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Old 07-10-2005, 16:07   #1
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engine size?

I recently read a sailboat's engine should ideally be 2 horsepower or more for every 1 thousand lbs. of displacement and yet many manufactures do not design to this criteria. However, I have not heard a single complaint from an owner about lack of horsepower. Any thoughts about engine size?

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Old 07-10-2005, 16:19   #2
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Scott,
There are a few rules of thumb on engine size.
I tend to use 1hp per 1K of displacement. Then I'll add for accesorise and underbody/windage. It ends up being around the 2hp/1k rule in the end.
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Old 07-10-2005, 16:44   #3
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This is a complex issue...

I started a thread on "Engine Size" a while ago. You should be able to find it at http://cruisersforum.com/showthread.php?threadid=1577
The main point that came out of that discussion was that you needed sufficient power for the area and conditions that your boat would be cruising in. For example in an area with strong currents a more powerful engine would be needed.
I have considered the matter more carefully and I would suggest that he following points may be just as, if not more, important than engine size.
1) Engine area ventilation-Diesels must have clean cool(not hot)air in accord with the manufacturers recommendation.
2)Fuel tank installation-The tank(s) with associated piping and filters must be able to supply clean fuel to the engine under all (and this includes when wildly tossing arround in a gale) conditions.
3)The exhaust system must be able to remove the exhaust gasses from the engine without exceeding the back pressure set by the engine manufacturer, and without allowing water to reach inside the engine.
4)The propellor should match the hull and the intended conditions.
5)The engine area should be clean, well lit, and enable easy servicing.
6)A selection of appropriate spare parts must be carried.
In paricular, looking a some old cruising boats for sale in Australia some had a very small engine, but huge fuel capacity.(1000+ litres)
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Old 07-10-2005, 17:30   #4
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New technology

New advances in inverter technology and motor technology has changed the "formula" for the amount of hp required to drive boats in addition to the design for electrical systems. Becaus dc motors can be designed to deliver constant torque vs rpm a motor-generator combination can be designed to drive a boat with less hp per displacement ton ALONG with better fuel consumption.

Fisher Panda has some units already in application on sailboats. The research arm of Glacier Bay (a big company not just in refrigeration) has made several brushless dc motors for use with dc generators to give variable speed drive. An inverter operating off of the same voltage "rail" gives controlled ac for the boat.

With such units 32 hp may be deemed sufficient for a 40 ft cruising monohull to drive hull speed at a fuel consumption on the order of two liters per hour!

Such technology will cause a ripple effect in the whole vessel design. For example, fuel tankage volume and weight will drop. The diesel location will change to provide more opportune placement since it no longer has to be attached to the prop shaft. The dc motor is very small and compact if water cooled. Bi-directional inverters can be used to "piggy-back" power to the diesel when surge power is needed. When it is not the inverter can charge the battery banks at high currents. Hawker batteries, for example, have been used in electric vehicle racing and recharged from "dead" in 1/2 hour for the next race. Such technology is going to change a lot in our world.
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Old 07-10-2005, 18:37   #5
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I have a 2QM15 Yanmar in my Tanzer 8.5m
15 hp, 7400 pounds 29 feet LOA.
10hp would be enough most of the time. 55 mph winds and 8 foot waves on the nose, required nearly all the power available. So would strong currents and strong headwinds on the nose.
The extra power would also turn a larger alternator. But 10 hp would be enough for practically everything accept extreme conditions.
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Old 07-10-2005, 19:46   #6
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Sustained 3hp per ton is the formula I use. Works out good in comparison to the 1hp per 1k adjusted to 2hp to 1k. Hybrid setups fr boats are definitely the future. Wish I had the bread to set up my trimaran that way.
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Old 07-10-2005, 21:54   #7
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51 HP, 33 foot monohull.

Displacement from the builder: 15,200 lbs.

26 years later and crusing (over) equipped, 23,000 lbs.

Plenty of reserve power when needed, would not trade for less power.

SUV Man..
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Old 08-10-2005, 13:43   #8
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You never know when you'll need that extra HP?

There could be a incident, when you may have to gun the engine to get your boat out of harm's way.

Then like what CSY Man said, "reserve power."

You should always have the power to move your baot along. When there is no wind. Or to go in or out of harbors. Especially, when navigating through heavily traffic. You'll need the power to avoid collisions with others?

Power is very important. Power is the way? Never enough. Or not enough to do the job. But, more is better, even when it is sailboat.


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Old 08-10-2005, 16:28   #9
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A couple of issues with overpowering a sail boat. One, is the weight. More power means bigger engine, and more weight. The other is structure. Too much power can put severe strains on the structure of a sail boat. CSY Man, the set up you have sounds in the range that is practicle for a motor sailor. Not overkill, but plenty for the currents in the San Juans, and other such areas. More power is only better to a point. An engine that will drive the boat at hull speed in calm waters is usually sufficient. If the hull starts to plow, you are probably over powered.
And Kevin, real sailors enter and leave harbor under sail
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Old 08-10-2005, 16:48   #10
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Yeah but weight and structure is not a problem with the CSY 33.

The smaller Westerbeke 30 is almost as heavy and the boat is so porky anyway that one won't notice the difference.

Ditto for structure, the hull is 1 1/2 inch thick, no flexing going on.

Have used all the power a couple of times to save the boat (and my bacon...When the bridges started to close before I was all the way through....Gone through bridges about 500 times here in Ft. Lauderdale..Big engine is good to have...Very good to have)
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Old 08-10-2005, 17:27   #11
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I understand on the structure issue, but you are within reason with the power. I am talking about a 95hp perkins in a 37' boat. Or bigger for that matter. As for beating the bridges, well, it would suck to have your mast scissored. My point overall is that there is a certain point where additional power will not add as much as it will take. 2 or 3 hp per thousand pounds is not bad, but when you have to start adding internal balast in the bow to balance the bigger engine, you might be headed for overkill.
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Old 08-10-2005, 21:21   #12
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Uh, yeah, but then ya are talking extreme...
Like the teenage kid adding a V-8 to his Honda Civic.

Not the case with my ship, but if I had a 120 HP Ford Lehman in the same boat, yup, overkill and too much weight, etc.

Now here is the question:

When I go high power in the ocean my wake is 90 degrees to the direction of travel, and the stern digs in..

It does not seem right..

So for Jeff and the other naval arks around here:

1) Am I approching hull speed?

2) Am I right at hull speed?

3) Or slightly past hull speed?

The speed is probably 6.5 knots...25 feet water line...Lots of power, in fact the engine over-heats after 10 mins...3000 RPM or more.
Prop is wide 3 blade 10 X 16....(Very wide blade, like a tug prop.)

Been experimenting with props and power settings over the years, finally ready to go back to the prop the previous owner recommended as being the best compromise...he was right all along, but I thought I was smarter...Guess I was wrong....

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Old 08-10-2005, 22:05   #13
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Hell man,

If fuel was much cheaper. I'd build me a PT patrol-type boat. And really go the hell off with HP and RPM's.

Create one hell-of-a-wake everytime I roar out of the harbor. Past the posted harbor speed limit signs. And such. :-)


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Old 08-10-2005, 23:59   #14
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Just a Bit of Context Here

The "I sail in an area where strong currents predominate" argument notwithstanding for the moment, I'd like to make a couple of observations.
  • ideas about the adequate amount of "auxilliary" power a sailboat needs, and about when to run our engines, has changed over time. Kai Nui jokes about sailing into the harbor, and when we laugh it may be at ourselves, because we're not the seamen our fathers were, i.e., if you have to charge your batteries anyway, what's the point in sailing off the hook? Engine dependence makes us more like the powerboaters than we'd want to admit. The Pardy's did not pay me to say this!
  • almost any size engine can be justified when "extreme conditions" is used as the rationale. Arguments that balance horsepower against displacement, windage, etc. have their place, and a of course a prudent skipper throws his purist impulses overboard when safety requires, but "a bigger engine is always better" thinking can be a trap that is just too tempting to avoid. This thinking helps fuel the "bigger engine/more weight/less storage/need bigger boat" fire.
The reliable marine engine has arguably opened the door of cruising to a much wider spectrum of people by shortening the period one must "pay his dues" before he has access to Tahitian sunsets; it has also made destinations that were before considered too risky to attempt at all, or only in a limited number of tide/current/weather conditions almost a mundane occurence; and completely changed the way the cruiser evaluates weather windows, transit times, etc. It's been a great boon to cruising, and it sure is nice to know you have a reliable engine under your deck. But we should fight the temptation it offers to make us lazier, less skilled or more careless.
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Old 09-10-2005, 00:23   #15
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Well said!
My 28 footer has no engine, so sailing in, and sailing on and off the hook is the only option. I feel it has made me a better sailor. On the other hand, I can think of many breezy warm days missed because the wind was in a bad direction to get out. Still, there is allot of pride in arriving at a crowded anchorage under sail, and dropping the hook in the perfect spot. Dropping the sails, and watching the admiration from the observers around.
My grandfather was a whaler, and a true seaman of the early 1900's. I know I am nowhere near the sailor he was, but in my own way, I hope my skills surpass the weekend warrior.
Oh, and who said I was joking?
As for charging batteries, Kittiwake has none to charge. We heat with wood, cook with propane, and light with oil. Including nav lights.
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