OK I am still confused on the prop/engine

[rainger]

Generally when a twin is run as a single, it will be "over propped"--that is lugging at WOT. The reason is, that when the boat is at WOT with twins, it will be on a plane.

If you propped the 40 hp engine correctly for a single, it would have a lower pitch prop, than when propped for twins (and planing).

Back to rainger's C-Hawk. Another issue is that the boat has bottom paint and enough marine growth to slow the boat down. Another reason to do nothing with the props, until the bottom is cleaned, old paint sanded smooth, or stripped, and a nice smooth coat of new paint applied. Painting may slow the boat a knot or two at the high end, but usually not enough to be an issue.

Also looking the photos of C-hawk, the props have some of the paint on the tips missing. I cannot see if there are any dings. I carry a hammer and coarse metal file, " just in case". It might pay to have these props scanned and trued--or you may not feel it is worth the trouble, if the blades are smooth, and no obvious sign of damage. Although it may not be as critical in the lower horsepower outboards, a scan will be sure the blades are properly pitched, and are identical, as well as balanced.

Yea I was taking into account the bottom. I have since pressure washed her and tomorrow she goes up on blocks and will be sanded and a new coat of Micron CSC is being applied.

The props have no dings just missing paint but I will be sure to have them looked at.

 

[C-Hawk]

WHAT :shock: another C-Hawk :?: How can that be?
I had twin 40s on my first boat (Fishtales)- good power and great economy. When I got my current boat (C-Hawk) I installed a single 90 with a 9.9 kicker. The 90 was a dog - no power and a heavy drinker of fuel. After 4 years of arguing with Honda, I finally told them to sell the 90 and installed twin 50s - yes, back to good power and great economy.
I hope you are able to figure out the best combination engine/prop and get out there and enjoy your boat.

 

[rainger]

WHAT :shock: another C-Hawk :?: How can that be?
I had twin 40s on my first boat (Fishtales)- good power and great economy. When I got my current boat (C-Hawk) I installed a single 90 with a 9.9 kicker. The 90 was a dog - no power and a heavy drinker of fuel. After 4 years of arguing with Honda, I finally told them to sell the 90 and installed twin 50s - yes, back to good power and great economy.
I hope you are able to figure out the best combination engine/prop and get out there and enjoy your boat.

Hey thanks. Yes I did look at your boat but I had to name her C Hawk anyways.

Like you I am a die hard Hawk fan. In one of my other threads you will see my Seahawk tattoo commemorating the super bowl win. BTW the Lombardi trophy and some Hawk players are here in town today. If it is not too crowded I am going to try to get in to see and take a picture of the trophy.

I also had a sailboat in the 80s I named Seahawk. Thank you for allowing me to share the name with you.

 

[Sunbeam]

Generally when a twin is run as a single, it will be "over propped"--that is lugging at WOT. The reason is, that when the boat is at WOT with twins, it will be on a plane.

If you propped the 40 hp engine correctly for a single, it would have a lower pitch prop, than when propped for twins (and planing).

Okay, that is the way I have always understood it. So: Run boat to WOT/planing with both engines running and prop so that the RPM attained in this situation is what is called for (probably something around 5,600 - 6,000 optimally, although I don't have the manual for that specific engine to hand). Then the engine won't run as efficiently as a single, but you have to choose one or the other, and if you wanted a single... :wink

 

[rjmcnabb]

I'm in the same boat as Sunbeam, I get fastest and smoothest ride with the single 50 trimmed all the way in. The wake is far smoother and well sculpted than when trimmed out, even just a little. Forward loading is light, and things may change as I accumulate cruising gear up there.

My real question: on a correctly propped motor, can you reach proper RPMs at WOT if the boat is not moving?

I would be surprised if you can, just as I am surprised that a single 40 would be given the same prop as each of a pair, if twinned. Likewise, I can't see how a single 40 propped with a 14 is equivalent to a pair of 40s spinning 7s.

I have these doubts because I can't help but think that the hydrodynamics of a prop must change as forward motion increases. Am I out to lunch, here?
Rod

 

[Sunbeam]

I'm in the same boat as Sunbeam, I get fastest and smoothest ride with the single 50 trimmed all the way in.

Hey, at least we're not alone! I too plan to tweak things and record data, as I suspect that when things are better set up one should not get the best efficiency when trimmed all the way down/in. Otherwise why even have a range of adjustment? I have seen where some riggers put a wedge on the C-Dory transom to effectively "start things" further out/up, but then most C-Dorys don't seem to have (or need?) this.

My real question: on a correctly propped motor, can you reach proper RPMs at WOT if the boat is not moving?

I wouldn't think you should rev like that not under load (i.e. in neutral), but maybe you are thinking tied to the dock and in gear (hence the dock lines are the load?). I've only tested my boat (and bigger inboards) while actually out on sea trial.

Sunbeam

 

[AstoriaDave]

rjmcnabb asked: My real question: on a correctly propped motor, can you reach proper RPMs at WOT if the boat is not moving?

No. Props "unload" as the boat accelerates, because the water the back of the prop pushes against is moving away from it, so that the prop has an easier time than if the boat is stationary in the water. This why folks here insist that you do your testing with the boat loaded as you expect to use it, and swap out props, looking for one that reaches the accepted WOT figure for best use of the engine's power capability.

The suggested pitch and diameter figures are a ballpark guess, because propellers differ, some being more cupped than others, and others having slightly different foils. It's trial and error. But, all is not lost, because you need at least one spare anyway, and if you load the boat more heavily one day, you might need a slightly different pitch to reach WOT.[/b]

 

[thataway]

I'm in the same boat Very Happy as Sunbeam, I get fastest and smoothest ride with the single 50 trimmed all the way in. The wake is far smoother and well sculpted than when trimmed out, even just a little. Forward loading is light, and things may change as I accumulate cruising gear up there.

Yes, some folks like to run bow down into chop--you will not get the best fuel efficiency--and that is your choice. I have found that the bow down does not give the fastest speed, nor WOT in many boats. Bow trimmed down into chop--good, when cross seas or down seas, not so good. The 16's may ride differently than the 22's

My real question: on a correctly propped motor, can you reach proper RPMs at WOT if the boat is not moving?

You probably will not be able to reach full WOT in a boat that is not moving. This is why special wheels are made to run the engine up when in a test tank. You will have a lot of prop slip if the boat is not planing, and running a single. You want minimal prop slip! The outboard prop is easily subject to slip--ventillation and other inefficiencies by prop position related to surface of water and the hull. The effects of over propping and running at high speed with an outboard are not immediately as obvious as in with larger inboards and large props, because of the amount of slip fairly easily developed by an outboard.

Larger boats, especially inboards will demonstrate the differences in props far more than outboards, where the props are limited in diameter by the size of the clearance in the foot. For example in my large cruising boats I might swing a 26" by 20" inch prop, with a 3:1 transmission with 90 hp. The slip will be low, and the speeds will be low. VS a 13" x 15" prop in the C Dory. You also have to be a bit careful, since different outboards are geared differently in the lower unit. For instance the 150 Suzuki will be running a 20" prop when 150 Honda may be running a 15" prop (with a slightly smaller diameter) , and the speeds will be about the same.

Tug boats are propped to achieve max pull at a very low speed. Large props, large reduction ration. Some boats have controllable pitch props to get the most efficient use of the prop at all speeds.

I would be surprised if you can, just as I am surprised that a single 40 would be given the same prop as each of a pair, if twinned. Likewise, I can't see how a single 40 propped with a 14 is equivalent to a pair of 40s spinning 7s.

Of course not. No one has claimed that cutting the pitch in half is correct. In fact, the difference in pitch is no where half. But remember what is said about planing, vs displacement speeds, and the "work" against hull resistance that is involved in each mode.

I have these doubts because I can't help but think that the hydrodynamics of a prop must change as forward motion increases. Am I out to lunch, here?

See above. The dynamics of a hull in displacement mode vs planing mode, or semi displacement speeds are going to be different. As for the hydrodynamics of props--I refer you to two books by David Gerr:
The Nature of Boats: Insights and Esoterica for the Nautically Obsessed, and The Propeller Handbook: The Complete Reference for Choosing, Installing, and Understanding Boat Propellers. Both are excellent reads. I strongly recommend owning both books, if you are interested in the dynamics of boats mooving thru the water. The hydrodynamics of a prop which is slipping 5% is far different than a prop which is slipping 40% (100% if the boat is not moving).
The outboard prop in a C Dory should be somewhere about 10% to 12% slip. You may be surprised by the decrease of slip and increase of speed with decreasing the pitch.

 

[Sunbeam]

I'm in the same boat Very Happy as Sunbeam, I get fastest and smoothest ride with the single 50 trimmed all the way in. The wake is far smoother and well sculpted than when trimmed out, even just a little. Forward loading is light, and things may change as I accumulate cruising gear up there.

Yes, some folks like to run bow down into chop--you will not get the best fuel efficiency--and that is your choice.

I can't speak for rjmcnabb, but on my boat, running with engine trimmed all the way down/in is the most efficient. But, it is not particularly bow down (to make it go bow down, which I do in chop, I use the trim tabs). What we (at least I think it's both of us) are talking about is running with the engine trimmed all the way down/in -- but the boat not (necessarily) bow down.

In other words, I'm going along, bow is not "down" but motor IS trimmed all the way down/in. I check my speed and RPM. Now, if I trim the motor up at all, I lose speed, and the engine sound gets noisier. If I trim it back to all the way down/in, I gain speed and the boat gets quieter. If I want to put the bow down I use the trim tabs, but again, the best setting for the engine seems to be all the way down/in. There is never any "better" setting for speed vs. a given RPM than motor trimmed all the way down/on (regardless of trim tab settings). I mean, at speed and not idling along, of course.

(If I want to get the bow down, say for chop, then I trim it that way with the trim tabs. But still with the engine all the way down/in for best efficiency (as gauged by speed for a given RPM.)

Now, I'm not saying this is how it should be, in a perfect world. Because when there is a range of adjustment on something, to me it's never right to always have it all the way to the end of the range (or why have a range?). My suspicion is that as currently mounted, the Permatrim is further beneath the surface of the water than it should be. Once I get that up closer to the surface I'm guessing that engine trim will once again be useful; but if not then I would consider a transom wedge.

Does this go along with what you find, rjmcnabb? In other words, running with the engine trimmed all the way down/in is not only the most efficient, but doesn't necessarily mean your boat is "bown down"?

 

[rjmcnabb]

Aye, Sunbeam - I believe we are on the same page; perhaps we should say we (you and me, anyway) are trimming for "stern up," not for "bow down." Our boats seem to differ: 16 vs. 22, I have no tabs, and a set of Doelfins ( two triangular foils that are not in the prop wash, as is your Permatrim.) What I find, running in smooth water, is that I can start with middle trim (on the trim display), set the RPMs to get me the speed I want, and let her settle down. The bow wave will come back somewhere under the v-berth (hard to be sure where, but judging by my feet and the view out the pilot's window,) and the wake/wash will bubble up close to the stern and stay close in. The kicker leg will be heavily dunked. Being alone, I cannot tell how deep are the Doelfins. When I bring the trim in, the bow wave cleans up but doesn't move a lot; the wake moves way astern, the quarter waves appear and move out, the leg of the kicker comes out of the water, and the din diminishes. At modest speeds, GPS determines that speed across the bottom increases 4-5 kt (say, from 15 to 19 kt) without any change in RPM. I take all these data to indicate that the stern is rising without depressing the bow.
I hope this is the running attitude that I have admired in some other small boats - very level, small bow wave, absolutely no rooster-tail, minimal wash, and apparently going like a bolt from hell. Still the kid in me.
Next comes trials with higher speed so I can get near WOT, but not scare the juice out of me. And then some longer distance runs to check fuel consumption. I have to weigh my tanks to do this, but limited tests last year gave me 2-4 lb/hr under a variety of conditions. Some of that might be getting up to some pretty good mileage (10-15 nm/gal.) :shock: I am dreaming on!
Carry on, Sunbeam.
Rod

 

[thataway]

What sort of fuel flow meters are you all using to measure efficiency? When I note efficiency, this implies better mileage. (As measured with fuel flow meters). Not sure how to measure efficiency unless you are able to document fuel usage at a specific speed--not as per a tank full, which does not measure at specific speeds.

 

[colbysmith]

I actually have a fuel flow meter on my single, as part of the Navman Unit. However, it's almost useless as the 115 4-stroke EFI must work in a way that cycles fuel into it's own system, as my meter will fluctuate at stable RPMs... But I think over time, just using gallons per hour gives one a pretty good idea how they are doing. And then maybe tracking miles per hour...
Colby

 

[Sunbeam]

What sort of fuel flow meters are you all using to measure efficiency? When I note efficiency, this implies better mileage. (As measured with fuel flow meters). Not sure how to measure efficiency unless you are able to document fuel usage at a specific speed--not as per a tank full, which does not measure at specific speeds.

Well maybe I'm all washed up as I am just going by sound of the boat, feel of the boat, and speed/RPM. Here is what occurs:

1) I get up on plane and am cruising along. The bow is not down; in fact it is "up" unless or until I use the trim tabs to trim it down (which I do in chop).

2) The engine is trimmed all the way down/in.

3) I am going at X speed (let's just say 25mph for the sake of example) and X RPM.

4) I decide to try trimming the engine up/out, as I know I'm "supposed" to be able to, and I want to see what happens.

5) What happens is that the boat/engine get noisier, and my speed goes down. I have not touched the throttle.

I gather from this (increased "fussier water" sounds from the engine, reduced speed) that it's less efficient. I don't have a fuel flow meter and actually the fuel consumption isn't my major focus (although of course it's nice to reduce it). I was just speaking of efficiency/pleasantness/speed of the boat running. Essentially, the boat runs "happier" with the motor trimmed all the way down/in, and if I trim up/out at at all, it gets less happy (noisier, slower, etc.).

 

[thataway]

Glad you have a "Happy" boat Sunbeam…!

Is your best WOT speed with engine trimmed down all of the way ?

Colby, with engines which recycle fuel you have to use a Flow Scan, which will measure fuel in and fuel returned (as also in diesels). With an E Tec, you should be able to get fuel burn off the NMEA 2000 computer output.

 

[Sea Wolf]

What sort of fuel flow meters are you all using to measure efficiency? When I note efficiency, this implies better mileage. (As measured with fuel flow meters). Not sure how to measure efficiency unless you are able to document fuel usage at a specific speed--not as per a tank full, which does not measure at specific speeds.

Well maybe I'm all washed up as I am just going by sound of the boat, feel of the boat, and speed/RPM. Here is what occurs:

1) I get up on plane and am cruising along. The bow is not down; in fact it is "up" unless or until I use the trim tabs to trim it down (which I do in chop).

2) The engine is trimmed all the way down/in.

3) I am going at X speed (let's just say 25mph for the sake of example) and X RPM.

4) I decide to try trimming the engine up/out, as I know I'm "supposed" to be able to, and I want to see what happens.

5) What happens is that the boat/engine get noisier, and my speed goes down. I have not touched the throttle.

I gather from this (increased "fussier water" sounds from the engine, reduced speed) that it's less efficient. I don't have a fuel flow meter and actually the fuel consumption isn't my major focus (although of course it's nice to reduce it). I was just speaking of efficiency/pleasantness/speed of the boat running. Essentially, the boat runs "happier" with the motor trimmed all the way down/in, and if I trim up/out at at all, it gets less happy (noisier, slower, etc.).

Sounds like you're ventilating!

With the engine trimmed down (forward), the boat thrust lifts the stern, but also keeps the anti-ventilation plate under the water flow off the stern of the hull.

With the motor tilted up (backward), the bow rises excessively, the water flows downward as it leaves the back of the stern from the pitched-up hull, and the anti-ventilation plate catches some air at the front, spins faster and more noisily (but not enough to raise the rpm over the redline excessively), you lose some of the thrust with the air mixed in, and the boat slows down.

I'd think this could be because the engine is mounted too high, with the anti-ventilation plate's high position being the real culprit.

This may or may not be right, but it's what I'd guess from your description.

Joe. :teeth :thup

 

[Sunbeam]

Glad you have a "Happy" boat Sunbeam…!

Is your best WOT speed with engine trimmed down all of the way ?

Yes it is. If I trim the engine up at all from "full down," my WOT speed is reduced.

 

[thataway]

Then I would suspect that there is an issue with the way the motor is mounted. My experience is that motor trimmed all of the way down is not going to get the highest RPM for WOT. (Specifically in two C Dory 22s and one C Dory 25--as well as a number of other small relatively fast boats). If your Permatrim is not running on the surface (which I believe you said it was not), then the motor is lower than ideal.

 

[Sunbeam]

Sounds like you're ventilating!

I'd think this could be because the engine is mounted too high, with the anti-ventilation plate's high position being the real culprit.

Actually, I believe the engine is set a bit too low. I say this only because, in another discussion here, someone posted links to photos showing how the Permatrim/anti-ventilation plate is supposed to ride (nearly at the surface of the water that you can see). I happened to just have taken the same angle of photo of my engine when running along at speed. My plate is much lower than they show. Also, Roy of "Roy & Dixie" has the same engine as me, in the same position, and found the same thing. He raised his engine two holes and found it overall improved (not sure about how he trims the engine though).

Too bad I didn't check it out more before adding the Permatrim, just for data. I only ran it that way on the initial sea trial though.

But, I'll experiment - starting with moving the engine up a couple of holes like Roy did - and see what happens. It would be nice to be able to make some use of the trim feature on the engine. Seems like if it is there, I should be able to put it to some good use.

 

[T.R. Bauer]

Since I have had twins (honda 45s) for around 2000 hours of usage, I can tell you I can travel with ease at 20-24 mph when loaded and turn around 4800-5000 rpms. My max rpm is really close to 6000 rpms and moves the boat a hair or two over 30 mph. Just a couple of days ago, the boat had 3 heavy adults and 2 kids on board and we came back with 200 pounds of halibut in the fish box I made for the splash well and still had no trouble going 23-25 mph on flat water. I could have even gone faster without trouble, but don't like going past 5000 rpms in general. Unless you are operating your boat at elevation or something, your performance numbers are way off as we are only talking about 10 hp (I think).

 

[Sea Wolf]

Sunbeam-

Sounds good to me!

Give it a try!

Joe. :teeth :thup