Outboard Motor

[cfelle2]

Displacement is the amount of weight of water a boat will displace while floating.  Boat manufacturers use the term incorrectly as boat weight.  That has always been a pet peeve of mine.   

Chris

[Shawn]

Chris,

The weight of the water displaced by a floating boat is equal to the weight of the boat (and everything in it).

The boat is pushing down by 3000 pounds, 3000 pounds of displaced water is pushing back up. Opposing forces are balanced, the boat is floating.

Shawn

[maynard]

The bigger prop is almost a requirement for efficiency and low revs. I just went across the gulf stream with a 4 hp mercury long shaft. When the seas moderated to 4' we were able to complete the last half of the crossing against 15 mph headwinds in a 23' Compac.  The sailpro model would be the best choice.

[brackish]

I found (and then subsequently lost) a marine designer calculation that gives the horsepower requirement for a displacement hull.  When I did the calculation for a ComPac 23 it came out to 5.4 HP and I'm fairly sure that has some condition safety factor included.  I would have already swapped my very heavy 9.9 for a SailPro, but waiting for the next planned road trip to do so.  Thought it would be BEER 2012, but a wedding may keep that from happening.

[maynard]

Weight was mentioned. For the tohatsu mercury motors it's the same power head until you get to the 8 hp. With the single cylinder motor one person is able to handle the motor even while the boat is afloat when you want to move the motor onto the dingy. The motor bracket on the boat is almost as important as the motor choice compare carefully. Fair winds

[LConrad]

I have an older Tohatsu 9.8 with the large diameter and very short pitch prop. Full throttle over revs by about 200 rpm and hits about 6.6 mph. Burns lots of gas compared to about 5 mph. 8 HP with short pitch prop should work fine even in the wind.

[mike gartland]

For five years I sailed in Galveston Bay with a 20 year old Tohatsu 9.9 hp 2-stroke extra-long shaft and very much appreciated the added horsepower when I was fighting a tidal current plus wind.  At other times it was overkill...but very nice to have when I needed it.  When I retired and returned home to Colorado I left the old smoking 2 stroke behind and replaced it with a modern 4 stroke Tohatsu 8 hp as I now sail much smaller lakes without the tidal currents.  The 8 seems more than adequate and just a standard long shaft...in place of the old 25 in extra-long...works just fine without the high chop I used to encounter in Galveston Bay.  So the bottom line is that the motor that the CP-23s needs depends quite a bit on where you sail.  I actually think a 6 hp would be fine where I am right now but the Admiral refused to go with a non-electric start motor so I was forced to go with an 8 hp instead of the 6 hp I would have preferred.

Fair winds,
WindRush CP-23/III

[fawsr]

When I bought my CP19 in February it came with a 9.9 electric start OB. The PO had been using it in the Mississippi river and thus needed the extra horsepower. I read that the 19 needed 4-6 hp to be happy. I ran the calculations and came up with the follwong:

3 knots require =   1.1 HP
4 knots require =   2.6 HP
5 knots require =   6.2 HP
6 knots require =   8.8 HP
7 knots require = 14.0 HP

Since I am in NE Arkansas my local venues will be lakes. If I happen to the Gulf I still won't be looking at significant tidal currents
so I was figuring on downsizing to 2.5 or 3.5 HP. Does this sound about right for those out there with 19's?
My biggest concern is the weight of the current OB.



Going Back to gfspencer's original question:

2.  Will a 6 hp 4-stroke be enough to push a Com-pac 23/3?  I'm on a lake so I don't have to worry about currents or tides.  (An 8 hp motor is $700 more expensive and a lot heavier.)

I calculated HP requirements for a CP 23 to be ...

2 knot     0.7 HP
3 knot     2.5 HP
4 knot     5.9 HP
5 knot   14.0 HP

If these calculations are correct, a 6HP OB should push a 23 about 4 knots. Is this consistent with other owners experience?

[brackish]

If these calculations are correct, a 6HP OB should push a 23 about 4 knots. Is this consistent with other owners experience?

Not my experience.  With my 9.9, lake conditions, I can achieve 5.5 knots with the throttle about 40% open.  80% gets me over six and WOT doesn't do much better than that with hull speed being 6.02 for a 23.  Can't tell you what the developed HP is at 40%, but doubt that it is anywhere near 6.

Interested to know about your calculation.  As mentioned in an earlier post I had found a calculation for HP requirement for displacement hull.  It was 5.4 HP for a 23, and that would take the boat to at least hull speed.

The big consideration for me is weight.  Removing and replacing the 104 lb. 9.9 for a tow requires me, if alone, to disconnect my truck from the trailer back it up to the motor, remove it, with difficulty and lay it down, then line up and reconnect the truck.  It takes at least two strong people to do it without using the truck, my wife does not qualify.  Additionally all that weight affects the trim of the boat.  It is a sailboat.  I'm more concerned with proper trim than to have enough emergency HP to get me through some condition I may never see.

I had a 4 and a 6 for my Columbia 24 challenger.  It had a displacement of 3900 lbs.  Normally we would choose the four, because once sailing, we could lift the motor, put a plug in the well and lay the motor on its side in the well.  Much harder to do with the six.  4 hp pushed the boat along fine.

[Tim Gardner]

Here's the calculation site on Pocketyachts

http://sites.google.com/site/pocketyachtcruising/sailing-references/sailboat-math

[Billy]

Fawsr,
GO HOGS! I'm originally from Fayetteville. Nice to see a fellow Arkansan on here.

I used to have a yamaha 2.5 hp 15" shaft on my 19 that worked fine with me. And that was with some fairly decent currents here in the gulf. The only 2 problems was the yamaha clogged the carbs anytime the gass was more than a week old. And I used stabil.  The other was the 15" short shaft. Couldn't go forward.

2-3 hp will be more than fine for a 19 in a lake. Plus it is lighter!

[brackish]

Here's the calculation site on Pocketyachts

http://sites.google.com/site/pocketyachtcruising/sailing-references/sailboat-math

Yep, that's the one, thanks.

Just redid my calc, comes to 5.14 @ design displacement and 6.09 with the added weight that the calc suggests.  That is for the 23 theoretical hull speed which is 6.02 knots, which i never travel at, fuel use for that extra half knot is not worth it.

Still planning to drop to a six for the weight difference,

[fawsr]

I'll send you the formulas I used this evening. On the road right now. Like you, my concern is weight & balance. At any rate looks like a 2.5 or 3.5 would do fine for me.

[Shawn]

"If these calculations are correct, a 6HP OB should push a 23 about 4 knots. Is this consistent with other owners experience?"

No, I can hit hull speed with my 6hp SailPro.

The numbers for the 19 also ignore hull speed. 14hp will not get the 19 to 7 knots as that would require getting it up on a plane. The Sailboat Company tried a 40hp engine on a 16 and it wouldn't plane with that.

Shawn

[HideAway]

I can hit hull speed easily with my very old Evinrude 2 stroke 8 hp.  Since we do have strong current/wind combinations I like the power.  The dealer who sold me the motor told me that 2 strokes were meant to run hard and that we sailors always have problems because we never do that.   So over the years we almost always run full or near full bore for a bit.   If I run full the boat digs a hole so 3/4 throttle is about it.

The Evinrude has a standard prop-it takes a lot of throttle in small spaces - turning around in a marina for instance.  It also burns a gallon an hour at hull speed creating fuel storage problems on long cruises.

We will have to replace the motor one of these days - That 6hp Sailpro looks like quite a bargain with electric start and alternator for under 2 k