Engine size for boat size

[TrentePieds]

@ SailorBob #29

You are right about the fast currents needing watching. However, your Engine/prop combination generates speed through the water, which is a consideration entirely discrete from that of speed over the ground. Once you are at hull speed in a displacement hull, more horsepower will not take you faster over the ground, and will therefore not benefit you in transiting the passes.

The two passes in the Gulf Islands that are particularly demanding of attention, for different reasons, are

1) Dodds Narrows because it is, indeed, very narrow, and has enuff of a wiggle in it that you cannot see from end to end. On any tide worthy of the name the current runs faster than your hull speed. The slack is about 15 minutes, and given that you have a mile to run on the flow you can JUST make it on the slack if no-one gets in your way :-).

But that isn't the real problem with Dodds. It carries a lot of commercial traffic, and if a Sunday sailor gets under the bows of a vessel of any size, that vessel may not be able to maneuver for fear of being set ashore. Good practice is to make a securite call before entering to announce your intentions so the commercial boys can warn you off if necessary.

2) Active Pass I avoid. Lots of little stinkpots getting in the way of the Tsawwassen to Sidney ferries, and there is no sense in giving the ferry skippers more to worry about than they already have. The transit is also longer than you can do on the slack. On the other hand, the flow is rather less than Dodds and is, in itself, nothing to worry about. If you are sailing out of Shelter Island or anywhere else on the river, Active is your closest pass, and it's okay as far as current is concerned.

Porlier and Gabriola Passes are the ones I like (since I'm out of West Van) and they are not worrisome at all despite the dockside talk. In both you will find whirlpools, but they are really quite easily circumvented. Porlier is really easy. I've gone through in ten minutes on a fair tide and at other times it's taken over an hour on a foul tide. Either way, it's easy and safe enuff.

TrentePieds

[hamburking]

Originally Posted by a64pilot

Bigger engines burn more fuel, are heavier, require bigger exhaust, more expensive props that have higher drag etc. Now if you want to motor at hull speed all the time, then you need a big motor, but knock off a half a knot, you don't need that big motor and fuel burn is way down and range way up.

Of course this is just my opinion

Its my opinion too.

Diesels run well under load. I see no benefit to an oversize engine.
If you need more speed...you need more waterline.

[ramblinrod]

Originally Posted by GordMay

According to Ted Brewer:
3-4 bhp per ton (1HP/500 - 600#) is adequate
5 bhp per ton (1HP/400#) is ample
6 bhp per ton (1HP/300#) is often too much.

Very good! I recommend the 1 HP / 400 LB.

length disp 1 hp/500 1 hp/400 1 hp/300
20 1600 3 4 5
26 3200 6 8 11
32 10000 20 25 33
40 20000 40 50 67
50 40000 80 100 133

Actually,

For the 1600 lb boat, 5 HP is about perfect.

For the 10K lb boat, 25 HP is about perfect.

And for the 40K lb boat, 80 HP is about perfect.

[TeddyDiver]

A few things to consider.
First you shouldn't compare length with hp. Instead look at the displacement/hp so you get better comparision between different sized boats. Hull length, waterline length to be precise has then more to do with the gear ratio and propeller how you get the best out of the hp you got.
Second thought. How much you use the engine? Lets say a boat needs 30hp to get to 'hull speed' on flat water. If you need that ability only occasionally and during short periods you may well manage with that 30hp engine using full throttle. On the other end of the scale you might need to maintain that speed for days, sometimes against wind and waves so you use 2/3 throttle with the addittional power needed against the weather so end up with 55hp engine. Both are right choices depending your needs or anything between. Remember also when continous duty you need heftier gear box too..

BR Teddy

[Astro2]

Many thanks to all. Your advise is sage. I love forums where experience sailors actually work to improve the sport/lifestyle.
I am much clearer about me needs and the questions I need to ask and by the way I would be sailing north out of Sydney, Australia not the British Isles. Blame my jargon of 14' dingy sailing lingo of younger days.
You have all been great.I continue to look forwards to your unrelated posts on this forum.

[TrentePieds]

Since the point was made several times that increasing engine power necessitates an increase in many other design parameters I thot the following might be useful for one or two new members:


My Vetus manual sez that my M3-10's specific Fuel Consumption (SFC) is 186, i.e. that at the maximum permitted RPM of 3,000, when developing 20 HP, I should expect to use 186 x 20 grammes of fuel per hour = 3,720 grammes.


The specific gravity of diesel fuel may be taken to be .8 meaning that a litre weighs 800 grammes. Therefore at 3K RPM I should expect to use 3,720/800 = 4.65 litres per hour.


However, I am propped so that at the top of the torque curve, which occurs at 1,800 RPM, the engine output is 12HP and TrentePieds is at cruising speed in flat water and zero wind. The anticipated fuel consumption is therefore 186 x 12/800 = 2.79 litres per hour. Monitoring fuel use by dividing the litres taken at each pressing up with the engine hours run as per the log I find that my actual fuel use under the variety of conditions one finds here in the Salish Sea on a one week passage is just a smidge over 2 litres per engine hour.


Theoretical hull speed is 6 ½ knots. Cruising speed is 5 knots. Generally I've had enuff after two watches back-to-back, and in the Salish Sea that is more than enuff to get from fuel dock to fuel dock. Therefore 8 hours x (say) 3 litres = 24 litres is all the tankage I really need for doing all my cruising 100% under power. 24 litres is just a smidge over 6 (US) gallons. Build in a safety factor of 3, and a 20 gallon tank will do the job.


Assuming a tank of triangular section to fit in the bilge, apex pointing down, that means a tank measuring 70L x 55W x 40H centimetres, or, IOW, 27 x 21 x 16 inches.


TrentePieds

[fatherchronica]

Joshua had an 8 hp 2 stroke lawnmower motor.

[wayne.b]

Originally Posted by TrentePieds

Since the point was made several times that increasing engine power necessitates an increase in many other design parameters I thot the following might be useful for one or two new members:


My Vetus manual sez that my M3-10's specific Fuel Consumption (SFC) is 186, i.e. that at the maximum permitted RPM of 3,000, when developing 20 HP, I should expect to use 186 x 20 grammes of fuel per hour = 3,720 grammes.


The specific gravity of diesel fuel may be taken to be .8 meaning that a litre weighs 800 grammes. Therefore at 3K RPM I should expect to use 3,720/800 = 4.65 litres per hour.


However, I am propped so that at the top of the torque curve, which occurs at 1,800 RPM, the engine output is 12HP and TrentePieds is at cruising speed in flat water and zero wind. The anticipated fuel consumption is therefore 186 x 12/800 = 2.79 litres per hour. Monitoring fuel use by dividing the litres taken at each pressing up with the engine hours run as per the log I find that my actual fuel use under the variety of conditions one finds here in the Salish Sea on a one week passage is just a smidge over 2 litres per engine hour.


Theoretical hull speed is 6 ½ knots. Cruising speed is 5 knots. Generally I've had enuff after two watches back-to-back, and in the Salish Sea that is more than enuff to get from fuel dock to fuel dock. Therefore 8 hours x (say) 3 litres = 24 litres is all the tankage I really need for doing all my cruising 100% under power. 24 litres is just a smidge over 6 (US) gallons. Build in a safety factor of 3, and a 20 gallon tank will do the job.


Assuming a tank of triangular section to fit in the bilge, apex pointing down, that means a tank measuring 70L x 55W x 40H centimetres, or, IOW, 27 x 21 x 16 inches.


TrentePieds

===

Here's where your calculation is going wrong for 1800 RPM. Actual horsepower developed varies as the square of the RPMs. Therefore at 1800 RPM your actual horsepower is approximately ((1800/3000)^2) * 20 which equals 7.2 HP, so 2 liters/hour is just about right.

[SkiprJohn]

Originally Posted by Astro2

I am in market for a 35' - 40' mono. Mostly coastal cruising and island hoping but over long distance - many months per year. I am looking at used production boats (5 -15 years). I have been "cruising" the forums for months and do not want to the " production boat" discussions, but am keen for advise on this topic. What size engine for what size boat. The online sales sites suggest commonly 40hp Yanmar for about 35' but when you go to 40' it is up to near 55 hp. That seems a big jump. I do not want to be going backwards when the winds are down in an opposing tide but not carry something so overpowered (and big) I will be running it just over idle. I know space will be a limiting issue but the 40 hp seems a little small for size. Keen on you experienced advice.

Many years back when I had my old wood 35 ketch with the Perkins 4-107/108 I learned that the 4-108 was used in nearly everything because it was easy to get for boat manufacturers. I'd seen it in 30 footers and 50 footers just because it was easy to get. You'd also notice that the horsepower rating was anywhere from 36 to 50 with the same engine. That's because the boat manufacturer just made up their own numbers for horsepower rating. I would think that nowadays marketers are even more devious so don't trust what the boat manufacturers say, check out the engine manufactures specifications. In my opinion, an honest 40hp engine in a 40 foot heavy displacement sailing vessel is more than adequate.

[Sailing55]

Originally Posted by SkiprJohn

Many years back when I had my old wood 35 ketch with the Perkins 4-107/108 I learned that the 4-108 was used in nearly everything because it was easy to get for boat manufacturers. I'd seen it in 30 footers and 50 footers just because it was easy to get. You'd also notice that the horsepower rating was anywhere from 36 to 50 with the same engine. That's because the boat manufacturer just made up their own numbers for horsepower rating. I would think that nowadays marketers are even more devious so don't trust what the boat manufacturers say, check out the engine manufactures specifications. In my opinion, an honest 40hp engine in a 40 foot heavy displacement sailing vessel is more than adequate.

The 4-108 is an upgraded version of the 4-107; the difference being the 4-108 no longer had wet liners, and "improved" injector pump, and higher horsepower. My early '70s 4-107 is rated at approximately 36 horsepower. Perkins did not make up numbers for horsepower, there were so many versions of those engines for so many applications that even the serial numbers cannot accurately date the year of manufacture, let alone the horsepower. And parts were/are available for those engines virtually anywhere.

I agree that 40 HP with the proper prop is more than likely plenty of power for heavy displacement sailing hulls of the size you mention. My boat is 10 tons on a 27 foot waterline, and that 4-107 is more than adequate...

[Steve Bean]

I recall reading an article by Jimmy Cornell after he surveyed several dozen cruisers somewhere way off in the Pacific. Nobody said, "My engine is too big.". Ha ha! He came up with a rough rule of thumb of one horsepower per foot of boat, up to a certain size, of course. That is what I ended up with in my Samson C- Mist 32, a ferrocement double-ender very very similar to a Westsail 32 in design and displacement. It was built with a 10 HP Volvo, which was fine in calm conditions but not in any substantial opposing wind, chop, or current. I repowered with a 32 HP 4-cylinder Universal diesel with a 17-inch 3-blade prop which I find about right.

[TheThunderbird]

I hate turbo engines

While they provide more HP/Kg, they must run at high rpm, which is what a sailing boat shouldn't do.

Taking the turbo off implies a change to the injectors, an expense that I avoid for now....but had I to buy new ones, I will reconsider to get rid of that stupid extra-power

[Dockhead]

Originally Posted by 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.

Same effect with my Autoprop. It gears down in really tough conditions, requiring a lot of revs. More than I would like it to.

But otherwise, it's brilliant.

[Dockhead]

Originally Posted by TheThunderbird

I hate turbo engines

While they provide more HP/Kg, they must run at high rpm, which is what a sailing boat shouldn't do.

Taking the turbo off implies a change to the injectors, an expense that I avoid for now....but had I to buy new ones, I will reconsider to get rid of that stupid extra-power

To each his own of course.

But why do you think turbo engines "must" be run at high RPM? There's no principle which requires this. Large ship engines have turbos and run at 50 RPM.

Power to weight ratio is very important for sailboats, and crucial for cats. But that's not the only benefit of turbos.

They have a significant advantage in specific fuel consumption, because they recapture and reuse some of the energy lost in the exhaust stream.

They are also smoother and quieter, because the exhaust turbine absorbs the exhaust impulses, and because the backpressure ahead of the exhaust turbine, and pressurized intake, reduces the difference in pressure between intake and exhaust tracts. This same effect is supposed to reduce stress on the engine, for a given amount of power produced.


One more advantage for sailboat use, where you may often use the engine at a fraction of its max power, is that the engine will be smaller for a given amount of power and so better loaded at the lower power setting. Let me explain: With a naturally aspirated diesel, you modulate the power output by modulating the amount of fuel injected. The mass of air inducted at a given RPM does not change. So at the top end of the power range you will reach overfueled or approaching overfueled, and at the bottom end you will have very cool combustion because you are injecting very little fuel into the same mass of air.

With a turbo engine, on the other hand, the mass of air varies depending on the power required up to 2x or 3x (depends on the boost pressure). So for a given engine size you have a wider range of possible power outputs without either overfueling or underfueling. This is better for the engine, and more fuel efficient.


Downside is only cost and slight increase in complexity. But marine turbos are very very simple, reliable devices, so I don't consider this much of a problem.


It is not an accident that naturally aspirated diesels are dying out. Diesels and turbos go together like cookies and cream.

[Hud3]

My Island Packet 380 displaced about 24,000 lb loaded for cruising. The engine was a 55 hp Yanmar 4JH3-BE. So that's 1 hp per 436 lb. It cruised at 6.5 knots at 2500 rpm, and averaged about 0.7 gallons per hour. It reached hull speed (7.5 kts) at about 3000 rpm.

It was over-powered in most circumstances, but I was happy to have the extra horses one day when a squall line came through as I was working my way off a lee shore in the Caribbean. It took most of those 55 horses to keep the boat motoring in place against sustained 50 to 55 knot winds for 20-30 minutes.

Don't scrimp on the horsepower!