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Old 28 April 2014, 03:41   #1
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30hp outboard on a sib marked as a 25hp max?

I'm just curious but I recently brought a 30hp outboard. I'm looking for a sib but a lot of the newer sib's are marked up to 25hp. Surely if 25 hp engines and 30hp are effectively the same there should be no problems that arise from it while at sea? Also curious if I put it on a sib marked as a 20hp max on transom. My budget is limited. I currently have it ready to go on my avon sib which is rated up to 40hp but the inflatable keel has given up with multiple leaks in hard the patch areas .
Any advice would be great
Cheers Steve
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Old 28 April 2014, 04:22   #2
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Quote:
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Surely if 25 hp engines and 30hp are effectively the same there should be no problems that arise from it while at sea?
Hi Steve - welcome to RIBnet

Maybe you could clarify what you mean by "effectively the same"

For example apart from the 20% difference in power and subsequent speed - a Yamaha F25LA weighs 81kg where as a Yamaha F30LA weighs 97kg - that is 16 kgs (that also is nearly 20%) all of which will make a huge difference on the back of a SIB.
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Old 28 April 2014, 05:20   #3
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Those older Avon SIB's were much higher rated in terms of HP than more recent PVC types.
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Old 28 April 2014, 06:29   #4
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From reading other previous post I come to understand that 25and 30hp engines are you same. I own an old Suzuki dt300 which I brought recently at a boat jumble sale in plymouth at a bargain price of 140 and it runs perfectly, however it feels exactly the same weight as a 15hp mariner 2 stroke I used to have on my sib. So if a sib rated 20hp max can can the 15hp surely it can take the 30hp without any issues?
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Old 28 April 2014, 06:38   #5
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Steve , you may just want to run the idea past your insurers before you go ahead ?
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Old 28 April 2014, 07:19   #6
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Quote:
Originally Posted by Stevenmb View Post
From reading other previous post I come to understand that 25and 30hp engines are you same. I own an old Suzuki dt300 which I brought recently at a boat jumble sale in plymouth at a bargain price of 140 and it runs perfectly, however it feels exactly the same weight as a 15hp mariner 2 stroke I used to have on my sib. So if a sib rated 20hp max can can the 15hp surely it can take the 30hp without any issues?
Why? The ratings are there for a reason - why should a SIB rated to 20hp be fine with a 30??!

There is no way a 25 or 30 weighs the same as a 15. The Mariner 15 2 stroke is about the lightest 15 out there at 34kg.
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Old 28 April 2014, 08:01   #7
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I think I now where you are coming from. i.e. A 1990 Yamaha 2 stroke 25hp Origin is the same engine as a 1990 2 stroke 30hp Yamaha, just with more power.

Lots of people over engine boats, but you need to make a considered judgement on what is safe.

First thing would be to check with your insurance, I dont just mean insure the boat with the bigger engine on the policy. You should tell them you want to use an engine that is Xhp over rated for the boat and get it in writing that they will cover you if it all goes wrong.

Consider how much you are going to over rate, take into account weight and power of the engine. In a few cases (the Yam above) the 25 is the same as the 30 but it most cases its a world of difference with the 25hp being 2 cylinder and the 30hp being 3 cylinder and 30-50% heavier.

What is the condition of the boat ? A 20 year old sib designed for a max 25hp 2stroke wont last long with a 30hp 4 stroke on it, it will either rip the transom support pads or shred the glue very quickly.

I respect that you have asked the question, but the fact that you have needed to ask tells me you shouldnt do it.

There are a few sibs that will take a 30hp, Honwave T40, Zodiac Futura, Xpro Commando etc, you just need to look for them. Or sell the 30hp for a profit and find a smaller one.
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Old 28 April 2014, 09:55   #8
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If you put a larger engine on the rear of a rib, or sib that it is rated for, you instantly give any insurance the excuse they require not to pay out if anything goes wrong.

If that was the case, and it was me you injured, I would come after you with a very big solicitor and take everything you owned, possibly even your freedom. limits and rating are there for a reason !!
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Old 28 April 2014, 10:55   #9
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with two stroke e.g. mercury, tohatsu amd mariner the 30hp and 25hp are the same engine, but the 25hp has a gasket restricting the air flow.

So a 30hp on a 25hp rated sib should be fine, can't see anything wrong if they are the same weight. The only issue you have is insurance unless you change the stickers to 25hp?

Henry
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Old 28 April 2014, 11:22   #10
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Changing the stickers would leave you criminally culpable for insurance fraud should the worst happen.

Find a friendly insurer that will accept you overpowering the boat, or change the boat/engine to something more suitable.
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