Hi all,
I've been having issues with obtaining the correct WOT with the original prop that was on my boat. The original was a 15.25 x 15 prop. With that prop I was getting 3900 rpm WOT and 34 mph both rpm and speed have been confirmed. RPM confirmed by timing light and speed by GPS. Yesterday I went to a 16 x 12 prop, I was expecting to lose a bit of top speed but not as much as I did. With the new prop I was getting 4300 rpm and a top speed of 25 mph. (This was loaded with 2 adults, and about 100 lbs of children and associated gear). With the original prop it didn't matter how much weight was in the boat it could be just me or the whole family and it would still do 3900 rpm and 34 mph.
Other info:
Engine: 3.0 litre merc (Compression is good with 160 - 165 on each cylinder)
Gears: 1.62:1 (somebody switched gears at one time)
Boat: 1991 Sea Ray 170 bow rider
An online speed calculator shows that my slippage with the original prop was .5% which is unrealistic, with the new prop it shows 17%.
The boat gets up on plane pretty fast and responds ok to trim.
My only explanation is that the original prop can not possibly be a 15 pitch (It has been repaired at least once by me, not sure about previous owner). However if it was more than a 15 pitch I would have expected a larger jump in rpm by dropping to a 12 pitch. A 400 - 500 rpm increase is about right for a 3 inch drop and increase in diameter.
Any ideas? Something just doesn't add up to me.
PS: I know the gears are wrong for the boat and I'll likely never get it to work the way it was designed and I've come to terms with that. (it is our first boat and we'll likely upgrade in a few years)
I've been having issues with obtaining the correct WOT with the original prop that was on my boat. The original was a 15.25 x 15 prop. With that prop I was getting 3900 rpm WOT and 34 mph both rpm and speed have been confirmed. RPM confirmed by timing light and speed by GPS. Yesterday I went to a 16 x 12 prop, I was expecting to lose a bit of top speed but not as much as I did. With the new prop I was getting 4300 rpm and a top speed of 25 mph. (This was loaded with 2 adults, and about 100 lbs of children and associated gear). With the original prop it didn't matter how much weight was in the boat it could be just me or the whole family and it would still do 3900 rpm and 34 mph.
Other info:
Engine: 3.0 litre merc (Compression is good with 160 - 165 on each cylinder)
Gears: 1.62:1 (somebody switched gears at one time)
Boat: 1991 Sea Ray 170 bow rider
An online speed calculator shows that my slippage with the original prop was .5% which is unrealistic, with the new prop it shows 17%.
The boat gets up on plane pretty fast and responds ok to trim.
My only explanation is that the original prop can not possibly be a 15 pitch (It has been repaired at least once by me, not sure about previous owner). However if it was more than a 15 pitch I would have expected a larger jump in rpm by dropping to a 12 pitch. A 400 - 500 rpm increase is about right for a 3 inch drop and increase in diameter.
Any ideas? Something just doesn't add up to me.
PS: I know the gears are wrong for the boat and I'll likely never get it to work the way it was designed and I've come to terms with that. (it is our first boat and we'll likely upgrade in a few years)