have about 150 horsepower (the minimum for a Hundested propeller), and will displace maybe 22 tons, which is what, 7 horsepower per ton? A bit more than the 5 horsepower per ton which I have now, which has been too little on a couple of occasions.

To use this power you need either a large propeller or a variable pitch one. The large prop will kill your sailing performance, so I'm going with the Hundested.

Those of you who only use your engine to get out of the slip before heading out into the bay -- will have a much smaller horsepower requirement. Those of us who sail long distances and in all conditions will sometimes really need the power. So to each his own.

[Dockhead]

Originally Posted by tkeithlu

Guys, hull speed has gotten too little attention and power to weight too much in this discussion. Reaching hull speed takes surprisingly little HP in even a very heavy boat. Exceeding it takes bunches. You're talking aout 40 to 45 foot boats. A 35' waterline has a hull speed of 7.9 knots, 40' waterline 8.5 knots, and a 45' 9.0. So, my 43' overall 30 ton boat does 6.5 knots at 1600 RPM, 7.5 at 2100, and 8.5 at screaming speed on a 140 HP engine. If you are staying at those gentle speeds, those of you with the seemingly small engines are doing just fine; those that have to fight a 7 knot current or want to beat a bridge closing are going to need a whopper of an engine.

The issue is not exceeding hull speed, which is extremely inefficient when motoring even in boats with very efficient hulls.

The issue is making progress against strong wind and a head sea. That's where you need lots of power, and speed is not the issue at all.

On a sailboat, it's usually easier to make lots of power, when you need lots of power, under sail. But there are cases when for some reason or another you need to do it with the engine. For example, when you need to get up a very narrow channel directly against the wind -- as happened to me once or twice.

[siamese]

Nope, I won't make the mistake of telling anyone what size engine they should have.

But for me....I sail Lake Michigan, where the waves can get steep and close together. Many times we've sailed North from our home port, spent a few days in a nice port, and when it's time to return to our home port, the wind's directly on our bow, and we opt to motor straight down the coast.

This is one of those situations where personal experience has been 100% illuminating.

The speed difference between 80% rpm on smooth water and 80% rpm in the steep waves is substantial. We have a Universal M25xp in our Catalina 28. Same engine our dock neighbors have in their Catalina 36. No way do I want to have less power than I currently have.

I don't see enough or any advantage to having a smaller engine. Small engine or big engine you still have a fuel tank, fuel lines, cooling systems, pistons, injectors, and so on. The complexity of diesel propulsion is there regardless, so why not scale it up to reasonable proportions and enjoy the extra ponies when you need/want them. The difference in fuel consumption will be negligible.

[tkeithlu]

Motoring against a headwind is partially a metter of hull speed because hull speed is relative to standing water, not GPS location. It appears that we've all been in different situations that lead to different conclusions, and perhaps should agree to disagree. I make two final observations: One, when cruising today I see a lot more boats being used a motor sailers than I did 30 years ago, which speaks for a robust engine and more fuel space. The other is MooGroc's point about shoe horning a bigger engine into a space designed for a small one. Routine maintenance space on the usual boat is pretty bad as is, and I wonder whether that bigger motor is going to get proper care.

[Dockhead]

Originally Posted by tkeithlu

Motoring against a headwind is partially a metter of hull speed because hull speed is relative to standing water, not GPS location.
. . .

True, but if you can get anywhere near hull speed bashing against a headwind and head seas, then this is not the situation we are talking about.

I have 100 horsepower for a 20 metric tons (light ship) boat, and all 100 horsepower was not enough for me to make more than 1 point something knots against a NWerly gale in Borkum Riff, in wind against tide. It was awful. One knot less of boat speed and I might have ended up on the beach. It was too narrow to sail and tack. I spent 5 hours clawing up 8 miles or something up to the gatt where I could at last turn.

You are right -- your personal experiences definitely shape your ideas about how much power you need.

[a64pilot]

You would be surprised in the difference in fuel consumption of a 100 HP engine making 40 HP or a 60 HP engine making 40 HP.
The difference is almost immeasurable, the biggest problems with having more engine than you will need come in at size, weight and cost, but going out on a limb here, but weight depending on type of boat and hull design, may not be insignificant, it may well be the difference between a fast, good sailing boat, and a Pig?

[Dockhead]

Originally Posted by a64pilot

You would be surprised in the difference in fuel consumption of a 100 HP engine making 40 HP or a 60 HP engine making 40 HP.
The difference is almost immeasurable, the biggest problems with having more engine than you will need come in at size, weight and cost, but going out on a limb here, but weight depending on type of boat and hull design, may not be insignificant, it may well be the difference between a fast, good sailing boat, and a Pig?

Indeed!

The weight issue is important, but power to weight ratios vary enormously in marine engines. Modern turbocharged engines offer as much power as you want, in no more weight than an old Perkins 4-108 like I had in my old boat.

Yes, my 2000cc Yanmar with 100 horsepower in my present boat, weighs the same as the 2000cc Perkins 4-108 in my previous boat.

[a64pilot]

I'm 25,000 lbs, and have a 44 HP engine, which I have about 25 maybe usable due to a drive line vibe I just can't figure out above 2500 RPM.
That's just 2 HP per ton
That will get me within a half a kt of hull speed and thankfully I have never needed more, I do lose more drive straight into waves and wind with the Autoprop than I did with a fixed prop, but even then she would make 5 kts into seas that were having the prop come out of the water often.
I have always been a proponent of having more water, fuel, engine than you need though

[tkeithlu]

It sounds as if we are getting into risk argument, you know, probability of event times severity of consequences if it happens. The severity side could be valued to the point of arguing for any bigger engine, because it is possible if unlikely that you could get into a situation where a bigger engine was your salvation. There are several such situations described above. The probabilty side could start with "don't sail on lee shores or into storms" and argue for smaller engines. Part of the problem may be that there are normal situations, such as crossing the gulf stream or getting out of a port against the tide and wind that are damned slow at hull speed for a 40' boat. But then, the smaller engine crowd would say "The tide waits for no man, but lots of men have waited for the tide." I don't think we're going to find a single correct answer.

[barnakiel]

Yes.

Your engine should be matched not just to the displacement (this we have heard maybe 100 times by now) but also to the way the boat is (powered) or (sailed). Have your pick.

No problem a 60 footer with a 35 h.p. engine, if that's what is best fit for her.

In the cruising context, many owners, for their varied and all valid reasons, will want plenty of hp, some will go for the mid-road 5 hp/tonne, fewer will go for less than 3hp per tonne.

The only situation when the engine is too small is when it cannot do the job envisioned by the owner of the boat.

Love&cold beer,
b.

[k. michael marquardt]

Fuel consumption as engine power connected to a small size prop is a third (sometimes 3.5) power ratio to speed.
As most small diesel engines in the 20 to 150hp range have closely the same efficiency there is not much of a difference in consumption between a 30hp diesel to a 100hp diesel by the same steaming conditions.

[EWOL Props]

The vibration above a certain rpm number is an hydrodynamic effect due to misalignment of blades.
If you like get in touch with me privately on CF or write me at info@ewoltech.com and I will tell you more.....

Originally Posted by a64pilot

I'm 25,000 lbs, and have a 44 HP engine, which I have about 25 maybe usable due to a drive line vibe I just can't figure out above 2500 RPM.
That's just 2 HP per ton
That will get me within a half a kt of hull speed and thankfully I have never needed more, I do lose more drive straight into waves and wind with the Autoprop than I did with a fixed prop, but even then she would make 5 kts into seas that were having the prop come out of the water often.
I have always been a proponent of having more water, fuel, engine than you need though