Minimum power to plane 22

[Professional Hobbyist]

We put our new 22 Cruiser "Last Frontier" in the water today for the first time. After some twin engine motoring I tried powering the boat on one engine. With four people aboard, half fuel, and a normal load of day trip stuff the boat would not plane on one 50 hp Suzuki engine. I would make 9 mph and turn only 4400 rpm.

Using both engines she came on plane at about 12 mph and the engines would turn 6000 rpm with a max speed of 29.8 mph on very smooth water in Lake Washington.

I was surprised she wouldn't plane on the port engine. I didn't try the starboard engine. I pulled the other engine out of the water for the test and played with both engine trim and the trim tabs, but could not get her above 4400 rpm or on plane.

Does this sound right to other 22 Cruiser owners with a similar load and hp?

Otherwise I am thrilled with our boat and can’t wait to get back on the water and do some serious exploring.

 

[tsturm]

We put our new 22 Cruiser "Last Frontier" in the water today for the first time. After some twin engine motoring I tried powering the boat on one engine. With four people aboard, half fuel, and a normal load of day trip stuff the boat would not plane on one 50 hp Suzuki engine. I would make 9 mph and turn only 4400 rpm.

Using both engines she came on plane at about 12 mph and the engines would turn 6000 rpm with a max speed of 29.8 mph on very smooth water in Lake Washington.

I was surprised she wouldn't plane on the port engine. I didn't try the starboard engine. I pulled the other engine out of the water for the test and played with both engine trim and the trim tabs, but could not get her above 4400 rpm or on plane.

Does this sound right to other 22 Cruiser owners with a similar load and hp?

Otherwise I am thrilled with our boat and can’t wait to get back on the water and do some serious exploring.

You are overworking the 1 motor, prop is taking too big a bite with that load. That load with 1 - 50 hp motor (as in single main motor) & a smaller pitch prop would be ok. :beer

 

[Sea Wolf]

professional hobbiest-

Your experience with trying to plane a 22 with one motor out of a set of twins is very consistent with previous discussions and experiences.

It takes at least 60 hp to plane the boat with a single motor of a set of twins, and the boat loading, weight wise, makes this marginal.

Quite a few folks have performed the same experiment you did, and found a single 40, 45, or 50 hp motor of a set of twins will not plane a 22.

Remove the other motor's weight, and the boat planes on 60 hp, maybe even a little less with one person and nothing much otherwise in it.

A pair of 60's wouldn't be too far off the new hp max. rating of 115, but probably not worth the extra weight and expense for most folks, and would be overpowered by the manufacturer's recommendations.

Also, as I remember it, a pair of 60's would weigh considerably more than a pair of 50's.

Joe. :teeth

 

[DaveS]

professional hobbiest-
Your experience with trying to plane a 22 with one motor out of a set of twins is very consistent with previous discussions and experiences.
Joe. :teeth

P.H.
Yep, your test is very consistent as stated by Joe. I have twin Johnsons (aka Suzukis) = same results.

 

[El and Bill]

We also confirm everything Joe said. Our second engine (40hp) we see simply as an integral part of our power system of twins -- and as a backup, to get us home with reasonable speed should there be a failure.

 

[forrest]

9 mph does seem low to me. With four people and gear, my 15 hp kicker with the 200hp main in the down position will push a 26' CC at 7.5 mph. Thats WOT though.
Forrest

 

[Not For Hire]

I think your numbers are good. It is true the second main will not push the boat much faster than a kicker. However, it will push it some faster than a kicker, will push it at lower rpm, and will push it with better manuverability in wind and waves, and from the helm (altho some kickers are set up that way). The single kicker at hull speed (6mph or less) is very economical and good for trolling above 1.5 mph. Some have experimented with getting on plane with both engines and then dropping one and have been able to stay on plane. That is just a fun experiment and not much use in the real world.

A single 50 mounted amidship would plane a lightly loaded 22.

Regards, Mark

 

[thataway]

I agree that probably with a lower pitch prop, that a 50 would plane a very lightly loaded 22. But it would not be worth carrying two extra props--and you would probably not have that "light" a boat.

The problem is that when you go over 2 x sq rt LWL, you are just starting to climb over the bow wave--which is comming up on a plane. That takes a lot more HP than going 1.5 x sq rt LWL which is 6.36 knots. Making an assumption that the LWL of a CD 22 is 18 feet, the 1.5 x sq rt=6.36 knots.
You are going close to 1.7 x sq rt LWL for 7.3 knots--and 9 knots is about 2.1 x sq rt LWL--but not yet planing--and is lugging the motor at 4400 RPM WOT.

A 26 foot CC will have a longer LWL than a C Dory--and 7.5 mph is 6.5 knots--so that is about right. With the low power you will probably be at 1.4 x sq rt LWL...

 

[forrest]

Bob,
Huh? The square root of what? All I'm saying is why would you ever want to go with twins if that's all the better one can do alone.
Forrest

 

[JamesTXSD]

Bob,
Huh? The square root of what? All I'm saying is why would you ever want to go with twins if that's all the better one can do alone.
Forrest

Hi Forrest,

LWL = length at the water line. So, you take the square root of that length times 1.34 to determine hull speed on a displacement hull.

Regarding the twins vs single - well, that debate will always go on here. We seem to be about evenly divided... and that's part of versatility of the boat: your money, your choice.

Best wishes,
Jim B.

 

[thataway]

Forrest--There are some empirical formulas which predict the hull speed of boats at displacement speed. These are frequently used to determine how much power is necessary.

The Square root of the Length water line, is the basis of these formulas. At 1 x sq rt LWL (Length Water Line--assumption is that it is 18 feet in the CD 22) the speed will be 4.24 knots. This is a minumal HP speed--and probaly can be achieved with 4 to 5 hp. An effecient speed is 1.2 x sq rt LWL or in the CD 22 is 5 knots. The max speed which is achieved with now power is generally accepted to be 1.34 x sq rt LWL, or 5.7 knots. Once you drive the boat faster than that, you are going into semi displacement speed. Any speed from 1.4 to somewhere in the mid 2's is semi displacement, and relitatively ineffecient. The C Dory 22 begins to plane in the lower range--at about 2.5 x sq rt LWL, or about at about 10.6 knots.

There a few books I recommend that power boaters own. One of these is "The Nature of Boats, by David Gerr NA. He gives the lower range of planing at 2.5 to 2.7 x sq rt LWL and this is for flat bottom boats, like the C Dory. Deep V's will be closer to 3.0 x sq. RT. LWL. Or in this case up around 15 knots.

Why use these numbers and formula? It helps to explain why the 50 hp is not enough to get the boat up on a plane--and why the 9 knots is not up to the planing speed.

The question why go with twins? Well, I am of the group, that feels that a single with a kicker is the best choice. But, a kicker may not be enough power to push the boat as fast as you like into wind and waves. So some folks say that they would rather use the twins, which will give them the 8 knots and enough reserve to push against the wind and ways, which you might occasionally find. This is a justified reason for twins. There are some boats which have big enough twins to get up on a plane. But the Tom Cat is not one of thse, nor is the CD 22--when loaded.

 

[forrest]

All math aside, I stand by my original query; If a 50 hp twin will push a 22' with the other motor up, 9 mph and a 15 hp will push a 26', 7.5 mph with the big motor down, why would you ever want a twin? That 7.5 mph was both in Puget Sound during breakin as well as out in the Pacific at Westport crossing the bar in a test of its capacity to control the boat.
Forrest

 

[hardee]

Thanks Dr. Bob, for that explanation.

"But, a kicker may not be enough power to push the boat as fast as you like into wind and waves. So some folks say that they would rather use the twins, which will give them the 8 knots and enough reserve to push against the wind and ways, which you might occasionally find. This is a justified reason for twins."

I knew there was some reason I liked my twin 40's :hug :hug2 besides that they really help in slow speed, tight quarters manoeuvering :thup :thup . And sometimes it is just fun to sit in one place and spin like a top :roll: 8) .

Harvey
SleepyC :moon

 

[Professional Hobbyist]

All math aside, I stand by my original query; If a 50 hp twin will push a 22' with the other motor up, 9 mph and a 15 hp will push a 26', 7.5 mph with the big motor down, why would you ever want a twin? That 7.5 mph was both in Puget Sound during breakin as well as out in the Pacific at Westport crossing the bar in a test of its capacity to control the boat.
Forrest

To my thinking there are two basic reasons to have two engines on your boat, secondary purpose, and safety. The use of a single large propulsion engine and a smaller kicker engine is often used when secondary propulsion is the primary concern. The kicker is most often used as a trolling engine, but also adds a measure of safety when the propulsion engine is inoperable. Matched twin engines are used when safety is the primary concern.

Your assumption that there is little difference between a 50 hp engine and a 15 hp engine based on top displacement speed is incorrect. A 40 to 50 hp engine and a 10 to 15 hp kicker engine are not equal even when operating at displacement speeds. The speed difference between the two sizes may be similar when operating at displacement speeds, but the thrust is dramatically different.

Neither are a C-Dory 22 and a CC 26 equal or suitable for comparison of displacement speeds. The CC 26 is longer, has a longer water line and a higher displacement hull speed. The longer the water line length the higher the displacement speed. We would need to know what speed a 10 or 15 hp engine pushes a C-Dory 22 to make an accurate comparison, but the difference will still be small.

It appears that we can not carry an auxiliary engine large enough to plane a C-Dory 22 with a normal cruising load, but a matched pair of mid sized engines still gives the best safety margin for an engine out situation. The 40 to 50 hp single gives the boat a slightly higher top displacement speed and more maneuvering power than a 10 to 15 hp kicker engine. The mid sized engine also trolls nicely at idle.

I plan to experiment with the idea of a smaller pitch prop which might allow one 50 hp engine to develop maximum rpm/hp and plane the boat. I would consider carrying this auxiliary prop to use in the unlikelihood of an engine out situation while cruising. If I were reduced to one engine I could change props and get home on one engine at planning speeds.

Does anyone have any thoughts on the necessary pitch required to get a single Suzuki 50 hp engine up to max rpm when mounted on a fully loaded C-Dory 22?

 

[Captains Cat]

"Professional Hobbyist
Your assumption that there is little difference between a 50 hp engine and a 15 hp engine based on top displacement speed is incorrect. A 40 to 50 hp engine and a 10 to 15 hp kicker engine are not equal even when operating at displacement speeds. The speed difference between the two sizes may be similar when operating at displacement speeds, but the thrust is dramatically different. "

The 40 to 50 hp single gives the boat a slightly higher top displacement speed and more maneuvering power than a 10 to 15 hp kicker engine.

Beg to differ. Displacement speed doesn't vary with the size of the engine, it's dependent on the shape/length of the hull (primarily length). It is very close to 1.33 X the length of the waterline. (In the case of a CD 22, if the LWL is 20 feet, that speed is about 6 knots or so) That is where the hull begins to break free of the wave caused by the bow. Then it starts to come "up" on plane. You could have a 115 on there or a 30 and the displacement speed would be the same. If the engine still has some power/thrust left when you get there, adding more power will propel it "up", or ahead of that wave and it comes out of the water and starts to plane.

I'm sure Thataway will chime in too!

 

[thataway]

The pitch of the props you would want would be that which will allow the single engine to achieve rated Wide open throttle. I would want a prop with the pitch which would give about 6000 RPM. If you have both right rotation engines, then one prop, would be satisfactory. If you have counter rotation engines, then you want two props; one for each rotation.
Since the origional poster was getting 4400 PRM, He could drop the pitch of the prop considerably--at least by 4", and maybe 6"--but then you want a "big foot" prop, to utalize a larger diameter prop, which will be more effecient.

There is no real "magic" number for displacement--but the 1.34 is usually the number touted for a real full displacement boat. One that will not go any faster no matter how much HP you apply. At least one of my sailboats quallified in that category--it would go aboout 9.5 knots at WOT--I had 90 hp, some of the sisterships had 130 hp--they all would go 9.5 knots max. This was because this was a double ender under the water, and there was no lifting area in the boats aft buttocks area (nautical term).
As you applied more hp, the boat dug a bigger hole in the water, but went no faster.

The C Dory is a different type of boat--since it has an essentially flat bottom, with a little rocker, it has a lot of lift, and will run at a semi displacement speed, right on up to a planing speed, with a modest amount of HP. Now, if you were able to develope the max hp (and torque) for the 50 hp engine/prop combo, it is entirely possiable that you could to 11 knots--or maybe 12 knots--but do you really want to run that engine at WOT, or near it, when the other engine has failed?


I had done a comparison of the Tom cat 24 and the Glacier Bay a couple of years ago--and Georgs reminded me of in a private post, unrelated to this thread. I had noted that the Glacier bay (a semidispacement cat), would go up to about 15 knots, and the Tom Cat (a planing cat) would not go beyond 9 knots. There are boats which are built to be semidisplacement boats--for example the Rosborough 246--and it probably will go faster with a single 50 than the CD 25 will (I haven't checked the exact speeds, but I remember some posts from the Rosie list suggesting that). It has a different hull form.

Keep in mind--that your 9.9 may drive that boat in flat calm water at 7.5 mph, but it will not in winds or head seas--were as the 50 will keep pushing the boat at 9 mph in head seas and wind. I am personally willing to accept the lower speeds with a kicker, but some people want the greater margin of safety.

One also has to analize the failure of modern outboards. Most likely it will be fuel related. If one engine is getting bad fuel--what is the chance that the second engine is? For that reason, I like to have a separate tank for the kicker. The other more common failure, is that the batteries run down--and then both 50 hp are out--I have never hand started a 50--but I used to hand start a 40 (before electric start was standard)--so it is possiable--at least when you are younger. Also the isolated back up battery is a very good idea, so that if you run down both batteries, you can still start the engine.

 

[iggy]

All these isues have been discussed countless times, of course, but they of such interest it seems worthwhile revisiting them often.

Dr. Bob describes the redundancy of fuel supplies and batteries that would minimize both large main engines failing simultaneously. However, one advantage of a large single and kicker that has not been mentioned (in this thread) is the following.

With both mains in the water at the same time, there is a reasonably good possibility of hitting a submerged object and taking out both props, or both engines at the same time. I suspect the probability would be significantly greater if one encountered a submerged line, or worse yet, a fishing net.

With a single large main out of commission you would have the option of lowering the kicker and proceeding (slowly) to safety. In other words, additional 'redundancy' is maintained by traveling with your second engine out of the water completely (and on a separate fuel supply, and separate battery). Granted, the kicker would have less oomph than a remaining 40 hp twin main if it is still functional, but as indicated, you really are limited to hull speed either way.

The debate continues . . .

e.g.

 

[Hunkydory]

With both mains in the water at the same time, there is a reasonably good possibility of hitting a submerged object and taking out both props, or both engines at the same time. I suspect the probability would be significantly greater if one encountered a submerged line, or worse yet, a fishing net.

e.g.

Very good point e.g.

An acquaintance of ours we met on Yellowstone Lake who has a duel propped approximately 30 ft. Rennell lost both props and suffered other damage when striking a log on the Inland Passage near Ketchikan and had to be towed in for repairs. We have many times with both motors down trapped debris between, that caused us to stop and raise one motor to release. With one motor the debris would have just slid around mostly unnoticed unless going to fast or the debris to big.

Our duel set up has served us well, but if buying used again a single with a kicker would not stop the deal. The many pros and cons of both set ups have been thoroughly discussed, so there is good information for those yet to commit or thinking about a refit.

Jay

 

[Professional Hobbyist]

"Professional Hobbyist
Your assumption that there is little difference between a 50 hp engine and a 15 hp engine based on top displacement speed is incorrect. A 40 to 50 hp engine and a 10 to 15 hp kicker engine are not equal even when operating at displacement speeds. The speed difference between the two sizes may be similar when operating at displacement speeds, but the thrust is dramatically different. "

The 40 to 50 hp single gives the boat a slightly higher top displacement speed and more maneuvering power than a 10 to 15 hp kicker engine.

Beg to differ. Displacement speed doesn't vary with the size of the engine, it's dependent on the shape/length of the hull (primarily length). It is very close to 1.33 X the length of the waterline. (In the case of a CD 22, if the LWL is 20 feet, that speed is about 6 knots or so) That is where the hull begins to break free of the wave caused by the bow. Then it starts to come "up" on plane. You could have a 115 on there or a 30 and the displacement speed would be the same. If the engine still has some power/thrust left when you get there, adding more power will propel it "up", or ahead of that wave and it comes out of the water and starts to plane.

I'm sure Thataway will chime in too!

Don't get confused with book learning. The theoretical hull speed of a displacement hull design is estimated by a mathematical formula. The C-Dory 22 is not a displacement hull, it's a planning hull. It can be propelled at any number of speeds, some displacement speeds, prior to planning.

The issue here is how fast can a 15 or 50 hp engine propel the C-Dory 22? Neither can propel it over the bow wave into planning speeds, but the 50 hp engine can propel it faster, maybe into semi-planning speeds. The 15 hp engine might not get the boat up to a theoretical hull speed.

 

[thataway]

Iggy's point is valid--and a very good reason to carry: at least one spare prop for each engine, a good serrated knife and a hack saw (to cut nets or poly prop lines).