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Old 09 June 2016, 18:07   #1
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Help With Porpoising

I have a 3.3 Rib with a 30hp motor, I also have a sport hydrofoil on motor and it keeps my rib from porpoising when I am in boat by myself at all speeds but when my wife gets on board and sits in front we cant get max speed because the boat will porpoise, if she sits in back with me we can get max speed if I play with the trim but unable to if she is in front.

So with that said will a 4 blade prop help with this or ??, its a small narrow boat and best she stays in front.
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Old 09 June 2016, 18:14   #2
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I put a 4 blade prop on a boat for the same reason which made no difference to porpoising. A set of trim tabs fixed the problem.

Jon
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Old 09 June 2016, 18:19   #3
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Quote:
Originally Posted by jonp View Post
I put a 4 blade prop on a boat for the same reason which made no difference to porpoising. A set of trim tabs fixed the problem.

Jon
Did the 4 blade hurt your top speed by chance, I would like to try the trim tabs but I don't have much room in back, this is a narrow rib.
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Old 09 June 2016, 18:31   #4
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Quite a big motor for a 3.1m what's the layout? Centre console? Do you have adjustable trim on motor ?
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Old 09 June 2016, 18:46   #5
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Originally Posted by HDAV View Post
Quite a big motor for a 3.1m what's the layout? Centre console? Do you have adjustable trim on motor ?
Yes its one of the smallest RIBs that can handle 30hp, its a 4 stroke with trim adjustments, like I said above by myself the boat is fine and I do have a hydrofoil on motor but when my wife rides along I cant control the porpoise at top speed, gets out of control.

Here is a pic of what it looks like
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Old 09 June 2016, 21:58   #6
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The 4 blade Solas prop did knock back the top end speed but it increased the low planeing speed and hole shot (take offs). It was on a bigger boat which helped keep the boat at a more constant speed when working big swells. I dropped back pitch sizes with the 4 blade from a 17 to a 15 to keep the same rpm.

I recently fitted some Bennett self levelling trim tabs to my rib which gave adverse problems, but they have worked great on other boats I've owned. Maybe get some L shapped brackets made to fit the small area where you can fit them.
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Old 10 June 2016, 04:55   #7
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Hi

Firstly....I'm not having a go at your wife....The reason for the porpoising is the centre of gravity has moved too far forward of the leading edge of the wetted surface of your hull which, with a 30 on a boat this size, will be pretty close to the stern. If standard trim, and weight redistribution won't sort it, my next move would be to see if the engine can be raised any on the transom. That would reduce it's tendency to lift the bow. If you go too far it'll ventilate but there might be some scope there.
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Old 10 June 2016, 10:10   #8
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How fast do it go wene running right
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Old 10 June 2016, 11:00   #9
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I guess the other way of looking at the situation - is your wife as much of a speed addict as you? if not, just don't open the throttle so far when she is aboard?

If she wants the vmax, then would removing the hydrofoil from the motor help when you are 2 up?
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Old 10 June 2016, 12:51   #10
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The bow is not an ideal, especially comfortable or particularly dry place to sit on a small boat anyway, can't she just sit beside you at the back since it sounds like it is fine then?
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