Thinking about props

A forum for discussing topics relating to older MacGregor/Venture sailboats.
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Curwen
Engineer
Posts: 154
Joined: Tue Aug 11, 2015 6:19 am
Sailboat: MacGregor 26D
Location: Salt Lake City, UT

Thinking about props

Post by Curwen »

Hello,

I have a 4 stroke Honda 9.9 hanging on the back of my 89 26D. I do not know if this is the motor that came with my boat. I don't know if it is a high thrust motor or a long shaft motor.

I am unhappy with my speed while motoring. I usually have my throttle full wide open and I feel like I am barely moving. I don't have any measurements of my SOG, so I really don't know if my motor is pushing my boat appropriately. I know when the wind dies, the other sailboats get back to the marina much faster than I do on their motors.

I don't like needing to have my throttle fully open. I've had the motor tuned up two of the last three seasons by a marine shop and I haven't noticed any real difference.

My three blade prop has a few nicks and dicks from bumping ground a time or two in shallow water. Should I get the prop smoothed out and would that make a big difference?

How do I determine if I am running the proper prop for my motor?
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Starscream
Admiral
Posts: 1388
Joined: Tue Nov 03, 2009 10:08 am
Sailboat: MacGregor 26X
Location: Montreal, Quebec. 2002 26X - Suzi DF90A

Re: Thinking about props

Post by Starscream »

Without an rpm gauge, it's going to be hard to trouble-shoot prop issues.

It's normal on a 26 ft sailboat to be among the slowest under power. Most sailboats can run at hull speed under power, and the longer the boat, the higher the hull speed. A 26 ft boat hS a hull speed of just over 6 knots, iirc. The Mac waterline is less than 26ft tho.

Is it possible to get a GPS reading of your speed? Even if the boat has no electronics, any cellphone can download a GPS app for SoG.

Its always BETTER to run with a clean prop without any dings. Less vibration means longer longevity, but a few small nicks are pretty common and not the end of the world. These small nicks and scratches don't usually affect the motor performance in a noticeable way.

More data (speed/rpm) and photos (prop) would help us give more accurate analysis.
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Curwen
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Joined: Tue Aug 11, 2015 6:19 am
Sailboat: MacGregor 26D
Location: Salt Lake City, UT

Re: Thinking about props

Post by Curwen »

The boat goes back in the water in March, so I won't be able to take any SOG readings until then.

Thanks for the info!

Curwen
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WinSome
Engineer
Posts: 160
Joined: Thu Jul 07, 2016 7:35 am
Sailboat: MacGregor 26X
Location: Hamilton Ontario Canada

Re: Thinking about props

Post by WinSome »

Here’s something to compare: winsome is a 1990 26C.
Shaft length- it's measured from the top of the clamp bracket to the anti-ventilation plate. our 26C has 25”. Considered extra long. It’s the best length if out in any serious waves.

Motor size: our 8hp Tohatsu (2 stroke) at WOT moves us at nearly 6 knot-hull speed. Assuming no head wind! We can sail as fast as we can cruise!
It’s loud and not so pleasant to converse. So use 3/4 throttle mostly. 4.8 knots. I’m on the water and really don’t care about the others.

Buddies 26D has a 9.9 four stroke and he doesn’t need use WOT.

Another local has a 15 hp and runs hull speed at half throttle. Nice and quiet. Motor weighs only slightly more.
Note that many 15 hp started design life as a 9.9hp.
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Tomfoolery
Admiral
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Joined: Tue Jul 05, 2011 7:42 am
Sailboat: MacGregor 26X
Location: Rochester, NY '99X BF50 'Tomfoolery'

Re: Thinking about props

Post by Tomfoolery »

WinSome wrote: Sat Feb 20, 2021 6:45 amNote that many 15 hp started design life as a 9.9hp.
Like the ubiquitous Johnson/Evinrude 9.9/15 hp 2-strokes that everybody had in the 70's, including me (9.9 hp). Same engine with just a few minor changes between the two versions, like maybe carburetion. I think the 15 spun faster.


Image
Tom
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Jonair222
Chief Steward
Posts: 67
Joined: Tue Jun 23, 2015 6:14 pm
Sailboat: Venture 2-22

Re: Thinking about props

Post by Jonair222 »

PITCH
yes you want a nice even non-bent prop
those 35$ ebay one are fine


its all about BLADE PITCH.
and size of course. Look for markings on back of prop.
my 323cc(15hp) uses a 9 3/4" prop with a 10Pitch
I up sized to eleven for better high speed cruising.
'Hole shot' not a priority on a sailboat.
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Curwen
Engineer
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Joined: Tue Aug 11, 2015 6:19 am
Sailboat: MacGregor 26D
Location: Salt Lake City, UT

Re: Thinking about props

Post by Curwen »

Got to test SOG on a windless day and WOT is 6 knots. Going out tonight, will test half throttle.
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Tomfoolery
Admiral
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Joined: Tue Jul 05, 2011 7:42 am
Sailboat: MacGregor 26X
Location: Rochester, NY '99X BF50 'Tomfoolery'

Re: Thinking about props

Post by Tomfoolery »

Curwen wrote: Thu May 06, 2021 5:54 am Got to test SOG on a windless day and WOT is 6 knots. Going out tonight, will test half throttle.
A Mac 26D has a LWL of 23.5 ft, which calculates to almost exactly 6.5 kts theoretical hull speed.* That's a max with low input power, so you're probably not going to get much better than what you have now.

Is the OB getting up to its design speed range? Ideally, it should be somewhere in that range at full throttle. If it's running slow, a little less pitch would bring it up, but it may not have any real effect on boat speed. Better for the engine, though.

Of course, the opposite is true, too. The engine may be up to its speed-limited redline, and a little more pitch can add a little more thrust while bringing the engine speed down into the design range.

*though it does actually depend on more than just wetted hull length in that simple formula.
Tom
Be seeing you . . .
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