The C-Brats Forum Index
HomeForumsMy TopicsCalendarEvent SignupsMemberlistOur C-DorysThe Brat MapPhotos

Propping Twins -- for single or twin power
Goto page 1, 2  Next
 
Post new topic   Reply to topic    The C-Brats Forum Index -> Propellers
View previous topic :: View next topic  
Author Message
hardee



Joined: 30 Oct 2006
Posts: 12632
City/Region: Sequim
State or Province: WA
C-Dory Year: 2005
C-Dory Model: 22 Cruiser
Vessel Name: Sleepy-C
Photos: SleepyC
PostPosted: Fri Aug 10, 2012 4:19 pm    Post subject: Propping Twins -- for single or twin power Reply with quote

This discussion came up in another thread and it is a totally new concept to me so I am moving this from another thread. I am curious about this because it my understanding was that WOT was WOT whether it was a single or half of a twin set up.

Quote:
Quote:
hardee wrote:
I didn't know that there is a difference between a prop and a "twin prop". I thought they were propped for the size of motor (hp) and the max RPM attained at wide open throttle (WOT). Can't really see how my twin 40's could be propped for a single, or in the reprop process, there was nothing mentioned about "are this running as a single on the boat or are they working as one of a pair of twins".



I'm not a pro, but the way I understand propping motors, there would be a difference. I may not be able to explain it correctly (or may not even be correct), but here is what I believe:

Any engine has a specified WOT RPM. If you can attain that RPM at WOT, then by definition you are running the engine at levels that will not "lug" the engine (which is bad for it). Instead you are running at a certain percentage of the available horsepower (think about running your car at 30 mph in 5th gear vs. doing the same in 2nd gear; I think running a twin at high speed alone on a "twin" prop would be more like the former).

If you are pushing a light boat (or have a large engine for the boat size), then you would use "X" prop to power it (determined by going to WOT and figuring out which prop gets you to the mfgr. stated top RPM). If you are pushing a heavy boat (or have a small engine for the boat size), then a different prop would get you to that sweet spot. By definition, the same prop would not really do it for both extremes.

A "small" twin (small because it is only half of your planned horsepower) would actually be propped as half of a team. That is, the prop is only able to turn the engine up to the proper RPM at WOT if it is operating in tandem with its "other half." If it's operating alone, but with the prop that makes it work properly as a set of twins, you would not be able to reach the proper RPM at WOT, and would thus be lugging the engine/running inefficiently. My guess is that if you take your boat out and run both twins together, you will get your mfgr-specified WOT at top speed, but that if you try the same thing with one twin alone, you won't. Probably both engines were run together to determine which props would work best for your setup.

OTOH, if you were propping the boat planning to run with a single 40 or 50 hp engine, I think you would choose a different prop (because you would be testing for the correct prop by running up to WOT with just the one engine, and going for the correct RPM that way). You would not get the same top speed (because, 40 hp can't do as much as the 80 hp you have with twin 40's), but you would probably get a higher speed, and more importantly, not be lugging the engine.

So my guess is that if one were going to use a single "twin" to get home in "any weather/tide/current condition,' that you might want to change to a "single" prop. Because I think (but don't know for sure) that if lugging the engine you would also not be getting the best power out of it, which you might well need in bad conditions (you would also be lugging, but if it's an emergency you probably would not "care" about hurting the engine).

Okay, now the people who really know can come in and point out where I'm mistaken (please do!).

Sunbeam


Brings up a good question. If... (Like that is a question Shocked ), I am running twins, do I need to prop them different somehow? Generally they are both in the water and running. Occasionally (skinny water, or extended slow cruising), I will run one only and the other one up and out of the water. Extremely rarely, would I ever be in a marina area or any docking situation without both down and running.

They are aluminum, 3 blade, 12.25" X 9", on 40 Yamiha's. These were new the end of last season, and seem to be an improvement over the ProPulse 4 blade that were there for the first 4 years.

So What is the real scoop on propping twins? (And this is not a question of should I be running a single or twins.) That decision has already been made.

Thanks for any help in understanding this concept.

Harvey
SleepyC Moon


_________________
Though in our sleep we are not conscious of our activity or surroundings, we should not, in our wakefulness, be unconscious of our sleep.
Back to top
View user's profile Send private message
Sunbeam



Joined: 23 Feb 2012
Posts: 3990
City/Region: Out 'n' About
State or Province: Other
C-Dory Year: 2002
C-Dory Model: 22 Cruiser
Photos: Sunbeam
PostPosted: Fri Aug 10, 2012 4:32 pm    Post subject: Reply with quote

I realize I may be repeating what was already said (and I'm not a mechanic either), but you are right in that an engine (whether twin or single) has a mfgr's recommended WOT RPM, which is the same regardless of whether it is going to be used as a single or half of a twin setup. However, to find that, you run the boat in the planned engine configuration and then prop the engine(s) to reach the proper RPM at WOT (then you can run them however you want after that, likely not at WOT very often).

So with twins, as I understand it, you would run them *both together* (on the boat) up to WOT and then prop them so that in the pair, running together (i.e. with the help of the "other twin"), the engines each reach their recommended RPM at WOT.

Given this, I don't see how one of the pair, alone, with the same prop that fits the above scenario, would reach the recommended RPM at WOT. It would still run, and still propel the boat, but the engine would be lugging and that's not good for it (just like a car).

On the other hand, if one were running, say, one single 40hp engine (not as a set of twins), then you would run that one engine alone up to WOT on the boat, and then adjust the prop as necessary so that you were reaching the recommended RPM (and then run the boat at whatever speeds you want after the test and getting the right prop on it).

So I guess what I'm saying is that it's a combination of the engine and the load that factor in. One 40 hp "twin" is only "half an engine" when you are running it as a planned pair making 80 hp; therefore you run your WOT tests with them both running together (80 hp). One 40 hp single is "a whole engine" and so would be run by itself during the WOT tests to determine the best prop. On the same boat I believe this would result in different props being the "right" one to best achieve the mfgr's recommended WOT RPM. That's why I figure that if one were planning long runs (especially if they would be in challenging conditions, as mentioned in the original thread) on a "single twin" that one might want to carry a prop that is the right one for a single (but of course you can still run with one twin on a "twin prop" - it's not the best for the engine but it's not going to be instant death either - it's just the I don't believe it would be the "best/right prop").

Sorry for any repetition and/or any errors (I'm completely open to correction).

Sunbeam
Back to top
View user's profile Send private message
Sunbeam



Joined: 23 Feb 2012
Posts: 3990
City/Region: Out 'n' About
State or Province: Other
C-Dory Year: 2002
C-Dory Model: 22 Cruiser
Photos: Sunbeam
PostPosted: Fri Aug 10, 2012 5:09 pm    Post subject: Reply with quote

hardee wrote:
If I am running twins, do I need to prop them different somehow?


Should have had this in my reply above: If it were me I would take the boat out with as typical a load as possible, run both engines together up to WOT, and see what RPM they are making. If the RPM is within the mfgr's recommended range for WOT, then the props are correct. I believe this is how twins would be sea-trialed initially (i.e. before putting on the first props).

I guess my theory from above could be tested by then running one twin engine alone up to WOT, and seeing what the RPM is, and whether it is within the mfgr's recommended range for WOT. My guess is that it wouldn't be - that it would be too low (but that a different prop could make it right).

Sunbeam
Back to top
View user's profile Send private message
Captains Cat



Joined: 03 Nov 2003
Posts: 7313
City/Region: Cod Creek>Potomac River>Chesapeake Bay
State or Province: VA
C-Dory Year: 2005
C-Dory Model: 22 Cruiser
Vessel Name: Captain's Choice II
Photos: Captain's Cat
PostPosted: Fri Aug 10, 2012 5:34 pm    Post subject: Reply with quote

Sunbeam wrote:
.

I believe this is how twins would be sea-trialed initially (i.e. before putting on the first props).


Sunbeam


I have a sneaking suspicion that if you sea trial a boat before you put props on, it won't go very fast at all.... Mr. Green

Charlie

_________________
CHARLIE and PENNY CBRAT #100
Captain's Cat II 2005 22 Cruiser
Thataway (2006 TC255 - Sold Aug 2013)
Captain's Cat (2006 TC255 - Sold January 2012)
Captain's Kitten (1995 CD 16 Angler- Sold June 2010)
Captain's Choice (1994 CD 22 Cruiser- Sold Jun 2007)
Potomac River/Chesapeake Bay
K4KBA
Back to top
View user's profile Send private message Send e-mail
hardee



Joined: 30 Oct 2006
Posts: 12632
City/Region: Sequim
State or Province: WA
C-Dory Year: 2005
C-Dory Model: 22 Cruiser
Vessel Name: Sleepy-C
Photos: SleepyC
PostPosted: Fri Aug 10, 2012 5:46 pm    Post subject: Reply with quote

Sunbeam wrote:
hardee wrote:
If I am running twins, do I need to prop them different somehow?


Should have had this in my reply above: If it were me I would take the boat out with as typical a load as possible, run both engines together up to WOT, and see what RPM they are making. If the RPM is within the mfgr's recommended range for WOT, then the props are correct. I believe this is how twins would be sea-trialed initially (i.e. before putting on the first props).

I guess my theory from above could be tested by then running one twin engine alone up to WOT, and seeing what the RPM is, and whether it is within the mfgr's recommended range for WOT. My guess is that it wouldn't be - that it would be too low (but that a different prop could make it right).

Sunbeam


OK, I'm game. I am going to try this, next time the boat goes into the water. (Maybe as soon as this weekend. Wink and we will see. I'm thinking the WOT is going to be the same in both instances (1 or 2 down) but the top speed is going to be severely different.

Charlie --> Laughing Laughing Laughing You are always right. Cool

Harvey
SleepyC Moon
Back to top
View user's profile Send private message
Grumpy



Joined: 10 Oct 2005
Posts: 1606
City/Region: Whidbey Is
State or Province: WA
Vessel Name: Kingfisher II
Photos: Kingfisher
PostPosted: Fri Aug 10, 2012 6:52 pm    Post subject: Reply with quote

Let me play confucious Twisted Evil

It seems to me the whole point about "propping" twins to both run at manufacturers rated rpm is that this is the best setting for running with twins.

Why are we suggesting that on a twin engined boat, you would want to achieve WOT rpm for any extended period on ONE engine ???

To me the whole point about having (identical) twins is that you can safely get to a port on one engine. Period. That is an exercise that will be done at some reasonable speed determined by need and conditions.

Will you be running at the most efficient setting ? No, but who cares, it is only a temporary measure until you get the other one fixed.

If you were only using one engine and keeping the other one as a spare then it might make sense to prop both for WOT when running individually. (Don't know of anyone who does this)

Which raises the question of how do you prop a non identical twin (aka. Kicker)??

Having fun yet ?

M

_________________
2006 CD-22 Kingfisher Sold Jan 08.
1987 Arima SeaChaser 17, Sea Star. Sold May 2010
2008 RF246 Kingfisher II Sold Apr 2013
Back to top
View user's profile Send private message Send e-mail
NORO LIM



Joined: 24 Apr 2008
Posts: 875
City/Region: Olympia
State or Province: WA
C-Dory Year: 2006
C-Dory Model: 23 Venture
Vessel Name: NORO LIM (sold 12/12/14)
Photos: NORO LIM
PostPosted: Fri Aug 10, 2012 6:58 pm    Post subject: Reply with quote

Harvey,

I doubt you will get close to the recommended WOT RPMs with only one engine running.

Just as is the case with propping for heavy or light loads, or sea level or high altitude, I think you should prop according to how you will use the boat mostly. Except for a real emergency, I'm not going to try to beat hull speed with one engine. No expert, but I don't think lugging is going to be much of an issue with low RPM, hull-speed operation on one engine.

I guess you could carry multiple sets of props, as an alternative. Might be you'd end up with some tiny little prop in order to get one engine up to the recommended WOT RPM.

_________________
Bill, Formerly on NORO LIM
2001 CD 16, 2001-2006
2006 CC 23, 2006-2014
Back to top
View user's profile Send private message
Matt Gurnsey
Dealer


Joined: 11 Nov 2008
Posts: 1532
City/Region: Port Orchard
State or Province: WA
Photos: Kitsap Marina
PostPosted: Fri Aug 10, 2012 7:39 pm    Post subject: Reply with quote

I'm trying to find a good analogy to help explain this, but it's really hard.

Let's try this-

Imagine you're riding a bike. ( I know we're in no shape for that, that's why we're imagining). You're going up a slight hill, and you've chosen a specific gear that allows your legs to move at the rythem you like.

Now pedel with just one leg (we're imagining that it can be done effectively). You've reduced your power to the pedals by half. Can that one leg maintain the same speed at the same gearing? No- it has to work harder, and as it works harder you'll be slowing down.

If you change gears, you could get that one leg back to the rtythem you like, but now the bike will be going slower as you've switched to a lower gear on the bike. The leg is putting out the same power, and at the same rythem, but it can't, by itself, move the bike at the same speed.

So let's go back to the boat. Your boat is set to have 80 horsepwer move 300 pounds (roughly) at 5500 WOT. From that number you can graph your cruise speed at various rpm's.

Remove one engine and now you have 40 horsepower moving the same 3000 pounds. Can half the horsepower move the same weight?

Changing props is like changing gears on the bike. A prop with less pitch would allow the single 40 to come up to the same 5500 rpm. But the boat won't be going as fast.

I'm not sure that was any clearer. Did it help?

_________________
Matt Gurnsey
Kitsap Marina
www.kitsapmarina.com
360-895-2193
(888) 293-7991
Back to top
View user's profile Send private message Send e-mail Visit poster's website
Matt Gurnsey
Dealer


Joined: 11 Nov 2008
Posts: 1532
City/Region: Port Orchard
State or Province: WA
Photos: Kitsap Marina
PostPosted: Fri Aug 10, 2012 7:43 pm    Post subject: Reply with quote

Or think of this-

It takes about 40 horsepower to push a 16 C-Dory to about 30 mph wide open.

It takes about twice that horsepower (either single 75-90 or twin 40-50) to push the 22 C-Dory to about 30 mph.

If you moved the prop from the 16 to one of the engines on the 22, it won't allow the engine to get to WOT. (In fact I'll bet (but don't know) that the prop on the single 40 powered 16 is about the same size as the twin powered 22).
Back to top
View user's profile Send private message Send e-mail Visit poster's website
hardee



Joined: 30 Oct 2006
Posts: 12632
City/Region: Sequim
State or Province: WA
C-Dory Year: 2005
C-Dory Model: 22 Cruiser
Vessel Name: Sleepy-C
Photos: SleepyC
PostPosted: Sat Aug 11, 2012 1:32 am    Post subject: Reply with quote

Oh this is fun. Yes Matt, the bicycle analogy does help, and I get that the boat will not be moving at the same speed when running on a single. I get that the engine will be working harder, but it is still the same prop on the 40, so it seems like it should still spin at the same speed for WOT should it not. Or is it that the boat is not moving as fast, so not traveling as far through the water per each revolution of the prop, (working harder) so might not spin up to the same top RPM level, ---- thus lugging.

I will be trying this SOON.

By the way, my 22 is not going 30MPH, not yet, anyway.

Thanks,

Harvey
SleepyC Moon
Back to top
View user's profile Send private message
Grumpy



Joined: 10 Oct 2005
Posts: 1606
City/Region: Whidbey Is
State or Province: WA
Vessel Name: Kingfisher II
Photos: Kingfisher
PostPosted: Sat Aug 11, 2012 11:17 am    Post subject: Reply with quote

Matt,

As an ex competitive bike rider, I like your analogy (and you are quite right, no way I could do it today.)

You are also quite correct that if I wanted to run on one engine and get to 5,500 rpm I would have to change the prop. (gear) and the boat would go slower than with two engines at 5,500 rpm.

My point is, why would anyone want to do that??

I have 2 BF 135's and if I run them both at 5,500 rpm I have no idea how fast I am going because the boat wasn't really designed to go that fast and logs etc come up rather quickly when you have your eyes closed.

Actually I can't quite get 5,500 on the counter rotating engine which I attribute to the drag from the extra gearing but who cares?

With both engines running I get about 14 Kts at 3,500 rpm.

In case of an engine problem I run one engine at 3,500 rpm and get 8 - 9 Kts. which is quite fast enough to get me to safety in bad weather / adverse current or even in good weather. That is why I have identical twins (I was also born in June in case that has any effect Laughing )

I also have a BF2 on the transom but that one is geared for the dinghy.

M
Back to top
View user's profile Send private message Send e-mail
Sunbeam



Joined: 23 Feb 2012
Posts: 3990
City/Region: Out 'n' About
State or Province: Other
C-Dory Year: 2002
C-Dory Model: 22 Cruiser
Photos: Sunbeam
PostPosted: Sat Aug 11, 2012 12:12 pm    Post subject: Reply with quote

Grumpy wrote:


Why are we suggesting that on a twin engined boat, you would want to achieve WOT rpm for any extended period on ONE engine ???


In the original thread (of which this topic was a tangent), we were talking about kickers. It was mentioned that if one had one of a pair of twins, it would be possible to get home in (paraphrasing) any conditions of wind, current, or waves. I asked if a "single" prop would be carried to cover that sort of scenario (vs. just getting oneself to safety, not expecting to be able to do so in any/all conditions, etc.). And the discussion went from there.

I was thinking that if I had twins (which I think are nifty), and I were expecting to be able to make longer treks in tougher conditions on a single, I might carry two spare props: One "twin" and one "single." Of course the "single" prop would just be for certain situations, because one can easily run a single twin on a "twin" prop - just not as efficiently or as good for the engine.

On the subject of WOT RPM: I was out on a (non-C-Dory) 25-footer yesterday that's powered with twin Honda 115's. At WOT with the pair running, the boat went 30+ knots (IIRC) and hit 5400 RPM. That's within the mfgr's specified 5000-6000 RPM WOT rating for that model engine, so the props are "proper." Doing the same with just one of the twins (which is easily enough power for the boat, at 115hp - some are running with less than that total number of hp in different engine configurations), we only got up to 17 knots and 4XXX RPM. So both speed and RPM were affected, and the RPM were no longer within mfgr's specs. We were essentially lugging the engine.

Captains Cat wrote:
Sunbeam wrote:
.

I believe this is how twins would be sea-trialed initially (i.e. before putting on the first props).


Sunbeam


I have a sneaking suspicion that if you sea trial a boat before you put props on, it won't go very fast at all.... Mr. Green


Ha!
Back to top
View user's profile Send private message
Grumpy



Joined: 10 Oct 2005
Posts: 1606
City/Region: Whidbey Is
State or Province: WA
Vessel Name: Kingfisher II
Photos: Kingfisher
PostPosted: Sat Aug 11, 2012 12:27 pm    Post subject: Reply with quote

[quote="Sunbeam"]
Grumpy wrote:


I was thinking that if I had twins (which I think are nifty), and I were expecting to be able to make longer treks in tougher conditions, I might carry two spare props: One "twin" and one "single." Of course the "single" prop would just be for certain situations, because one can easily run a single twin on a "twin" prop - just not as efficiently or as good for the engine.

On the subject of WOT RPM: I was out on a (non-C-Dory) 25-footer yesterday that's powered with twin Honda 115's. At WOT with the pair running, the boat went around 30 knots (IIRC) and hit 5400 RPM. That's within the mfgr's specified 5000-6000 RPM WOT rating for that engine and propped as they should be for twins. Doing the same with just one of the twins (which is easily enough power for the boat, at 115hp - some are running with less than that total number of hp), we only got up to 17 knots and 4XXX RPM. So both speed and RPM were affected, and the RPM were no longer within mfgr's specs. We were essentially lugging the engine.


I absolutely agree with you that if I tried to run on one engine at WOT I would not get into the 5-6000 rpm range without changing the prop, and that is quite logical.

I fully understand your idea of re-propping under certain conditions where you are going to use the boat as a "single" and want to run at optimal conditions.
Unfortunately for me that would be an expensive option. I already have to carry two spare props (as I have a counter rotating set-up) so to cover all bases I would now need 4 spares Shocked even in Aluminum that is a lot of space and $$.

There is another issue in running with one engine and that is the thrust is now off center and must be less efficient plus you have to counteract with steering. That is not much of an issue at low speed but once you are up in the 17 Kt range it would be significant.

M
Back to top
View user's profile Send private message Send e-mail
Sunbeam



Joined: 23 Feb 2012
Posts: 3990
City/Region: Out 'n' About
State or Province: Other
C-Dory Year: 2002
C-Dory Model: 22 Cruiser
Photos: Sunbeam
PostPosted: Sat Aug 11, 2012 12:38 pm    Post subject: Reply with quote

I hear you on the space and $$ for you if you carried "extra" spares Shocked In reality, I'm used to just trying to get to the closest safe spot where I can anchor and wait for favorable conditions (with past boats, and when inshore), so that's probably what I would set myself up for. But it's nice to know what the options are, and what they give or don't give you.

As it is I have an 80hp single and an 8hp kicker; but if I re-powered I might go to twins just 'cause I think they're nifty Cool
Back to top
View user's profile Send private message
Matt Gurnsey
Dealer


Joined: 11 Nov 2008
Posts: 1532
City/Region: Port Orchard
State or Province: WA
Photos: Kitsap Marina
PostPosted: Sat Aug 11, 2012 1:09 pm    Post subject: Reply with quote

I love these thgeoretical discussions. I don't have to leave the keyboard or break a sweat.
Back to top
View user's profile Send private message Send e-mail Visit poster's website
Display posts from previous:   
Post new topic   Reply to topic    The C-Brats Forum Index -> Propellers All times are GMT - 5 Hours
Goto page 1, 2  Next
Page 1 of 2

 
     Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You cannot download files in this forum



Page generation time: 0.0823s (PHP: 83% - SQL: 17%) - SQL queries: 33 - GZIP disabled - Debug on