One more prop question

mochosla

Petty Officer 2nd Class
Joined
Jan 3, 2004
Messages
112
Hi,

I have a boat and need to know if the prop it has is the right size. I tend to doubt it is as it cabitates.
Boat:
2001 Striper WA with hard top
Engine:
2006 BF 150A HP Engine with whale aluminum hydrofoil
Prop
Honda SS S3X14 3/4X16R.

I use the boat in the ocean in So Cal and usually have 4 people with fishing gear, bait tank full etc. The engine goes up to 5900 RPMs and up to 32 knots p/h (under very good and flat conditions).

When heavy, the boat has a slow out of the hole and sometimes will not plane and it feels as if the boat was dragging something VERY heavy and will not go any faster than 10 knots until the prop "catches up" and then it gets going. I believe this is cavitation.

Any help will be greatly appreciated.

Thanks.
 

hwsiii

Commander
Joined
Jan 25, 2009
Messages
2,639
Re: One more prop question

Mo, I assum you have a 21' Striper from the motor you have and the speeds you reached and I also found your motor to have a WOT range of between 5,000 and 6,000 RPM with a gear ratio of 2.14:1, please verify this. You gave us the speed and RPM lightly loaded, but you didn't give us any numbers for when you are heavily loaded, we need those as well. I show your boat heavily loaded to be over 5,100 pounds, and at that point you have only 1 HP for every 34 pounds of weight in the boat and that is a very low power to weight ratio. I need to know what your propeller slip is at when you are fully loaded, that is where your problem is, not lightly loaded. The cavitation you are experiencing is coming from the heavy load you carry and not enough blade geometry to handle that load as you try to come up to planing speed and get over the hump. I do not know the blade geometry for the prop you have on the boat now, but I can tell you that when you are lightly loaded that prop is working ok, but when you have it heavily loaded it does not have enough blade surface area and the correct blade geometry to plane the boat in a reasonable time period. In my opinion this is most probably caused by having too much rake in the prop you currently have and not enough stern lift, because when you are heavily loaded it causes the stern to sink and the bow to rise which just exacerbates the problem you have trying to get on plane with the wrong attack angle to let the boat flatten out and get over the hump.
You need two different props for maximum performance of your boat when you carry such different loads, the one you have now for light loads and a Solas Amita 4 blade Aluminum 15" pitch prop for the heavy loads when you have a full boat. This prop will lift the stern out of the water and lower your attack angle when you transition to plane with a heavy load and will have a much better hole shot as well as cruising MPG, less ventilation in rough water because of the extra blade surface area and different blade geometry and it will also have much better acceleration throughout your entire RPM range.
When you are lightly loaded this prop could possibly over rev and you will have to be watchful if you use it for all around use.

Boat and Motor

mohoslaBoatandMotor.jpg



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