Optimize outboard to inflateable

[jfkal]

I am not happy with the performance of my inflatable a West Marine 3.1m with inflatable keel and a Suzuki 6 HP engine (4 stroke).
I added Hydro foils (some improvements) added trimtabs (worse).
I get it on a plane with one person in no time (half throttle). But as soon a second person is on board it is displacement mode only. Engine revs do not increase any more from 3/4 throttle up to full throttle. Would a prop with a shallower pitch help?
Any structured approach to optimize the performance, matching boat to engine, engine angle and prop specs?

Many thanks for your help.

Joerg

 

[exfinnsailor]

Bigger engine needed

 

[jfkal]

Bigger Engine

Bigger engine needed

I should have added (without spending more money for a bigger engine) :-(

 

[Mr Cassandra]

I should have added (without spending more money for a bigger engine) :-(

Diet?

 

[FishyInverness]

The "Rule of thumb" is that if you have a more laden boat, you should reduce the pitch and increase the diameter on your prop, increasing your low end torque but reducing your top end speed. The other option is a Four Blade prop, if one is available for your engine. (EDIT : Can't seem to find one for the DF6 in the range i'm afraid...) You need to bear in mind that you should be staying within the recommended rev range for the engine to the craft you're using though, to avoid excessive wear on your outboard....I'll see if I can look up an optimum prop using the documents i've got hanging around somehwere....

(Incidentally I have the same problem with a Suzuki DT4 on a 3m air keel inflatable - but seeing as I only potter around i'm not too bothered! )

Ok, i've gone through the documents - This is working over two manufacturers (Honda and Suzuki) and unfortunately my Honda prop suggestion guide only goes down to an 8hp - but based on that with your size of inflatable, it suggests a prop with a 9.25 inch diameter (diameter being the key if you're looking for more low-down power) and 8 inch pitch - the Suzuki DF6 appears to only have one diameter size of prop, being a 7.5" diameter and a choice of 7, 6.5 or 6 inch pitch which could limit your options somewhat

 

[jfkal]

Diet

Diet?

Who? Me? As I said if it only me in the boat it is planing. And I am not overweight, only under-tall.

 

[jfkal]

Pitch

The "Rule of thumb" is that if you have a more laden boat, you should reduce the pitch and increase the diameter on your prop, increasing your low end torque but reducing your top end speed. The other option is a Four Blade prop, if one is available for your engine. (EDIT : Can't seem to find one for the DF6 in the range i'm afraid...) You need to bear in mind that you should be staying within the recommended rev range for the engine to the craft you're using though, to avoid excessive wear on your outboard....I'll see if I can look up an optimum prop using the documents i've got hanging around somehwere....

(Incidentally I have the same problem with a Suzuki DT4 on a 3m air keel inflatable - but seeing as I only potter around i'm not too bothered! )

Ok, i've gone through the documents - This is working over two manufacturers (Honda and Suzuki) and unfortunately my Honda prop suggestion guide only goes down to an 8hp - but based on that with your size of inflatable, it suggests a prop with a 9.25 inch diameter (diameter being the key if you're looking for more low-down power) and 8 inch pitch - the Suzuki DF6 appears to only have one diameter size of prop, being a 7.5" diameter and a choice of 7, 6.5 or 6 inch pitch which could limit your options somewhat

Thanks that does help already a lot. Any other insights, tips and tricks anyone?

 

[landlockedpirate]

Sorry, but you just dont have enough power. A 3.1 m inflatable with a heavy 4 stroke 6 HP motor won't do it, regardless of prop or trim. You need at least an 8 HP to get over the hump with 2 adults on board.

 

[FishyInverness]

^

I think that's what I was also trying to say, but ended up on a circuitous route without reaching a real outcome!

 

[GrahamM376]

We have 2.9m inflatable keel & floor dinghy (42 kgs) with Suzuki DF6 and it planes well 2 up with some shopping but with 2 people + 2 x 25L water drums it's a struggle. Prop C611 is 7.5 x 6.5 which is smaller pitch than standard 7.5 x 7. Also added wings/hydrofoils which helped. Don't like the engine, wouldn't buy another.

 

[Rigger Mortice]

Our inflatabubble is a similar size. We used to use a 15hp Yamaha with it. Two up would plane without any problem. Three up would just plane but with difficulty.
We now use it as a tender and with a 3.3 Mariner I can just get it to plane one up.....but only just and when conditions are OK.

As others have said, I doubt that with a 6hp engine you will ever get it to plane reliably. You need a bigger engine.

 

[Haven't-a-Clue]

I had a Honwave 3.2m inflatable with V-floor and an 8hp Honda 4-stroke (which tips the scales at 48kg, I've still got it) and one up it'd plane no trouble and do 16kts, however, add another person and it took full throttle to get on the plane and would top out at 12kts, but didn't need full whack to keep it there. We did have to sit well f'ward to get on the plane 2 up, but could sit back when planing. I'd agree with the others, you need more power to plane two up. We now have a 2.4 version and with a 3.5 hp Mercury it will JUST plane 1 up, 2 up it's displacement only.

 

[jfkal]

We have 2.9m inflatable keel & floor dinghy (42 kgs) with Suzuki DF6 and it planes well 2 up with some shopping but with 2 people + 2 x 25L water drums it's a struggle. Prop C611 is 7.5 x 6.5 which is smaller pitch than standard 7.5 x 7. Also added wings/hydrofoils which helped. Don't like the engine, wouldn't buy another.

I will try that. And totally second [Don't like the engine, wouldn't buy another]. Nothing but trouble. Every time when I had to really rely on it, it gave up the ghost. Spent more on repairs than on buying the engine itself. Will use it as a mooring anchor when it packs up one more time and buy a 8HP as suggested by others.

 

[Rigger Mortice]

and buy a 8HP as suggested by others.

If planing two up is important to you (and it was the reason you initially posted), I would seriously think about going bigger than 8hp. It will only be on the margin and you may be dissappointed.

 

[Csail]

bigger engine needed

true , all the ones i use have min. Of 10 hp.