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Old 13-01-2014, 08:42   #1
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Max Engine size?

When looking at the specs for various tenders they all post a max hp size for the outboard. Is that due to weight or power? For example there are several that say max 8hp but on Mercury's the 8 and 9.9 weigh the same. Could you put the 9.9 on or is there some other factor that I am unaware of?
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Old 13-01-2014, 16:03   #2
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Re: Max Engine size?

Power. It's about the loads on the transom when pushing the hull and payload through the water.
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Old 13-01-2014, 16:07   #3
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Re: Max Engine size?

I think there is some kind of formula that is followed too. It's real common to see high power Bass boats and other that have motors that are WAY bigger than allowed on the decal.
If it were me and an 8 and a 9.9 were the same weight, I'd definitely go for the 9.9.
You know where 9.9 came from don't you?
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Old 13-01-2014, 16:39   #4
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Re: Max Engine size?

The Code of Federal Regulations spells out the maximum hp based on a formula combined with a chart and subject to exceptions (under certain circumstances). See eCFR — Code of Federal Regulations for the details.

Even given this is a pretty stupid way to figure out what size engine a boat should have its what we have. However designers do tend to then design boats to the specifications found here, so it is something of a self fulfilling prophesy in that if the USCG says the max hp you can have is a 9.9, then it doesn't make any sense to design the boat to use a 50hp motor anyway.
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Old 13-01-2014, 16:53   #5
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Re: Max Engine size?

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Originally Posted by ontherocks83 View Post
When looking at the specs for various tenders they all post a max hp size for the outboard. Is that due to weight or power? For example there are several that say max 8hp but on Mercury's the 8 and 9.9 weigh the same. Could you put the 9.9 on or is there some other factor that I am unaware of?
Someone else mentioned loads on the transom. Its a good point, especially if the weight of the engine is identical. This would only be an issue with a heavily laden tender , attempting high thrust in a lumpy chop with considerable and fluctuating loads on the transom.

But for every day running to and from the shore, no problems. Especially considering that the max recommended loads will still have some room to play .
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Old 13-01-2014, 16:58   #6
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Re: Max Engine size?

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You know where 9.9 came from don't you?
No I don't. I assumed it was for insurance reasons but was never sure. Whats the reason?
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Old 13-01-2014, 17:29   #7
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Re: Max Engine size?

My understanding is the the reason is stability as well as structural.

I took a quick look at the formula a while ago and beam was a key component, as was the number of persons to be carried.

Having had the interesting experience of putting a tiller hard over as a result of misinterpreting the force needed when using a tiller extension I can attest that even a correctly sized motor can give a real scare.

Yes, an experienced skilled driver can get away with more power than recommended, but disaster may only be half a heatbeat away.
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Old 13-01-2014, 17:49   #8
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Re: Max Engine size?

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No I don't. I assumed it was for insurance reasons but was never sure. Whats the reason?

Several years ago many small lakes and other bodies of water rules were passed prohibiting engines 10Hp and larger, so the manufacturers not to be outdone, re-labeled engines 9.9.
Now I can't prove that, but it was what I was told years ago.
Maybe urban legend for all I can prove.
Only difference exterior dimension wise for many modern engines is the stickers, I know a few that have put 9.9 stickers on engines up to 20 HP or so. without looking hard at the data plate who can tell?
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Old 13-01-2014, 17:59   #9
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Re: Max Engine size?

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My understanding is the the reason is stability as well as structural.

I took a quick look at the formula a while ago and beam was a key component, as was the number of persons to be carried.

Having had the interesting experience of putting a tiller hard over as a result of misinterpreting the force needed when using a tiller extension I can attest that even a correctly sized motor can give a real scare.

Yes, an experienced skilled driver can get away with more power than recommended, but disaster may only be half a heatbeat away.

I agree with your statement, but the rules suppose a fool, assume a 50 lb kid running an empty boat at full throttle, I believe a prudent adult can keep their speed in check, you don't have to twist it wide open just because you can. Plus it's my opinion that an engine run at say 70% power will outlast one often run wide open. I'll go the bigger HP if it's the same weight as a lower HP every time.
National speed limit is 70 MPH? But autos are sold that can exceed 200? Same thing in my book, just because it's there doesn't mean it has to be used.
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Old 13-01-2014, 18:32   #10
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Re: Max Engine size?

> Several years ago many small lakes and other bodies of water rules were passed prohibiting engines 10Hp and larger, so the manufacturers not to be outdone, re-labeled engines 9.9.

In many jurisdictions, it is also the cut-off point for whether the operator and/or boat needs a licence or not.
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