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Speeds under power

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D Wrate
1st Mate

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38 Posts

Initially Posted - 07/07/2004 : 19:26:07

Hello all again! Thanks so much for your help on my reversing under power questions...now a different question...

Curious as to your o/b size & prop combo, max & cruise speeds under power for a wing or fin keel 25. Again we are not sailing a Cat but curious about what kind of speeds you are seeing. I cannot power faster than 3.5 to 3.75 kts as indicated on my Standard Horizon knotmeter.

Just trying to determine if I have a prop issue, or a knotmeter calibration issue. Impeller is clean, hull is clean... displacement 3500#. Similiar beam, LWL 19.5 ft...
Thanks
David

Edited by - on

ClamBeach
Master Marine Consultant

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3072 Posts

Response Posted - 07/07/2004 : 20:08:48

That's pretty slow... how big is your outboard?

Only way to really find out for sure is to either borrow a GPS or mark off a half mile course somewhere on your waters... make a timed run one way and immediately turn around and run back on a reciprocal course. Average the two calculated speeds. Slack tide or a low current area helps.

Edited by - on

Bill Holcomb
Admiral

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USA
769 Posts

Response Posted - 07/07/2004 : 20:37:41

Remember David that your boat - like ours - is limited in tops speed to the design's theoretical hull speed. For displacement boats like sailboats the THS = 1.34 times the square root of the water line length.

Bill Holcomb - C25 Snickerdoodle #4839

Edited by - on

D Wrate
1st Mate

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38 Posts

Response Posted - 07/07/2004 : 20:55:19

We have a Honda BF75, 7.5 HP. Theoretical hull speed is 5.91 kts ( I assume this is kts...) or is it mph?

The real reason I am wondering about this is that I see posts where statements are made to the effect... "I see 5 kts @ 1/2 throttle with my Nissan 8" or similiar... I couldn't go an indicated 5 kts under power no way, no how at this point. Note I say indicated...

David

Edited by - on

oldsalt
Admiral

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USA
578 Posts

Response Posted - 07/07/2004 : 21:31:36

Unless you calibrate your knotmeter (I also have a Standard Horizon model) with a GPS or measured course you really can't rely on its accuracy but your 7.5 should push you a lot better than 3.75 knots if it's running well. Another factor is whether or not you have a high thrust prop which supplies its extra thrust at the expense of top speed due to its lower pitch. I have a 9.9 2 stroke Johnson Sailmaster with a high thrust prop and it will give me a little over 5.6 knots versus a theoretical hull speed of about 6.9. Lastly, you might just have a prop with an unacceptably low pitch.

If you do get to the bottom of this please let us know.

Edited by - on

D Wrate
1st Mate

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38 Posts

Response Posted - 07/07/2004 : 21:58:03

I know that my prop is 9.5" diameter X 9" pitch.

I have considered changing it to a lower pitch since I don't think I am reaching the reccommnded rpm. Once I accidently knocked her out of gear whilst running at the throttle setting that gives me my max speed, not wide open since that just burns fuel with no increase in speed. When she went into neutral, the rpm's increased like crazy, probably 2x faster which led me to think that it is way over-propped and cannot reach proper operating rpm.

I have yet to take a tach with me to see what kind of rpm it runs at 3.5 kts.

Anyone else running a BF75?

David

Edited by - on

Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 07/07/2004 : 22:03:51

With a waterline length of 22', the theoretcal hull speed is about 6.3 knots. That Honda should push you to 5.5 at around half throttle. Going above 5.5 is just a waste of gas and unneeded noise. I generally cruise at about 5 knots with the engine running somewhere in the third-to-half throttle range. Use a GPS to check your speed, but be aware that it is also registering the effect of any current (with or against).

BTW, are you sure the trailer isn't still strapped to the boat?

(There's a blonde-joke circulating the Web as a "true story" about that...)

Edited by - Dave Bristle on 07/07/2004 22:13:29

ddlyle
Captain

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302 Posts

Response Posted - 07/07/2004 : 22:29:31

The first time I took my GPS on my boat to measure my speed, I was going really fast.
I had great winds, reefed for the first time and the telltales were really flying.
My wife put the batteries in the GPS and tried to turn it on.
She wasn't feeling well 'cause of the winds, and following seas.
I put the motor on to head home (motor sailing on a broad reach) and we were practically surfing.
I had to mind the tiller to make the best of things 'cause the admiral wasn't feeling well.
And the friggin GPS wouldn't turn on !
I knew after two grown sons she knew how to put the AA batteries in an electrical toy, so I didn't want to make her feel bad ('cause she was already sea sick.)
But God I really wanted to see my speed. Knew I was going over 7 knots !!
But I couldn't open the gps to look (minding the tiller).
Sure enough, when I got home,
my BLOND wife had put the AA batteries in wrong.

I wanted so bad to see the knots.

Edited by - on

Doug
Captain

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USA
457 Posts

Response Posted - 07/07/2004 : 23:39:37

My Standard gauges only show about 4.5 knots with my Yamaha T8. My GPS shows things to be much faster. I assume my knot meter is running a little slow. Have to remember that GPS speed is speed over the surface of the earth (landspeed) and the knotmeter is speed of the boat through the water. If and when I get the boat into a current free lake I'll try to get the two to match.

Edited by - on

D Wrate
1st Mate

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38 Posts

Response Posted - 07/08/2004 : 00:24:36

Just out of curiosity, do GPS's show speed in knots as well as mph & kph?

David

Edited by - on

lancej
1st Mate

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81 Posts

Response Posted - 07/08/2004 : 06:45:50

My Garmin shows speed in knots or MPH, it is a setting within the GPS software. I have an Autohelm knotmeter, and it is a little more optomistic than the GPS, but I use it because it takes current into effect, ect.

That being said, my (well loaded) C25 (5000+ lbs displacement) will cruise along at 4 - 4.5 knots with the old 4.5 HP Merc, and 5.5 knots with my Evinrude 9.9 HP and the "high thrust" prop. The "high thrust" prop actually serves to slow you down.

Sounds like you may have some issues, though I cannot imagine what.

Lance
85 FK/SR

Edited by - on

frankr
Captain

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256 Posts

Response Posted - 07/08/2004 : 08:08:37

GPS's report speed in either knots or MPH relative to a point on the earth whereas knot meters report speed relative to the water. If you are going with the current the knot meter should read less than a GPS since it can not sense the extra kick of the current(if you are just drifting with a 2 kts current the knot meter should read 0 while the GPS should read 2 kts). If you are going against the current then the knot meter should read more than the GPS since the boat must over come the current to go anywhere (if the current is 2kts you need 2 kts of speed just to stay in the same place - the GPS will read 0 in this case).

Edited by - on

Jonathan Cuff
Navigator

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Cayman Island
173 Posts

Response Posted - 07/08/2004 : 09:31:16

On my WK I get 5.2 knots (via GPS) with a new Honda 5hp 20". If it gets windy or rough that figure will drop. However then I'm sailing!!

Cheers

Edited by - on

f_sondergaard
1st Mate

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USA
92 Posts

Response Posted - 07/08/2004 : 09:55:25

Our boat, a wing keel has a 15hp honda (I know...its a bigger motor than we needed, but the price was right

) makes 5.8-6.2 knots registering on the knotmeter.
Jason and Fran
1982 Catalina
swing to wing

Edited by - on

RoofRoof
Navigator

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USA
186 Posts

Response Posted - 07/08/2004 : 11:22:02

My SK with a 2001 Honda 8hp 4 stroke 25" shaft and standard prop does 5.5-6.5 knots per my GPS. 1/2 to full throttle.

Edited by - on

dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 07/08/2004 : 12:29:01

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by D Wrate
 ...Curious as to your o/b size & prop combo, max & cruise speeds under power for a wing or fin keel 25. Again we are not sailing a Cat but curious about what kind of speeds you are seeing. I cannot power faster than 3.5 to 3.75 kts as indicated on my Standard Horizon knotmeter...<hr height="1" noshade id="quote"></blockquote id="quote">

At the beginning of the season, I switched from a stock 9" x 9" prop to a 9 3/4" x 6.5" prop on my 8hp Mercury 2-stroke and I'm not pleased with the results. My top speed dropped a bit, which isn't a big deal, but my fuel economy is absolutely abysmal. I think I've used more gas so far this season than I did all of last year.

As far as manuevering performance goes, I haven't noticed any difference between the two props, but then again, around the docks I only use enough engine to get up to 1 knot or so, which means I don't need the thrust capacity of a tug boat.

Since the vast majority of my engine on-time (about 99%) is spent at cruising speeds, I'll be switching back to the old prop for both the increased fuel economy and top end speed.

Edited by - on

JimB517
Past Commodore

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USA
3285 Posts

Response Posted - 07/08/2004 : 12:51:02

Indiscipline (78 fin keel, Standard Rig) 8 HP "classic" Honda 4 stroke with full water/fuel cruising load:

2 knots at idle
3.75 knots with the throttle to the "start" line - just above idle
4.75 knots at about 1/4 throttle (engine running smooth, very quiet)
5.5 knots at 1/3 throttle - this is my cruising speed
5.75 knots at 1/2 throttle (engine roaring)
6 to 6.3 knots at full throttle (engine roaring, stern squatting)

I always have the main up and sheeted in and the jib rolled when motoring

Edited by - on

Martin
Deckhand

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USA
16 Posts

Response Posted - 07/08/2004 : 14:41:02

I always have the main up and sheeted in and the jib rolled when motoring

JimB517,

Why do you have your main always up when motoring?

Thanks, Martin

Edited by - on

D Wrate
1st Mate

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38 Posts

Response Posted - 07/08/2004 : 16:11:53

Jim are those speeds via GPS or knotmeter?

David

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D Wrate
1st Mate

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38 Posts

Response Posted - 07/08/2004 : 16:17:11

I also generally motor with sails up & sheeted in to reduce the rolling motion. Unless I am entering after having sailed there or exiting harbours before I set sails.

It also can boost the speed a tad as the forward motion from motoring creates apparent wind which will generate forward motion.

David

Edited by - on

D Wrate
1st Mate

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38 Posts

Response Posted - 07/08/2004 : 16:20:52

D Lucier, I think I am leaning towards having an inaccurately calibrated knotmeter. You and I are basically running the same motor combo and our boats are not so different to cause the degree of difference I am seeing.

Good opportunity to purchase a baisc gps unit...

David

Edited by - on

JimB517
Past Commodore

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USA
3285 Posts

Response Posted - 07/08/2004 : 17:33:14

Those speeds are under knotmeter, which generally agrees with my GPS. I use the difference between the GPS and the knotmeter to estimate current speed (also by looking at the bouys).

I find that motoring with the main up reduces rolling a lot and I can usually get at least a little drive out of the main motorsailing. Tack just as you would without the motor except you can point a lot higher and still keep the mainsail full. You'll find that you can then back off the motor a little for fuel economy. Keep an eye on your apparant wind indicator. Even motoring downwind in light air the apparant wind will move forward of the beam. Adjust your main accordingly.

Edited by - on

Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 07/08/2004 : 21:49:34

Motoring with the mains'l up is common practice for cruisers--primarily for reducing roll, but partially to pick up any boost a cat's-paw might provide. In dead-still air, the apparent wind is on the nose, so there is no power generated by the main. But if there's a tiny air movement on the beam, the apparent wind can be like a beat, and a little power can be generated.

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John Ace
Deckhand

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USA
24 Posts

Response Posted - 07/09/2004 : 08:17:54

I've been running my engine, Johnson 9.9, full to charge the battery and get 6.2-6.3 knt.

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dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 07/10/2004 : 23:18:50

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by D Wrate
 D Lucier, I think I am leaning towards having an inaccurately calibrated knotmeter. You and I are basically running the same motor combo and our boats are not so different to cause the degree of difference I am seeing.

Good opportunity to purchase a basic gps unit...

David
<hr height="1" noshade id="quote"></blockquote id="quote">

David,

My knotmeter was .8kts off (reading low) too, but I'm still getting cruddy fuel economy from my low pitch prop.

Edited by - on

D Wrate
1st Mate

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38 Posts

Response Posted - 07/12/2004 : 11:35:32

Was on the baot yesterday, no sailing, just doing some maintenance which included an inspection of the knotmeter impeller (totally clean) and the calibration knob on the guage itself. Turns out the knob was turned all the way to one end of its range. Spinning the impeller manually I could not get past 6 or so knots. I turned it all the way to to the other end spun the impeller & lo and behold... 10+ kts!!!

Ok so that probably isn't right either... so I put it in the middle of the range and will see how that goes on our next sail.

David

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