Planning with a SIB, impact on the bottom/keel

PeterInSa · 21 December 2017, 07:11

I have an 2.7m SIB with a inflatable keel and hard floor, powered by a Watersnake T18S ( ie 18lbs of thrust) connected to a 60Amp Lithium battery. Wind tide/river flow impact on my speed so am looking at a 5.8HP Parsun Outboard or a smaller Parson from memory 3.8HP.

I have a 1/2 Cabin Boat with a largish outboard and when planning the fibreglass boat goes thump thump on waves, am wondering what the impact of Thump thump on the bottom of my SIB ditto the inflatable keel ie will it stretch the Poly? fabric?. The SIB is rated at 7.5HP max outboard.

Thanks

Peter

MCL · 21 December 2017, 09:00

SIBs that are designed to plane won't suffer any damage from planning. What kind of floor does yours have? Plywood/aluminium, high pressure air, or slatted.

PeterInSa · 22 December 2017, 06:54

Re Floor:To me its some form of wood coated with a plastic material (and heavy), In 4 sections the front V piece then 3 rectangle pieces. Have AL on each side of these 3 rectangle and V piece to slot into each other. Need to fit AL pieces on each side of the boat to hold the 3 rectangle pieces flat.

Mikefule · 22 December 2017, 07:45

If the hull is rated for a given engine size, you can assume that it will cope with the stresses of using that engine size at full revs in choppy water. In reality, you could almost certainly cope with "one size up" because they have allowed a safety margin, but I would not recommend it.

Inflatables are very robust, and their inherent flexibility acts as a shock absorber. It may be loud but that is because there is a lot of surface area to transmit the sound.

Assemble the boat carefully, check that the boards and stringers (side pieces) are snuggly fitted and that the tubes and keel are at the right pressure. Balance the load in the boat and ensure that items like the anchor box are secure. Build up your confidence incrementally. Don't just go out in the biggest waves you can find at full speed the first time!

After a few hours of using it, you will have forgotten what you were worried about.

Within the size range that you have mentioned, buy the biggest engine you can afford. It is easier to use a big engine at 50% throttle than a small engine at 150%!

Have fun. That's a nice size of boat.

rossymtb · 23 December 2017, 20:08

I personally wouldn't have anything less than a 6hp to get up on the plane. Even then we are usually talking single cylinder. Twin would be much better a 8hp.
But then if you look at the parsun 8hp it is the sake weight as a 9.8hp so makes no sense to buy the lower hp one but the bigger version and just use less throttle.

rossymtb · 23 December 2017, 20:10

Sorry we are talking electricity. My point regarding larger engine is invalid then as I was on about petrol powered.

But I would still buy biggest electric I could.

PeterInSa · 24 December 2017, 05:01

Hi Rossy, The 5.8HP is a Petrol unit the 6HP in Oz costs significantly more than the 5.8. Parsun in Oz is not a Boating Forum Favorite but to me good value for money. eg I have 2 Mercs on my 1/2 cabin boat.

My SIB has an engine limit of 7.5HP.

My 60Amp Lithium with a bigger motor would reduce my run time on the water and I would hate to run out of UMPH.

Peter

rossymtb · 24 December 2017, 07:39

I know what you are saying with Parsun not being "the best" and many people dislike them. But personally apart from a CDI break down on the older one I had last year it was a great engine. I now have the 15hp version

In your case I think the 5.8hp engine would be a great option. It's light enough 20kg has integral fuel tank with the option for a remote tank for longer distance to. And if you are anything like me no chance of forgetting to charge the battery before a trip out.