High Altitude prop choice

surfcitydude

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Nov 19, 2014
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I just purchased a 16', Smoker-craft Lodge SS boat with a 2009 Honda 50hp, 4 stroke engine. The boat does not have a tach. The previous owner had the original prop on the engine, a 11.1x11, 4 blade aluminum Solas prop. The boat was always run at low elevations, under 1000'. I plan on using the boat above 5000 ft., in the Sieras. My question is, how do I determine which prop for higher elevations without having a tach?
 

fishrdan

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Jan 25, 2008
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6,989
I would get a tach first, a Tiny Tach is around $50. Your outboard's manual might have recommendations for high altitude, so check that too.

Most likely you will need to drop pitch going to a higher altitude, and the tach will help identify where you are at with the current setup at sealevel, or 1000'. Once you have the numbers, WOT RPM, speed along with the other specs you provided, a prop choice can be made for 5000'. You're looking to stay with in the WOT RPM range of the outboard and I'd try to keep at the top end for high altitude operation.
 

steelespike

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Apr 26, 2002
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A 2009 Lodge is 17 ft and about 1,000 lbs dry weight.
As suggested a tach is a must.Both to help determine what prop is needed and to tell how well the selection is working.
The motor needs to rev freely within its rpm range 5500-6000.
Once the correct prop is working you can operate at whatever rpm you prefer.
If I had to guess I would suggest a 9" pitch.
 

Sea Rider

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Sep 20, 2008
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Check with tach manufacturer before buying, most TT and Hardline induction tachs are limited to just 2 cylinder engines, if your engine is a 3 cylinder, tach probably won't read accurately or work as intended.

Happy Boating
 

steelespike

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Apr 26, 2002
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I don't think the TT is limited to only 2 cylinders.There are settings for the number of cylinders,I think up to 8, and you do have take into consideration wasted spark ignition.If you talk to TinyTach I'm sure they can help you figure if a 3 cylinder setting isn't available and how to compensate for it.
 
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rallyart

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Jun 7, 2008
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1,175
You may not be reading this anymore surfcitydude, but you are dropping your horsepower more than 20% so the quick answer is to drop your pitch by 20% to get some of your power back. If you've got a 21" then put on a 17" pitch. That's not the perfect way to figure it out but it's close enough if you're in a hurry to make a decision. Your top speed is down anyway due to the Hp loss so you should not have to worry too much about not being fast enough.
 

Sea Rider

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I don't think the TT is limited to only 2 cylinders.There are settings for the number of cylinders,I think up to 8.


Happen to have both brands in current use, sorry, the stated settings are all about firing sequence, not number of cylinders. For 3 and up cylinders usually you must buy a standard big tach in which you can program the number of cylinders/pulses in use.

Happy Boating
 
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Silvertip

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Sep 22, 2003
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28,758
Tiny tachs don't care how many cylinders and engine has since the sense lead hooks to only one plug wire. The only issue is whether the engine has what's called a "wasted spark" ignition system. These systems fire the plug twice rather than once. I would also not pay $50 for a Tiny Tach when a genuine full size tach is available for not much more. Those tachs also do not care how many cylinders an engine has unless it happens to be an I/O or inboard with an auto derived engine where sense pulses come from the ignition system. Full size tachs have settings for 4P, 5P and 6P and perhaps 4, 6, and 8 cylinders. Outboards derive a tach signal from the charging system which may have 4, 5 or 6 pulses per engine revolution. This makes the number of cylinders irrelevant. Most more recent outboards will have a 6P sense signal.
 
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