15hp is too much for a little SIB

[nabiul]

After a string of disappointing under-powered or half working motors for two fat guys on a little 10ft inflatable I decided to make the jump to a 15hp motor. I took it out by myself to test the motor and at first I was wondering why it couldn't match the worn out 9.5hp I had before. After about 20 minutes I went to adjust the trim setting and when I looked down I realized that my launch wheels were still in the water, so I pulled them up and gunned it. I wasn't ready for what happened next, the boat immediately jumped up on a plane and started smashing into the waves, hard. I was on lake ontario and a gentle breeze was making half a foot to 1ft swells, I thought the glued floor seam would rip right apart and the floor boards would split from the battering, I was getting thrown all over the place. I had to stop every 5 minutes of running and take a breather since there was no in between speed, either in the water at a crawl or on a plane and getting jack hammered. I was worried about my weight smashing into the floor so I sat backwards on the bench seat at the front while reaching back to steer; bad idea, the seat slipped off after a couple of hits and I fell backwards into the bow and twisted my shoulder while the motor was still going at WOT. I had enough, I turned the boat around and headed back to the safety of port at a crawl.

Inside the bay on still waters I clocked the boat at 40km/h, even then I was catching air and taking a battering. I don't think these soft floppy boats were meant to be abused like this.

 

[mrdrh99]

What kind of boat? I had a Saturn and properly inflated it handled a 15 very well! I bet you could easily trade that 15 for a mint condition 9.9

 

[Sea Rider]

Some issues :

If being heavy try sitting on deck right in middle towards bow with extended arm if solo boating, if with 2, one in each tube in front of the other, right in middle tubes.

Have learned that you should not wot on choppy windy water cond, will give a extreme dangerous bumpy ride. Wear kill cord on wrist at all times.

That 10 footer must be top air inflated along keel, that's with a pressure gauge to minimum 3.0 PSI once floating to perform as expected.

OB must be trimmed for combo to ride parellel to water level, provided that boaters weight is well distributed on deck, usually secon/third hole out transom.

Stay with that 15, that small sib has more hull drag than a larger one, an underpowered 9.8 HP will probably struggle to plane that sib with 2 heavy champs on board. Better is to have more HP than less, with trials and learning curve will master the demon shortly.

Which OB and Sib brand is it. It?s an air, wooden or alum deck ?

Happy Boating

 

[nabiul]

It's a bombard tropik 305 with wood floor boards and a johnson 15, both are mid 90's.

I inflated the keel to where my body weight wouldn't move the foot pump anymore and re-inflated on the water, although I should've probably done it a third time as the lake water is much colder than the bay.

 

[Chopperbill]

It was my second season before I figured out weight distribution. Wife lays at the bow and I ain't going to change that. You can slide your ass one foot either way and it make a difference in boat performance. Don't give up on that 15 yet.

 

[gwozhog]

When I use my 15hp on my 10 ft Achilles I sit on the floor when I'm solo and she flies at 29mph. Even at that speed it is very controllable.

 

[Sea Rider]

Moons back (1996) used to own a Zodiac made OMC-305 powered with a Evi 96, 15 HP OB, top air inflated with gauge performed very nice, had an excellent factory OB/Transom height match, combo flew with 2 on flat calm no wind water cond. What disliked, probably same as the Tropik 305 was bow tube had less diam compared to side tubes, nose was too pointy for my particular taste, lost too much available bow space which is a must have. Those were the glorious boating days were you had to extensively water test how these things worked properly in order to have fun.



Pumping an inflatable till foot has come to a stop does not guarantee it's fully inflated to its factory working pressure, will depend entirely on boater's weight, a skinny one will inflate much less than a heavy weight champ, you can test that with 2 different weight boaters along a pressure gauge. To pump fully any inflatable after some minutes rest on water for pressure to stabilize, is to use a good hand pump and pump it up to at least 3.0 PSI, this pressure will guarantee extraordinary water fun and excellent combo behavior at speed. Correct trim, & weight distribution on deck is also a must have...

Happy Boating

 

[nabiul]

I have the exact same boat and motor; I think the bombard 305 is just a zodiac rebrand. My equipment bag eats all the bow space so we sit on the side tubes while moving and then one of us moves upfront on the bench seat while fishing. I'm a pretty big guy around 245lbs, maybe I need to get an inline pressure gauge.

 

[Sea Rider]

If you been pumping your sib to a drum beat sound when banging hand against tubes or by finger gauge touch, you will definitely be way off from recommended working pressure. Need to have all 3 air chambers evenly inflated for combo to perform as expected. Assume Bombard sibs has those special diaphragm air valves compared to asian ones that uses Halkey Roberts type, if so, will need to buy a propietary air gauge from them as the mentioned one will not fit or work.

Happy Boating

 

[nabiul]

I retract my statement, my buddy and I finally had a chance to take this motor out onto the water and 15hp is not enough! LoL. We weigh as much as three regular sized people and we really had to rock back and forth to get out of the hole, when it finally caught plane though it was doing 35km/h.

Maybe one of those hydrofoil doodads will help.

Also the prop bushing or something gave out after we were done fishing and the propeller would just slip when we tried to throttle it up past like walking speed.

 

[Sea Rider]

Having owned that same size sib, didn't experienced planing issues with 2, definitely not light feather champ boaters with just an Evi 10 HP. You seem to have a prop with spun hub, make a spun hub test to check that. Useless to keep boating with prop issues. You can for go for a prop maximization to optimize hole shot with heavier boaters.

Happy Boating

 

[nabiul]

I got the prop off and I think the hub is spun. Also the size and pitch are crazy, 9.5x12.5 while the maximum I could find online for this motor was a 11" pitch. I'm taking this to the prop forum.