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I have a 13 whaler with a 45hp 2 stroke 4 cylinder 1987 mariner - my mechanic put a new mercury 10.3 x 15p prop and it does great at sea level - 1500ft - tows 2 of my 3 boys on a tube while my 3rd boy, wife and dog are on the boat - no issues. But when we got to our cabin further up in the sierras and on the lake at 5500 ft it couldn't get past 3000 rpms with just me unless i hung off the bow and steered with my feet.

i have heard and read that a 10 3/8 x 11p would do the trick but an other mechanic mentioned that i only need to drop to a 13p - any suggestions -- dont need a ton of top end speed, its a smaller lake and we just go fishing but its fun to cruise with everyone as well.

I understand there will be some loss of of overall power at altitude of course.

Thoughts?

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You may have to have the carbs jetted differently for running at high elevations ,if that is a possibility. Up in the hills there isn't enough good old oxygen to mix with the amount of fuel you have getting sucked into your engine.

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thanks Bill - we did also adjust the idle on the carbs - on this motor the jets are fixed for cruising however.

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Sounds like you will have to wait for the pack of prop gurus to help out, good luck!

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Air density decreases with altitude and internal combustion engines need the oxygen in air to operate and the higher the altitude, the less oxygen here is so there is less power available.

Going from 1500 feet to 5500 feet will turn that 45 HP outboard into a 38 HP outboard so prop accordingly.

A 13" pitch prop of the same model will more than compensate for the gain in altitude. An 11" prop would be w a y too short.

But consider going to a four blade prop with less pitch as well. I like the Solas Amita 4 aluminum four blades for small boats like these. They are inexpensive too. You would want their part # 1313-105-13

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thanks Tom - appreciate the help, I'm going to try the 3blade 13 - would i notice a difference between 3/4 blade with the same pitch?

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Most likely, yes.