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Speed Under Power Question...?

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dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Initially Posted - 05/05/2004 : 12:56:34

Recently I changed the prop on my 8hp Merc 2-stroke from a 9" x 9" to a 9 3/4" x 6.5", which of course has changed certain performance aspects, one of them being top end boatspeed.

In another post, Jerry of Pretty Penny II reported an average speed of 5.4KTS (6.2 MPH) at no greater than half throttle with his new motor. In the last week, I have been observing the performance of my new prop and, in a calm channel, I can only get about 4.1 KTS with the motor getting pretty close to its top rpm. With my old prop, I believe I had a higher topspeed at closer to 1/2 throttle. If this turns out to be the case, I may switch back to my older prop for the better fuel economy.

Anyway, my questions is, what is your topspeed and approximate rpm with your particular engine/prop setup?

Don Lucier

North Star SR/FK

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osmepneo
Past Commodore

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USA
1420 Posts

Response Posted - 05/05/2004 : 16:36:25

The idea of the 6.5" pitch is lower end power, in situations where full throttle is not necessary. On displacement hulls. One place this advantage would be observed would be in "tight" quarters. You're reporting exactly what is supposed to happen. Just a greater amount of difference than I would have expected. Of course your pitch change is pretty dramatic, ca 33%, but that seems like a big difference.

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osmepneo
Past Commodore

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USA
1420 Posts

Response Posted - 05/05/2004 : 16:38:03

I'll test mine when I launch and reprt how mine is performing. Maybe with the standard 7.5 " pitched prop and then with the 6.5 " pitched prop.

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Derek Crawford
Master Marine Consultant

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USA
3323 Posts

Response Posted - 05/05/2004 : 18:28:10

I switched to a 6.5" pitch prop about a year ago. My speed at 1/2 - 3/4 throttle has definitely dropped (about 1/2 - 1k) but for close quarter manoevering and docking it gives much greater control at low revs - which is why I switched

Derek

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matsche
Captain

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USA
280 Posts

Response Posted - 05/05/2004 : 18:44:23

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">Anyway, my questions is, what is your topspeed and approximate rpm with your particular engine/prop setup?<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">

Tohatsu 6hp 4 stroke, standard prop (7.8/8). It will do about 4.5 knots @ half throttle, 5.5 knots at full throttle. It takes a lot to get that last knot! There is an optional 7.8/7 prop for this motor, but the standard one is fine for what I do. It will easily back out of the slip with a 20 knot wind on my tail.

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dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 05/05/2004 : 23:08:52

Thanks for the input guys...

I'll keep evaluating the new prop, but with the motor now running as high as it does, it makes me feel like I'm getting nowhere...slow! Additionally, with the motor maxed out, I feel as if I don't have that little extra "oomphh" in reserve.

I'm still not sure how much better this prop is at low speed either. Being one who doesn't use the motor much around the docks, the advantages of the low pitch prop aren't that evident to me.

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lcharlot
Master Marine Consultant

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Antigua and Barbuda
1301 Posts

Response Posted - 05/06/2004 : 01:43:47

To use a low-pitch prop effectively, you need to increase the prop diameter, which you can't really do on a outboard designed for a narrow range of prop diameters. This issue of prop pitch vs. diameter is what makes the Yamaha T8 a superior outboard for a small sailboat - it was designed from the outset to use a very large 12" prop, with a low gear ratio and less pitch to reduce cavitation at the low speeds of a displacement hull sailboat. Too bad they put a puny, almost worthless 5 amp alternator on it. If it had the 13 amp alternator of the original Yamaha 9.9HT, it would be the perfect Catalina 25/250 motor, bar none.

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frog0911
Master Marine Consultant

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USA
1349 Posts

Response Posted - 05/06/2004 : 09:23:31

The 6.2 MPH was the average speed with a 16 knot quartering headwind and the 2 foot waves associated with it. I have not try it in calm water yet to see what the top speed would equal. The standard prop on the engine is 8.9" dia. with 8.3" pitch. Half throttle runs at around 3000RPMs with full throttle being around 5. Reverse thrust seems to be ample even with the through prop exhaust. It stopped the boat with ease docking with a 10 kt tail wind in Palatka.

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Gary B.
Admiral

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USA
969 Posts

Response Posted - 05/06/2004 : 10:07:58

I have the Yamaha 9.9 high thrust four stroke. While admittedly overkill, the Yamaha will move the C25 at something a bit OVER hull speed should the need arise. I see 7.2 or better on my KM all the time, but I know it reads a bit fast: maybe 1/2 knot or so, but since I am always on "moving" water of the river or ocean, I cannot compare with the GPS accurately. I DO know that 6 plus knots is EASY at about 1/2 throttle, and it backs and stops with authority.

I am going on a 200 mile yacht race from Astoria, Oregon to Victoria, B.C. next week. I took the Yammy off my boat and put it on the 7000# Ericson 27 that I will be crewing on because it has SO much more power than the owner's Mercury 8 2 stroke. I hope we won't need the engine, but if we do, I am confident we have about as much thrust as one could get in a sailboat auxiliary. LOVE the Yammy, but wish it was lighter!

Same old, same old,

Gary B.
s/v Encore! #685 SK/SR
Vice Commodore

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RichardG
Admiral

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USA
990 Posts

Response Posted - 05/06/2004 : 15:50:30

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">Tohatsu 6hp 4 stroke. It will do about 4.5 knots @ half throttle, 5.5 knots at full throttle<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">

That's speed a slip neighbor on my dock claims he gets in calm conditions with the same motor on his Bristol Channel Cutter 28. It's also the same that I get on my C25 with my old 6 hp Suzuki 2-stroke.

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