Right props for CD-22 for full 5500-6000 RPM at WOT

[Rokjok777]

Yes I know the answer is "it depends". Depends on loading, water conditions, motor HP, manufacturer etc. So let's assume moderately to fully loaded (2-3 people with gas, water and gear); twin Honda 50's; used in fairly rough inshore and offshore conditions; and not used for towing skiers etc.

I've got 11-1/2 x 13 props on her now. A local dealer told me I should be able to get to WOT RPM's (on flat water) with just one engine running, but I can only get to around 4500 RPM's. With both running, she maxes out at 5500-6000, which is fine.

Is this dealer right? Or am I OK with my current props? Thanks as always, Squidward is getting tuned up with all the great advice I'm getting...I appreciate it.

[Les Lampman]

Hi Rokjok777,

Boy, this is like jumping in for a swim when you know there are sharks in the water! But since I've heard they won't attack unless provoked...here goes!

In a literal sense you will get to WOT rpm's with one engine running since whatever rpm you obtain with the throttle to the stop is your WOT rpm. I don't think that's what you meant though.

So the real answer is... getting to the same rpm with one engine running as with two can't happen unless you've got an extremely high horsepower-to-weight ratio (I mean a crazy amount); that isn't the case with the C-Dory 22's (and shouldn't be since that's not what the boat is about). When you shut down one engine the load factor on the second one is really high; it can't possibly reach the same rpm as it did as the second half of a twin set.

You've got the props that I would have suggested and your rpm is just where it ought to be for the BF50's; I wouldn't change a thing.

This does point out something though; with one engine running your new WOT rpm for that engine is 4500 rpm (or whatever you do get); to be kind to your engine, if you find yourself running home on one, don't continuously run at more than 3500 to 3800 rpm. For optimum engine life you don't want to pull more than 80% of available horsepower out of the engine on a continuous basis (an easy thing to remember is just stay 1000 rpm below WOT). Obviously, in an emergency situation I wouldn't worry about it but otherwise don't run the single with the throttle against the stop.

[Rokjok777]

Yes I do like to run at 1000 RPM's or so below the maximum...I guess I just didn't understand what that "maximum" was supposed to mean. When I run twins, since they achieve 5500 or so, I like to run at 4500 or so. But the point you make is that if 4500 is the max for one engine, then when I'm running just one engine (which is almost never) I should try and run at 3500 or so (80% of max). So now I understand what WOT is: just what it states, the throttle all the way down. It doesn't relate to RPM.

Great to finally understand all this, sounds like the local guy was misinformed (or not as good at explaining as you are...). I'll keep the props I've got, thanks again Les, I wish you were my "local guy"!

[Chuck S]

You can get a different propeller for single engine running. Honda website show the 22 Cruiser planing on one engine. Probably not 1 of 2, but planing seems possible. Maybe not on your current propeller though.

Changing a prop at sea is difficult, but if you're on the beach or moored and need to come home on one engine a prop which gets you on a plane will make several hours difference in transit.

Unless you can get the hull to break free of the surface, though, a different prop will just spin the engine faster but you'll go no faster. If you can't plane a 9.9 kicker engine will usually run just as fast as a 50.

I see lots of discussion on this subject, but I've not seen any experiments in getting a 22 Cruiser to plane on one engine with the other tilted out of the water.

I don't see any issues running 4500 rpm endlessly on one engine (or two). On the Honda 50 this is right at the torque peak. At 4500 the engine is producing 40.4HP, right at 80% of it's rated HP. Available HP at sealevel is still 50 regardless of the load.

(HP = torque * rpm/5252 = 47.1 * 4500/5252 = 40.4)

-- Chuck

[Les Lampman]

A CD22 with a single BF50 doesn't do all that badly and planes easily. A single BF50 as a set of twins won't plane the boat; there's too much weight in the stern. I have been able to run a twin engine CD22 up to cruise speed and then with careful coordination shut down one while tilting it clear of the water. The boat will stay on plane but it takes full throttle. And, of course, this isn't the likely scenario...more likely that you're already stopped and trying to achieve a plane.

Running a single BF50 (in a set of twins) will push the CD22 considerably faster than a 8 to 9.9 kicker will; we're not strictly limited to hull speed because the hull is not a displacement hull. The bow will rise over its bow wave and the BF50 has enough power to push the speed/length ratio into the 'semi-displacement' (or semi-planing) range.

When trying to maximize engine life you're not concerned with 'rated' horsepower but rather maximum achievable horsepower. An engine that obtains its rated horsepower at say, 5500 rpm, is not going to be doing that at 4500 rpm. A vacuum gauge is good indication of how hard an engine is working; if you're towing a heavy load up a grade in high gear (or too high a gear) with your foot on the floor, the needle is just about pegged low and the engine is working very hard. Same thing happens on outboards (which is really why we have WOT ratings in the first place). If you've got the throttle wide open and can't obtain rated WOT rpm you're working that engine very hard. Even though the engine would normally be happy at 4500 from an rpm standpoint, it's now working extremely hard to achieve that and we're pulling much more than 80% of obtainable horsepower...in fact, we're asking for 100% output if 4500 rpm is our WOT rpm. That's why it's important to get the propping right and to get the WOT rpm where it belongs; the prop really determines the final drive ratio and it's just as important as getting the proper rear end ratio in a truck. We're pre-determining what load the engine can reasonably handle; it's pretty common knowledge that a truck with a 4.11 read end can handle a heavy load better that one with a 2.54 rear end...the prop does exactly the same thing on an outboard. For our trucks, most of us might settle for a compromise 3.73 or so for all-round use and that's sort of where most propping ends up on C-Dory type boats; we're not trying to milk that last mph out of the engine and we don't need to pull a barge so something 'in the middle' typically works pretty well.

Since we're doing all this on-the-fly when we're out running our boats and since we've got no vacuum gauge available some 'rules-of-thumb' have come to be. The 'quick and dirty' rule-of-thumb for max continuous cruise rpm is 1,000 rpm under maximum obtainable rpm. That's not strictly accurate (and is on the conservative side) but given the rpm ranges that our engines are typically operating in, it suffices pretty well.

For a pair of BF50's, 5500 is about the minimum WOT throttle I'd like to see; if the boat isn't as heavy as it will typically be then we're going to drop below that when the weight is added (depending, of course, on how much). If that drops to say, 5200 or so and we cruise at 4500 we're working the engine harder then we need to and pulling a higher percentage of available horsepower. For occasional use this wouldn't be a bad situation but if this is the 'normal' configuration I'd want a bit less prop to allow the engines to reach at least 5500 when loaded the way it normally would be. And, experience on the water indicates that the BF50 is much happier when it can 'turn up' freely. [The 'number' for the twin BF40's would be at least 5000 and preferably 5200 to 5400.]

[Chuck S]

Les --

I'm going to need more convincing that running the engine at 80% power, and in this case at the engine's torque peak, has any negative effects on it. Available horsepower at sealevel is 50 regardless of the load in the boat. You can't spin the propeller fast enough in this case to get maximum horsepower, but the engine will spin up to it's torque peak and it it has adequate cooling she should be fine. If you change the propeller so the engine spins up to 5500 rpm with this load you get the same speed number of propeller rpms at 5500 that you got at 4500, therefore the same speed thru the water, only at a higher rpm. If you then drop back to 80% power you get a lot less speed thru the water.

Torque is really what counts and at these rpms you're not lugging the engine. Yeah, she's working harder than if the load was lighter, but that's what engines are paid to do.

The temptation here is to push the throttle to the firewall and on a carb-type engine can load up the cylinders with excess fuel, so there's a need to throttle back once she's running at maximum rpm for the load.

-- Chuck

[Captains Cat]

Just put Capt's Choice in the water last weekend and the engines run much better than they did last year. Had some carb work which must explain it.

Have found now, with them running well, that my WOT value will exceed 6000 RPMs (I didn't try to go to the top but had some throttle left when I got to 6000!) I have twin 45HP Honda 4 Strokes. Replaced the props last year with Hustler Alum props, 10 3/4" X 12 Pitch. They have other pitches available, this is the type with a hub and all you have to do is to replace the outer ring that has the blades (They're still $91 from iBoats though)

Any recommendations as to which to try next, to bring the WOT value down? The boat had a typical load, two humans (large size) and a full load of gas.

They have

10 1/2" x 13
10 3/8" x 14
and
10 1/8" x 15

I was thinking maybe the 14 pitch ones?

Thanks...Charlie

[Les Lampman]

Hi Charlie,

It would be unusual to run 14" props on your setup but it's obviously not possible to say that they wouldn't work; just too many variables and I've no familiarity with those props. Typically I'm running 13" on the twin Honda setups but I'm using the Solas (Honda OEM) 3-bladed props. I don't have any reason to suspect that one is 'better' than the other; seems you're in the 'buy and try' mode!

Les

[Les Lampman]

[Captains Cat]

Les, many thanks. I think the old props that I got when I bought the boat in Dec 02 (it's a '94) were the original, Solas ones. Although I couldn't see a brand, they were 13" pitch. I think I'll go with the 13's and keep these 12's as spares.

Charlie

[Pat Anderson]

OK, let's revive this topic! C-Dory publishes a chart that includes speed and fuel economy at various rpms. Daydream does not as far as I can tell come close to either. We are using the standard factory-issue props (11.5" x 13" aluminum three blade). Our WOT on twin BF40s is 5500 rpm. Our first year, we ran a lot at or close to WOT, and when I checked, mileage (did not check burn rate) was south of 4.0 mpg and speed was 23 - 25 mph. Just started running at 4500 rmp, and mileage has climbed up to about 4.3 mpg and giving us 18 - 20 mph (reasonably good conditions too). Here is the chart (doesn't line up with cut and paste but you can figure it out). At 4500 rmp I feel I should get a little better top speed, and a LOT better mpgs. I suspect we are probably loaded heavier than the test setup (don't really know). Bottom line, should I be considering a change in prop size or pitch to improve either or both of mileage and speed at a given rpm?
Performance ~ C-Dory 22' Cruiser

Twin Honda 40 Hp, Four Stroke
11 1/2" X 13" Prop, 3 blade, Aluminum

800 Lb. People, Fuel & Gear.

Fuel consumption, is for "both" motors

Miles Knots
R.P.M. G.P.H. M.P.H. M.P.G. N.M.P.H. N.M.P.G.
3000 1.75 10.9 4.98 9.5 5.42
3500 3.00 15.0 5.01 13.1 4.36
4000 3.50 18.7 5.34 16.3 4.65
4500 4.20 22.5 5.35 19.5 4.65
5000 4.90 26.0 5.31 22.6 4.62
5500 6.75 29.4 4.35 25.5 3.78
6000 7.80 31.1 3.98 27.0 3.47


--------------------------------------------------------------------------------

Single Honda 75 Four Stroke

13 3/4" X 15" Prop, 3 blade, aluminum

800 Lb. People, Fuel & Gear.

Miles Knots
R.P.M. G.P.H. M.P.H. M.P.G. N.M.P.H. N.M.P.G.
2500 1.50 8.5 5.67 7.3 4.87
3000 2.40 12.5 5.21 10.8 4.48
3500 3.10 16.2 5.23 14.0 4.49
4000 3.85 20.3 5.27 17.5 4.53
4500 4.70 24.7 5.26 21.3 4.52
5000 5.60 27.6 4.93 23.7 4.24
5500 6.80 30.4 4.47 26.2 3.84

[Sawdust]

Pat,

I'll jump on this one a litttle because I've run lots of tests with the 75 Honda on a 22.

I've never been able to come close to the factory published figures -- and that's understandable. Too many variables that affect fuel consumption. Boat weight, boat trim, boat speed, sea conditions, and the list goes on.

Even measuring actual miles you travel is difficult. And an autopilot that will hold +/- 1 or 2 degrees will do much better than a skipper who lets the heading wander 4 or 5 degrees. Every helm correction induces drag and increases fuel consumption.

After several tests from Deception to Pt. Angeles and Sequim, Deception to Jones, and other fairly long runs, I feel safe in using 3.8 NMPG for planning at zero current. Under ideal conditions I can beat 4.0, but not by much. As I remember, El and Bill do nearly 4.0 -- and with their heavy boat that's very good.

My short trips to Lopez Island, Smith Island, etc. run about 3.5 from dock to dock.

All this probably doesn't help a bit, and hope there will be lots of skippers adding to the numbers. And I would expect a wide spread, because we all do it our own way.

Dusty

[B~C]

Pat, start packing the "light beer "and your speed and milage will improve

[C-Worthy]

Does it help gas mileage to bring the bow down in chop? My wife and I differ on if the extra drag from the motors being down cancels out any savings in increased speed at the same RPM. We get 1-2nmph more if the motors are all the way down in a straight-on chop (and the ride is smoother).

22' Cruiser, twin Honda45s, three batteries, dog, extra anchor,, etc...

[Sea Wolf]

C-Worthy-

To get a final answer to this question, you're going to have to have a fuel flow meter coupled to a GPS that together can give you instantaneous mileage figures as you change trim. Several C-Brats have this type of set up, and they should be able to give you figures they have derived from their experiences.

Triming the motors down under the boat to raise the stern and lower the bow increases the surface area exposed while softening the ride by breaking the chop with the sharpness of the bow. The extra drag should decrease your mileage figures, but the point may well be relatively moot if you value the comfort of the ride that way. Also contributing to the mileage loss is the inefficient thrust angle of the motor(s).

Shifting weight forward can have the same effect of lowering the bow and achieving a smoother ride with less mileage loss from the inefficient motor thrust angle created when trim is used to lower the bow. 100 ft of 1/4" chain (74 lbs) does this very efficiently. although there are other caveats of which to be aware. Is there any thing or anyone that can be moved forward?

The ultimate mileage figures in planing boats are usually achieved with the bow trimed up as far as possible to minimize surface area with the boat planing only on the very back of the hull bottom, but the mass and balance of the C-Dory doesn't permit this very easily and the resultant light bow is totally incapable of smoothly breaking choppy waves.

Let's get some hard numbers now from those who have the instrumentation to measure it. HTH. Joe.