What is the minimum HP for a 22 or a 25?

H

hommeland

New member
What would be the best way to drive the MPG to the highest possible level?

What happens if I put 60 hp on a 22? or 80 hp on a 25? I know the performance at high (or not so high) speeds would suffer; and the resale value would suffer, but I'm looking for the best MPG at slow speeds. Would I get even better MPG at very slow speeds if the power was twins and only ran one at very low RPM like a kicker?
 
we have twin 40s and one engine can run our heavily loaded 22 at up to about 10kts so if mpg is all you are thinking about a single 40hp will move the boat quite easily.
 
hommeland":1ncms8df said:
What would be the best way to drive the MPG to the highest possible level?

What happens if I put 60 hp on a 22? or 80 hp on a 25? I know the performance at high (or not so high) speeds would suffer; and the resale value would suffer, but I'm looking for the best MPG at slow speeds. Would I get even better MPG at very slow speeds if the power was twins and only ran one at very low RPM like a kicker?
Click to expand...

The best mileage is when the boat is at displacement speeds...that's about 6 knots and under on the C-Dory 22 and about 6.4 knots on the CD25. In calm water it takes about one horsepower for every 500 pounds of boat weight to achieve hull speed. For a CD22 that's about 8 horsepower, for the CD25 about 15 horsepower (these numbers are a bit conservative as I've based them on a "loaded" boat). You need more horsepower to maintain hull speed when you have wind and wave action, especially from forward. How much "reserve" horsepower is needed depends on the area of operation and the operator's desires. My experience would say that a 15-hp engine on the CD22 would drive it to hull speed in almost all conditions; on a CD25 I'd likely recommend a 25 hp engine.

The moment that hull speed is exceeded you can not obtain the highest possible MPG, that only exists at slower speeds. In fact hull speed won't be the highest obtainable either (though very close).

The graph of necessary power escalates exponentially as you exceed hull speed. As an example, on a CD22 you could run at hull speed (6 knots) on 8 hp. To increase the speed to 8 knots means one horsepower is required for every 200 pounds of weight, which means 20 horsepower is required. Put another way, to gain a 35% increase in speed (in this example) a 150% increase in horsepower is required.

With the advent of modern fuel injected outboards that don't pay the penalty from low rpm operation that carburetor equipped engines did you won't find much savings (in terms of fuel burn) just by using a smaller engine. That is to say, you're not going to get a meaningful savings in fuel burn running a 15-hp engine on a CD22 versus a 60-hp (or even a 90) if running at displacement speeds. And if you exceed the speeds at which the 15 hp engine is efficient it will actually use more fuel than the 60 (or 90).

What you have to do is select a speed at which you'd like to cruise (remembering that any speed above hull speed gets exponentially more expensive). The required horsepower can then be calculated and an engine chosen that produces that amount of horsepower at its most efficient rpm.
 
Great answer Les. Is there gonna be a test? :roll: :mrgreen:

Seriously, it was. Hommeland needs to analyze how he wants to use the boat. He can get great mileage with a little engine but not go very far or fast. He can have lots of gas left when he's miles from a safe port and the weather blows up and not get there quickly.

It's all a tradeoff. Make a decision, buy a boat, go have fun! (want a TomCat?)

Charlie
 
Interesting topic, and (Les) thanks for some excellent guidance on the hp necessary to drive a vessel at hull speed.

I suspect that in the years to come we will all be exploring ways to go boating without breaking the bank.

One solution might be to use minimum hp for hull speed (but obviously that restricts one's ability to get on-plane and into safe harbor).

Another is to explore equipment that makes displacement cruising more comfortable. I've found that, particularly at hull speed, an autopilot really makes going slow cruising much easier. You can cruise along without constantly jockeying the wheel, listen to the radio, chat with passenger's; it's really quite enjoyable (if you're not in a hurry).

Another item to consider is how heavily your vessel is loaded. If you're going to be in Southeast Alaska for three months you'll have a heavy vessel. If you're out for the weekend and loaded as though you're headed for Alaska, you might consider off-loading some stuff! Your fuel saving's may be miniscule, but you'll feel better.

And naturally the benefit's of route planning are significant (avoid useless back and forth travel).

And best-of-all, time at anchor doesn't burn ANY fuel!

In any case - Just Go!

Casey
 
When we ordered our boat, Jeff Messmer (now with Ranger Tugs, then with C-Dory) asked us how we intended to use our boat. We had been sailors, so getting the most speed out of the boat was less important to us than enjoying the scenery. He recommended a Honda 135; pretty much the same engine as the 150, with less top end. Good advice.

There have been a few times, namely when at higher elevations, that I wish I had more horsepower. Most of our cruising is at sea level, and the 135 does all we ask of it, however we are loaded. There is definitely a speed and fuel penalty to be paid for carrying extra weight... and we seldom travel light (except when around our home).

When traveling at displacement speed, usually around 1800 RPM on our boat, we are getting better than 5 mpg... according to our fuel flow meter, we are burning less than 1 gallon per hour at 6 knots. Pretty decent fuel burn for a boat... that offers the accommodations the 25 does. No doubt, the 22 will do even better... physics doesn't care so much whether it's a Honda, Suzuki, or a Yamaha. :wink: For the record, I considered twin 90s for our boat, but fuel injected Honda 90s didn't exist, yet... thus the decision to go with the 135.

I have mentioned this in the past, but owning and using a boat is about the SMILES per gallon. Compared to almost anything that rolls on land, boats are not fuel efficient... it's all relative. Oh, and moving the boat on a trailer gets you to the great cruising areas more efficiently. 8)

Best wishes,
Jim B.
 
We have twin 50's on our 22 and cruise at trawler speeds 95% of the time with just 1 motor down at a time. I just change motors from time to time to keep the hours the same. The advantage for us is that we can go 30mph when needed and also get 7mpg at other times. Also, with rotating between motors we always know that they both are working good and maintained.
 
I have said this before--and will probably say this agin. I have gone thousands of miles on a boat which is bigger than a C Dory25, (weight, depth, displacement more--length OA and LWL and beam the same--with a 5 hp outboard. The boat cruised efficiently at about 5 knots--full out maybe 6 knots. You are not going to drive a C Dory 25 much more than 6.3 with low hp. This gets back to 1.34 x sq root Length Water line (or about 4.7 in the 25) If you have adverse current, you still will not go faster thru the water. If you have head winds or chop, it will slow the boat down--but it does not take long until these are high enough to get the boat speed down less than the 6 knots. So the practical displacement speed is about 6 knots for a 25--a bit less for a 22. The C Dory's flat hull will go into a faster semi displacement mode fairly easily, but requires considerable more HP (and fuel) to go into planing mode at a low speed.

You really loose out in resale by owning lower HP engines. One of the beauties of the C Dory in the PNW is that you can beat the current and get thru passes much more easily than with a slower displacement only vessel. Also there is the safety factor. You cannot beat a storm usually, but you can get to safe shelter faster before the storm over whelms you.
 
Back
Top