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Old 14 July 2016, 19:21   #1
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How is the hp of an inflatable decided? Boat recommendations?

How is the hp rating of an inflatable decided? I can understand how the weight limit is determined, but not the power.

For example 15hp and 20hp Suzuki's weigh in the same at 44kgs, there's only 170 difference. Unfortunately I need to suffer a 23kg weight penalty because a 9.9hp is not going to allow full use from the boat I would buy.

I figure with one person, a 15hp with 11" prop will probably be faster than a 20hp with 10" prop so is it really so dangerous to go over a hp rating if you are within the transom weight?

I'm dead set on the DF20AS and want the cheapest and lightest way to use it while avoiding a craft that would pull apart after one or two season's use. I am forced to transport it on roof bars because of towing a caravan and would consider a RIB but prefer an inflatable with V air deck. I'd really like to get pulled up on a wakeboard (doing nothing extreme) and tow a tube around but other times the boat will be heavily loaded 2-3 adults +2 dogs and beach gear.

In 2017 I will be returning to the water 37 years after being a teenager in dad's twin hull rigid Sportyak II with 3.5hp Evinrude (lol!). I'm currently considering a SD330 Excel rated to 15hp and think I prefer the SD330 over the 20hp SD365.


Thanks for your input!
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Old 15 July 2016, 00:49   #2
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I think 3 adults 2 dogs and beach gear is going to make a 3.3 metre sib feel very small
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Old 15 July 2016, 02:46   #3
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I think you are right, especially because I would like a bow canopy to give the pooches some protection. 3.6m seems minimum size but I'm still intrigued about hp rating.
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Old 15 July 2016, 03:20   #4
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if it were me i would buy the 20 be sensible with it on your current boat and when you upgrade buy the boat that suits the engine.
but consider insurance if you hit someone if you use it on your current boat the risk you take might be worth it to you.

cheers
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Old 15 July 2016, 03:39   #5
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Limecc welcome to the forum. My uncle owned a couple of Sportyyaks over the years... powered by a Seagull so a pretty sluggish outfit.

When you ask about how the HP of an outboard is decided do you mean by the user or the makers approved power?

If the user it's usually the most powerful outboard for a given weight that is manageable in each person's specific circumstances. That is assuming price isn't a consideration.

If the maker then it's down to factors such as transom strength, its strength of attchment to the tubes and the stability of the SIB with a maximum power. When you are powering through choppy water with a 20hp it puts a lot of twisting strain on a transom... I've had a 15hp cause a Zodiac (brand new boat!) transom to delaminate in such circumstances.

If you look at (for example) the Zodiac models that are rated to 10/15hp compared to those rated to 25hp the extra build quality of the higher rated transoms is very evident.

Also note that often (certainly Zodiac) there is an advised hp rating and a maximum allowed... there is often a 5-10hp difference in this figure. You could assume that going to a maximum hp and running hard all the time is more likely to cause problems than keeping within the advised range.

You will see I have a new (in March) Suzuki DF20AS. It's a great outboard for our outfit and was chosen after experience with several motors in the 10-25hp range. However... I would never choose a specific outboard and then try and match a SIB to it. The SIB itself is far more important in getting one that suits the way you need to store/transport... how many folks... what kit you carry... the area of water you operate in and your budget.

As an example last year on holiday we were using a 10yr old Mariner 15hp 2-stroke with our Aerotec. This year we'll have the Suzuki DF20AS. If you had a ride out in both combinations you'd find the experience broadly the same as it's determined far more by the boat itself.

Or to flip the theory over last year we had both the Aerotec and a Zodiac 3.4 Acti-V air floor which were both used with the 15hp Mariner... massive difference in experience due to the SIB differences despite being powered by the same outboard.
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Old 15 July 2016, 05:15   #6
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Just a small point re-reading your OP. You mention wanting a SIB with an inflatable V floor and then that you are looking at the Excel SD330.

I think that model has the flat air floor and may not give the experience you're lookig for in a V floor.

I also agree with Beerbelly that 3 folks, two dogs plus kit will make a 330 feel small at sea.
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Old 15 July 2016, 05:18   #7
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In days of old max HP was determined by either a designer or more likely a wise old guy with lots of experience. There is however now an ISO standard, and manufacturers only exceed it's calculation if they show through real life testing that it is ok. I don't think it's just about speed, if it was it would just state a max speed.

Will a 20 on a 15 rated transom be OK, possibly. In fact if you drive it with caution probably. I have however seen one 15 rated transom get a nasty crack when it was given "full beans" with an 18. It wasn't the beefiest looking transom anyway and perhaps would have failed with a 15, but the warranty option was not there to get it fixed.

People make a big deal about insurance for over rated boats, but it is possible to insure.
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Old 15 July 2016, 05:38   #8
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OMO but i look toward buying a boat that will do most if not all i need it for hence i bought a heavy duty boat designed to carry the weight mostly for the diving fraternity with heavy kit.
even then its rated at only 25 hp, flat out it shifts along around 22 knts in perfect conditions how ever get a bit of sea and you've got to throttle back. so what i am saying is speed , conditions , power available, boat spec, even the correct engine is too much in certain circumstances.
the boats heavier build does stop me throwing it around on the beach but it suits me.
as you have said same weight on the transom keep the throttle back until you get your new boat.

cheers
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Old 15 July 2016, 07:52   #9
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Thanks for all the responses.

After chatting with Terry Wright today he gave a very competitive price on a SD360/DF20AS package so I think I will go with that 8 months early - at the moment he has stock and can hold pre-Brexit prices which I foresee will rise due to exchange rates next year.

I looked at a used Aerotec 380 on Ebay but figure I'm better off with a new Excel for the same money. They both seem to be good quality, I'm not going for the aluminium floor version unless you guys give me advice to the contrary, the dogs blunt claws would not grip it too well.

I have lots of newbie questions which are off topic so I'll start a new threads for those. Thanks guys.


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Old 15 July 2016, 08:07   #10
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Quote:
Originally Posted by Fenlander View Post
Just a small point re-reading your OP. You mention wanting a SIB with an inflatable V floor and then that you are looking at the Excel SD330.

I think that model has the flat air floor and may not give the experience you're lookig for in a V floor.

I also agree with Beerbelly that 3 folks, two dogs plus kit will make a 330 feel small at sea.
Wise words about the size but I thought all the Excel range had the RIB style V floor?

Excel Inflatable Boat Features.
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