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Old 09-08-2010, 16:38   #1
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Motor Size

How do you determine what size motor to install in a boat?
Guess that would be the question in general.
More specifically I'm looking at a 34 foot Columbia with a 10 HP Volvo installed in it. Is this large enough?
Thanks Jeremy
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Old 09-08-2010, 16:40   #2
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No, it's not large enough; and it's a Volvo.
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Old 09-08-2010, 16:58   #3
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2.5 HP per ton

I read somewhere that 2.5 HP/ton was a good rule of thumb. I don't remember where I got that figure, Nigel Calder?
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Old 09-08-2010, 16:59   #4
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That's enough for getting out of the marina. But if you need to motor into a breeze or seas you will find yourself very short of power.

You need 18-20hp. I'd guess.
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Old 09-08-2010, 17:02   #5
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It all depends on what you want the engine to do. If you wqant to pull water skiers, by all means get a larger engine. If you want the engine to get you in and out of the slip and push the boat along at a decent speed when the winds don't blow, it's fine. We were very happy with an MD2b Volvo in our 20,000# Westsail for many 10's of thousands of miles. It pushed the boat at 5k plus in a calm and we could motorsail to windward in quite nasty conditions.

Some people feel you need an engine large enough to push a boat at hull speed into a hurricane. If you want to spend the money in both initial cost and fuel, have at it. Actually think the trend to ever bigger motors in Sailboats is leading to a lot of serious accidents from people who back themselves into a corner expecting the iron jib to pull their bacon from the fire.
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Old 09-08-2010, 17:03   #6
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Typically these had an Atomic 4 or the 28 HP Palmer P-60. A 10 HP might get her to 6 knots, but you will not have any reserve to handle chop, contrary currents, windage, etc. 20 HP would be a minimum, 30 HP ideal for just about every situation and 35 - 40 HP if you do a lot of motor sailing in rough conditions.
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Old 09-08-2010, 17:33   #7
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Typically these had an Atomic 4 or the 28 HP Palmer P-60. A 10 HP might get her to 6 knots, but you will not have any reserve to handle chop, contrary currents, windage, etc. 20 HP would be a minimum, 30 HP ideal for just about every situation and 35 - 40 HP if you do a lot of motor sailing in rough conditions.
INDEED !
A modern Yanmar 2GM (whatever they're calling them now, but @ 18/20HP) would be a good BASIC choice. Larger could be better, depending ...
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Old 09-08-2010, 17:39   #8
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Thanks for the input.
Kind of seemed on the small side but my needs may not necessitate a larger motor.
Thanks Jeremy
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Old 09-08-2010, 17:40   #9
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2 horses per 1,000 lbs. displacement is a good rule of thumb to start with. I won't go into the "which brand is best" discussion....
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Old 10-08-2010, 02:22   #10
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I dislike rules of thumb, they always let you hit the barn door, but not much else.

Using the "rules" listed here we get 15 to 24 HP (for the 6 ton Tripp C-34) which is as imprecise as my deaf uncle's piano playing. This of course assumes it's the blister deck Bill Tripp design, not the Wirth Monroe C-33 turned into a C-34 with cut down sheer, reshaped appendages and conventional deck structures.
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Old 10-08-2010, 02:56   #11
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Don't forget the horse power loss from compressor fro the fridge/freezer and the alternator
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Old 10-08-2010, 07:53   #12
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Rules of thumb are okay for getting you into the ballpark, but beyond that they don't tell you much.

Here's an example. Last weekend I was over at Indian Rocks Beach, on the Gulf coast of Florida. It was a beautiful day. Perfect for sailing, with a nice 10 knots or so breeze and hardly any chop on the Gulf. Sitting there on the beach, I watch a lovely ketch coming up the coast. When he gets close enough I realize that, although his sails are up, they are just flapping in the breeze. On a perfect day like today he is motoring! What the hell does he even have his sails up for, if he's not going to use them?

This is one of those guys who likes the look of a sailboat, but really is just a motor-boater. I used to have a slip in a marina next to a guy like that. He never went anywhere without his motor running. Sails up or down didn't matter. Wide open water or going through a draw-bridge didn't matter. He started the motor almost first thing after he arrived at the boat and he didn't shut it down until he was back in his slip and almost ready to go home. He thought I was reckless and a danger to everyone around me when I would sail into my slip (even though I only did it when the wind was right and I NEVER had any issue with it).

Now, no offense intended if you recognize yourself here, but if you don't feel comfortable going anywhere without the motor running then you need a much bigger motor--not to mention a VERY MUCH bigger gas tank!--than someone who bought a sailboat because they don't want the noise and stink of a motor.

On the other hand, if you see a motor as a necessary evil, to be used only when there is no alternative, then you can probably get by with a smaller motor. Of course, you'll also require a correspondingly smaller gas tank. Lots of people have sailed all around the world (still do, actually) without any motor, which makes the case for zero horsepower as the smallest motor you should consider!

So, the general rule of thumb is that you should have about 2 hp for every 1,000 lbs. of displacement. A Columbia 34, depending on the exact model and year of mfg., will have a displacement somewhere between 10,000 and about 13,000 lbs. That would indicate 20 hp on the low end, up to 26 hp on the high end based on the rule of thumb. But if you intend to motor a lot you should probably consider 30 hp as the minimum. If you are willing to accept the limitations that come with it, you could do without a motor at all, or get by on the 10 hp that is in there.

But you MUST be aware that there are plenty of limitations that come with an under-powered engine! You cannot motor into any significant wind or head seas. You are going to be limited on how fast you can motor. There will be times when you want to go that way, but must instead go this way. I would add that MOST boat buyers will consider this boat severely under-powered, and so the resale value is going to be dramatically affected. It should be priced with the expectation that you will re-power (whether or not you choose to re-power is another matter).

Good luck.
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Old 10-08-2010, 13:18   #13
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Thanks for the rules of thumb. That does help. In hearing stories of people going motor less I often wonder how they do it. Or if I should not start mine up some someday and see how it goes. I don't feel I motor excessively, I would rather sail. But, it is nice to motor in to the cove for the night and for docking.
Jeremy
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Old 10-08-2010, 13:19   #14
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Good point on the resale also
Jeremy
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Old 10-08-2010, 14:14   #15
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It might also be worth mentioning that, in a boat like this, the main engine is likely your primary source of electricity. On a 10 hp, I'd expect an alternator that can keep the starting battery topped up and perhaps run a few lights and a radio. If your electrical demands are much more than that, you might find yourself idling the engine for very long periods to top up the batteries. A bigger engine can swing a bigger alternator, which- depending on your cruising style- might be important.
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