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Old 15 September 2013, 09:39   #1
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which is the right outboard hp

Hi all I would like to know if there is any formula to work out the right engine hp by lenght and weight. Thanks
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Old 15 September 2013, 11:37   #2
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Look at the spec of the boat and see what it's rated for, different manufacturers with the same length boat may have very different maximum engine size, go over it and you can get into all sorts of problems.
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Old 15 September 2013, 13:13   #3
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Hi Thomas my rib spec indicates max hp is 50 and I have 60hp installed but I found another rib that is built by the same company but with an other name and its spec say 50 hp max recommended but 80hp max allowed and I would like to upgrade to 90hp as it is the same weight as a 70 hp
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Old 15 September 2013, 14:27   #4
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What length is the RIB? I can only go buy my limited experience but I had a 4m Avon a while ago with a 50hp Yamaha on it, I'd have thought between 4m and 4.5m a 50 would be ok, 4.5 to 4.8 then a 60 but so much depends on the boat.
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Old 15 September 2013, 14:32   #5
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Rib is 16 ft and it weight around 340kg rib only
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Old 15 September 2013, 14:37   #6
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Quote:
Originally Posted by thomas View Post
Look at the spec of the boat and see what it's rated for, different manufacturers with the same length boat may have very different maximum engine size, go over it and you can get into all sorts of problems.
+ 1 hull design, transom thickness play a part in the size and power of engine that can be fitted don't think you can just broad brush because of the length of boat
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Old 16 September 2013, 04:56   #7
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If your goal is max sped then engine size will not be the limiting factor but the ability of the hull and the cox to control that boat. Chine walking will be an issue. I remember that you posted some time ago bemoning the speed of your rib which I happened to think was quite fact. It should also be noted that a greater weight on the transom will upset the weight distribution on your boat.

TSM
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Old 17 September 2013, 10:53   #8
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Quote:
Originally Posted by Jason calleja View Post
Hi Thomas my rib spec indicates max hp is 50 and I have 60hp installed but I found another rib that is built by the same company but with an other name and its spec say 50 hp max recommended but 80hp max allowed
What are the build dates of these boats? if yours says "max 50" and the other is "max 80" that doesn't sound like the same boat to me!

Take mine for example. Ignoring the name / branding, it's essentially a MK1 version of twhat is now the O-Pro 2m Beam. Mine takes max 60Hp.

The new ones will take 90. If I hung a 90 on mine the last I would see of the engine would be accompanied by the sound of snapping plywood! Having spoken to Humber about weight capacity they confirmed it was thre same basic hull shape, but as said above, the transom is now thicker amongst other subtle structural mods.



Quote:
Originally Posted by Jason calleja View Post
and I would like to upgrade to 90hp as it is the same weight as a 70 hp

You are proposing to nearly double the max hosses allowed on the transom. That can only lead to a structural failure.

Also the weight - I have a 5m boat, and the handling in big waves alters substantially depending on whether I store the 30L fuel tank at the bow or stern. Regardless of the 70/90 being the same weight, both will be heavier than your version of the hull was designed for.....


If it works, you'll have an absolute hoot of a ride, but be aware it could end up as a very messy & expensive breakage...... Probably worth checking with the builder what the story is regarding hosses & weight.
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Old 17 September 2013, 11:12   #9
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What I would like to know is how a manufacturer works out what size engine goes on what size RIB. Is it all in the design work or just trial, error and experience?
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