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Old 20-02-2005, 00:26   #1
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Engine Size

Does anyone have any thoughts on using a small diesel engine in a cruising yacht.
A small engine coupled with an autoprop has advantages that should include:
Lower installation and running cost
Easier installation
Lower maintenance(could be used at 80% power all the time)
Less noise
More suitable for use as a generator
Less fuel needed to be carried.
I'm thinking here of using an 18hp or 28 hp engine in a 45' 15 ton yacht.
The big question is: will the small engine provide enough power in difficult situations? ie off a lee shore in a gale.
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Old 20-02-2005, 02:29   #2
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The rough Rule of thumb for determining Horsepower Required for a Sailboat : one horsepower per 500 pounds ( 50lbs ) loaded.

Expect you'll want a lot more than 28 HP (> 55 HP).

PS:
Just came accross this in my files:

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. That would mean a 38 ft. sailboat with a displacement of 15,000 lb should have a 30 hp engine for coastal cruising and a 37.5 hp engine for offshore.
In your case: 15 Long Tons = 33600#
Hence: 33. x 2 = 67.2 HP ~or~ 33.6 x 2.5 = 84 HP.


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... that would mean 38 ft. sailboat with a displacement of 15,000 lb should have a 30 hp engine.
In your case 33 x 2 = 66 HP

Some people suggest 1 hp for every ft. of boat waterline length... that would mean a 38 ft sailboat with a waterline length of 30 ft should have a 30 hp engine.
In your case 41 x1 = 41 HP

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)... that would mean a boat with a displacement of 15,000 lb should have a 20 - 33 hp engine. For offshore conditions some people suggest the engine be larger than the size calculated for coastal cruising, possibly + 25%... so in this example the engine size would be increased to 25 - 41 hp respectively.
In your case up to 5 x 15 = 45 HP

How do you measure horsepower?
Power is a measurement of the ability to do a unit of work in a specific time. It is calculated, not measured. Horsepower and watts (kilowatts are 1,000s of watts) are units of power. Power is calculated by measuring engine torque at a specific crankshaft rpm and multiplying the 2 figures together.

Engine Horsepower/Torque Formulae

Horsepower = ( Torque in Lb/Ft x Crankshaft Speed in RPM ) 5252

Power = Torque X RPM
Torque = Power RPM
RPM = Power Torque
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Old 20-02-2005, 11:09   #3
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Just to be Gords second fiddle again, the horsepower required is not often needed to simply move the boat along. It is when the going gets tuff, or you need to manouvre in a tight place or as many find out the wrong way, stop the momentum in the fastest possible time, like docking in a birth, that the HP becomes of real importance.
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Old 20-02-2005, 23:18   #4
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I know that lots of horsepower is nice, but does this apply to ordinary cruising.
We spend heaps of money on good sails, rigging, mast etc and we expect that this is going to do the job most of the time.
Is it really necessary to drag what is an extra 500Kg halfway arround the world and pay an extra thousand or so each year just to gain a little convenience when docking or to power out of difficult situations quickly.
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Old 21-02-2005, 05:32   #5
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Chris having enough power is very important, your safety is involved. I was amazed at what wind and waves can do to the proformance of my boat under power. My engine can push the boat at 8.5 knots with no problem but when it has 3 foot waves on the nose it gets knocked down to 2.5-3.5 kt. I'm sure that you will run into inlets and currents and any number of conditions that will make you wish you had more power. I by no means am unhappy with my engine choice but I do think I am on the low end of what HP I should have. I think many of Gords formulas are correct.
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Old 21-02-2005, 11:41   #6
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I am not sure what will incure the extra thousand or so per year. It may cost more for the initial investment, but...
OK, a small motor working hard is going to use as much fuel as a larger one working lighter. The small motor won't last as long. The small motor won't get you out of trouble when needed. Example... Here in NZ and I am sure we are not the only country with these problems, we can have currents traveling at upto 7knts. That makes any motor work very hard. Even my big 130HP perkins 6 cyc will over heat if I have to ring it's neck to get through a channel with 6-7knts flowing. I am 26t.
The words of a freind that has just returned from 10yrs sailing was... Have you got a good engine/gearbox? You will need it. there is little wind out there. They were suprised at how many sea miles they ended up doing under motor.
And my final response is, you have to view that extra horsepower as a safety device. I presume you have an EPIRB, Danbouy, Liferaft, Flares etc etc. I bet you will never need to use anyone of them. Just imagine the saving you could make by not carrying them. But would you put to sea with out them?? That same rule applies to the engine. You need to be powered correctly.
Hey even running a geny while under motor power is going to tax an under powered motor.
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Old 21-02-2005, 16:58   #7
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WOW! I must be more under powered than I thought! My boat is 32 tons and I have a 90 hp diesel.
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Old 21-02-2005, 23:26   #8
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32T?? ouch. That's a heavey mother. What length??
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Old 22-02-2005, 01:26   #9
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horsepower

We also have strong tides and is why this new boat i have put in 85hp for 16.5 ton.Greg
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Old 22-02-2005, 04:49   #10
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Our boat is 56' over the deck and 61' overall. It was power by a 85 HP perkins 4-236 that was damaged from being under water when I bought the boat. I repowered with a 90hp Westerbeke. I felt that the boat was pushed for many years with a 85 hp that the 90hp would be fine.
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Old 22-02-2005, 11:43   #11
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And it probably will be OK. I wouldn't lose sleep over it. But it is interesting to see so many manufacturers having so many views on what is the best power/weight ratio. Add to that mix, the lesser known fact that the published HP of an engine can also be just as varied. Most think an engine that is promoted as say 95HP actually is 95HP. But it may be more and it maybe less. It depends on how those ponies where measured. For example, my Perkins 6.354 is rated as 115SHP@2800RPM. But it can't sustain 2800RPM. That is an intermittent rating. It's actuall is only 95SHP@2400RPM. Now add to the mix, SHP is different to SAE. The SAE rating is 131HP@2800RPM. Thats a big jump. But wait there's more. The DIN rating is 113ps@2800/94ps@2400.
So the engine is promoted as a 130HP engine. So it depends on how your engine is spec'd.
Then the next step in the equation, is what is the torque. I believe this is the most important figure and yet one that is seldom shown, except when you delve deep into the specs of the engine. Big slow reving monsters usually have big torque. Torque and RPM equal what the engine can actually achieve. A high reving engine may have a high HP figure, but the torque figure can be much lower. Thus the ability to do the hard work isn't there. The torque is also affected by gear ratio. Think of it this way. A slow revolving large diameter prop will have greater control and effect on starting and stopping in the water. It may not produce the top end speed, but at low speeds, it is very effective. The mass and friction of it's size in the water makes it a poor performer for higher speed. All the motors energy goes into trying to spin the thing in the water instead of propelling the boat forward. So a large prop is ideal for slow heavey displacement vessels, where the slow turning large diameter prop requires large amounts of torque. A high reving motor placed on to a very high ration gear box, thus dropping the RPM at the prop, is also not a good scenario. The more you gear something, the more friction is introduced and the more loss of power into running the gearbox. A direct motor to prop connection is one of least resistance, thus greatest power transfer. But it often isn't as simple as that.
So choosing a motor is not as simple as just buying the best name you can afford. A lot of homework needs to be done on what is the best match for your boat.
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Old 22-02-2005, 14:25   #12
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Chris, I'm more inclined to be supportive of your philosophy here. This is apparently a minority view, but I still think of an engine as an auxiliary, not a primary propulsion system, on a sailboat.
Small engines are not necessarily less durable than larger ones, and are not hurt at all by being run at their design (i.e. continuous) rating. In fact, it's better, by definition, to run an engine in the power range for which it was designed and optimized.
I'm not sure that you'd need more power offshore. Coastal conditions often involve tides and currents, close manouevring and more dangerous conditions than encountered offshore with plenty of sea room.
Many people seem to feel that the engine should be powerful enough to push the boat at hull speed in the worst conditions imaginable. I think it may be better to adapt to the conditions, rather than trying to overpower Mother Nature.
Finally, my guess is that you'll make better decisions and generally be safer if your insurance policy is allowing for the conditions, having a well-rigged boat and practising good seamanship. I'd rather do that than rely on an oversized engine, which may or may not get me out of trouble. The worse things get, the more I'm happy it's a sailboat and not a powerboat.
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Old 22-02-2005, 19:38   #13
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Big is better

My opinion anyway:

Have a 51 HP Perkins in a 33 foot sail boat and love it.

Many a times I have to go "pedal to the metal" to survive bridges closing when I am right there, and also muscle my way out of sandy groundings in the Bahamas.

Big engine and big fat prop has probably saved my bacon over the years.

Yes, a smaller engine would push me across calm water at the same speed, but with higher RPM and perhaps more wear and tear.
Just wont have the excess "war power" in case things get close..

On the other hand, ya learn to live with what ya got..

Normal cruise power around here is 2100 RPM.
That gives 5.2 knots and 0.6 Gallons per hour.

Next step up is 2400 RPM @ 5.6 knots and 0.7 GPH.

At 2800 RPM I am doing around 6.5 something and burning a gallon.

3000 RPM is max continoues and making noise and soaking fuel.
So close to hull speed that the drag is way up there and no gain or benefits.

In tight spots I can go up to 4000 RPM, and did today...(Well 3800 was all the thing would turn, but the motor is rated at 4K
if ya get 4K ya are under-propped)

The railroad bridge was about to close so I firewalled the poor ol Perkins and we took off up the river and cleared smartly 'cause of the excess power.
Would not have puilled the same stunt with the 27 HP Yanmar that the modern 33 feet boats come equipped with.

(The board counting minuttes and seconds was un-readable so we got into a point of no-return situation..Either try to stop or go full forward. We did)

Based on the above..: Bigger is better, around here anyway.
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Old 25-02-2005, 12:38   #14
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As an added thought on the weight vs power notion, I replaced the OEM 37 hp cast iron block engine on my 26,000lb sailboat with a 58 hp engine that has an aluminum block. The weight differential is +30 lbs. The original 4-107 weighed 435lb. The Westerbeke 58 weighs 465.

At cruising RPM (2100) in reasonably flat water, the boat does 5.5 kts at .7 gals/hour driving a 19x11 Maxprop through a 2.5:1 Hurth trans.

Just my 2 cents...
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Old 06-03-2005, 14:41   #15
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Engine Size

I'm confused! I have a 40 foot full keeled ketch that weighs 28,000 lbs. I am replacing the engine with a rebuilt Perkins 4-154 that allegedly puts out 62 hp. Now I'm not sure if it will acutally deliver 62 hp and if it does will that be enough power for the boat??? Help.

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