Performance of inflatable with new 6hp Mercury ?

[Capt'n Caution]

I am planning to buy a new 10' x 5' inflatable of the usual configuration with tail cones on 16" tubes and an inflatable keel. Has anyone had personal experience with the how this boat would perform with a new 6 hp , 4 cycle Merc? The boat will have an air floor, will weigh 100 pounds with basic essential gear; engine, gas and tools etc. about 100 pounds; 2 people and gear about 300 pounds. This totals 500 pounds max. Will the boat actually plane, or just do a fast or slow mush? The reason is we both have bad backs, plus we cannot trailer it to where we want to fish. I already have a new Merc 10 hp which would be perfect, but it is a bit too heavy at 84 pounds for us to safely manage.

 

[ondarvr]

You aren't going to be going anywhere quickly, but you will get there. While the small single cylinder 4 strokes are lighter, they also tend to be louder and vibrate a great deal compared to what you have now. But I understand the weight issue with a bad back.

 

[QBhoy]

Hi
I'm afraid to say that I'm pretty sure it won't plane. If by some miracle it does, it will require clever weight distribution and the engine flat out.
Having had loads of different inflatables and small ribs, I can tell you the following tips and experiences;
If your inflatable (given that it's modern) says the max hp is xxhp, it will be referring to a recent 4 stroke engine.
This leads me onto my next point...an average 4 stroke 4/5/6hp has next to no guts or torque, but weigh the same or more than a 8hp 2 stroke. The latter having much more power and an extra cylinder. It's worth noting that late vintage 2 stroke engines of this size are like hens teeth (very rare) to the market, for this reason.
In my opinion, the way forward for your situation is to get a good 8hp or 10hp 2 stroke. You should then expect easy planing, sustained plaining at half to 3/4 throttle and speeds of up to and perhaps exceeding 20mph.
Generally the longer the inflatable, the easier it is to plane.
Long story short, get yourself a good 2 stroke.

 

[Sea Rider]

Have in use a 320 Mtr Sib with inflatable keel with a 5 HP OB with maximized prop, only good for fast displacement speeds on flat calm no wind water with 2 up due to size & hull drag. What you can do if wanting to get the max out of that 6 HP is install a Hardline induction hour/tach , inflate Sib's tubes and deck to its factory working pressure, go for a wot spin as usually loaded on flat calm no wind water cond and check achieved max wot rpm at full grip.

Can maximize a less prop pitch to pull wot revs up towards max range, impossible to know it that 10 footer will plane or not due to size and hull drag, at least OB will perform much better throughout the whole throttle range than with current delivered prop.

Happy Boating

 

[Capt'n Caution]

This forum is a remarkable source of experienced information. Previously, I talked to several inflatable and motor dealers, and got no good information. Now, in such a short time I have reliable, experienced advisors who have willingly given me the benefit of their time and wisdom. Very much appreciated!

Since I believe that a fully planing inflatable, not necessarily very fast, is safer and more efficient, I am going to use my current 9.9 Merc. I just have to figure how to build a device to allow both of us to safely lift, and carry the engine to and from the car and the boat.

Thanks!!

 

[Sea Rider]

Are we talking about a 10 footer air deck or alum deck inflatable ? if you haven't bough it yet, would it be an issue going tad larger ? a larger Sib will perform much better with a 6 or 10 HP that a shorter 10 footer. Will plane faster, is roomier, more maneuverable and easy to control at speed.

Happy Boating

 

[ronaldj]

I am planning to buy a new 10' x 5' inflatable of the usual configuration with tail cones on 16" tubes and an inflatable keel. Has anyone had personal experience with the how this boat would perform with a new 6 hp , 4 cycle Merc? The boat will have an air floor, will weigh 100 pounds with basic essential gear; engine, gas and tools etc. about 100 pounds; 2 people and gear about 300 pounds. This totals 500 pounds max. Will the boat actually plane, or just do a fast or slow mush? The reason is we both have bad backs, plus we cannot trailer it to where we want to fish. I already have a new Merc 10 hp which would be perfect, but it is a bit too heavy at 84 pounds for us to safely manage.

I basically set up my boat for having a bad back
My boat is 84lbs and 11 feet and running a 5hp

Just so you no Zodiac makes a 10 foot rib boat that has a folding transom, folding tubes and fits in a 7 foot bag
That will run better with your 6hp than your sib with a 9.9hp

You may also want to look at a 13,14 or 15 foot Kaboat.
They are specifically made for low hp outboards

The thing about having a bad back is that
It will not get better as you get older

Your only real options are to change the boat or get a trailer

 

[km1125]

I have an older Nissan version of that same 6HP Merc and it's on a 8' inflatable keel SIB.

The big trick to planing was to use a high-thrust prop and it will get on plane with 2 people, a dog and small cooler without much of an issue. The 7 and 8 pitch props are too big to get the engine into its power range, but the 6 pitch "elephant ears" or "mouse ears" prop works great. I can cruise about 15 mph depending on conditions.

 

[Sea Rider]

When cruising at 15 mph asume at full throttle, what's the max wot rpm achieved ?

Happy Boating

 

[km1125]

When cruising at 15 mph asume at full throttle, what's the max wot rpm achieved ?

Happy Boating

I'm assuming that question was for me. I did not have a good tach to measure. Thinking of picking up a tinytach this year so I'll know better. Yes, 15mph would be full throttle, but whether or not I'm at full throttle depends on who's onboard and what the water is like. I can still be up on plane at 11ish with some throttle to spare.

 

[Sea Rider]

KM,

Installing an induction tach on a smaller inflatable with a under powered OB is the way to go. Will see rpm differences boating with 1 or with 2 up on flat calm no wind water cond. Ideal is going for individual wot runs with 1 and 2 up to know wot rpm achieved on both runs Have you tested a 7 prop to say that won't get OB into it's power range. On the other side, could be over revving OB at full throttle with a 6 prop and not knowing anything about it.

Happy Boating