Engine size for boat size

[Cheechako]

My 44 footer had a 51 hp (Perkins 4-108)and it was plenty.... surprisingly. My 47 footer had 80 HP (Nanni/Mercedes).
My only caution would be to watch for modern engines whose HP ratings are at quite high RPM. It's not a good comparison to an older slower turning engine rated at lower rpms.
By comparison my HC38 only had a 25 HP diesel. That boat needed a much bigger engine in any kind of seaway at all. Just wouldn't move thru chop. Yet did fine in flat water.

[Stu Jackson]

Good observations here.

I know you said you're in the 35' - 40' mono range, but just for example, over the 25+ years of production runs for my boat, they started with a 21 hp M25, then a 23 hp M25XP, finally a four banger M35 30 hp. Based on motoring next to friends with the larger engines, we don't see much difference, and they don't throttle back to let us keep up. a64 has some very good points.

The reality is that you will find a range of hps on boats of that size. In addition to access and condition, you should be very careful about the makes and models of each engine. Once you get somewhat closer, ask specifically. For example, many folks have Volvo engines, and love them, but the parts prices are obscene, while Yanmars (John Deere) and Universals (Kubota) can use parts from tractor engines.

Happy hunting & good luck.

[denverd0n]

Originally Posted by a64pilot

I think loaded I fall to the less than adequate point, but can easily make hull speed etc.

Going into a stiff breeze and significant waves?

As mentioned before, even a very small engine can usually get a sailboat up near hull speed in flat water with no wind. I think what Brewer calls an "adequate" engine is his estimate of what will probably be needed to keep the boat moving well in adverse conditions.

On the other hand, bigger is very definitely NOT always better. As already pointed out, a larger engine is heavier, costs more to buy, costs more to maintain, and uses more fuel. Going overboard in the size of an engine is just kind of dumb. After all, you're not going to be water-skiing behind that sailboat! (Which I mention mainly because I recently saw an ad for a 17,000 lbs. sailboat that had a 95hp turbo diesel installed in it. That's just stupid!)

Personally, I would consider 1 hp per 1,000 lbs. of displacement to be an absolute minimum. 1.5 is reasonable, but on the low side. 2 is quite adequate. 3 is more than enough. Anything above that and you're just carrying excess weight, and burning excess fuel, for no benefit.

[Sailing55]

OP, you are getting some good advice here, but the subject of horsepower requirements involves a lot of variables.

Waterline length, waterline beam, draft, displacement, how the displacement is distributed along the w/l, is the entry full or fine, does the boat drag a big quarter wave behind it or does it leave a flat wake, etc. The diameter and pitch of the prop, [whether or not the prop is matched to the most efficient engine rpm], these things are all interrelated...

One VERY important point brought up by a couple of folks here is accessibility to the engine. It is imperative that one has the ability to reach and maintain the engine!

If you find a boat that you like and it moves around under its' own power, and there are a lot of boats of that class with the same engine, then go have some fun. You can always make "adjustments" if your needs change...

[Cheechako]

Trying to think of examples I have personal knowledge experience with:
West sail 32, (heavy) 25 hp Definitely not enough.
Seawind 31, (med heavy) 17 hp volvo not enough
38 Hans Christian, 25 HP not enough.
44 Tanton, 51 Hp good
Rawson 30, (medium displacement) 25 HP old Volvo fine.
42 Cat, (2) 3GM30 Yanmars, not enough in tough conditions. (engines rated at 3600 rpm)
All were fine in flat water.

[a64pilot]

With the old three blade prop she punched through even 4 to 5 ft waves and 20 kts of wind at 6 kts, waves were big enough to where the prop was occasionally being lifted out of the water, that was as much as I wanted, six kts was as fast as you wanted in those seas, and might have been a little too much. I think heavier boats do better punching through seas, (Inertia)
But with the new Autoprop, I think I lost some drive, the prop reduces pitch, sort of gears down, I just need more RPM as I'm in a lower "gear" I guess.

[TrentePieds]

Here are some hard numbers that I've been watching since TrentePieds is fairly new to me. They may be a basis for the OP's ruminations, bearing in mind that (by rule of thumb) increase in displacement rises as the third power of the increase in length, but that increase in wetted surface rises only as the square of the increase in length.

TrentePieds is 24 Feet on the WL and displaces 10K lbs (more or less). Engine is a Vetus (Mitsubishi) rated at 20HP at 3.0K RPM (max) with torque peaking at 1,850 or so. The power curve sez that at 1,800 the HP output is 12. Documents inherited say she wears a 14" Campbell "Sailor" prop pitched 10" driven through a Hurth tranny reducing 2 to 1 (close enuff).

Ted Brewer's numbers by way of Gord May, seem verified. 1HP per 500 lbs displacement.

On a LWL of 24 feet the theoretical hull speed is 6 1/2 Knts.

I get about 4 knots at 1,500 or so. The numbers say that I should only get 3 knots on a 10" pitch, so something is wonkey. Either the "slip factor" is not 50% or the pitch is not 10". I'll know after I haul in the spring.

At 2K she begins to get skittish and climb her bow wave. Theoretically, with a 10" pitch she should only get just over 4 knts rather than the 6 1/2 that makes her skittish. To get 6 1/2 knts at 2K would require a 16" pitch, assuming a slip factor of 50%. To advance at 6 1/2 knots on a 10" pitch would require a slip factor of 20%, and that just isn't plausible.

With 20 knts of wind on the nose, no canvass up, I still get my 4 knts at 1,800. In 20 knts from the NW (the "hardest" weather we Sunday Sailors have to endure in the Salish Sea) the seas are never much more that 18" unless it's been blowing hard for days, and, given the short fetch, swells aren't much to write home about, so lifting her displacement a coupla feet is really never necessary and I could happily do with a smaller engine, say 15HP.

TrentePieds

[sailor1924]

I believe this is one of the most difficult questions in boat design. There are so many differing criteria that can affect the way boats act under power. So I'll add a bit of my observation.
In my experience with mostly deep draft displacement hulls, every contradictory opinion expressed in this thread carries some truth and should carried into the decision making process.
I've had several boats and sailed both with and without an engine.
I used to own an 40 ton narrow beamed deep drafted ketch with 45 foot waterline and built with an offset propeller. I repowered this boat two times and prior owners had done so twice as well. The original engine was a 25 HP gas engine was replaced with a 35 HP Fordson Major diesel that weighed in at 900#s.
When I bought the bought it the engine ran but access was such a bear that it was nearly impossible to service it adequately. I promptly managed to improperly seat the old spring loaded oil filter and seized the engine the first time I started it up to move the boat. So having learned it the hard way I have to say maintenance access is crucial in choosing a new boat.
I sailed this boat for 3 years without an engine using a an old inflatable with a 4 HP outboard as a yawl boat when I wanted an engine. I found that the dink could get her going at almost 6 knots once she started moving, if anything it was more then adequate in a flat calm. So those assert low HP speak some truth.
I eventually found a rebuilt diesel identical to the one I'd seized and being mostly broke in those days I bought it and installed it and was able to experience the joys of running an underpowered boat for several years. So I can assure you that a little less then 1 HP per ton is not an adequate power equation in many conditions including backing off an accidental grounding. But it was very economical with fuel and could push the boat to about 7 knots.
I was able buy and install an 88 HP Modern Yanmar diesel, still offset but with a Maxprop and, boy oh boy, I could backup. And bull into a headwind if needed, though it slowed and consumed more fuel, and occasionally when in gear slow, running the windlass, recovering the anchor in heavy wind and wave, it might labor. 2.+ HP per ton seems a workable formula at least for an old plank on edge design. The Yanmar only weighed 630#s so it permitted a more effective sound proofing of the engine compartment without a weight penalty.

My current boat was called a full powered sail boat by the designer, John Alden, definitely not a trawler with sails and weighs in at 15 tons and has 109 HP naturally aspirated Hercules diesel it's second engine installed in 1973.
It's engine has good access for maintenance and consequently over its history has received good maintenance and is in excellent condition. I have found the power to weight equation for this boat to be excellent making her a pleasure to handle in all conditions with adequate power for propulsion and to run the heavy alternators (80 amp start, and 200 amp house) in all conditions including a headwind. By some of the advices offered in this thread this boat is definitely overpowered at 6 HP per ton but perhaps it has contributed to the 42 year longevity of the engine.
And of course the cleanliness of the bottom will affect engine performance as well.
Good luck and many happy hours thinking about what engine to choose.

[Chuck Hawley]

I have a 38' yawl with a 30' waterline, which supposedly weighs 12,400. I have the completely conventional Yanmar 3GM30 (24hp), and I would say that the boat is slightly underpowered. Top speed is 6.5knots, but it falls quickly when powering into a head sea or head wind. Plus, I have to run the engine at 3,000 rpm or more to get to hull speed. The boat has a very efficient hull shape (between racer and cruiser). I think I have a 16" prop.

Chuck Hawley

[TrentePieds]

One or two bits of terminology makes me think you sail in British waters :-) Compared with what I remember from my early years in Denmark, the waters here in the Salish Sea are positively benign, and powering considerations tend to reflect that.

Hull form makes a tremendous difference twixt boats, given that, assuming flat water, there are two major considerations: Area resistance and frictional resistance. Without looking at specific sections on the lines drawings we can say nothing that isn't mere guesswork.

Things that have not yet been mentioned is that prop diameter (swept area) is what absorbs the horsepower, and that is why the prop diameter has to be chosen with regard to engine output. The prop doesn't really care whence the horses come as long as they come. "Overprop" (too big a diameter) and you can drop the RPM to below the "max torque" point and begin to labour.

In small boats you have to go for a "best fit" since HP output is "lumpy" in increments of 5 HP. That makes a 30% difference in the smaller sizes, with all that that means for balancing one thing against another.

Once the anticipated RPM is determined through appropriate diameter selection as a function of engine and tranny selection, we can begin to think about pitch. A 50% "slip factor" is often assumed and advocated for preliminary calculations, but obviously the actual slip factor can only be determined empirically for each installation. For yotties that would seem to be overkill.

Seems to me that if you "max out" at 3K RPM as I do, then pitching for hull speed at 80% = 2.4K should give quite acceptable results.

TrentePieds has an interesting and irritating resonant rattle/squeak at 2.1K so that's my audible RPM warning. I go there and then back off a bit.

When I haul in the spring I'm gonna measure up the prop and see what we've really got, and then, while I'm upside down in the engine compartment, I'll have a look at the tranny.

As others have said, access is a real issue, and the Vetus, as someone also said, is VERY badly arranged. I had a "no start" resulting from a bad toy-type button in the cockpit panel. I myself absolutely couldn't get at the terminal on the solenoid, and had to call on someone younger and more agile to wire the terminal to a gown-up button on the engine casing.

The oil filter is a horizontal mount on the aft end of the engine that I can barely reach, and I cannot see the filter so it all has to be done by feel. A remotely mounted filter is on the agenda for haul-out. The manual "kill" button on the fuel injector pump is also out of sight, and while I can reach it and I find it by feel, I'm never keen on getting my whiskers too near to the alternator belt, and I hate to think what could happen if a young lady with, say, a ponytail should have to reach for that button.

I'll run the Vetus till she's done. Then I think it'll have to be a Beta because that block is set up so things are accessible from the fore-end of the beast. TrentePieds was originally bought as a bare hull and "owner finished". The appointments are quite acceptable, but the evidence is everywhere that the original owner didn't have the training or the experience to foresee all the design considerations that are part of naval architecture.

But that made the price right, and my beloved just loves the accommodations. Happy wife, happy life, wot? :-)!

TrentePieds

[Glen Ballou]

I have a 75 hp turbo diesel in my Beneteau 461. Weighs 22,000 lbs empty. We living on it for 4 months so it is considerably heavier. 218 gal of water adds another 1800 lbs. Cruising at 6-7 kts on smooth water requires only a little power, but I am glad I have 75 hp when bucking headwinds and high seas as is often the case in the Virgin Islands.

[Sandero]

My Contest 36s with probably 18,000 displacement has a 36hp (1985 Volvo) with a 2 blade fixed prop and can motor at 7+ knots at 2000k rpm with a clean bottom.

[ebsail]

Had an Ericson 35 for 9 years. It had a 24 HP Universal. Was fine for 99% of the time. However when punching into a tall chop and 30 knot headwind, We got down to 2.5 knots. I sure wished for another 10 HP to get directly up wind to a protected overnight harbor

[Sailorbob8599]

My Passage 24/30 with a 6 m waterline weighs in at 9,000 lb (cruising weight). She is equipped with a BMW D12 hp. single cyl diesel which seems to reflect Ted Brewers recommendations.
I built this boat myself from a bare hull, having no prior knowledge of 'how to', but a strong desire to own such a pocket cruiser. Given this, and prior to engine purchase, I searched out all the advice I could from those I considered to be knowledgeable people. This included a local NA who modified the vessel from the original plans by Tom Gilmer NA; the BMW engine dealer in Seattle; the Campbell-Sailer Prop shop in Vancouver BC whom I eventually contracted to engineer the prop to both boat and engine; and finally an independent local NA. just for an additional opinion.
I received various advice, some suggesting I go with the BMW D7 hp engine, but as in all things, I knew it would eventually come down to ME making the final decision.
In consideration of very fast currents encountered in passages between the islands in the northern end of The Salish Sea (formerly The Strait of Georgia) ... some shown on the charts as high as 16 knts ... I elected to go with the larger D12.
The boat was only launched August of 2015, therefore I have not had the opportunity to take her through any of these channels. So far, we have only run in open waters between the mouth of The Fraser River and Boundary Bay. The boat appears to perform well in a chop, pushing along at 5.5 - 6 knots as measured with a simple manual type knot meter, towed about 15' behind the boat.
I'm not sure how accurate that might be, but it seems to be fairly close as checked on 'time over a fixed mile' distance ... which I'm sure must be close around max hull speed for this heavy little full keeled cruising boat.
I hasten to say I would not be entering those channels at full flood, but quite often, one must enter a channel against the flood tide in order to clear the passage with the aid of the ebb before it reaches OMG 16 knot flow. Otherwise, it would be like exiting the passage on sling-shot. All very exciting for both master and crew.

[Sandero]

Originally Posted by ebsail

Had an Ericson 35 for 9 years. It had a 24 HP Universal. Was fine for 99% of the time. However when punching into a tall chop and 30 knot headwind, We got down to 2.5 knots. I sure wished for another 10 HP to get directly up wind to a protected overnight harbor

That sounds about right...