Anemic Performance

[julian hood]

I have an Oday 27 with a late model A-4. It has an after market 3 blade prop designed for the A-4. When shifting into forward I have to back down on the throttle for the the tranny to positively engage into gear. Once that is done you can tell that the engine is under load. I am getting 5 MPH tops out of the engine. Should I be expecting more? When in reverse seems that I get a lot more muscle. Any ideas.

[wlevin]

I think I'd need to know more to figure out if you have any problem. For example, do you know how many RPM you are turning at 5 MPH? Are you sure it is MPH and not Knots you are measuring?

When I put our engine in forward gear I always throttle down also. Also, when put into gear I can tell the engine is under load. It is. For a 27 foot sailboat with a ? waterline length, 5 MPH sounds pretty decent. We regularly motor our 34 foot sailboat at 5.5 knots at about 1800 rpm, if that helps.
Bill and Jeanne
T-34C #453
Otter

[julian hood]

Anemic Performance

Thanks for your response. It is 5 mph based on my GPS. I don't know how many rpm's as my tach is on the fritze.

Julian

[keithems]

gps gives "ground speed", not speed in relation to the water. which is what hull speed is -- if there is a current, wind, etc. ground speed can be much less or more than water speed [the exact term escapes me right now, but in aviation we call it airspeed vs groundspeed.] 5 mph or knots sounds fine for a 27' boat [your waterline is likely only 23'] -- the approx. formula is about 1.3 X square root of lwl [length at the water line] -- also late in the season bottom fouling becomes a factor, and i think -- not sure -- that things like sails add to wind resistance, further cutting speed. -- the problem is the a-4 is advestised [rated] at 30 hp, but unless u add a 3 or 4 speed transmission, it's like driving a car in third gear only. i keep reminding myself that i got into all this cuz i'm a sailor -- on a sailboat -- and sailing is what i love -- the motor is there mostly to get me in and out of the slip and through the canal locks.

[sastanley]

keithems,
I found the original A-4 owners manual last evening..it claims that it also only puts out 30HP at 3,500 RPM! I don't know if would ever spin that fast, even with no prop or load on the motor
If I recall, at 1,800 RPM, which seems to be a common 'cruising RPM', it is putting out about 12-14 HP. Don likely has the book memorized, so I am sure he'll know.

-Shawn

[Don Moyer]

Shawn,

We cobbled the attached spec sheet together from various Universal documents which might help pull things together for you. The term "Recommended max available power" might be a bit strange to engineers, but in this context it really relates to a recommendation to select a prop that allows the engine to achieve 2400 RPM at wide open throttle (WOT). Universal never published a maximum available RPM, but we backed into 2400 RPM starting with Universal's recommendation that the best cruising RPM was 2000 OR 80% of maximum available RPM - 2000 RPM works out to be very close to 80% of 2400.

Don

[wlevin]

performance curve

When we fixed our tachometer we were able to plot rpm's against speed (at a time of dead water and flat conditions when speed over the ground-gps-equaled speed through the water-knotmeter) to produce a performance curve for our motor and prop. It helped us realize that the original prop on the boat was too big, and it confirmed for us that 1800 rpm was our most efficient cruising setting.
Bill and Jeanne
T34C #453
Otter

[sastanley]

The graph is a great idea. I am interested in this discussion because I am experiencing anemic performance from my A4 as well, but since I am a new owner, I don't know if that is caused by neglect, internal damage (drowning!), or mis-tune, etc...

Now it is out of the water, so ability to test 'improvements' are limited. On the way to the travel lift the best smooth RPM I could muster was about 1,500-1,600. Then it just started running worse with additional throttle.

The biggest thing that helped up to that point was to clean the prop..as mentioned in other threads, the fastest she'll ever be is after first splash! I'll take some notes, and maybe make a graph similar to Bill & Jeanne's after a few hours of motoring. When I grew up on this boat, there was never a tach on the motor, but I seem to recall that 4.5 - 5.0 knots on the knotmeter was my father's typical cruising speed, and max speed was 5.9-6.0 (accuracy of the knotmeter unknown!) For the few weeks I had the boat in the water, the paddlewheel was too gunked up to work, and I had bigger problems than to bother cleaning it.

I do not know if this is of interest to anyone, but there was a small table in my A4 owner's manual which states HP output at various RPM.

[msauntry]

It helped us realize that the original prop on the boat was too big, and it confirmed for us that 1800 rpm was our most efficient cruising setting.
Bill and Jeanne
T34C #453
Otter

Bill and Jeanne,
What prop do you have now and what prop did you switch from?
Any further thoughts on your current prop?
Thanks,
Micah

[wlevin]

prop type and size

Micah
We have one of those plastic (composite?) CDI props, though we also have an Indigo three blade bronze one as a backup. They both work well, but we prefer the CDI because we can hide it behind our keel and so we sail a bit better with it.

As for sizing, our first prop was way too big for the 27 foot boat we owned before our current one. (Maybe 12 X 9?) I can't remember the size, but it was obviously too big because we could only get the engine to 1500 rpm even though it seemed to be running well. When we had the prop repitched one size (9 to 8, I think) for not much money, we gained 200-250 rpm right away. When we ordered the CDI we left it up to the maker to send us the right one. I never did get the sizing and don't think it's on the prop.

As for sizing a prop for your boat, you could leave it up to the expertise of the people you buy from, although I have read some stories of people who were sold incorrectly sized props. If you want to get some expertise of your own, I'd suggest doing a search on Don's site for "prop size." You'll find lots of threads with information about specific props on specific boats and how they performed for some very informed A4 owners.
Bill and Jeanne
T34C #453
Otter

[wlevin]

more data

In my posting with the rpm by kts data I didn't make clear that the curve was for our current boat with the correct sized prop. I've added the original curve showing the performance of the oversized prop that couldn't achieve better than 1500 rpm
Bill and Jeanne
T34C #453
Otter

[sastanley]

props/gearing/cruise rpm

thanks for the add'l info. I have considered one of those CDI props, although I recall adequate motoring for the time I cruised on this boat as a kid...Allowing the pitch to decrease to allow increased RPM (more HP to the shaft) sounds pretty neat..I wonder if it puts additional load on the motor, or if it likes running a few hundred RPM faster? I personally would never (I know that is a strong word) consider a 3-bladed prop, mostly because of the detriment to the sailing performance.

I guess the question is whether or not the throttle position is the same for a given RPM between the different props?

I will comment that I am also a Honda (Civic) enthusiast, and I've chatted with some other enthusiasts that get better mileage with a transmission that is geared for higher RPM than one with the RPM lower at highway speed..those things seem to like 3,000 RPM! My current '91 Civic cruises at 2,800 at 60 MPH and I get 33-36 mpg depending on the 'city' driving I do, but I have a tranny with a different final drive ratio (lower, for slightly higher RPM, about 3,050 at 60MPH) that I am considering trying out to see the difference.

[msauntry]

We had a thread going a little while ago with a few of us full keeled boats with the prop in an aperature (Triton, Vanguard, Albergs, etc.) This presents a challenge for prop selection with the limited space and a rather large deadwood in front of the prop.

I've got a Columbia 13x8 two blade prop off my old Triton (Columbia was a prop manufacturer, not refering to the boat). When I got my Alberg 35, its prop was mangled from electrolysis so I used that one and it wouldn't reach proper rpm's. I've had the Indigo on for a few years and like its performance, but I would like a lower drag option. I found a "Hydralign" 3-blade feathering prop that might fit the aperature, but the price is pretty high. I think I'll do what you did and get my 13x8 repitched and see where that leaves me.

CDI prop: I'm curious how you keep it from fouling? Sand, prime, and paint? Slather it in silicon?

[sastanley]

Micah,
This is from CDI's website regarding barnacles...the short answer is they have a 'special' paint.

Question: What about Barnacles?

Answer: Barnacles grow on plastic just like they grow on your fiberglass hull. We have a special paint which is a much harder finish than bottom paint, doesn't use poisons, and lasts for about a year even with extensive motoring. We sell it in kits, with everything you need to do the job.

Traditional bottom paints don't work very well on any kind of prop. They are made to ablate (i.e., slough off) at 5 - 10 knots, revealing new poison. Prop blades spin much faster than 10 knots, so by the time you've motored for awhile, most of the poison is gone, and the barnacles can grow.