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Old 04 June 2016, 16:36   #1
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Ribcraft low speed trim trouble

I'm having some hassle with the handling on my ribcraft.

Its fine dead slow < 6knots and fine on the plane > 15 knots but for the sakes of trekking up and down 10 knot speed limits the bow rises up and no amount of trimming down will level her out.

Today she was OK with two heavy mates moved right to the front - sat quite nicely then, but I dont really want to have a permanent 200kg counterweight in the front!

I've a suzuki 140 on the back and an 8HP aux. I've now just fitted a removable bench seat near the back too which will undoubtedly make matters worse, but I need this for taking my wife and young son out.

The engine wont trim any lower. Does anyone else have any suggestions?

Thanks
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Old 04 June 2016, 16:44   #2
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Ribcraft low speed trim trouble

That's normal & no amount of trim fiddling will cure it, the boat is trying to climb "over the hump" I.e. Get on to the plane. All planing hulls ( not just RC) have their planing speeds where they go from displacement to planing. Yours is around the 10kn mark which is about normal.


Sh1t happens
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Old 05 June 2016, 04:29   #3
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Yes got to agree with PD all the way it's annoying but it is what it is. Maybe for that sort of situation you may have to lessen your speed and take s little longer.


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Old 05 June 2016, 10:18   #4
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Are the low trim stop bolts set too high?
( if it actually has any which i guess it does)
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Old 05 June 2016, 10:58   #5
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Quote:
Originally Posted by jambo View Post
Maybe for that sort of situation you may have to lessen your speed and take s little longer.

Sometimes there is some hysteresis and if you can get it on the plane it will stay up there at a lower speed than it transitioned at in the first place. If circumstances allow you might be able to get on the plane, then drop back to ten knots and stay there, making less wake than sitting at 9.9 knots trying to get over the hump.
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Old 05 June 2016, 16:39   #6
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Yes all valid points, thanks. Frustrating as anything over 6 knots is creating massive wash for such a small boat, and anything under 10 knots seems to take forever to get out of the harbour. Think there is a trim stop bolt, might remove that might give a tiny bit extra travel.
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Old 05 June 2016, 18:11   #7
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Ribs aren't really designed for speeds between displacement pootling and the plane at usually 12kts+....I have hole between 5 & 12kt - annoying when harbour speed limits are 6kts and are pushed our further every year.
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Old 05 June 2016, 20:10   #8
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John on here developed an app to help with this, not sure if it was ever released but I have the beta version on my phone as I tested it.

As others have said above your boat creates a hump of water in front of you. However the speed you can travel at before you start to climb the hump relates to you boat length. So assuming you have about 4.85m of hull in the water you can only travel at 5.3 knots before you start to fight against the hump.

Once over it though you're away

So if you're in a 6 knot limit there's no point in doing 6 knots as you're wasting fuel and creating loads of wash

Here's a pic of the app assuming that you have 4.85m of hull in the water...

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