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Maximum Outboard Horsepower Capacity (Guide and how-to)
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<blockquote data-quote="jdsgrog" data-source="post: 204614" data-attributes="member: 4307"><p>The transom is measured at the "widest point" on the transom. Doesn't matter if it's top middle or bottm. This formula is a general formula for those who have homemade boats or have boats without USCG capacity tags, especially on the old boats that didn't require them at the time. I'm not certain if this is an absolute "industry" standard used by boat manufacturers, but maybe someone in the industry can chime in. Bottom line, if you have a USCG tag, the formula is a moot point.</p><p></p><p>One point of correction, you need to round up to the next multiple of 5 hp. Like for my boat, the actual rating based on the formula is 33hp. But I would round up to 35hp which my boat is actually rated for based on the USCG tag.</p><p></p><p>In terms of hp rating, I believe pre-mid eighties hp was measured at the shaft and thereafter was measured at the prop. There was about a 15% loss from shaft to prop, so for older outboards, their actual rating would be about 15% less than modern outboards (e.g. an older 25hp is about equivalent to a 21-22hp by modern standards). But hp ratings can be a little misleading vs actual performance since actual performance is actually based on a lot of other factors (prop pitch, gear ratio, motor weight to hp ratio, etc).</p></blockquote><p></p>
[QUOTE="jdsgrog, post: 204614, member: 4307"] The transom is measured at the "widest point" on the transom. Doesn't matter if it's top middle or bottm. This formula is a general formula for those who have homemade boats or have boats without USCG capacity tags, especially on the old boats that didn't require them at the time. I'm not certain if this is an absolute "industry" standard used by boat manufacturers, but maybe someone in the industry can chime in. Bottom line, if you have a USCG tag, the formula is a moot point. One point of correction, you need to round up to the next multiple of 5 hp. Like for my boat, the actual rating based on the formula is 33hp. But I would round up to 35hp which my boat is actually rated for based on the USCG tag. In terms of hp rating, I believe pre-mid eighties hp was measured at the shaft and thereafter was measured at the prop. There was about a 15% loss from shaft to prop, so for older outboards, their actual rating would be about 15% less than modern outboards (e.g. an older 25hp is about equivalent to a 21-22hp by modern standards). But hp ratings can be a little misleading vs actual performance since actual performance is actually based on a lot of other factors (prop pitch, gear ratio, motor weight to hp ratio, etc). [/QUOTE]
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Maximum Outboard Horsepower Capacity (Guide and how-to)
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