smaller prop on 11' inflatable 1983 7.5HP Johnson

[merc20076]

Hi guys!

Finally, this past Saturday I went fishing with my new 11' D330 Brig SIB. These kinds of boats are really stable!

Ok I was alone and tried to balance the weight in the boat. My 7.5HP had a hard time get it on plane but when it got, it went 11-12mph. I had the bigger prop for that mtr, a 8.5X9P. I have a smaller prop, the 8X7 P, do you think this will do a big difference?

For Saturday fishing results I got a big 13 pounds lake trout!

 

[steelespike]

Re: smaller prop on 11' inflatable 1983 7.5HP Johnson

Yes the 7" will make a big difference.In my very limited experience the typical small inflatable
can be difficult to get on plane and is likely to run slower than a ridged hull of about the same size.Your 7.5 on a more typical boat would run about 15-18 mph.A 2" pitch change is somewhat extreme and should be very effective.You should get something like a Tinytach to check your rpm. rpm and speed is the only way to be sure the motor is running within its rpm range.Important for the health of the motor.

 

[Sea Rider]

Re: smaller prop on 11' inflatable 1983 7.5HP Johnson

Yes as stated by steelespike buy a Tiny Tach or Hardline tach, both are tach/hour meters and induction installation, once installed try both props at full throttle. Check min & max rpm factory parameters for that engine and see which prop gives medium range between min and max rpm, that way will have better hole shot and speed, not top speed though as you cannot have both at same time. That sib is rated for a 15 HP engine, so probably will need to add trim tabs to help plane faster if planning on keeping the 7.5 HP engine.

Happy Boating

 

[merc20076]

Re: smaller prop on 11' inflatable 1983 7.5HP Johnson

What could be the speed range with this prop? Yes I know, I should get a 15HP but buying a trailer for it is much more important for now. Wayyyy to long to set-up and set-down even with a good electric pump. Wasted money for a 2.5PSi electric pump and launching wheels...

It says on the tiller handle, 7.5HP @ 5000 rpm. Max 6000.

 

[Sea Rider]

Re: smaller prop on 11' inflatable 1983 7.5HP Johnson

Make a wot test using a GPS to check max speed achieved with both props, standard factory delivered prop should give middle range between 5 to 6 K rpm at wot. Both props must be inside those factory parameters at wot for the engine to work safely.

Happy Boating

 

[jspringator]

Re: smaller prop on 11' inflatable 1983 7.5HP Johnson

Boatstogo recommends a hydrofoil to make it easier to plane. Do you have a foil? I've been wanting to try the one that attaches to the skeg and protects the prop from grounding.

http://www.hydro-shield.com/

 

[Sea Rider]

Re: smaller prop on 11' inflatable 1983 7.5HP Johnson

It's not only about hydrofoils, trim tabs recommendations to plane faster, it's a matter of sib lenght, passengers to carry, overall weight to move and engine power. This idea of Hydroshield is nice, hope it's not a garbagge collector once installed.

 

[merc20076]

Re: smaller prop on 11' inflatable 1983 7.5HP Johnson

Update, maybe an issue last weekend.

Now that a have a trailer, I set up the boat again today, blew it with electric pump and then top it off with my foot pump. I kept pumping untill it was hard to pump. Is this over inflated, I did not force. Maybe it was not enough inflated last week. Anyways , second test tomorrow and I'll have my 2 props.

 

[Sea Rider]

Re: smaller prop on 11' inflatable 1983 7.5HP Johnson

To top max with a foot pump is the way to go untill rear tube's cones flexes slightly or use a pressure gauge. You need to be a real heavy weight champ to overinflate a sib that way.

Happy Boating

 

[merc20076]

Re: smaller prop on 11' inflatable 1983 7.5HP Johnson

Upadate!

Went back to Memphremagog lake yesterday and tryed my 8X7P prop and I turned right back to put back my standard prop. At WOT I just go up in the air and I go at arround 5 MPH. I think It has a spun prop hub.

With original prop it did plane better (still little slow) since it was at the correct pressure in the tubes, but on plane with a calm lake it is fast and fun! Do you think a hydro stabilizer would help plane it faster? I have a 3 gallons fuel tank and I almost have no choice to put it in the back.

 

[Sea Rider]

Re: smaller prop on 11' inflatable 1983 7.5HP Johnson

That's why manufacturers sends engines when new with medium range pitch, performs better as you have tested, that engine is not that powerfull to plane faster with hydrofoils, before you invest on them, simpley move yourself towards bow, sit in middle deck, not on tubes and give a full throttle, definetly will plane much faster on calm waters. If you choose a different prop to plane faster, will end losing actual top speed.

Happy Boating

 

[Helmuth]

Re: smaller prop on 11' inflatable 1983 7.5HP Johnson

Hi,

You may want to look at Maxi Marine Lifters trim tabs (http://www.maximarine.com).

Regards,

Helmuth
Maxi Marine

 

[Sea Rider]

Re: smaller prop on 11' inflatable 1983 7.5HP Johnson

By adding those extremely long lifters you are in a way extending the sib's water contact lenght, can those lifters be folded towards transom as not to interfere with back transom entanglement issues, somebody bumping against them when not in use ?

Happy Boating

 

[merc20076]

Re: smaller prop on 11' inflatable 1983 7.5HP Johnson

I can easily reach 16-17MPH with this 7.5HP when I'm alone! But it's slow to start to get it on plane but once it starts it accelerates pretty fast.


I use my 8.5X9 propeller, the smaller one did really not work fine.

I wonder how fast i'd reach with a modern 2 stroke 15HP?

So far I really like the stability of this boat!

 

[Sea Rider]

Re: smaller prop on 11' inflatable 1983 7.5HP Johnson

With a 15 HP standard prop and driver, will have probably 20 MPH compared to a 7.5, will gain better hole shot to plane faster and carry bigger loads.

Happy Boating

 

[Helmuth]

Re: smaller prop on 11' inflatable 1983 7.5HP Johnson

By adding those extremely long lifters you are in a way extending the sib's water contact lenght, can those lifters be folded towards transom as not to interfere with back transom entanglement issues, somebody bumping against them when not in use ?

Happy Boating

Yes, Lifters increase the water contact length or planing area which improves the weight carrying performance with passengers. They generally don't extend much beyond the outside tubes for entanglement or bumping to be an issue, and if desired, they can quickly be detached from the transom in the water (or for roll-up of the inflatable).

Regards,

Helmuth
Maxi Marine