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Old 20 November 2004, 15:01   #1
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Country: UK - Scotland
Town: Lochgelly
Boat name: Purrdy
Make: Northcraft
Length: 5m +
Engine: Mariner 135hp ob
MMSI: 235029446
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Posts: 46
New to the forums

Hi folks, new here and a few questions.
Just bought my first rib having used club ribs for the last 10 years, Northcraft 5.5 with a Mariner 135 v6 2.0 litre.Complete package 1999 with 50 hours on the engine. She seems very sensitive to throttle unlike my previous experience. In choppy conditions if you back off a little she drops off the plane very quickly and slams to a standstill in the waves. You can`t run her hard in these conditions since even in about a force 5 she wants to either do 6 knots or 40 knots your choice on the throttle switch. In rough conditions she seems to gather herself for a second or two before launching onto the plane and then she is just too fast for the conditions. In calm water she is on the plane in her own length and will hit 45 knots in a few seconds at about 5700 revs.
Stainless prop with 21 stamped on it. Thoughts on dropping to a shallower pitch or fitting cav plate hydrofoils?
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Old 20 November 2004, 15:40   #2
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Hi and welcome.

Not suprised she does 45 knots thats a huge amount of power for a 5.5m rib, especially if it doesn't have much weight in it. If its the standard throttle box for Mariner they are awful, you could consider changing it to something better which would give a smoother throttle action.

The prop sounds about right if its 5700 revs at WOT. that would drop slightly with some people on board, a smaller 19" would be even faster to accelerate and you would be pulling wheelies don't think the fins are going to do much either at that speed.

Pete
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Old 21 November 2004, 05:05   #3
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Country: UK - England
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Boat name: Sea Quell
Make: Picton Cobra
Length: 5m +
Engine: Mercury 150 4 Stroke
MMSI: 235038298
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Hi and welcome,
You should find the forum friendly, interesting and helpful - sounds like you will need to take care in the Forth in your pocket rocket, but I am sure you will get used to her.
I'll keep a look out for you when I am next up diving the May - drop a photo on the forum or mail me.
Enjoy life without the hassles of club boat diving !! I did
Jeff
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Old 21 November 2004, 05:19   #4
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Country: UK - Scotland
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Boat name: Purrdy
Make: Northcraft
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Engine: Mariner 135hp ob
MMSI: 235029446
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Posts: 46
The Throttle box is a Quicksilver, I think, since the boat is undercover and under a inch thick layer of sleet and ice this morning. Glad I took her out for a spin yesterday rather than today. I`ll look out for you Jeff round the Isle of May.
Mine is yellow and black tubes on a black hull. I will post some pics of her soon.
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Old 21 November 2004, 07:52   #5
tue
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You could try fitting some Quicksilver Platinum control cables. They are much smoother running with less friction so can take sharper turns and should allow you much finer throttle control. I fitted them to mine and you can control the revs by +/-200-300rpm!!

Also check your propeller for some rubber plugs behind each blade. Mine were open when I first got it and it would rocket onto the plane and drop off very quickly with slight throttle decrease. Since I put the bungs in it has improved the slow planning speed of the boat. 0 to planning is slighly reduced but with an engine that size its not going to struggle.

EDIT what ever you do DO NOT fit hydro foil fins
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Old 21 November 2004, 14:06   #6
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Prop and cables

Quote:
Originally Posted by tue
You could try fitting some Quicksilver Platinum control cables. They are much smoother running with less friction so can take sharper turns and should allow you much finer throttle control. I fitted them to mine and you can control the revs by +/-200-300rpm!!

Also check your propeller for some rubber plugs behind each blade. Mine were open when I first got it and it would rocket onto the plane and drop off very quickly with slight throttle decrease. Since I put the bungs in it has improved the slow planning speed of the boat. 0 to planning is slighly reduced but with an engine that size its not going to struggle.

EDIT what ever you do DO NOT fit hydro foil fins
Agree with all of the above. Had the same symptoms on the Searider. The rubber plugs are available from your Mercury / Mariner dealer - about 30p each.
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Old 22 November 2004, 03:26   #7
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The prop is a Laser II and has the 3 holes each with a bung in it with a hole about 7 mm in the centre of each bung. I assume these holes are calibrated and allow exhaust out to allow the prop a little cavitation at low revs, high throttle so the motor doesn`t bog down thus allowing faster accel from standstill. Are you saying that you recommend no hole? Or that some people remove the bung completely and that is a problem? Also why do you not recommend foils, I have had good results on 150 Johnsons with the rib becoming easier to steer and planing much happier when heavily loaded.

Oh and thanks for all the advice
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Old 22 November 2004, 06:03   #8
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Holes

Quote:
Originally Posted by Nyarla
The prop is a Laser II and has the 3 holes each with a bung in it with a hole about 7 mm in the centre of each bung. I assume these holes are calibrated and allow exhaust out to allow the prop a little cavitation at low revs, high throttle so the motor doesn`t bog down thus allowing faster accel from standstill. Are you saying that you recommend no hole? Or that some people remove the bung completely and that is a problem? Also why do you not recommend foils, I have had good results on 150 Johnsons with the rib becoming easier to steer and planing much happier when heavily loaded.

Oh and thanks for all the advice
I believe that you can get bungs with different size holes - but my prop was a small hub with only one choice of bungs - no holes in them.

You are correct about how the holes work - allowing the prop to ventilate to prevent the motor bogging down. However, with 135hp, I don't think that this would be a problem for you anyway. My boat worked best / most predictably with the holes closed.
Anyway, its a cheap and easy thing to try out first.

Have never used the foils but am sceptical of anything that can be added to "improve" an engine that the manufacturers don't use. Still, relatively cheap to try out and may calm things down a bit.

Is your throttle cable stiff as has been suggested? If so this would be a worthwhile route of investigation.
Try undoing the throttle cable friction screw on the throttle box. If the throttle does not slow ly back itself off then chances are the cable is a bit sticky.

Some engines have a very definite point where they come into their power band and with a sticky throttle it is very difficult to fine adjust the speed in the range where the engine is just starting to make good power.

Regards
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Old 22 November 2004, 08:15   #9
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Quote:
Originally Posted by Nyarla
Also why do you not recommend foils, I have had good results on 150 Johnsons with the rib becoming easier to steer and planing much happier when heavily loaded.
I fitted "hydrofoils" early on - something not to declare in public !!! - and they certainly improved the ride - quicker on to the plane, less tendancy to drop off at lower speeds, and a generally better attitude.
Certainly they are no substitute for the perfect RIB, rigged to give perfect balance - but on a modest 5.5m, carrying overweight divers and kit, there is a limit how you can design the layout. ......... bigger would be better - but the banker at home didn't understand !

Jeff
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Old 22 November 2004, 14:23   #10
tue
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Quote:
Originally Posted by Nyarla
Also why do you not recommend foils, I have had good results on 150 Johnsons with the rib becoming easier to steer and planing much happier when heavily loaded.

Oh and thanks for all the advice
Low Down on Dol-fins Look at this thread and do a search for doe-lins/hydrofoils and you will find enough info to tell you not to fit them. Especially on that setup of big motor light boat.

Plastic bungs in Prop This one is the prop bungs.

Cheers
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