need help selecting a prop

bouillai

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Joined
Oct 8, 2013
Messages
2
i have a 21' lobster boat with a 2008 90hp yamaha four stroke. the current prop is a 13.25x17 3 blade aluminum and most rpms i can get is 4700. i cruise at about 4000 rpms which is about 12 knts. top speed is around 15-16 knts. not sure how much boat weighs but i know its heavy and always has at least 1000lbs of equipment onboard. looking to get closer to the max 6000 rpm and gain a little cruise speed i know i need to go down in pitch but not sure how much and if i shoud stay with 3 blade or go to 4 blade. any help woud be appreciated. thanks
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: need help selecting a prop

To make about 6,000 rpm you would need to reduce your pitch to about 12" pitch.
Your present slip is about 44%,extremely high perhaps it has to do with your weight.
Is the hull intended to plane?Increasing the rpm should help the slip.
Do you know the brand and year of the boat?Is there an opportunity to weigh the boat?
a 12" prop could produce about 24 mph if the slip is reduced to about 20%
It may be wishful thinking if the boat isn't intended to plane.
Do you know the rated hp of the boat?
 
Last edited:

bouillai

Recruit
Joined
Oct 8, 2013
Messages
2
Re: need help selecting a prop

the boat is a semi displacement hull. the boat is a 1986 ronald newall. its a custom boat builder so finding information such as weight, is almost impossible and i dont have a chance to weigh the boat as it is a work boat and it will be staying in the water until next spring not sure of max hp rating but would think its around 150hp. i was able to try a 14x13 michigan wheel that gives me about a 13 knt cruise at 4600 rpm but dont know what the max rpms are because if i try to throttle up past 4800 the prop blows out and loses grip. thanks again for your help
 
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