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consumedbynature
United Kingdom
108 Posts |
 Posted - 09 Feb 2014 : 23:10:42
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Hi. I have a 1982 Shetland Black Prince with a 120hp inboard engine, power trim and trim tabs. It weighs 2280kg plus however many persons on board. There seems to be a boomerang shaped fin on the bottom of the leg.
The boat is used at River speeds up to 6mph and has a 14 x 13 prop which I presume was fitted by the last owner as the boat was used on the broads. The original paperwork shows the boat was fitted with a 14 x 17. I find that I have to use 2000-2500 rpm to travel 6mph. The current engine was fitted after 1992 and is well serviced.
In the summer whilst on tidal waters I tried to get the boat onto the plane without success. It just made a lot of splash and noise and the gps on my phone registered a top speed of 9mph.
My question is will the boat go faster if I change to a 14x17 prop and will it get on the plane or is it likely to be too heavy?
Thanks
Billy
Shetland Black Prince -Fizzy - Worcestershire Avon
(Formerly owned a Norman 20) |
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Deano
United Kingdom
1843 Posts |
Posted - 09 Feb 2014 : 23:25:08
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My gut feeling is that with 120bhp on tap there should be a prop size that will get it on the plane. I don't know it 14x17 is correct though?
What engine is it? what is the max revs when you achieved 9mph? What is the max revs on load in the engine spec?
People talk of under propped and over propped. You need to be at max revs and still just pushing with the prop, for optimum plane potential.
Dean - Boating on the Great Ouse.
Freeman 32 "Liberty"
See the photos http://www.flickr.com/photos/54758027@N00/ |
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consumedbynature
United Kingdom
108 Posts |
Posted - 12 Feb 2014 : 16:08:27
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The boat has a 2.3ltr volvo Petrol engine on a sp A1 leg with cd trim.
I pushed the boat up to 3800 revs but due to the digging in and amount of wake and heat I didn't give it anymore
I believe the recommended max revs is 4700 with cruising revs of 4500.
Thanks
Billy
Shetland Black Prince -Fizzy - Worcestershire Avon
(Formerly owned a Norman 20) |
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consumedbynature
United Kingdom
108 Posts |
Posted - 12 Feb 2014 : 16:09:19
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The boat has a 2.3ltr volvo Petrol engine on a sp A1 leg with cd trim.
I pushed the boat up to 3800 revs but due to the digging in and amount of wake and heat I didn't give it anymore
I believe the recommended max revs is 4700 with cruising revs of 4500.
Thanks
Billy
Shetland Black Prince -Fizzy - Worcestershire Avon
(Formerly owned a Norman 20) |
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mike
United Kingdom
80 Posts |
Posted - 14 Feb 2014 : 17:08:56
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Its a few years since i drove a ski boat but my thoughts are you need to break the problem down into sections.
First worry is when you say heat I assume you mean the engine? why is it getting hot in a short time.
Second and apologies if I am telling you something you already know, but when you get a boat up on to the plane as you open the throttle the bow lifts up, the stern digs down and a big wake is created, if you keep the throttle open the boat should clime its own bow wave and come up on the plane. It can be a bit disconcerting the first few times because when you open the throttle you just see sky for a few seconds.
This brings us to the third section outdrive trim, at slow speeds within reason it wont make much difference, but at full throttle if the leg is trimmed out it can push the stern down.
Fourth sort out the prop, if none of the above are the problem, pour a large scotch (other beverages are available)go on google, and type in outdrive prop sizes or anything similar, it will keep you quiet for hours - there is no magic answer about props.
Sorry if i have gone over stuff you already know but hope it helps
Mike
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Deano
United Kingdom
1843 Posts |
Posted - 15 Feb 2014 : 00:41:51
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How clean is the bottom of the boat??? It needs to be quite smooth to plane. If there is too much fouling that can create enough drag to stop it getting over the hump if it is borderline.
Once on the plane you can reduce the throttle and stay there. I need 13 knots to get over the hump, but she cruises at 11 knots once up there.
Dean - Boating on the Great Ouse.
Freeman 32 "Liberty"
See the photos http://www.flickr.com/photos/54758027@N00/ |
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cliveshep
Thailand
1324 Posts |
Posted - 16 Feb 2014 : 18:28:57
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Your boat is severely under-propped I'd say. I used to plane a Norman 20 with a 8ft grp dinghy on davits, plus a 3hp motor, with a large Seagull auxillary in the stern locker, full water and fuel tank, 4 large adults, dog, cat, and all the gear.
It probably did in the order of 25-30 knots, on fixed trim tabs, with an Enfield stern drive driven by a 1600cc Ford cross-flow.
If you can't plane a Black Prince with 2300cc something is severely wrong.
If the engine roars up to max revs and the boat crawls - put a coarser pitch prop on it and keep trying coarser and coarser until the maximum revs are ALL it will do. With tabs the boat should be then be flying!
The cost of boating is insignificant compared to costs of a young wife and two teenage kids! |
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Prop size. Will she plane?
