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Old 12-12-2010, 16:58   #1
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Propeller Pitch

i have just purchased two 90 hp two stroke Tohatsu outboards.
The boat is fast displacement (6ton) and max speed so for is 12 knots.
However I am only getting 4,500 rpm max out of each motor, when they should max out at 5,500rpm
The problem is that Tohatsu do not make a smaller pitch prop than 11inch pitch (props are 14 X 11)
What should I do,
One suggestion was to grind down the diamater from 14 to 13.75"???
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Old 12-12-2010, 17:10   #2
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This site lists a 9" that will fit.
9” Pitch Tohatsu Outboard Boat Propellers 90 HP 1987 & Newer - iboats.com
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Old 12-12-2010, 17:10   #3
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If you cut down the props...

You may get more RPM's but not likely any more speed. It sounds like you need more hp for a 6 ton boat.

When you say fast displacement, is that a planing hull, hard chine w/ a square transom and trim-tabs. And very little draft. If not, then you pushing/dragging too much water.

What is the designed hull speed?
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Old 12-12-2010, 18:25   #4
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delmarrey,
If you look at some of my previous posts you will see what I mean.

There is NO hull speed limitation other than friction drag, with this design,which is basically if you want to double your speed you have to use four times more power.
With 2 Honda 30 hp max speed was 7.5 knots, with 2 X 90 hp I now get 12 knots but I am hoping for a bit more.
With one 90 hp at a 3,800rpm I cruise at 8 knots
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Old 12-12-2010, 19:11   #5
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The 9" pitch will give you the 5500 rpm required. as Captain Bill said.

Solas make a 14 x 9 x 3 in Alloy for $292 + delivery
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Old 13-12-2010, 11:01   #6
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Quote:
Originally Posted by beau View Post
delmarrey,
If you look at some of my previous posts you will see what I mean.

There is NO hull speed limitation other than friction drag, with this design,which is basically if you want to double your speed you have to use four times more power.
With 2 Honda 30 hp max speed was 7.5 knots, with 2 X 90 hp I now get 12 knots but I am hoping for a bit more.
With one 90 hp at a 3,800rpm I cruise at 8 knots
Well, it seems this is not a planing hull and your drawing says that. So the only way you can get more speed would be more hp. But if your after more RPM's then the different prop is your only option.

You must have a lot of wetted surface to drag. On my 35' LWL w/14K# and a 7' draft fin I can do 8 kt w/just 39 hp.
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